University of Southern California Dissertations and Theses

Development of a family of semi-random multichromophoric polymers for application in organic solar cells

(USC Thesis Other)

Development of a family of semi-random multichromophoric polymers for application in organic solar cells

PDF

Download a page range

Download transcript

Copy asset link

Request this asset

Transcript (if available)

Content DEVELOPMENT OF A FAMILY OF SEMI-RANDOM
MULTICHROMOPHORIC POLYMERS FOR APPLICATION IN ORGANIC
SOLAR CELLS

by

Beate Burkhart

A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(CHEMISTRY)

May 2013

Copyright 2013 Beate Burkhart
ii
DEDICATION

To Felix for always believing in me
iii
ACKNOWLEDGMENTS
First and foremost I would like to thank my advisor and mentor Prof Barry C.
Thompson for making this thesis possible and giving me the opportunity to work in the
exciting field of polymer photovoltaics. I’m grateful for his outstanding support and
mentorship during the last 4 1/2 years. I have learned invaluable lessons from him, not
only about research but also about leadership and personal ethics, which will continue to
guide me in my career and life.
I am very thankful to Petr P. Khlyabich, whose contribution to this work cannot
be valued highly enough. Only because of his extensive knowledge and dedication to the
fabrication and measurement of polymer solar cells it was possible to realize the potential
of semi-random polymers and achieve the outstanding efficiencies presented in this work.
I’m also deeply grateful for innumerable challenging scientific discussions, laughter and
simply his friendship.
I would like to thank Prof. G. K. Surya Prakash and Prof Mark E. Thompson for
their important guidance and generous support throughout this whole process. My
gratitude furthermore extends to Prof. Thieo E. Hogen-Esch and Prof. Andrea Armani for
being members of my qualifying exam committee and to Michelle Dea, Katie McKissick,
David Hunter, Jessy May, Carole Phillips, Allan Kershaw and Ross Lewis for all their
help over the last years.
For their scientific support and invaluable guidance especially in the first years
many thanks go to: Dr. Somesh Kumar, Dr. Miklós Czaun, Dr. Parag Jog, Dr William
Wilson and Dr. Tûba Çakir-Çanak.
iv
I also want to thank our collaborators: Prof. Steve E. Bradforth, Prof. Alex V.
Benderskii, Prof. Joe C. Campbell (University of Virginia) and Prof. Frederik C. Krebs
(DTU Risø).
To all my friends and colleagues at USC and especially Hema Krishnan, Petr P.
Khlyabich, Somesh Kumar, Alejandra Aviles, Andrey Rudenko, John-Paul Jones, Dr.
Maria Frushicheva, Janet Olsen and Dr. Tûba Çakir-Çanak. Thank you for your
friendship and mentorship. I could not have done it without you (and the coffee breaks).
Thank you very much to my whole family and especially to my parents for all the
opportunities and support they have offered me over the years. Thank you to my
grandfather for taking me serious as a child and gifting me his chemistry books. Special
thanks to my sister Sabine and my brother-in-law Franz for always being there for me
(even if it is 2 o’clock in the morning) and cheering me up.
Felix, I’m proud to be your wife. Thank you for all your patience, support and
love. And most importantly: thank you for always believing in me.

v

TABLE OF CONTENTS
DEDICATION……. ..................................................................................................... ii
ACKNOWLEDGMENTS ........................................................................................... iii
LIST OF TABLES ..................................................................................................... viii
LIST OF FIGURES .......................................................................................................x
ABSTRACT……. .................................................................................................... xviii
CHAPTER 1 Design of Conjugated Polymers for Organic Photovoltaics ............1
1.1 Semi-Random Conjugated Polymers in the Context of Organic Solar
Cells… ............................................................................................................1
1.2 General Introduction: Organic Solar Cells ......................................................2
1.3 Design Principles for Band Gap and Energy Level Control of Polymers .....11
1.4 State-of-the-Art Polymer Donors for Organic Solar Cells ............................17
1.4.1 Homopolymers .......................................................................................18
1.4.2 Perfectly Alternating Donor/Acceptor Copolymers ..............................21
1.4.2.1 Benzothiadiazole Based D/A Copolymers .....................................23
1.4.2.2 Thienopyrroledione Based D/A Copolymers .................................32
1.4.2.3 Diketopyrrolopyrrole and Isoindigo Based D/A Copolymers ........36
1.4.2.4 Quinoxaline Based D/A Copolymers .............................................41
1.4.2.5 Quinoidal Acceptors: Thienothiophene, Thienopyrazine and
Isothianaphthene ............................................................................43
1.4.3 Random Donor/Acceptor Copolymers ..................................................48
1.5 A Novel Family of Donor Materials: Semi-Random Copolymers ...............53
1.6 References Chapter 1 ....................................................................................55
CHAPTER 2 Semi-Random Multichromophoric rr-P3HT Analogues: Design
and First Generation .....................................................................................................69
2.1 Introduction ...................................................................................................69
2.2 Synthesis of Semi-Random Copolymers ......................................................73
2.3 Characterization and Solar Cell Performance ...............................................75
2.4 Conclusion .....................................................................................................80
2.5 References Chapter 2 ....................................................................................81
CHAPTER 3 Efficient Solar Cells from Semi-Random P3HT Analogues
Incorporating Diketopyrrolopyrrole.............................................................................83
vi
3.1 Introduction ...................................................................................................83
3.2 Synthesis, Characterization and Solar Cell Performance of DPP
Containing Semi-Random Polymers.............................................................84
3.3 Conclusion .....................................................................................................94
3.4 References Chapter 3 ....................................................................................95
CHAPTER 4 Semi-Random Two Acceptor Copolymers: Influence of the
Acceptor Composition on Physical Properties and Solar Cell Performance ...............98
4.1 Introduction ...................................................................................................98
4.2 Synthesis, Characterization and Solar Cell Performance ..............................99
4.3 Conclusion ...................................................................................................111
4.4 References for Chapter 4 .............................................................................113
CHAPTER 5 Semi-Random Two-Acceptor Copolymers: Elucidating
Electronic Trends Though Multiple Acceptor Combinations ....................................115
5.1 Introduction .................................................................................................115
5.2 Results and Discussion ................................................................................116
5.3 Conclusions .................................................................................................130
5.4 References Chapter 5 ..................................................................................132
CHAPTER 6 Solar Cells Based on Semi-Random P3HT Analogues
Containing Dithienopyrrole: Influence of Incorporating a Strong Donor .................133
6.1 Introduction .................................................................................................133
6.2 Results and Discussion ................................................................................134
6.3 Conclusion ...................................................................................................146
6.4 References for Chapter 6 .............................................................................148
CHAPTER 7 Influence of the Ethylhexyl Side-Chain Content on the Open-
Circuit Voltage in rr-Poly(3-hexylthiophene-co-3-(2-ethylhexyl)thiophene)
Copolymers……… ....................................................................................................150
7.1 Introduction .................................................................................................150
7.2 Results and Discussion ................................................................................156
7.3 Summary and Conclusions ..........................................................................170
7.4 References for Chapter 7 .............................................................................174
CHAPTER 8 Conclusions and Outlook .............................................................177
8.1 Semi-Random Copolymers: Summary and Future Directions ....................177
8.2 General Thoughts and Conclusions on Polymer Solar Cells ......................181
8.3 References Chapter 8 ..................................................................................183
BIBLIOGRAPHY……. .............................................................................................185
APPENDIX 1 Semi-Random Multichromophoric rr-P3HT Analogues: Design
and First Generation ...................................................................................................214
vii
A1.1 Materials and Methods ................................................................................214
A1.2 Synthesis......................................................................................................216
A1.3 Polymer Characterization ............................................................................223
A1.4 Device Fabrication and Characterization ....................................................229
A1.5 References Appendix 1 ...............................................................................232
APPENDIX 2 Efficient Solar Cells from Semi-Random P3HT Analogues
Incorporating Diketopyrrolopyrrole...........................................................................233
A2.1 Synthesis......................................................................................................233
A2.2 Polymer Characterization ............................................................................239
A2.3 Device Fabrication and Characterization ....................................................246
A2.4 References Appendix 2 ...............................................................................250
APPENDIX 3 Semi-Random Two Acceptor Copolymers: Influence of the
Acceptor Composition on Physical Properties and Solar Cell Performance .............251
A3.1 Synthesis......................................................................................................251
A3.2 Polymer Characterization ............................................................................255
A3.3 Device Fabrication and Characterization ....................................................261
A3.4 References Appendix 3 ...............................................................................263
APPENDIX 4 Semi-Random Two-Acceptor Copolymers: Elucidating
Electronic Trends Though Multiple Acceptor Combinations ....................................264
A4.1 Synthesis......................................................................................................264
A4.2 Polymer Characterization ............................................................................267
A4.3 Device Fabrication and Characterization ....................................................272
APPENDIX 5 Solar Cells Based on Semi-Random P3HT Analogues
Containing Dithienopyrrole: Influence of Incorporating a Strong Donor .................274
A5.1 Synthesis......................................................................................................274
A5.2 Device Fabrication and Characterization ....................................................278
A5.3 References Appendix 5 ...............................................................................279
APPENDIX 6 Influence of the Ethylhexyl Side-Chain Content on the Open-
Circuit Voltage in rr-Poly(3-hexylthiophene-co-3-(2-ethylhexyl)thiophene)
Copolymer……..........................................................................................................280
A6.1 Synthesis......................................................................................................280
A6.2 Polymer Characterization ............................................................................284
A6.3 Device Fabrication and Characterization ....................................................293
A6.4 References Appendix 6 ...............................................................................293

viii
LIST OF TABLES
Table 1.1 Structure and solar cell performance of select homopolymers. ..................18
Table 1.2 Structure and solar cell performance of select PCPDTBT analogs. ...........24
Table 1.3 Structure and solar cell performance of select PCDTBT analogs. ..............27
Table 1.4 Structure and solar cell performance of benzodithiophene/ benzothia-
diazole copolymers. .....................................................................................................29
Table 1.5 Structure and solar cell performance of polymers with benzothiadia-
zole analogs as the acceptor. ........................................................................................31
Table 1.6 Structure and solar cell performance of thienopyrroledione based co-
polymers. ......................................................................................................................35
Table 1.7 Structure and solar cell performance of diketopyrrolopyrrole based
copolymers. ..................................................................................................................39
Table 1.8 Structure and solar cell performance of quinoxaline based copo-
lymers. ..........................................................................................................................42
Table 1.9 Solar cell performance of select PTB copolymers. .....................................45
Table 1.10 Structure and solar cell performance of thienothiophene based co-
polymers. ......................................................................................................................47
Table 1.11 Solar cell performance of TPT based random copolymers. ......................49
Table 1.12 Solar cell performance of TPTPT based copolymers. ..............................50
Table 1.13 Structure and solar cell performance of PDPP-T-DPP. ............................51
Table 1.14 Solar cell performance of benzotrithiophene based polymers. .................52

Table 2.1 Molecular weights (PDI’s), electrochemical HOMO values, optical
band gaps and SCLC mobilities of synthesized polymers. ..........................................79

Table 3.1 Photovoltaic properties of P3HT, P3HTT-DPP-5%, P3HTT-DPP-10%
and P3HTT-DPP-15% with PC
61
BM as an acceptor. ..................................................90

Table 4.1 Electronic and photovoltaic properties of P3HTT-TPD-10%, P3HTT-
TPD-15%, P3HTT-TPD-DPP (1:1), P3HTT-TPD-DPP (2:1) and P3HTT-TPD-
DPP (1:2) as well as optimized solar cell results of P3HT, P3HTT-DPP-10%
and P3HTT-DPP-15% with PC
61
BM as an acceptor. ................................................107
ix

Table 5.1 Molecular weights (PDI’s), electrochemical HOMO values and
optical band gaps of two-acceptor semi-random copolymers. ...................................122
Table 5.2 Electronic and photovoltaic properties of optimized two-acceptor
semi-random copolymers. ..........................................................................................124

Table 6.1 Molecular weights (PDI’s), electrochemical HOMO values and
optical band gaps of synthesized polymers and for comparison P3HTT-DTP-
10% and P3HTT-BTD. ..............................................................................................137
Table 6.2 Photovoltaic properties of P3HT, P3HTT-DTP, P3HTT-BTD-DTP,
P3HTT-TP-DTP, P3HTT-DPP-DTP and P3HTT-DPP-10% with PC
61
BM as the
acceptor ......................................................................................................................143

Table 7.1 Molecular and electronic properties of polymers. .....................................158
Table 7.2 Photovoltaic properties of P3HT, P3HT
90
-co-EHT
10
, P3HT
75
-co-
EHT
25
, P3HT
50
-co-EHT
50
and P3EHT with PC
61
BM as the acceptor. ......................165

Table A2.1 Optical properties of P3HT, P3HTT-DPP-5%, P3HTT-DPP-10%
and P3HTT-DPP-15% in o-DCB and thin films, spin-coated from o-DCB. .............243
Table A2.2 Raw short-circuit current densities (Jsc), spectral-mismatch factor
(M), spectral mismatch-corrected short-circuit current densities (Jsc,corr) and
integrated short-circuit current densities (Jsc,EQE). .................................................249

Table A3.1 Raw short-circuit current densities (J
sc
), spectral-mismatch factor
(M), spectral mismatch-corrected short-circuit current densities (J
sc,corr
) and
integrated short-circuit current densities (J
sc,EQE
). .....................................................262

Table A4.1 Raw short-circuit current densities (J
sc
), spectral-mismatch factor
(M), spectral mismatch-corrected short-circuit current densities (J
sc,corr
) and
integrated short-circuit current densities (J
sc,EQE
). .....................................................273

x
LIST OF FIGURES
Figure 1.1 Schematic Illustration of a) a donor-acceptor bilayer solar cell; b) a
BHJ solar cell; c) typical current-voltage curves for dark and light currents in
solar cells with the output parameters: J
sc
(short circuit current density), V
oc

(open circuit voltage), FF (fill factor) and J
m
and V
m
(current and voltage at
maximum power point) - the power conversion efficiency ( η) and FF are defined
below the J-V-curve; and d) simplified energy level diagram illustrating the
HOMO and LUMO energies of a donor and acceptor pair, the band gap of the
donor (E
g
), the V
oc
which is empirically related to the HOMO
D
– LUMO
A
offset
and the necessary driving force for exciton dissociation (LUMO
D
– LUMO
A
>
0.3 eV)............................................................................................................................5
Figure 1.2 Photon flux density from the sun (AM 1.5G) as a function of
wavelength. The red curve represents the integrated short circuit current density
which is on the y-axis at the right. .................................................................................8
Figure 1.3 a) Non-degenerate ground states of conjugated polymers: the
aromatic and quinoidal form of poly(thiophene) and poly(isothianaphthene) are
shown as examples, b) charge separated structure of a D/A copolymer
(PBTTPD
86
) where thienopyrroledione is the acceptor and dodecylthiophene the
donor unit, c) head-to-tail, head-to-head and tail-to-tail connection of substituted
thiophenes and steric hindrance between o-hydrogen atoms of neighboring
phenyl units (torsional twists around single bonds are indicated with a curved
arrow). ..........................................................................................................................13
Figure 1.4 a) Common donor monomers, b) common acceptor monomers and c)
quinoidal acceptors. .....................................................................................................14
Figure 1.5 Literature examples for a) Stille polymerization, b) Suzuki
polymerization

and c) direct arylation polymerization. ...............................................22
Figure 1.6 Benzothiadiazole analogs used in efficient polymer:fullerene BHJ
solar cells. ....................................................................................................................30
Figure 1. 7 Structures for PTB1, PTB3, PTB4, PTB7 and PTB9. ..............................44
Figure 1.8 TPT-based random copolymers. ................................................................49
Figure 1.9 Ladder type TPTPT based random copolymers. .......................................50
Figure 1.10 Benzotrithiophene based D/A copolymers: BTT-DPP (perfectly
alternating) and P1 (random). ......................................................................................52

xi
Figure 2.1 Concept of multichromophoric semi-random copolymers (a) and a
representative synthetic scheme (b), along with the structures of the described
first generation polymers (c). .......................................................................................71
Figure 2.2 UV/vis absorption of all five polymers in (a) solution (o-DCB) and
(b) thin film (spin-coated from o-DCB and annealed for 30 min at 60
o
C under
N
2
) where (i) is P3HT, (ii) is P3HTT, (iii) is P3HTT-BTD, (iv) is P3HTT-TP
and (v) is P3HTT-TP-BTD. .........................................................................................76
Figure 2.3 GIXRD of thin films spin-coated from CB before and after annealing
at 150
o
C for 30 min under N
2
. The inset shows the region around 2Θ = 5 -7
o
in
greater detail. Polymers shown are P3HT ((i)a, (i)b), P3HTT-BTD ((iii)a) and
P3HTT-TP-BTD ((v)a, (v)b) where b stands for before annealing and a for after
annealing. .....................................................................................................................78

Figure 3.1 Synthesis and structures of P3HTT-DPP-5%, P3HTT-DPP-10% and
P3HTT-DPP-15%. .......................................................................................................85
Figure 3.2 UV/vis absorption spectra of polymers in (a) solution (o-DCB) and
(b) thin film (spin-coated from o-DCB) where (i) is P3HT (annealed at 150 ºC
for 30 min for the thin films), (ii) is P3HTT-DPP-5% (thin film as-cast), (iii) is
P3HTT-DPP-10% (thin film as-cast) and (iv) is P3HTT-DPP-15% (thin film as-
cast).. ............................................................................................................................88
Figure 3.3 EQE of the BHJ solar cells based on P3HT (black squares), P3HTT-
DPP-5% (red circles), P3HTT-DPP-10% (green triangles) and P3HTT-DPP-
15% (blue stars) with PC
61
BM as the acceptor, under optimized conditions for
device fabrication. ........................................................................................................93

Figure 4.1 Synthesis of (a) TPD containing semi-random polymers P3HTT-
TPD-10% and P3HTT-TPD-15% and (b) two-acceptor polymers containing
TPD and DPP (P3HTT-TPD-DPP (1:1), P3HTT-TPD-DPP (2:1) and P3HTT-
TPD-DPP (1:2)). ........................................................................................................101
Figure 4.2 UV/vis absorption spectra of polymers in thin films (spin-coated
from o-DCB and solvent annealed for 20 minutes under N
2
) where a) are
polymers containing one acceptor (either TPD or DPP) and P3HT synthesized
by the same method for reference and b) are polymers containing two acceptors
(TPD and DPP). (i) is P3HT (black line), (ii) is P3HTT-TPD-10% (red line), (iii)
is P3HTT-TPD-15% (orange line), (iv) is P3HTT-DPP-10% (dark green line),
(v) is P3HTT-DPP-15% (light green line), (vi) is P3HTT-TPD-DPP (2:1) (cyan
line), (vii) is P3HTT-TPD-DPP (1:1) (purple line) and (viii) is P3HTT-TPD-
DPP (1:2) (magenta line). ..........................................................................................104
xii
Figure 4.3 EQE of the BHJ solar cells based on P3HT (black open squares),
P3HTT-TPD-10% (red open circles), P3HTT-TPD-15% (orange diamonds),
P3HTT-DPP-10% (dark green stars), P3HTT-DPP-15% (light green circles),
P3HTT-TPD-DPP (1:1) (purple triangles), P3HTT-TPD-DPP (2:1) (cyan
squares) and P3HTT-TPD-DPP (1:2) (magenta upside down triangles) with
PC
61
BM as the acceptor, under optimized conditions for device fabrication. ...........110

Figure 5.1 Synthesis of semi-random polymers and structures of two-acceptor
copolymers P3HTT-BTD-DPP, P3HTT-TP-DPP, P3HTT-BTD-TPD and
P3HTT-TP-TPD. ........................................................................................................117
Figure 5.2 UV/vis absorption spectra of two-acceptor polymers (solid lines) and
corresponding one-acceptor polymers (dashed lines) in thin films. a) P3HTT-
BTD-DPP (solid blue), P3HTT-BTD (dashed purple) and P3HTT-DPP (dashed
light blue), b) P3HTT-TP-DPP (solid orange), P3HTT-TP (dashed light green)
and P3HTT-DPP (dashed light blue), c) P3HTT-BTD-TPD (solid green),
P3HTT-BTD (dashed purple) and P3HTT-TPD (dashed black), d) P3HTT-TP-
TPD (solid red), P3HTT-TP (dashed light green) and P3HTT-TPD (dashed
black)..........................................................................................................................118
Figure 5.3 GIXRD of thin films annealed for 45 min at 145 °C under N
2
.
Polymers shown are (i) P3HT (black), (ii) P3HTT-BTD-DPP (blue), (iii)
P3HTT-TP-DPP (orange), (iv) P3HTT-BTD-TPD (green) and (v) P3HTT-TP-
TPD (red). The inset shows the region around 2θ = 4° - 8° in more detail. ..............120
Figure 5.4 CV traces for the oxidation of (i) P3HT (black), (ii) P3HTT-BTD-
DPP (blue), (iii) P3HTT-TP-DPP (orange), (iv) P3HTT-BTD-TPD (green) and
(v) P3HTT-TP-TPD (red). Ferrocene was used as a reference and values were
converted to the vacuum scale using the approximation that the ferrocene redox
couple is 5.1 eV relative to vaccum. ..........................................................................123
Figure 5.5 J-V curves of BHJ solar cells based on (i) P3HT (black), (ii) P3HTT-
BTD-DPP (blue), (iii) P3HTT-TP-DPP (orange), (iv) P3HTT-BTD-TPD (green)
and (v) P3HTT-TP-TPD (red) with PC
61
BM as the acceptor under AM 1.5G
illumination (100 mW/cm
2
) at the optimal conditions for solar cell performance. ...126
Figure 5.6 EQE of the BHJ solar cells based on P3HT (black upside down
triangles), P3HTT-BTD-DPP (blue circles), P3HTT-TP-DPP (orange squares),
P3HTT-BTD-TPD (green triangles) and P3HTT-TP-TPD (red stars) with
PC
61
BM as the acceptor under optimized conditions for device fabrication. ............128
Figure 5.7 TEM images of polymer:PC
61
BM films (optimized conditions for
best solar cell performance were used to make the films). P3HT:PC
61
BM is
shown for reference....................................................................................................129

xiii
Figure 6.1 Synthesis of 2,6-dibromo-N-(1-butylpentyl)dithieno[3,2-b:2’,3’-d]-
pyrrole (DTP) (4) (a) and a representative synthetic scheme for semi-random
Stille polymerization (b). ...........................................................................................135
Figure 6.2 Structures of P3HTT-DTP, P3HTT-BTD-DTP, P3HTT-TP-DTP and
P3HTT-DPP-DTP (a) and for comparison P3HTT-DPP-10% and P3HTT-BTD
(b). ..............................................................................................................................136
Figure 6.3 UV/vis absorption spectra of polymers in (a) solution (o-DCB) and
(b) thin film (spin-coated from o-DCB) where (i) is P3HT (black line) (annealed
at 150 C for thin film), (ii) is P3HTT-DTP (red line), (iii) is P3HTT-BTD-DTP
(blue line), (iv) is P3HTT-TP-DTP (light green line), (v) is P3HTT-DPP-DTP
(purple line) and (vi) is P3HTT-DPP-10% (dark green line) (thin film as cast).
For all DTP containing polymers the thin films are annealed at 60 C for 30 min. ..139
Figure 6.4 GIXRD of thin films. Polymers shown are (i) P3HT (red line) spin-
coated from o-DCB and annealed at 150 C for 30 min under N
2
, (ii) P3HT
(black line) spin-coated from CB and annealed at 150 C for 30 min under N
2
,
(iii) P3HTT-DPP-DTP (dark green line) spin coated from o-DCB and annealed
at 150 C for 30 min under N
2
and (iv) P3HTT-DPP-DTP (purple line) spin
coated from o-DCB and solvent annealed for 20 min under N
2
. The inset shows
the region around 2θ = 4  - 7 in more detail. ...........................................................141
Figure 6.5 CV traces for the oxidation of (i) P3HT (black line), (ii) P3HTT-
DTP (red line), (iii) P3HTT-BTD-DTP (blue line), (iv) P3HTT-TP-DTP (green
line) and (v) P3HTT-DPP-DTP (purple line). Ferrocene was used as a reference
and values were converted to the vacuum scale using the approximation that the
ferrocene redox couple is 5.1 eV relative to vacuum .................................................142
Figure 6.6 J-V curves of the BHJ solar cells based on (i) P3HT (black line) , (ii)
P3HTT-DTP (red line), (iii) P3HTT-BTD-DTP (blue line), (iv) P3HTT-TP-DTP
(green line) and (v) P3HTT-DPP-DTP (purple line) with PC
61
BM as the
acceptor under AM 1.5G illumination (100 mW/cm
2
) at the optimal conditions
for solar cell performance. .........................................................................................143
Figure 6.7 EQE of the BHJ solar cells based on P3HT (black squares), P3HTT-
DTP (red circles), P3HTT-BTD-DTP (blue triangles), P3HTT-TP-DTP (green
stars) and P3HTT-DPP-DTP (purple triangles) with PC
61
BM as the acceptor
under optimized conditions for device fabrication. ...................................................146

Figure 7.1 Synthesis of monomer 2-bromo-5-trimethyltin-3-(2-ethylhexyl)thio-
phene (3) (a) and Stille polymerization for poly(3-hexylthiophene-co-3-(2-
ethylhexyl)thiophene) (b). ..........................................................................................158
xiv
Figure 7.2 UV/vis absorption of all five polymers in (a) solution (CB) and (b)
thin films (spin coated from CB and annealed for 30 min under N
2
at 150 ºC for
(i), (ii) and (iii), 100 ºC for (iv) and 40 ºC for (v)) where (i) is P3HT (purple
line), (ii) is P3HT
90
-co-EHT
10
(green line), (iii) is P3HT
75
-co-EHT
25
(blue line),
(iv) is P3HT
50
-co-EHT
50
(red line) and (v) is P3EHT (orange line)..........................160
Figure 7.3 GIXRD of thin films spin-coated from CB and annealed for 30 min
under N
2
(annealing temperature was 150 ºC for (i)-(iii), 100 ºC for (iv) and 40
ºC for (v)) are shown where (i) is P3HT (purple line), (ii) is P3HT
90
-co-EHT
10

(green line), (iii) is P3HT
75
-co-EHT
25
(blue line), (iv) is P3HT
50
-co-EHT
50
(red
line) and (v) is P3EHT (orange line). The inset shows the region around 2θ = 4º
- 8º in greater detail. ...................................................................................................162
Figure 7.4 Melting points of polymers as measured by DSC. ..................................163
Figure 7.5 J-V curves of the BHJ solar cells based on (i) P3HT (purple line), (ii)
P3HT
90
-co-EHT
10
(green line), (iii) P3HT
75
-co-EHT
25
(blue line), (iv) P3HT
50
-
co-EHT
50
(red line) and (v) P3EHT (orange line) with PC
61
BM as the acceptor
under AM 1.5G illumination (100 mW/cm
2
) at the optimal conditions for solar
cell performance.........................................................................................................166
Figure 7.6 HOMO levels of the polymers in the solid state (filled squares) and
V
oc
(circles) of the optimized solar cells as a function of the amount of 2-
ethylhexyl side-chains in the polymer backbone. ......................................................168
Figure 7.7 TEM images of polymer:PC
61
BM films (optimized conditions for
best solar cell performance were used to make the films) where a) is P3HT, b) is
P3HT
90
-co-EHT
10
, c) is P3HT
75
-co-EHT
25
, d) is P3HT
50
-co-EHT
50
and e) is
P3EHT........................................................................................................................170

Figure A1.1 Synthesis of 5,7-dibromo-thieno[3,4-b]pyrazine. .................................216
Figure A1.2 Synthesis of 4,7-dibromo-2,13-benzothiadiazole. ................................218
Figure A1.3 Synthesis of 2-bromo-5-trimethyltin-3-hexylthiophene. ......................219
Figure A1.4 Synthesis of 2,5-bis(trimethyltin)thiophene. ........................................220
Figure A1.5 Synthesis of P3HT and semi-random copolymers P3HTT, P3HTT-
BTD, P3HTT-TP and P3HTT-TP-BTD. ...................................................................221
Figure A1.6
1
H NMR of P3HT in CDCl
3
. ................................................................223
Figure A1.7
1
H NMR of P3HTT in CDCl
3
. ..............................................................224
Figure A1.8
1
H NMR of P3HTT-BTD in CDCl
3
......................................................225
xv
Figure A1.9
1
H NMR of P3HTT-TP in CDCl
3
. ........................................................226
Figure A1.10
1
H NMR of P3HTT-TP-BTD in tetrachloroethane-d2. .....................227
Figure A1.11 CV traces for the oxidation of P3HT, P3HTT, P3HTT-BTD,
P3HTT-TP and P3HTT-TP-BTD. .............................................................................228
Figure A1.12 DSC traces of P3HTT and P3HTT-BTD. ...........................................229
Figure A1.13 J-V curves of solar cells based on P3HT, P3HTT-TP, P3HTT-
BTD and P3HTT-TP-BTD. .......................................................................................232

Figure A2.1 Synthesis of 2,5-diethylhexyl-3,6-bis(5-bromothiophene-2-yl)-
pyrrolo[3,4-c]-pyrrole-1,4-dione (3). .........................................................................233
Figure A2.2 Synthesis of semi-random copolymers P3HTT-DPP-5%, P3HTT-
DPP-10% and P3HTT-DPP-15%. .............................................................................236
Figure A2.3
1
H NMR of P3HTT-DPP-5% in CDCl
3
................................................239
Figure A2.4
1
H NMR of P3HTT-DPP-10% in CDCl
3
..............................................240
Figure A2.5
1
H NMR of P3HTT-DPP-15% in tetrachloroethane-d2. .....................241
Figure A2.6 CV traces for the oxidation of a. P3HTT-DPP-5%, b. P3HTT-DPP-
10% and c. P3HTT-DPP-15%. ..................................................................................244
Figure A2.7 GIXRD of thin films of (i) P3HT (spin-coated from CB and
annealed at 150 ºC for 30 min under N
2
) (black line), (ii) P3HTT-DPP-5% (spin-
coated from o-DCB and annealed at 150 ºC for 30 min under N
2
) (red line), (iii)
P3HTT-DPP-10% (spin-coated from o-DCB and tested as-cast) (green line), (iv)
P3HTT-DPP-10% (spin-coated from o-DCB and annealed at 150

ºC for 30 min
under N
2
) (purple line) and (v) P3HTT-DPP-15% (spin-coated from o-DCB and
annealed at 150

ºC for 30 min under N
2
) (blue line). .................................................245
Figure A2.8 TEM images of (a) P3HT:PC
61
BM, (b) P3HTT-DPP-5%:PC
61
BM,
(c) P3HTT-DPP-10%:PC
61
BM and (d) P3HTT-DPP-15%:PC
61
BM prepared
under optimal solar cells conditions. .........................................................................246
Figure A2.9 J-V curves of the BHJ solar cells based on P3HT (black line),
P3HTT-DPP-5% (red line), P3HTT-DPP-10% (green line) and P3HTT-DPP-
15% (blue line) with PC
61
BM as the acceptor under AM 1.5G illumination (100
mW/cm
2
) at the optimal conditions for solar cell performance. ................................249

Figure A3.1
1
H NMR of P3HTT-TPD-10% in tetrachloroethane-d2 at 60 °C. .......255
xvi
Figure A3.2
1
H NMR of P3HTT-TPD-15% in tetrachloroethane-d2 at 60 °C. .......255
Figure A3.3
1
H NMR of P3HTT-TPD-DPP (1:1) in tetrachloroethane-d2 at 60
°C. ..............................................................................................................................256
Figure A3.4
1
H NMR of P3HTT-TPD-DPP (2:1) in tetrachloroethane-d2 at 60
°C. ..............................................................................................................................256
Figure A3.5
1
H NMR of P3HTT-TPD-DPP (1:2) in tetrachloroethane-d2 at 60
°C. ..............................................................................................................................257
Figure A3.6 UV/vis spectra of polymer solutions in o-DCB with P3HTT-DPP-
10% and P3HTT-DPP-15% for reference..................................................................257
Figure A3.7 CV traces for the oxidation of thin films (as cast) in acetonitrile
with ferrocene as the reference (Fc/Fc
+
= 5.1 eV relative to vacuum). .....................258
Figure A3.8 GIXRD of thin films spin coated from o-DCB. P3HT was solvent
annealed for 20 minutes under N
2
and P3HTT-TPD-10%, P3HTT-TPD-DPP
(1:1), P3HTT-TPD-DPP (2:1), P3HTT-TPD-DPP (1:2) were thermally annealed
at 150 °C for 45 minutes under N
2
. P3HTT-TPD-15% did not show any
crystalline features. ....................................................................................................258
Figure A3.9 GIXRD of thin films spin coated from o-DCB and solvent
annealed for 20 minutes under N
2
. Only P3HTT-TPD-DPP (1:1) and P3HTT-
TPD-DPP (1:2) show crystalline features whereas P3HTT-TPD-10%, P3HTT-
TPD-15% and P3HTT-TPD-DPP (2:1) are completely amorphous. .........................259
Figure A3.10 TEM images of a) P3HT:PC
61
BM, b) P3HTT-DPP-10%:
PC
61
BM, c) P3HTT-DPP-15%:PC
61
BM, d) P3HTT-TPD-10%:PC
61
BM, e)
P3HTT-TPD-15%:PC
61
BM, f) P3HTT-TPD-DPP (1:1):PC
61
BM, g) P3HTT-
TPD-DPP (2:1):PC
61
BM, h) P3HTT-TPD-DPP (1:2):PC
61
BM prepared under
optimal solar cell conditions. .....................................................................................260
Figure A3.11 J-V curves of all polymer:PC
61
BM BHJ solar cells under AM
1.5G illumination (100 mW/cm
2
) at optimized conditions for solar cell
performance. P3HT is shown for reference. ..............................................................263

Figure A4.1
1
H NMR of P3HTT-BTD-DPP in tetrachloroethane-d2 at ..................267
Figure A4.2
1
H NMR of P3HTT-TP-DPP in tetrachloroethane-d2 at .....................268
Figure A4.3
1
H NMR of P3HTT-BTD-TPD in tetrachloroethane-d2 at .................269
Figure A4.4
1
H NMR of P3HTT-TP-TPD in tetrachloroethane-d2 at .....................270
xvii
Figure A4.5 UV/vis absorption spectra of two-acceptor copolymers in o-DCB
solution. (i) is P3HT (black), (ii) is P3HTT-BTD-DPP (blue), (iii) is P3HTT-TP-
DPP (orange), (iv) is P3HTT-BTD-TPD (green) and (v) is P3HTT-TP-TPD
(red). ...........................................................................................................................271
Figure A4.6 GIXRD of thin films spin coated from o-DCB and solvent
annealed for 20 min under N
2
. (i) is P3HT (black), (ii) is P3HTT-BTD-DPP
(blue) and (iii) is P3HTT-TP-DPP (orange). .............................................................271
Figure A4.7 Linear correlation between HOMO energies and V
oc
of semi-
random two-acceptor polymers (P3HTT-TP-BTD, P3HTT-TPD-DPP (1:1),
P3HTT-TPD-DPP (1:2), P3HTT-TPD-DPP (2:1), P3HTT-BTD-DPP, P3HTT-
TP-DPP, P3HTT-BTD-TPD and P3HTT-TP-TPD. The data point at 5.29 eV/
0.36 V corresponding to P3HTT-TP-TPD is excluded when plotting the linear
correlation (red line, R = 0.889).................................................................................273

Figure A6.1
1
H NMR of P3HT in CDCl
3
. ................................................................284
Figure A6.2
1
H NMR of P3HT
90
-co-EHT
10
in CDCl
3
. .............................................285
Figure A6.3
1
H NMR of P3HT
75
-co-EHT
25
in CDCl
3
. .............................................286
Figure A6.4
1
H NMR of P3HT
50
-co-EHT
50
in CDCl
3
. .............................................287
Figure A6.5
1
H NMR of P3EHT in CDCl
3
. ..............................................................288
Figure A6.6 UV/vis absorption of all five polymers in as cast thin films (spin
coated from CB) where P3HT is purple line, P3HT
90
-co-EHT
10
is green line,
P3HT
75
-co-EHT
25
is blue line, P3HT
50
-co-EHT
50
is red line and P3EHT is
orange line. .................................................................................................................289
Figure A6.7 DSC curves where a) is P3HT, b) is P3HT
90
-co-EHT
10
, c) is
P3HT
75
-co-EHT
25
and d) is P3HT
50
-co-EHT
50
. .........................................................290
Figure A6.8 CV traces for the oxidation of thin films (as cast) where a) is
P3HT, b) is P3HT
90
-co-EHT
10
, c) is P3HT
75
-co-EHT
25
, d) is P3HT
50
-co-EHT
50

and e) is P3EHT. Ferrocene was used as a reference and values were converted
to the vacuum scale using the approximation that the ferrocene redox couple is
5.1 eV relative to vacuum. .........................................................................................291
Figure A6.9 CV traces for the oxidation of polymers in solution (CHCl
3
with
tetrabutylammonium tetrafluoroborate as supporting electrolyte) where a) is
P3HT, b) is P3HT
90
-co-EHT
10
, c) is P3HT
75
-co-EHT
25
, d) is P3HT
50
-co-EHT
50

and e) is P3EHT. Ferrocene was used as a reference and values were converted
to the vacuum scale using the approximation that the ferrocene redox couple is
5.1 eV relative to vacuum. .........................................................................................292
xviii
ABSTRACT
The energy demand of the world population is predicted to increase up to 40% by
2035 and one of the biggest tasks in recent history is to find technologies that can meet
this challenge. The use of solar energy is a very promising research direction because
enough photons reach the earth within one hour to meet the energy demand of the human
population for an entire year. The vision of organic photovoltaics, as opposed to silicon
solar cells, is that of low cost, flexible and lightweight solar cells which are easily
integrated in existing infrastructure.
The main focus of this dissertation are semi-random regioregular poly(3-
hexylthiophene) (rr-P3HT) based polymers, which are a novel class of broadly absorbing,
conjugated polymers for efficient polymer:fullerene bulk heterojunction solar cells. Semi-
random polymers combine a low band gap characteristic of the donor/acceptor (D/A)
approach with the broad absorption profile of random copolymers all while retaining high
hole mobilities and favorable mixing with fullerene acceptors. This new family of
polymers is able to overcome drawbacks of both perfectly alternating D/A and random
copolymers.
The concept of semi-random copolymers is introduced in chapter 2 and shows
that the combination of randomized sequence distribution and P3HT-like character allows
these polymers to have an unprecedented set of properties important for efficient
polymer:fullerene solar cells. Their multichromophoric nature enables broad absorption
of the solar photon flux using only small amounts (5% to 17.5%) of acceptor monomers
xix
while important qualities of P3HT such as semicrystallinity, high hole mobility and
mixing at favorable ratios with fullerenes are retained.
Subsequently, chapter 3 focuses on the influence the amount of acceptor, in this
case diketopyrrolopyrrole (DPP), has on the properties of semi-random copolymers.
Importantly a broad photocurrent response, representative of the polymer absorption
profile confirms that semi-random D/A copolymers are an effective platform for
improving light harvesting in BHJ solar cells and results in efficient polymer:fullerene
solar cells with P3HTT-DPP-10% reaching an efficiency of 5.7%.
Of special interest among semi-random copolymers are those containing multiple
distinct acceptor monomers. Consequently, in chapter 4 a family of semi-random two-
acceptor copolymers containing thienopyrroledione (TPD) and DPP is introduced which
illustrates that the observed broad absorption profiles and high absorption coefficients
can translate into a strong and broad photocurrent response. External quantum efficiency
values of up to 40% at 800 nm for P3HTT-TPD-DPP(1:2) and current densities of over
16 mA/cm
2
are achieved, which are among the highest currents ever reported with
PC
61
BM as the acceptor. Further work on two-acceptor polymers (chapter 5) focuses on
additional acceptor combinations in order to gain insight on general trends of polymer
properties such as absorption profiles and frontier energy levels.
The strong donor dithienopyrrole was incorporated into semi-random copolymers
(chapter 6), but no broadening of the absorption profile was observed. Instead this study
emphasizes that monomer combinations must be carefully selected in order to retain the
xx
semicrystalline nature of P3HT and to maintain a HOMO energy suitable for targeting
reasonable values of the open circuit voltage (V
oc
) in bulk heterojunction solar cells.
Finally, chapter 7 focuses on a simple poly-3-alkylthiophene based model system
in order to study the influence of branched alkyl side chains on the V
oc
. Results from this
study suggest that the introduction of small amounts of 2-ethylhexyl chains is a viable
way of tuning the V
oc
in polymers with almost no change in all other properties, which
could be an especially promising strategy for semi-random copolymers.

1
CHAPTER 1 Design of Conjugated Polymers for Organic
Photovoltaics
1.1 Semi-Random Conjugated Polymers in the Context of Organic Solar
Cells
Semi-Random regioregular poly(3-hexylthiophene) (rr-P3HT) based polymers,
which are the focus of this dissertation, are a novel class of conjugated polymers which
can be used in efficient polymer:fullerene bulk heterojunction (BHJ) solar cells. The
combination of a randomized sequence distribution with a P3HT-like character allows
semi-random polymers to have an unprecedented combination of characteristics
important for efficient polymer:fullerene solar cells. The multichromophoric nature of
these polymers enables broad absorption of the solar photon flux using only small
amounts of acceptor monomer, while important qualities of P3HT such as
semicrystallinity, high hole mobility and mixing at favorable ratios with fullerenes are
retained. Band gaps (E
g
) as low as 1.27 eV, short circuit currents (J
sc
) of over 16 mA/cm
2

and external quantum efficiency (EQE) of 40% at 800 nm as well as efficiencies (AM
1.5G) of close to 6% have been achieved with semi-random copolymers and [6,6]-
phenyl-C
61
-butyric acid methylester (PC
61
BM) as the acceptor.
1–3
This work was
motivated by several fundamental shortcomings in the current state-of-the-art polymers
such as narrow absorption breadth, the necessity of high fullerene loadings with perfectly
alternating donor/acceptor (D/A) copolymers, and low charge carrier mobilities in
2
random copolymers. These drawbacks have been successfully addressed here via the
development of semi-random copolymers.
In order to put these achievements into a broader context, chapter 1 will briefly
introduce organic solar cells as well as discuss important design principles for polymer
donors before summarizing the characteristics of current state-of-the-art
polymer:fullerene solar cells. The field of solution processable organic solar cells has
long been dominated by conjugated polymers, mainly because of their superior film
forming capability, and even though efficiencies of small molecule solar cells are rapidly
improving, a thorough discussion is outside the scope of this work.
4–6

1.2 General Introduction: Organic Solar Cells
With the world population growing rapidly the energy demand is predicted to
increase up to 40% by 2035 and one of the biggest tasks in recent history is to find
technologies that can meet this challenge. Even though fossil fuels currently dominate the
energy market it is projected that renewable energy sources will play a much bigger role
by 2035 due to increasing prices and limited resources of non-renewables as well as
environmental concerns.
7

Enough solar energy reaches the earth within one hour to meet the energy demand
of the human population for an entire year
8
but electricity generation from non-hydro
based renewable energy sources was only about 3% in 2009 with solar being a small
fraction of that. The primary reason for this is that the photovoltaic market is currently
3
dominated by polycrystalline silicon solar cells (> 80%) which have a high cost, are slow
to manufacture and take up a lot of space making them unattractive for the consumer.
9–12

The vision of organic photovoltaics, on the other hand, is that of low cost, flexible
and lightweight solar cells which are easily integrated in existing infrastructure and
visually pleasing.
13–15
In the last decade this motivation has spurred enormous research
efforts on conjugated semi-conducting materials and power conversion efficiencies (η)
have improved from 2.5% in 2001 to over 9% in 2012.
16–29

Compared to inorganic semiconductors, organic materials such as polymers and
small molecules have several advantages. The very high absorption coefficients (likely to
exceed 10
5
cm
-1
) are ~1000 times higher than for polycrystalline silicon which makes it
possible to use very thin films of active material (between 50 and 200 nm) and still
absorb photons efficiently.
30,31
Additionally, organic materials have much lower densities
than inorganic semi-conductors (P3HT = 1.1 g/cm
2
vs. silicon 2.33 g/cm
2
).
32,33
The
combination of these two factors ensures that the amount of material used in organic solar
cells is considerably smaller than for inorganic solar cells thus reducing cost and weight.
In inorganic semiconductors, photon absorption produces free electrons and holes
(due to the high dielectric constant of the material) which are then collected at the
respective electrodes. Organic semiconductors, on the other hand, have a much lower
dielectric constant (generally between 2 and 4) and upon absorption of a photon with
appropriate wavelength a so-called exciton (a tightly bound electron-hole pair) is
formed.
34
In 1986 Tang introduced a bilayer solar cell (the general architecture of a
current bilayer solar cell is shown in Figure 1.1a) featuring an interface between an
4
electron donor (copper phthalocyanine) and electron acceptor (a perylene tetracarboxylic
derivative which has a higher electron affinity) providing the necessary driving force for
exciton separation.
35
Due to short singlet exciton diffusion lengths (generally believed to
be ~10 nm) the efficiency of bilayer solar cells is inherently limited, as the excitons either
recombine before reaching the donor-acceptor interface or the film thickness is too thin to
absorb a relevant portion of the sun light.
34
This problem was solved by the introduction
of the BHJ solar cell (Figure 1.1b) by Yu et al. and Halls et al. in 1995, where the donor
and acceptor materials are intimately mixed forming a bicontinuous network with small-
scale phase separation.
36,37
This small-scale phase separation allows all excitons to reach
a donor-acceptor interface and dissociate into electrons and holes even with active layer
thicknesses of 100-200 nm, which are thick enough to absorb the majority of incident
photons.
38
The BHJ is currently by far the dominant solar cell geometry for organic solar
cells offering the advantage of a single solution processing step to form the active layer,
which is easily adaptable for large scale fabrication at low temperature and ambient
pressure using minimal energy.
39,40

In this chapter I will consequently focus on binary blend BHJ solar cells with a
single, solution processable active layer, as this is also the geometry which was used to
test conjugated polymers synthesized during this work (chapters 2-7) whereas other
solution-processed device architectures, such as tandem solar cells, are not discussed
here.
41–43

5
Figure 1.1 Schematic Illustration of a) a donor-acceptor bilayer solar cell; b) a BHJ solar
cell; c) typical current-voltage curves for dark and light currents in solar cells with the
output parameters: J
sc
(short circuit current density), V
oc
(open circuit voltage), FF (fill
factor) and J
m
and V
m
(current and voltage at maximum power point) - the power
conversion efficiency ( η) and FF are defined below the J-V-curve; and d) simplified
energy level diagram illustrating the HOMO and LUMO energies of a donor and acceptor
pair, the band gap of the donor (E
g
), the V
oc
which is empirically related to the HOMO
D
–
LUMO
A
offset and the necessary driving force for exciton dissociation (LUMO
D
–
LUMO
A
> 0.3 eV).

Since Tang’s seminal discov ery,
35
fullerenes have emerged as the most important
class of electron acceptors and are currently used almost exclusively. For solution
processed solar cells functionalized fullerenes such as PC
61
BM are predominantly used,
with other acceptors (both small molecules and polymers) showing much lower
6
efficiencies in BHJ solar cells.
44–58
The considerable improvement of organic solar cell
efficiencies over the last decade is thus predominantly due to huge efforts focused on the
development of new donor materials such as small molecules and especially polymers. In
this chapter an overview will be given of the polymer characteristics required for high
efficiencies and the progress towards embodying these characteristics as well as the
design principles employed for the synthesis of novel polymers. In addition to the
synthetic approach, further optimization of efficiencies is frequently achieved through
modification of the active layer morphology (for example using solvent additives) as well
as solar cell architecture (introduction of interlayers, improved electrodes, inverted device
structure).
59–64

To understand the guiding principles of polymer design their characteristics can
be correlated to the solar cell output parameters which are briefly introduced in the
following paragraph. The power conversion efficiency ( η or PCE) of an organic solar cell
is determined by several parameters, namely the J
sc
, the open circuit voltage (V
oc
) as well
as the fill factor (FF) and can be expressed as η(%) = (J
sc
x V
oc
x FF)/(Input Power) when
standard AM1.5G conditions with light intensity of 100 mW/cm
2
are used (equations and
generalized J-V curve in Figure 1.1c).
65
When designing new semiconducting materials
the goal is to simultaneously optimize all parameters and consequently maximize η.
Necessary characteristics of the donor component in solution processable BHJ
solar cells can be divided into electronic requirements (such as a low band gap for high
J
sc
and correct positioning of frontier energy levels for high V
oc
and sufficient driving
force for exciton dissociation) and morphological requirements (such as optimal mixing
7
with fullerenes, a certain degree of crystallinity and high charge carrier mobility). The
control over the electronic properties of donor materials has been the driving force behind
the synthesis of a staggering number of new materials whereas principles governing the
morphology of the active layer are less understood and have been less frequently
targeted. Below we will briefly discuss the influence the band gap as well as frontier
energy levels (HOMO and LUMO) have on the output parameters of a solar cell, such as
J
sc
and V
oc
, and in the following subchapter we will introduce the design principles
available for the synthetic chemist to tune the electronic properties of a material and thus
optimize those parameters.
To achieve high J
sc
the active layer needs to absorb sunlight broadly and intensely
across the solar spectrum (from the UV/vis into the near infrared (NIR), photon flux
shown in Figure 1.2) because J
sc
is proportional to the product of spectral absorption
breadth and absorption intensity of the active layer.
17
As illustrated by the red curve in
Figure 1.2, a polymer with an absorption onset of 650 nm can only reach a limited
maximum theoretical J
sc
of 16.77 mA/cm
2
whereas a polymer with a low band gap and an
absorption onset of 950 nm can theoretically achieve a J
sc
of 35.72 mA/cm
2

(numbers are
calculated by integrating the area underneath the solar photon flux in Figure 1.2
assuming an EQE of 100% across the absorption breadth). Considering that most
fullerenes primarily absorb in the UV, and often to a lesser degree in the visible region of
the solar spectrum, an important design principle for novel polymers is to lower the band
gap, allowing absorption of longer wavelength light and thus high J
sc
. Often polymers
with a band gap lower than P3HT, which is still considered the benchmark for conjugated
8
polymers and has a band gap of 1.9 eV (see section 1.3.1), are called low band gap.
Originally though, the term was used for polymers with band gaps lower than 1.5 eV
which are able to absorb NIR photons.
66
Mixing of donor materials with the fullerene
acceptor at favorable ratios is also beneficial for high J
sc
because, in combination with the
fact that fullerenes do not absorb in the NIR, the absorption coefficients of polymers in
the visible spectra are generally considerably higher than for fullerenes. Within the
constraints of a defined thickness of the active layer, dictated by the relatively low charge
carrier mobilities of organic materials, a large amount of fullerene would thus be
unfavorable because it dilutes the absorption of the polymer potentially leading to lower
J
sc
.

Figure 1.2 Photon flux density from the sun (AM 1.5G) as
a function of wavelength. The red curve represents the
integrated short circuit current density which is on the y-
axis at the right.
9
It also has to be taken into account that the frontier energy levels of the polymer
(HOMO and LUMO) have to be positioned correctly relative to the fullerene acceptor in
order to obtain maximum V
oc
and to provide sufficient driving force for exciton
dissociation. The V
oc
is empirically related to the magnitude of the HOMO
D
- LUMO
A
(Figure 1.1d) offset although this is a very simplified picture and multiple reports point
to the importance of the charge transfer state in determining the V
oc
.
67–70
Based on this
premise, newly designed polymers tend to have lower HOMO levels than P3HT (which is
most commonly reported as 5.2 eV) in order to increase V
oc
but also retain a LUMO
D
–
LUMO
A
offset > 0.3 eV (Figure 1.1d) which is thought necessary as a downhill driving
force for exciton dissociation (and thus efficient charge generation).
20
An alternative
approach to increase the V
oc
is the use of an acceptor which has a higher lying LUMO
than commonly used PCBM. This has recently been shown successfully with ICBA (a
bis-indene-fullerene adduct) which in combination with P3HT shows a V
oc
of 0.84 V
instead of 0.63 V for P3HT:PCBM.
49,50,71,72
So far though, efficient solar cells combining
ICBA with other polymers than P3HT remain relatively rare.
73
As mentioned above, the
V
oc
is related to the HOMO
D
- LUMO
A
offset and the HOMO and LUMO energies of
donor materials are generally determined using cyclic voltammetry (CV). Considerable
inconsistencies in the literature regarding the use of the standard redox couple
ferrocenium/ferrocene (Fc
+
/Fc) in CV measurements make it very difficult to compare
values reported for HOMO and LUMO levels.
74
Consequently, during the following
discussion of state-of-the art polymers the exact numbers will not be mentioned in most
cases but rather the general trends and the more reliable value of V
oc
itself.
10
Finally, the FF is affected by the hole mobility of the polymer and is strongly tied
to the morphology of the polymer:fullerene blend, (degree of phase separation between
polymer and acceptor, aggregation, crystallinity) with a high fill factor (FF > 0.6)
indicating balanced, trap-free charge transport through the bulk and favorable
morphology. The FF can be improved by extensive optimization of device fabrication
through methods such as solvent annealing, thermal annealing or the use of
additives.
59,61,62

This brief overview illustrates that for the optimization of solar cell efficiencies a
careful fine tuning of the energy levels of polymers, also with respect to fullerene energy
levels, is necessary in order to achieve an optimal balance between J
sc
and V
oc
because
maximization of either one would result in a decrease of the other parameter. For
example, in order to drastically lower the band gap of a polymer (which would maximize
the theoretically attainable J
sc
) and still obtain a working solar cell, the HOMO energy
level has to eventually shift upwards to retain a large enough driving force for exciton
dissociation (LUMO
D
- LUMO
A
offset > 0.3 eV) resulting in a loss of V
oc
and likely no
overall improvement of PCE. From theoretical calculations it was estimated that the
optimal polymer will have a band gap of ~1.5 eV and HOMO level of ~5.4 eV when
properly balanced and can reach efficiencies over 10% - 12% in binary blend BHJ solar
cells with a single active layer.
20,41,75
The focus of the introduction is thus on the design
principles for donor materials that aim to achieve this balance.

11
1.3 Design Principles for Band Gap and Energy Level Control of
Polymers
On the following pages the design principles which are available for the synthetic
chemist to control the electronics of polymers will be introduced. Specifically, the control
of energy levels and band gap, through structural modifications and thus the possibility to
tune solar cell output parameters will be discussed.
Conjugated polymers possess an inherent band gap because of the bond length
alternation (BLA) between single and double bonds in the polymer backbone. In the
(hypothetical) case of a polymer with completely delocalized electrons the band gap
would be zero, but such systems are unstable (Peierls instability) leading to the
localization of electrons and consequently BLA.
76–78

Several design principles have been established in order to decrease the BLA (and
thus the band gap) in conjugated polymers; specifically the stabilization of the polymer
quinoidal form and the D/A approach. In aromatic polymers two non-degenerate ground
states exist, with the quinoidal form having a higher energy but lower band gap than the
aromatic form (Figure 1.3a). In order to stabilize the quinoidal form (and lower its
energy below the aromatic form) fused ring systems such as isothianaphthene,
thienopyrazine and thienothiophene have been developed. In the first two cases the
aromatic stabilization energy is lower for thiophene than for the fused ring system
(benzene or pyrazine), leading to dearomatization of thiophene (and consequently
aromatization of benzene or pyrazine) and increased delocalization of electrons along the
backbone (Figure 1.3a shows poly(isothianaphthene) as an example).
79,80
For
12
poly(thienothiophene), however it was predicted that the aromatic and quinoidal forms
have approximately the same energy and are thus both contributing to the polymer
structure.
81,82
This contribution of the quinoidal form not only decreases BLA but also
planarizes the polymer backbone, with both effects resulting in a lowered band gap.
As mentioned, another design principle that can be used is the D/A approach
illustrated in Figure 1.3b, also called push-pull approach, where electron rich monomers
and electron poor monomers (common examples are shown in Figure 1.4) are
incorporated in an alternating fashion into the polymer backbone.
76–78,83,84
This
alternating structure can stabilize the quinoidal form because it increases the double bond
character between the monomers by favoring the charge separated resonance structure
(positive charge on the electron rich monomer and negative charge on the electron poor
monomer). The D/A approach also offers the advantage that the HOMO and LUMO level
of polymers can, to a certain degree, be individually tuned because they are located on
different parts of the polymer, with the LUMO almost exclusively on the acceptor unit
whereas the HOMO is more delocalized but often predominantly on the donor unit.
23,85

The D/A approach is currently by far the most dominant approach to make efficient low
band gap polymers and small molecules and Figure 1.3b shows one such example (D/A
copolymer PBTTPD with an efficiency of 7.3%).
86

13

Figure 1.3 a) Non-degenerate ground states of conjugated polymers: the aromatic and
quinoidal form of poly(thiophene) and poly(isothianaphthene) are shown as examples, b)
charge separated structure of a D/A copolymer (PBTTPD
86
) where thienopyrroledione is
the acceptor and dodecylthiophene the donor unit, c) head-to-tail, head-to-head and tail-
to-tail connection of substituted thiophenes and steric hindrance between o-hydrogen
atoms of neighboring phenyl units (torsional twists around single bonds are indicated
with a curved arrow).
14

As mentioned above, the backbone of conjugated polymers consists of alternating
single and double bonds with the p
z
-orbitals of the carbon atoms overlapping. If the
backbone is twisted out of plane around single bonds the p
z
-orbitals no longer overlap
and the delocalization of electrons is even further limited resulting in an increased band
gap.
76–78
In addition to stabilizing the quinoidal form and applying the D/A approach

Figure 1.4 a) Common donor monomers, b) common acceptor monomers and c)
quinoidal acceptors.
15
several other design principles have been developed to ensure planarity of the polymer
backbone and the most common approaches are discussed below.
Although most monomers are designed to be symmetric, automatically resulting
in regiosymmetric polymers, in some important cases monomers are unsymmetric.
Unsymmetrical monomers substituted with an alkyl chain (for example 3-
hexylthiophene) are generally copolymerized in a regioregular (rr) fashion (with the side
chains predominantly oriented head-to-tail) as opposed to randomly (where the backbone
contains head-to-tail, head-to-head and tail-to-tail linkages) in order to reduce
unfavorable steric interactions between side chains (Figure 1.3c). The best example for
this is P3HT, where regiorandom (ra) P3HT not only has a much higher band gap (due to
torsional twists) than rr-P3HT but other properties such as the hole mobility, are also
negatively impacted.
87,88
On the other hand, the nature of the side-chains is important for
both symmetric and unsymmetric monomers, with linear side chains introducing less
steric hindrance than bulky side chains. Often though, a trade-off is required between
planarity of the backbone and solubility of the polymer in common organic solvents to
ensure processability.
23
One approach to planarize the backbone, even when branched,
bulky side chains are used, is the introduction of spacers, such as a nonsubstituted
thiophene, between the monomers.
89
This is also important when neighboring monomers
both contain phenyl groups in the backbone in order to avoid torsional twists between o-
hydrogen atoms (for example carbazole donor copolymerized with benzothiadiazole
acceptor) (Figure 1.3c).
60,90
Additionally, fused ring systems such as
cyclopentadithiophene, carbazole and benzodithiophene which rigidify the backbone and
16
suppress rotational disorder around single bonds (ladder polymers take this concept to the
extreme) are often used in D/A materials to planarize the polymer backbone.
25,60,91

In general the most direct influence a synthetic chemist has on the energy levels
(HOMO and LUMO) of conjugated polymers and small molecules is through the use of
substituents. When an electron donating group, such as an alkoxy group, is used electron
density is pushed into the conjugated backbone and the HOMO level of the material is
raised, often accompanied by a reduction in band gap because the LUMO is less affected.
On the other hand, when electron withdrawing groups are used, such as a cyano group or
fluorine atom, the electron density in the backbone is reduced lowering the HOMO level
and increasing the electron affinity.
26,92
Beyond the relatively straightforward influence
alkyl side chains have on the solubility as well as planarity of polymers they can also
have a pronounced impact on material energy levels, and thus V
oc
, although the effect is
less understood.
93,94

Finally, if considering a polymer in the solid state, favorable intermolecular
interactions can also have a considerable influence on the band gap by further planarizing
the backbone. This is often apparent through a considerable red-shift of the UV/vis
absorption spectrum when going from solution to thin film. Strategies outlined above to
planarize the polymer backbone generally also improve intermolecular packing leading to
higher degrees of crystallinity. Another important factor when considering intermolecular
packing is the molecular weight and polydispersity index (PDI) of polymers, which has
been shown to have considerable impact on the morphology of the active layer and thus
17
solar cell efficiency and can be influenced by choosing an appropriate polymerization and
purification procedure.
60

The application of the above design principles in the pursuit of optimal donor
materials will be discussed in the following sections.

1.4 State-of-the-Art Polymer Donors for Organic Solar Cells
In the last paragraphs the electronic characteristics necessary for donor materials
in order to achieve higher solar cell efficiencies were discussed (low band gap, correct
positioning of frontier energy levels) and the important design principles available to tune
these characteristics and optimize solar cell output parameters (especially J
sc
and V
oc
)
were introduced. In the following sections the focus will be on demonstrating the
progress towards embodying those characteristics by summarizing the state-of-the-art
polymer and small molecule donors. Polymer donors are divided into homopolymers
(containing either only one monomer or only donor monomers), perfectly alternating D/A
copolymers (where a donor monomer is copolymerized with an acceptor monomer
resulting in a perfectly alternating monomer sequence in the polymer backbone) and
finally random D/A copolymers (which also contain donor and acceptor monomers but
are less ordered than perfectly alternating D/A copolymers).

18
1.4.1 Homopolymers
Poly(3-alkylthiophenes) (P3ATs), which are the most common homopolymers,
are generally made either via Grignard Metathesis (GRIM) or Rieke polymerization
methods, both of which give polymers with high regioregularities (percentage of head-to-
tail connections in the polymer backbone) and molecular weights. GRIM, which is
predominantly used on the laboratory scale, is a nickel-based living chain-growth
polymerization and affords low PDI polymers. Other methods such as Stille and Suzuki
coupling have also been used for poly(alkylthiophenes) but have the disadvantage that
monomer synthesis is more complicated.
88,95,96
Table 1.1 summarizes the solar cell
performance of some of the most effective P3AT’s reported to date.

Table 1.1 Structure and solar cell performance of select homopolymers.
Polymer J
sc
(mA/cm
2
) V
oc
(V) FF η
max
( η
avg
) % Ref.

P3HT:PC
61
BM (1:0.8)
9.5 0.63 0.68 5.08
(AM1.5G, 80
mW/cm
2
)
71

P3HT:IC
70
BA (1:1)
11.35 0.87 0.75 7.4 (7.2)
97

P3HDTTT:PC
61
BM (1:1)
6.33 0.82 0.66 3.40
98

PDOTT:PC
71
BM (1:1)
7.90 0.75 0.71 4.23 (4.03)
99

a
average value

19
rr-P3HT, which has a HOMO level of 5.2 eV and E
g
of 1.9 eV is by far the most
studied polymer for organic solar cells and, with PCBM as the acceptor, efficiencies of
up to 5% (Table 1.1) have been achieved but most reported PCE values are between 3
and 4%.
71,100,101
Higher efficiencies have been published with indene-C60-bisadduct
(IC
60
BA) (6.5%) and indene-C70-bisadduct (IC
70
BA) (7.4%) due to the increased V
oc

because of the higher acceptor LUMO level.
72,97
This dominance of rr-P3HT is due to a
combination of favorable properties such as semicrystallinity, high hole mobility as well
as the formation of a small-scale phase separated, stable BHJ morphology which makes it
an almost ideal candidate for organic solar cells. Out of all investigated P3ATs the hexyl
side chain of P3HT seems to offer the best compromise between solubility
(processability), morphology (crystallinity and polymer:fullerene mixing) and
chromophore density of the polymer with only poly(3-pentylthiophene) achieving similar
solar cell efficiencies.
102–106
As mentioned above, ra-P3HT is amorphous and has much
lower hole mobility than rr-P3HT due to the unfavorable steric interactions of the side
chains.
87
Even though it has been claimed that the regioregularity of P3HT has to be very
high (~95%) in order to achieve high efficiencies, no significant effect on the
performance can be detected for regioregularity between 90 and 98%.
100,107
On the
contrary, it has been found that lower rr-P3HT (86%) still has sufficient electronic
properties for high efficiency in combination with a higher thermal stability of the
polymer:fullerene BHJ.
108,109
Another factor that influences the performance of P3HT
solar cells is the molecular weight. It has been reported that a M
n
(number average
molecular weight) of over 10 000 g/mol is necessary in order to achieve high efficiencies
20
predominantly because of low hole mobility for low M
n
P3HT.
110–112
On the other hand
when M
n
gets too high processability decreases due to gellation of the polymer solution
and it has been suggested that a mixture of high and low M
n
polymer performs best.
71

rr-P3HT has several shortcomings which prevent it from achieving even higher
efficiencies such as the moderate V
oc
(~0.6 V with a HOMO energy of 5.2 eV) as well as
the narrow absorption breadth (absorption onset ~650 nm) resulting in a limited J
sc
.
Several attempts have been made to improve the efficiency of poly(3-alkylthiophenes) by
improving the V
oc
and some approaches are discussed below (attempts to broaden the
absorption fall under the category of D/A copolymers). Hou et al. reported that by
decreasing the alkyl side chain density on a thiophene backbone using a long branched
alkyl chain on every third ring (P3HDTTT) they were able to increase V
oc
considerably
compared to P3HT. However no improvement of efficiency was achieved compared to
P3HT:PCBM solar cells (Table 1.1).
98
Ko et al. used a similar strategy by only putting
alkyl chains on the 3- and 4-position of every third thiophene ring which resulted in a
lowered HOMO energy and increased V
oc
(0.75 V). In this case the obtained efficiencies
of up to 4.23% with PC
71
BM (PDOTT, Table 1.1) were found to exceed those observed
for the reference solar cell with P3HT.
99
A follow up study focused on the degree of
twisting along the backbone in a family poly(hexylthiophenes) where polymers only
differ by the number and placement of the alkyl chains. They found that a certain degree
of twisting is beneficial due to an increase in the V
oc
together with almost unchanged J
sc
,
FF and hole mobility and thus increased PCE. Larger degrees of twisting, on the other
hand, led to increased band gaps because of the reduced conjugation length and thus
21
reduced J
sc
.
113
A slightly different approach to increase the V
oc
in P3ATs will be
discussed in chapter 7.
94

1.4.2 Perfectly Alternating Donor/Acceptor Copolymers
Palladium-catalyzed cross-coupling reactions are used almost exclusively for the
synthesis of well defined D/A copolymers and current high performing D/A polymers are
either synthesized by Stille (stanyllated monomers) or Suzuki-Miyaura (boronic
monomers) coupling which allow controlled C
sp2
-C
sp2
bond formation.
114–117
The
resulting polymers generally have high molecular weights and are of high quality
whereas certain drawbacks such as unreacted end-groups (such as trimethyltin) as well as
residual catalyst can be solved by end-capping and careful purification.
118,119

Nevertheless, the monomer synthesis often involves toxic reagents and direct arylation
has recently been introduced as an economically and ecologically attractive alternative
where unfunctionalized heteroaromatic monomers can be directly coupled through C-H
bond cleavage.
120
Although it has been reported that the properties of a polymer made by
direct arylation can be considerably different from the properties of the same polymer
made by conventional cross-coupling polymerization it has recently been shown for the
first time that a polymer made by direct arylation can be successfully (6.1%) used in solar
cells which is promising for future use of this approach.
121,122

22

In the following pages D/A polymers are classified on the basis of acceptor
monomer, distinguishing further between quinoidal (predominantly thienothiophene) and
non-quinoidal acceptors (benzothiadiazole, thienopyrroledione, diketopyrrolopyrrole,
isoindigo and quinoxaline). Acceptors can be qualified by their electron-accepting ability
which can be empirically determined based on the E
g
of homologous D/A polymers
(common donor unit): the smaller E
g
, the stronger the electron-accepting ability.
126
Here
we will focus predominantly on strong and medium strength acceptors (rather than weak
acceptors) as the corresponding copolymers generally give better photovoltaic
performance.

Figure 1.5 Literature examples for a) Stille polymerization
123
, b) Suzuki
polymerization
124
and c) direct arylation polymerization
125
.
23
1.4.2.1 Benzothiadiazole Based D/A Copolymers
Benzothiadiazole (BTD) (Figure 1.4b) is a strong acceptor monomer due to the
presence of the electron-withdrawing imine nitrogens. It is likely the most used acceptor
monomer for conjugated D/A copolymers and can, depending on the donor comonomer,
lead to polymers with very low band gaps (and high J
sc
) or very low HOMO levels (and
high V
oc
). BTD is symmetric, compact and can be functionalized on the benzene unit in
order to tune energy levels or increase solubility.
PCPDTBT shown in Table 1.2, contains cyclopentadithiophene (CPDT) as the
planar, strong donor monomer and BTD as the acceptor monomer and was first
introduced by Muehlbacher et al. in 2006 showing promising properties such as a low
band gap of 1.40 eV due to the strong D/A effect, an EQE onset beyond 900 nm and PCE
of ~3.2%.
127
The solar cell performance was further optimized by Peet et al. using 1,8-
octanedithiol as an additive to improve the morphology of the BHJ achieving an average
PCE of 5.5% and J
sc
of 16.2 mA/cm
2
(Table 1.2) thus proving that low band gap
polymers can be very efficient in harvesting the solar spectrum and are able to achieve
high efficiencies.
128
In order to increase the V
oc
of PCPDTBT the fluorinated analog of
BTD was used (PCPDTFBT) which resulted in a downshift of the HOMO energy
because of the electron withdrawing nature of fluorine. Additionally, the presence of the
fluorine atom improved the morphology of the BHJ considerably, allowing for efficient
(5.51%) solar cell performance without the use of additives. The PCE was further
increased to 5.81% (Table 1.2) by introducing a fullerene based electron-collecting
layer.
129

24

Table 1.2 Structure and solar cell performance of select PCPDTBT analogs.
Polymer J
sc
(mA/cm
2
) V
oc
(V) FF η
max
( η
avg
) % Ref.

PCPDTBT:PC
71
BM (1:2 to 1:3)
16.2
a
0.62
a
0.55
a
5.8 (5.5)
128

PCPDTFBT: PC
71
BM (1:2)
15.0 0.76 0.51 5.81
129

PSBTBT :PC
71
BM (1:1)
14.9 0.57 0.61 5.24
130

12.7
a
0.68
a
0.55
a
5.1 (4.7)
131

P2: PC
71
BM (1:1)
17.3
a
0.57
a
0.61
a
6.0 (5.9)
132

PGeBTBT: PC
71
BM (1:1)
18.4
a
0.56
a
0.42
a
4.32
a

133

a
average value

The silicon analog of PCPDTBT, where the bridge C-atom in the donor monomer
CPDT has been replaced by a Si-atom, has been studied in detail by several groups (and
named respectively PSBTBT
131,134
or Si-PCPDTBT
130
). While the absorption profile
stays approximately constant the morphology of the material changes considerably.
PCPDTBT is an amorphous material whereas PSBTBT is semicrystalline with short π -π-
stacking distances and hole mobilities twice as high. The better packing of PSBTBT is
attributed to the significantly longer C-Si bond which reduces the steric hindrance
25
between the bulky side chains and the adjacent thiophene rings. Solar cell efficiencies of
5.1% (PSBTBT) and 5.24% (Si-PCPDTBT) were achieved (Table 1.2) compared to
5.5% for PCPDTBT.
130,131,134
Coffin et al. on the other hand, not only exchanged the
bridge atom of the donor from C to Si but also studied the impact of the alkyl side chains
by changing them from branched (2 -ethylhexyl) to linear (dodecyl) which lowered the
band gap slightly and resulted in an average PCE of 5.9% for P2 (Table 1.2) due to
extremely high J
sc
of 17.3 mA/cm
2
. They also showed the considerable impact the
molecular weight has on solar cell efficiency with an increase from 1.2% to 5.9% when
the M
n
was increased from 7000 g/mol to 34 000 g/mol.
132

The germanium analog of PCPDTBT, PGeBTBT, has recently been synthesized
and studied in polymer:PC
71
BM solar cells. The C-Ge bond is less polarized than the C-
Si bond due to the smaller electronegativity difference between C and Ge making
arylgermanes more stable against bases and nucleophiles than aryl silanes and allowing
the use of environmentally friendlier Suzuki polymerization. The C-Ge bond is slightly
longer than the C-Si bond also resulting in semicrystalline packing in the solid state with
a short π -π-stacking distance (3.51 Å). The combination of high hole mobility (0.08
cm
2
/(Vs)) and low band gap (1.47 eV) is reflected in the solar cell performance with
unprecedented J
sc
of 18.4 mA/cm
2
and η = 4.5% (Table 1.2).
133

Dithienopyrrole (DTP), where the C-bridge atom of CPDT is replaced by
electron-rich nitrogen, has also been used in PCPDTBT analogs but because of its
stronger electron donating ability HOMO levels are higher leading to decreased V
oc
and
PCE even though the band gaps are reduced.
135
Gibson et al. took a different approach by
26
systematically replacing the sulfur atom on BTD in PCPDTBT with selenium and
tellurium. They showed that the band gap of polymers in solution systematically
decreases from 1.59 eV to 1.06 eV when replacing sulfur with the heavier chalcogens but
no solar cell data were reported.
136

PCDTBT shown in Table 1.3, contains thiophene flanked carbazole as the donor
unit and BTD as the acceptor and was introduced by Blouin et al. in 2007 showing an
efficiency of 3.6% and high V
oc
of 0.89 V.
137
Since its initial report the solar cell
performance of PCDTBT has been optimized several times, mainly by improving the
device architecture, resulting in a maximum PCE of 7.2% (Table 1.3).
63,138,139
Carbazole
is a dibenzene unit bridged by a nitrogen atom and is a weaker donor than DTP or CPDT
due to the stronger aromaticity of the benzene units, resulting in polymers with lower
HOMOs and higher V
oc
. Carbazole is usually flanked by two unsubstituted thiophene
units in order to avoid unfavorable steric interactions between monomers which could
cause twists in the polymer backbone. Analogs of PCDTBT containing fluorene,
silafluorene or germafluorene as the donor monomer instead of carbazole have also been
synthesized (BisDMO-PFDTBT, PSiF-DBT and PGFDTBT). Even though considerable
V
oc
of up to 1 V was reported, they show limited efficiencies, except for PSiF-DBT
(5.4%, Table 1.3), likely due to very low hole mobilities impeding charge transport.
140–
142
In order to improve the packing in the solid state Du et al. replaced fluorene with 9-
alkylidene-9H-fluorene where the bridge sp
3
-C is replaced with a sp
2
-hybridized C atom
planarizing the monomer. In combination with two octyloxy chains on BTD this ensured
27
that PAFDTBT is both soluble in commonly used organic solvents but also has a high
hole mobility resulting in a PCE of 6.2% (Table 1.3).
143

Chen et al. copolymerized BTD with a donor monomer TPT
(thiophene/phenylene/thiophene), which is a hybrid between fluorene and CPDT
resulting in a-PTPTBT. The V
oc
is with 0.85 V intermediate between BisDMO-PFDTBT
(0.97 V) and PCPDTBT (0.62 V) but the FF is very high (0.67) because of the improved
morphology. Solvent annealed a-PTPTBT:PC
61
BM BHJ solar cells gave a maximum
efficiency of 6.4% (Table 1.3) exceeding both BisDMO-PFDTBT (Table 1.3) and
PCPDTBT (Table 1.2).
144

Table 1.3 Structure and solar cell performance of select PCDTBT analogs.
Polymer J
sc
(mA/cm
2
) V
oc
(V) FF η
max
( η
avg
) % Ref.

PCDTBT:PC
71
BM (1:4)
11.95 0.91 0.66 7.2 (6.5)
139

BisDMO-PFDTBT:PC
71
BM (1:3)
9.1 0.97 0.51 4.5
140

PSiF-DBT:PC
61
BM (1:2)
9.5 0.90 0.51 5.4
(AM1.5G,
80 mW/cm
2
)
142

PGFDTBT: PC
71
BM
6.9 0.79 0.51 2.8
141

PAFDTBT: PC
71
BM (1:3)
9.9 0.89 0.70 6.2
143

28
Table 1.3 Continued
Polymer J
sc
(mA/cm
2
) V
oc
(V) FF η
max
( η
avg
) % Ref.

a-PTPTBT: PC
71
BM (1:1)
11.2 0.85 0.67 6.41 (6.1)
144

Benzodithiophene (BDT) is a relatively weak donor monomer which has been
used in multiple copolymers with BTD and which allows for low HOMO levels in the
resulting polymers as well as enhanced delocalization of electrons due to backbone
planarization.
145–147
Zhou et al. copolymerized BDT with difluorinated dithienyl-
benzothiadiazole resulting in PBnDT-DTffBT which showed an efficiency of 7.2% in the
polymer:PC
61
BM BHJ solar cell (Table 1.4). Compared to nonfluorinated polymer
PBnDT-DTBT, not only the V
oc
was increased (0.91 V vs. 0.87 V) due to the lowered
HOMO level but also the J
sc
(12.91 mA/cm
2
vs. 10.03 mA/cm
2
) due to increased EQE
even though the band gap remained practically identical.
147
Both Huo et al. and Peng et
al. attached thiophene groups to the central phenyl ring of BDT in order to increase the
conjugation of the polymer backbone.
145,146
Huo et al. synthesized PBDTTBT and
achieved an efficiency of 5.66% with PC
71
BM as the acceptor (Table 1.4).
145
Peng et al.,
on the other hand, noticed that the introduction of a fluorine atom on BTD lowers the
HOMO without increasing the band gap thus resulting in a slightly higher efficiency for
the fluorinated polymer PBDT-FBT (6.21%, Table 1.4) compared to the alkoxy
substituted polymer PBDT-OBT (5.64%).
146

29
Finally, Ong et al. synthesized a copolymer based on dialkylquarterthiophene and
BTD which not only retains the high hole mobility of poly(thiophenes) but also has a
reduced band gap because of the D/A effect between BTD and electron rich
alkylthiophene resulting in a solar cell efficiency of 6.26% (Table 1.4).
148

Table 1.4 Structure and solar cell performance of benzodithiophene/ benzothiadiazole
copolymers.
Polymer J
sc
(mA/cm
2
) V
oc
(V) FF η
max
( η
avg
) % Ref.

PBnDT-DTffBT:PC
61
BM (1:1)
12.91 0.91 0.61 7.2 (6.9)
147

PBDTTBT:PC
71
BM (1:2)
10.7 0.92 0.57 5.66
145

PBDT-FBT:PC
71
BM (1:2)
12.05 0.86 0.59 6.21 (6.08)
146

POD2T-DTBT:PC
71
BM (1:1)
12.3 0.72 0.70 6.26
148

Multiple BTD analogs (Figure 1.6), which can be either stronger or weaker
acceptors than BTD have been successfully used in D/A copolymers (Table 1.5). Zhou et
al. used pyridalthiadiazole (PT), which is a stronger acceptor than BTD because pyridine
is more electron deficient than benzene, and copolymerized it with several weak donor
30
monomers. The resulting copolymers had low band gaps (1.51 – 1.56 eV) and the highest
efficiency (6.32%, Table 1.5) was achieved with PBnDT-DTPyT containing BDT as the
donor monomer.
149
PT is an unsymmetric acceptor and simple copolymerization with
donor monomers results in regiorandom copolymers. Ying et al. showed that the hole
mobility of regiorandom PT copolymers is several orders of magnitude lower than that of
regioregular copolymers (0.005 cm
2
/(V*s) vs 0.6 cm
2
/(V*s)) emphasizing the importance
of the regioregularity when using unsymmetric monomers.
150

Naphthobisthiadiazole (NT) is a slightly stronger acceptor than BTD and
additionally has an enlarged planar aromatic structure which may facilitate interchain
packing by rigidifying the polymer backbone. Wang et al. compared analogous NT and
BTD based copolymers and found that the band gap of PBDT-DTNT was reduced by
0.15 eV compared to PBDT-DTBT due to the larger acceptor strength of NT.
151
After
considerable device optimization and introduction of a conjugated polyelectrolyte
interlayer, efficiencies of up to 8.6% were achieved with PBDT-DTNT (Table 1.5).
152

Benzotriazole (TAZ), on the other hand, is a weaker acceptor, because the lone electron

Figure 1.6 Benzothiadiazole analogs used in efficient
polymer:fullerene BHJ solar cells.
31
pair on the nitrogen atom is more basic than the lone pair on sulphur in BTD thus
increasing the electron density in the ring.
153,154
This causes TAZ-based copolymers to
have a wider band gap than BTD based copolymers. Price et al. successfully used
fluorinated TAZ to make PBnDT-FTAZ (with BDT as the donor monomer) which, even
though it has a larger band gap than P3HT (2 eV vs. 1.9 eV), shows efficiencies of up to
7.1% with PC
61
BM because of a considerable increase in V
oc
and very high FF (Table
1.5).
154
Nie et al. successfully used benzoxadiazole (BO) in a D/A copolymer with BDT
which also resulted in a polymer (P1) with high V
oc
and a maximum efficiency of 6.05%
(Table 1.5).
155

Table 1.5 Structure and solar cell performance of polymers with benzothiadiazole
analogs as the acceptor.
Polymer J
sc
(mA/cm
2
) V
oc
(V) FF η
max
( η
avg
) % Ref.

PBnDT-DTPyT:PC
61
BM (1:1)
12.78 0.85 0.58 6.32 (6.11)
149

PBDT-DTNT:PC
71
BM (1:1.5)
17.4
a
0.75
a
0.61
a
8.6 (8.4)
(AM1.5G, 95
mW/cm
2
)
152

PBnDT-FTAZ:PC
61
BM (1:2)
12.45 0.79 0.72 7.1 (6.81)
154

P1:PC
71
BM (1:1.5)
13.6 0.88 0.51 6.05 (5.65)
155

a
average value
32
1.4.2.2 Thienopyrroledione Based D/A Copolymers
Thienopyrroledione (TPD) (Figure 1.4b) is an acceptor of medium strength due
to the electron withdrawing carbonyl groups. TPD based copolymers tend to have very
low HOMO levels (and consequently high V
oc
) but due to the medium acceptor strength
band gaps are rarely very low and with most donor monomers they tend to be ~1.7 eV.
TPD can be substituted with alkyl chains on the nitrogen atom in order to increase the
solubility of polymers.
In 2010 Zou et al. and Piliego et al. almost simultaneously reported PBDTTPD
(simply called P3 by Piliego et al.) which is a copolymer of BDT and TPD shown in
Table 1.6.
156,157
Both groups reported the same high V
oc
(0.85 V) but Piliego et al.
achieved a higher PCE (6.8% vs. 5.5%) possibly due to a combination of much smaller
device area (0.03 cm
2
vs. 1 cm
2
) and more thorough device optimization. Piliego et al.
also studied the influence that the alkyl side chain attached to TPD has on the device
performance, concluding that a long, linear side chain, such as octyl, gives better
properties than branched side chains because of increased order in the thin film. They
also found that these TPD-based polymers have a face on orientation on the substrate
(with the backbones parallel to the substrate) which is beneficial for charge transport in
the device.
157
Recently Aïch et al. succeeded in increasing the efficiency of PBDTTPD to
7.1% (Table 1.6) for solar cells with an active area of 1 cm
2
and PC
61
BM as the acceptor
by using solvent additives to fine-tune the morphology.
158
Li et al. replaced BDT with the
stronger donor CPDT in order to reduce the band gap and achieve a higher J
sc
. But, as
mentioned in the introduction, even though J
sc
was increased (14.1 mA/cm
2
vs. 10.89
33
mA/cm
2
for PBDTTPD) V
oc
simultaneously decreased (0.75 V vs. 0.93 V for PBDTTPD)
because of the higher HOMO level and the efficiency of PCPDTTPD-Oc:PC
71
BM
(6.41%, Table 1.6) was not improved compared to PBDTTPD.
159
Replacing CPDT with
dithienosilole (DTS), analogous to PSBTBT described above, gave PDTSTPD which has
a slightly larger band gap (1.73 eV) but lower HOMO level compared to PCPDTTPD-
Oc. By blending PDTSTPD with PC
71
BM a solar cell efficiency of 7.3% was achieved.
160

A careful study on the influence of the molecular weight of PDTSTPD and the length of
the alkyl side chain on TPD showed that the octyl side chain is optimal and that a high
molecular weight is necessary in order to obtain high efficiencies. Whereas a M
n
of 10
000 g/mol gave a low PCE of 3.1%, M
n
increase to 31 000 g/mol allowed for an
efficiency of 7.7% (Table 1.6) predominantly due to a simultaneous increase in hole
mobility by two orders of magnitude and improved morphology.
161
Amb et al. argued
that replacing the Si bridge atom with the larger Ge would result in further enhancement
of ordering because the C-Ge bond is even longer and steric hindrance even more
reduced. The Ge-based polymer (PDTG-TPD) has a somewhat higher HOMO level
compared to the Si-based polymer together with a slightly lowered band gap.
162
In an
inverted device geometry with PC
71
BM as the acceptor PDTG-TPD initially achieved a
solar cell efficiency of 7.3% which was further increased to 8.5% (Table 1.6) by the
addition of an electron-transporting interlayer.
29,162

Yuan et al. successfully copolymerized TPD and the symmetric donor unit
bi(dodecyl)thiophene and obtained PBTTPD which is semicrystalline and has a low
HOMO level. Even though the polymer had a low M
n
of 9 700 g/mol the solar cell
34
efficiency obtained with PC
61
BM was up to 4.7%.
163
They further optimized this
efficiency to 7.3% (Table 1.6) by switching to PC
71
BM and optimizing the bulk
morphology by using solvent additives.
86
Jo et al. synthesized a very similar polymer (P2,
Table 1.6) using a relatively novel approach, direct arylation polymerization, which is a
low-cost, green alternative to the standard methods such as Stille and Suzuki
polymerization. They were not only able to achieve high M
n
(41 000 g/mol) but also
demonstrated for the first time that polymers made by this approach can be successfully
used in very efficient solar cells (up to 6.1%).
121
Finally, Najari et al. showed that the
TPD unit can be used in combination with carbazole as the donor monomer in order to
make polymers with very low HOMO and high V
oc
. They obtained PCDTTPD (Table
1.6) which has an impressive V
oc
of 1.07 V but only moderate PCE in combination with
PC
61
BM.
164

35
Table 1.6 Structure and solar cell performance of thienopyrroledione based copolymers.
Polymer J
sc
(mA/cm
2
) V
oc
(V) FF η
max
( η
avg
) % Ref.

PBDTTPD:PC
61
BM (1:1.5)
10.89 0.93 0.70 7.1
158

PCPDTTPD-Oc:PC
71
BM (1:2)
14.1 0.75 0.61 6.41 (6.31)
159

PDTSTPD:PC
71
BM (1:2)
12.13 0.91 0.70 7.7
161

PDTG-TPD:PC
71
M (1:1.5)
14.4 0.86 0.69 8.5 (8.1)
29

PBTTPD:PC
71
BM (1:1.5)
13.1 0.92 0.61 7.3 (7.1)
86

P2:PC
71
BM (1:1.2)
10.51 0.92 0.63 6.1
121

PCDTTPD:PC
61
BM (1:1)
4.72 1.07 0.36 1.82
164

36
1.4.2.3 Diketopyrrolopyrrole and Isoindigo Based D/A Copolymers
Diketopyrrolopyrrole (DPP) has been widely used as a pigment in inks, paints and
plastics and is a very strong electron acceptor which is most commonly flanked by two
thiophene units.
165,166
DPP containing polymers generally have very low band gaps (1.5
eV and lower), are semicrystalline and have high charge carrier mobilities (both hole and
electron) which make them ideal candidates for OPV and field effect transistor (FET)
devices. DPP can be substituted with alkyl side chains on both nitrogen atoms in order to
tune the solubility of the polymers.
One of the first efficient DPP containing D/A copolymers was published by
Wienk et al. in 2008. Electron rich quarterthiophene (substituted with alkyl side chains)
was combined with electron poor DPP to give pBBTDPP2 (Table 1.7) which has a band
gap of 1.4 eV. Despite this low band gap the J
sc
was only 11.3 mA/cm
2
which, in
combination with moderate FF and V
oc
, restricted the PCE to 4%.
167
Removing the alkyl
substituents on the quaterthiophene unit induces additional planarity in the polymer
backbone which can enhance packing and charge carrier mobility. In order to retain
solubility DPP was substituted with much longer, branched alkyl side chains and the
resulting polymer, pDPP (Table 1.7), achieved a high J
sc
of almost 15 mA/cm
2
and PCE
of 5.62%.
168

Bijleveld et al. used a terthiophene unit instead of quaterthiophene and PDPP3T
(Table 1.7) exhibited high hole and electron mobilities (0.04 cm
2
/(V*s) and 0.01
cm
2
/(V*s)) but only a relatively moderate PCE of 4.7%.
169
Replacement of one of the
thiophene units with a phenyl ring gave PDPPTPT (Table 1.7) which has a reduced
37
HOMO level and increased band gap compared to PDPP3T. This resulted in a
considerably increased V
oc
(0.8 V vs. 0.65 V) and an improved solar cell efficiency of
5.5%.
170
Jung et al. replaced a thiophene unit with dithienothiophene which, due to its
highly planar heteroarene structure, favors strong π -π-interactions and thus high charge
carrier mobilities. PDTTDPP (Table 1.7) has a very low band gap with an absorption
onset at 1055 nm and high hole mobility of up to 0.68 cm
2
/(V*s). In combination with
PC
71
BM as the acceptor a maximum efficiency of 6.05% and J
sc
of 13.7 mA/cm
2
was
obtained.
171
Very recently Gevaerts et al. have synthesized PDPP5T (Table 1.7) which,
compared to PDPP3T, contains two additional alkyl substituted thiophene units. In a
single junction solar cell with PC
61
BM an efficiency of 5.3% was obtained and in a
tandem solar cell in combination with high band gap polymer PCDTBT (Table 1.3) an
efficiency of 7.5% was reached.
43

Dou et al. synthesized a family of four D/A copolymers, using a combination of
DPP and two different BDT donor monomers (substituted on the central phenyl ring with
either alkylated phenyl or thienyl units), in order to find an optimal low band gap
polymer for tandem solar cells. BDT is not as strongly electron donating as thiophene
resulting in polymers with lower HOMO levels and consequently higher V
oc
than those of
DPP based D/A polymers discussed above. It is interesting to note that the nature of the
alkyl side chains on both DPP and BDT was critical for the solar cell performance and
only the specific combination of 2-ethylhexyl chains on BDT and 2-butyloctyl on DPP
gave high efficiencies. For the single layer BHJ solar cells with PC
71
BM a maximum
efficiency of 6.6% was achieved for PBDTT-DPP (Table 1.7) with three of the
38
synthesized polymers showing efficiencies of 5.8% or higher. Inverted tandem solar cells,
with P3HT as the high band gap polymer showed maximum efficiency of 8.8%.
42
Jung et
al. completely removed the substituents on BDT resulting in PBDTDPP (Table 1.7)
which has an even lower HOMO level and high V
oc
of 0.82 V but due to the considerably
reduced J
sc
the PCE was limited to 5.16%.
172

As mentioned above, the electron deficient DPP core is almost always flanked by
two thiophene units and very few studies focus on the effect of modifying these units.
Bronstein et al. recently replaced the thiophenes with larger thienothiophene units which
can enhance the coplanarity of the polymer and improve the charge carrier mobility. As a
result P1 (Table 1.7) shows a FET hole mobility of almost 2 cm
2
/(V*s) and a PCE of
5.4% with PC
71
BM as the acceptor (primarily limited by the moderate V
oc
).
173
Yiu et al.
replaced the DPP flanking thiophenes with furan units which significantly increased the
solubility of the polymers, allowing them to use linear instead of branched alkyl side
chains on the DPP unit. Polymers with linear side chains were shown to have a shorter π -
π stacking di stance than those with branched side chains and outperformed them
considerably in BHJ solar cells. A maximum PCE of 6.5% was obtained for PDPP2FT-
C
14
(Table 1.7) which is almost 2% higher than for the thiophene based analog PDPP3T
(Table 1.7) substituted with hexyldecyl side chains for sufficient solubility.
169,174

39
Table 1.7 Structure and solar cell performance of diketopyrrolopyrrole based
copolymers.
Polymer J
sc
(mA/cm
2
) V
oc
(V) FF η
max
( η
avg
) % Ref.

pBBTDPP2:PC
71
BM (1:2)
11.5 0.61 0.58 4.0
167

pDDP: PC
71
BM (1:1)
14.84 0.63 0.60 5.62
168

PDPP3T: PC
71
BM (1:2)
11.8 0.65 0.60 4.7
169

PDPPTPT: PC
71
BM (1:2)
10.8
a
0.80
a
0.65
a
5.6 (5.5)
170

PDTTDPP:PC
71
BM (1:1.5)
13.7
a
0.66
a
0.66
a
6.05
a

171

PDPP5T:PC
61
BM (1:2)
14.0 0.58 0.65 5.3
43

PBDTT-DPP: PC
71
BM (1:2)
14.0 0.73 0.65 6.6 (6.5)
42

PBDTDPP: PC
71
BM (1:1)
10.49 0.82 0.60 5.16
172

40

Table 1.7 Continued
Polymer J
sc
(mA/cm
2
) V
oc
(V) FF η
max
( η
avg
) % Ref.

P1: PC
71
BM (1:2)
15.0 0.58 0.61 5.4
173

PDPP2FT-C 14:PC 71BM (1:3)
14.8
a
0.65
a
0.64
a
6.5 (6.2)
174

P3TI:PC
71
BM (1:1.5)
13.1 0.70 0.69 6.3
175

a
average value

Isoindigo is another strong acceptor monomer that has been used in the dye
industry and can be easily synthesized from widely available precursors. Isoindigo was
used for the first time in D/A copolymers by Stalder et al. and they showed that,
depending on the electron-donating strength of the comonomers the polymers have
promising optical and electrochemical properties.
176
Despite this promising first report,
solar cell efficiencies of isoindigo based D/A polymers have mostly stayed well below
those reported for DPP based copolymers.
177–180
One notable exception is P3TI (Table
1.7) containing a planar terthiophene unit as the donor which is beneficial for π-π
stacking of the polymer backbones. P3TI has a low band gap of 1.5 eV in combination
with a low HOMO energy and with PC
71
BM as the acceptor an efficiency of 6.3% was
achieved.
175

41
1.4.2.4 Quinoxaline Based D/A Copolymers
Quinoxaline is a strong acceptor due to the electron withdrawing imine groups
and was first used in a semiconducting polymer in 2007 by Gadisa et al. where it was
copolymerized with fluorene to give APFO-15 (Table 1.8). Due to the very large V
oc
a
respectable efficiency of 3.7% was achieved.
181
Interestingly, no further efficiency
increase in quinoxaline based polymers was reported until 2010 when Wang et al.
published TQ1 (Table 1.8).
182–184
TQ1 is a very simple structure synthesized by
copolymerizing quinoxaline (substituted with two octyloxyphenyl groups) with thiophene
and has a medium band gap (1.70 eV) combined with a very low HOMO level (5.7 eV).
A maximum efficiency of 6.0% was achieved with PC
71
BM as the acceptor.
184
Zhang et
al. argued that if the phenyl substituents on quinoxaline could be connected with a single
bond between the ortho positions the resulting monomer would not only be a stronger
acceptor but also facilitate intermolecular packing. D/A copolymer PIDT-phanQ (using
indacenodithiophene as the donor monomer) had a lower band gap and slightly higher
hole mobility than the analogous polymer PIDT-diphQ where the phenyl substituents on
quinoxaline are not connected and consequently showed an improved PCE of 6.24%
compared to 5.69% for PIDT-diphQ.
185
This efficiency was subsequently improved
multiple times up to 6.63% (Table 1.8) by extensive device optimization.
186,187
In a
similar fashion Zhang et al. used quinoxaline with a benzodithiophene unit fused on top
and copolymerized it with indacenodithiophene. The resulting polymer P1 had an
efficiency of 6.06% (Table 1.8).
188
He et al. copolymerized thiophene flanked
quinoxaline with 4,5-ethylene-2,7-carbazole resulting in PECz-DTQx which, after
42
extensive device optimization and incorporation of a polyfluorene interlayer at the
cathode, gave a PCE of 6.07% (Table 1.8).
189

Table 1.8 Structure and solar cell performance of quinoxaline based copolymers.
Polymer J
sc
(mA/cm
2
) V
oc
(V) FF η
max
( η
avg
) % Ref.

APFO-15:PC
61
BM (1:3)
6.0 1.0 0.63 3.7
181

TQ1:PC
71
BM (1:3)
10.5 0.89 0.64 6.0
184

PIDT-phanQ: PC
71
BM (1:3)
11.41 0.88 0.66 6.63
186

P1:PC
71
BM (1:3)
11.6 0.83 0.63 6.06
188

PECz-DTQx:PC
71
BM (1:4)
11.4 0.81 0.66 6.07 (6.00)
189

43
1.4.2.5 Quinoidal Acceptors: Thienothiophene, Thienopyrazine and
Isothianaphthene
Thienothiophene (TT), isothianaphthene (ITN) and thienopyrazine (TP) are all
monomers which stabilize the quinoidal form of conjugated polymers and are thus able to
lower the band gap by decreasing the BLA. On the other hand only TP is an acceptor
monomer in the actual sense of the word because it contains electron withdrawing imine
groups. Neither TT nor ITN are inherently electron poor but TT is frequently substituted
with electron withdrawing groups such as ester, ketone, fluorine and sulfonyl (or a
combination of those) to increase its acceptor strength and lower the HOMO level of
resulting polymers. TP based conjugated D/A copolymers, and to a lesser degree ITN
based copolymers, have been studied extensively for the application in organic solar cells
but only low to moderate efficiencies have been achieved so far.
66,190–193
TT based
polymers, on the other hand, are currently some of the highest performing polymers and
are discussed in detail below.
28,194–196

Yu et al. developed a large family of copolymers PTBX (Figure 1.7 and Table
1.9) based on alkoxy substituted BDT and ester substituted thienothiophene (TT).
Whereas for PTB1 linear alkyl side chains were used both on the BDT and TT units the
solubility of PTB2-PTB7 was considerably enhanced by switching to branched side
chains such as 2-ethylhexyl. A detailed study also showed that the thin film morphology
and the π-π-stacking distance are strongly influenced by the type of side chains used in
the polymers. A positive correlation was found between strong π -π-interactions and high
44
FF in the BHJ solar cells and it was concluded that branched side chains have no negative
impact on the π -π-stacking distance if they are attached to the TT unit.
197

Multiple approaches were used in order to lower the high HOMO levels of PTBX
polymers and consequently increase the V
oc
. In PTB3 the alkoxy side chains on BDT
were replaced by less electron donating alkyl side chains whereas in PTB4 and PTB7 the
TT unit was substituted with a fluorine atom in order to decrease the HOMO level
without interrupting the packing of the polymer.
196,198,199
Detailed studies showed that
monofluorination of TT results in the best device performances whereas perfluorination
of the polymer backbone greatly reduced PCE, probably due to poor compatibility with
PC
71
BM molecules.
200
This careful optimization process resulted in a PCE of 7.4% for
PTB7 (Table 1.9) which was further improved to 8.4% by introducing a cathode
interlayer in the solar cell and very recently to 9.2% by using an inverted device structure
and a polymer interlayer.
27,194,196

Figure 1. 7 Structures for PTB1, PTB3, PTB4,
PTB7 and PTB9.
196,198,199

45
Table 1.9 Solar cell performance of select PTB copolymers.
Polymer J
sc
(mA/cm
2
) V
oc
(V) FF η
max
( η
avg
) % Ref.
PTB1:PC
61
BM (1:1.2) 15.6 0.56 0.63 5.6
199

PTB3:PC
61
BM (1:1) 13.9 0.72 0.58 5.85
198

PTB4:PC
61
BM (1:1) 13.0 0.74 0.61 5.90
198

PTB7:PC
71
BM (1:1.5) 14.5 0.74 0.69 7.40
196

15.75 0.75 0.70 8.37
194

17.5 0.75 0.70 9.2
27

PTB9: PC
71
BM (1:1.5) 14.3 0.60 0.66 5.66 (5.54)
201

Replacing the ester group on TT with a ketone group is an alternative to
substitution with fluorine which, even though it is effective in lowering the HOMO level,
is also costly and synthetically more challenging. PBDTTT-C (Table 1.10) displayed an
increased V
oc
of 0.7 V (compared to 0.6 eV for the ester substituted analog) and a PCE of
6.58% with PC
71
BM.
202
Additional substitution of TT with a fluorine atom as well as a
linear alkyl chain instead of branched 2-ethylpentyl resulted in PBDTTT-CF (Table 1.10)
which has an even further decreased HOMO level as well as higher hole mobility (a
higher hole mobility than for most other PTBX copolymers was also observed for
fluorine substituted PTB4 and PTB7 likely due to improved packing).
196,198,203

Consequently an increased efficiency of 7.73% was observed for PBDTTT-CF.
203

Huang et al. used the strongly electron withdrawing sulfonyl group in order to
replace the combination of ester (ketone)/fluorine on TT and obtained PBDTTT-S (Table
1.10) which showed a high V
oc
(0.76 V) and a PCE of 6.22%.
204
PBDTTT-S is an
amorphous solid because of the disorder introduced by the bulky sulfonyl side chain and
in order to improve packing alkylthienyl substituents were introduced on the central
phenyl ring of BDT as well as thiophene units between BDT and TT. The thiophene
spacers between monomers decrease steric hindrance between side chains whereas the
46
alkylthienyl substituents on BDT can increase the two-dimensional conjugation and
intermolecular π-π-overlap. PBDTDTTT-S-T (Table 1.10) is, opposite to PBDTT-S, a
semicrystalline solid with a hole mobility more than three times as high which led to an
increased PCE of 7.81%.
195
Using a similar strategy Huo et al. replaced alkoxy
substituted BDT in PBDTTT-C with alkylthienyl substituted BDT resulting in PBDTTT-
C-T (Table 1.10), which not only has a lower band gap as a consequence of increased
conjugation but also an improved thermal and oxidative stability. Additionally, the hole
mobility of PBDTTT-C-T was considerably increased compared to PBDTTT-C resulting
in an efficiency of 7.59%.
205
Extensive device optimization by Li et al. through addition
of gold nanoparticles and the use of a silver nanograting electrode led to further
efficiency increase and a very high PCE of 8.79%.
28

Saadeh et al. described a new series of D/A copolymers based on
selenophenoselenophene (the Se based analog of TT) as well benzodiselenophene (the Se
based analog of BDT) and compared them with the analogous sulfur based polymer
PTB9 (Figure 1.7 and Table 1.9). PSeB2 (Table 1.10), where all sulfur atoms are
replaced by selenium atoms, has a considerably lowered band gap compared to PTB9 as
well as much higher hole mobility probably due to the higher polarizability of Se
compared to S and thus increased Se/Se interchain interactions. For PSeB2 a PCE of
6.87% was achieved which is a 21% increase over the analogous S containing polymer
PTB9.
201

47
Table 1.10 Structure and solar cell performance of thienothiophene based copolymers.
Polymer J
sc
(mA/cm
2
) V
oc
(V) FF η
max
( η
avg
) % Ref.

PBDTTT-C:PC
71
BM (1:1.5)
14.7 0.70 0.64 6.58 (6.30)
202

PBDTTT-CF: PC
71
BM (1:1.5)
15.2 0.76 0.67 7.73 (7.40)
203

PBDTTT-S:PC
71
BM (1:1.5)
14.1 0.76 0.58 6.22
204

PBDTDTTT-S-T:PC
71
BM (1:1)
16.35
a
0.69
a
0.66
a
7.81 (7.48)
195

PBDTTT-C-T:PC
71
BM (1:1.5)
17.48 0.74 0.59 7.59
205

18.39
a
0.76
a
0.63
a
8.79
a

28

PSeB2:PC
71
BM (1:1.2)
16.8 0.64 0.64 6.87 (6.46)
201

48
1.4.3 Random Donor/Acceptor Copolymers
As opposed to perfectly alternating D/A copolymers, random copolymers have a
randomized sequence distribution of donor and acceptor monomers. Even though the
perfectly alternating D/A copolymers described above have lowered band gaps and often
have good overlap with the maximum solar photon flux (at ~700 nm) leading to
improved efficiencies compared to P3HT, the absorption is generally not truly broadened
and short wavelength photons are sacrificed, which in some cases limits the achievable
J
sc
or necessitates high loadings of fullerene. On the other hand it has been shown that
random structures are an effective way of broadening the absorption profiles of
conjugated polymers due to the presence of multiple chromophores in the polymer
backbones.
123,206–209
In 2008 Chen et al. randomly copolymerized thiophene-phenylene-
thiophene (TPT) with BTD as the acceptor and thiophene as the third monomer
(necessary to balance the number of functional groups) (Figure 1.8). They obtained
polymers which had lower band gaps than P3HT (1.7 eV vs. 1.9 eV) without diminishing
the absorption of short wavelength photons. Even though they used TPT which is a
planar monomer and should enhance π-π–conjugation as well as intermolecular stacking
the measured hole mobilities were an order of magnitude below values measured for
P3HT. The maximum PCE achieved for PTPTBT with PC
71
BM was 4.4% (Table
1.11).
208
In a follow up study the same group looked at the use of other acceptor
monomers and achieved efficiencies of over 4% (Figure 1.8 and Table 1.11) using
analogous quinoxaline and DPP based random copolymers. PTPTDPP showed an
49
especially broad and intense absorption profile but interestingly this did not translate into
uniform photoresponse with the majority of current due to short wavelength photons.
210

Table 1.11 Solar cell performance of TPT based random copolymers.
Polymer J
sc
(mA/cm
2
) V
oc
(V) FF η
avg
( η
max
) %
Ref.
PTPTBT:PC
71
BM (1:3) 10.1 0.80 0.53 4.3 (4.4)
208

PTPTQ: PC
71
BM (1:3) 9.0 0.81 0.57 4.2
210

PTPTDPP: PC
71
BM (1:3) 10.3 0.75 0.54 4.2
210

Figure 1.8 TPT-based random copolymers.
208,210

50
More recently the TPT core was extended even further by attaching another
phenyl and thiophene unit (TPTPT) in hopes of lowering the band gap and increasing
hole mobility by planarizing the polymer backbone (Figure 1.9). TPTPT was
copolymerized with either DPP or BTD and thiophene as the third comonomers
(analogous to Figure 1.8) and the ratio between acceptor and donor monomers was
adjusted in order to find the copolymer with optimal performance. PTPTPTDPP12
(which has a DPP to TPTPT ratio of 1:2) has a low band gap (1.37 eV), broad absorption
profile and high efficiency of 5.1% with PC
71
BM (Table 1.12). On the other hand, the
SCLC hole mobility is low (2.6x10
-5
cm
2
/(V*s)) and, opposite to what the UV/vis
absorption profile suggests, the EQE shows only very low photoresponse at longer
wavelengths limiting the achievable efficiency.
211

Table 1.12 Solar cell performance of TPTPT based copolymers.
Polymer J
sc
(mA/cm
2
) V
oc
(V) FF η
avg
% Ref.
PTPTPTDPP12:PC
71
BM
(1:4)
11.71 0.70 0.62 5.1
211

PTPTPTDPP13:PC
71
BM
(1:3)
10.97 0.68 0.61 4.6
211

Figure 1.9 Ladder type TPTPT based random copolymers.
211

51
Li et al. copolymerized 3,5-dialkyldithienothiophene with DPP and thiophene and
the resulting copolymer PDPP-T-DTT (Table 1.13) has a low band gap (1.37 eV) as well
as a very uniform absorption profile from 370-900 nm. Interestingly the hole mobility of
PDPP-T-DTT was an order of magnitude lower than for an analogous perfectly
alternating copolymer, which the authors attributed to the disorder introduced by the
random monomer linkages. With PC
71
BM an efficiency of 5.02% was achieved for
PDPP-T-DTT which was predominantly limited by the moderate V
oc
(0.58 V).
212

Table 1.13 Structure and solar cell performance of PDPP-T-DPP.
Polymer J
sc
(mA/cm
2
) V
oc
(V) FF
η % Ref.

PDPP-T-DTT:PC
71
BM (1:2)
12.76 0.58 0.67 5.02
212

The above mentioned examples of random copolymerizations have in common
that of the three monomers used in a polymer only one is an acceptor. Nielsen et al. chose
to take a different approach by introducing a second acceptor monomer into the polymer
backbone. Using benzotrithiophene as the donor monomer and DPP as the acceptor and
then adding either BTD or TPD (or alternatively increasing the benzotrithiophene
content) allowed them to fine tune optical and electronic properties of the random
copolymers (Figure 1.10). All four novel random polymers not only show a higher
efficiency than the perfectly alternating reference polymer BTT-DPP but also an
increased EQE in both the short and long wavelength region. Copolymer P1 containing
52
benzotrithiophene, DPP and BTD achieved an efficiency of 5.14% in a BHJ solar cell
with PC
71
BM whereas BTT-DPP only has a PCE of 2.68% (Table 1.14).
213

Table 1.14 Solar cell performance of benzotrithiophene based polymers.
Polymer J
sc
(mA/cm
2
) V
oc
(V) FF η % Ref.
BTT-DPP:PC
71
BM (1:2) 6.30 0.71 0.60 2.68
213

P1: PC
71
BM (1:2) 10.95 0.68 0.69 5.14
213

From this short overview it is clear, that even though random D/A copolymers
generally have a broad absorption profile, the number of corresponding polymer:fullerene
solar cells which show high efficiencies is very limited compared to perfectly alternating
D/A copolymers. As mentioned above, due to the structural disorder of completely
random polymers they are often amorphous resulting in low hole mobilities.
Additionally, most random copolymers necessitate high loadings of fullerenes for

Figure 1.10 Benzotrithiophene based D/A copolymers:
BTT-DPP (perfectly alternating) and P1 (random).
213

53
optimized device efficiencies and consequently, even though the polymers broadly
absorb the solar photon flux, contribution of long wavelength photons to J
sc
is very
limited.
123,209,214–216

1.5 A Novel Family of Donor Materials: Semi-Random Copolymers
Reflecting on the different groups of polymers discussed in chapter 1,
homopolymers, perfectly alternating D/A copolymers and random D/A copolymers, it is
obvious that each group has different advantages and disadvantages. Homopolymers, and
specifically the so-called “fruitfly” (although a misnomer) of conjugated polymers P3HT,
has a high hole mobility, is semicrystalline and forms stable active layers with PC
61
BM at
favorable ratios. On the other hand its band gap is high and efficiency is thus inherently
limited. D/A copolymers generally have lower band gaps than P3HT and have achieved
champion efficiencies as a result of high J
sc
and V
oc
. Often though, absorption profiles are
relatively narrow and solar cell performance relies heavily on PC
71
BM. Additionally, all
champion efficiencies have been only achieved after very extensive device optimization.
Finally, random D/A copolymers which tend to have very broad absorption profiles due
to their multichromophoric nature, often suffer from low hole mobilities because of the
increased structural disorder.
Here a novel family of semi-random copolymers with a set of unprecedented
properties will be introduced, which draws elements from the three major polymer groups
discussed above in order to simultaneously optimize all polymer properties.
1–3,217
Semi-
54
random polymers consist mainly of rr-P3HT with small amounts of acceptor monomer
randomly distributed in the backbone thus lowering the band gap through the D/A effect.
Similar to completely random copolymers the randomized sequence distribution of
monomers in semi-random copolymers generates broadly absorbing, multichromophoric
polymers. Importantly though, they retain a larger degree of structural order than random
copolymers as well as many of the attractive properties of P3HT. The following chapters
of this dissertation will discuss the concept, synthesis, characterization and solar cell
performance of semi-random copolymers in detail.
55
1.6 References Chapter 1
(1) Burkhart, B.; Khlyabich, P. P.; Thompson, B. C. ACS Macro Lett. 2012, 1, 660–
666.
(2) Burkhart, B.; Khlyabich, P. P.; Cakir Canak, T.; LaJoie, T. W.; Thompson, B. C.
Macromolecules 2011, 44, 1242–1246.
(3) Khlyabich, P. P.; Burkhart, B.; Ng, C. F.; Thompson, B. C. Macromolecules
2011, 44, 5079–5084.
(4) Mishra, A.; Bäuerle, P. Angew. Chem. Int. Ed. 2012, 51, 2020–2067.
(5) Sun, Y.; Welch, G. C.; Leong, W. L.; Takacs, C. J.; Bazan, G. C.; Heeger, A. J.
Nat. Mater. 2011, 11, 44–48.
(6) van der Poll, T. S.; Love, J. A.; Nguyen, T.-Q.; Bazan, G. C. Adv. Mater. 2012,
24, 3646–3649.
(7) International Energy Agency; Organisation for Economic Co-operation and
Development World energy outlook 2011.; IEA, International Energy Agency :
OECD: Paris, 2011.
(8) Jäger-Waldau, A. Green 2011, 1, 277–290.
(9) García-Valverde, R.; Cherni, J. A.; Urbina, A. Prog. Photovolt: Res. Appl. 2010,
18, 535–558.
(10) Wolden, C. A.; Kurtin, J.; Baxter, J. B.; Repins, I.; Shaheen, S. E.; Torvik, J. T.;
Rockett, A. A.; Fthenakis, V. M.; Aydil, E. S. J. Vac. Sci. Technol. A 2011, 29,
030801.
(11) Avrutin, V.; Izyumskaya, N.; Morkoç, H. Superlattices Microstruct. 2011, 49,
337–364.
(12) Nayak, P. K.; Garcia-Belmonte, G.; Kahn, A.; Bisquert, J.; Cahen, D. Energy
Environ. Sci. 2012, 5, 6022–6039.
(13) Espinosa, N.; Hösel, M.; Angmo, D.; Krebs, F. C. Energy Environ. Sci. 2012, 5,
5117–5132.
(14) Lunt, R. R.; Osedach, T. P.; Brown, P. R.; Rowehl, J. A.; Bulović, V. Adv. Mater.
2011, 23, 5712–5727.
56
(15) Lizin, S.; Van Passel, S.; De Schepper, E.; Vranken, L. Sol. Energy Mater. Sol.
Cells 2012, 103, 1–10.
(16) Shaheen, S. E.; Brabec, C. J.; Sariciftci, N. S.; Padinger, F.; Fromherz, T.;
Hummelen, J. C. Appl. Phys. Lett. 2001, 78, 841–843.
(17) Bundgaard, E.; Krebs, F. Sol. Energy Mater. Sol. Cells 2007, 91, 954–985.
(18) Thompson, B. C.; Fréchet, J. M. J. Angew. Chem. Int. Ed. 2008, 47, 58–77.
(19) Gendron, D.; Leclerc, M. Energy Environ. Sci. 2011, 4, 1225–1237.
(20) Thompson, B. C.; Khlyabich, P. P.; Burkhart, B.; Aviles, A. E.; Rudenko, A.;
Shultz, G. V.; Ng, C. F.; Mangubat, L. B. Green 2011, 29–54.
(21) Chochos, C. L.; Choulis, S. A. Prog. Polym. Sci. 2011, 36, 1326–1414.
(22) Beaujuge, P. M.; Fréchet, J. M. J. J. Am. Chem. Soc. 2011, 133, 20009–20029.
(23) Zhou, H.; Yang, L.; You, W. Macromolecules 2012, 45, 607–632.
(24) Li, G.; Zhu, R.; Yang, Y. Nat. Photonics 2012, 6, 153–161.
(25) Bian, L.; Zhu, E.; Tang, J.; Tang, W.; Zhang, F. Prog. Polym. Sci. 2012, 37,
1292–1331.
(26) Li, Y. Acc. Chem. Res. 2012, 45, 723–733.
(27) He, Z.; Zhong, C.; Su, S.; Xu, M.; Wu, H.; Cao, Y. Nat. Photonics 2012, 6, 593–
597.
(28) Li, X.; Choy, W. C. H.; Huo, L.; Xie, F.; Sha, W. E. I.; Ding, B.; Guo, X.; Li, Y.;
Hou, J.; You, J.; Yang, Y. Adv. Mater. 2012, 24, 3046–3052.
(29) Small, C. E.; Chen, S.; Subbiah, J.; Amb, C. M.; Tsang, S.-W.; Lai, T.-H.;
Reynolds, J. R.; So, F. Nat. Photonics 2011, 6, 115–120.
(30) Günes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. Rev. 2007, 107, 1324–1338.
(31) Walker, B.; Tamayo, A. B.; Dang, X.-D.; Zalar, P.; Seo, J. H.; Garcia, A.;
Tantiwiwat, M.; Nguyen, T.-Q. Adv. Funct. Mater. 2009, 19, 3063–3069.
(32) Yang, X.; Loos, J.; Veenstra, S. C.; Verhees, W. J. H.; Wienk, M. M.; Kroon, J.
M.; Michels, M. A. J.; Janssen, R. A. J. Nano Lett. 2005, 5, 579–583.
57
(33) van Bavel, S.; Sourty, E.; de With, G.; Frolic, K.; Loos, J. Macromolecules 2009,
42, 7396–7403.
(34) Blom, P. W. M.; Mihailetchi, V. D.; Koster, L. J. A.; Markov, D. E. Adv. Mater.
2007, 19, 1551–1566.
(35) Tang, C. W. Appl. Phys. Lett. 1986, 48, 183–185.
(36) Halls, J. J. M.; Walsh, C. A.; Greenham, N. C.; Marseglia, E. A.; Friend, R. H.;
Moratti, S. C.; Holmes, A. B. Nature 1995, 376, 498–500.
(37) Yu, G.; Heeger, A. J. J. Appl. Phys. 1995, 78, 4510–4515.
(38) Coakley, K. M.; McGehee, M. D. Chem. Mater. 2004, 16, 4533–4542.
(39) Krebs, F. C. Sol. Energy Mater. Sol. Cells 2009, 93, 394–412.
(40) Søndergaard, R.; Hösel, M.; Angmo, D.; Larsen-Olsen, T. T.; Krebs, F. C. Mater.
Today 2012, 15, 36–49.
(41) Kotlarski, J. D.; Blom, P. W. M. Appl. Phys. Lett. 2011, 98, 053301.
(42) Dou, L.; Gao, J.; Richard, E.; You, J.; Chen, C.-C.; Cha, K. C.; He, Y.; Li, G.;
Yang, Y. J. Am. Chem. Soc. 2012, 134, 10071–10079.
(43) Gevaerts, V. S.; Furlan, A.; Wienk, M. M.; Turbiez, M.; Janssen, R. A. J. Adv.
Mater. 2012, 24, 2130–2134.
(44) Hummelen, J. C.; Knight, B. W.; LePeq, F.; Fred, W. J. Org. Chem. 1995, 60,
532–538.
(45) Kraabel, B.; Hummelen, J. C.; Vacar, D.; Moses, D.; Sariciftci, N. S.; Heeger, A.
J.; Wudl, F. J. Chem. Phys. 1996, 104, 4267.
(46) Brabec, C. J.; Zerza, G.; Cerullo, G.; De Silvestri, S.; Luzzati, S.; Hummelen, J.
C.; Sariciftci, S. Chem. Phys. Lett. 2001, 340, 232–236.
(47) Wienk, M. M.; Kroon, J. M.; Verhees, W. J. H.; Knol, J.; Hummelen, J. C.; van
Hal, P. A.; Janssen, R. A. J. Angew. Chem. Int. Ed. 2003, 42, 3371–3375.
(48) Ai, X.; Beard, M. C.; Knutsen, K. P.; Shaheen, S. E.; Rumbles, G.; Ellingson, R.
J. J. Phys. Chem. B 2006, 110, 25462–25471.
(49) He, Y.; Chen, H.-Y.; Hou, J.; Li, Y. J. Am. Chem. Soc. 2010, 132, 1377–1382.
(50) He, Y.; Zhao, G.; Peng, B.; Li, Y. Adv. Funct. Mater. 2010, 20, 3383–3389.
58
(51) He, Y.; Li, Y. Phys. Chem. Chem. Phys. 2011, 13, 1970–1983.
(52) Li, C.-Z.; Chueh, C.-C.; Yip, H.-L.; Zou, J.; Chen, W.-C.; Jen, A. K.-Y. J. Mater.
Chem. 2012, 22, 14976–14981.
(53) Holcombe, T. .; oo, C. H.; avulak, D. F. J.; Thompson, B. C.; Fr chet, J. .
J. J. Am. Chem. Soc. 2009, 131, 14160–14161.
(54) McNeill, C. R.; Greenham, N. C. Adv. Mater. 2009, 21, 3840–3850.
(55) Zhou, E.; Cong, J.; Wei, Q.; Tajima, K.; Yang, C.; Hashimoto, K. Angew. Chem.
Int. Ed. 2011, 50, 2799–2803.
(56) Mori, D.; Benten, H.; Ohkita, H.; Ito, S.; Miyake, K. ACS Appl. Mater. Interfaces
2012, 4, 3325–3329.
(57) Bloking, J. T.; Han, X.; Higgs, A. T.; Kastrop, J. P.; Pandey, L.; Norton, J. E.;
Risko, C.; Chen, C. E.; Brédas, J.-L.; McGehee, M. D.; Sellinger, A. Chem.
Mater. 2011, 23, 5484–5490.
(58) Sonar, P.; Fong Lim, J. P.; Chan, K. L. Energy Environ. Sci. 2011, 4, 1558–1574.
(59) Peet, J.; Senatore, M. L.; Heeger, A. J.; Bazan, G. C. Adv. Mater. 2009, 21, 1521–
1527.
(60) Brabec, C. J.; Heeney, M.; McCulloch, I.; Nelson, J. Chem. Soc. Rev. 2011, 40,
1185–1199.
(61) Lou, S. J.; Szarko, J. M.; Xu, T.; Yu, L.; Marks, T. J.; Chen, L. X. J. Am. Chem.
Soc. 2011, 133, 20661–20663.
(62) Liu, F.; Gu, Y.; Jung, J. W.; Jo, W. H.; Russell, T. P. J. Polym. Sci. B Polym.
Phys. 2012, 50, 1018–1040.
(63) Park, S. H.; Roy, A.; Beaupré, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.;
Leclerc, M.; Lee, K.; Heeger, A. J. Nat. Photonics 2009, 3, 297–302.
(64) Murray, I. P.; Lou, S. J.; Cote, L. J.; Loser, S.; Kadleck, C. J.; Xu, T.; Szarko, J.
M.; Rolczynski, B. S.; Johns, J. E.; Huang, J.; Yu, L.; Chen, L. X.; Marks, T. J.;
Hersam, M. C. J. Phys. Chem. Lett. 2011, 2, 3006–3012.
(65) Kumar, P.; Chand, S. Prog. Photovolt: Res. Appl. 2012, 20, 377–415.
(66) Rasmussen, S. C.; Schwiderski, R. L.; Mulholland, M. E. Chem. Commun. 2011,
47, 11394–11410.
59
(67) Vandewal, K.; Gadisa, A.; Oosterbaan, W. D.; Bertho, S.; Banishoeib, F.; Van
Severen, I.; Lutsen, L.; Cleij, T. J.; Vanderzande, D.; Manca, J. V. Adv. Funct.
Mater. 2008, 18, 2064–2070.
(68) Veldman, D.; Meskers, S. C. J.; Janssen, R. A. J. Adv. Funct. Mater. 2009, 19,
1939–1948.
(69) Clarke, T. M.; Durrant, J. R. Chem. Rev. 2010, 110, 6736–6767.
(70) Piliego, C.; Loi, M. A. J. Mater. Chem. 2012, 22, 4141.
(71) Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J. Adv. Funct. Mater. 2005, 15,
1617–1622.
(72) Zhao, G.; He, Y.; Li, Y. Adv. Mater. 2010, 22, 4355–4358.
(73) Xin, H.; Subramaniyan, S.; Kwon, T.-W.; Shoaee, S.; Durrant, J. R.; Jenekhe, S.
A. Chem. Mater. 2012, 24, 1995–2001.
(74) Cardona, C. M.; Li, W.; Kaifer, A. E.; Stockdale, D.; Bazan, G. C. Adv. Mater.
2011, 23, 2367–2371.
(75) Scharber, M. C.; Mühlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A.
J.; Brabec, C. J. Adv. Mater. 2006, 18, 789–794.
(76) Roncali, J. Chem. Rev. 1997, 173–205.
(77) Roncali, J. Macromol. Rapid Commun. 2007, 28, 1761–1775.
(78) Kroon, R.; Lenes, M.; Hummelen, J.; Blom, P.; de Boer, B. Polymer Revs. 2008,
48, 531–582.
(79) Wudl, F.; Kobayashi, M.; Heeger, A. J. J. Org. Chem. 1984, 49, 3382–3384.
(80) Nayak, K.; Marynick, D. S. Macromolecules 1990, 23, 2237–2245.
(81) Hong, S. Y.; Marynick, D. S. Macromolecules 1992, 25, 4652–4657.
(82) Pomerantz, M.; Gu, X. Synth. Met. 1997, 84, 243–244.
(83) Havinga, E. E.; ten Hoeve, W.; Wynberg, H. Polym. Bull. 1992, 119–126.
(84) Havinga, E. E.; ten Hoeve, W.; Wynberg, H. Synth. Met. 1993, 55, 299–306.
60
(85) Mondal, R.; Ko, S.; Norton, J. E.; Miyaki, N.; Becerril, H. A.; Verploegen, E.;
Toney, M. F.; Brédas, J.-L.; McGehee, M. D.; Bao, Z. J. Mater. Chem. 2009, 19,
7195–7197.
(86) Su, M.-S.; Kuo, C.-Y.; Yuan, M.-C.; Jeng, U.-S.; Su, C.-J.; Wei, K.-H. Adv.
Mater. 2011, 23, 3315–3319.
(87) Mauer, R.; Kastler, M.; Laquai, F. Adv. Funct. Mater. 2010, 20, 2085–2092.
(88) Osaka, I.; McCullough, R. D. Acc. Chem. Res. 2008, 41, 1202–1214.
(89) Ku, S.-Y.; Liman, C. D.; Burke, D. J.; Treat, N. D.; Cochran, J. E.; Amir, E.;
Perez, L. A.; Chabinyc, M. L.; Hawker, C. J. Macromolecules 2011, 44, 9533–
9538.
(90) Boudreault, P.-L. T.; Najari, A.; Leclerc, M. Chem. Mater. 2011, 23, 456–469.
(91) Scherf, U. J. Mater. Chem. 1999, 9, 1853–1864.
(92) Uy, R. L.; Price, S. C.; You, W. Macromol. Rapid Commun. 2012, 33, 1162–
1177.
(93) Yang, L.; Zhou, H.; You, W. J. Phys. Chem. C 2010, 114, 16793–16800.
(94) Burkhart, B.; Khlyabich, P. P.; Thompson, B. C. Macromolecules 2012, 45,
3740–3748.
(95) Okamoto, K.; Luscombe, C. K. Polym. Chem. 2011, 2, 2424.
(96) Stefan, M. C.; Bhatt, M. P.; Sista, P.; Magurudeniya, H. D. Polym. Chem. 2012,
3, 1693–1701.
(97) Guo, X.; Cui, C.; Zhang, M.; Huo, L.; Huang, Y.; Hou, J.; Li, Y. Energy Environ.
Sci. 2012, 5, 7943–7949.
(98) Hou, J.; Chen, T. L.; Zhang, S.; Huo, L.; Sista, S.; Yang, Y. Macromolecules
2009, 42, 9217–9219.
(99) Ko, S.; Verploegen, E.; Hong, S.; Mondal, R.; Hoke, E. T.; Toney, M. F.;
McGehee, M. D.; Bao, Z. J. Am. Chem. Soc. 2011, 133, 16722–16725.
(100) Dang, M. T.; Hirsch, L.; Wantz, G. Adv. Mater. 2011, 23, 3597–3602.
(101) Marrocchi, A.; Lanari, D.; Facchetti, A.; Vaccaro, L. Energy Environ. Sci. 2012,
5, 8457–8474.
61
(102) Gadisa, A.; Oosterbaan, W. D.; Vandewal, K.; Bolsée, J.-C.; Bertho, S.; D’Haen,
J.; Lutsen, L.; Vanderzande, D.; Manca, J. V. Adv. Funct. Mater. 2009, 19, 3300–
3306.
(103) Nguyen, L. H.; Hoppe, H.; Erb, T.; Günes, S.; Gobsch, G.; Sariciftci, N. S. Adv.
Funct. Mater. 2007, 17, 1071–1078.
(104) Troshin, P.; Susarova, D.; Khakina, E.; Goryachev, A. E.; Ponomarenko, S.;
Razumov, V.; Sariciftci, N. S. J. Mater. Chem. 2012, 22, 18433–18441.
(105) Sun, Y.; Cui, C.; Wang, H.; Li, Y. Adv. Energy Mater. 2012, 2, 966–969.
(106) Wu, P.-T.; Xin, H.; Kim, F. S.; Ren, G.; Jenekhe, S. A. Macromolecules 2009,
42, 8817–8826.
(107) Kim, Y.; Cook, S.; Tuladhar, S. M.; Choulis, S. A.; Nelson, J.; Durrant, J. R.;
Bradley, D. D. C.; Giles, M.; McCulloch, I.; Ha, C.-S.; Ree, M. Nat. Mater. 2006,
5, 197–203.
(108) Sivula, K.; Luscombe, C. K.; Thompson, B. C.; Fréchet, J. M. J. J. Am. Chem.
Soc. 2006, 128, 13988–13989.
(109) oo, C. H.; Thompson, B. C.; im, B. J.; Toney, . F.; Fr chet, J. . J. J. Am.
Chem. Soc. 2008, 130, 16324–16329.
(110) Schilinsky, P.; Asawapirom, U.; Scherf, U.; Biele, M.; Brabec, C. J. Chem.
Mater. 2005, 17, 2175–2180.
(111) Kline, R. J.; McGehee, M. D.; Kadnikova, E. N.; Liu, J.; Fréchet, J. M. J.; Toney,
M. F. Macromolecules 2005, 38, 3312–3319.
(112) Chang, J.-F.; Clark, J.; Zhao, N.; Sirringhaus, H.; Breiby, D.; Andreasen, J.;
Nielsen, M.; Giles, M.; Heeney, M.; McCulloch, I. Phys. Rev. B 2006, 74,
115318.
(113) Ko, S.; Hoke, E. T.; Pandey, L.; Hong, S.; Mondal, R.; Risko, C.; Yi, Y.;
Noriega, R.; McGehee, M. D.; Brédas, J.-L.; Salleo, A.; Bao, Z. J. Am. Chem.
Soc. 2012, 134, 5222–5232.
(114) Babudri, F.; Farinola, G. M.; Naso, F. J. Mater. Chem. 2004, 14, 11–34.
(115) Cheng, Y.-J.; Luh, T.-Y. J. Organomet. Chem. 2004, 689, 4137–4148.
(116) Carsten, B.; He, F.; Son, H. J.; Xu, T.; Yu, L. Chem. Rev. 2011, 111, 1493–1528.
(117) Hoyos, M.; L. Turner, M.; Navarro, O. Curr. Org. Chem. 2011, 15, 3263–3290.
62
(118) Park, J. K.; Jo, J.; Seo, J. H.; Moon, J. S.; Park, Y. D.; Lee, K.; Heeger, A. J.;
Bazan, G. C. Adv. Mater. 2011, 23, 2430–2435.
(119) Camaioni, N.; Tinti, F.; Franco, L.; Fabris, M.; Toffoletti, A.; Ruzzi, M.;
Montanari, L.; Bonoldi, L.; Pellegrino, A.; Calabrese, A.; Po, R. Org. Electron.
2012, 13, 550–559.
(120) Facchetti, A.; Vaccaro, L.; Marrocchi, A. Angew. Chem. Int. Ed. 2012, 51, 3520–
3523.
(121) Jo, J.; Pron, A.; Berrouard, P.; Leong, W. L.; Yuen, J. D.; Moon, J. S.; Leclerc,
M.; Heeger, A. J. Adv. Energy Mater. 2012, ASAP.
(122) Rudenko, A. E.; Wiley, C. A.; Stone, S. M.; Tannaci, J. F.; Thompson, B. C. J.
Polym. Sci. A Polym. Chem. 2012, 50, 3691–3697.
(123) Zhu, Z.; Waller, D.; Gaudiana, R.; Morana, M.; Mühlbacher, D.; Scharber, M.;
Brabec, C. Macromolecules 2007, 40, 1981–1986.
(124) Bijleveld, J. C.; Shahid, M.; Gilot, J.; Wienk, M. M.; Janssen, R. A. J. Adv. Funct.
Mater. 2009, 19, 3262–3270.
(125) Kowalski, S.; Allard, S.; Scherf, U. ACS Macro Lett. 2012, 1, 465–468.
(126) Zhang, Z.; Wang, J. J. Mater. Chem. 2012, 22, 4178–4187.
(127) Mühlbacher, D.; Scharber, M.; Morana, M.; Zhu, Z.; Waller, D.; Gaudiana, R.;
Brabec, C. Adv. Mater. 2006, 18, 2884–2889.
(128) Peet, J.; Kim, J. Y.; Coates, N. E.; Ma, W. L.; Moses, D.; Heeger, A. J.; Bazan,
G. C. Nat. Mater. 2007, 6, 497–500.
(129) Zhang, Y.; Zou, J.; Cheuh, C.-C.; Yip, H.-L.; Jen, A. K.-Y. Macromolecules
2012, 45, 5427–5435.
(130) Scharber, M. C.; Koppe, M.; Gao, J.; Cordella, F.; Loi, M. A.; Denk, P.; Morana,
M.; Egelhaaf, H.-J.; Forberich, K.; Dennler, G.; Gaudiana, R.; Waller, D.; Zhu,
Z.; Shi, X.; Brabec, C. J. Adv. Mater. 2010, 22, 367–370.
(131) Hou, J.; Chen, H.-Y.; Zhang, S.; Li, G.; Yang, Y. J. Am. Chem. Soc. 2008, 130,
16144–16145.
(132) Coffin, R. C.; Peet, J.; Rogers, J.; Bazan, G. C. Nat. Chem. 2009, 1, 657–661.
(133) Fei, Z.; Kim, J. S.; Smith, J.; Domingo, E. B.; Anthopoulos, T. D.; Stingelin, N.;
Watkins, S. E.; Kim, J.-S.; Heeney, M. J. Mater. Chem. 2011, 21, 16257–16263.
63
(134) Chen, H.-Y.; Hou, J.; Hayden, A. E.; Yang, H.; Houk, K. N.; Yang, Y. Adv.
Mater. 2010, 22, 371–375.
(135) Yue, W.; Zhao, Y.; Shao, S.; Tian, H.; Xie, Z.; Geng, Y.; Wang, F. J. Mater.
Chem. 2009, 19, 2199–2206.
(136) Gibson, G. L.; McCormick, T. M.; Seferos, D. S. J. Am. Chem. Soc. 2012, 134,
539–547.
(137) Blouin, N.; Michaud, A.; Leclerc, M. Adv. Mater. 2007, 19, 2295–2300.
(138) Liu, J.; Shao, S.; Fang, G.; Meng, B.; Xie, Z.; Wang, L. Adv. Mater. 2012, 24,
2774–2779.
(139) Sun, Y.; Takacs, C. J.; Cowan, S. R.; Seo, J. H.; Gong, X.; Roy, A.; Heeger, A. J.
Adv. Mater. 2011, 23, 2226–2230.
(140) Chen, M.-H.; Hou, J.; Hong, Z.; Yang, G.; Sista, S.; Chen, L.-M.; Yang, Y. Adv.
Mater. 2009, 21, 4238–4242.
(141) Allard, .; A ch, R. B.; endron, D.; Boudreault, P. -L. T.; Tessier, C.; Alem, S.;
Tse, S.-C.; Tao, Y.; Leclerc, M. Macromolecules 2010, 43, 2328–2333.
(142) Wang, E.; Wang, L.; Lan, L.; Luo, C.; Zhuang, W.; Peng, J.; Cao, Y. Appl. Phys.
Lett. 2008, 92, 033307.
(143) Du, C.; Li, C.; Li, W.; Chen, X.; Bo, Z.; Veit, C.; Ma, Z.; Wuerfel, U.; Zhu, H.;
Hu, W.; Zhang, F. Macromolecules 2011, 44, 7617–7624.
(144) Chen, Y.-C.; Yu, C.-Y.; Fan, Y.-L.; Hung, L.-I.; Chen, C.-P.; Ting, C. Chem.
Commun. 2010, 46, 6503–6505.
(145) Huo, L.; Hou, J.; Zhang, S.; Chen, H.-Y.; Yang, Y. Angew. Chem. Int. Ed. 2010,
49, 1500–1503.
(146) Peng, Q.; Liu, X.; Su, D.; Fu, G.; Xu, J.; Dai, L. Adv. Mater. 2011, 23, 4554–
4558.
(147) Zhou, H.; Yang, L.; Stuart, A. C.; Price, S. C.; Liu, S.; You, W. Angew. Chem.
Int. Ed. 2011, 50, 2995–2998.
(148) Ong, K.-H.; Lim, S.-L.; Tan, H.-S.; Wong, H.-K.; Li, J.; Ma, Z.; Moh, L. C. H.;
Lim, S.-H.; de Mello, J. C.; Chen, Z.-K. Adv. Mater. 2011, 23, 1409–1413.
(149) Zhou, H.; Yang, L.; Price, S. C.; Knight, K. J.; You, W. Angew. Chem. Int. Ed.
2010, 49, 7992–7995.
64
(150) Ying, L.; Hsu, B. B. Y.; Zhan, H.; Welch, G. C.; Zalar, P.; Perez, L. A.; Kramer,
E. J.; Nguyen, T.-Q.; Heeger, A. J.; Wong, W.-Y.; Bazan, G. C. J. Am. Chem.
Soc. 2011, 133, 18538–18541.
(151) Wang, M.; Hu, X.; Liu, P.; Li, W.; Gong, X.; Huang, F.; Cao, Y. J. Am. Chem.
Soc. 2011, 133, 9638–9641.
(152) Yang, T.; Wang, M.; Duan, C.; Hu, X.; Huang, L.; Peng, J.; Huang, F.; Gong, X.
Energy Environ. Sci. 2012, 5, 8208–8214.
(153) Patel, D. G. (Dan); Feng, F.; Ohnishi, Y.; Abboud, K. A.; Hirata, S.; Schanze, K.
S.; Reynolds, J. R. J. Am. Chem. Soc. 2012, 134, 2599–2612.
(154) Price, S. C.; Stuart, A. C.; Yang, L.; Zhou, H.; You, W. J. Am. Chem. Soc. 2011,
133, 4625–4631.
(155) Nie, W.; MacNeill, C. M.; Li, Y.; Noftle, R. E.; Carroll, D. L.; Coffin, R. C.
Macromol. Rapid Commun. 2011, 32, 1163–1168.
(156) ou, .; ajari, A.; Berrouard, P.; Beaupr , S.; R da A ch, B.; Tao, .; Leclerc,
M. J. Am. Chem. Soc. 2010, 132, 5330–5331.
(157) Piliego, C.; Holcombe, T. W.; Douglas, J. D.; Woo, C. H.; Beaujuge, P. .;
Fr chet, J. . J. J. Am. Chem. Soc. 2010, 132, 7595–7597.
(158) Aïch, B. R.; Lu, J.; Beaupré, S.; Leclerc, M.; Tao, Y. Org. Electron. 2012, 13,
1736–1741.
(159) Li, Z.; Tsang, S.-W.; Du, X.; Scoles, L.; Robertson, G.; Zhang, Y.; Toll, F.; Tao,
Y.; Lu, J.; Ding, J. Adv. Funct. Mater. 2011, 21, 3331–3336.
(160) Chu, T.-Y.; Tsang, S.-W.; Zhou, J.; Verly, P. G.; Lu, J.; Beaupré, S.; Leclerc, M.;
Tao, Y. Sol. Energy Mater. Sol. Cells 2012, 96, 155–159.
(161) Chu, T.-Y.; Lu, J.; Beaupré, S.; Zhang, Y.; Pouliot, J.-R.; Zhou, J.; Najari, A.;
Leclerc, M.; Tao, Y. Adv. Funct. Mater. 2012, 22, 2345–2351.
(162) Amb, C. M.; Chen, S.; Graham, K. R.; Subbiah, J.; Small, C. E.; So, F.;
Reynolds, J. R. J. Am. Chem. Soc. 2011, 133, 10062–10065.
(163) Yuan, M.-C.; Chiu, M.-Y.; Liu, S.-P.; Chen, C.-M.; Wei, K.-H. Macromolecules
2010, 43, 6936–6938.
(164) Najari, A.; Berrouard, P.; Ottone, C.; Boivin, M.; Zou, Y.; Gendron, D.; Caron,
W.-O.; Legros, P.; Allen, C. N.; Sadki, S.; Leclerc, M. Macromolecules 2012, 45,
1833–1838.
65
(165) Hao, Z.; Iqbal, A. Chem. Soc. Rev. 1997, 26, 203–213.
(166) Qu, S.; Tian, H. Chem. Commun. 2012, 48, 3039–3051.
(167) Wienk, M. M.; Turbiez, M.; Gilot, J.; Janssen, R. A. J. Adv. Mater. 2008, 20,
2556–2560.
(168) Liu, F.; Gu, Y.; Wang, C.; Zhao, W.; Chen, D.; Briseno, A. L.; Russell, T. P. Adv.
Mater. 2012, 24, 3947–3951.
(169) Bijleveld, J. C.; Zoombelt, A. P.; Mathijssen, S. G. J.; Wienk, M. M.; Turbiez,
M.; de Leeuw, D. M.; Janssen, R. A. J. J. Am. Chem. Soc. 2009, 131, 16616–
16617.
(170) Bijleveld, J. C.; Gevaerts, V. S.; Di Nuzzo, D.; Turbiez, M.; Mathijssen, S. G. J.;
de Leeuw, D. M.; Wienk, M. M.; Janssen, R. A. J. Adv. Mater. 2010, 22, E242–
E246.
(171) Jung, J. W.; Liu, F.; Russell, T. P.; Jo, W. H. Energy Environ. Sci. 2012, 5, 6857–
6861.
(172) Woong Jung, J.; Woong Jo, J.; Liu, F.; Russell, T. P.; Ho Jo, W. Chem. Commun.
2012, 48, 6933–6935.
(173) Bronstein, H.; Chen, Z.; Ashraf, R. S.; Zhang, W.; Du, J.; Durrant, J. R.; Shakya
Tuladhar, P.; Song, K.; Watkins, S. E.; Geerts, Y.; Wienk, M. M.; Janssen, R. A.
J.; Anthopoulos, T.; Sirringhaus, H.; Heeney, M.; McCulloch, I. J. Am. Chem.
Soc. 2011, 133, 3272–3275.
(174) Yiu, A. T.; Beaujuge, P. M.; Lee, O. P.; Woo, C. H.; Toney, M. F.; Fréchet, J. M.
J. J. Am. Chem. Soc. 2012, 134, 2180–2185.
(175) ang, .; a, .; hang, .; Vandewal, .; Henriksson, P.; Ingan s, O.; hang,
F.; Andersson, M. R. J. Am. Chem. Soc. 2011, 133, 14244–14247.
(176) Stalder, R.; Mei, J.; Reynolds, J. R. Macromolecules 2010, 43, 8348–8352.
(177) Stalder, R.; Grand, C.; Subbiah, J.; So, F.; Reynolds, J. R. Polym. Chem. 2012, 3,
89–92.
(178) Ma, Z.; Wang, E.; Jarvid, M. E.; Henriksson, P.; Inganäs, O.; Zhang, F.;
Andersson, M. R. J. Mater. Chem. 2012, 22, 2306–2314.
(179) Zhang, G.; Fu, Y.; Xie, Z.; Zhang, Q. Macromolecules 2011, 44, 1414–1420.
66
(180) Liu, B.; Zou, Y.; Peng, B.; Zhao, B.; Huang, K.; He, Y.; Pan, C. Polym. Chem.
2011, 2, 1156–1162.
(181) Gadisa, A.; Mammo, W.; Andersson, L. M.; Admassie, S.; Zhang, F.; Andersson,
M. R.; Inganäs, O. Adv. Funct. Mater. 2007, 17, 3836–3842.
(182) Zhang, F.; Bijleveld, J.; Perzon, E.; Tvingstedt, K.; Barrau, S.; Inganäs, O.;
Andersson, M. R. J. Mater. Chem. 2008, 18, 5468–5474.
(183) Lindgren, L. J.; hang, F.; Andersson, .; Barrau, S.; Hellstr m, S.; ammo,
.; Per on, .; Ingan s, O.; Andersson, . R. Chem. Mater. 2009, 21, 3491–
3502.
(184) Wang, E.; Hou, L.; Wang, Z.; Hellstroem, S.; Zhang, F.; Inganaes, O.;
Andersson, M. R. Adv. Mater. 2010, 22, 5240–5244.
(185) Zhang, Y.; Zou, J.; Yip, H.-L.; Chen, K.-S.; Zeigler, D. F.; Sun, Y.; Jen, A. K.-Y.
Chem. Mater. 2011, 23, 2289–2291.
(186) Li, C.-Z.; Chueh, C.-C.; Yip, H.-L.; O’ alley, . .; Chen, W.-C.; Jen, A. K.-Y.
J. Mater. Chem. 2012, 22, 8574–8578.
(187) Zou, J.; Yip, H.-L.; Zhang, Y.; Gao, Y.; Chien, S.-C.; O’ alley, .; Chueh, C. -
C.; Chen, H.; Jen, A. K.-Y. Adv. Funct. Mater. 2012, 22, 2804–2811.
(188) Zhang, Y.; Zou, J.; Yip, H.-L.; Chen, K.-S.; Davies, J. A.; Sun, Y.; Jen, A. K.-Y.
Macromolecules 2011, 44, 4752–4758.
(189) He, Z.; Zhang, C.; Xu, X.; Zhang, L.; Huang, L.; Chen, J.; Wu, H.; Cao, Y. Adv.
Mater. 2011, 23, 3086–3089.
(190) Shaheen, S. E.; Vangeneugden, D.; Kiebooms, R.; Vanderzande, D.; Fromherz,
T.; Padinger, F.; Brabec, C. J.; Sariciftci, N. S. Synth. Met. 2001, 121, 1583–
1584.
(191) Vangeneugden, D. L.; Vanderzande, D. J. M.; Salbeck, J.; van Hal, P. A.;
Janssen, R. A. J.; Hummelen, J. C.; Brabec, C. J.; Shaheen, S. E.; Sariciftci, N. S.
J. Phys. Chem. B 2001, 105, 11106–11113.
(192) Qin, Y.; Kim, J. Y.; Frisbie, C. D.; Hillmyer, M. A. Macromolecules 2008, 41,
5563–5570.
(193) Douglas, J. D.; Griffini, G.; Holcombe, T. W.; Young, E. P.; Lee, O. P.; Chen, M.
S.; Fréchet, J. M. J. Macromolecules 2012, 45, 4069–4074.
67
(194) He, Z.; Zhong, C.; Huang, X.; Wong, W.-Y.; Wu, H.; Chen, L.; Su, S.; Cao, Y.
Adv. Mater. 2011, 23, 4636–4643.
(195) Huang, Y.; Guo, X.; Liu, F.; Huo, L.; Chen, Y.; Russell, T. P.; Han, C. C.; Li, Y.;
Hou, J. Adv. Mater. 2012, 24, 3383–3389.
(196) Liang, Y.; Xu, Z.; Xia, J.; Tsai, S.-T.; Wu, Y.; Li, G.; Ray, C.; Yu, L. Adv. Mater.
2010, 22, E135–E138.
(197) Szarko, J. M.; Guo, J.; Liang, Y.; Lee, B.; Rolczynski, B. S.; Strzalka, J.; Xu, T.;
Loser, S.; Marks, T. J.; Yu, L.; Chen, L. X. Adv. Mater. 2010, 22, 5468–5472.
(198) Liang, Y.; Feng, D.; Wu, Y.; Tsai, S.-T.; Li, G.; Ray, C.; Yu, L. J. Am. Chem.
Soc. 2009, 131, 7792–7799.
(199) Liang, Y.; Wu, Y.; Feng, D.; Tsai, S.-T.; Son, H.-J.; Li, G.; Yu, L. J. Am. Chem.
Soc. 2009, 131, 56–57.
(200) Son, H. J.; Wang, W.; Xu, T.; Liang, Y.; Wu, Y.; Li, G.; Yu, L. J. Am. Chem.
Soc. 2011, 133, 1885–1894.
(201) Saadeh, H. A.; Lu, L.; He, F.; Bullock, J. E.; Wang, W.; Carsten, B.; Yu, L. ACS
Macro Lett. 2012, 1, 361–365.
(202) Hou, J.; Chen, H.-Y.; Zhang, S.; Chen, R. I.; Yang, Y.; Wu, Y.; Li, G. J. Am.
Chem. Soc. 2009, 131, 15586–15587.
(203) Chen, H.-Y.; Hou, J.; Zhang, S.; Liang, Y.; Yang, G.; Yang, Y.; Yu, L.; Wu, Y.;
Li, G. Nat. Photonics 2009, 3, 649–653.
(204) Huang, Y.; Huo, L.; Zhang, S.; Guo, X.; Han, C. C.; Li, Y.; Hou, J. Chem.
Commun. 2011, 47, 8904–8906.
(205) Huo, L.; Zhang, S.; Guo, X.; Xu, F.; Li, Y.; Hou, J. Angew. Chem. Int. Ed. 2011,
50, 9697–9702.
(206) Liang, Y.; Xiao, S.; Feng, D.; Yu, L. J. Phys. Chem. C 2008, 112, 7866–7871.
(207) Beaujuge, P. M.; Ellinger, S.; Reynolds, J. R. Nat. Mater. 2008, 7, 795–799.
(208) Chen, C.-P.; Chan, S.-H.; Chao, T.-C.; Ting, C.; Ko, B.-T. J. Am. Chem. Soc.
2008, 130, 12828–12833.
(209) He, Y.; Wang, X.; Zhang, J.; Li, Y. Macromol. Rapid Commun. 2009, 30, 45–51.
68
(210) Yu, C.-Y.; Chen, C.-P.; Chan, S.-H.; Hwang, G.-W.; Ting, C. Chem. Mater.
2009, 21, 3262–3269.
(211) Chen, C.-H.; Cheng, Y.-J.; Chang, C.-Y.; Hsu, C.-S. Macromolecules 2011, 44,
8415–8424.
(212) Li, J.; Ong, K.-H.; Lim, S.-L.; Ng, G.-M.; Tan, H.-S.; Chen, Z.-K. Chem.
Commun. 2011, 47, 9480–9482.
(213) ielsen, C. B.; Ashraf, R. S.; Schroeder, B. C.; D’Angelo, P.; atkins, S. .;
Song, K.; Anthopoulos, T. D.; McCulloch, I. Chem. Commun. 2012, 48, 5832–
5834.
(214) Tsai, J.-H.; Chueh, C.-C.; Chen, W.-C.; Yu, C.-Y.; Hwang, G.-W.; Ting, C.;
Chen, E.-C.; Meng, H.-F. J. Polym. Sci. A Polym. Chem. 2010, 48, 2351–2360.
(215) Peng, Q.; Liu, X.; Qin, Y.; Xu, J.; Li, M.; Dai, L. J. Mater. Chem. 2011, 21,
7714–7722.
(216) Song, J.; Zhang, C.; Li, C.; Li, W.; Qin, R.; Li, B.; Liu, Z.; Bo, Z. J. Polym. Sci. A
Polym. Chem. 2010, 48, 2571–2578.
(217) Burkhart, B.; Khlyabich, P. P.; Thompson, B. C. J. Photon. Energy 2012, 2,
021002.

69
CHAPTER 2 Semi-Random Multichromophoric rr-P3HT
Analogues: Design and First Generation
2.1 Introduction
To introduce the concept of semi-random copolymers in more detail the next
pages will focus predominantly on the premises semi-random polymers are based on, the
synthetic strategies used to achieve well defined structures as well as their thorough
characterization, whereas the following chapters will put more emphasis on structure
property relationships.
As discussed in section 1.3, to achieve longer wavelength absorption the D/A
approach is frequently applied.
1
Fewer papers take an alternate approach to low band gap
polymers by using a random sequence of electron rich and electron poor monomers
instead of the mentioned perfectly alternating structure.
2–5
Most commonly in this
random approach, two donor monomers are copolymerized with one acceptor monomer
whereas it is less common that two different acceptors are polymerized with one donor
monomer (see section 1.4.3).
2,6–10
Many of these random copolymers show a
considerably broadened absorption and/or two distinct absorption peaks in the short and
long wavelength regions illustrating the large potential of random polymer structures to
increase J
sc
in solar cells in the future.
The so-called semi-random structure introduced here, is defined by a random
polymerization that is based on a restricted linkage pattern of the monomers due to
70
regiospecific placement of the reactive functional groups. As a consequence, the
polymers are designed to retain a higher degree of structural order than purely random
analogues. Figure 2.1a illustrates the concept of semi-random polymers by showing a
possible segment of one of the synthesized semi-random polymer chains (the synthetic
scheme is illustrated in Figure 2.1b). It can be seen that no repeat unit can easily be
defined because many different monomer sequences are possible and as a consequence
multiple chromophores can be envisioned depending on the actual effective conjugation
length of the polymer backbone. Figure 2.1a schematically shows two possible
chromophores as an example, with chromophore I consisting of pure rr-P3HT and thus
absorbing at shorter wavelengths and chromophore II containing acceptor monomer BTD
which shifts the absorption onset to longer wavelengths. Semi-random copolymers are
thus expected to have a broader absorption than perfectly alternating copolymers (which
contain a single chromophore) because they contain multiple distinct chromophores,
which can absorb in different regions of the solar spectrum. As mentioned already in
section 1.5, semi-random copolymers are chosen over random copolymers because they
allow better control over the polymer structure and still offer the advantage of multiple
chromophores. Additionally, semi-random copolymers are designed to favor short-range
crystallinity, (whereas random copolymers tend to be amorphous solids
3,5
) over long-
range crystallinity which is known to be advantageous in the case of rr-P3HT, where a
reduction in regioregularity prevents large-scale phase separation between the polymer
and fullerene but still allows for high hole mobility.
11,12

71

Figure 2.1 Concept of multichromophoric semi-random copolymers (a) and a
representative synthetic scheme (b), along with the structures of the described first
generation polymers (c).
72
The design of this novel class of polymers is based on P3HT in order to capitalize
on the attractive properties of this well known conjugated polymer. P3HT shows a good
miscibility with fullerenes, forms bicontinuous networks with PCBM after thermal
annealing and is semicrystalline which leads to high hole mobilities.
13
Additionally,
composites with PCBM show very good long term stability which is important for
commercial success of organic solar cells.
14,15
The major weakness of P3HT, the narrow
spectral absorption breadth from 350-650 nm (limiting J
sc
to a theoretical maximum of
~14.3mA/cm
2
)
16
is solved in this approach by copolymerizing with a small amount of
acceptor monomers. Multiple acceptors can be easily used in semi-random conjugated
polymers to adjust the band gap and position of the HOMO and LUMO energies while
still retaining a polymer backbone that is P3HT-like.
Several papers are worth mentioning here as they are related to polymer structures
synthesized in this work (Figure 2.1c). The polymers of Li et al.
4
show similarity to the
structures in Figure 2.1c as they contain similar monomer units as well as randomized
structures, but differ from the polymers presented here in several critical aspects. Most
importantly, use of 2,5-dibromo-3-hexylthiophene as a monomer in the work of Li et al,
does not allow for the formation of hexylthiophene-hexylthiophene bonds and also
prevents any control over regioregularity in the polymers. In contrast, the use of the 2-
bromo-5-trimethylstannyl-3-hexylthiophene monomer in semi-random copolymers
ensures the formation of head-to-tail 3-hexylthiophene sequences. In addition, the content
of hexylthiophene in the polymers published by Li et al, is considerably lower than in the
present case (25-30% vs. 65-80% respectively). As a consequence semi-random
73
copolymers are unique and are rr-P3HT analogues, which represent a simple perturbation
of the rr-P3HT structure, but maintain critical elements of the polymeric structure. In
comparison, several other examples are known of polymers containing extended
oligothiophene segments, which are copolymerized with an acceptor monomer to give
perfectly alternating polymer structures. These polymers tend to be crystalline and
generally show broadened absorption spectra but have the disadvantage of complicated
and lengthy monomer synthesis contrasting with the here presented approach of using a
simple synthetic procedure to obtain P3HT-like semi-random D/A copolymers.
17–19

2.2 Synthesis of Semi-Random Copolymers
Polymers were designed with a number of structural features in mind: First,
monomers for semi-random Stille polymerization shown in Figure 2.1b are carefully
chosen to inherently avoid sterically unfavorable linkages. Reaction of the monomer 2-
bromo-5-trimethyltin-hexylthiophene (1) with itself gives head-to-tail P3HT, which is
desirable for efficient solar cells and is illustrated in chromophore I in Figure 2.1a.
Figure 2.1b also shows that acceptor monomers such as 4,7-dibromo-2,1,3-
benzothiadiazole (3) can only react with donor monomers ((1) or 2,5-
bis(trimethyltin)thiophene (2)) but not with themselves, which avoids large (insoluble)
segments of acceptor in polymers. These restrictions in monomer connectivities help to
overcome some of the limitations random copolymers have, namely an amorphous
morphology and likely low hole mobilities as a consequence.
3,5
Second, as can be seen in
74
Figure 2.1c between 65 and 80% of 2-bromo-5-trimethyltin-hexylthiophene (1) is used in
the polymer chains in order to retain the favorable properties of P3HT. Third, the amount
of acceptor is kept small (between 10 and 17.5%) but the strength of acceptors is varied
from the weaker acceptor BTD to the strong acceptor TP and finally a combination of the
two acceptors in order to tune the energy levels. The synthesis of all monomers is
described in appendix 1.
Figure 2.1b shows an example for the established semi-random Stille
polymerization (using DMF as the solvent and Pd(PPh
3
)
4
as the catalyst) which was used
to synthesize polymers P3HTT-BTD, P3HTT-TP and P3HTT-TP-BTD which are shown
in Figure 2.1c. The polymerization method is highly reproducible and independent of the
order of mixing of reactants and careful purification using a standard soxhlet extraction
gives high quality polymers. P3HT and P3HTT were made as reference polymers using
the exact same polymerization conditions. The acronyms stand for poly-3-
hexylthiophene thiophene (P3HTT) to take into account the additional non-alkylated
thiophene ring in the backbone with the second part of the acronym indicating the
respective acceptor(s) TP or BTD. For the synthesis of P3HTT-BTD, P3HTT-TP and
P3HTT-TP-BTD monomers 1 and 2 were copolymerized with the selected ratios of
dibromo-benzothiadiazole and/or dibromo-thienopyrazine (see appendix 1). Molecular
weights were determined by dissolving polymers in o-dichlorobenzene (o-DCB) using
gel permeation chromatography (GPC) against polystyrene standards. The results are
summarized in Table 2.1 and show that the M
n
are all in the same range (15,000 - 17,000
g/mol) with the exception of P3HTT. Integration via
1
H NMR indicates that composition
75
of the polymers matches the monomer feed ratios (see appendix 1 for polymer NMR’s
and appendix 2 for a section on how to calculate the monomer incorporation ratio).

2.3 Characterization and Solar Cell Performance
UV/vis absorption spectra were recorded for all five polymers in solution (o-
DCB) and in annealed thin films. The results are summarized in Figure 2.2. As can be
seen from the UV/vis absorption spectra all three D/A semi-random copolymers have
considerably broadened absorption spectra compared to P3HT and P3HTT. Red shifts of
the absorption peaks when comparing solution spectra to thin film spectra are between 50
and 100 nm, which indicates ordering and planarization of the polymer chains in the solid
state. The largest shift (103 nm) is observed for the long wavelength absorption peak of
P3HTT-TP-BTD. In the solid state the area underneath the absorption curve of P3HTT-
TP-BTD is nearly twice as large as for P3HT, which means that twice as many photons
are absorbed in films of the same thickness and J
sc
can potentially be much higher.
16

Because the absorption of P3HTT-TP-BTD blankets the whole visible region with a high
absorption coefficient, it is a “black” polymer. The optical band gaps defined by the onset
of absorption in the thin film spectra are summarized in Table 2.1 and are considerably
lower for the D/A copolymers than for P3HT.
76

In order to validate the concept that the semi-random approach allows the
retention of attractive properties of P3HT, such as semicrystallinity, while broadening the
absorption, the copolymers were analyzed by grazing-incidence X-ray diffraction
(GIXRD). As can be seen in Figure 2.3, P3HT and P3HTT-TP-BTD exhibit peaks in the

Figure 2.2 UV/vis absorption of all five
polymers in (a) solution (o-DCB) and (b) thin
film (spin-coated from o-DCB and annealed for
30 min at 60
o
C under N
2
) where (i) is P3HT,
(ii) is P3HTT, (iii) is P3HTT-BTD, (iv) is
P3HTT-TP and (v) is P3HTT-TP-BTD.
77
2 range of 5-7°, without thermal treatment, which are referenced as (100) in relation to
P3HT.
13
These peaks become even more pronounced after thermal annealing, which is
evidence for increased crystallinity. P3HTT-BTD on the other hand only develops
semicrystalline features after annealing at 150
o
C. As has already been mentioned this is
different from random copolymers and a considerable advantage of the semi-random
concept.
3,5
The inter-chain packing distance (100) for P3HTT-BTD is calculated as 15.6
Å and for P3HTT-TP-BTD as 14.4 Å (for comparison the inter-chain packing distance of
P3HT was measured as 16.4 Å).
13
Interestingly, P3HTT displayed a possible melting
transition in differential scanning calorimetry (DSC) measurements (see appendix 1) but
showed no evidence of crystallinity in X-ray studies. P3HTT-TP on the other hand seems
to be completely amorphous as neither X-ray nor DSC studies show any signs of
crystallinity, which could however be due to non-optimized processing conditions.
78

HOMO levels of the polymers were determined by CV vs. ferrocene and the
values are summarized in Table 2.1 (see appendix 1 for CV traces). P3HTT-BTD shows
a considerably lower HOMO than P3HT (5.41 vs. 5.17 eV), which is expected due to the
electron-withdrawing effects of BTD. Interestingly, the HOMO level of P3HTT-TP-BTD
(5.11 eV) is higher than the HOMO levels of both P3HTT-TP (5.23 eV) and P3HTT-
BTD (5.41 eV) and close to the value measured for P3HT (5.17 eV).
Hole mobilities were determined using space-charge limit current (SCLC) method
and are (except for P3HTT) extremely close to the hole mobility of P3HT (see Table 2.1
for values), which further validates the concept of these semi-random polymers. High

Figure 2.3 GIXRD of thin films spin-coated
from chlorobenzene (CB) before and after
annealing at 150
o
C for 30 min under N
2
. The
inset shows the region around 2Θ = 5 -7
o
in
greater detail. Polymers shown are P3HT ((i)a,
(i)b), P3HTT-BTD ((iii)a) and P3HTT-TP-BTD
((v)a, (v)b) where b stands for before annealing
and a for after annealing.
79
hole mobilities are important in order to match the electron mobility of fullerenes and
avoid space charge build up which limits device performance.
20

Table 2.1 Molecular weights (PDI’s), electrochemical HOMO values, optical band gaps
and SCLC mobilities of synthesized polymers.
M
n
(PDI)
a
g/mol HOMO
b
(eV) E
g
(optical)
c

(eV)
SCLC hole mobility
(cm
2
V
-1
s
-1
)
P3HT 17,180 (2.74) 5.17 1.91 2.30 x 10
-4 d

P3HTT 47,850 (1.75) 5.29 1.96 8.21 x 10
-5 e

P3HTT-BTD 15,310 (2.45) 5.41 1.62 2.06 x 10
-4 f

P3HTT-TP 16,680 (2.35) 5.23 1.36 2.50 x 10
-4 e

P3HTT-TP-BTD 16,320 (2.05) 5.11 1.27 2.35 x 10
-4 g

a
Determined by GPC with polystyrene as standard and o-DCB as eluent.
b
CV (vs
Fc/Fc
+
) in acetonitrile containing 0.1M TBAPF
6
.
c
Optical band gaps from onset of
absorption in UV/vis spectra in films.
d
Spin cast from CB, post annealed (after Al
deposition) at 150
o
C for 60 min under N
2
.
e
spin cast from o-DCB, pre annealed (before
Al deposition) at 100
o
C for 10 min under vacuum.
f
As cast from o-DCB.
g
Spin cast
from CB, pre annealed (before Al deposition) at 100
o
C for 30 min under vacuum.

The photovoltaic properties of semi-random polymers were studied in BHJ solar
cells using PC
61
BM as the acceptor in a conventional device configuration of
ITO/PEDOT:PSS/polymer:PC
61
BM/Al. A detailed device fabrication process is
described in appendix 1. All three D/A polymers exhibit promising preliminary
photovoltaic properties under simulated AM 1.5G conditions (100 mW/cm
2
). A PCE of
0.71% was observed for the P3HTT-TP:PC
61
BM solar cells (polymer:PC
61
BM, w/w
1:0.8) with a V
oc
of 0.44 V, a J
sc
of 3.22 mA/cm
2
, and a FF of 0.50. The maximum
efficiency of P3HTT-BTD:PC
61
BM (w/w 1:5) solar cells reaches 0.75% with a V
oc
of
80
0.79 V, a J
sc
of 2.87 mA/cm
2
, and a FF of 0.33. P3HTT-TP-BTD:PCB
61
BM (w/w 1:0.8)
showed an efficiency of 0.43% with a V
oc
of 0.39 V, a J
sc
of 3.04 mA/cm
2
, and a FF of
0.37. For reference, P3HT:PCBM solar cells were found to give a peak efficiency of
3.89% with a V
oc
of 0.59 V, a J
sc
of 10.22 mA/cm
2
, and a FF of 0.64.

2.4 Conclusion
In summary, a family of semi-random D/A copolymers based on rr-P3HT was
described and the investigations on this first family of polymers have shown that the
attractive properties of P3HT are retained. Despite the randomized polymerization, semi-
crystalline polymers with high charge carrier mobilities are realized. In addition, broad
and intense spectral absorption is achieved with only a limited content (10-17.5%) of
acceptor units in the polymer backbone using a simple, robust and reproducible
polymerization method based on easily synthesized monomers.
After this initial proof of concept regarding the promising properties of semi-
random polymers the following chapters will focus on the synthesis of tailored structural
analogues with improved performance in polymer:fullerene solar cells.
81
2.5 References Chapter 2
(1) Kroon, R.; Lenes, M.; Hummelen, J.; Blom, P.; de Boer, B. Polymer Revs. 2008,
48, 531–582.
(2) Zhu, Z.; Waller, D.; Gaudiana, R.; Morana, M.; Mühlbacher, D.; Scharber, M.;
Brabec, C. Macromolecules 2007, 40, 1981–1986.
(3) Chen, C.-H.; Hsieh, C.-H.; Dubosc, M.; Cheng, Y.-J.; Hsu, C.-S. Macromolecules
2010, 43, 697–708.
(4) He, Y.; Wang, X.; Zhang, J.; Li, Y. Macromol. Rapid Commun. 2009, 30, 45–51.
(5) Song, J.; Zhang, C.; Li, C.; Li, W.; Qin, R.; Li, B.; Liu, Z.; Bo, Z. J. Polym. Sci. A
Polym. Chem. 2010, 48, 2571–2578.
(6) Chen, C.-P.; Chan, S.-H.; Chao, T.-C.; Ting, C.; Ko, B.-T. J. Am. Chem. Soc.
2008, 130, 12828–12833.
(7) Li, J.; Ong, K.-H.; Lim, S.-L.; Ng, G.-M.; Tan, H.-S.; Chen, Z.-K. Chem.
Commun. 2011, 47, 9480–9482.
(8) ielsen, C. B.; Ashraf, R. S.; Schroeder, B. C.; D’Angelo, P.; atkins, S. .;
Song, K.; Anthopoulos, T. D.; McCulloch, I. Chem. Commun. 2012, 48, 5832.
(9) Song, J.; Zhang, C.; Li, C.; Li, W.; Qin, R.; Li, B.; Liu, Z.; Bo, Z. J. Polym. Sci. A
Polym. Chem. 2010, 48, 2571–2578.
(10) Tsai, J.-H.; Chueh, C.-C.; Chen, W.-C.; Yu, C.-Y.; Hwang, G.-W.; Ting, C.;
Chen, E.-C.; Meng, H.-F. J. Polym. Sci. A Polym. Chem. 2010, 48, 2351–2360.
(11) Sivula, K.; Luscombe, C. K.; Thompson, B. C.; Fréchet, J. M. J. J. Am. Chem.
Soc. 2006, 128, 13988–13989.
(12) oo, C. H.; Thompson, B. C.; im, B. J.; Toney, . F.; Fr chet, J. . J. J. Am.
Chem. Soc. 2008, 130, 16324–16329.
(13) Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J. Adv. Funct. Mater. 2005, 15,
1617–1622.
(14) Hauch, J. A.; Schilinsky, P.; Choulis, S. A.; Childers, R.; Biele, M.; Brabec, C. J.
Sol. Energy Mater. Sol. Cells 2008, 92, 727–731.
82
(15) Jørgensen, M.; Norrman, K.; Krebs, F. C. Sol. Energy Mater. Sol. Cells 2008, 92,
686–714.
(16) Bundgaard, E.; Krebs, F. Sol. Energy Mater. Sol. Cells 2007, 91, 954–985.
(17) Yue, W.; Zhao, Y.; Tian, H.; Song, D.; Xie, Z.; Yan, D.; Geng, Y.; Wang, F.
Macromolecules 2009, 42, 6510–6518.
(18) Zoombelt, A. P.; Gilot, J.; Wienk, M. M.; Janssen, R. A. J. Chem. Mater. 2009,
21, 1663–1669.
(19) Liang, F.; Lu, J.; Ding, J.; Movileanu, R.; Tao, Y. Macromolecules 2009, 42,
6107–6114.
(20) Mihailetchi, V.; Wildeman, J.; Blom, P. Phys. Rev. Lett. 2005, 94, 126602.

83
CHAPTER 3 Efficient Solar Cells from Semi-Random P3HT
Analogues Incorporating Diketopyrrolopyrrole
3.1 Introduction
After introducing semi-random polymers
1
as well as the unprecedented
combination of favorable properties they possess in chapter 2, this chapter describes the
first examples of semi-random P3HT analogues that give high efficiency in BHJ solar
cells. Thiophene flanked DPP (see section 1.4.2.3 ) was chosen as the acceptor unit and
the effect of acceptor content on polymer properties and photovoltaic response is
described in the following pages. The choice of the DPP unit as the acceptor was
influenced by a growing number of recent reports of high efficiency in BHJ solar cells
with polymers (see section1.4.2.3) and small molecules
2
containing this unit, albeit with
variable N-alkyl substitution. Introduction of the DPP unit into polymer backbones with
different donor units
3–6
led to low optical band gaps (1.2 – 1.6 eV), photocurrent response
up to 1100 nm,
7,8
J
sc
of up to 14 mA/cm
2
and solar cell efficiencies of 5 – 6.5%.
9–13
The
presence of one thiophene on each side of the DPP acceptor minimizes steric hindrance
and induces planarity, which enhances chain packing and intermolecular - 
interaction.
5,14–16
As a result, high hole mobilities are observed for DPP-containing
polymers.
17
These physical and electronic properties of DPP-based polymers, together
with their observed thermal stability,
17,18
make DPP a very attractive unit for
incorporation into semi-random copolymers.
84
3.2 Synthesis, Characterization and Solar Cell Performance of DPP
Containing Semi-Random Polymers
Three novel D/A polymers with different percentages of the DPP unit (5 - 15%)
were synthesized. The synthesis of all three P3HTT-DPP polymers (P3HTT-DPP stands
for poly(3-hexylthiophene-thiophene-diketopyrrolopyrrole)) were carried out in analogy
to the polymerization procedure described in chapter 2 and is illustrated in Figure 3.1.
1

Copolymerization of 2-bromo-5-trimethyltin-3-hexylthiophene (1) with 2,5-
bis(trimethyltin)thiophene (2) and varying amounts of dibromo-bisthiophene-
diketopyrrolopyrrole (3) in DMF at 95 C with Pd(PPh
3
)
4
as the catalyst, gave polymers
with M
n
17,500 – 24,500 g/mol (see appendix 2 for polymer characterization data).
Polymer structures were confirmed by
1
H NMR and it was found that polymer
composition matched the monomer feed ratios, as was previously observed with the
analogous semi-random polymers described in chapter 2.
1
The resulting polymers are
represented by the acronyms P3HTT-DPP-X%, where the percentage of DPP monomer is
indicated, giving P3HTT-DPP-5%, P3HTT-DPP-10%, and P3HTT-DPP-15%. Synthetic
procedures for the monomers, as well as detailed synthesis and purification procedures
for the polymers are given in appendix 2.
85

The optical properties of semi-random P3HTT-DPP polymers in o-DCB solutions
and thin films were studied using UV/vis spectroscopy as shown in Figure 3.2. As a
reference, data is shown for P3HT synthesized using analogous Stille polymerization
conditions. The introduction of DPP into the P3HT backbone is observed to significantly
decrease the optical bandgap and lead to the formation of a distinct dual band absorption
in solutions and thin films. This type of absorption profile is often ascribed to -  (short
wavelength band) and ICT (intramolecular charge transfer) transitions (long wavelength
band).
19,20
In the case of semi-random polymers, the dual band absorption could more
specifically be assigned to   transitions of segments in the randomized polymer that

Figure 3.1 Synthesis and structures of P3HTT-DPP-5%, P3HTT-DPP-10% and
P3HTT-DPP-15%.
86
are thiophene-rich (short wavelength band) and ICT transitions in segments that are rich
in D/A linkages (long wavelength band). This is especially evident in solution (Figure
3.2a) where pristine P3HT has a single absorption band with a peak at 463 nm and this
absorption band is retained in all three P3HTT-DPP copolymers. With increasing content
of DPP acceptor, the intensity and breadth of the long wavelength absorption band (D/A
or ICT band) increases at the expense of the short wavelength band (thiophene - *).
More specifically, an increase of the DPP content leads to a red-shift and intensity
increase of the ICT band, accompanied by a blue-shift and intensity decrease in the - *
band, which is even more pronounced in thin films (Figure 3.2b).
Absorption coefficients in thin films of the ICT band of P3HTT-DPP-10% and
especially P3HTT-DPP-15% are approaching 10
5
cm
-1
and are comparable to the peak
value of P3HT. Furthermore, the ICT peak positions of the P3HTT-DPP polymers are
located close to the maximum of the photon flux from the sun, which is at 700 nm (1.8
eV).
21
It is notable that small red-shifts of the ICT peak positions for P3HTT-DPP
polymers (10 – 15 nm), when going from solution to film, imply that already in solution
the polymers adopt a planar conformation and upon film formation only a small
reorganization and increase in packing between the polymer chains occurs (especially in
the case of P3HTT-DPP-15%). This is in contrast to P3HT, which displays a 96 nm red-
shift induced by significant ordering in the solid state. As a further point, thermal
annealing is observed to enhance the thin film absorption of P3HT and affect the
absorption profile.
22
The P3HTT-DPP thin film absorption spectra were collected with
thin films spin-coated from o-DCB solution. It was observed that the films exhibited a
87
rapid color change from dark green to grey-purple immediately after spin-coating was
completed. Unlike P3HT, neither thermal annealing nor slow solvent evaporation had any
effect on the absorption profiles. Overall, increasing the content of DPP in the polymer
backbone gives rise to a relatively uniform absorption profile at low DPP content (5%),
analogous to the multichromophoric description that was proposed for semi-random D/A
copolymers in chapter 2. However, as the DPP content increases (10 – 15%), the polymer
absorption profile begins to converge toward that observed for perfectly alternating
thiophene-DPP polymers, which contain significantly higher contents (50%) of DPP
acceptor.
9,11,15–17
Another interesting feature in the thin film absorption spectra is the
presence of the vibronic features in the ICT band. The same vibrational shoulders were
observed in the case of other DPP-based polymers and small molecules,
2,6
and were
ascribed to the high degree of ordering and strong intermolecular ( - ) interactions.
23,24

88

To verify the formation of semi-crystalline polymer thin films, GIXRD was used
(see appendix 2 for data). P3HTT-DPP polymers were spin-coated from o-DCB solutions
under identical conditions used for the preparation of films for absorption spectra.

Figure 3.2 UV/vis absorption spectra of
polymers in (a) solution (o-DCB) and (b) thin
film (spin-coated from o-DCB) where (i) is
P3HT (annealed at 150 ºC for 30 min for the
thin films), (ii) is P3HTT-DPP-5% (thin film as-
cast), (iii) is P3HTT-DPP-10% (thin film as-
cast) and (iv) is P3HTT-DPP-15% (thin film as-
cast).
89
P3HTT-DPP-10% was found to exhibit evidence of crystallinity in the as-cast films with
an interchain distance (100) of 14.7 Å (for comparison, the P3HT interchain distance was
measured as 16.6 Å). In contrast, P3HTT-DPP-5% and P3HTT-DPP-15% show features
indicative of crystalline order only upon annealing at 150

C showing 16.0 and 16.2 Å
interchain distances, respectively. Under the same thermal annealing condition, P3HTT-
DPP-10% shows a more intense peak than that observed in the as-cast film and a larger
interchain distance of 15.3 Å. Slow solvent evaporation was observed to have no effect
for any of the P3HTT-DPP polymers. However, all of the P3HTT-DPP polymers showed
evidence of a semi-crystalline structure.
The SCLC method was employed to determine the hole mobilities of the P3HTT-
DPP polymers. High hole mobilities in the range of 1 – 2.3 x 10
-4
cm
2
/(V*s) (Table 3.1)
were obtained, which are close to that of P3HT and are attributed to the semi-crystalline
nature of the polymers revealed with the GIXRD. In contrast to GIXRD measurements,
the maximum mobilities were obtained when a slow solvent evaporation technique was
employed, where thin films were placed in a N
2
cabinet for 20 minutes before aluminum
deposition.
The HOMO and LUMO energy levels for P3HT and the P3HTT-DPP polymers
were measured by CV with ferrocene as a reference, and converted to the vacuum scale
using the approximation that the ferrocene redox couple is 5.1 eV relative to vacuum (see
appendix 2 for the CV traces).
25,26
All P3HTT-DPP polymers, independent of the DPP
content, showed a HOMO level of 5.2 eV, which is equivalent to that of P3HT. The
90
measured position of the HOMO levels also indicates that the polymers should be
resistive to air oxidation and thus facilitate device operational lifetime.
27,28

Table 3.1 Photovoltaic properties of P3HT, P3HTT-DPP-5%, P3HTT-DPP-10% and
P3HTT-DPP-15% with PC
61
BM as an acceptor.
Polymer:PC
61
BM
(ratio)
Thickness
(nm)
c
SCLC hole
mobility
(cm
2
/(V*s)
d

J
sc
(mA/cm
2
)
e V
oc

(V)
FF

 
(%)
P3HT (1:1)
a
95 2.3 x 10
-4
9.49 0.61 0.61 3.42
P3HTT-DPP-5% (1:1)
b
74 1.1 x 10
-4
9.57 0.66 0.58 3.60
P3HTT-DPP-10% (1:1.3)
b
71 2.3 x 10
-4
13.87 0.57 0.63 4.94
P3HTT-DPP-15% (1:2.6)
b
75 1.3 x 10
-4
13.44 0.50 0.60 4.10
a
Spin-coated from CB and annealed at 150

C for 30 min under N
2
after aluminum
deposition.
b
Spin-coated from o-DCB and tested after spending 20 min in a N
2
cabinet
before aluminum deposition.
c
Measured by X-ray reflectivity.
d
Measured for neat
polymer films.
e
Mismatch corrected
29
(see appendix 2).

The observed characteristics (high M
n
, low E
g
, high absorption coefficient, high
hole mobility) of the P3HTT-DPP polymers make them excellent candidates for
photovoltaic devices. BHJ solar cells in a conventional device configuration of
ITO/PEDOT:PSS/polymer:PC
61
BM/Al were fabricated in air (see appendix 2 for the
detailed device fabrication procedures). The optimized polymer:PC
61
BM weight ratios
for P3HTT-DPP-5%, P3HTT-DPP-10%, P3HTT-DPP-15% were found to be 1:1, 1:1.3
and 1:2.6, respectively. Optimal processing conditions include slow solvent evaporation
(solvent annealing) from the polymer:PC
61
BM composites for 20 minutes in a N
2
cabinet
after spin-coating and prior to aluminum deposition, analogous to the conditions observed
to give the highest mobilities for the polymers in the SCLC measurements. Interestingly
91
the same solvent annealing process was not observed to give any changes in the GIXRD
data or in the absorption spectra of the polymers. Shorter or longer solvent annealing
times for the solar cells led to a decrease in J
sc
and thermal annealing across a range of
temperatures was also observed to decrease the performance of the solar cells. Table 3.1
lists the average values of , V
oc
, FF, and mismatch corrected
29
J
sc
obtained under
simulated AM 1.5G illumination (100 mW/cm
2
) (J-V curves are provided in appendix 2).
High values of FF for all devices can be attributed to the high hole mobilities of the
polymers, presumably leading to balanced charge transport in the devices and a reduction
of recombination.
30–32
It is observed that the V
oc
of the solar cells varies from 0.66 V with
P3HTT-DPP-5% to 0.57 V with P3HTT-DPP-10%, to 0.50 V with the P3HTT-DPP-
15%. The 160 mV range in the V
oc
cannot be explained by differences between the
HOMO of the donor and LUMO of the acceptor,
33
because the positions of the HOMO
levels of the three P3HTT-DPP polymers are the same. One possible explanation for the
changes in the V
oc
could be related to the increase of the degree of aggregation, when
going from P3HTT-DPP-5% to P3HTT-DPP-10% to P3HTT-DPP-15%, supported by the
observed decrease in the solubility of the polymers in o-DCB with increasing DPP
content.
34
As such, the recombination rate could increase with increasing DPP content,
34–
36
thus slowing down the kinetics of molecular electron transfer
37
and leading to the
observed V
oc
reduction. However, it should be noted that V
oc
values observed for
perfectly alternating DPP copolymers, which utilize thiophenes as the donor units were
0.63 – 0.68 V in PC
61
BM blends.
15,16
As such, the origins of this trend in the V
oc
are
under further investigation.
92
A more easily explained trend is that of the J
sc
, where the decrease of the
polymer band gap, relative to P3HT (J
sc
= 9.49 mA/cm
2
, mismatch correction M = 1.05),
results in significant increases in the J
sc
, giving 9.57 mA/cm
2
for P3HTT-DPP-5% (M =
0.87), 13.87 mA/cm
2
for P3HTT-DPP-10% (M = 0.76) and 13.44 mA/cm
2
for P3HTT-
DPP-15% (M = 0.71). The highest observed J
sc
value for P3HTT-DPP-10% can
explained by the more balanced, intense absorption across the visible and near-infrared
regions with respect to the other two DPP containing polymers and the smaller quantity
of PC
61
BM needed to optimize device performance in comparison to P3HTT-DPP-15%.
The photocurrent response for all the optimized BHJ solar cells is shown in
Figure 3.3. All devices showed strong photocurrent response in the range 350 – 850 nm,
with EQE values of 41% and 46% at 750 nm for P3HTT-DPP-10% and P3HTT-DPP-
15%, respectively. Photocurrent peaks around 400 nm are assigned to PC
61
BM light
absorption, while photocurrent responses in the longer wavelength regions are attributed
primarily to the polymers. The integrated photocurrents from the EQE measurement
match within 5% to that of the mismatch corrected photocurrents measured under
simulated AM 1.5G illumination (see appendix 2 for mismatch corrected (J
sc,corr
) and
integrated (J
sc,EQE
) photocurrents). A uniformly strong photocurrent response from the
P3HTT-DPP-10% in the 350 – 850 nm range is explained by the favorable ratio of
polymer to PC
61
BM of 1:1.3.
As further characterization of the BHJ solar cells, transmission electron
microscopy (TEM) images (see appendix 2) show the presence of uniform, bicontinuous
thin films, with small length-scales of phase separation in PC
61
BM blends for all the
93
P3HTT-DPP polymers. The observed morphologies are indistinguishable from optimized
blends of P3HT and PC
61
BM and result in an apparent large interfacial area for efficient
charge separation, which helps to explain the high attainable FFs and J
sc
values for P3HT
and P3HTT-DPP polymers. In general, the semi-random approach allows favorable
morphology formation without application of any solvent additives
11,23
or thermal
annealing
38
at a close to 1:1 polymer:fullerene ratio, when the acceptor content in the
polymer backbone is low.

Figure 3.3 EQE of the BHJ solar cells based on
P3HT (black squares), P3HTT-DPP-5% (red
circles), P3HTT-DPP-10% (green triangles) and
P3HTT-DPP-15% (blue stars) with PC
61
BM as
the acceptor, under optimized conditions for
device fabrication.
94

3.3 Conclusion
In summary, a family of novel semi-random P3HTT-DPP copolymers containing
different contents (5 – 15 %) of the DPP acceptor unit was synthesized. These polymers
combine broad absorption profiles, high absorption coefficients, high hole mobilities and
semi-crystalline structures similar to P3HT. In BHJ solar cells with PC
61
BM, the
polymers show effective film formation with optimized polymer:fullerene ratios that vary
based on the content of DPP in the polymer backbone and efficiencies of nearly 5.0% are
observed for P3HTT-DPP-10% at a 1:1.3 polymer:fullerene ratio. A broad photocurrent
response, representative of the polymer absorption profile confirms that semi-random
donor-acceptor copolymers are an effective platform for improving light harvesting in
BHJ solar cells that further benefits from a simple and highly modular synthetic protocol.
95
3.4 References Chapter 3
(1) Burkhart, B.; Khlyabich, P. P.; Cakir Canak, T.; LaJoie, T. W.; Thompson, B. C.
Macromolecules 2011, 44, 1242–1246.
(2) Walker, B.; Tamayo, A. B.; Dang, X.-D.; Zalar, P.; Seo, J. H.; Garcia, A.;
Tantiwiwat, M.; Nguyen, T.-Q. Adv. Funct. Mater. 2009, 19, 3063–3069.
(3) ou, .; endron, D.; Badrou -A ch, R.; ajari, A.; Tao, .; Leclerc, .
Macromolecules 2009, 42, 2891–2894.
(4) Bijleveld, J. C.; Karsten, B. P.; Mathijssen, S. G. J.; Wienk, M. M.; de Leeuw, D.
M.; Janssen, R. A. J. J. Mater. Chem. 2011, 21, 1600.
(5) Huo, L.; Hou, J.; Chen, H.-Y.; Zhang, S.; Jiang, Y.; Chen, T. L.; Yang, Y.
Macromolecules 2009, 42, 6564–6571.
(6) Zoombelt, A. P.; Mathijssen, S. G. J.; Turbiez, M. G. R.; Wienk, M. M.; Janssen,
R. A. J. J. Mater. Chem. 2010, 20, 2240–2246.
(7) Zhou, E.; Yamakawa, S.; Tajima, K.; Yang, C.; Hashimoto, K. Chem. Mater.
2009, 21, 4055–4061.
(8) Zhou, E.; Wei, Q.; Yamakawa, S.; Zhang, Y.; Tajima, K.; Yang, C.; Hashimoto,
K. Macromolecules 2010, 43, 821–826.
(9) Yiu, A. T.; Beaujuge, P. M.; Lee, O. P.; Woo, C. H.; Toney, M. F.; Fréchet, J. M.
J. J. Am. Chem. Soc. 2012, 134, 2180–2185.
(10) Liu, F.; Gu, Y.; Wang, C.; Zhao, W.; Chen, D.; Briseno, A. L.; Russell, T. P. Adv.
Mater. 2012, 24, 3947–3951.
(11) Bijleveld, J. C.; Gevaerts, V. S.; Di Nuzzo, D.; Turbiez, M.; Mathijssen, S. G. J.;
de Leeuw, D. M.; Wienk, M. M.; Janssen, R. A. J. Adv. Mater. 2010, 22, E242–
E246.
(12) Jung, J. W.; Liu, F.; Russell, T. P.; Jo, W. H. Energy Environ. Sci. 2012, 5, 6857–
6861.
(13) Dou, L.; Gao, J.; Richard, E.; You, J.; Chen, C.-C.; Cha, K. C.; He, Y.; Li, G.;
Yang, Y. J. Am. Chem. Soc. 2012, 134, 10071–10079.
(14) Qu, S.; Tian, H. Chem. Commun. 2012, 48, 3039–3051.
96
(15) Wienk, M. M.; Turbiez, M.; Gilot, J.; Janssen, R. A. J. Adv. Mater. 2008, 20,
2556–2560.
(16) Bijleveld, J. C.; Zoombelt, A. P.; Mathijssen, S. G. J.; Wienk, M. M.; Turbiez,
M.; de Leeuw, D. M.; Janssen, R. A. J. J. Am. Chem. Soc. 2009, 131, 16616–
16617.
(17) Bronstein, H.; Chen, Z.; Ashraf, R. S.; Zhang, W.; Du, J.; Durrant, J. R.; Shakya
Tuladhar, P.; Song, K.; Watkins, S. E.; Geerts, Y.; Wienk, M. M.; Janssen, R. A.
J.; Anthopoulos, T.; Sirringhaus, H.; Heeney, M.; McCulloch, I. J. Am. Chem.
Soc. 2011, 133, 3272–3275.
(18) Tieke, B.; Rabindranath, A. R.; Zhang, K.; Zhu, Y. Beilstein J. Org. Chem. 2010,
6, 830–845.
(19) Zhu, Y.; Champion, R. D.; Jenekhe, S. A. Macromolecules 2006, 39, 8712–8719.
(20) Beaujuge, P. M.; Amb, C. M.; Reynolds, J. R. Acc. Chem. Res. 2010, 43, 1396–
1407.
(21) Bundgaard, E.; Krebs, F. Sol. Energy Mater. Sol. Cells 2007, 91, 954–985.
(22) Kim, Y.; Cook, S.; Tuladhar, S. M.; Choulis, S. A.; Nelson, J.; Durrant, J. R.;
Bradley, D. D. C.; Giles, M.; McCulloch, I.; Ha, C.-S.; Ree, M. Nat. Mater. 2006,
5, 197–203.
(23) Woo, C. H.; Beaujuge, P. M.; Holcombe, T. W.; Lee, O. P.; Fr chet, J. . J. J.
Am. Chem. Soc. 2010, 132, 15547–15549.
(24) Jo, J.; Gendron, D.; Najari, A.; Moon, J. S.; Cho, S.; Leclerc, M.; Heeger, A. J.
Appl. Phys. Lett. 2010, 97, 203303.
(25) Thompson, B. C.; Kim, Y.-G.; McCarley, T. D.; Reynolds, J. R. J. Am. Chem.
Soc. 2006, 128, 12714–12725.
(26) Cardona, C. M.; Li, W.; Kaifer, A. E.; Stockdale, D.; Bazan, G. C. Adv. Mater.
2011, 23, 2367–2371.
(27) Thompson, B. C.; Kim, Y.-G.; Reynolds, J. R. Macromolecules 2005, 38, 5359–
5362.
(28) Jørgensen, M.; Norrman, K.; Krebs, F. C. Sol. Energy Mater. Sol. Cells 2008, 92,
686–714.
(29) Shrotriya, V.; Li, G.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. Adv. Funct.
Mater. 2006, 16, 2016–2023.
97
(30) Koster, L. J. A.; Mihailetchi, V. D.; Blom, P. W. M. Appl. Phys. Lett. 2006, 88,
052104.
(31) Kotlarski, J. D.; Moet, D. J. D.; Blom, P. W. M. J. Polym. Sci. B Polym. Phys.
2011, 49, 708–711.
(32) Kim, M.-S.; Kim, B.-G.; Kim, J. ACS Appl. Mater. Interfaces 2009, 1, 1264–
1269.
(33) Thompson, B. C.; Khlyabich, P. P.; Burkhart, B.; Aviles, A. E.; Rudenko, A.;
Shultz, G. V.; Ng, C. F.; Mangubat, L. B. Green 2011, 1, 29–54.
(34) Kim, B.-G.; Jeong, E. J.; Park, H. J.; Bilby, D.; Guo, L. J.; Kim, J. ACS Appl.
Mater. Interfaces 2011, 3, 674–680.
(35) Maurano, A.; Hamilton, R.; Shuttle, C. G.; Ballantyne, A. M.; Nelson, J.;
O’Regan, B.; hang, .; cCulloch, I.; A imi, H.; orana, .; Brabec, C. J.;
Durrant, J. R. Adv. Mater. 2010, 22, 4987–4992.
(36) Deibel, C.; Strobel, T.; Dyakonov, V. Adv. Mater. 2010, 22, 4097–4111.
(37) Schlenker, C. W.; Thompson, M. E. Chem. Commun. 2011, 47, 3702–3716.
(38) van Bavel, S.; Sourty, E.; de With, G.; Frolic, K.; Loos, J. Macromolecules 2009,
42, 7396–7403.

98
CHAPTER 4 Semi-Random Two Acceptor Copolymers:
Influence of the Acceptor Composition on Physical
Properties and Solar Cell Performance
4.1 Introduction
Reviewing the last two chapters it is clear that, among the semi-random polymers,
P3HTT-TP-BTD containing two distinct acceptors, BTD and TP (with a total acceptor
content of 17.5%) is of special interest.
1
This polymer displayed unprecedented strong
and uniform light absorption properties with an absorption onset at 1000 nm and a peak
absorption coefficient at 750 nm of almost 9 x 10
4
cm
-1
. Despite this, solar cell efficiency
was very low at only 0.43%. For comparison, few other random conjugated polymers
with multiple acceptor monomers have been studied in polymer solar cells with
efficiencies reaching slightly over 5%.
2–4
Even though the currents reported in these
examples do not necessarily reflect the broad absorption profiles, and in some cases can
be primarily attributed to absorption by PC
71
BM, the overall solar cell performance gives
reason to believe that this class of polymers as a whole is very promising.
DPP and TPD were chosen as the acceptor units and the effect of varying the ratio
between acceptor monomers on polymer properties as well as solar cell performance will
be discussed in this chapter. Analogous to chapter 3, DPP was chosen as one of the
acceptors as it minimizes steric hindrance and induces planarity, thus enhancing the
crystallinity of the polymers.
5–7
Additionally, semi-random copolymers containing DPP
99
have shown good properties as well as high solar cell efficiencies (see chapter 3) and are
thus a promising starting point for novel two-acceptor polymers.
8
TPD on the other hand
has recently gained significant attention (see section 1.4.2.2) and has been used in many
D/A copolymers with high solar cell efficiencies generally showing large V
oc
and strong
absorption in the visible region of the solar spectrum.
9–14
As the previously studied
P3HTT-DPP-15% (with the same acceptor content as the investigated two-acceptor
polymers) suffers from moderate V
oc
(0.51 eV), and also unbalanced absorption that is
weaker in the visible, TPD was chosen as the complementary acceptor in order to
increase both the V
oc
and absorption intensity at shorter wavelengths.
In the following pages the first examples of semi-random copolymers containing
multiple distinct acceptor monomers that show high efficiencies (~5%) and impressive
currents (> 16 mA/cm
2
) in polymer:PC
61
BM solar cells will be described.

4.2 Synthesis, Characterization and Solar Cell Performance
For this study five novel semi-random D/A copolymers were synthesized and the
synthesis of all five new polymers was carried out using the synthetic protocol for semi-
random copolymers described in chapter 2.
1
P3HTT-TPD-10% and P3HTT-TPD-15%
(were P3HTT-TPD stands for poly(3-hexylthiophene-thiophene-thienopyrroledione) and
the number indicates the acceptor content in percent) were synthesized by
copolymerizing 2-bromo-5-trimethyltin-3-hexylthiophene with 2,5-
bis(trimethyltin)thiopene and either 10 or 15% of dibromo-thienopyrroledione in DMF at
100
95 °C with Pd(PPh
3
)
4
as the catalyst (Figure 4.1a). Two-acceptor copolymers were made
in an identical manner (Figure 4.1b) with the addition of dibromo-bisthiophene-
diketopyrrolopyrrole and varying the ratio of acceptors for a total acceptor content of
15% resulting in P3HTT-TPD-DPP (1:1) (7.5% TPD and 7.5% DPP), P3HTT-TPD-DPP
(2:1) (10% TPD and 5% DPP) and P3HTT-TPD-DPP (1:2) (5% TPD and 10% DPP).
The total acceptor monomer content was chosen as 15% in order to ensure good
solubility of the resulting two-acceptor copolymers as well as to retain their P3HT-like
character while providing a broad enough range over which to vary the relative content of
the two acceptor monomers. As with above discussed examples of semi-random
polymers, the two-acceptor polymers benefit from a highly reproducible synthesis and
batch-to-batch consistency of polymer properties.
1,8,15

101

Figure 4.1 Synthesis of (a) TPD containing semi-random polymers P3HTT-TPD-
10% and P3HTT-TPD-15% and (b) two-acceptor polymers containing TPD and
DPP (P3HTT-TPD-DPP (1:1), P3HTT-TPD-DPP (2:1) and P3HTT-TPD-DPP
(1:2)).
102
Molecular weights determined by GPC (calibrated with polystyrene standards)
after soxhlet purification are between 11,730 and 22,630 g/mol (see synthetic procedures
in appendix 3) with the differences mainly due to decreased solubility of some of the
polymers. Polymer structures, and especially acceptor contents, were confirmed using
1
H
NMR by comparing the integration of distinct acceptor peaks (both in the aromatic and
alkyl region) with the benzylic CH
2
peaks of 3-hexylthiophene at ~2.7 ppm (appendix 3).
Importantly monomer content and especially acceptor ratios in the polymers match the
monomer feed ratio.
The optical properties of all synthesized semi-random copolymers in o-DCB
solutions (see appendix 3) and thin film (Figure 4.2) were studied using UV/vis
spectroscopy. P3HTT-TPD-10% and P3HTT-TPD-15% show a slight broadening of the
absorption (Figure 4.2a) compared to P3HT and only one absorption peak with
absorption onsets of 680 nm and 690 nm, respectively. P3HTT-DPP-10% and P3HTT-
DPP-15% have been described in chapter 3,
8
and exhibit a significantly decreased optical
band gap relative to P3HT with a broad absorption profile showing two distinct
absorption bands which can be attributed to the π-π* band and ICT band. Figure 4.2a
illustrates that semi-random copolymers containing either TPD or DPP have
complementary absorption profiles which would allow for strong and uniform absorption
of the solar spectrum if combined. The absorption profiles of the copolymers containing
both TPD and DPP are shown in Figure 4.2b with all three polymers absorbing the solar
spectrum very broadly into the NIR and with high peak absorption coefficients reaching 8
x 10
4
cm
-1
at ~700 nm for P3HTT-TPD-DPP (1:2). P3HTT-TPD-DPP (1:1) has an
103
absorption onset at 836 nm, corresponding to an optical band gap of 1.48 eV, with a
uniform absorption profile enveloping both the absorption profile of P3HTT-TPD-10%
and P3HTT-DPP-10%. Increasing the TPD content to 10% gives P3HTT-TPD-DPP (2:1)
which shows a distinct absorption peak at 583 nm, slightly red-shifted from the
absorption peak of P3HTT-TPD-10% and P3HTT-TPD-15% around 540 nm, whereas the
long wavelength absorption coefficient is considerably reduced compared to P3HTT-
TPD-DPP (1:1). The absorption onset is blue-shifted compared to P3HTT-TPD-DPP
(1:1) with an optical band gap of 1.50 eV, which is likely due to the increased amount of
TPD acceptor, considering that P3HTT-TPD-10% has a larger band gap than P3HTT-
DPP-10%. On the other hand, P3HTT-TPD-DPP (1:2) which contains twice as much
DPP (10%) as TPD (5%), has a distinct absorption peak at 693 nm corresponding to the
ICT band of P3HTT-DPP-10% and P3HTT-DPP-15%, together with a vibronic shoulder
at 761 nm. P3HTT-TPD-DPP (1:2) also has the lowest optical band gap (1.47 eV), with
an absorption onset at 846 nm. The vibronic shoulder visible in both P3HTT-DPP-10%
and P3HTT-DPP-15% is retained in all three of the two-acceptor polymers, suggesting
that the semi-crystalline nature of these polymers is preserved. This is further supported
by the fact that all polymers show a red-shift going from o-DCB solution (appendix 3) to
thin film, indicating increased order in the solid state. Overall, adding a second acceptor
gives polymers with intense and broad absorption profiles mimicking the weighted sum
of the corresponding one-acceptor polymer absorption profiles with absorption peaks
rising or falling according to the change in ratio of acceptors.
104

Figure 4.2 UV/vis absorption spectra of polymers
in thin films (spin-coated from o-DCB and solvent
annealed for 20 minutes under N
2
) where a) are
polymers containing one acceptor (either TPD or
DPP) and P3HT synthesized by the same method
for reference and b) are polymers containing two
acceptors (TPD and DPP). (i) is P3HT (black
line), (ii) is P3HTT-TPD-10% (red line), (iii) is
P3HTT-TPD-15% (orange line), (iv) is P3HTT-
DPP-10% (dark green line), (v) is P3HTT-DPP-
15% (light green line), (vi) is P3HTT-TPD-DPP
(2:1) (cyan line), (vii) is P3HTT-TPD-DPP (1:1)
(purple line) and (viii) is P3HTT-TPD-DPP (1:2)
(magenta line).
105
To verify the formation of semi-crystalline polymer films, GIXRD was used (see
appendix 3). P3HTT-TPD-10% and P3HTT-TPD-DPP (2:1) are both amorphous when
solvent annealed but semi-crystalline when thermally annealed at 145 °C for 45 minutes
with an interchain distance (100) of 17.00 Å and 16.36 Å, respectively. Both P3HTT-
TPD-DPP (1:1) and P3HTT-TPD-DPP (1:2) are already semi-crystalline when solvent
annealed with interchain distances of 16.50 Å and 15.96 Å. This distance decreases
slightly for P3HTT-TPD-DPP (1:1) after thermal annealing (from 16.50 Å to 16.08 Å),
whereas it remains the same for P3HTT-TPD-DPP (1:2), although the intensity of the
diffraction peak for P3HTT-TPD-DPP (1:2) increases considerably upon annealing.
P3HTT-TPD-15% on the other hand is completely amorphous even after thermal
annealing and it is also observed that increasing amounts of TPD in the two-acceptor
polymers reduces the intensity of the diffraction peaks, indicating that in semi-random
copolymers TPD hinders order in the solid state (see appendix 3). The interchain
distances of the two-acceptor copolymers are comparable to P3HTT-DPP-15% (16.2 Å
after annealing) but are considerably larger than that of P3HTT-DPP-10% (14.7 Å as cast
and 15.3 Å after annealing). This is likely due to a combination of the increased amount
of longer and branched alkyl side-chains compared to P3HTT-DPP-10% as well as the
introduction of TPD which gives a larger (100) interchain distance (17.00 Å) in P3HTT-
TPD-10%.
The HOMO levels of all polymers in thin film were measured by CV with
ferrocene as a reference (Fc/Fc
+
= 5.1 eV)
16,17
and the values are summarized in Table
4.1. P3HTT-TPD-10% and P3HTT-TPD-15% show a HOMO of 5.40 eV which is
106
considerably lower than that of both P3HTT-DPP-10% and P3HTT-DPP-15% (measured
previously as 5.20 eV)
8
and consistent with literature values for TPD-containing perfectly
alternating D/A copolymers.
10–12,14
Interestingly this is consistent with previous findings
that the type of acceptor monomer rather than the amount influences the HOMO energy
of semi-random copolymers.
1,8
As expected P3HTT-TPD-DPP (1:1) and P3HTT-TPD-
DPP (2:1) have intermediate HOMO levels with values of 5.35 eV and 5.30 eV,
respectively. P3HTT-TPD-DPP (1:2) on the other hand has the same HOMO level as
P3HTT-DPP-10% and P3HTT-DPP-15% at 5.20 eV suggesting that DPP has a stronger
influence on the HOMO energy level than TPD.
Hole mobilities for neat polymers determined with the SCLC method are all on
the same order of magnitude as P3HT, which was measured as 2.6 x 10
-4
cm
2
/(V*s) (see
Table 4.1). It is observed that polymers containing DPP as an acceptor (either by itself or
in combination with TPD) have a higher mobility than polymers containing only TPD
(for example P3HTT-TPD-DPP (2:1) has a mobility of 1.6 x 10
-4
cm
2
/(V*s) and P3HTT-
TPD-15% has a mobility of 0.7 x 10
-4
cm
2
/(V*s)).
This is generally consistent with the results from the GIXRD measurements and
the observed trend that the degree of crystallinity goes down with increasing amounts of
TPD. An interesting case is P3HTT-TPD-10% which has a lower mobility than P3HTT-
TPD-DPP (2:1) even though both polymers have the same TPD content and qualitatively
similar level of crystallinity when thermally annealed. One possible explanation for this
is that the DPP unit facilitates π-π-stacking in the solid state thus improving charge
transport.
6,18

107
Table 4.1 Electronic and photovoltaic properties of P3HTT-TPD-10%, P3HTT-TPD-
15%, P3HTT-TPD-DPP (1:1), P3HTT-TPD-DPP (2:1) and P3HTT-TPD-DPP (1:2) as
well as optimized solar cell results of P3HT, P3HTT-DPP-10% and P3HTT-DPP-15%
with PC
61
BM as an acceptor.
Polymer: PC
61
BM
(ratio)
HOMO
(eV)
a
E
g

(optical)
(eV)
b
SCLC hole
mobility
(cm
2
/(V*s))
c
J
sc
(mA/cm
2
)
d
V
oc

(V)
FF η
avg

( η
peak
)
(%)
P3HT
e,f

(1:0.9)
5.20 1.90 2.6 x 10
-4
9.87 0.60 0.64 3.79
(3.90)
P3HTT-TPD-10%
h

(1:1.5)
5.40 1.82 0.8 x 10
-4
5.38 0.72 0.58 2.22
(2.30)
P3HTT-TPD-15%
h

(1:1.3)
5.40 1.80 0.7 x 10
-4
5.33 0.68 0.56 2.02
(2.08)
P3HTT-DPP-10%
e,f

(1:1.3)
5.20 1.51 2.3 x 10
-4
14.62 0.59 0.64 5.53
(5.73)
P3HTT-DPP-15%

e,f

(1:2.6)
5.20 1.46 1.3 x 10
-4
14.28 0.51 0.65 4.66
(4.72)
P3HTT-TPD-DPP
(1:1)
e,g

(1:1.7)
5.35 1.48 1.5 x 10
-4
15.26 0.51 0.64 4.93
(5.03)
P3HTT-TPD-DPP
(2:1)
e,g

(1:1.5)
5.30 1.50 1.6 x 10
-4
11.67 0.55 0.62 3.94
(4.11)
P3HTT-TPD-DPP
(1:2)
e,g

(1:2.0)
5.20 1.47 1.9 x 10
-4
16.37 0.50 0.61 4.92
(4.97)
a
CV (vs Fc/Fc
+
) in acetonitrile containing 0.1 M TBAPF
6
.
b
Optical band gaps from
onset of absorption in UV/vis spectra of solvent annealed films.
c
Measured for neat
polymer films.
d
Mismatch corrected.
e
Spin-coated from o-DCB and placed to the N
2

cabinet before aluminum deposition for
f
30 min and
g
20 min.
h
Spin-coated from CB and
tested as-cast.

Considering the broad and intense absorption into the NIR, semi-crystalline
nature, high hole mobilities and lower-lying HOMO levels compared to P3HTT-DPP-
10% and P3HTT-DPP-15%, the investigated two-acceptor polymers are promising
candidates as donors in combination with a fullerene acceptor in organic BHJ solar cells.
BHJ solar cells in a conventional device configuration of
ITO/PEDOT:PSS/polymer:PC
61
BM/Al were fabricated in air (see appendix 3 for detailed
108
solar cell fabrication procedures). The optimized polymer:PC
61
BM weight ratios are
shown in Table 4.1 and range between 1:1.5 and 1:2.0 for two-acceptor polymers.
Interestingly, these ratios are intermediate relative to the limiting cases of one-acceptor
polymers P3HTT-TPD-15% (1:1.3) and P3HTT-DPP-15% (1:2.6), which have the same
overall acceptor monomer content. Optimal processing conditions include slow solvent
evaporation (solvent annealing) from the polymer:PC
61
BM blends after spin-coating and
prior to aluminum deposition for P3HT, two-acceptor and DPP-containing polymers.
Active layer thickness for all solar cells is between 75 and 85 nm. Table 4.1 lists η, V
oc
,
FF, and mismatch corrected
19
J
sc
obtained under simulated AM 1.5G illumination (100
mW/cm
2
) (J-V curves are shown in appendix 3). P3HT:PC
61
BM solar cells were
fabricated as a point of reference and an average efficiency of 3.79% was measured with
a peak efficiency of 3.90%, which is slightly lower than literature champion values.
20

This difference is primarily attributed to solar cell fabrication and measurement in air as
opposed to a protected environment in a glove box. FFs of all solar cells are extremely
high, in the range of 0.56 to 0.65, indicating that the devices work extremely well due to
balanced charge-carrier mobilities and optimized morphologies.
21,22
P3HTT-TPD-10%
and P3HTT-TPD-15% show moderate solar cell performance of 2.22% and 2.02%
attributed to their relatively narrow and weak absorption profiles, which are reflected in
the moderate J
sc
values.
As expected from the measured HOMO levels, the V
oc
of P3HTT-TPD-10% and
P3HTT-TPD-15% is increased to 0.72 V and 0.68 V compared to 0.59 V and 0.51 V for
P3HTT-DPP-10% and P3HTT-DPP-15% (Table 4.1). In both cases the V
oc
decreases
109
when going from 10 to 15% acceptor content even though this change is not reflected in
the measured HOMO levels.
8
Optimized processing conditions have led to a considerable
increase in efficiency compared to earlier reported values for both P3HTT-DPP-10% and
P3HTT-DPP-15% from 4.94% to 5.53% (average value) and 4.10% to 4.66% (average
value), respectively.
8
The measured peak efficiency of P3HTT-DPP-10% of 5.73% is
among the highest reported efficiencies for DPP-containing conjugated polymers and, to
the best of our knowledge, the highest efficiency ever reported for a DPP-containing
polymer when using PC
61
BM as the acceptor.
5–7,18
Two-acceptor copolymers P3HTT-
TPD-DPP (1:1), P3HTT-TPD-DPP (2:1) and P3HTT-TPD-DPP (1:2) show high average
efficiencies of 4.93%, 3.94% and 4.92%, respectively. Contrary to what the HOMO
energies indicate, only P3HTT-TPD-DPP (2:1) has an increased V
oc
compared to P3HTT-
DPP-15% (0.55 V vs. 0.51 V), whereas both P3HTT-TPD-DPP (1:1) and P3HTT-TPD-
DPP (1:2) have V
oc
values which are lower at 0.51 V and 0.50 V, respectively. This
seems to indicate that the V
oc
in

these semi-random two-acceptor copolymers is much
more determined by DPP than TPD, although the reason for that is still unclear. In
agreement with the recorded UV/vis spectra in thin films (Figure 4.2), both P3HTT-
TPD-DPP (1:1) and P3HTT-TPD-DPP (1:2) have very large J
sc
values of 15.26 and
16.37 mA/cm
2
, respectively, which are among the highest published values for polymer
solar cells
23–25
and considerably higher than J
sc
values published previously for BHJ solar
cells using two-acceptor polymers as the donor.
2–4

P3HTT-TPD-DPP (2:1) has a lower J
sc
of 11.67 mA/cm
2
, mainly due to the
reduced absorption at long wavelengths as well as a slightly increased band gap because
110
of the small amount of DPP acceptor in the polymer. The lower J
sc
is the reason for the
lower efficiency of P3HTT-TPD-DPP (2:1) compared to the other two-acceptor polymers
even though the FF is comparable and the V
oc
is slightly higher.

The photocurrent response for all optimized BHJ solar cells is shown in Figure
4.3. P3HTT-TPD-10% and P3HTT-TPD-15% show photocurrent response up to 700 nm
with moderate peak EQE of 40% at 510 nm and 37% at 490 nm, respectively. On the
other hand, all two-acceptor polymers as well as the previously reported P3HTT-DPP-

Figure 4.3 EQE of the BHJ solar cells based on
P3HT (black open squares), P3HTT-TPD-10%
(red open circles), P3HTT-TPD-15% (orange
diamonds), P3HTT-DPP-10% (dark green stars),
P3HTT-DPP-15% (light green circles), P3HTT-
TPD-DPP (1:1) (purple triangles), P3HTT-TPD-
DPP (2:1) (cyan squares) and P3HTT-TPD-DPP
(1:2) (magenta upside down triangles) with
PC
61
BM as the acceptor, under optimized
conditions for device fabrication.
111
10% and P3HTT-DPP-15% show very strong and uniform photocurrent response all the
way from 350 nm out to 850 nm (with the peak at 400 nm due to PC
61
BM light
absorption). P3HTT-TPD-DPP (1:1) and P3HTT-TPD-DPP (1:2) have a peak efficiency
of 61% and 68% at 680 nm, respectively and at 800 nm show EQE values of 29% and
40%, which are rarely achieved at this wavelength with low band gap conjugated
polymers. The integrated photocurrents from the EQE measurements match within 5% of
those from the mismatch corrected photocurrents measured under simulated AM 1.5G
illumination (see appendix 3 for mismatch factors).
For further characterization the BHJ morphology of the solar cells was analyzed
by TEM and the recorded images are shown in appendix 3. All images show uniform,
bicontinuous thin films with small length-scales of phase separation between PC
61
BM

and the polymers. The observed morphologies are almost indistinguishable from
P3HT:PC
61
BM, which confirms the fact that the newly synthesized polymers retain a
similar miscibility with PC
61
BM and also explains the high observed FF and J
sc
values.

4.3 Conclusion
In summary, a family of five new semi-random copolymers was described, three
of which contain two different acceptor monomers (TPD and DPP) in varying ratios, but
at a fixed overall content (15%). The two-acceptor polymers P3HTT-TPD-DPP (1:1),
P3HTT-TPD-DPP (2:1) and P3HTT-TPD-DPP (1:2) show very broad and uniform
absorption of the solar spectrum with high absorption coefficients. This translates into
112
broad and strong photocurrent responses from 350 nm into the NIR with high EQE values
of up to 40% at 800 nm for P3HTT-TPD-DPP (1:2). Efficiencies of close to 5% in BHJ
solar cells are observed for two-acceptor semi-random copolymers with current densities
of over 16 mA/cm
2
, rivaling the highest observed values in the literature. This study
shows that semi-random copolymers containing multiple distinct acceptor monomers are
a very promising class of polymers able to achieve high currents and high efficiencies in
solar cells due to broad, uniform and strong absorption of the solar spectrum. It also
highlights that fine-tuning of acceptor monomer ratios is paramount to achieve the best
possible efficiencies.
In order to gain further insight into semi-random two-acceptor polymers the
following chapter will focus on expanding this family of polymers by investigating other
acceptor monomer combinations. Primarily this will allow one to draw conclusions on
the general trends and scope of two-acceptor polymers as well as establish areas that
require deeper investigation.
113
4.4 References for Chapter 4
(1) Burkhart, B.; Khlyabich, P. P.; Cakir Canak, T.; LaJoie, T. W.; Thompson, B. C.
Macromolecules 2011, 44, 1242–1246.
(2) Song, J.; Zhang, C.; Li, C.; Li, W.; Qin, R.; Li, B.; Liu, Z.; Bo, Z. J. Polym. Sci. A
Polym. Chem. 2010, 48, 2571–2578.
(3) Tsai, J.-H.; Chueh, C.-C.; Chen, W.-C.; Yu, C.-Y.; Hwang, G.-W.; Ting, C.;
Chen, E.-C.; Meng, H.-F. J. Polym. Sci. A Polym. Chem. 2010, 48, 2351–2360.
(4) ielsen, C. B.; Ashraf, R. S.; Schroeder, B. C.; D’Angelo, P.; atkins, S. .;
Song, K.; Anthopoulos, T. D.; McCulloch, I. Chem. Commun. 2012, 48, 5832.
(5) Qu, S.; Tian, H. Chem. Commun. 2012, 48, 3039–3051.
(6) Jung, J. W.; Liu, F.; Russell, T. P.; Jo, W. H. Energy Environ. Sci. 2012, 6857–
6861.
(7) Bijleveld, J. C.; Gevaerts, V. S.; Di Nuzzo, D.; Turbiez, M.; Mathijssen, S. G. J.;
de Leeuw, D. M.; Wienk, M. M.; Janssen, R. A. J. Adv. Mater. 2010, 22, E242–
E246.
(8) Khlyabich, P. P.; Burkhart, B.; Ng, C. F.; Thompson, B. C. Macromolecules
2011, 44, 5079–5084.
(9) Zhang, Q. T.; Tour, J. M. J. Am. Chem. Soc. 1997, 119, 5065–5066.
(10) ou, .; ajari, A.; Berrouard, P.; Beaupr , S.; R da A ch, B.; Tao, .; Leclerc,
M. J. Am. Chem. Soc. 2010, 132, 5330–5331.
(11) Piliego, C.; Holcombe, T. .; Douglas, J. D.; oo, C. H.; Beaujuge, P. .;
Fr chet, J. . J. J. Am. Chem. Soc. 2010, 132, 7595–7597.
(12) Chu, T. - .; Lu, J.; Beaupr , S.; hang, .; Pouliot, J. -R.; Wakim, S.; Zhou, J.;
Leclerc, M.; Li, Z.; Ding, J.; Tao, Y. J. Am. Chem. Soc. 2011, 133, 4250–4253.
(13) Amb, C. M.; Chen, S.; Graham, K. R.; Subbiah, J.; Small, C. E.; So, F.;
Reynolds, J. R. J. Am. Chem. Soc. 2011, 133, 10062–10065.
(14) Najari, A.; Berrouard, P.; Ottone, C.; Boivin, M.; Zou, Y.; Gendron, D.; Caron,
W.-O.; Legros, P.; Allen, C. N.; Sadki, S.; Leclerc, M. Macromolecules 2012, 45,
1833–1838.
114
(15) Burkhart, B.; Khlyabich, P. P.; Thompson, B. C. J. Photon. Energy 2012, 2,
021002.
(16) Cardona, C. M.; Li, W.; Kaifer, A. E.; Stockdale, D.; Bazan, G. C. Adv. Mater.
2011, 23, 2367–2371.
(17) Thompson, B. C.; Kim, Y.-G.; McCarley, T. D.; Reynolds, J. R. J. Am. Chem.
Soc. 2006, 128, 12714–12725.
(18) Bronstein, H.; Chen, Z.; Ashraf, R. S.; Zhang, W.; Du, J.; Durrant, J. R.; Shakya
Tuladhar, P.; Song, K.; Watkins, S. E.; Geerts, Y.; Wienk, M. M.; Janssen, R. A.
J.; Anthopoulos, T.; Sirringhaus, H.; Heeney, M.; McCulloch, I. J. Am. Chem.
Soc. 2011, 133, 3272–3275.
(19) Shrotriya, V.; Li, G.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. Adv. Funct.
Mater. 2006, 16, 2016–2023.
(20) Dang, M. T.; Hirsch, L.; Wantz, G. Adv. Mater. 2011, 23, 3597–3602.
(21) Kim, M.-S.; Kim, B.-G.; Kim, J. ACS Appl. Mater. Interfaces 2009, 1, 1264–
1269.
(22) Kotlarski, J. D.; Moet, D. J. D.; Blom, P. W. M. J. Polym. Sci. B Polym. Phys.
2011, 49, 708–711.
(23) Huo, L.; Zhang, S.; Guo, X.; Xu, F.; Li, Y.; Hou, J. Angew. Chem. Int. Ed. 2011,
50, 9697–9702.
(24) Coffin, R. C.; Peet, J.; Rogers, J.; Bazan, G. C. Nat. Chemistry 2009, 1, 657–661.
(25) Saadeh, H. A.; Lu, L.; He, F.; Bullock, J. E.; Wang, W.; Carsten, B.; Yu, L. ACS
Macro Lett. 2012, 1, 361–365.

115
CHAPTER 5 Semi-Random Two-Acceptor Copolymers:
Elucidating Electronic Trends Through Multiple Acceptor
Combinations
5.1 Introduction
After expanding the family of two-acceptor semi-random copolymers from
P3HTT-TP-BTD (chapter 2) to include three novel, promising polymers (chapter 4),
which have very broad absorption and strong photoresponse, this chapter will focus on
further exploring the influence of the acceptor combination in order to establish general
features of this class of polymers as well as to gain further insight into the design of semi-
random polymers in general.
In addition to DPP and TPD acceptor monomers, which have already been used in
efficient semi-random copolymers (chapter 4), TP and BTD were chosen respectively
because they have either shown to give very low band gap polymers (absorbing into the
NIR of the solar spectrum) or high efficiencies in polymer solar cells.
1–4
Additionally, all
four acceptor monomers (as well as both co-monomers, 2-bromo-5-trimethyltin-3-
hexylthiophene and 2,5-bis(trimethyltin)thiophene) are straightforward to synthesize in
no more than 3-4 steps, which is important for future industrial applications, where long
and involved synthetic procedures could be potentially cost and time prohibitive.
Four new polymers containing a combination of TPD, DPP, TP or BTD with a
total acceptor content of 15% were synthesized and the influence the specific acceptor
116
combination has on the optical and electronic properties of the two-acceptor semi-random
copolymers as well as the solar cell performance will be described in the following pages.
More importantly, by taking into account not only results from this study but also data
presented in chapters 2 and 4
5,6
it is possible identify general trends for UV/vis absorption
profile, HOMO energy levels and V
oc
which are valuable for gaining insight into this
complex family of polymers.

5.2 Results and Discussion
The synthesis of all four new polymers was carried out using the Stille
polymerization described in chapter 2.
5
As shown in Figure 5.1 2-bromo-5-trimethyltin-
3-hexylthiophene was copolymerized with 2,5-bis(trimethyltin)thiophene and two
dibrominated acceptor monomers (TPD, DPP, TP or BTD) in DMF at 95 °C with
Pd(PPh
3
)
4
as the catalyst to give P3HTT-BTD-DPP, P3HTT-TP-DPP, P3HTT-BTD-TPD
and P3HTT-TP-TPD.

117

The acceptor content was set, as previously,
6
to 15% in order to ensure good
solubility and retain the P3HT-like character of polymers with the acceptor ratio constant
at 1:1. Molecular weights were determined after careful soxhlet purification by GPC and
range between 12,580 and 22,770 g/mol with PDI’s between 2.4 and 3.3 ( Table 5.1).
Polymer structures were analyzed using 500 MHz
1
H NMR and monomer feed ratios
match the monomer incorporation in the polymer backbone exactly (see appendix 4 for
integrated polymer R’s).

Figure 5.1 Synthesis of semi-random polymers and structures of two-
acceptor copolymers P3HTT-BTD-DPP, P3HTT-TP-DPP, P3HTT-BTD-
TPD and P3HTT-TP-TPD.
118

The optical properties of the two-acceptor semi-random copolymers were studied
using UV/vis spectroscopy and are shown Figure 5.2a-d. Each two-acceptor polymer
(solid lines) is shown together with the respective “parent” one -acceptor polymers
containing 10% of individual acceptor (dashed lines), for comparison. The introduction
of acceptor monomers into the P3HT dominated backbone broadens the absorption and

Figure 5.2 UV/vis absorption spectra of two-acceptor
polymers (solid lines) and corresponding one-acceptor
polymers (dashed lines, 10% acceptor content) in thin
films. a) P3HTT-BTD-DPP (solid blue), P3HTT-BTD
(dashed purple) and P3HTT-DPP (dashed light blue), b)
P3HTT-TP-DPP (solid orange), P3HTT-TP (dashed light
green) and P3HTT-DPP (dashed light blue), c) P3HTT-
BTD-TPD (solid green), P3HTT-BTD (dashed purple) and
P3HTT-TPD (dashed black), d) P3HTT-TP-TPD (solid
red), P3HTT-TP (dashed light green) and P3HTT-TPD
(dashed black).
119
decreases the optical E
g
both in solution and thin film considerably. Except for P3HTT-
BTD-TPD all polymers show an absorption profile with two maxima, likely due to π -π*-
transition in the 3-hexylthiophene rich polymer segments as well as ICT transitions in
acceptor rich regions. Going from o-DCB solution (see appendix 4) to thin film all
polymers show a pronounced red-shift indicating increased order in the solid state. Both
P3HTT-BTD-DPP and P3HTT-TP-DPP show a shoulder (at 776 nm and 785 nm,
respectively) in solution which disappears in the thin film absorption profile, possibly
indicating aggregation of the polymer chains in solution. The absorption profile of
P3HTT-TP-TPD in solution on the other hand is tailing off after 810 nm at a very low
absorptivity also indicative of aggregation. In thin films both P3HTT-BTD-DPP and
P3HTT-TP-DPP retain the vibronic shoulder present in DPP-containing semi-random
copolymers described in chapters 3 and 4 thus indicating good order in the solid state.
6,7

In reference to the thin-film spectra in Figure 5.2, P3HTT-BTD-DPP displays a
E
g
of 1.47 eV, which is almost identical to P3HTT-DPP with a band gap of 1.51 eV at
10% DPP content and 1.46 eV at 15%.
5,7
P3HTT-TP-DPP on the other hand has a much
lower E
g
(1.32 eV) due to the introduction of TP (P3HTT-TP has a band gap of 1.36 eV)
5

which is not only generally considered a strong electron acceptor but also stabilizes the
quinoidal form of the polymer thus lowering the band gap.
4
P3HTT-TP-TPD also has a
very low band gap at 1.32 eV and the shape of the absorption profile as well as E
g
are
dominated by the stronger acceptor TP. Not surprisingly P3HTT-BTD-TPD has a
considerably larger band gap (1.64 eV) and narrower absorption which is consistent with
the more narrow absorption profiles (in combination with higher band gaps) of both
120
P3HTT-BTD and P3HTT-TPD (both at 10% and 15% acceptor content).
5,6
From this
discussion of the UV/vis absorption and previously described data (chapter 4) on two-
acceptor semi-random copolymers
6
it is apparent that both the shape of the absorption
profile as well as the band gap of a novel two-acceptor copolymer can be predicted from
the respective “parent” one -acceptor semi-random copolymers and that this linear
combination of the absorption profiles is a general feature of two-acceptor copolymers. In
addition to the acceptor combination, the acceptor ratio has also been shown to have a
similar effect on the absorption profile, with the two-acceptor profile being the weighted
sum of one-acceptor profiles, but in order to generalize this observation further acceptor
combinations with different acceptor ratios need to be studied.
6

Figure 5.3 GIXRD of thin films annealed for 45
min at 145 °C under N
2
. Polymers shown are (i)
P3HT (black), (ii) P3HTT-BTD-DPP (blue),
(iii) P3HTT-TP-DPP (orange), (iv) P3HTT-
BTD-TPD (green) and (v) P3HTT-TP-TPD
(red). The inset shows the region around 2θ = 4°
- 8° in more detail.
121

A key goal of all semi-random polymers is the retention of the P3HT character. In
order to verify the semicrystalline nature of the polymers indicated by the pronounced
red-shift in the UV/vis spectra going from solution to thin film as well as the presence of
vibronic shoulders, grazing incidence GIXRD was used. Figure 5.3 shows diffraction
traces of the thermally annealed films and data for the solvent annealed films can be
found in appendix 4. Both P3HTT-BTD-DPP and P3HTT-TP-DPP are already
semicrystalline when solvent annealed (see appendix 4) with the peak intensity increasing
slightly after thermal annealing. The interchain distances (100) are 15.30 Å and 15.35 Å,
respectively, for both solvent and thermally annealed films (as a reference P3HT was
measured as 15.56 Å after thermal annealing). P3HTT-BTD-TPD and P3HTT-TP-TPD
on the other hand are completely amorphous when solvent annealed and show less
intense diffraction peaks than DPP-containing polymers after thermal annealing. Their
interchain packing distances are larger than for polymers containing DPP ,with P3HTT-
BTD-TPD having a packing distance of 17.05 Å and P3HTT-TP-TPD of 16.70 Å. The
higher degree of crystallinity of DPP-based polymers is consistent with previously
described DPP based semi-random copolymers and can be explained by the tendency of
DPP to reduce steric hindrance and planarize the polymer backbone leading to better
packing.
6–8
The larger interchain packing distances (100) of TPD based two-acceptor
polymers (17.05 Å for P3HTT-BTD-TPD vs. 15.30 Å for P3HTT-BTD-DPP) have also
been found in the parent polymers P3HTT-TPD (with both 10% and 15%) and are likely
due to the longer octyl side-chain in TPD.
6

122

Table 5.1 Molecular weights (PDI’s), electrochemical HOMO values and optical band
gaps of two-acceptor semi-random copolymers.
M
n
(g/mol)
a
PDI
a
HOMO
b

(eV)
E
g
(optical)
(eV)
c
P3HT 18,240 2.9 5.19 1.90
P3HTT-BTD-DPP 15,350 3.4 5.22 1.47
P3HTT-TP-DPP 22,770 2.4 5.14 1.32
P3HTT-BTD-TPD 16,770 3.0 5.40 1.64
P3HTT-TP-TPD 12,580 3.3 5.29 1.32
a
Determined by GPC with polystyrene as standard and o-DCB as eluent;
b
CV (versus
Fc/Fc
+
) in acetonitrile containing 0.1 M TBAPF
6
;
c
Optical band gaps from onset of
absorption in UV/vis spectra in films.

The HOMO levels (Table 5.1 and Figure 5.4) of all polymers were measured
using CV with ferrocene as reference (P3HT is included in Figure 5.4 as point of
reference and was measured as 5.19 eV).
9,10
The HOMO level of P3HTT-BTD-DPP was
measured as 5.22 eV which is (within measuring error) identical with P3HTT-DPP (5.20
eV for both 10% and 15% acceptor content) but considerably higher than P3HTT-BTD
(5.41 eV). P3HTT-TP-DPP on the other hand has a higher HOMO energy (5.14 eV) than
either of the parent one-acceptor polymers (P3HTT-TP: 5.23 eV).
5,7

123

A similar behavior (two-acceptor polymer has a higher HOMO level than either
of the one-acceptor polymers) was noticed already with P3HTT-TP-BTD, which has a
HOMO level of 5.11 eV.
5
The HOMO level of P3HTT-BTD-TPD was measured as 5.40
eV, not surprisingly considering both P3HTT-BTD and P3HTT-TPD (both 10% and 15%
acceptor content) also have a HOMO energy around 5.40 eV. Finally P3HTT-TP-TPD
has a HOMO level (5.29 eV) which has an intermediate energy between the parent one-
acceptor polymers P3HTT-TP and P3HTT-TPD.
5,6
Additionally, two-acceptor polymers
containing both TPD and DPP which are described in chapter 4 also have intermediate
HOMO levels.
6
It is clear that in cases where the two “parent” one -acceptor polymers
have energetically different HOMO levels the HOMO level of the two-acceptor

Figure 5.4 CV traces for the oxidation of (i)
P3HT (black), (ii) P3HTT-BTD-DPP (blue), (iii)
P3HTT-TP-DPP (orange), (iv) P3HTT-BTD-
TPD (green) and (v) P3HTT-TP-TPD (red).
Ferrocene was used as a reference and values
were converted to the vacuum scale using the
approximation that the ferrocene redox couple is
5.1 eV relative to vaccum.
124
copolymer is at least higher than the lowest HOMO level of the one-acceptor polymers
(e.g. P3HTT-TP-TPD has a HOMO level of 5.29 eV which is above the HOMO level of
P3HTT-TPD (both 10% and 15%) but below that of P3HTT-TP) and in some cases it is
even higher than for both one-acceptor polymers (e.g. P3HTT-TP-DPP). The only
exception to this general observation is P3HTT-BTD-TPD where both one-acceptor
copolymers as well as P3HTT-BTD-TPD have the same HOMO level.
Hole mobilities (Table 5.2) for neat polymers determined with the SCLC method
are all on the same order of magnitude as P3HT (measured as 2.6 x 10
-4
cm
2
/(V*s)),
which should allow for balanced charge transport in the solar cells and confirms the
P3HT-like character of two-acceptor semi-random copolymers. P3HTT-BTD-DPP shows
the highest hole mobility (1.9 x 10
-4
cm
2
/(V*s)) of the four two-acceptor copolymers
whereas P3HTT-TP-DPP surprisingly has the lowest (1.1 x 10
-4
cm
2
/(V*s)) even though
its degree of crystallinity (see Figure 5.3) is higher than all of the other polymers and the
DPP unit generally facilitates π -π-stacking and thus charge transport.
8,11

Table 5.2 Electronic and photovoltaic properties of optimized two-acceptor semi-random
copolymers.
polymer:PC
61
BM
(ratio)
a

SCLC hole
mobility
(cm
2
/(V*s))
b
J
sc
(mA/cm
2
)
c
V
oc
(V) FF η
avg
( η
peak
) (%)
P3HT (1:0.9)
d
2.6 x 10
-4
9.87 0.60 0.64 3.79 (3.90)
P3HTT-BTD-DPP
(1:1.0)
e

1.9 x 10
-4
10.91 0.50 0.55 2.97 (3.00)
P3HTT-TP-DPP
(1:1.3)
e

1.1 x 10
-4
7.94 0.36 0.48 1.37 (1.39)
P3HTT-BTD-TPD
(1:1.3)
e

1.5 x 10
-4
6.89 0.58 0.62 2.48 (2.53)
P3HTT-TP-TPD
(1:1.1)
e

1.3 x 10
-4
4.56 0.36 0.48 0.78 (0.81)
a
Spin-coated from o-DCB and placed in a N
2
cabinet for
d
30 min and
e
20 min;
b

Measured for neat polymer films;
c
Mismatch corrected.

125
Considering the broad and intense UV/vis absorption as well as the high hole
mobilities of all four novel two-acceptor semi-random copolymers BHJ solar cells in a
conventional device configuration (ITO/PEDOT:PSS/Polymer:PC
61
BM/Al) were
fabricated in air and the J-V curves are shown in Figure 5.5. As can be seen in Table 5.2
all polymers mix at very low ratios with PC
61
BM, which is favourable for strong light
absorption in the NIR. It is interesting to note that even though P3HTT-BTD solar cells
were optimized at an unfavourable polymer:PC
61
BM ratio of 1:5 (chapter 2), two-
acceptor polymers containing BTD as one of the acceptors do not suffer the same
problem.
5
Optimal processing conditions consist of slow solvent evaporation (solvent
annealing) from the polymer:PC
61
BM blend after spin-coating and prior to aluminium
deposition. The active layer thickness for solar cells is between 75 and 85 nm. Values for
η, V
oc
, mismatch corrected
12
J
sc
(obtained under simulated AM 1.5G, 100 mW/cm
2
) and
FF for all two-acceptor polymers as well as P3HT as a point of reference are listed in
Table 5.2.
126

Efficiencies of TP containing two-acceptor copolymers are relatively low with
1.37% for P3HTT-TP-DPP and 0.78% for P3HTT-TP-TPD. This is predominantly due to
the low V
oc
found in both solar cells (0.36 V compared to 0.60 V for P3HT) as well as
low J
sc
in combination with a moderate FF of 0.48. Considering the V
oc
of P3HTT-TP
was previously measured as 0.44 V and TP-containing polymers in general show a low
V
oc
this is not surprising but, especially in the case of P3HTT-TP-TPD, it is not reflected
in the HOMO level which is slightly lower than that of P3HT (5.29 eV for P3HTT-TP-
TPD vs. 5.19 eV for P3HT).
4,5
This suggests that effects beside the HOMO
D
- LUMO
A

(see chapter 1) offset play a role in determining the V
oc
in these polymers.
13,14

Figure 5.5 J-V curves of BHJ solar cells based
on (i) P3HT (black), (ii) P3HTT-BTD-DPP
(blue), (iii) P3HTT-TP-DPP (orange), (iv)
P3HTT-BTD-TPD (green) and (v) P3HTT-TP-
TPD (red) with PC
61
BM as the acceptor under
AM 1.5G illumination (100 mW/cm
2
) at the
optimal conditions for solar cell performance.
127
Both P3HTT-BTD-DPP and P3HTT-BTD-TPD have higher efficiencies than
P3HTT-TP-DPP and P3HTT-TP-TPD with 3% and 2.53%, respectively. For P3HTT-
BTD-DPP this is predominantly due to an increased J
sc
of 10.91 mA/cm
2
(exceeding the
J
sc
measured for P3HT) as well as a moderate V
oc
and a higher FF. P3HTT-BTD-TPD on
the other hand has a relatively low J
sc
(6.89 mA/cm
2
) but the highest V
oc
(0.58 V) of all
four two-acceptor polymers as well as a very high FF of 0.62. Considering that both
P3HTT-BTD and P3HTT-TPD (both 10% and 15% acceptor content) have much higher
V
oc
(0.79 V, 0.72 V and 0.68 V, respectively) than P3HTT-BTD-TPD, even though they
all have the same HOMO energy, this could potentially indicate that HOMO levels of
two-acceptor polymers are unreliable predictors for the V
oc
of the corresponding BHJ
solar cells.
5,6
However, plotting the HOMO energies of both previously reported as well
as novel two-acceptor polymers, versus the V
oc
(see appendix 4) reveals a linear
correlation (R = 0.877) with P3HTT-TP-TPD as the only exception to this general trend.
This linear correlation between the HOMO energy and the V
oc
should ultimately allow
predictability of the V
oc
of a solar cell by measuring the HOMO levels of two-acceptor
semi-random polymers.
In order to better understand the low J
sc
values of some of the reported polymers,
especially considering their broad and intense UV/vis absorption profiles, the
photocurrent response for all BHJ solar cells was measured and is shown in Figure 5.6.
128

As can be seen in Figure 5.6 P3HTT-BTD-DPP has strong and uniform
photoresponse with a peak EQE value of 50% at 600 nm and a relatively high EQE of
17% at 800 nm which is reflected in the higher J
sc
(10.91 mA/cm2). P3HTT-TP-DPP,
P3HTT-BTD-TPD and P3HTT-TP-TPD on the other hand have much weaker
photoresponses with peak maxima at 32%, 39% and 28%, respectively. P3HTT-TP-DPP
still has an EQE of 15% at 800 nm but rapidly decreases to 3% at 900 nm whereas
P3HTT-TP-TPD is not able to convert NIR photons efficiently into current with an EQE
of only 3% at 800 nm.
For further characterization the BHJ morphology of the solar cells was analyzed
by TEM and the images are shown in Figure 5.7. All images show bicontinuous

Figure 5.6 EQE of the BHJ solar cells based on
P3HT (black upside down triangles), P3HTT-
BTD-DPP (blue circles), P3HTT-TP-DPP
(orange squares), P3HTT-BTD-TPD (green
triangles) and P3HTT-TP-TPD (red stars) with
PC
61
BM as the acceptor under optimized
conditions for device fabrication.
129
structures of polymer and PC
61
BM as well as small scale phase separation favorable for
exciton dissociation. The morphologies found for all two-acceptor polymer:PC
61
BM BHJ
films are indistinguishable from P3HT:PC
61
BM which is shown at the bottom of Figure
5.7 for reference.

Figure 5.7 TEM images of polymer:PC
61
BM films (optimized conditions
for best solar cell performance were used to make the films).
P3HT:PC
61
BM is shown for reference.
130
5.3 Conclusions
In summary we have synthesized a family of four semi-random D/A copolymers
containing two different acceptor monomers with a total acceptor content of 15%
(P3HTT-BTD-DPP, P3HTT-TP-DPP, P3HTT-BTD-TPD and P3HTT-TP-TPD). We
have found that all polymers have a considerably broadened absorption spectrum with
band gaps as low as 1.32 eV for TP-containing polymers. In general, both the shape of
the absorption profile as well as the band gap of a novel two-acceptor copolymer can be
predicted through a linear combination of absorption profiles of the respective one-
acceptor semi-random copolymers. HOMO level energies on the other hand, are
generally higher than the lowest measured value for the “parent” one -acceptor polymers
and in some cases even higher than values for both one-acceptor polymers. Importantly,
HOMO energies are correlated with the V
oc
of the corresponding BHJ solar cell in a
linear fashion in almost all cases, ultimately allowing the V
oc
of a solar cell to be
predicted from the HOMO level of a polymer. These novel two-acceptor copolymers also
reemphasize that semi-random copolymers generally and broadly retain the good
properties of P3HT such as high hole mobilities, semi-crystallinity and favorable mixing
at low ratios with PC
61
BM to give a bicontinuous, small-scale phase separated
morphology in the BHJ blends.
5–7
Power conversion efficiencies of up to 3% for P3HTT-
BTD-DPP due to a broad photoresponse together with a good FF have been found,
confirming the promise of semi-random two-acceptor copolymers.
In future work a combination of theoretical and spectroscopic approaches will
focus on elucidating the fundamental reasons behind the large variability in the solar cell
131
photoresponse of semi-random copolymers, even though they consistently have broad
and intense absorption profiles. Moreover a better understanding of factors influencing
the HOMO energy levels of semi-random copolymers will be an important focus.
132
5.4 References Chapter 5
(1) Park, S. H.; Roy, A.; Beaupré, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.;
Leclerc, M.; Lee, K.; Heeger, A. J. Nat. Photonics 2009, 3, 297–302.
(2) Coffin, R. C.; Peet, J.; Rogers, J.; Bazan, G. C. Nat. Chem. 2009, 1, 657–661.
(3) Huo, L.; Hou, J.; Zhang, S.; Chen, H.-Y.; Yang, Y. Angew. Chem. Int. Ed. 2010,
49, 1500–1503.
(4) Rasmussen, S. C.; Schwiderski, R. L.; Mulholland, M. E. Chem. Commun. 2011,
47, 11394–11410.
(5) Burkhart, B.; Khlyabich, P. P.; Cakir Canak, T.; LaJoie, T. W.; Thompson, B. C.
Macromolecules 2011, 44, 1242–1246.
(6) Burkhart, B.; Khlyabich, P. P.; Thompson, B. C. ACS Macro Lett. 2012, 1, 660–
666.
(7) Khlyabich, P. P.; Burkhart, B.; Ng, C. F.; Thompson, B. C. Macromolecules
2011, 44, 5079–5084.
(8) Qu, S.; Tian, H. Chem. Commun. 2012, 48, 3039–3051.
(9) Cardona, C. M.; Li, W.; Kaifer, A. E.; Stockdale, D.; Bazan, G. C. Adv. Mater.
2011, 23, 2367–2371.
(10) Thompson, B. C.; Kim, Y.-G.; McCarley, T. D.; Reynolds, J. R. J. Am. Chem.
Soc. 2006, 128, 12714–12725.
(11) Jung, J. W.; Liu, F.; Russell, T. P.; Jo, W. H. Energy Environ. Sci. 2012, 6857–
6861.
(12) Shrotriya, V.; Li, G.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. Adv. Funct.
Mater. 2006, 16, 2016–2023.
(13) Perez, M. D.; Borek, C.; Forrest, S. R.; Thompson, M. E. J. Am. Chem. Soc.
2009, 131, 9281–9286.
(14) Yang, L.; Zhou, H.; You, W. J. Phys. Chem. C 2010, 114, 16793–16800.

133
CHAPTER 6 Solar Cells Based on Semi-Random P3HT
Analogues Containing Dithienopyrrole: Influence of
Incorporating a Strong Donor
6.1 Introduction
As described in the previous chapters, several different acceptor monomers with
varying acceptor strength have been incorporated in semi-random polymers with great
success,
1–3
however the influence of incorporating a small amount of strong donor
monomer into the polymer backbone has yet to be studied. DTP donors have been used in
various D/A polymers
4–13
as they offer the same advantages as other fused ring systems
(maximi ation of π -orbital overlap by planarizing the polymer backbone and potentially
improved π -π-stacking). When compared to other fused-ring systems, such as CPDT,
DTP is much more electron rich, potentially allowing for a more pronounced D/A
effect.
6,12,14–16
Additionally, the solubility of the polymer can be tuned through the alkyl
substituents on the nitrogen in DTP without a negative influence on the polymer
backbone.
6
DTP based D/A polymers have shown very large peak absorption coefficients
(10
5
cm
-1
) as well as photoresponse into the near-infrared region (1100 nm) suggesting
that the DTP unit is an ideal candidate for strong donor incorporation in semi-random
polymers.
4,13
Here the influence of the strong donor DTP, in combination with three
different acceptors (BTD, TP and DPP), on the optical and electronic properties of the
134
synthesized semi-random D/A copolymers and the solar cell performance will be
demonstrated.

6.2 Results and Discussion
The synthetic routes to the monomer 2,6-dibromo-N-(1-butylpentyl)dithieno[3,2-
b:2’,3’-d]pyrrole (4) (DTP) and the polymers are outlined in Figure 6.1. Starting from 5-
nonanol, the alcohol is first transformed into 5-bromononane (1) and then reacted to
nonane-5-amine (2) using Gabriel synthesis. Palladium catalyzed Buchwald-Hartwig
coupling between (2) and 3,3’-dibromo-2,2’-bithienyl gave N-(1-
butylpentyl)dithieno[3,2-b:2’,3’-d]pyrrole (3) in good yield, followed by bromination
with N-bromosuccinimide to give monomer (4). The synthesis of all four DTP containing
polymers was carried out analogous to the previously described Stille polymerization
procedure for semi-random polymers (chapter 2).
1
Monomers (4), (5), (6) and the
applicable acceptor were dissolved in DMF and the polymerization was carried out at 95
C in the presence of Pd(PPh
3
)
4
for 48 hours analogous to chapter 2.

135

After careful soxhlet purification, polymers were obtained with molecular weights
of ~20,000 g/mol and PDI’s ranging from 2.9 -3.6 (Table 6.1) (values were determined
by GPC calibrated with polystyrene standards). Polymer structures were analyzed using
500 MHz
1
H NMR and not only is the synthetic procedure highly reproducible but the
monomer feed ratio matches the monomer ratio in the polymer backbone (see appendix 5
for polymer R’s). Polymers are represented by the acronyms P3HTT-DTP, P3HTT-
BTD-DTP, P3HTT-TP-DTP and P3HTT-DPP-DTP (Figure 6.2). The detailed synthetic
procedures for the monomer as well as synthesis and purification of polymers can be
found in appendix 5. Polymers are represented in Figure 6.2 and for comparison

Figure 6.1 Synthesis of 2,6-dibromo-N-(1-butylpentyl)dithieno[3,2-
b:2’,3’-d]pyrrole (DTP) (4) (a) and a representative synthetic scheme for
semi-random Stille polymerization (b).
136
previously synthesized polymers P3HTT-DPP-10%
2
and P3HTT-BTD
1
(chapters 2 and
3) are also included (Figure 6.2b).

Figure 6.2 Structures of P3HTT-DTP, P3HTT-BTD-DTP,
P3HTT-TP-DTP and P3HTT-DPP-DTP (a) and for comparison
P3HTT-DPP-10% and P3HTT-BTD (b).
137

Table 6.1 Molecular weights (PDI’s), electrochemical HOMO values and optical band
gaps of synthesized polymers and for comparison P3HTT-DTP-10% and P3HTT-BTD.
1,2

M
n
(g/mol)
c
PDI
c
HOMO (eV)
d
E
g
(optical)
e
(eV)
P3HT 22,320 2.5 5.19 1.91
P3HTT-DTP 20,150 3.5 5.22 1.96
P3HTT-BTD-DTP 20,030 2.8 5.15 1.61
P3HTT-TP-DTP 21,200 3.0 5.05 1.38
P3HTT-DPP-DTP 19,620 3.6 5.18 1.47
P3HTT-DPP-10%
a
24,750 2.3 5.20 1.51
P3HTT-BTD
b
15,310 2.45 5.41 1.62
a
From reference
2
;
b
From reference
1
;
c
Determined by GPC with polystyrene as standard
and o-DCB as eluent;
d
CV (vs Fc/Fc
+
) in acetonitrile containing 0.1M TBAPF
6
;
e
Optical
band gaps from onset of absorption in UV/vis spectra in films.

The optical properties of DTP containing semi-random polymers were studied
using UV/vis spectroscopy in solution (o-DCB) and thin films (spin coated from o-DCB
and annealed at 60 C). For comparison the absorption profiles of P3HT and P3HTT-
DPP-10% are included in Figure 6.3. As can be seen from Figure 6.3 all polymers
containing an acceptor have a broadened absorption compared to P3HT and P3HTT-DTP
due to a lowered band gap caused by strong D/A effects as well as the multichromophoric
nature of the polymers. The optical E
g
(Table 6.1) was estimated from the onset of
absorption of the thin film spectra. Interestingly the introduction of the strong electron
donor DTP does not appear to have a very large influence on the band gaps as they
remain practically unchanged, comparing for example P3HTT-BTD and P3HTT-BTD-
DTP. The D/A interactions can also be seen in the dual nature of the absorption profiles
(except for P3HTT-BTD-DTP where the two bands seem to merge), which are generally
assigned to π -π*-transitions (short wavelength region) and ICT transitions (long
wavelength region).
17,18
Especially in thin films the short wavelength absorption of all
138
polymers overlap with that observed for P3HT, indicating thiophene-rich polymer
segments, whereas ICT bands are more likely in segments with higher concentration of
acceptor and thus more pronounced D/A effects. Going from solution to thin film the
absorption maxima of DTP-containing polymers red-shift considerably indicating
increased ordering in the solid state. Comparing the absorption profiles of P3HTT-DPP-
DTP with P3HTT-DPP-10% in thin films the introduction of DTP has led to a marked
increase of the absorption coefficient in the short wavelength region whereas the ICT
band retains the same intensity with a slightly less pronounced vibronic shoulder. Overall
this results in a more uniform absorption profile and should allow for the absorption of
more photons with P3HTT-DPP-DTP and ultimately a larger J
sc
.

139

Figure 6.3 UV/vis absorption spectra of
polymers in (a) solution (o-DCB) and (b) thin
film (spin-coated from o-DCB) where (i) is
P3HT (black line) (annealed at 150 C for thin
film), (ii) is P3HTT-DTP (red line), (iii) is
P3HTT-BTD-DTP (blue line), (iv) is P3HTT-
TP-DTP (light green line), (v) is P3HTT-DPP-
DTP (purple line) and (vi) is P3HTT-DPP-10%
(dark green line) (thin film as cast). For all DTP
containing polymers the thin films are annealed
at 60 C for 30 min.
140
In the previous chapters it has been shown that semi-random polymers tend to be
semi-crystalline which translates into high hole mobilities.
1,2
GIXRD analysis of the DTP
containing polymers showed that P3HTT-DPP-DTP is semi-crystalline after solvent
annealing and displays a peak at 5.75 degrees corresponding to an interplane distance
(100 direction) of 15.35 Å (for comparison the interplane distance of P3HT was
measured as 16.68 Å as can be seen in Figure 6.4). This peak does not become
significantly more pronounced after thermal annealing but shifts slightly to 5.76

degrees
corresponding to an interplane distance of 15.14 Å which is somewhat smaller than for
the solvent annealed film. On the other hand all other polymers are observed to be
amorphous, suggesting that the addition of DTP, which features (relatively short) alkyl
side chains, is enough to disrupt the ordering that is, for example, found in P3HTT-BTD.
1

The solid state ordering of the synthesized polymers is reflected in the hole mobilities
measured using the SCLC technique where only P3HTT-DPP-DTP (1.40 x 10
-4
cm
2
V
-1
s
-
1
) has a mobility approaching that of P3HT (2.3 x 10
-4
cm
2
V
-1
s
-1
) (and very similar to
P3HTT-DPP-10%),
2
whereas all other polymers display mobilities almost one order of
magnitude lower (Table 6.2).

141

The HOMO levels of all polymers were measured using CV with ferrocene as a
reference (P3HT is included in Figure 6.5 as well and was measured as 5.19 eV).
Interestingly the introduction of the DTP unit into the polymer backbone has raised the
HOMO energy for P3HTT-TP-DTP compared to P3HTT-TP (5.23 eV) and especially
P3HTT-BTD-DTP compared to P3HTT-BTD (5.41 eV) considerably, whereas almost no
change was found for P3HTT-DTP and P3HTT-DPP-DTP compared to P3HTT (5.29 eV)
and P3HTT-DPP-10% (5.2 eV).
1,2
Due to the very electron rich nature of DTP an
increase in HOMO energy upon introduction of DTP into the polymer is not surprising

Figure 6.4 GIXRD of thin films. Polymers
shown are (i) P3HT (red line) spin-coated from
o-DCB and annealed at 150 C for 30 min under
N
2
, (ii) P3HT (black line) spin-coated from CB
and annealed at 150 C for 30 min under N
2
, (iii)
P3HTT-DPP-DTP (dark green line) spin coated
from o-DCB and annealed at 150 C for 30 min
under N
2
and (iv) P3HTT-DPP-DTP (purple line)
spin coated from o-DCB and solvent annealed
for 20 min under N
2
. The inset shows the region
around 2θ = 4  - 7 in more detail.
142
and can be consistently found throughout the literature, although the strong acceptor DPP
appears to override this effect of DTP.
6,10,12,19

BHJ solar cells were fabricated in air using the conventional device configuration
of ITO/PEDOT:PSS/Polymer:PC
61
BM/Al and the J-V curves are shown in Figure 6.6.
As can be seen in Table 6.2, P3HTT-BTD-DTP, P3HTT-TP-DTP and P3HTT-DPP-DTP
show optimized solar cell performance at a 1:1 weight ratio with PC
61
BM (which is the
same ratio used for P3HT), while P3HTT-DTP devices are optimized at a much less
favorable 1:3 ratio.

Figure 6.5 CV traces for the oxidation of (i)
P3HT (black line), (ii) P3HTT-DTP (red line),
(iii) P3HTT-BTD-DTP (blue line), (iv) P3HTT-
TP-DTP (green line) and (v) P3HTT-DPP-DTP
(purple line). Ferrocene was used as a reference
and values were converted to the vacuum scale
using the approximation that the ferrocene redox
couple is 5.1 eV relative to vacuum.
20,21

143
Table 6.2 Photovoltaic properties of P3HT, P3HTT-DTP, P3HTT-BTD-DTP, P3HTT-
TP-DTP, P3HTT-DPP-DTP and P3HTT-DPP-10% with PC
61
BM as the acceptor.
2

Polymer:PC
61
BM
(ratio)
SCLC hole
mobility
(cm
2
/(V*s))
d

J
sc
(mA/cm
2
)
e
V
oc
(V) FF η
(%)
P3HT (1:1)
a
2.3 x 10
-4
9.10 0.61 0.58 3.19
P3HTT-DTP (1:3)
b
5.06 x 10
-5
1.81 0.63 0.33 0.36
P3HTT-BTD-DTP (1:1)
c
6.21 x 10
-5
1.48 0.47 0.31 0.21
P3HTT-TP-DTP (1:1)
c
7.03 x 10
-5
1.81 0.35 0.28 0.18
P3HTT-DPP-DTP (1:1)
c
1.4 x 10
-4
10.77 0.53 0.50 2.83
P3HTT-DPP-10% (1:1.3)
c
2.3 x 10
-4
13.87 0.57 0.63 4.94
a
Spin-coated from CB and annealed at 145 C for 30 min under N
2
after aluminum
deposition;
b
Spin-coated from CB and tested as-cast;
c
Spin-coated from o-DCB and
tested after spending 20 min in a N
2
cabinet before aluminum deposition;
d
Measured for
neat polymer films;
e
Mismatch corrected.
2

Figure 6.6 J-V curves of the BHJ solar cells
based on (i) P3HT (black line) , (ii) P3HTT-
DTP (red line), (iii) P3HTT-BTD-DTP (blue
line), (iv) P3HTT-TP-DTP (green line) and (v)
P3HTT-DPP-DTP (purple line) with PC
61
BM as
the acceptor under AM 1.5G illumination (100
mW/cm
2
) at the optimal conditions for solar cell
performance.
144
A high percentage of polymer in the active layer is expected to be beneficial for
the device performance since the absorption coefficient of polymers in the visible range
and NIR are much stronger than of the acceptor PC
61
BM and as a result large amounts of
acceptor should limit J
sc
. P3HTT-DTP, P3HTT-TP-DTP and P3HTT-BTD-DTP show
moderate performance with average efficiencies of 0.18 – 0.36%. A decrease in V
oc
is
observed upon incorporation of the acceptor units into the polymer backbone, which
follows the same trend as observed for the HOMO energies as measured by CV. In
comparison to P3HTT-BTD and P3HTT-TP (chapter 2)
1
efficiencies of P3HTT-BTD-
DTP and P3HTT-TP-DTP have decreased, which can be mainly attributed to
considerably lower V
oc
values (due to higher HOMO energies because of strongly
electron donating dithienopyrrole) as well as decreased mobilities due to amorphous
active layers. Values of FF below 33% can also be explained based on the observed low
hole mobilities, which likely facilitate build-up of space-charge due to unbalanced charge
transport and unfavorable recombination dynamics.
22
Furthermore, low hole mobilities
can be responsible for the low J
sc
, even though photoresponse up to 800 nm was recorded
for P3HTT-TP-DTP, as shown in Figure 6.7.
Upon addition of the acceptor DPP into the polymer backbone, a completely
different result is observed. Noticeably higher hole mobilities, likely due to the tendency
to crystallize (as can be seen from GIXRD, Figure 6.4), help to improve the efficiency of
P3HTT-DPP-DTP solar cells to 2.83%. A mismatch corrected J
sc
(M = 0.81) of 10.77
mA/cm
2
for P3HTT-DPP-DTP exceeds the current obtained for P3HT (M = 1.06) (9.10
mA/cm
2
), which is mainly attributed to the decrease of the optical E
g
of P3HTT-DPP-
145
DTP and as a result strong photoresponse up to 850 nm (Figure 6.7). The absorption
profile (Figure 6.3) of P3HTT-DPP-DTP matches well with the EQE spectrum (Figure
6.7), thus the photocurrent response in the long wave region is primarily attributed to the
polymer. The integrated photocurrents from the EQE spectra match within 5% to that of
mismatch corrected J
sc
measured under AM 1.5G illumination. Comparing P3HTT-DPP-
DTP to P3HTT-DPP-10%, it is interesting to note that even though P3HTT-DPP-DTP
shows a slightly larger integrated area underneath the absorption curve (Figure 6.3) this
does not translate into a larger J
sc
(10.77 vs. 13.87 mA/cm
2
). In addition the lower V
oc
as
well as the smaller FF for P3HTT-DPP-DTP relative to P3HTT-DPP-10% lead to an
overall reduction in the efficiency upon incorporation of DTP from 4.94% to 2.83%.
Nonetheless, the 2.83% efficiency obtained without extensive optimization for P3HTT-
DPP-DTP and the broad photocurrent response once again indicate the great potential of
semi-random copolymers as effective light harvesting polymers for BHJ solar cells.

146

6.3 Conclusion
In summary, a novel family of semi-random D/A copolymers based on rr-P3HT
was synthesized and the influence of introducing the strong donor DTP into the polymer
backbone in combination with three different acceptor monomers (BTD, TP and DPP)
has been analyzed. It was found that in the cases of P3HTT-DTP, P3HTT-BTD-DTP and
P3HTT-TP-DTP the presence of DTP leads to amorphous polymers with low hole
mobilities and very moderate solar cell efficiencies. Additionally V
oc
tends to be reduced
in the D/A analogues P3HTT-BTD-DTP and P3HTT-TP-DTP compared to previous
results
1
with analogous polymers (differing only by the absence of DTP) due to the
strongly electron donating nature of the DTP unit and thus increased HOMO levels. A

Figure 6.7 EQE of the BHJ solar cells based on
P3HT (black squares), P3HTT-DTP (red circles),
P3HTT-BTD-DTP (blue triangles), P3HTT-TP-
DTP (green stars) and P3HTT-DPP-DTP (purple
triangles) with PC
61
BM as the acceptor under
optimized conditions for device fabrication.
147
completely different result has been observed with P3HTT-DPP-DTP, which is a semi-
crystalline polymer with hole mobilities on the order of P3HT and a broad photocurrent
response giving a solar cell efficiency of 2.83%. These results indicate that monomer
combinations must be carefully selected in order to retain the semicrystalline nature of
P3HT and to maintain a HOMO energy suitable for targeting reasonable values of V
oc
.
Additionally, no broadening of the absorption profile was observed through the
introduction of the strong donor, which is likely due to the fact that the strong donor and
acceptor cannot bond to each other. Preliminary results from a study which is currently
underway show a considerable red-shift of the absorption profile of up to 100 nm. In
these novel semi-random polymers the strong donor (DTP or EDOT) can directly bond to
the acceptor (DPP or PDT), and even a polymer with the exact same composition as
P3HTT-DPP-DTP discussed above, shows a 100 nm red-shift, thus emphasizing the
importance of monomer connectivity. These early results not only show the potential of
strong donor containing semi-random polymers as NIR absorbing active materials for
polymer:fullerene solar cells, but more importantly the versatility of the semi-random
platform in general.
148
6.4 References for Chapter 6
(1) Burkhart, B.; Khlyabich, P. P.; Cakir Canak, T.; LaJoie, T. W.; Thompson, B. C.
Macromolecules 2011, 44, 1242–1246.
(2) Khlyabich, P. P.; Burkhart, B.; Ng, C. F.; Thompson, B. C. Macromolecules
2011, 44, 5079–5084.
(3) Burkhart, B.; Khlyabich, P. P.; Thompson, B. C. ACS Macro Lett. 2012, 1, 660–
666.
(4) Yue, W.; Zhao, Y.; Shao, S.; Tian, H.; Xie, Z.; Geng, Y.; Wang, F. J. Mater.
Chem. 2009, 19, 2199–2206.
(5) Ashraf, R. S.; Gilot, J.; Janssen, R. A. J. Sol. Energy Mater. Sol. Cells 2010, 94,
1759–1766.
(6) Zhang, S.; Guo, Y.; Fan, H.; Liu, Y.; Chen, H.-Y.; Yang, G.; Zhan, X.; Liu, Y.;
Li, Y.; Yang, Y. J. Polym. Sci. A Polym. Chem. 2009, 47, 5498–5508.
(7) Zhang, X.; Steckler, T. T.; Dasari, R. R.; Ohira, S.; Potscavage, W. J.; Tiwari, S.
P.; Coppée, S.; Ellinger, S.; Barlow, S.; Brédas, J.-L.; Kippelen, B.; Reynolds, J.
R.; Marder, S. R. J. Mater. Chem. 2010, 20, 123–134.
(8) Zhou, E.; Nakamura, M.; Nishizawa, T.; Zhang, Y.; Wei, Q.; Tajima, K.; Yang,
C.; Hashimoto, K. Macromolecules 2008, 41, 8302–8305.
(9) Shi, M.-M.; Deng, D.; Chen, L.; Ling, J.; Fu, L.; Hu, X.-L.; Chen, H.-Z. J. Polym.
Sci. A Polym. Chem. 2011, 49, 1453–1461.
(10) Zhou, E.; Cong, J.; Tajima, K.; Yang, C.; Hashimoto, K. Macromol. Chem. Phys.
2011, 212, 305–310.
(11) Hu, X.; Shi, M.; Zuo, L.; Nan, Y.; Liu, Y.; Fu, L.; Chen, H. Polymer 2011, 52,
2559–2564.
(12) Zhang, Y.; Zou, J.; Yip, H.-L.; Sun, Y.; Davies, J. A.; Chen, K.-S.; Acton, O.;
Jen, A. K.-Y. J. Mater. Chem. 2011, 21, 3895.
(13) Zhou, E.; Wei, Q.; Yamakawa, S.; Zhang, Y.; Tajima, K.; Yang, C.; Hashimoto,
K. Macromolecules 2010, 43, 821–826.
149
(14) Koeckelberghs, G.; De Cremer, L.; Vanormelingen, W.; Verbiest, T.; Persoons,
A.; Samyn, C. Macromolecules 2005, 38, 4545–4547.
(15) Koeckelberghs, G.; De Cremer, L.; Persoons, A.; Verbiest, T. Macromolecules
2007, 40, 4173–4181.
(16) Ogawa, K.; Rasmussen, S. C. Macromolecules 2006, 39, 1771–1778.
(17) Zhu, Y.; Champion, R. D.; Jenekhe, S. A. Macromolecules 2006, 39, 8712–8719.
(18) Beaujuge, P. M.; Amb, C. M.; Reynolds, J. R. Acc. Chem. Res. 2010, 43, 1396–
1407.
(19) Piliego, C.; Holcombe, T. .; Douglas, J. D.; oo, C. H.; Beaujuge, P. .;
Fr chet, J. . J. J. Am. Chem. Soc. 2010, 132, 7595–7597.
(20) Cardona, C. M.; Li, W.; Kaifer, A. E.; Stockdale, D.; Bazan, G. C. Adv. Mater.
2011, 23, 2367–2371.
(21) Thompson, B. C.; Kim, Y.-G.; McCarley, T. D.; Reynolds, J. R. J. Am. Chem.
Soc. 2006, 128, 12714–12725.
(22) Mihailetchi, V.; Wildeman, J.; Blom, P. Phys. Rev. Lett. 2005, 94, 126602.

150
CHAPTER 7 Influence of the Ethylhexyl Side-Chain
Content on the Open-Circuit Voltage in rr-Poly(3-
hexylthiophene-co-3-(2-ethylhexyl)thiophene) Copolymers
7.1 Introduction
Chapters 2-6 have focused in great detail on novel semi-random copolymers
which are able to broadly absorb the solar spectrum due to their multichromophoric
nature.
1–4
Importantly these polymers are able to translate the strong and broad absorption
profiles into high J
sc
. On the other hand, the V
oc
of most semi-random copolymers
investigated to date is lower than that of P3HT. Considering that the PCE of a
polymer:fullerene solar cell is not only proportional to J
sc
but also V
oc
it is important to
find ways to increase both output parameters. The following pages will center around a
simple but effective way of increasing the V
oc
of P3ATs which is potentially applicable in
semi-random D/A copolymers.
Among the great number of conjugated polymers that have been investigated, rr-
P3HT is the most studied conjugated polymer. As discussed in chapter 1 (section 1.4.1)
P3HT offers a unique property set and has a semi-crystalline structure and high hole
mobility as well as a favorable mixing ratio with fullerene acceptors such as PCBM (~1:1
as opposed to ratios of 1:3 or even 1:4 for many D/A copolymers), which allows for
maximum amounts of polymer, and consequently strong light absorption, in active layers
with a constrained thickness.
5
The combination of these positive factors leads to solar cell
151
efficiencies of up to 5% for P3HT:PCBM.
6
However, due to unfavorable positioning of
the frontier energy levels (HOMO and LUMO) and a wide E
g
, the J
sc
as well as the V
oc
in
fullerene blends are restricted to relatively small values thus limiting the achievable
power conversion efficiency of P3HT-based solar cells.
7

The most commonly used approaches to higher efficiency solar cells are a
decrease in the polymer E
g
and thus increase in J
sc
by absorbing more photons and/or the
lowering of the polymer HOMO to achieve a higher V
oc
(which can be roughly
approximated by the HOMO
D
- LUMO
A
offset).
8
For both strategies the D/A approach is
often used, which is described in detail in chapter 1 (section 1.3) and generally leads to
lower E
g
polymers and at the same time gives control over the position of the HOMO
(and LUMO) level thus potentially also allowing for an increase of the V
oc
.
9
As a
consequence, P3HT and P3ATs in general, have lately mainly been used to study
fundamental behavior such as polymer and fullerene crystallization or interdiffusion with
fullerenes and are less associated with strategies to increase efficiency (evidenced by the
much shorter section about homopolymers in chapter 1 (section 1.4.1) compared to
perfectly alternating D/A copolymers (section 1.4.2)).
10–13

Until recently in the design of new conjugated polymers for solar cells it was
commonly accepted that the conjugated backbone of a polymer determines the electronic
properties whereas the attached (non-conjugated) alkyl side-chains were considered
merely a necessity for the solubility of polymers in organic solvents to allow for solution
processing.
14
Nevertheless, considerable influence of side-chains on the morphology
(crystalline vs. amorphous, lamellar stacking distance, mixing with fullerenes) as well as
152
thermal properties (glass transition temperature and melting point) of the polymer has
been widely known.
15,16
A common example for the pronounced influence of alkyl side-
chains is rr-P3HT (head-to-tail ordering of 3-hexylthiophene, see chapter 1), which
shows high hole mobility, semi-crystallinity and high solar cell efficiencies, whereas ra-
P3HT is amorphous with considerably lower efficiencies.
17,18
P3HT also exemplifies the
fact that the alkyl chain length is important as out of all investigated P3ATs the hexyl
side-chain seems to be the optimal compromise between solubility (processability), good
morphology (e.g. phase separation between polymer and fullerene and crystallinity) as
well as chromophore density.
19
It has also been found with P3ATs that the optimal
polymer:fullerene ratio is influenced by the side-chain density on the conjugated
backbone, where a lower density allows not only for interdigitation of side-chains but
also leads to intercalation of fullerene and thus higher required ratios of acceptor in the
solar cells.
20
As such the identity, content and distribution of alkyl side-chains play a
strong role in the solar cell performance of P3ATs and are slowly drawing more
attention, although no comprehensive or conclusive picture has yet emerged of the
specific influences of all those structural variations.
Recently a more profound influence of alkyl side-chains on polymer properties
has been reported for both D/A copolymers and P3ATs. Several studies found a
correlation between the alkyl side-chain length and the V
oc
of the polymer:fullerene solar
cells where typically a longer side-chain leads to a lower HOMO level of the polymer
(while E
g
remains constant) and larger V
oc
while keeping the conjugated backbone
constant.
21,22
It needs to be mentioned though, that these changes in V
oc
and HOMO level
153
tend to be relatively small (often within 100 mV) while processing conditions such as
annealing can also have a large influence on V
oc
. In addition, other reports show less clear
trends so that it is difficult to draw any general conclusions.
19,23,24
With P3ATs, Hou et
al. reported that by decreasing the alkyl side-chain density on a thiophene backbone using
a long branched alkyl chain on every third thiophene ring they were able to increase the
V
oc
considerably compared to P3HT (as a consequence of a lower HOMO level of the
polymer) without increasing the band gap. However, no improvement of efficiency was
achieved compared to P3HT solar cells.
25
Ko et al. used a similar approach with P3ATs
to lower the HOMO energy and increase the V
oc
by only putting alkyl chains on the 3 and
4 position of every third thiophene ring. In this case obtained efficiencies of up to 4.2%
with PC
71
BM were found to exceed those observed with P3HT.
26
In both of these cases
with P3ATs the decrease in density of electron donating alkyl side-chains was cited as the
reason for a lower HOMO and consequently a larger V
oc
. Interestingly in the study by Ko
et al. the V
oc
was also observed to increase with side-chain length (going from hexyl side-
chain to dodecyl) whereas the HOMO levels remained almost constant (but lower than
P3HT). Note that this is not consistent with the trend reported for P3ATs where longer
side-chains lead to a lower HOMO level.
21,22
This result suggests that side-chain length
has an effect on V
oc
that is more complex than simply via an influence on the HOMO
level.
In D/A copolymers it is even more difficult to identify the influence of alkyl side-
chains as not only the type (length, branched or linear) but also the position (e.g. on the
donor or acceptor moiety) of the solubilizing groups can be changed. In addition studies
154
often do not contain enough data points or too many parameters are changed
simultaneously to draw any general conclusions on the impact of side-chains on the
V
oc
.
27–30
However, there are a several reports in the literature which show a pronounced
influence of side-chains on the V
oc
with D/A polymers in polymer:fullerene solar cells. Li
et al. published a family of CPDT-TPD copolymers with varying alkyl chains where they
found that longer/branched alkyl chains on CPDT lead to lower HOMO energies and
increased V
oc
.
31
Yang et al. reported a study on naphtodithiophene (NDT)-
dithiophenebenzothiadiazole (DTBT) copolymers where they changed the alkyl side-
chains on the NDT and DTBT units, varying the length (octyl, dodecyl) as well as the
branching (2-hexyldecyl, 2-ethylhexyl) of the side-chains but leaving placement
constant.
32
In this case the authors found a pronounced influence of the type of side-
chains on the V
oc
as well as J
sc
of the polymer:PCBM devices, where long and branched
side-chains lead to large V
oc
but small J
sc
values. A correlation between the
preexponential term J
so
(from the generalized Shockley equation for solar cells) and the
V
oc
of the polymer:PCBM devices was found (similar observations on small molecule
solar cells had already been made by Perez et al.
33
) and it was stated that long branched
alkyl chains weaken intermolecular interaction between polymer and fullerene and thus
lead to small J
so
values and consequently large V
oc
values. It has to be mentioned though
that HOMO levels of polymers in this study were not constant with different side-chain
substitution and that for most cases the HOMO level of these polymers follows the same
trend as the V
oc
of the polymer:PCBM devices, with the highest HOMO level belonging
to the polymer with short and linear side-chains (which also has the smallest V
oc
value)
155
whereas longer as well as branched side chains lower the HOMO level and give higher
V
oc
. It was concluded that short, branched alkyl side-chains (such as 2-ethylhexyl) are
optimal for polymer:fullerene device performance as they allow for both a large V
oc
and
large J
sc
and thus the highest efficiency.
From this short literature overview it can easily be seen that no comprehensive
picture of the effect of side-chain positioning, length and branching on polymer:fullerene
solar cell performance has evolved. In addition there seems to be no general agreement
on the reasons for the observed effects of the alkyl side-chains on the V
oc
(influence of
interfacial interactions (J
so
) vs. lowering of polymer HOMO level). In order to address
some of these questions, and specifically the influence of branched alkyl side-chains, a
simple and modular model system of random copolymers based on rr-P3HT was
designed. This system is intended to help gain insight into the effect the introduction of
varying amounts of short and branched alkyl side-chains (2-ethylhexyl) have on the
properties and solar cell performance (and especially the V
oc
) of the resulting polymers.
This system takes into account the beneficial impact of 2-ethylhexyl chains on device
properties that has been shown in D/A copolymers as well as the fact that a certain degree
of disorder in the polymer backbone is beneficial for solar cell performance.
14,32,34,35
The
influence of increasing amounts of branched 3-(2-ethylhexyl)thiophene (10, 25 and 50%)
in rr-poly(3-hexylthiophene-co-3-(2-ethylhexyl)thiophene) copolymers on important
properties such as UV/vis absorption, polymer crystallinity, HOMO energy levels and
polymer:PC
61
BM solar cell performance (especially the V
oc
) is studied and compared to
the results of the corresponding homopolymers P3HT and poly(3-(2-
156
ethylhexyl)thiophene) (P3EHT). It should be noted that P3EHT has received relatively
little attention and no solar cell data has been reported to date. Somanathan et al.
characterized P3EHT, which was synthesized by chemical oxidative polymerization,
36

whereas Hashimoto et al. studied copolymers between 3-hexylthiophene and 3-(2-
ethylhexyl)thiophene (block as well as random copolymers) and were able to find a
distinct influence of the ratio of comonomers on the polymer film morphology and the
field-effect hole mobilities for block copolymers as well as random copolymers.
37–39

Segalman et al. demonstrated that P3EHT has comparable optical and charge transport
properties as P3HT but did not report solar cell data.
40

Here it will be shown that even small amounts of 3-(2-ethylhexyl)thiophene
added as a comonomers to 3-hexylthiophene lead to an increased V
oc
of the
polymer:PC
61
BM solar cells (because of lowered HOMO levels in the solid state) while
J
sc
remains high due to high absorption coefficients ultimately leading to an efficiency
increase compared to P3HT.

7.2 Results and Discussion
Polymers were synthesized using the same Stille coupling
41,42
procedure as for
semi-random copolymers (chapter 2), with 2-bromo-5-trimethyltin-3-hexylthiophene and
2-bromo-5-trimethyltin-3-(2-ethylhexyl)thiophene as co-monomers to insure
polymerization with a head-to-tail regioregularity as is shown in Figure 7.1b. Synthesis
of 2-bromo-5-trimethyltin-3-hexylthiophene has been reported previously
41
and 2-bromo-
157
5-trimethyltin-3-(2-ethylhexyl)thiophene (3) was synthesized starting from 3-
bromothiophene using Kumada coupling
38
to give (1) followed by bromination in the
two-position using N-bromosuccinimide to give (2) and finally stannylation using LDA
and trimethyltinchloride to give the monomer (3) (Figure 7.1a). The ratio of monomers
was varied starting from polymerizations with only 2-bromo-5-trimethyltin-3-
hexylthiophene to give pure P3HT and then introducing 10, 25 and 50% of 2-bromo-5-
trimethyltin-3-(2-ethylhexyl)thiophene. Polymers are named P3HT
(100-x)%
-co-EHT
x%
in
order to indicate the respective amounts of 3-hexylthiophene and 3-(2-
ethylhexyl)thiophene in the copolymers. In all cases very soluble polymers with high
molecular weights (determined via GPC using polystyrene standards) and good yields
were obtained after soxhlet purification (Table 7.1). The polymer structures were
analyzed using
1
H NMR and match closely with previously reported spectra of
copolymers of 3-hexylthiophene and 3-(2-ethylhexyl)thiophene.
37
By integrating the
aromatic peaks at 6.96 ppm (aromatic CH from 3-hexylthiophene) as well as 6.94 ppm
(aromatic CH from 3-(2-ethylhexyl)thiophene) and comparing with the integration of the
benzylic protons of both monomers at ~2.8 ppm it was confirmed that the monomer feed
ratio is equivalent to the incorporation of monomers in the backbone (all polymer
1
H
NMR’s with integrations can be seen in appendix 6) .

158

Table 7.1 Molecular and electronic properties of polymers.
Polymer M
n
g/mol
(PDI)
a
HOMO (eV)
(solution)
b
HOMO (eV)
(thin film)
c
E
g
(optical)
(eV)
d
P3HT 24,240 (2.7) 5.25 5.17 1.9
P3HT
90
-co-EHT
10
21,330 (2.5) 5.25 5.30 1.9
P3HT
75
-co-EHT
25
26,120 (2.5) 5.30 5.43 1.9
P3HT
50
-co-EHT
50
40,130 (2.0) 5.32 5.48 1.9
P3EHT 22,180 (2.9) 5.28 5.57 2.0
a
Determined by GPC with polystyrene as standard and o-DCB as eluent.
b
CV (vs
Fc/Fc
+
) in chloroform containing 0.1 M TBABF
4
.
c
CV (vs Fc/Fc
+
) in acetonitrile
containing 0.1 M TBAPF
6
.
d
Optical band gaps from onset of absorption in UV/vis
spectra in annealed films.

UV/vis absorption spectra were recorded for all five polymers in solution (CB),
as-cast films (see appendix 6) and annealed thin films (annealing temperatures were
chosen to be below the melting point of the polymer). The results are summarized in

Figure 7.1 Synthesis of monomer 2-bromo-5-trimethyltin-3-
(2-ethylhexyl)thiophene (3) (a) and Stille polymerization for
poly(3-hexylthiophene-co-3-(2-ethylhexyl)thiophene) (b).
159
Figure 7.2. As can be seen from the UV/vis absorption in solution P3HT, P3HT
90
-co-
EHT
10
and P3HT
75
-co-EHT
25
all have the same peak absorption wavelength (456 nm)
whereas the peak positions of P3HT
50
-co-EHT
50
and P3EHT are slightly blue shifted
(454 and 445 nm respectively) (Figure 7.2a). The absorptivity decreases with increasing
ethylhexyl side-chain content, which can be explained as a dilution effect of the
chromophore due to the increase in larger side-chains with increasing ethylhexyl content.
In thin films the absorption maxima red-shift for all five polymers relative to the solution
measurement (100 nm for P3HT, 68 nm for P3HT
90
-co-EHT
10
, 65 nm for P3HT
75
-co-
EHT
25
, 64 nm for P3HT
50
-co-EHT
50
and 40 nm for P3EHT), which is an indication of
planarization of the polymer backbones in the thin film (Figure 7.2b). All absorption
profiles show vibronic features after annealing in the longer wavelength region, also
indicating ordering in the solid state and suggesting a semi-crystalline structure (with the
exception of P3HT
50
-co-EHT
50
all polymers show vibronic features already in the
unannelaed films, see appendix 6). The onset of absorption in the thin films is the same
for all polymers except for P3EHT, which has a slightly higher E
g
of 2.0 eV probably due
to more significant steric interactions, which inhibit backbone planarity. All other
polymers have an E
g
of 1.9 eV, which is identical with P3HT. Interestingly in thin films
the absorption coefficients of all polymers (except P3EHT) are virtually the same
showing almost no dilution effect of the chromophore due to the side-chains. It can be
concluded that the introduction of 50% or less of 2-ethylhexyl side-chains does not
considerably alter the absorption properties of the thin films compared to P3HT.

160

Thin polymer films prepared under identical annealing conditions as for UV/vis
were analyzed by GIXRD to confirm their semi-crystalline nature. As can be seen in
Figure 7.3 all polymers exhibit peaks in the 2θ range of 5º-7º which are referenced as

Figure 7.2 UV/vis absorption of all five polymers
in (a) solution (CB) and (b) thin films (spin coated
from CB and annealed for 30 min under N
2
at 150
ºC for (i), (ii) and (iii), 100 ºC for (iv) and 40 ºC
for (v)) where (i) is P3HT (purple line), (ii) is
P3HT
90
-co-EHT
10
(green line), (iii) is P3HT
75
-co-
EHT
25
(blue line), (iv) is P3HT
50
-co-EHT
50
(red
line) and (v) is P3EHT (orange line).
161
(100) reflections in relation to P3HT. The interplane packing distance (100) for P3HT is
calculated as 16.7 Å, for P3HT
90
-co-EHT
10
as 17.0 Å, for P3HT
75
-co-EHT
25
as 16.9 Å,
for P3HT
50
-co-EHT
50
as 16.0 Å and for P3EHT as 14.9 Å. P3EHT has a considerably
smaller interplane distance than P3HT, which is consistent with the literature
37,40
and is
likely due to the significant steric interactions introduced by the 2-ethylhexyl side-chains
which inhibit backbone planarity and thus decrease the density of side-chains in the (100)
direction and lead to a shorter interplane packing distance. That effect can also already be
seen in P3HT
50
-co-EHT
50
where the interplane distance is decreased to 16.0 Å. The
slightly larger interplane distances for P3HT
90
-co-EHT
10
, P3HT
75
-co-EHT
25
compared to
P3HT can be explained by the introduction of the bulkier 2-ethylhexyl side-chains which
require more space, but at low percentages do not introduce significant steric interactions
so that the polymer backbone remains planar and the density of side-chains in the (100)
direction is not reduced. As a consequence the stacking distance in the (100) direction
increases as determined by the larger 2-ethylhexyl side-chains. The degree of
crystallinity, as inferred from the peak intensity, of the polymer thin films decreases with
introduction of 2-ethylhexyl side-chains as can be seen in Figure 7.3 and is lowest for
P3EHT. This can be explained by the unfavorable steric interactions introduced by the 2-
ethylhexyl side-chains, which are expected to inhibit crystallization.
162

In order to expand on the results obtained by GIXRD and analyze the melting and
crystallization behavior of the polymers, DSC traces were recorded. Melting points (see
appendix 6 for DSC curves) decrease with increasing percentage of 2-ethylhexyl side-
chains as expected from the literature and match well for those polymers which have
been reported previously (Figure 7.4).
37
A melting transition for P3EHT was not
observed, although it has been previously reported in the literature (two peaks at ~70 and
~90 ºC attributed to two coexisting crystal structures), which is probably due to its
relatively slow crystallization kinetics.
40,43
The presented GIXRD and DSC data confirms
that even though the polymers contain branched 2-ethylhexyl side-chains and have a

Figure 7.3 GIXRD of thin films spin-coated from
CB and annealed for 30 min under N
2
(annealing
temperature was 150 ºC for (i)-(iii), 100 ºC for (iv)
and 40 ºC for (v)) are shown where (i) is P3HT
(purple line), (ii) is P3HT
90
-co-EHT
10
(green line),
(iii) is P3HT
75
-co-EHT
25
(blue line), (iv) is
P3HT
50
-co-EHT
50
(red line) and (v) is P3EHT
(orange line). The inset shows the region around
2θ = 4º - 8º in greater detail.
163
random distribution of 3-hexylthiophene and 3-(2-ethylhexyl)thiophene co-monomers,
the semi-crystalline nature of rr-P3HT is retained and even P3EHT shows a certain
degree of lamellar packing (as evidenced by GIXRD).

.
In order to gain insight on the effect of the 2-ethylhexyl side-chain content on the
polymer HOMO levels, CV vs. ferrocene (see appendix 6 for CV traces) was employed
in both solution and thin films. The values are summarized in Table 7.1. HOMO levels
for polymers in solution are virtually identical (within the range of 5.25-5.32 eV),
whereas in the solid state an increase in the amount of 2-ethylhexyl side-chains leads to a
marked and correlated decrease in the HOMO level. In thin films the introduction of only
10% 2-ethylhexyl chains led to a 0.13 eV decrease in the HOMO level compared to
P3HT, while P3EHT has a HOMO level of 5.57 eV (compared to 5.17 for P3HT).

Figure 7.4 Melting points of polymers as
measured by DSC.
164
Considering that the optical band gap does not increase for P3HT
90
-co-EHT
10
, P3HT
75
-
co-EHT
25
and P3HT
50
-co-EHT
50
compared to P3HT, the LUMO level is most likely
shifting down simultaneously with the HOMO level. The fact that HOMO levels of all
five polymers are essentially identical in solution implies that the observed decrease in
HOMO level energy in thin films is a solid state organization effect and dependent on the
bulk properties of the polymers. This decrease in HOMO levels, especially for P3HT
90
-
co-EHT
10
, P3HT
75
-co-EHT
25
and P3HT
50
-co-EHT
50
comes with almost no decrease in
the absorption coefficient and with an E
g
equivalent to P3HT. These results are consistent
with previous literature reports that show lowered HOMO levels (and constant E
g
) when
longer side-chains were introduced into polymers and are not only expected to allow for
larger V
oc
in solar cells but also make the polymers more stable against oxidation in
air.
21,22,44

The SCLC method was employed to determine the hole mobilities of all five
polymers and the values are summarized in Table 7.2. With increasing 2-ethylhexyl
content the mobility decreases, which correlates well with the decreased degree of
crystallinity of the polymer films. However, with the exception of P3EHT, the measured
mobilities are still extremely close to the hole mobility of P3HT.

165
Table 7.2 Photovoltaic properties of P3HT, P3HT
90
-co-EHT
10
, P3HT
75
-co-EHT
25
,
P3HT
50
-co-EHT
50
and P3EHT with PC
61
BM as the acceptor.
Polymer:PC
61
BM
(ratio)
SCLC hole
mobility
(cm
2
/(V*s))
f

J
sc
(mA/cm
2
)

V
oc
(V) FF  (%)
P3HT
(1:1)
a

2.3 x 10
-4
9.67 0.60 0.60 3.48
P3HT
90
-co-EHT
10

(1:0.8)
b

1.77 x 10
-4

9.26 0.63 0.51 2.80
P3HT
75
-co-EHT
25

(1:0.8)
c

1.39 x 10
-4

9.85 0.69 0.57 3.85
P3HT
50
-co-EHT
50

(1:3.5)
d

1.07 x 10
-4
2.52 0.85 0.35 0.74
P3EHT
(1:3.0)
e
2.87 x 10
-5
2.54 0.90 0.36 0.83
a
Spin-coated from CB and annealed at 145

C for 60 min under N
2
after aluminum
deposition.
b
Spin-coated from CB and annealed at 110

C for 60 min under N
2
after
aluminum deposition.
c
Spin-coated from CB and annealed at 110

C for 30 min under N
2

after aluminum deposition.
d
Spin-coated from CB and annealed at 110

C for 10 min
under N
2
after aluminum deposition.
e
Spin-coated from CB and tested as-cast.
f
Measured
for neat polymer films.

After characterization of the neat polymers in solution and thin films, all polymers
were used in photovoltaic devices with PC
61
BM as the acceptor. BHJ solar cells in a
conventional device configuration of ITO/PEDOT:PSS/polymer:PC
61
BM/Al were
fabricated in air. P3HT
90
-co-EHT
10
and P3HT
75
-co-EHT
25
have a optimized
polymer:PC
61
BM ratios of 1:0.8, which is close to that found here for P3HT:PC
61
BM at
1:1. As mentioned earlier, this ratio is very favorable as it allows for a maximum amount
of polymer (and consequently strong light absorption) in an active layer with constrained
thickness and it also implies that the behavior of P3HT
90
-co-EHT
10
and P3HT
75
-co-
EHT
25
in blends is similar to P3HT showing good miscibility with the fullerene. Upon
further increase of the 2-ethylhexyl side-chain content (P3HT
50
-co-EHT
50
and P3EHT),
166
significant changes in the optimal polymer:fullerene ratios were observed and an excess
of PC
61
BM is necessary for peak device operation (1:3.5 and 1:3 for P3HT
50
-co-EHT
50

and P3EHT respectively), indicating a fundamental change in the behavior of these
polymers compared to P3HT. Average values of , V
oc
, FF, and J
sc
obtained under
simulated AM 1.5G illumination (100 mW/cm
2
) for all polymers are listed in Table 7.2
and J-V curves are shown in Figure 7.5.

Figure 7.5 J-V curves of the BHJ solar cells
based on (i) P3HT (purple line), (ii) P3HT
90
-co-
EHT
10
(green line), (iii) P3HT
75
-co-EHT
25
(blue
line), (iv) P3HT
50
-co-EHT
50
(red line) and (v)
P3EHT (orange line) with PC
61
BM as the
acceptor under AM 1.5G illumination (100
mW/cm
2
) at the optimal conditions for solar cell
performance.
167
The V
oc
of the optimized solar cells increases upon introduction of the 2-
ethylhexyl side-chains into the polymer backbone as shown in Figure 7.6 following the
trend suggested by the decreasing HOMO levels as measured by CV of the thin films.
This seems to indicate that the increase in V
oc
for this family of polymers is primarily
determined by the HOMO
D
- LUMO
A
offset and less by other effects suggested in the
literature (such as the polymer:fullerene interface interactions represented by J
so
).
8,32,45

Even 10% of 2-ethylhexyl side-chains are enough to enhance V
oc
from 0.60 V for P3HT
to 0.63 V and this value is even further increased to 0.69, 0.85 and 0.90 V in case of
P3HT
75
-co-EHT
25
, P3HT
50
-co-EHT
50
and P3EHT. While the increases in V
oc
with
increasing 2-ethylhexyl content are well correlated with the concomitantly decreasing
HOMO energies of the polymers as shown in Figure 7.6, the increase in V
oc
from 0.69 V
with P3HT
75
-co-EHT
25
to 0.85 V with P3HT
50
-co-EHT
50
of 160 mV is significantly
larger than the corresponding decrease in HOMO energy of only 50 meV. No explanation
for this is evident, but it is possible that this large step in V
oc
is indicative of influences on
the V
oc
in addition to the increased HOMO
D
- LUMO
A
offset, as noted above, such as
donor-acceptor interfacial effects induced by bulky polymer side chains. However, the
significant changes in other solar cell properties between P3HT
75
-co-EHT
25
and P3HT
50
-
co-EHT
50
, including the change in optimal polymer-fullerene ratio, make it difficult to
decouple these parameters and assign an underlying influence on the increase in V
oc
beyond the clearly observed correlation with decreasing HOMO energy.

168

At the same time the J
sc
values for P3HT, P3HT
90
-co-EHT
10
and P3HT
75
-co-
EHT
25
are high and close to each other in the range of 9.26 – 9.85 mA/cm
2
while a
noticeable drop in J
sc
was recorded for P3HT
50
-co-EHT
50
and P3EHT. A high FF (around
60%) for P3HT

and P3HT
75
-co-EHT
25
suggests a favorable morphology of the
polymer:PC
61
BM blends as well as balanced charge transport in the devices. This
combination of increased V
oc
and high J
sc
as well as a large FF ultimately allows P3HT
75
-
co-EHT
25
:PC
61
BM to exceed the efficiency of P3HT:PC
61
BM (3.85 vs. 3.48%). It is
unclear why the fill factor of P3HT
90
-co-EHT
10
, after extensive solar cell optimization,

is
only 51% even though the absorption coefficient, degree of crystallinity, hole mobility
and mixing ratio are comparable to P3HT

and P3HT
75
-co-EHT
25
. Ultimately the lower FF
causes the efficiency of the optimized P3HT
90
-co-EHT
10
device to drop to 2.80% even
though J
sc
is high and the V
oc
is increased compared to P3HT. For P3HT
50
-co-EHT
50
and
P3EHT the combination of a decrease in the absorption coefficient, the degree of
.
Figure 7.6 HOMO levels of the polymers in the
solid state (filled squares) and V
oc
(circles) of the
optimized solar cells as a function of the amount of
2-ethylhexyl side-chains in the polymer backbone.
169
crystallinity, and the hole mobility, as well as the unfavourable mixing ratios with PCBM
(less polymer in the active layer) explains the low J
sc
, FF and ultimately the low
efficiencies. Nevertheless the observed trend in V
oc
with increasing 2-ethylhexyl side-
chain content is still maintained.
For further characterization the BHJ morphology of the solar cells was analyzed
by TEM and the recorded images are shown in Figure 7.7. P3HT and P3HT
90
-co-EHT
10

(Figure 7.7a and b) appear to have almost the same morphology with an overall
homogeneous distribution of polymer and PC
61
BM whereas P3HT
75
-co-EHT
25
and
P3HT
50
-co-EHT
50
(Figure 7.7c and d) show a slightly finer phase separation. Apparently
the introduction of small amounts of 2-ethylhexyl side-chains allows for the good
properties of P3HT:PC
61
BM blends, such as the formation of bicontinuous pathways for
charge transport and domain sizes favourable for exciton splitting, to be retained. In
contrast, P3EHT (Figure 7.7e) shows large, (roughly on the scale of 100 nm), round,
dark regions throughout the entire film, which appear to be PC
61
BM aggregates. These
large fullerene aggregates likely prevent efficient exciton dissociation and hinder charge
transport.
170

7.3 Summary and Conclusions
The work presented here has been motivated by the desire to demonstrate and
understand the influence of conjugated polymer alkyl side-chains on the performance of
polymer:fullerene BHJ solar cells. Specifically the relationship between alkyl side-chain

Figure 7.7 TEM images of polymer:PC
61
BM films (optimized conditions for best
solar cell performance were used to make the films) where a) is P3HT, b) is
P3HT
90
-co-EHT
10
, c) is P3HT
75
-co-EHT
25
, d) is P3HT
50
-co-EHT
50
and e) is
P3EHT.
171
identity (length, branching) and distribution (patterns of alkyl-substituted and non-
substituted rings) and the V
oc
is of greatest interest for not only the design of new
polymers, but also for furthering understanding of the mechanism of operation in
polymer:fullerene solar cells. In order to address this incredibly complex issue, a model
system of random alkylthiophene copolymers was developed in order to logically test one
variable, in particular the influence of branched alkyl chain content (specifically 2-
ethylhexyl) on P3ATs. With this model system it was possible to demonstrate a
correlation between increasing branched alkyl chain content, a physical property of the
polymer (decreasing HOMO energy measured in the solid state), and the increasing V
oc

of polymer:fullerene BHJ solar cells. The observed decrease of HOMO levels in the solid
state, and simultaneous increase of the V
oc
upon increasing branched side-chain content is
consistent with some previously reported work on P3ATs and, according to the presented
results, implies a solid state organization effect of the polymer, as the introduction of
even small amounts of 2-ethylhexyl side-chains leads to a marked decrease of the HOMO
levels of polymers in the solid state, whereas they are virtually identical in solution.
21,22

While the physical origin of the decreasing HOMO energy in the solid state is not clear,
an important observation is that even with significant branched side-chain content (up to
50%), the decreasing HOMO energy is not accompanied by any change in the polymer
optical band gap. Further, it is clear that, for the polymers studied here, the V
oc
is strongly
correlated to the HOMO
D
- LUMO
A
offset as the decrease in HOMO levels is reflected in
the measured increase in V
oc
values of the polymer:PC
61
BM solar cells. More detailed
explanations, such as interfacial interactions between the polymer and fullerene, which
172
other literature reports show to be relevant for increased V
oc
are not necessary to explain
the observed results.
32,33

Further work is clearly needed on the path to developing a comprehensive picture
of alkyl side-chain effects on polymer:fullerene solar cells and specifically on the V
oc
.
The present study does not address the influence of alkyl chain length and distribution
nor does it address the potential differences between polymers with homogenous
backbone content (e.g. P3ATs) or copolymers, such as D/A polymers. However, it is
clear that logical model studies of isolated variables, analogous to that presented here,
can provide an important path toward demonstrating the influence of alkyl chain
substitution in conjugated polymers and opening the door to a unified description and
ultimately a detailed fundamental understanding. As a further interesting consequence of
the model study presented here, it was found that the good properties of P3HT (high
absorption coefficient, semi-crystallinity and high hole mobility) and the optical band gap
are essentially retained in P3HT
90
-co-EHT
10
, P3HT
75
-co-EHT
25
and P3HT
50
-co-EHT
50
.
The relatively large increases in the V
oc
of the polymer:fullerene solar cells with
introduction of only small amounts of 2-ethylhexyl chains, with almost no change in all
other properties, suggests that this is also a viable way of tuning the V
oc
in other polymers
as a method for seeking higher efficiencies. This could be an especially promising
strategy for semi-random copolymers, where replacing small amounts of hexylthiophene
with 2-ethylhexylthiophene could potentially lead to an increase of the moderate V
oc
of
some of these polymers without negatively impacting all other favourable properties.
Combining the here discussed approach to increase V
oc
with the unprecedented
173
combination of favourable properties of semi-random copolymers offers the potential of
increasing efficiencies far beyond what has been observed so far and is a very promising
direction for future work.
174
7.4 References for Chapter 7
(1) Burkhart, B.; Khlyabich, P. P.; Thompson, B. C. ACS Macro Lett. 2012, 1, 660–
666.
(2) Burkhart, B.; Khlyabich, P. P.; Thompson, B. C. J. Photon. Energy 2012, 2,
021002.
(3) Khlyabich, P. P.; Burkhart, B.; Ng, C. F.; Thompson, B. C. Macromolecules
2011, 44, 5079–5084.
(4) Burkhart, B.; Khlyabich, P. P.; Cakir Canak, T.; LaJoie, T. W.; Thompson, B. C.
Macromolecules 2011, 44, 1242–1246.
(5) Thompson, B. C.; Khlyabich, P. P.; Burkhart, B.; Aviles, A. E.; Rudenko, A.;
Shultz, G. V.; Ng, C. F.; Mangubat, L. B. Green 2011, 1, 29–54.
(6) Dang, M. T.; Hirsch, L.; Wantz, G. Adv. Mater. 2011, 23, 3597–3602.
(7) Bundgaard, E.; Krebs, F. Sol. Energy Mater. Sol. Cells 2007, 91, 954–985.
(8) Clarke, T. M.; Durrant, J. R. Chem. Rev. 2010, 110, 6736–6767.
(9) Kroon, R.; Lenes, M.; Hummelen, J.; Blom, P.; de Boer, B. Polymer Revs. 2008,
48, 531–582.
(10) Agostinelli, T.; Lilliu, S.; Labram, J. G.; Campoy-Quiles, M.; Hampton, M.;
Pires, E.; Rawle, J.; Bikondoa, O.; Bradley, D. D. C.; Anthopoulos, T. D.;
Nelson, J.; Macdonald, J. E. Adv. Funct. Mater. 2011, 21, 1701–1708.
(11) Treat, N. D.; Shuttle, C. G.; Toney, M. F.; Hawker, C. J.; Chabinyc, M. L. J.
Mater. Chem. 2011, 21, 15224–15231.
(12) Treat, N. D.; Brady, M. A.; Smith, G.; Toney, M. F.; Kramer, E. J.; Hawker, C.
J.; Chabinyc, M. L. Adv. Energy Mater. 2011, 1, 82–89.
(13) Snyder, C. R.; Henry, J. S.; DeLongchamp, D. M. Macromolecules 2011, 44,
7088–7091.
(14) Thompson, B. C.; Kim, B. J.; Kavulak, D. F.; Sivula, K.; Mauldin, C.; Fréchet, J.
M. J. Macromolecules 2007, 40, 7425–7428.
(15) Szarko, J. M.; Guo, J.; Liang, Y.; Lee, B.; Rolczynski, B. S.; Strzalka, J.; Xu, T.;
Loser, S.; Marks, T. J.; Yu, L.; Chen, L. X. Adv. Mater. 2010, 22, 5468–5472.
175
(16) Wu, P.-T.; Ren, G.; Jenekhe, S. A. Macromolecules 2010, 43, 3306–3313.
(17) Osaka, I.; McCullough, R. D. Acc. Chem. Res. 2008, 41, 1202–1214.
(18) Mauer, R.; Kastler, M.; Laquai, F. Adv. Funct. Mater. 2010, 20, 2085–2092.
(19) Nguyen, L. H.; Hoppe, H.; Erb, T.; Günes, S.; Gobsch, G.; Sariciftci, N. S. Adv.
Funct. Mater. 2007, 17, 1071–1078.
(20) Kline, R. J.; DeLongchamp, D. M.; Fischer, D. A.; Lin, E. K.; Richter, L. J.;
Chabinyc, M. L.; Toney, M. F.; Heeney, M.; McCulloch, I. Macromolecules
2007, 40, 7960–7965.
(21) Alibrahim, M.; Roth, H.; Schroedner, M.; Konkin, A.; Zhokhavets, U.; Gobsch,
G.; Scharff, P.; Sensfuss, S. Org. Electron. 2005, 6, 65–77.
(22) Gadisa, A.; Oosterbaan, W. D.; Vandewal, K.; Bolsée, J.-C.; Bertho, S.; D’Haen,
J.; Lutsen, L.; Vanderzande, D.; Manca, J. V. Adv. Funct. Mater. 2009, 19, 3300–
3306.
(23) Friedel, B.; McNeill, C. R.; Greenham, N. C. Chem. Mater. 2010, 22, 3389–3398.
(24) Stevens, D. M.; Speros, J. C.; Hillmyer, M. A.; Frisbie, C. D. J. Phys. Chem. C
2011, 115, 20806–20816.
(25) Hou, J.; Chen, T. L.; Zhang, S.; Huo, L.; Sista, S.; Yang, Y. Macromolecules
2009, 42, 9217–9219.
(26) Ko, S.; Verploegen, E.; Hong, S.; Mondal, R.; Hoke, E. T.; Toney, M. F.;
McGehee, M. D.; Bao, Z. J. Am. Chem. Soc. 2011, 133, 16722–16725.
(27) Zoombelt, A. P.; Leenen, M. A. M.; Fonrodona, M.; Nicolas, Y.; Wienk, M. M.;
Janssen, R. A. J. Polymer 2009, 50, 4564–4570.
(28) ang, .; Hou, L.; ang, .; a, .; Hellstr m, S.; huang, .; hang, F.;
Ingan s, O.; Andersson, . R. Macromolecules 2011, 44, 2067–2073.
(29) Biniek, L.; Fall, S.; Chochos, C. L.; Anokhin, D. V.; Ivanov, D. A.; Leclerc, .;
L v ue, P.; Heiser, T. Macromolecules 2010, 43, 9779–9786.
(30) Li, Z.; Zhang, Y.; Tsang, S.-W.; Du, X.; Zhou, J.; Tao, Y.; Ding, J. J. Phys.
Chem. C 2011, 115, 18002–18009.
(31) Li, W.; Qin, R.; Zhou, Y.; Andersson, M.; Li, F.; Zhang, C.; Li, B.; Liu, Z.; Bo,
Z.; Zhang, F. Polymer 2010, 51, 3031–3038.
176
(32) Yang, L.; Zhou, H.; You, W. J. Phys. Chem. C 2010, 114, 16793–16800.
(33) Perez, M. D.; Borek, C.; Forrest, S. R.; Thompson, M. E. J. Am. Chem. Soc.
2009, 131, 9281–9286.
(34) oo, C. H.; Thompson, B. C.; im, B. J.; Toney, . F.; Fr chet, J. . J. J. Am.
Chem. Soc. 2008, 130, 16324–16329.
(35) Sivula, K.; Luscombe, C. K.; Thompson, B. C.; Fréchet, J. M. J. J. Am. Chem.
Soc. 2006, 128, 13988–13989.
(36) Somanathan, N.; Radhakrishnan, S.; Thelakkat, M.; Schmiedt, H. W. Macromol.
Mater. Eng. 2002, 287, 236–242.
(37) Zhang, Y.; Tajima, K.; Hashimoto, K. Macromolecules 2009, 42, 7008–7015.
(38) Zhang, Y.; Tajima, K.; Hirota, K.; Hashimoto, K. J. Am. Chem. Soc. 2008, 130,
7812–7813.
(39) Zhang, Y.; Tajima, K.; Hashimoto, K. Synth. Met. 2011, 161, 225–228.
(40) Ho, V.; Boudouris, B. W.; Segalman, R. A. Macromolecules 2010, 43, 7895–
7899.
(41) Burkhart, B.; Khlyabich, P. P.; Cakir Canak, T.; LaJoie, T. W.; Thompson, B. C.
Macromolecules 2011, 44, 1242–1246.
(42) Khlyabich, P. P.; Burkhart, B.; Ng, C. F.; Thompson, B. C. Macromolecules
2011, 44, 5079–5084.
(43) Boudouris, B. W.; Ho, V.; Jimison, L. H.; Toney, M. F.; Salleo, A.; Segalman, R.
A. Macromolecules 2011, 44, 6653–6658.
(44) Thompson, B. C.; Kim, Y.-G.; Reynolds, J. R. Macromolecules 2005, 38, 5359–
5362.
(45) Holcombe, T. .; orton, J. .; Rivnay, J.; oo, C. H.; oris, L.; Piliego, C.;
riffini, .; Sellinger, A.; Br das, J. -L.; Salleo, A.; Fr chet, J. . J. J. Am. Chem.
Soc. 2011, 133, 12106–12114.

177
CHAPTER 8 Conclusions and Outlook
8.1 Semi-Random Copolymers: Summary and Future Directions
A new class of D/A copolymers called semi-random polymers, consisting mainly
of rr-P3HT with small amounts (between 5% and 17.5%) of acceptor monomer randomly
distributed in the backbone, was introduced. Similar to completely random copolymers
the randomized sequence distribution of monomers in semi-random copolymers
generates broadly absorbing, multichromophoric polymers. Importantly though, the semi-
random polymers retain a larger degree of structural order than random copolymers due
to regiospecific placement of the reactive functional groups resulting in a restricted
linkage pattern of the monomers in the polymer backbone which inherently avoids
sterically unfavorable linkages. Not only do the resulting polymers in almost all cases
show very broad and intense absorption profiles, but due to the small amounts of acceptor
moieties they also retain many attractive properties of rr-P3HT such as semicrystallinity,
high hole mobility on the order of P3HT and optimal blending with PC
61
BM at favorable
ratios. This is in contrast to many of the above mentioned random D/A copolymers
(section 1.4.3), which despite broadened absorption, often suffer from low hole mobilities
due to a disordered polymer structure as well as the necessity of unfavorable
polymer:fullerene ratios with high fullerene loadings.
After the initial proof of concept (using BTD or TP as the acceptors – chapter 2)
which resulted in polymers with very promising optical and electronic properties but only
moderate solar cell efficiencies, DPP was used for a second generation of efficient semi-
178
random copolymers (chapter 3).
1,2
The influence of the acceptor content was studied and
it was shown that P3HTT-DPP-10% with 10% DPP strikes the best balance between low
band gap and a broad and uniform absorption profile whereas for P3HTT-DPP-15% the
absorption at shorter wavelengths is already considerably reduced. All three polymers are
semicrystalline and have high hole mobilities with P3HTT-DPP-10% reaching a
maximum efficiency of 5.73% in a BHJ solar cell with PC
61
BM (chapter 4).
3

Of special interest among the semi-random copolymers are those containing
multiple distinct acceptor monomers because they have especially intense and broad
UV/vis absorption profiles. A family of two-acceptor copolymers containing TPD and
DPP (chapter 4) was synthesized,
3
were the acceptor content was kept constant at 15%
but the ratio of acceptors was varied. Importantly, those semi-random two-acceptor
copolymers showed that the observed broad absorption profiles and high absorption
coefficients can translate into a strong and broad photocurrent response from 350 nm into
the NIR with EQE values of up to 40% at 800 nm for P3HTT-TPD-DPP(1:2).
Additionally it was shown that changing the acceptor ratio, even at such a small overall
acceptor content, allowed to fine tune optical and electronic properties of the polymers
resulting in optimized solar cell performance. Efficiencies of ~5% and currents of over 16
mA/cm
2
were achieved which are among the highest currents ever reported with PC
61
BM
as the acceptor. Further work on two-acceptor polymers (chapter 5) using additional
acceptor combinations showed that both the shape of the absorption profile as well as the
band gap of a novel two-acceptor copolymer can be predicted through a linear
combination of absorption profiles of the respective one-acceptor semi-random
179
copolymers. HOMO level energies on the other hand, are generally higher than the
lowest measured value for the “parent ” one -acceptor polymers and in some cases even
higher than values for both one-acceptor polymers. Additionally they are correlated with
the V
oc
of the corresponding BHJ solar cell in a linear fashion in almost all cases,
ultimately allowing the V
oc
of a solar cell to be predicted from the HOMO level of a
polymer.
Incorporation of the strong donor DTP into semi-random copolymers (chapter 6)
did not lead to the expected broadening of the absorption profile, likely due to the fact
that the strong donor and acceptor were not able to directly bond to each other.
4

Additionally, the polymers showed decreased η mostly due to low V
oc
and weak
photoresponse emphasizing that monomer combinations must be carefully selected in
order to retain the semicrystalline nature of P3HT and to maintain a HOMO energy
suitable for targeting reasonable values of V
oc
. Preliminary results of novel semi-random
polymers not described in this thesis, where the strong donor and acceptor can bond to
each other, show a considerable red-shift of the absorption profile of ~100 nm which
emphasizes the importance of monomer connectivity. These early results clearly show the
potential of semi-random copolymers incorporating a strong donor as NIR absorbing
active materials in polymer:fullerene solar cells and emphasize that these materials
should be pursued in the future.
In future work a combination of theoretical and spectroscopic approaches will
focus on elucidating the fundamental reasons behind the large variability in the solar cell
photoresponse of semi-random copolymers, even though they consistently have broad
180
and intense absorption profiles. Moreover a better understanding of factors influencing
the HOMO energy levels of semi-random copolymers will be an important focus. Work
in this area is essential in order to reach the ultimate goal – semi-random copolymers
which combine a large J
sc
(achieved through broad and intense absorption) with a high
V
oc
and overcome current efficiency limits of semi-random polymers.
A potential approach to increasing the V
oc
in semi-random copolymers using a
synthetic strategy is to replace small amounts of the hexyl-chains on thiophene with
branched 2-ethylhexyl side-chains. As shown in chapter 7 the V
oc
of rr-poly(3-
hexylthiophene-co-3-(2-ethylhexyl)thiophene):fullerene solar cells increases with
introduction of only small amounts of 2-ethylhexyl chains, with almost no change in all
other properties, which suggests that this is also a viable way of tuning the V
oc
in other
polymers.
5

Furthermore, it has not only been shown that the use of semi-random copolymers
in ternary blend solar cells can lead to an increased V
oc
compared to the corresponding
binary blend solar cell – thus solving one of the biggest remaining challenges of semi-
random copolymers – but also to an overall η increase compared to both limiting binary
blend systems.
6
The reported initial ternary model system of P3HTT-DPP-10%, P3HT
75
-
co-EHT
25
and PC
61
BM highlights not only the tremendous potential of ternary blend
BHJ solar cells but also that semi-random copolymers are promising candidates for these
devices due to their good miscibility with fullerenes and P3ATs.
In conclusion, semi-random copolymers are promising candidates for future
application in very efficient, solution processable BHJ solar cells. They can be made by
181
combining easily accessible and structurally simple monomers in a reliable, robust and
reproducible polymer synthesis which is important for future industrial applications. The
resulting polymers unite an unprecedented set of properties such as semicrystallinity,
high hole mobilities, broad absorption profiles and favourable mixing with fullerene
acceptors and have already shown efficiencies of close to 6%. I am convinced that in the
future a combination of theoretical, spectroscopic and synthetic efforts will allow the
synthesis of semi-random copolymers capable of outperforming current state-of-the-art
perfectly alternating D/A polymers.

8.2 General Thoughts and Conclusions on Polymer Solar Cells
Over the last decade tremendous progress has been made in the field of BHJ solar
cells, to the point that currently efficiencies of over 6% are published frequently whereas
even 4 years ago they were still relatively rare. Through careful fine tuning of electronic
and morphological properties V
oc
of over 1V, J
sc
of over 18 mA/cm
2
and FF of up to 0.75
has been achieved in BHJ cells with fullerene acceptors, although not with the same
donor material.
7–11
The current champion efficiency of 9.2% (J
sc
= 17.5 mA/cm2, V
oc
=
0.75 V and FF = 0.70) for BHJ solar cells was achieved with PTB7 (which is a perfectly
alternating D/A copolymer composed of thienothiophene and benzodithiophene, Figure
1.6) after very careful device optimization.
12
Even though reported efficiencies of over
9% are quickly approaching theoretically predicted limits of 10-12% and
commercialization of organic solar cells is currently under way, considerable challenges
182
remain for the synthetic chemist.
12–14
So far it seems that the ideal donor material, which
unites favorable optoelectronic properties such as a low band gap, broad absorption and
low HOMO level with optimal morphological properties, has not been found. Often very
high efficiencies are primarily achieved through extensive device optimization such as
solvent additives and interlayers and these more complicated solar cell architectures are
potentially difficult to realize on an industrial scale and also remove some of the
important advantages of solution processable BHJ solar cells such as easy and fast
fabrication as well as low cost.
10,12,15,16
Not only is the ideal donor material still elusive
but it has also been predicted that by increasing the dielectric constant of organic
materials efficiencies of over 20% can be achieved and so far few advances have been
made in this direction.
17
Additionally, areas such as efficient water soluble donor
materials for truly environmentally friendly organic solar cells as well as approaches to
increase the long-term stability of polymer:fullerene composites through cleavable or
crosslinkable polymers still have much room for improvement.
18,19
Finally, the
application of donor materials in ternary blend BHJ solar cells as well as tandem solar
cells also has the potential to lead to considerably increased efficiencies in the
future.
6,13,20,21

183

8.3 References Chapter 8
(1) Burkhart, B.; Khlyabich, P. P.; Cakir Canak, T.; LaJoie, T. W.; Thompson, B. C.
Macromolecules 2011, 44, 1242–1246.
(2) Khlyabich, P. P.; Burkhart, B.; Ng, C. F.; Thompson, B. C. Macromolecules
2011, 44, 5079–5084.
(3) Burkhart, B.; Khlyabich, P. P.; Thompson, B. C. ACS Macro Lett. 2012, 1, 660–
666.
(4) Burkhart, B.; Khlyabich, P. P.; Thompson, B. C. J. Photon. Energy 2012, 2,
021002.
(5) Burkhart, B.; Khlyabich, P. P.; Thompson, B. C. Macromolecules 2012, 45,
3740–3748.
(6) Khlyabich, P. P.; Burkhart, B.; Thompson, B. C. J. Am. Chem. Soc. 2012, 134,
9074–9077.
(7) Fei, Z.; Kim, J. S.; Smith, J.; Domingo, E. B.; Anthopoulos, T. D.; Stingelin, N.;
Watkins, S. E.; Kim, J.-S.; Heeney, M. J. Mater. Chem. 2011, 21, 16257–16263.
(8) Gadisa, A.; Mammo, W.; Andersson, L. M.; Admassie, S.; Zhang, F.; Andersson,
M. R.; Inganäs, O. Adv. Funct. Mater. 2007, 17, 3836–3842.
(9) Guo, X.; Cui, C.; Zhang, M.; Huo, L.; Huang, Y.; Hou, J.; Li, Y. Energy Environ.
Sci. 2012, 5, 7943–7949.
(10) Li, X.; Choy, W. C. H.; Huo, L.; Xie, F.; Sha, W. E. I.; Ding, B.; Guo, X.; Li, Y.;
Hou, J.; You, J.; Yang, Y. Adv. Mater. 2012, 24, 3046–3052.
(11) Najari, A.; Berrouard, P.; Ottone, C.; Boivin, M.; Zou, Y.; Gendron, D.; Caron,
W.-O.; Legros, P.; Allen, C. N.; Sadki, S.; Leclerc, M. Macromolecules 2012, 45,
1833–1838.
(12) He, Z.; Zhong, C.; Su, S.; Xu, M.; Wu, H.; Cao, Y. Nat. Photonics 2012, 6, 593–
597.
(13) Kotlarski, J. D.; Blom, P. W. M. Appl. Phys. Lett. 2011, 98, 053301.
184
(14) Scharber, M. C.; Mühlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A.
J.; Brabec, C. J. Adv. Mater. 2006, 18, 789–794.
(15) He, Z.; Zhong, C.; Huang, X.; Wong, W.-Y.; Wu, H.; Chen, L.; Su, S.; Cao, Y.
Adv. Mater. 2011, 23, 4636–4643.
(16) Small, C. E.; Chen, S.; Subbiah, J.; Amb, C. M.; Tsang, S.-W.; Lai, T.-H.;
Reynolds, J. R.; So, F. Nat. Photonics 2011, 6, 115–120.
(17) Koster, L. J. A.; Shaheen, S. E.; Hummelen, J. C. Adv. Energy Mater. 2012,
ASAP.
(18) Helgesen, M.; Søndergaard, R.; Krebs, F. C. J. Mater. Chem. 2010, 20, 36–60.
(19) Søndergaard, R.; Helgesen, M.; Jørgensen, M.; Krebs, F. C. Adv. Energy Mater.
2011, 1, 68–71.
(20) Ameri, T.; Min, J.; Li, N.; Machui, F.; Baran, D.; Forster, M.; Schottler, K. J.;
Dolfen, D.; Scherf, U.; Brabec, C. J. Adv. Energy Mater. 2012, ASAP.
(21) Yang, L.; Zhou, H.; Price, S. C.; You, W. J. Am. Chem. Soc. 2012, 134, 5432–
5435.

185
BIBLIOGRAPHY
Agostinelli, T., Lilliu, S., Labram, J. G., Campoy-Quiles, M., Hampton, M., Pires, E.,
Rawle, J., Bikondoa, O., Bradley, D. D. C., Anthopoulos, T. D., Nelson, J., and
Macdonald, J. E. (2011) Real-Time Investigation of Crystallization and Phase-
Segregation Dynamics in P3HT:PCBM Solar Cells During Thermal Annealing,
Adv. Funct. Mater., 21, pp. 1701–1708.
Ai, X., Beard, M. C., Knutsen, K. P., Shaheen, S. E., Rumbles, G., and Ellingson, R. J.
(2006) Photoinduced Charge Carrier Generation in a Poly(3-hexylthiophene) and
Methanofullerene Bulk Heterojunction Investigated by Time-Resolved Terahertz
Spectroscopy, J. Phys. Chem. B, 110, pp. 25462–25471.
Aïch, B. R., Lu, J., Beaupré, S., Leclerc, M., and Tao, Y. (2012) Control of the active
layer nanomorphology by using co-additives towards high-performance bulk
heterojunction solar cells, Org. Electron., 13, pp. 1736–1741.
Alibrahim, M., Roth, H., Schroedner, M., Konkin, A., Zhokhavets, U., Gobsch, G.,
Scharff, P., and Sensfuss, S. (2005) The influence of the optoelectronic properties
of poly(3-alkylthiophenes) on the device parameters in flexible polymer solar
cells, Org. Electron., 6, pp. 65–77.
Allard, ., A ch, R. B., endron, D., Boudreault, P. -L. T., Tessier, C., Alem, S., Tse, S.-
C., Tao, Y., and Leclerc, M. (2010) Germafluorenes: New Heterocycles for
Plastic Electronics, Macromolecules, 43, pp. 2328–2333.
Amb, C. M., Chen, S., Graham, K. R., Subbiah, J., Small, C. E., So, F., and Reynolds, J.
R. (2011) Dithienogermole As a Fused Electron Donor in Bulk Heterojunction
Solar Cells, J. Am. Chem. Soc., 133, pp. 10062–10065.
Ameri, T., Min, J., Li, N., Machui, F., Baran, D., Forster, M., Schottler, K. J., Dolfen, D.,
Scherf, U., and Brabec, C. J. (2012) Performance Enhancement of the
P3HT/PCBM Solar Cells through NIR Sensitization Using a Small-Bandgap
Polymer, Adv. Energy Mater., ASAP.
Ashraf, R. S., Gilot, J., and Janssen, R. A. J. (2010) Fused ring thiophene-based
poly(heteroarylene ethynylene)s for organic solar cells, Sol. Energy Mater. Sol.
Cells, 94, pp. 1759–1766.
Avrutin, V., Izyumskaya, N., and Morkoç, H. (2011) Semiconductor solar cells: Recent
progress in terrestrial applications, Superlattices Microstruct., 49, pp. 337–364.
186
Babudri, F., Farinola, G. M., and Naso, F. (2004) Synthesis of conjugated oligomers and
polymers: the organometallic way, J. Mater. Chem., 14, pp. 11–34.
van Bavel, S., Sourty, E., de With, G., Frolic, K., and Loos, J. (2009) Relation between
Photoactive Layer Thickness, 3D Morphology, and Device Performance in
P3HT/PCBM Bulk-Heterojunction Solar Cells, Macromolecules, 42, pp. 7396–
7403.
Beaujuge, P. M., Amb, C. M., and Reynolds, J. R. (2010) Spectral engineering in π -
conjugated polymers with intramolecular donor−acceptor interactions, Acc.
Chem. Res., 43, pp. 1396–1407.
Beaujuge, P. M., Ellinger, S., and Reynolds, J. R. (2008) The donor–acceptor approach
allows a black-to-transmissive switching polymeric electrochrome, Nat. Mater.,
7, pp. 795–799.
Beaujuge, P. M., and Fréchet, J. M. J. (2011) Molecular Design and Ordering Effects in
π-Functional Materials for Transistor and Solar Cell Applications, J. Am. Chem.
Soc., 133, pp. 20009–20029.
Bian, L., Zhu, E., Tang, J., Tang, W., and Zhang, F. (2012) Recent progress in the design
of narrow bandgap conjugated polymers for high-efficiency organic solar cells,
Prog. Polym. Sci., 37, pp. 1292–1331.
Bijleveld, J. C., Gevaerts, V. S., Di Nuzzo, D., Turbiez, M., Mathijssen, S. G. J., de
Leeuw, D. M., Wienk, M. M., and Janssen, R. A. J. (2010) Efficient Solar Cells
Based on an Easily Accessible Diketopyrrolopyrrole Polymer, Adv. Mater., 22,
pp. E242–E246.
Bijleveld, J. C., Karsten, B. P., Mathijssen, S. G. J., Wienk, M. M., de Leeuw, D. M., and
Janssen, R. A. J. (2011) Small band gap copolymers based on furan and
diketopyrrolopyrrole for field-effect transistors and photovoltaic cells, J. Mater.
Chem., 21, pp. 1600.
Bijleveld, J. C.; Shahid, M.; Gilot, J.; Wienk, M. M.; and Janssen, R. A. J. (2009)
Copolymers of Cyclopentadithiophene and Electron-Deficient Aromatic Units
Designed for Photovoltaic Applications, Adv. Funct. Mater., 19, pp. 3262–3270.
Bijleveld, J. C., Zoombelt, A. P., Mathijssen, S. G. J., Wienk, M. M., Turbiez, M., de
Leeuw, D. M., and Janssen, R. A. J. (2009) Poly(diketopyrrolopyrrole-
terthiophene) for Ambipolar Logic and Photovoltaics, J. Am. Chem. Soc., 131, pp.
16616–16617.

187
Biniek, L., Fall, S., Chochos, C. L., Anokhin, D. V., Ivanov, D. A., Leclerc, ., L v ue,
P., and Heiser, T. (2010) Impact of the Alkyl Side Chains on the Optoelectronic
Properties of a Series of Photovoltaic Low-Band-Gap Copolymers,
Macromolecules, 43, pp. 9779–9786.
Bloking, J. T., Han, X., Higgs, A. T., Kastrop, J. P., Pandey, L., Norton, J. E., Risko, C.,
Chen, C. E., Brédas, J.-L., McGehee, M. D., and Sellinger, A. (2011) Solution-
Processed Organic Solar Cells with Power Conversion Efficiencies of 2.5% using
Benzothiadiazole/Imide-Based Acceptors, Chem. Mater., 23, pp. 5484–5490.
Blom, P. W. M., Mihailetchi, V. D., Koster, L. J. A., and Markov, D. E. (2007) Device
Physics of Polymer:Fullerene Bulk Heterojunction Solar Cells, Adv. Mater., 19,
pp. 1551–1566.
Blouin, N., Michaud, A., and Leclerc, M. (2007) A Low-Bandgap Poly(2,7-Carbazole)
Derivative for Use in High-Performance Solar Cells, Adv. Mater., 19, pp. 2295–
2300.
Boudouris, B. W., Ho, V., Jimison, L. H., Toney, M. F., Salleo, A., and Segalman, R. A.
(2011) Real-Time Observation of Poly(3-alkylthiophene) Crystallization and
Correlation with Transient Optoelectronic Properties, Macromolecules, 44, pp.
6653–6658.
Boudreault, P.-L. T., Najari, A., and Leclerc, M. (2011) Processable Low-Bandgap
Polymers for Photovoltaic Applications, Chem. Mater., 23, pp. 456–469.
Brabec, C. J., Heeney, M., McCulloch, I., and Nelson, J. (2011) Influence of blend
microstructure on bulk heterojunction organic photovoltaic performance, Chem.
Soc. Rev., 40, pp. 1185–1199.
Brabec, C. J., Zerza, G., Cerullo, G., De Silvestri, S., Luzzati, S., Hummelen, J. C., and
Sariciftci, S. (2001) Tracing photoinduced electron transfer process in
conjugated polymer/fullerene bulk heterojunctions in real time, Chem. Phys.
Lett., 340, pp. 232–236.
Bronstein, H., Chen, Z., Ashraf, R. S., Zhang, W., Du, J., Durrant, J. R., Shakya
Tuladhar, P., Song, K., Watkins, S. E., Geerts, Y., Wienk, M. M., Janssen, R. A.
J., Anthopoulos, T., Sirringhaus, H., Heeney, M., and McCulloch, I. (2011)
Thieno[3,2-b]thiophene−Diketopyrrolopyrrole-Containing Polymers for High-
Performance Organic Field-Effect Transistors and Organic Photovoltaic Devices,
J. Am. Chem. Soc., 133, pp. 3272–3275.
Bundgaard, E., and Krebs, F. (2007) Low band gap polymers for organic photovoltaics,
Sol. Energy Mater. Sol. Cells, 91, pp. 954–985.
188
Burkhart, B., Khlyabich, P. P., Cakir Canak, T., LaJoie, T. W., and Thompson, B. C.
(2011) “Semi -Random” ultichromophoric rr-P3HT Analogues for Solar Photon
Harvesting, Macromolecules, 44, pp. 1242–1246.
Burkhart, B., Khlyabich, P. P., and Thompson, B. C. (2012) Influence of the Acceptor
Composition on Physical Properties and Solar Cell Performance in Semi-Random
Two-Acceptor Copolymers, ACS Macro Lett., 1, pp. 660–666.
Burkhart, B., Khlyabich, P. P., and Thompson, B. C. (2012) Influence of the Ethylhexyl
Side-Chain Content on the Open-Circuit Voltage in rr-Poly(3-hexylthiophene-co-
3-(2-ethylhexyl)thiophene) Copolymers, Macromolecules, 45, pp. 3740–3748.
Burkhart, B., Khlyabich, P. P., and Thompson, B. C. (2012) Solar cells based on semi-
random P3HT analogues containing dithienopyrrole: influence of incorporating a
strong donor, J. Photon. Energy, 2, pp. 021002.
Camaioni, N., Tinti, F., Franco, L., Fabris, M., Toffoletti, A., Ruzzi, M., Montanari, L.,
Bonoldi, L., Pellegrino, A., Calabrese, A., and Po, R. (2012) Effect of residual
catalyst on solar cells made of a fluorene-thiophene-benzothiadiazole copolymer
as electron-donor: A combined electrical and photophysical study, Org. Electron.,
13, pp. 550–559.
Cardona, C. M., Li, W., Kaifer, A. E., Stockdale, D., and Bazan, G. C. (2011)
Electrochemical considerations for determining absolute frontier orbital energy
levels of conjugated polymers for solar cell applications, Adv. Mater., 23, pp.
2367–2371.
Carsten, B., He, F., Son, H. J., Xu, T., and Yu, L. (2011) Stille Polycondensation for
Synthesis of Functional Materials, Chem. Rev., 111, pp. 1493–1528.
Chang, J.-F., Clark, J., Zhao, N., Sirringhaus, H., Breiby, D., Andreasen, J., Nielsen, M.,
Giles, M., Heeney, M., and McCulloch, I. (2006) Molecular-weight dependence
of interchain polaron delocalization and exciton bandwidth in high-mobility
conjugated polymers, Phys. Rev. B, 74, pp. 115318.
Chen, C.-H., Cheng, Y.-J., Chang, C.-Y., and Hsu, C.-S. (2011) Donor–Acceptor
Random Copolymers Based on a Ladder-Type Nonacyclic Unit: Synthesis,
Characterization, and Photovoltaic Applications, Macromolecules, 44, pp. 8415–
8424.
Chen, C.-H., Hsieh, C.-H., Dubosc, M., Cheng, Y.-J., and Hsu, C.-S. (2010) Synthesis
and Characterization of Bridged Bithiophene-Based Conjugated Polymers for
Photovoltaic Applications: Acceptor Strength and Ternary Blends,
Macromolecules, 43, pp. 697–708.
189
Chen, C.-P., Chan, S.-H., Chao, T.-C., Ting, C., and Ko, B.-T. (2008) Low-Bandgap
Poly(Thiophene-Phenylene-Thiophene) Derivatives with Broaden Absorption
Spectra for Use in High-Performance Bulk-Heterojunction Polymer Solar Cells,
J. Am. Chem. Soc., 130, pp. 12828–12833.
Chen, H.-Y., Hou, J., Hayden, A. E., Yang, H., Houk, K. N., and Yang, Y. (2010) Silicon
Atom Substitution Enhances Interchain Packing in a Thiophene-Based Polymer
System, Adv. Mater., 22, pp. 371–375.
Chen, H.-Y., Hou, J., Zhang, S., Liang, Y., Yang, G., Yang, Y., Yu, L., Wu, Y., and Li,
G. (2009) Polymer solar cells with enhanced open-circuit voltage and efficiency,
Nat. Photonics, 3, pp. 649–653.
Chen, M.-H., Hou, J., Hong, Z., Yang, G., Sista, S., Chen, L.-M., and Yang, Y. (2009)
Efficient Polymer Solar Cells with Thin Active Layers Based on Alternating
Polyfluorene Copolymer/Fullerene Bulk Heterojunctions, Adv. Mater., 21, pp.
4238–4242.
Chen, Y.-C., Yu, C.-Y., Fan, Y.-L., Hung, L.-I., Chen, C.-P., and Ting, C. (2010) Low-
bandgap conjugated polymer for high efficient photovoltaic applications, Chem.
Commun., 46, pp. 6503–6505.
Cheng, Y.-J., and Luh, T.-Y. (2004) Synthesizing optoelectronic heteroaromatic
conjugated polymers by cross-coupling reactions, J. Organomet. Chem., 689, pp.
4137–4148.
Chochos, C. L., and Choulis, S. A. (2011) How the structural deviations on the backbone
of conjugated polymers influence their optoelectronic properties and photovoltaic
performance, Prog. Polym. Sci., 36, pp. 1326–1414.
Chu, T. - ., Lu, J., Beaupr , S., hang, ., Pouliot, J. -R., Wakim, S., Zhou, J., Leclerc,
M., Li, Z., Ding, J., and Tao, Y. (2011) Bulk Heterojunction Solar Cells Using
Thieno[3,4- c]pyrrole-4,6-dione and Dithieno[3,2-b:2′,3′-d]silole Copolymer with
a Power Conversion Efficiency of 7.3%, J. Am. Chem. Soc., 133, pp. 4250–4253.
Chu, T.-Y., Lu, J., Beaupré, S., Zhang, Y., Pouliot, J.-R., Zhou, J., Najari, A., Leclerc,
M., and Tao, Y. (2012) Effects of the Molecular Weight and the Side-Chain
Length on the Photovoltaic Performance of Dithienosilole/Thienopyrrolodione
Copolymers, Adv. Funct. Mater., 22, pp. 2345–2351.
Chu, T.-Y., Tsang, S.-W., Zhou, J., Verly, P. G., Lu, J., Beaupré, S., Leclerc, M., and
Tao, Y. (2012) High-efficiency inverted solar cells based on a low bandgap
polymer with excellent air stability, Sol. Energy Mater. Sol. Cells, 96, pp. 155–
159.
190
Clarke, T. M., and Durrant, J. R. (2010) Charge Photogeneration in Organic Solar Cells,
Chem. Rev., 110, pp. 6736–6767.
Coakley, K. M., and McGehee, M. D. (2004) Conjugated Polymer Photovoltaic Cells,
Chem. Mater., 16, pp. 4533–4542.
Coffin, R. C., Peet, J., Rogers, J., and Bazan, G. C. (2009) Streamlined microwave-
assisted preparation of narrow-bandgap conjugated polymers for high-
performance bulk heterojunction solar cells, Nat. Chem., 1, pp. 657–661.
Dang, M. T., Hirsch, L., and Wantz, G. (2011) P3HT:PCBM, Best Seller in Polymer
Photovoltaic Research, Adv. Mater., 23, pp. 3597–3602.
Deibel, C., Strobel, T., and Dyakonov, V. (2010) Role of the Charge Transfer State in
Organic Donor-Acceptor Solar Cells, Adv. Mater., 22, pp. 4097–4111.
Dou, L., Gao, J., Richard, E., You, J., Chen, C.-C., Cha, K. C., He, Y., Li, G., and Yang,
Y. (2012) Systematic Investigation of Benzodithiophene- and Diketo-
pyrrolopyrrole-Based Low-Bandgap Polymers Designed for Single Junction and
Tandem Polymer Solar Cells, J. Am. Chem. Soc., 134, pp. 10071–10079.
Douglas, J. D., Griffini, G., Holcombe, T. W., Young, E. P., Lee, O. P., Chen, M. S., and
Fréchet, J. M. J. (2012) Functionalized Isothianaphthene Monomers That
Promote Quinoidal Character in Donor–Acceptor Copolymers for Organic
Photovoltaics, Macromolecules, 45, pp. 4069–4074.
Du, C., Li, C., Li, W., Chen, X., Bo, Z., Veit, C., Ma, Z., Wuerfel, U., Zhu, H., Hu, W.,
and Zhang, F. (2011) 9-Alkylidene-9H-Fluorene-Containing Polymer for High-
Efficiency Polymer Solar Cells, Macromolecules, 44, pp. 7617–7624.
Espinosa, N., Hösel, M., Angmo, D., and Krebs, F. C. (2012) Solar cells with one-day
energy payback for the factories of the future, Energy Environ. Sci., 5, pp. 5117-
5132.
Facchetti, A., Vaccaro, L., and Marrocchi, A. (2012) Semiconducting Polymers Prepared
by Direct Arylation Polycondensation, Angew. Chem. Int. Ed., 51, pp. 3520–
3523.
Fei, Z., Kim, J. S., Smith, J., Domingo, E. B., Anthopoulos, T. D., Stingelin, N., Watkins,
S. E., Kim, J.-S., and Heeney, M. (2011) A low band gap co-polymer of
dithienogermole and 2,1,3-benzothiadiazole by Suzuki polycondensation and its
application in transistor and photovoltaic cells, J. Mater. Chem., 21, pp. 16257–
16263.
191
Friedel, B., McNeill, C. R., and Greenham, N. C. (2010) Influence of Alkyl Side-Chain
Length on the Performance of Poly(3-alkylthiophene)/Polyfluorene All-Polymer
Solar Cells, Chem. Mater., 22, pp. 3389–3398.
Gadisa, A., Mammo, W., Andersson, L. M., Admassie, S., Zhang, F., Andersson, M. R.,
and Inganäs, O. (2007) A New Donor–Acceptor–Donor Polyfluorene Copolymer
with Balanced Electron and Hole Mobility, Adv. Funct. Mater., 17, pp. 3836–
3842.
Gadisa, A., Oosterbaan, W. D., Vandewal, K., Bolsée, J.-C., Bertho, S., D’Haen, J.,
Lutsen, L., Vanderzande, D., and Manca, J. V. (2009) Effect of Alkyl Side-Chain
Length on Photovoltaic Properties of Poly(3-alkylthiophene)/PCBM Bulk
Heterojunctions, Adv. Funct. Mater., 19, pp. 3300–3306.
García-Valverde, R., Cherni, J. A., and Urbina, A. (2010) Life cycle analysis of organic
photovoltaic technologies, Prog. Photovolt: Res. Appl., 18, pp. 535–558.
Gendron, D., and Leclerc, M. (2011) New conjugated polymers for plastic solar cells,
Energy Environ. Sci., 4, pp. 1225–1237.
Gevaerts, V. S., Furlan, A., Wienk, M. M., Turbiez, M., and Janssen, R. A. J. (2012)
Solution Processed Polymer Tandem Solar Cell Using Efficient Small and Wide
bandgap Polymer:Fullerene Blends, Adv. Mater., 24, pp. 2130–2134.
Gibson, G. L., McCormick, T. M., and Seferos, D. S. (2012) Atomistic Band Gap
Engineering in Donor–Acceptor Polymers, J. Am. Chem. Soc., 134, pp. 539–547.
Gronowitz, S. (1961) Halogen-metal interconversion with dibromobithienyls, Acta.
Chem. Scand., 15, pp. 1393–1395.
Günes, S., Neugebauer, H., and Sariciftci, N. S. (2007) Conjugated Polymer-Based
Organic Solar Cells, Chem. Rev., 107, pp. 1324–1338.
Guo, X., Cui, C., Zhang, M., Huo, L., Huang, Y., Hou, J., and Li, Y. (2012) High
efficiency polymer solar cells based on poly(3-hexylthiophene)/indene-C70
bisadduct with solvent additive, Energy Environ. Sci., 5, pp. 7943–7949.
Halls, J. J. M., Walsh, C. A., Greenham, N. C., Marseglia, E. A., Friend, R. H., Moratti,
S. C., and Holmes, A. B. (1995) Efficient photodiodes from interpenetrating
polymer networks, Nature, 376, pp. 498–500.
Hao, Z., and Iqbal, A. (1997) Some aspects of organic pigments, Chem. Soc. Rev., 26,
pp. 203–213.
192
Hauch, J. A., Schilinsky, P., Choulis, S. A., Childers, R., Biele, M., and Brabec, C. J.
(2008) Flexible organic P3HT:PCBM bulk-heterojunction modules with more
than 1 year outdoor lifetime, Sol. Energy Mater. Sol. Cells., 92, pp. 727–731.
Havinga, E. E., ten Hoeve, W., and Wynberg, H. (1992) A new class of small band gap
organic polymer conductors, Polym. Bull., pp. 119–126.
Havinga, E. E., ten Hoeve, W., and Wynberg, H. (1993) Alternate Donor-Acceptor
Small-Band-Gap Semiconducting Polymers; Polysquaraines and
Polycroconaines, Synth. Met., 55, pp. 299–306.
He, Y., Chen, H.-Y., Hou, J., and Li, Y. (2010) Indene−C60Bisadduct: A ew Acceptor
for High-Performance Polymer Solar Cells, J. Am. Chem. Soc., 132, pp. 1377–
1382.
He, Y., and Li, Y. (2011) Fullerene derivative acceptors for high performance polymer
solar cells, Phys. Chem. Chem. Phys., 13, pp. 1970–1983.
He, Y., Wang, X., Zhang, J., and Li, Y. (2009) Low Bandgap Polymers by
Copolymerization of Thiophene with Benzothiadiazole, Macromol. Rapid
Commun., 30, pp. 45–51.
He, Y., Zhao, G., Peng, B., and Li, Y. (2010) High-Yield Synthesis and Electrochemical
and Photovoltaic Properties of Indene-C70 Bisadduct, Adv. Funct. Mater., 20, pp.
3383–3389.
He, Z., Zhang, C., Xu, X., Zhang, L., Huang, L., Chen, J., Wu, H., and Cao, Y. (2011)
Largely Enhanced Efficiency with a PFN/Al Bilayer Cathode in High Efficiency
Bulk Heterojunction Photovoltaic Cells with a Low Bandgap Polycarbazole
Donor, Adv. Mater., 23, pp. 3086–3089.
He, Z., Zhong, C., Huang, X., Wong, W.-Y., Wu, H., Chen, L., Su, S., and Cao, Y.
(2011) Simultaneous Enhancement of Open-Circuit Voltage, Short-Circuit
Current Density, and Fill Factor in Polymer Solar Cells, Adv. Mater., 23, pp.
4636–4643.
He, Z., Zhong, C., Su, S., Xu, M., Wu, H., and Cao, Y. (2012) Enhanced power-
conversion efficiency in polymer solar cells using an inverted device structure,
Nat. Photonics.
Helgesen, M., Søndergaard, R., and Krebs, F. C. (2010) Advanced materials and
processes for polymer solar cell devices, J. Mater. Chem., 20, pp. 36–60.
193
Ho, V., Boudouris, B. W., and Segalman, R. A. (2010) Tuning Polythiophene
Crystallization through Systematic Side Chain Functionalization,
Macromolecules, 43, pp. 7895–7899.
Holcombe, T. W., Norton, J. E., Rivnay, J., Woo, C. H., oris, L., Piliego, C., riffini,
., Sellinger, A., Br das, J. -L., Salleo, A., and Fr chet, J. . J. (2011) Steric
Control of the Donor/Acceptor Interface: Implications in Organic Photovoltaic
Charge Generation, J. Am. Chem. Soc., 133, pp. 12106–12114.
Holcombe, T. ., oo, C. H., avulak, D. F. J., Thompson, B. C., and Fr chet, J. . J.
(2009) All-Polymer Photovoltaic Devices of Poly(3-(4-n-octyl)-phenylthiophene)
from Grignard Metathesis (GRIM) Polymerization, J. Am. Chem. Soc., 131, pp.
14160–14161.
Hong, S. Y., and Marynick, D. S. (1992) Understanding the Conformational Stability and
Electronic Structures of Modified Polymers Based on Polythiophene,
Macromolecules, 25, pp. 4652–4657.
Hou, J., Chen, H.-Y., Zhang, S., Chen, R. I., Yang, Y., Wu, Y., and Li, G. (2009)
Synthesis of a Low Band Gap Polymer and Its Application in Highly Efficient
Polymer Solar Cells, J. Am. Chem. Soc., 131, pp. 15586–15587.
Hou, J., Chen, H.-Y., Zhang, S., Li, G., and Yang, Y. (2008) Synthesis, Characterization,
and Photovoltaic Properties of a Low Band Gap Polymer Based on Silole-
Containing Polythiophenes and 2,1,3-Benzothiadiazole, J. Am. Chem. Soc., 130,
pp. 16144–16145.
Hou, J., Chen, T. L., Zhang, S., Huo, L., Sista, S., and Yang, Y. (2009) An Easy and
Effective Method To Modulate Molecular Energy Level of Poly(3-
alkylthiophene) for High-V
oc
Polymer Solar Cells, Macromolecules, 42, pp.
9217–9219.
Hoyos, M., L. Turner, M., and Navarro, O. (2011) Recent Advances in Polythiophene
Synthesis by Palladium-Catalyzed Cross-Coupling Reactions, Curr. Org. Chem.,
15, pp. 3263–3290.
Hu, X., Shi, M., Zuo, L., Nan, Y., Liu, Y., Fu, L., and Chen, H. (2011) Synthesis,
characterization, and photovoltaic property of a low band gap polymer alternating
dithienopyrrole and thienopyrroledione units, Polymer, 52, pp. 2559–2564.
Huang, Y., Guo, X., Liu, F., Huo, L., Chen, Y., Russell, T. P., Han, C. C., Li, Y., and
Hou, J. (2012) Improving the Ordering and Photovoltaic Properties by Extending
π-Conjugated Area of Electron-Donating Units in Polymers with D-A Structure,
Adv. Mater., 24, pp. 3383–3389.
194
Huang, Y., Huo, L., Zhang, S., Guo, X., Han, C. C., Li, Y., and Hou, J. (2011) Sulfonyl:
a new application of electron-withdrawing substituent in highly efficient
photovoltaic polymer, Chem. Commun., 47, pp. 8904–8906.
Hummelen, J. C., Knight, B. W., LePeq, F., and Fred, W. (1995) Preparation and
Characterization of Fulleroid and Methanofullerene Derivatives, J. Org. Chem.,
60, pp. 532–538.
Huo, L., Hou, J., Chen, H.-Y., Zhang, S., Jiang, Y., Chen, T. L., and Yang, Y. (2009)
Bandgap and Molecular Level Control of the Low-Bandgap Polymers Based on
3,6-Dithiophen-2-yl-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione toward Highly
Efficient Polymer Solar Cells, Macromolecules, 42, pp. 6564–6571.
Huo, L., Hou, J., Zhang, S., Chen, H.-Y., and Yang, Y. (2010) A Polybenzo[1,2-b:4,5-
b′]dithiophene Derivative with Deep HO O Level and Its Application in High -
Performance Polymer Solar Cells, Angew. Chem. Int. Ed., 49, pp. 1500–1503.
Huo, L., Zhang, S., Guo, X., Xu, F., Li, Y., and Hou, J. (2011) Replacing Alkoxy Groups
with Alkylthienyl Groups: A Feasible Approach To Improve the Properties of
Photovoltaic Polymers, Angew. Chem. Int. Ed., 50, pp. 9697–9702.
International Energy Agency, and Organisation for Economic Co-operation and
Development. (2011) World energy outlook 2011. IEA, International Energy
Agency : O CD, Paris.
Jäger-Waldau, A. (2011) Photovoltaics: Status and Perspectives until 2020, Green, 1, pp.
277–290.
Jo, J., Gendron, D., Najari, A., Moon, J. S., Cho, S., Leclerc, M., and Heeger, A. J.
(2010) Bulk heterojunction solar cells based on a low-bandgap carbazole-
diketopyrrolopyrrole copolymer, Appl. Phys. Lett., 97, pp. 203303.
Jo, J., Pron, A., Berrouard, P., Leong, W. L., Yuen, J. D., Moon, J. S., Leclerc, M., and
Heeger, A. J. (2012) A New Terthiophene-Thienopyrrolodione Copolymer-Based
Bulk Heterojunction Solar Cell with High Open-Circuit Voltage, Adv. Energy
Mater., pp. ASAP.
Jørgensen, M., Norrman, K., and Krebs, F. C. (2008) Stability/degradation of polymer
solar cells, Sol. Energy Mater. Sol. Cells, 92, pp. 686–714.
Jung, J. W., Liu, F., Russell, T. P., and Jo, W. H. (2012) A high mobility conjugated
polymer based on dithienothiophene and diketopyrrolopyrrole for organic
photovoltaics, Energy Environ. Sci., 5, pp. 6857–6861.
195
Kenning, D. D., Mitchell, K. A., Calhoun, T. R., Funfar, M. R., Sattler, D. J., and
Rasmussen, S. C. (2002) Thieno[3,4-b]pyrazines: Synthesis, Structure, and
Reactivity, J. Org. Chem., 67, pp. 9073–9076.
Khlyabich, P. P., Burkhart, B., Ng, C. F., and Thompson, B. C. (2011) Efficient solar
cells from semi-random P3HT analogues incorporating diketopyrrolopyrrole,
Macromolecules, 44, pp. 5079–5084.
Khlyabich, P. P., Burkhart, B., and Thompson, B. C. (2012) Compositional Dependence
of the Open-Circuit Voltage in Ternary Blend Bulk Heterojunction Solar Cells
Based on Two Donor Polymers, J. Am. Chem. Soc., 134, pp. 9074–9077.
Kim, B.-G., Jeong, E. J., Park, H. J., Bilby, D., Guo, L. J., and Kim, J. (2011) Effect of
Polymer Aggregation on the Open Circuit Voltage in Organic Photovoltaic Cells:
Aggregation-Induced Conjugated Polymer Gel and its Application for Preventing
Open Circuit Voltage Drop, ACS Appl. Mater. Interfaces, 3, pp. 674–680.
Kim, M.-S., Kim, B.-G., and Kim, J. (2009) Effective Variables To Control the Fill
Factor of Organic Photovoltaic Cells, ACS Appl. Mater. Interfaces, 1, pp. 1264–
1269.
Kim, Y., Cook, S., Tuladhar, S. M., Choulis, S. A., Nelson, J., Durrant, J. R., Bradley, D.
D. C., Giles, M., McCulloch, I., Ha, C.-S., and Ree, M. (2006) A strong
regioregularity effect in self-organizing conjugated polymer films and high-
efficiency polythiophene:fullerene solar cells, Nat. Mater., 5, pp. 197–203.
Kline, R. J., DeLongchamp, D. M., Fischer, D. A., Lin, E. K., Richter, L. J., Chabinyc,
M. L., Toney, M. F., Heeney, M., and McCulloch, I. (2007) Critical Role of Side-
Chain Attachment Density on the Order and Device Performance of
Polythiophenes, Macromolecules, 40, pp. 7960–7965.
Kline, R. J., McGehee, M. D., Kadnikova, E. N., Liu, J., Fréchet, J. M. J., and Toney, M.
F. (2005) Dependence of Regioregular Poly(3-hexylthiophene) Film Morphology
and Field-Effect Mobility on Molecular Weight, Macromolecules, 38, pp. 3312–
3319.
Ko, S., Hoke, E. T., Pandey, L., Hong, S., Mondal, R., Risko, C., Yi, Y., Noriega, R.,
McGehee, M. D., Brédas, J.-L., Salleo, A., and Bao, Z. (2012) Controlled
Conjugated Backbone Twisting for an Increased Open-Circuit Voltage while
Having a High Short-Circuit Current in Poly(hexylthiophene) Derivatives, J. Am.
Chem. Soc., 134, pp. 5222–5232.

196
Ko, S., Verploegen, E., Hong, S., Mondal, R., Hoke, E. T., Toney, M. F., McGehee, M.
D., and Bao, Z. (2011) 3,4-Disubstituted Polyalkylthiophenes for High-
Performance Thin-Film Transistors and Photovoltaics, J. Am. Chem. Soc., 133,
pp. 16722–16725.
Koeckelberghs, G., De Cremer, L., Persoons, A., and Verbiest, T. (2007) Influence of
the substituent and polymerization methodology on the properties of chiral
poly(dithieno[3,2-b:2‘,3‘-d]pyrrole)s, Macromolecules, 40, pp. 4173–4181.
Koeckelberghs, G., De Cremer, L., Vanormelingen, W., Verbiest, T., Persoons, A., and
Samyn, C. (2005) Synthesis and properties of polydithieno[3,2-b :2‘,3‘-
d]pyrroles: a class of soluble (chiral) conjugated polymers with a stable oxidized
state, Macromolecules, 38, pp. 4545–4547.
Koster, L. J. A., Mihailetchi, V. D., and Blom, P. W. M. (2006) Bimolecular
recombination in polymer/fullerene bulk heterojunction solar cells, Appl. Phys.
Lett., 88, pp. 052104.
Koster, L. J. A., Shaheen, S. E., and Hummelen, J. C. (2012) Pathways to a New
Efficiency Regime for Organic Solar Cells, Adv. Energy Mater., ASAP.
Kotlarski, J. D., and Blom, P. W. M. (2011) Ultimate performance of polymer:fullerene
bulk heterojunction tandem solar cells, Appl. Phys. Lett., 98, pp. 053301.
Kotlarski, J. D., Moet, D. J. D., and Blom, P. W. M. (2011) Role of balanced charge
carrier transport in low band gap polymer:Fullerene bulk heterojunction solar
cells, J. Polym. Sci. B Polym. Phys., 49, pp. 708–711.
Kowalski, S.; Allard, S.; Scherf, U. (2012) Synthesis of Poly(4,4-dialkyl-cyclopenta[2,1-
b:3,4-b′]dithiophene-alt-2,1,3-benzothiadiazole) (PCPDTBT) in a Direct
Arylation SchemeACS Macro Lett., 1, pp. 465–468.
Kraabel, B., Hummelen, J. C., Vacar, D., Moses, D., Sariciftci, N. S., Heeger, A. J., and
Wudl, F. (1996) Subpicosecond photoinduced electron transfer from conjugated
polymers to functionalized fullerenes, J. Chem. Phys., 104, pp. 4267–4273.
Krebs, F. C. (2009) Fabrication and processing of polymer solar cells: A review of
printing and coating techniques, Sol. Energy Mater. Sol. Cells, 93, pp. 394–412.
Kroon, R., Lenes, M., Hummelen, J., Blom, P., and de Boer, B. (2008) Small bandgap
polymers for organic solar cells (polymer material development in the last 5
years), Polymer Revs., 48, pp. 531–582.

197
Ku, S.-Y., Liman, C. D., Burke, D. J., Treat, N. D., Cochran, J. E., Amir, E., Perez, L. A.,
Chabinyc, M. L., and Hawker, C. J. (2011) A Facile Synthesis of Low-Band-Gap
Donor–Acceptor Copolymers Based on Dithieno[3,2-b:2′,3′-d
]thiophene, Macromolecules, 44, pp. 9533–9538.
Kumar, P., and Chand, S. (2012) Recent progress and future aspects of organic solar
cells, Prog. Photovolt: Res. Appl., 20, pp. 377–415.
Li, C.-Z., Chueh, C.-C., Yip, H.-L., O’ alley, . ., Chen, . -C., and Jen, A. K.-Y.
(2012) Effective interfacial layer to enhance efficiency of polymer solar cells via
solution-processed fullerene-surfactants, J. Mater. Chem., 22, pp. 8574–8578.
Li, C.-Z., Chueh, C.-C., Yip, H.-L., Zou, J., Chen, W.-C., and Jen, A. K.-Y. (2012)
Evaluation of structure–property relationships of solution-processible fullerene
acceptors and their n-channel field-effect transistor performance, J. Mater.
Chem., 22, pp. 14976–14981.
Li, G., Zhu, R., and Yang, Y. (2012) Polymer solar cells, Nat. Photonics, 6, pp. 153–
161.
Li, J., Ong, K.-H., Lim, S.-L., Ng, G.-M., Tan, H.-S., and Chen, Z.-K. (2011) A random
copolymer based on dithienothiophene and diketopyrrolopyrrole units for high
performance organic solar cells, Chem. Commun., 47, pp. 9480–9482.
Li, W., Qin, R., Zhou, Y., Andersson, M., Li, F., Zhang, C., Li, B., Liu, Z., Bo, Z., and
Zhang, F. (2010) Tailoring side chains of low band gap polymers for high
efficiency polymer solar cells, Polymer, 51, pp. 3031–3038.
Li, X., Choy, W. C. H., Huo, L., Xie, F., Sha, W. E. I., Ding, B., Guo, X., Li, Y., Hou, J.,
You, J., and Yang, Y. (2012) Dual Plasmonic Nanostructures for High
Performance Inverted Organic Solar Cells, Adv. Mater., 24, pp. 3046–3052.
Li, Y. (2012) Molecular Design of Photovoltaic Materials for Polymer Solar Cells:
Toward Suitable Electronic Energy Levels and Broad Absorption, Acc. Chem.
Res., 45, pp. 723–733.
Li, Z., Tsang, S.-W., Du, X., Scoles, L., Robertson, G., Zhang, Y., Toll, F., Tao, Y., Lu,
J., and Ding, J. (2011) Alternating Copolymers of Cyclopenta[2,1-b;3,4-
b′]dithiophene and Thieno[3,4 -c]pyrrole-4,6-dione for High-Performance
Polymer Solar Cells, Adv. Funct. Mater., 21, pp. 3331–3336.
Li, Z., Zhang, Y., Tsang, S.-W., Du, X., Zhou, J., Tao, Y., and Ding, J. (2011) Alkyl
Side Chain Impact on the Charge Transport and Photovoltaic Properties of
Benzodithiophene and Diketopyrrolopyrrole-Based Copolymers, J. Phys. Chem.
C, 115, pp. 18002–18009.
198
Liang, F., Lu, J., Ding, J., Movileanu, R., and Tao, Y. (2009) Design and Synthesis of
Alternating Regioregular Oligothiophenes/Benzothiadiazole Copolymers for
Organic Solar Cells, Macromolecules, 42, pp. 6107–6114.
Liang, Y., Feng, D., Wu, Y., Tsai, S.-T., Li, G., Ray, C., and Yu, L. (2009) Highly
Efficient Solar Cell Polymers Developed via Fine-Tuning of Structural and
Electronic Properties, J. Am. Chem. Soc., 131, pp. 7792–7799.
Liang, Y., Wu, Y., Feng, D., Tsai, S.-T., Son, H.-J., Li, G., and Yu, L. (2009)
Development of New Semiconducting Polymers for High Performance Solar
Cells, J. Am. Chem. Soc., 131, pp. 56–57.
Liang, Y., Xiao, S., Feng, D., and Yu, L. (2008) Control in Energy Levels of Conjugated
Polymers for Photovoltaic Application, J. Phys. Chem. C, 112, pp. 7866–7871.
Liang, Y., Xu, Z., Xia, J., Tsai, S.-T., Wu, Y., Li, G., Ray, C., and Yu, L. (2010) For the
Bright Future-Bulk Heterojunction Polymer Solar Cells with Power Conversion
Efficiency of 7.4%, Adv. Mater., 22, pp. E135–E138.
Lindgren, L. J., hang, F., Andersson, ., Barrau, S., Hellstr m, S., ammo, .,
Per on, ., Ingan s, O., and Andersson, . R. (2009) Synthesis,
Characterization, and Devices of a Series of Alternating Copolymers for Solar
Cells, Chem. Mater., 21, pp. 3491–3502.
Liu, B., Zou, Y., Peng, B., Zhao, B., Huang, K., He, Y., and Pan, C. (2011) Low
bandgap isoindigo-based copolymers: design, synthesis and photovoltaic
applications, Polym. Chem., 2, pp. 1156–1162.
Liu, F., Gu, Y., Jung, J. W., Jo, W. H., and Russell, T. P. (2012) On the morphology of
polymer-based photovoltaics, J. Polym. Sci. B Polym. Phys., 50, pp. 1018–1040.
Liu, F., Gu, Y., Wang, C., Zhao, W., Chen, D., Briseno, A. L., and Russell, T. P. (2012)
Efficient Polymer Solar Cells Based on a Low Bandgap Semi-crystalline DPP
Polymer-PCBM Blends, Adv. Mater., 24, pp. 3947–3951.
Liu, J., Shao, S., Fang, G., Meng, B., Xie, Z., and Wang, L. (2012) High-Efficiency
Inverted Polymer Solar Cells with Transparent and Work-Function Tunable
MoO3-Al Composite Film as Cathode Buffer Layer, Adv. Mater., 24, pp. 2774–
2779.
Lizin, S., Van Passel, S., De Schepper, E., and Vranken, L. (2012) The future of organic
photovoltaic solar cells as a direct power source for consumer electronics, Sol.
Energy Mater. Sol. Cells, 103, pp. 1–10.
199
Lou, S. J., Szarko, J. M., Xu, T., Yu, L., Marks, T. J., and Chen, L. X. (2011) Effects of
Additives on the Morphology of Solution Phase Aggregates Formed by Active
Layer Components of High-Efficiency Organic Solar Cells, J. Am. Chem. Soc.,
133, pp. 20661–20663.
Lunt, R. R., Osedach, T. P., Brown, P. R., Rowehl, J. A., and Bulović, V. (2011)
Practical Roadmap and Limits to Nanostructured Photovoltaics, Adv. Mater., 23,
pp. 5712–5727.
Ma, W., Yang, C., Gong, X., Lee, K., and Heeger, A. J. (2005) Thermally stable,
efficient polymer solar cells with nanoscale control of the interpenetrating
network morphology, Adv. Funct. Mater., 15, pp. 1617–1622.
Ma, Z., Wang, E., Jarvid, M. E., Henriksson, P., Inganäs, O., Zhang, F., and Andersson,
M. R. (2012) Synthesis and characterization of benzodithiophene–isoindigo
polymers for solar cells, J. Mater. Chem., 22, pp. 2306–2314.
Marrocchi, A., Lanari, D., Facchetti, A., and Vaccaro, L. (2012) Poly(3-hexylthiophene):
synthetic methodologies and properties in bulk heterojunction solar cells, Energy
Environ. Sci., 5, pp. 8457–8474.
Mauer, R., Kastler, M., and Laquai, F. (2010) The Impact of Polymer Regioregularity on
Charge Transport and Efficiency of P3HT:PCBM Photovoltaic Devices, Adv.
Funct. Mater., 20, pp. 2085–2092.
aurano, A., Hamilton, R., Shuttle, C. ., Ballantyne, A. ., elson, J., O’Regan, B.,
Zhang, W., McCulloch, I., Azimi, H., Morana, M., Brabec, C. J., and Durrant, J.
R. (2010) Recombination Dynamics as a Key Determinant of Open Circuit
Voltage in Organic Bulk Heterojunction Solar Cells: A Comparison of Four
Different Donor Polymers, Adv. Mater., 22, pp. 4987–4992.
McNeill, C. R., and Greenham, N. C. (2009) Conjugated-Polymer Blends for Opto-
electronics, Adv. Mater., 21, pp. 3840–3850.
Mihailetchi, V., Wildeman, J., and Blom, P. (2005) Space-Charge Limited Photocurrent,
Phys. Rev. Lett., 94, pp. 126602.
Mishra, A.; and Bäuerle, P. (2012) Small Molecule Organic Semiconductors on the
Move: Promises for Future Solar Energy Technology, Angew. Chem. Int. Ed., 51,
pp. 2020–2067.
Mondal, R., Ko, S., Norton, J. E., Miyaki, N., Becerril, H. A., Verploegen, E., Toney, M.
F., Brédas, J.-L., McGehee, M. D., and Bao, Z. (2009) Molecular design for
improved photovoltaic efficiency: band gap and absorption coefficient
engineering, J. Mater. Chem., 19, pp. 7195–7197.
200
Mori, D., Benten, H., Ohkita, H., Ito, S., and Miyake, K. (2012) Polymer/Polymer Blend
Solar Cells Improved by Using High-Molecular-Weight Fluorene-Based
Copolymer as Electron Acceptor, ACS Appl. Mater. Interfaces, 4, pp. 3325–3329.
Mühlbacher, D., Scharber, M., Morana, M., Zhu, Z., Waller, D., Gaudiana, R., and
Brabec, C. (2006) High Photovoltaic Performance of a Low-Bandgap Polymer,
Adv. Mater., 18, pp. 2884–2889.
Murray, I. P., Lou, S. J., Cote, L. J., Loser, S., Kadleck, C. J., Xu, T., Szarko, J. M.,
Rolczynski, B. S., Johns, J. E., Huang, J., Yu, L., Chen, L. X., Marks, T. J., and
Hersam, M. C. (2011) Graphene Oxide Interlayers for Robust, High-Efficiency
Organic Photovoltaics, J. Phys. Chem. Lett., 2, pp. 3006–3012.
Najari, A., Berrouard, P., Ottone, C., Boivin, M., Zou, Y., Gendron, D., Caron, W.-O.,
Legros, P., Allen, C. N., Sadki, S., and Leclerc, M. (2012) High Open-Circuit
Voltage Solar Cells Based on New Thieno[3,4-c]pyrrole-4,6-dione and 2,7-
Carbazole Copolymers, Macromolecules, 45, pp. 1833–1838.
Nayak, K., and Marynick, D. S. (1990) The Interplay between Gemetrie and Electronic
Structures in Polyisothianaphthene, Polyisonaphthothiophene, Polythieno(3,4-
b)thienopyrazine, and Polythieno(3,4-b)quinoxaline, Macromolecules, 23, pp.
2237–2245.
Nayak, P. K., Garcia-Belmonte, G., Kahn, A., Bisquert, J., and Cahen, D. (2012)
Photovoltaic efficiency limits and material disorder, Energy Environ. Sci., 5, pp.
6022–6039.
Nguyen, L. H., Hoppe, H., Erb, T., Günes, S., Gobsch, G., and Sariciftci, N. S. (2007)
Effects of Annealing on the Nanomorphology and Performance of
Poly(alkylthiophene):Fullerene Bulk-Heterojunction Solar Cells, Adv. Funct.
Mater., 17, pp. 1071–1078.
Nie, W., MacNeill, C. M., Li, Y., Noftle, R. E., Carroll, D. L., and Coffin, R. C. (2011) A
Soluble High Molecular Weight Copolymer of Benzo[1,2-b:4,5-b′]dithiophene
and Benzoxadiazole for Efficient Organic Photovoltaics, Macromol. Rapid
Commun., 32, pp. 1163–1168.
ielsen, C. B., Ashraf, R. S., Schroeder, B. C., D’Angelo, P., atkins, S. ., Song, .,
Anthopoulos, T. D., and McCulloch, I. (2012) Random benzotrithiophene-based
donor–acceptor copolymers for efficient organic photovoltaic devices, Chem.
Commun., 48, pp. 5832–5834.
Ogawa, K., and Rasmussen, S. C. (2006) N-Functionalized poly(dithieno[3,2-b:2‘,3‘-
d]pyrrole)s: highly fluorescent materials with reduced band gaps,
Macromolecules, 39, pp. 1771–1778.
201
Okamoto, K., and Luscombe, C. K. (2011) Controlled polymerizations for the synthesis
of semiconducting conjugated polymers, Polym. Chem., 2, pp. 2424–2434.
Ong, K.-H., Lim, S.-L., Tan, H.-S., Wong, H.-K., Li, J., Ma, Z., Moh, L. C. H., Lim, S.-
H., de Mello, J. C., and Chen, Z.-K. (2011) A Versatile Low Bandgap Polymer
for Air-Stable, High-Mobility Field-Effect Transistors and Efficient Polymer
Solar Cells, Adv. Mater., 23, pp. 1409–1413.
Osaka, I., and McCullough, R. D. (2008) Advances in Molecular Design and Synthesis of
Regioregular Polythiophenes, Acc. Chem. Res., 41, pp. 1202–1214.
Park, J. K., Jo, J., Seo, J. H., Moon, J. S., Park, Y. D., Lee, K., Heeger, A. J., and Bazan,
G. C. (2011) End-Capping Effect of a Narrow Bandgap Conjugated Polymer on
Bulk Heterojunction Solar Cells, Adv. Mater., 23, pp. 2430–2435.
Park, S. H., Roy, A., Beaupré, S., Cho, S., Coates, N., Moon, J. S., Moses, D., Leclerc,
M., Lee, K., and Heeger, A. J. (2009) Bulk heterojunction solar cells with internal
quantum efficiency approaching 100%, Nat. Photonics, 3, pp. 297–302.
Patel, D. G. (Dan), Feng, F., Ohnishi, Y., Abboud, K. A., Hirata, S., Schanze, K. S., and
Reynolds, J. R. (2012) It Takes More Than an Imine: The Role of the Central
Atom on the Electron-Accepting Ability of Benzotriazole and Benzothiadiazole
Oligomers, J. Am. Chem. Soc., 134, pp. 2599–2612.
Peet, J., Kim, J. Y., Coates, N. E., Ma, W. L., Moses, D., Heeger, A. J., and Bazan, G. C.
(2007) Efficiency enhancement in low-bandgap polymer solar cells by processing
with alkane dithiols, Nat. Mater., 6, pp. 497–500.
Peet, J., Senatore, M. L., Heeger, A. J., and Bazan, G. C. (2009) The Role of Processing
in the Fabrication and Optimization of Plastic Solar Cells, Adv. Mater., 21, pp.
1521–1527.
Peng, Q., Liu, X., Qin, Y., Xu, J., Li, M., and Dai, L. (2011) Pyrazino[2,3-g]quinoxaline-
based conjugated copolymers with indolocarbazole coplanar moieties designed
for efficient photovoltaic applications, J. Mater. Chem., 21, pp. 7714–7722.
Peng, Q., Liu, X., Su, D., Fu, G., Xu, J., and Dai, L. (2011) Novel Benzo[1,2-b:4,5-
b′]dithiophene-Benzothiadiazole Derivatives with Variable Side Chains for High-
Performance Solar Cells, Adv. Mater., 23, pp. 4554–4558.
Perez, M. D., Borek, C., Forrest, S. R., and Thompson, M. E. (2009) Molecular and
Morphological Influences on the Open Circuit Voltages of Organic Photovoltaic
Devices, J. Am. Chem. Soc., 131, pp. 9281–9286.
202
Piliego, C., Holcombe, T. W., Douglas, J. D., oo, C. H., Beaujuge, P. ., and Fr chet,
J. M. J. (2010) Synthetic Control of Structural Order in N-Alkylthieno[3,4-
c]pyrrole-4,6-dione-Based Polymers for Efficient Solar Cells, J. Am. Chem. Soc.,
132, pp. 7595–7597.
Piliego, C., and Loi, M. A. (2012) Charge transfer state in highly efficient polymer–
fullerene bulk heterojunction solar cells, J. Mater. Chem., 22, pp. 4141–4150.
van der Poll, T. S.; Love, J. A.; Nguyen, T.-Q.; and Bazan, G. C. (2012) Non-Basic High-
Performance Molecules for Solution-Processed Organic Solar Cells, Adv. Mater.,
24, pp. 3646–3649.
Pomerantz, M., and Gu, X. (1997) Poly(2-decylthieno[3,4-b]thiophene). A Soluble Low-
Bandgap Conducting Polymer, Synth. Met., 84, pp. 243–244.
Price, S. C., Stuart, A. C., Yang, L., Zhou, H., and You, W. (2011) Fluorine Substituted
Conjugated Polymer of Medium Band Gap Yields 7% Efficiency in
Polymer−Fullerene Solar Cells, J. Am. Chem. Soc., 133, pp. 4625–4631.
Qin, Y., Kim, J. Y., Frisbie, C. D., and Hillmyer, M. A. (2008) Distannylated
Isothianaphthene: A Versatile Building Block for Low Bandgap Conjugated
Polymers, Macromolecules, 41, pp. 5563–5570.
Qu, S., and Tian, H. (2012) Diketopyrrolopyrrole (DPP)-based materials for organic
photovoltaics, Chem. Commun., 48, pp. 3039–3051.
Rasmussen, S. C., Schwiderski, R. L., and Mulholland, M. E. (2011) Thieno[3,4-
b]pyrazines and their applications to low band gap organic materials, Chem.
Commun., 47, pp. 11394–11410.
Roncali, J. (1997) Synthetic Principles for Bandgap Control in Linear pi-Conjugated
Systems, Chem. Rev., pp. 173–205.
Roncali, J. (2007) olecular ngineering of the Band ap of π -Conjugated Systems:
Facing Technological Applications, Macromol. Rapid Commun., 28, pp. 1761–
1775.
Rudenko, A. E., Wiley, C. A., Stone, S. M., Tannaci, J. F., and Thompson, B. C. (2012)
Semi-random P3HT analogs via direct arylation polymerization, J. Polym. Sci. A
Polym. Chem., 50, pp. 3691–3697.
Saadeh, H. A., Lu, L., He, F., Bullock, J. E., Wang, W., Carsten, B., and Yu, L. (2012)
Polyselenopheno[3,4-b]selenophene for Highly Efficient Bulk Heterojunction
Solar Cells, ACS Macro Lett., 1, pp. 361–365.
203
Scharber, M. C., Koppe, M., Gao, J., Cordella, F., Loi, M. A., Denk, P., Morana, M.,
Egelhaaf, H.-J., Forberich, K., Dennler, G., Gaudiana, R., Waller, D., Zhu, Z.,
Shi, X., and Brabec, C. J. (2010) Influence of the Bridging Atom on the
Performance of a Low-Bandgap Bulk Heterojunction Solar Cell, Adv. Mater., 22,
pp. 367–370.
Scharber, M. C., Mühlbacher, D., Koppe, M., Denk, P., Waldauf, C., Heeger, A. J., and
Brabec, C. J. (2006) Design Rules for Donors in Bulk-Heterojunction Solar
Cells—Towards 10 % nergy -Conversion Efficiency, Adv. Mater., 18, pp. 789–
794.
Scherf, U. (1999) Ladder-type materials, J. Mater. Chem., 9, pp. 1853–1864.
Schilinsky, P., Asawapirom, U., Scherf, U., Biele, M., and Brabec, C. J. (2005) Influence
of the Molecular Weight of Poly(3-hexylthiophene) on the Performance of Bulk
Heterojunction Solar Cells, Chem. Mater., 17, pp. 2175–2180.
Schlenker, C. W., and Thompson, M. E. (2011) The molecular nature of photovoltage
losses in organic solar cells, Chem. Commun., 47, pp. 3702–3716.
Shaheen, S. E., Brabec, C. J., Sariciftci, N. S., Padinger, F., Fromherz, T., and
Hummelen, J. C. (2001) 2.5% efficient organic plastic solar cells, Appl. Phys.
Lett., 78, pp. 841–843.
Shaheen, S. E., Vangeneugden, D., Kiebooms, R., Vanderzande, D., Fromherz, T.,
Padinger, F., Brabec, C. J., and Sariciftci, N. S. (2001) Low band-gap polymeric
photovoltaic devices, Synth. Met., 121, pp. 1583–1584.
Shahid, M., Ashraf, R. S., Klemm, E., and Sensfuss, S. (2006) Synthesis and Properties
of Novel Low-Band-Gap Thienopyrazine-Based Poly(heteroarylenevinylene)s,
Macromolecules, 39, pp. 7844–7853.
Shi, M.-M., Deng, D., Chen, L., Ling, J., Fu, L., Hu, X.-L., and Chen, H.-Z. (2011)
Design and synthesis of dithieno[3,2-b:2′3′-d]pyrrole-based conjugated polymers
for photovoltaic applications: consensus between low bandgap and low HOMO
energy level, J. Polym. Sci. A Polym. Chem., 49, pp. 1453–1461.
Shrotriya, V., Li, G., Yao, Y., Moriarty, T., Emery, K., and Yang, Y. (2006) Accurate
Measurement and Characterization of Organic Solar Cells, Adv. Funct. Mater.,
16, pp. 2016–2023.
Sivula, K., Luscombe, C. K., Thompson, B. C., and Fréchet, J. M. J. (2006) Enhancing
the Thermal Stability of Polythiophene:Fullerene Solar Cells by Decreasing
Effective Polymer Regioregularity, J. Am. Chem. Soc., 128, pp. 13988–13989.
204
Small, C. E., Chen, S., Subbiah, J., Amb, C. M., Tsang, S.-W., Lai, T.-H., Reynolds, J.
R., and So, F. (2011) High-efficiency inverted dithienogermole-
thienopyrrolodione-based polymer solar cells, Nat. Photonics, 6, pp. 115–120.
Snyder, C. R., Henry, J. S., and DeLongchamp, D. M. (2011) Effect of Regioregularity
on the Semicrystalline Structure of Poly(3-hexylthiophene), Macromolecules, 44,
pp. 7088–7091.
Somanathan, N., Radhakrishnan, S., Thelakkat, M., and Schmiedt, H. W. (2002) Studies
on 3-(2-Ethylhexyl)thiophene Polymers, Macromol. Mater. Eng., 287, pp. 236–
242.
Son, H. J., Wang, W., Xu, T., Liang, Y., Wu, Y., Li, G., and Yu, L. (2011) Synthesis of
Fluorinated Polythienothiophene-co-benzodithiophenes and Effect of
Fluorination on the Photovoltaic Properties, J. Am. Chem. Soc., 133, pp. 1885–
1894.
Sonar, P., Fong Lim, J. P., and Chan, K. L. (2011) Organic non-fullerene acceptors for
organic photovoltaics, Energy Environ. Sci., 4, pp. 1558–1574.
Søndergaard, R., Helgesen, M., Jørgensen, M., and Krebs, F. C. (2011) Fabrication of
Polymer Solar Cells Using Aqueous Processing for All Layers Including the
Metal Back Electrode, Adv. Energy Mater., 1, pp. 68–71.
Søndergaard, R., Hösel, M., Angmo, D., Larsen-Olsen, T. T., and Krebs, F. C. (2012)
Roll-to-roll fabrication of polymer solar cells, Mater. Today, 15, pp. 36–49.
Song, J., Zhang, C., Li, C., Li, W., Qin, R., Li, B., Liu, Z., and Bo, Z. (2010) Conjugated
polymers with broad absorption: Synthesis and application in polymer solar cells,
J. Polym. Sci. A Polym. Chem., 48, pp. 2571–2578.
Stalder, R., Grand, C., Subbiah, J., So, F., and Reynolds, J. R. (2012) An isoindigo and
dithieno[3,2-b:2′,3′-d]silole copolymer for polymer solar cells, Polym. Chem., 3,
pp. 89–92.
Stalder, R., Mei, J., and Reynolds, J. R. (2010) Isoindigo-Based Donor−Acceptor
Conjugated Polymers, Macromolecules, 43, pp. 8348–8352.
Stefan, M. C., Bhatt, M. P., Sista, P., and Magurudeniya, H. D. (2012) Grignard
metathesis (GRIM) polymerization for the synthesis of conjugated block
copolymers containing regioregular poly(3-hexylthiophene), Polym. Chem., 3, pp.
1693–1701.

205
Stevens, D. M., Speros, J. C., Hillmyer, M. A., and Frisbie, C. D. (2011) Relationship
between Diode Saturation Current and Open Circuit Voltage in Poly(3-
alkylthiophene) Solar Cells as a Function of Device Architecture, Processing
Conditions, and Alkyl Side Chain Length, J. Phys. Chem. C, 115, pp. 20806–
20816.
Su, M.-S., Kuo, C.-Y., Yuan, M.-C., Jeng, U.-S., Su, C.-J., and Wei, K.-H. (2011)
Improving Device Efficiency of Polymer/Fullerene Bulk Heterojunction Solar
Cells Through Enhanced Crystallinity and Reduced Grain Boundaries Induced by
Solvent Additives, Adv. Mater., 23, pp. 3315–3319.
Sun, Y., Cui, C., Wang, H., and Li, Y. (2012) High-Efficiency Polymer Solar Cells
Based on Poly(3-pentylthiophene) with Indene-C70 Bisadduct as an Acceptor,
Adv. Energy Mater., 2, pp. 966–969.
Sun, Y.; Welch, G. C.; Leong, W. L.; Takacs, C. J.; Bazan, G. C.; and Heeger, A. J.
(2011) Solution-processed small-molecule solar cells with 6.7% efficiency Nat.
Mater., 11, pp. 44–48.
Sun, Y., Takacs, C. J., Cowan, S. R., Seo, J. H., Gong, X., Roy, A., and Heeger, A. J.
(2011) Efficient, Air-Stable Bulk Heterojunction Polymer Solar Cells Using
MoOx as the Anode Interfacial Layer, Adv. Mater., 23, pp. 2226–2230.
Szarko, J. M., Guo, J., Liang, Y., Lee, B., Rolczynski, B. S., Strzalka, J., Xu, T., Loser,
S., Marks, T. J., Yu, L., and Chen, L. X. (2010) When Function Follows Form:
Effects of Donor Copolymer Side Chains on Film Morphology and BHJ Solar
Cell Performance, Adv. Mater., 22, pp. 5468–5472.
Tang, C. W. (1986) Two-layer organic photovoltaic cell, Appl. Phys. Lett., 48, pp. 183–
185.
Thompson, B. C., and Fréchet, J. M. J. (2008) Polymer–Fullerene Composite Solar Cells,
Angew. Chem. Int. Ed., 47, pp. 58–77.
Thompson, B. C., Khlyabich, P. P., Burkhart, B., Aviles, A. E., Rudenko, A., Shultz, G.
V., Ng, C. F., and Mangubat, L. B. (2011) Polymer-Based Solar Cells: State-of-
the-Art Principles for the Design of Active Layer Components, Green, pp. 29–54.
Thompson, B. C., Kim, B. J., Kavulak, D. F., Sivula, K., Mauldin, C., and Fréchet, J. M.
J. (2007) Influence of Alkyl Substitution Pattern in Thiophene Copolymers on
Composite Fullerene Solar Cell Performance, Macromolecules, 40, pp. 7425–
7428.

206
Thompson, B. C., Kim, Y.-G., McCarley, T. D., and Reynolds, J. R. (2006) Soluble
Narrow Band Gap and Blue Propylenedioxythiophene-Cyanovinylene Polymers
as Multifunctional Materials for Photovoltaic and Electrochromic Applications, J.
Am. Chem. Soc., 128, pp. 12714–12725.
Thompson, B. C., Kim, Y.-G., and Reynolds, J. R. (2005) Spectral Broadening in
MEH-PPV:PCBM-Based Photovoltaic Devices via Blending with a Narrow Band
ap Cyanovinylene−Dioxythiophene Polymer, Macromolecules, 38, pp. 5359–
5362.
Tieke, B., Rabindranath, A. R., Zhang, K., and Zhu, Y. (2010) Conjugated polymers
containing diketopyrrolopyrrole units in the main chain, Beilstein J. Org. Chem.,
6, pp. 830–845.
Treat, N. D., Brady, M. A., Smith, G., Toney, M. F., Kramer, E. J., Hawker, C. J., and
Chabinyc, M. L. (2011) Interdiffusion of PCBM and P3HT Reveals Miscibility
in a Photovoltaically Active Blend, Adv. Energy Mater., 1, pp. 82–89.
Treat, N. D., Shuttle, C. G., Toney, M. F., Hawker, C. J., and Chabinyc, M. L. (2011) In
situ measurement of power conversion efficiency and molecular ordering during
thermal annealing in P3HT:PCBM bulk heterojunction solar cells, J. Mater.
Chem., 21, pp. 15224–15231.
Troshin, P., Susarova, D., Khakina, E., Goryachev, A. E., Ponomarenko, S., Razumov,
V., and Sariciftci, N. S. (2012) Material solubility and molecular compatibility
effects in the design of the fullerene/polymer composites for organic bulk
heterojunction solar cells, J. Mater. Chem., 22, pp. 18433–18441.
Tsai, J.-H., Chueh, C.-C., Chen, W.-C., Yu, C.-Y., Hwang, G.-W., Ting, C., Chen, E.-C.,
and Meng, H.-F. (2010) New thiophene-phenylene-thiophene acceptor random
conjugated copolymers for optoelectronic applications, J. Polym. Sci. A Polym.
Chem., 48, pp. 2351–2360.
Uy, R. L., Price, S. C., and You, W. (2012) Structure-Property Optimizations in Donor
Polymers via Electronics, Substituents, and Side Chains Toward High Efficiency
Solar Cells, Macromol. Rapid Commun., 33, pp. 1162–1177.
Vandewal, K., Gadisa, A., Oosterbaan, W. D., Bertho, S., Banishoeib, F., Van Severen,
I., Lutsen, L., Cleij, T. J., Vanderzande, D., and Manca, J. V. (2008) The Relation
Between Open ‐Circuit Voltage and the Onset of Photocurrent Generation by
Charge ‐Transfer Absorption in Polymer : Fullerene Bulk Heterojunction Solar
Cells, Adv. Funct. Mater., 18, pp. 2064–2070.

207
Vangeneugden, D. L., Vanderzande, D. J. M., Salbeck, J., van Hal, P. A., Janssen, R. A.
J., Hummelen, J. C., Brabec, C. J., Shaheen, S. E., and Sariciftci, N. S. (2001)
Synthesis and Characterization of a Poly(1,3-dithienylisothianaphthene)
Derivative for Bulk Heterojunction Photovoltaic Cells, J. Phys. Chem. B, 105, pp.
11106–11113.
Veldman, D., Meskers, S. C. J., and Janssen, R. A. J. (2009) The Energy of Charge-
Transfer States in Electron Donor-Acceptor Blends: Insight into the Energy
Losses in Organic Solar Cells, Adv. Funct. Mater., 19, pp. 1939–1948.
Walker, B., Tamayo, A. B., Dang, X.-D., Zalar, P., Seo, J. H., Garcia, A., Tantiwiwat,
M., and Nguyen, T.-Q. (2009) Nanoscale Phase Separation and High Photovoltaic
Efficiency in Solution-Processed, Small-Molecule Bulk Heterojunction Solar
Cells, Adv. Funct. Mater., 19, pp. 3063–3069.
Wang, E., Hou, L., Wang, Z., Hellstroem, S., Zhang, F., Inganaes, O., and Andersson, M.
R. (2010) An easily synthesized blue polymer for high-performance polymer
solar cells, Adv. Mater., 22, pp. 5240–5244.
ang, ., Hou, L., ang, ., a, ., Hellstr m, S., huang, ., hang, F., Ingan s, O.,
and Andersson, M. R. (2011) Side-Chain Architectures of 2,7-Carbazole and
Quinoxaline-Based Polymers for Efficient Polymer Solar Cells, Macromolecules,
44, pp. 2067–2073.
ang, ., a, ., hang, ., Vandewal, ., Henriksson, P., Ingan s, O., Zhang, F., and
Andersson, M. R. (2011) An Easily Accessible Isoindigo-Based Polymer for
High-Performance Polymer Solar Cells, J. Am. Chem. Soc., 133, pp. 14244–
14247.
Wang, E., Wang, L., Lan, L., Luo, C., Zhuang, W., Peng, J., and Cao, Y. (2008) High-
performance polymer heterojunction solar cells of a polysilafluorene derivative,
Appl. Phys. Lett., 92, pp. 033307.
Wang, M., Hu, X., Liu, P., Li, W., Gong, X., Huang, F., and Cao, Y. (2011) Donor–
Acceptor Conjugated Polymer Based on Naphtho[1,2-c:5,6-c
]bis[1,2,5]thiadiazole for High-Performance Polymer Solar Cells, J. Am. Chem.
Soc., 133, pp. 9638–9641.
Wen, L., Nietfeld, J. P., Amb, C. M., and Rasmussen, S. C. (2008) Synthesis and
Characterization of New 2,3-Disubstituted Thieno[3,4-b]pyrazines: Tunable
Building Blocks for Low Band Gap Conjugated Materials, J. Org. Chem., 73, pp.
8529–8536.

208
Wienk, M. M., Kroon, J. M., Verhees, W. J. H., Knol, J., Hummelen, J. C., van Hal, P.
A., and Janssen, R. A. J. (2003) Efficient Methano[70]fullerene/MDMO-PPV
Bulk Heterojunction Photovoltaic Cells, Angew. Chem. Int. Ed., 42, pp. 3371–
3375.
Wienk, M. M., Turbiez, M., Gilot, J., and Janssen, R. A. J. (2008) Narrow ‐Bandgap
Diketo-Pyrrolo ‐Pyrrole Polymer Solar Cells: The Effect of Processing on the
Performance, Adv. Mater., 20, pp. 2556–2560.
Wolden, C. A., Kurtin, J., Baxter, J. B., Repins, I., Shaheen, S. E., Torvik, J. T., Rockett,
A. A., Fthenakis, V. M., and Aydil, E. S. (2011) Photovoltaic manufacturing:
Present status, future prospects, and research needs, J. Vac. Sci. Technol. A, 29,
pp. 030801.
Woo, C. H., Beaujuge, P. M., Holcombe, T. W., Lee, O. P., and Fr chet, J. . J. (2010)
Incorporation of Furan into Low Band-Gap Polymers for Efficient Solar Cells, J.
Am. Chem. Soc., 132, pp. 15547–15549.
Woo, C. H., Thompson, B. C., Kim, B. J., Toney, . F., and Fr chet, J. . J. (2008) The
Influence of Poly(3-hexylthiophene) Regioregularity on Fullerene-Composite
Solar Cell Performance, J. Am. Chem. Soc., 130, pp. 16324–16329.
Woong Jung, J., Woong Jo, J., Liu, F., Russell, T. P., and Ho Jo, W. (2012) A low band-
gap polymer based on unsubstituted benzo[1,2-b:4,5-b′]dithiophene for high
performance organic photovoltaics, Chem. Commun., 48, pp. 6933–6935.
Wu, P.-T., Ren, G., and Jenekhe, S. A. (2010) Crystalline Random Conjugated
Copolymers with Multiple Side Chains: Tunable Intermolecular Interactions and
Enhanced Charge Transport and Photovoltaic Properties, Macromolecules, 43,
pp. 3306–3313.
Wu, P.-T., Xin, H., Kim, F. S., Ren, G., and Jenekhe, S. A. (2009) Regioregular Poly(3-
pentylthiophene): Synthesis, Self-Assembly of Nanowires, High-Mobility Field-
Effect Transistors, and Efficient Photovoltaic Cells, Macromolecules, 42, pp.
8817–8826.
Wudl, F., Kobayashi, M., and Heeger, A. J. (1984) Poly(isothianaphthene), J. Org.
Chem., 49, pp. 3382–3384.
Xin, H., Subramaniyan, S., Kwon, T.-W., Shoaee, S., Durrant, J. R., and Jenekhe, S. A.
(2012) Enhanced Open Circuit Voltage and Efficiency of Donor–Acceptor
Copolymer Solar Cells by Using Indene-C60 Bisadduct, Chem. Mater., 24, pp.
1995–2001.
209
Yang, L., Zhou, H., Price, S. C., and You, W. (2012) Parallel-like Bulk Heterojunction
Polymer Solar Cells, J. Am. Chem. Soc., 134, pp. 5432–5435.
Yang, L., Zhou, H., and You, W. (2010) Quantitatively Analyzing the Influence of Side
Chains on Photovoltaic Properties of Polymer−Fullerene Solar Cells, J. Phys.
Chem. C, 114, pp. 16793–16800.
Yang, T., Wang, M., Duan, C., Hu, X., Huang, L., Peng, J., Huang, F., and Gong, X.
(2012) Inverted polymer solar cells with 8.4% efficiency by conjugated
polyelectrolyte, Energy Environ. Sci., 5, pp. 8208–8214.
Yang, X., Loos, J., Veenstra, S. C., Verhees, W. J. H., Wienk, M. M., Kroon, J. M.,
Michels, M. A. J., and Janssen, R. A. J. (2005) Nanoscale Morphology of High-
Performance Polymer Solar Cells, Nano Lett., 5, pp. 579–583.
Ying, L., Hsu, B. B. Y., Zhan, H., Welch, G. C., Zalar, P., Perez, L. A., Kramer, E. J.,
Nguyen, T.-Q., Heeger, A. J., Wong, W.-Y., and Bazan, G. C. (2011)
Regioregular Pyridal[2,1,3]thiadia ole π -Conjugated Copolymers, J. Am. Chem.
Soc., 133, pp. 18538–18541.
Yiu, A. T., Beaujuge, P. M., Lee, O. P., Woo, C. H., Toney, M. F., and Fréchet, J. M. J.
(2012) Side-Chain Tunability of Furan-Containing Low-Band-Gap Polymers
Provides Control of Structural Order in Efficient Solar Cells, J. Am. Chem. Soc.,
134, pp. 2180–2185.
Yu, C.-Y., Chen, C.-P., Chan, S.-H., Hwang, G.-W., and Ting, C. (2009)
Thiophene/Phenylene/Thiophene-Based Low-Bandgap Conjugated Polymers for
Efficient Near-Infrared Photovoltaic Applications, Chem. Mater., 21, pp. 3262–
3269.
Yu, G., and Heeger, A. J. (1995) Charge separation and photovoltaic conversion in
polymer composites with internal donor/acceptor heterojunctions, J. Appl. Phys.,
78, pp. 4510–4515.
Yuan, M.-C., Chiu, M.-Y., Liu, S.-P., Chen, C.-M., and Wei, K.-H. (2010) A Thieno[3,4-
c]pyrrole-4,6-dione-Based Donor−Acceptor Polymer xhibiting High
Crystallinity for Photovoltaic Applications, Macromolecules, 43, pp. 6936–6938.
Yue, W., Zhao, Y., Shao, S., Tian, H., Xie, Z., Geng, Y., and Wang, F. (2009) Novel
NIR-absorbing conjugated polymers for efficient polymer solar cells: effect of
alkyl chain length on device performance, J. Mater. Chem., 19, pp. 2199–2206.

210
Yue, W., Zhao, Y., Tian, H., Song, D., Xie, Z., Yan, D., Geng, Y., and Wang, F. (2009)
Poly(oligothiophene-alt-benzothiadiazole)s: Tuning the Structures of Oligo-
thiophene Units toward High- obility “Black” Conjugated Polymers,
Macromolecules, 42, pp. 6510–6518.
Zhang, F., Bijleveld, J., Perzon, E., Tvingstedt, K., Barrau, S., Inganäs, O., and
Andersson, M. R. (2008) High photovoltage achieved in low band gap polymer
solar cells by adjusting energy levels of a polymer with the LUMOs of fullerene
derivatives, J. Mater. Chem., 18, pp. 5468–5474.
Zhang, G., Fu, Y., Xie, Z., and Zhang, Q. (2011) Synthesis and Photovoltaic Properties of
New Low Bandgap Isoindigo-Based Conjugated Polymers, Macromolecules, 44,
pp. 1414–1420.
Zhang, Q. T., and Tour, J. M. (1997) Low Optical Bandgap Polythiophenes by an
Alternating Donor/Acceptor Repeat Unit Strategy, J. Am. Chem. Soc., 119, pp.
5065–5066.
Zhang, S., Guo, Y., Fan, H., Liu, Y., Chen, H.-Y., Yang, G., Zhan, X., Liu, Y., Li, Y.,
and Yang, Y. (2009) Low bandgap π -conjugated copolymers based on fused
thiophenes and benzothiadiazole: synthesis and structure-property relationship
study, J. Polym. Sci. A Polym. Chem., 47, pp. 5498–5508.
Zhang, X., Steckler, T. T., Dasari, R. R., Ohira, S., Potscavage, W. J., Tiwari, S. P.,
Coppée, S., Ellinger, S., Barlow, S., Brédas, J.-L., Kippelen, B., Reynolds, J. R.,
and Marder, S. R. (2010) Dithienopyrrole-based donor–acceptor copolymers:
low band-gap materials for charge transport, photovoltaics and electrochromism,
J. Mater. Chem., 20, pp. 123–134.
Zhang, Y., Tajima, K., and Hashimoto, K. (2009) Nanostructure Formation in Poly(3-
hexylthiophene- block -3-(2-ethylhexyl)thiophene)s, Macromolecules, 42, pp.
7008–7015.
Zhang, Y., Tajima, K., and Hashimoto, K. (2011) Field-effect hole mobility of poly(3-
hexylthiophene-block-3-(2-ethylhexyl)thiophene)s, Synth. Met., 161, pp. 225–
228.
Zhang, Y., Tajima, K., Hirota, K., and Hashimoto, K. (2008) Synthesis of All-Conjugated
Diblock Copolymers by Quasi-Living Polymerization and Observation of Their
Microphase Separation, J. Am. Chem. Soc., 130, pp. 7812–7813.
Zhang, Y., Zou, J., Cheuh, C.-C., Yip, H.-L., and Jen, A. K.-Y. (2012) Significant
Improved Performance of Photovoltaic Cells Made from a Partially Fluorinated
Cyclopentadithiophene/Benzothiadiazole Conjugated Polymer, Macromolecules,
45, pp. 5427–5435.
211
Zhang, Y., Zou, J., Yip, H.-L., Chen, K.-S., Davies, J. A., Sun, Y., and Jen, A. K.-Y.
(2011) Synthesis, Characterization, Charge Transport, and Photovoltaic
Properties of Dithienobenzoquinoxaline- and Dithienobenzopyridopyrazine-
Based Conjugated Polymers, Macromolecules, 44, pp. 4752–4758.
Zhang, Y., Zou, J., Yip, H.-L., Chen, K.-S., Zeigler, D. F., Sun, Y., and Jen, A. K.-Y.
(2011) Indacenodithiophene and Quinoxaline-Based Conjugated Polymers for
Highly Efficient Polymer Solar Cells, Chem. Mater., 23, pp. 2289–2291.
Zhang, Y., Zou, J., Yip, H.-L., Sun, Y., Davies, J. A., Chen, K.-S., Acton, O., and Jen, A.
K.-Y. (2011) Conjugated polymers based on C, Si and N-bridged dithiophene and
thienopyrroledione units: synthesis, field-effect transistors and bulk
heterojunction polymer solar cells, J. Mater. Chem., 21, pp. 3895.
Zhang, Z., and Wang, J. (2012) Structures and properties of conjugated Donor–Acceptor
copolymers for solar cell applications, J. Mater. Chem., 22, pp. 4178–4187.
Zhao, G., He, Y., and Li, Y. (2010) 6.5% Efficiency of Polymer Solar Cells Based on
poly(3-hexylthiophene) and Indene-C60 Bisadduct by Device Optimization, Adv.
Mater., 22, pp. 4355–4358.
Zhou, E., Cong, J., Tajima, K., Yang, C., and Hashimoto, K. (2011) Synthesis and
photovoltaic properties of donor-acceptor copolymer based on dithienopyrrole
and thienopyrroledione, Macromol. Chem. Phys., 212, pp. 305–310.
Zhou, E., Cong, J., Wei, Q., Tajima, K., Yang, C., and Hashimoto, K. (2011) All-
Polymer Solar Cells from Perylene Diimide Based Copolymers: Material Design
and Phase Separation Control, Angew. Chem. Int. Ed., 50, pp. 2799–2803.
Zhou, E., Nakamura, M., Nishizawa, T., Zhang, Y., Wei, Q., Tajima, K., Yang, C., and
Hashimoto, K. (2008) Synthesis and photovoltaic properties of a novel low band
gap polymer based on N-substituted dithieno[3,2-b:2′,3′-d]pyrrole,
Macromolecules, 41, pp. 8302–8305.
Zhou, E., Wei, Q., Yamakawa, S., Zhang, Y., Tajima, K., Yang, C., and Hashimoto, K.
(2010) Diketopyrrolopyrrole-based semiconducting polymer for photovoltaic
device with photocurrent response wavelengths up to 1.1 μm, Macromolecules,
43, pp. 821–826.
Zhou, E., Yamakawa, S., Tajima, K., Yang, C., and Hashimoto, K. (2009) Synthesis and
Photovoltaic Properties of Diketopyrrolopyrrole-Based Donor−Acceptor
Copolymers, Chem. Mater., 21, pp. 4055–4061.
212
Zhou, H., Yang, L., Price, S. C., Knight, K. J., and You, W. (2010) Enhanced Photo-
voltaic Performance of Low-Bandgap Polymers with Deep LUMO Levels,
Angew. Chem. Int. Ed., 49, pp. 7992–7995.
Zhou, H., Yang, L., Stuart, A. C., Price, S. C., Liu, S., and You, W. (2011) Development
of Fluorinated Benzothiadiazole as a Structural Unit for a Polymer Solar Cell of
7 % fficiency, Angew. Chem. Int. Ed., 50, pp. 2995–2998.
Zhou, H., Yang, L., and You, W. (2012) Rational Design of High Performance
Conjugated Polymers for Organic Solar Cells, Macromolecules, 45, pp. 607–632.
Zhu, Y., Champion, R. D., and Jenekhe, S. A. (2006) Conjugated donor−acceptor
copolymer semiconductors with large intramolecular charge transfer: synthesis,
optical properties, electrochemistry, and field effect carrier mobility of
thienopyrazine-based copolymers, Macromolecules, 39, pp. 8712–8719.
Zhu, Z., Waller, D., Gaudiana, R., Morana, M., Mühlbacher, D., Scharber, M., and
Brabec, C. (2007) Panchromatic Conjugated Polymers Containing Alternating
Donor/Acceptor Units for Photovoltaic Applications, Macromolecules, 40, pp.
1981–1986.
Zoombelt, A. P., Gilot, J., Wienk, M. M., and Janssen, R. A. J. (2009) Effect of Extended
Thiophene Segments in Small Band Gap Polymers with Thienopyrazine, Chem.
Mater., 21, pp. 1663–1669.
Zoombelt, A. P., Leenen, M. A. M., Fonrodona, M., Nicolas, Y., Wienk, M. M., and
Janssen, R. A. J. (2009) The influence of side chains on solubility and
photovoltaic performance of dithiophene–thienopyrazine small band gap
copolymers, Polymer, 50, pp. 4564–4570.
Zoombelt, A. P., Mathijssen, S. G. J., Turbiez, M. G. R., Wienk, M. M., and Janssen, R.
A. J. (2010) Small band gap polymers based on diketopyrrolopyrrole, J. Mater.
Chem., 20, pp. 2240–2246.
Zou, J., Yip, H.-L., Zhang, Y., Gao, Y., Chien, S.-C., O’ alley, ., Chueh, C. -C., Chen,
H., and Jen, A. K.-Y. (2012) High-Performance Inverted Polymer Solar Cells:
Device Characterization, Optical Modeling, and Hole-Transporting
Modifications, Adv. Funct. Mater., 22, pp. 2804–2811.

213
ou, ., endron, D., Badrou -A ch, R., ajari, A., Tao, ., and Leclerc, . (2009) A
High-Mobility Low-Bandgap Poly(2,7-carbazole) Derivative for Photovoltaic
Applications, Macromolecules, 42, pp. 2891–2894.
ou, ., ajari, A., Berrouard, P., Beaupr , S., R da A ch, B., Tao, ., and Leclerc, .
(2010) A Thieno[3,4- c]pyrrole-4,6-dione-Based Copolymer for Efficient Solar
Cells, J. Am. Chem. Soc., 132, pp. 5330–5331.

214
APPENDIX 1 Semi-Random Multichromophoric rr-P3HT
Analogues: Design and First Generation
A1.1 Materials and Methods
All reagents from commercial sources were used without further purification,
unless otherwise noted. All reactions were performed under dry N
2
, unless otherwise
noted. All reactions were performed with glassware that was oven dried and then flamed
under high vacuum and backfilled with N
2
. Solvents were purchased from VWR and used
without further purification. Flash chromatography was performed using a Teledyne
CombiFlash R
f
instrument in combination with RediSep R
f
normal phase disposable
columns. Solvents were purchased from VWR and used without further purification
except for THF which was dried over sodium/benzophenone before being distilled.
All compounds were characterized by
1
H NMR (400 MHz) and
13
C NMR (100
MHz) on a Mercury 400. Polymer
1
H NMR’s (500 MHz) were obtained on a Varian
VNMRS-500. For polymer molecular weight determination, polymer samples were
dissolved in HPLC grade o-DCB at a concentration of 1 mg/ml, briefly heated and then
allowed to turn to room temperature prior to filtering through a 0.2 μm PTF filter. S C
was performed using HPLC grade o-DCB at a flow rate of 1 ml/min on one 300 × 7.8
mm TSK-Gel GMH
H R
-H column (Tosoh Corporation) at 70
o
C using a Viscotek GPC
Max VE 2001 separation module and a Viscotek TDA 305 RI detector. The instrument
215
was calibrated vs. polystyrene standards (1,050 – 3,800,000 g/mol) and data was
analyzed using OmniSec 4.6.0 software.
CV was collected using an Princeton Applied Research VersaStat3 potentiostat
under the control of VersaStudio Software. A standard three electrode cell based on a Pt
wire working electrode, a silver wire pseudo reference electrode (calibrated vs. Fc/Fc
+

which is taken as 5.1 eV vs. vacuum)
1,2
and a Pt wire counter electrode was purged with
nitrogen and maintained under nitrogen atmosphere during all measurements. For CV of
thin films acetonitrile was distilled over CaH
2
prior to use and tetrabutyl ammonium
hexafluorophosphate (0.1 M) was used as the supporting electrolyte. Polymer films were
made by repeatedly dipping the Pt wire in a 1% (w/w) o-DCB solution and dried under
nitrogen prior to measurement. For CV of solutions chloroform was distilled over CaH
2

prior to use and tetrabutyl ammonium tetrafluoroborate (0.1 M) was used as the
supporting electrolyte.
UV/vis absorption spectra were obtained on a Perkin-Elmer Lambda 950
spectrophotometer. Thickness of the samples and GIXRD measurements were obtained
using Rigaku Diffractometer Ultima IV using a Cu Kα radiation source (λ = 1.54 Å) in
the reflectivity and GIXRD mode, respectively. For thin film measurements polymers
were spin coated onto pre-cleaned glass slides from o-DCB solutions (7 mg/ml).
DSC traces were obtained using a Perkin Elmer DSC 8000 with a scan rate of 10
ºC/min. Sample size was ~3 mg and polymers were used as obtained after purification.
TEM was performed on the JEOL JEM-2100 microscope equipped with the
Gatan Orius CCD camera. The accelerating voltage was 200 kV. Films for the TEM
216
measurements were prepared from the CB or o-DCB solutions of polymer:PC
61
BM blend
at the optimized ratios and processing conditions. Films for TEM were prepared by first
spin-casting on KBr plates, which were then placed in de-ionized water and upon salt
dissolution the floated polymer:PC
61
BM films were picked up with the 600 hex mesh
copper grid (Electron Microscopy Sciences).

A1.2 Synthesis

2,5-Dibromo-3,4-dinitrothiophene (1) Compound was prepared through a
previously reported procedure.
3
80 ml sulphuric acid, 120 ml fuming sulphuric acid and
70 ml fuming nitric acid were combined in a 500 ml round bottom flask. The solution
was cooled with an ice bath to 15 °C. 46.09 g (0.19 mol) 2,5-dibromothiophene was
added slowly to keep the temperature below 30 °C. The solution turned orange and a
precipitate formed. It was than warmed to r.t. and stirred for 3 h. The yellow precipitate

Figure A1.1 Synthesis of 5,7-dibromo-thieno[3,4-b]pyrazine.

217
was filtered and recrystallized from MeOH. 29.96 g (0.09 mol, 47%) of light yellow solid
was obtained.
13
C NMR (100 MHz, CDCl
3
) δ 112.81, MALDI 389.4 (331.9 + 57).
3,4-Diaminothiophene hydrochloride (2) Compound was prepared through a
previously reported procedure.
4
10.00 g (30 mmol) 2,5-dibromo-3,4-dinitrothiophene was
suspended in 180 ml conc. HCl . The suspension was cooled with an ice bath. 24.90 g
(200 mmol) tin was added in small portions to keep the temperature between 15 and 20
°C. The suspension was than stirred over night in an ice bath. The solid was separated by
vacuum filtration and washed with Et
2
O and acetonitrile. 4.85 g of 3,4-diaminothiophene
hydrochloride was obtained as a white solid.
1
H NMR (400 MHz, DMSO-d
6
) δ 8.43 (br s,
6 H), 7.10 (s, 2H).
Thieno[3,4-b]pyrazine (3) Compound was prepared through a modified literature
procedure.
5
850 mg (4.54 mmol) 3,4-diaminothiophene hydrochloride was dissolved in
17 ml 5 % Na
2
CO
3
(degassed) solution. 0.23 ml (5.00 mmol) glyoxal was added and the
solution was left stirring for one hour. It was than extracted with Et
2
O several times. The
combined organic phases were washed with water and dried over Na
2
SO
4
. The solvent
was evaporated in vacuo. The product was obtained as a dark brown solid (230 mg, 45%
yield).
1
H NMR (400 MHz, CDCl
3
) δ 8.51 (s, 1H), 8.03 (s, 1H);
13
C NMR (100 MHz,
CDCl
3
) δ 144.44, 142.69, 117.92.
5,7-Dibromo-thieno[3,4-b]pyrazine (4) 200 mg (1.47 mmol) thieno [3,4-
b]pyrazine 3 was dissolved in 30 ml DMF and cooled to -20
o
C. 547 mg (3.08 mmol) N-
bromosuccinimide was added in one portion. The reaction mixture was stirred for 2 h at -
20
o
C. Water was added and the precipitate was redissolved in diethyl ether. The organic
218
phase was washed several times with water, dried with MgSO
4
and the solvent was
evaporated in vacuo. The crude product was purified using flash chromatography (DCM)
to give a yellow solid (212 mg, 49%).
1
H NMR (400 MHz, CDCl
3
) δ 8.53 (s, 1H);
13
C
NMR (100 MHz, CDCl
3
) δ 145.61, 140.60, 105.78.

2,1,3-Benzothiadiazole (5) 5.00 g (46.23 mmol) o-phenylenediamine was
dissolved in 150 ml DCM and 26 ml triethylamine. 6.05 g (3.7 ml, 50.85 mmol) thionyl
chloride was added slowly via addition funnel and the resulting mixture was refluxed for
20 h. After washing several times with 1 M HCl the organic phase was dried over MgSO
4

and the solvent was removed in vacuo. The product was obtained as a brown solid (3.33
g, 24.23 mmol, 53%) and was used without further purification for the next step.
1
H
NMR (400 MHz, CDCl
3
) δ 8.01 (m, 2H), 7.60 (m, 2H).
4,7-Dibromo-2,13-benzothiadiazole (6) To a solution of 3.00 g (22.00 mmol)
2,1,3-benzothiadiazole in 45 ml HBr (48 %) a solution of 10 g (65.00 mmol) Br
2
in 30 ml
HBr (48 %) was added very slowly. After complete addition the mixture was refluxed for
6 h during which time an orange precipitate formed. After cooling to r.t. a saturated
solution of Na
2
SO
3
was added and the suspension was filtered to obtain a light yellow
solid. Recrystallization from hexanes in the presence of activated charcoal gave the

Figure A1.2 Synthesis of 4,7-dibromo-2,13-benzothiadiazole.

219
product as a white solid (6.08 g, 20.68 mmol, 94%).
1
H NMR (400 MHz, CDCl
3
) δ 7.73
(s, 2H).
13
C NMR (100 MHz, CDCl
3
) δ 152.93, 132.32, 113.89.

2-Bromo-3-hexylthiophene (7) 10.29 g (61.25 mmol) 3-hexylthiophene was
dissolved in 83 ml acetic acid. 10.9 g (61.25 mmol) NBS was added in one portion. After
all the NBS dissolved the reaction mixture turned yellow and warmed up to 35
o
C. After
approximately 10 min the color turned back to colorless and the solution was left to cool
to rt. The reaction mixture was poured in water and extracted with Et
2
O. The combined
organic phases were washed multiple times with 10 % NaOH, dried over MgSO
4
and the
solvent was removed in vacuo. The product was obtained as a clear oil after vacuum
distillation (13.42 g, 88%).
1
H NMR (400 MHz, CDCl
3
) δ 7.18 (d, 1H), 6.79 (d, 1H), 2.56
(t, 2H), 1.57 (m, 2H), 1.31 (m, 6H), 0.89 (t, 3H).
13
C NMR (100 MHz, CDCl
3
) δ 142.02,
128.22, 125.06, 108.70, 31.59, 29.71, 29.39, 28.85, 22.55, 14.07.
2-Bromo-5-trimethyltin-3-hexylthiophene (8) Two 3-neck flasks were flame
dried. In flask number one a LDA solution was prepared by dissolving 1.5 ml freshly
distilled diisopropylamine in 7.5 ml dry THF. The solution was cooled to -78
o
C, 5.7 ml
(9.13 mmol) n-BuLi was added dropwise and the mixture was stirred for 30 min at -78
o
C. In flask number two 2.169 g (8.78 mmol) 2-bromo-3-hexylthiophene was dissolved in

Figure A1.3 Synthesis of 2-bromo-5-trimethyltin-3-hexylthiophene.

220
16 ml dry THF and cooled to -78
o
C. Then the contents of flask number one were
transferred via cannula to flask number two. The reaction mixture was stirred for 45 min
at -78
o
C and then 10.79 ml (10.79 mmol) trimethyltin chloride was added slowly. The
mixture was allowed to come to r.t. and stirred overnight. Water was added to quench the
reaction and the aqueous phase was extracted several times with Et
2
O. The combined
organic phases were dried over MgSO
4
and the solvent was evaporated in vacuo. Vacuum
distillation gave the product (2.843 g, 6.93 mmol, 79%) as a colorless liquid.
1
H NMR
(400 MHz, CDCl
3
) δ 6.85 (s, 1H), 1.57 (m, 2H), 1.32 (m, 6H), 0.89 (m, 3H), 0.35 (s, 9H).
13
C NMR (100 MHz, CDCl
3
) δ 143.14, 137.96, 136.32, 113.45, 31.67, 29.91, 29.29,
29.10, 22.64, 14.13, - 8.19.

2,5-Bis(trimethyltin)thiophene (9) 1.40 g (16.7 mmol) freshly distilled
thiophene was combined with 4.06 g (35 mmol) TMEDA in a 100 ml 3-neck flask. 12.1
ml (35 mmol) n-Buli was added slowly and the solution was refluxed for 90 min. It was
then cooled to -78
o
C and 35 ml (35 mmol) Me
3
SnCl was added. After stirring at -78
o
C
for 10 min the reaction mixture was brought to r.t. and left stirring over night. It was then
poured in water and extracted with Et
2
O. The combined organic phases were washed with
water, dried and the solvent was evaporated in vacuo. After recrystallization from EtOH a

Figure A1.4 Synthesis of 2,5-bis-
(trimethyltin)thiophene.
221
white solid was obtained (4.5239 g, 66%).
1
H NMR (400 MHz, CDCl
3
) δ 7.38 (s, 2H),
0.37 (s, 18H).

General Procedure for Stille Polymerization All monomers were dissolved in
dry DMF to give a 0.04 M solution. The solution was then degassed for 20 min. 4 mol%
Pd(PPh
3
)
4
was added in one portion, the solution was degassed for 10 additional minutes

Figure A1.5 Synthesis of P3HT and semi-random copolymers P3HTT, P3HTT-
BTD, P3HTT-TP and P3HTT-TP-BTD.
222
and then heated for 48 hours at 95
o
C. Then the reaction mixture was cooled to r.t. and
precipitated in MeOH. Purification was achieved via soxhlet extraction using MeOH,
hexanes and chloroform for all polymers. For P3HTT, P3HTT-BTD, P3HTT-TP and
P3HTT-TP-BTD an additional extraction with DCM was used prior to the CHCl
3
step.
For P3HTT-TP-BTD chlorobenzene was used as a final soxhlet solvent. P3HT: Yield:
92%, M
n
= 17,180, PDI = 2.74.
1
H NMR (500 MHz, CDCl
3
) δ 6.97 (s, 0.1H), 2.80 (m,
2H), 2.55 (s, 0.2H), 1.70 (m, 2H), 1.43 (m, 2H), 1.35 (m, 4H), 0.89 (m, 3H). P3HTT:
Yield: 48%, M
n
= 47,850, PDI = 1.75.
1
H NMR (500 MHz, CDCl
3
) δ 7.11 – 6.98 (m,
1.55 H), 2.80 (s, 2H), 2.57 (0.08H), 1.71 (s, 2.16H), 1.42, 1.34 (m, 7H), 0.92, (s, 3.4H).
P3HTT-BTD: Yield: 84%, M
n
= 15,310, PDI = 2.45.
1
H NMR (500 MHz, CDCl
3
) δ 7.98
(s, 0.25H), 7.82 (s, 0.25H), 7.08, 6.98 (m, 1H), 2.81 (m, 2H), 2.59 (s, 0.09H), 1.71 (s,
2.21H), 1.36 (m, 6.9H), 0.92 (s, 3.4H). P3HTT-TP: Yield: 73%, M
n
= 16,680, PDI =
2.35.
1
H NMR (500 MHz, CDCl
3
) δ 8.52 (s, 0.22H), 7.44 (s, 0.22 H), 7.11 (m, 0.45H),
6.97 (s, 0.65H), 2.80 (s, 2H), 2.57 (s, 0.1H), 1.71 (s, 2.22H), 1.35-1.44 (m, 8H), 0.91 (s,
4H). P3HTT-TP-BTD: Yield: 66%, M
n
= 16,320, PDI = 2.05.
1
H NMR (500 MHz,
Tetrachloroethane-d2, 100
o
C) δ 8.47 (s, 0.24H), 7.96 (s, 0.25H), 7.80 (s, 0.27H), 7.49 (s,
0.26H), 6.99-7.18 (m, 1H), 2.81 (s, 2H), 2.60 (s, 0.07H), 1.72 (s, 2.11H), 1.35-1.45 (m,
7.7H), 0.90 (s, 2.91H).

223
A1.3 Polymer Characterization

Figure A1.6
1
H NMR of P3HT in CDCl
3
.
224

Figure A1.7
1
H NMR of P3HTT in CDCl
3
.
225

Figure A1.8
1
H NMR of P3HTT-BTD in CDCl
3
.
226

Figure A1.9
1
H NMR of P3HTT-TP in CDCl
3
.
227

Figure A1.10
1
H NMR of P3HTT-TP-BTD in tetrachloroethane-d2.
228

Figure A1.11 CV traces for the oxidation of P3HT, P3HTT, P3HTT-BTD, P3HTT-TP
and P3HTT-TP-BTD.
229

Figure A1.12 DSC traces of P3HTT and P3HTT-BTD.

A1.4 Device Fabrication and Characterization
ITO-coated glass substrates (20 / , Thin Film Devices Inc.) were sequentially
cleaned by sonication in detergent, de-ionized water, tetrachloroethylene, acetone, and
isopropyl alcohol, and dried in a nitrogen stream. A thin layer of PEDOT:PSS (Baytron
®

P VP AI 4083, filtered at 0.45 μm) was first spin -coated on the pre-cleaned ITO-coated
glass substrates and then baked at 130 °C for 60 minutes under N
2
. Separate solutions of
polymer and PC
61
BM were prepared in o-DCB or trichlorobenzene solvents. The
solutions were stirred for 24 hrs before they were mixed at the desired ratios and stirred
for 24 hrs to form a homogeneous mixture. Subsequently, the polymer:PC
61
BM active
layer was spin-coated on top of the PEDOT:PSS layer. The P3HTT-TP:PC
61
BM layer
was spin cast from a solution of 1.7% octanedithiol (by volume) in o-DCB containing 7
mg/ml P3HTT-TP and 5.53 mg/ml PC
61
BM. The solution of P3HTT-BTD:PC
61
BM was
prepared by dissolving the polymer (7 mg/ml) and PCBM (35 mg/ml) in o-DCB. The
solution of P3HTT-TP-BTD:PC
61
BM was prepared by dissolving the polymer (10
230
mg/ml) and PCBM (8 mg/ml) in trichlorobenzene. The substrates with P3HTT-TP-
BTD:PC
61
BM blend were annealed at 100 °C for 30 minutes under vacuum and cooled to
room temperature prior to electrode deposition. At the final stage, the substrates were
pumped down to high vacuum (< 7×10
-7
Torr) and aluminum (100 nm) was thermally
evaporated at 2-3 Å/sec using a Denton Benchtop Turbo IV Coating System onto the
active layer through shadow masks to define the active area of the devices as of 4.9 mm
2
.
Thermal annealing of P3HTT-BTD:PC
61
BM blends was carried out by directly placing
the completed devices in the vacuum oven for 10 min at 100 °C. After annealing, the
devices were cooled down to room temperature before measurements were carried out.
P3HTT-TP:PC
61
BM BHJ solar cell was tested without thermal treatment.
The current-voltage (I-V) characteristics of photovoltaic devices were measured
under ambient conditions using a Keithley 2400 source-measurement unit. An Oriel
®

Sol3A class AAA solar simulator with xenon lamp (450 Watt) and an AM1.5 G filter was
used as the solar simulator. An Oriel PV reference cell system 91150V was used as the
reference cell. To calibrate the light intensity of the solar simulator, the power of the
xenon lamp was adjusted to make the J
sc
of the reference cell under simulated sun light as
high as it was under the calibration condition.
Polymer mobility was measured using a hole-only device configuration of
ITO/PEDOT:PSS/Polymer/Al in the space charge limited current regime. The devices
preparations were the same as described above for solar cells. The dark current was
measured under ambient conditions. At sufficient potential the mobilities of charges in
231
the device can be determined by fitting the dark current to the model of SCL current and
described by:
2
0
3
9
8
S C L C R
V
J
L
   

where J
SCLC
is the current density, ε
0
is the permittivity of space, ε
R
is the dielectric
constant of the polymer (assumed to be 3), μ is the zero-field mobility of the majority
charge carriers, V is the effective voltage across the device (V = V
applied
– V
bi
- V
r
), and L
is the polymer layer thickness. The series and contact resistance of the device (25-35 Ω)
was measured using a blank (ITO/PEDOT/Al) configuration and the voltage drop due to
this resistance (V
r
) was subtracted from the applied voltage. The built-in voltage (V
bi
),
which is based on the relative work function difference of the two electrodes, was also
subtracted from the applied voltage. The built-in voltage can be determined from the
transition between the ohmic region and the SCL region and is found to be about 1 V.
Polymer film thickness was measured using GIXRD.
232

A1.5 References Appendix 1
(1) Cardona, C. M.; Li, W.; Kaifer, A. E.; Stockdale, D.; Bazan, G. C. Adv. Mater.
2011, 23, 2367–2371.
(2) Thompson, B. C.; Kim, Y.-G.; McCarley, T. D.; Reynolds, J. R. J. Am. Chem.
Soc. 2006, 128, 12714–12725.
(3) Kenning, D. D.; Mitchell, K. A.; Calhoun, T. R.; Funfar, M. R.; Sattler, D. J.;
Rasmussen, S. C. J. Org. Chem. 2002, 67, 9073–9076.
(4) Wen, L.; Nietfeld, J. P.; Amb, C. M.; Rasmussen, S. C. J. Org. Chem. 2008, 73,
8529–8536.
(5) Shahid, M.; Ashraf, R. S.; Klemm, E.; Sensfuss, S. Macromolecules 2006, 39,
7844–7853.
-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
-10
-5
0
5
10
Current density (mA/cm
2
)
Voltage (V)
P3HT:PCBM
P3HTT-TP:PCBM
P3HTT-BTD:PCBM
P3HTT-TP-BTD:PCBM

Figure A1.13 J-V curves of solar cells based on
P3HT, P3HTT-TP, P3HTT-BTD and P3HTT-
TP-BTD.
233
APPENDIX 2 Efficient Solar Cells from Semi-Random P3HT
Analogues Incorporating Diketopyrrolopyrrole
A2.1 Synthesis
For general materials and methods section see appendix 1. Synthetic procedures
for the synthesis of 2-bromo-5-trimethyltin-3-hexylthiopehene and 2,5-
bis(trimethyltin)thiophene were used without modifications as reported in the literature.
1

Figure A2.1 Synthesis of 2,5-diethylhexyl-3,6-bis(5-
bromothiophene-2-yl)pyrrolo[3,4-c]-pyrrole-1,4-dione (3).
234
3,6-Dithiophene-2-yl-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (1) 11.2 g
(100 mmol) t-BuOK was added to a 500 ml round bottom flask. A solution of 6.54 g (60
mmol) 2-thiophene carbonitrile in 50 ml t-amyl alcohol was added and the reaction
mixture was heated to 110 ºC. A solution of dimethyl succinate (2.92 g, 20 mmol) in 16
ml t-amyl alcohol was added dropwise over a period of two hours. The reaction mixture
was then left to stir for an additional 2 hours at 110 ºC before cooling below 65 ºC and
diluting with 100 ml methanol. The mixture was neutralized with approximately 5 ml
glacial acetic acid, filtered and washed with methanol followed by H
2
O. The product was
obtained as a dark purple solid with 52% yield (3.075 g, 10.2 mmol) and used directly
without additional purification.
1
H NMR (400 MHz, DMSO-D
6
) δ 11.22 (s, 2H), 8.21
(dd, 2H), 7.96 (dd, 2H), 7.30 (dd, 2H);
13
C NMR (100 MHz, DMSO-D
6
) δ 161.47,
136.00, 132.52, 131.12, 130.64, 128.56, 108.40.
2,5-Diethylhexyl-3,6-dithiophene-2-yl-pyrrolo[3,4-c]pyrrole-1,4-dione (2)
Modified from the literature.
2
1.75 g (5.83 mmol) of 1 and 3.24 g (23 mmol) K
2
CO
3
were
dissolved in 34 ml DMF in a 100 ml round bottom flask equipped with a condenser. The
mixture was heated to 145 ºC and 5.21 g (27 mmol) 2-ethylhexyl bromide was added to
the solution. The system was stirred for 15 hours at 145 ºC. After cooling to r.t. the
mixture was poured into 75 ml of cold water, filtered and washed with H
2
O and methanol
several times. Purification via column chromatography (CH
2
Cl
2
) gave a purple solid as
the product (0.652 g, 1.24 mmol, 20%).
1
H NMR (400 MHz, CDCl
3
) δ 8.89 (dd, 2H) 7.62
(dd, 2H), 7.27 (dd, 2H), 4.03 (m, 4H), 1.86 (m, 2H), 1.38 – 1.25 (m, 16H), 0.85 (m, 12H);
235
13
C NMR (100 MHz, CDCl
3
) δ 161.75, 140.42, 135.25, 130.49, 129.83, 128.41, 107.94,
45.86, 39.08, 30.23, 28.36, 23.55, 23.05, 14.01, 10.48.
2,5-Diethylhexyl-3,6-bis(5-bromothiophene-2-yl)pyrrolo[3,4-c]-pyrrole-1,4-
dione (3) Modified from the literature.
3
350 mg (0.668 mmol) of 2 was dissolved in 15
ml CHCl
3
and stirred for 15 minutes. 240 mg (1.35 mmol) of N-bromosuccinimide was
added in one part and the reaction mixture was left stirring in the dark for 48 hours. The
reaction mixture was then poured in 100 ml methanol, stirred for one hour, filtered and
washed with hot methanol and hot H
2
O several times. After drying, 379 mg (0.56 mmol,
83%) of the purple product were obtained.
1
H NMR (400 MHz, CDCl
3
) δ 8.65 (d, 2H),
7.23 (d.2H), 3.94 (m, 4H), 1.84 (m, 2H), 1.35 – 1.26 (m, 16H), 0.87 (m, 12H);
13
C NMR
(100 MHz, CDCl
3
) δ 161.39, 139.39, 135.35, 131.45, 131.15, 118.99, 108.00, 46.01,
39.10, 30.18, 28.32, 23.56, 23.02, 14.01, 10.46.
236

Stille Polymerization for P3HTT-DPP-5%, P3HTT-DPP-10% and P3HTT-
DPP-15% (Figure 2.2):
P3HTT-DPP-5% 541 mg (1.32 mmol) 2-bromo-5-trimethyltin-3-
hexylthiophene, 30 mg (0.073 mmol) 2,5-bis(trimethyltin)thiophene and 50 mg (0.073
mmol) 2,5-diethylhexyl-3,6-bis(5-bromothiophene-2-yl)pyrrolo[3,4-c]-pyrrole-1,4-dione
were dissolved in 37 ml of dry DMF to give a 0.04 M solution. The solution was then
degassed for 20 min before 73 mg (4 mol%) Pd(PPh
3
)
4
was added in one portion. The
solution was degassed for 10 additional minutes and then heated for 44 hours at 95 ºC.
Then the reaction mixture was cooled to room temperature and precipitated into
methanol. Purification was achieved via soxhlet extraction using methanol followed by

Figure A2.2 Synthesis of semi-random copolymers P3HTT-DPP-5%,
P3HTT-DPP-10% and P3HTT-DPP-15%.
237
hexanes, a mixture of CHCl
3
: methanol (60:40) and finally CHCl
3
. The polymer was then
reprecipitated into methanol, vacuum filtered and dried. Yield: 62% (160 mg), M
n
=
19,000, PDI = 2.8.
1
H NMR (500 MHz, CDCl
3
) δ 8.93 (s, 0.11H), 7.15, 7.11 (d, 0.23H),
6.99 (m, 0.99H), 4.06 (s br, 0.20H), 2.80 (s, 2H), 2.58 (s, 0.07H), 1.94 (s, 0.03H), 1.70
(m, 2.04H), 1.44, 1.35 (m, 8.46H), 0.91 (s, 4.46H).
P3HTT-DPP-10% 526 mg (1.28 mmol) 2-bromo-5-trimethyltin-3-
hexylthiophene, 66 mg (0.16 mmol) 2,5-bis(trimethyltin)thiophene and 110 mg (0.16
mmol) 2,5-diethylhexyl-3,6-bis(5-bromothiophene-2-yl)pyrrolo[3,4-c]-pyrrole-1,4-dione
were dissolved in dry 40 ml DMF to give a 0.04 M solution. The solution was then
degassed for 20 min before 80 mg (4 mol%) Pd(PPh
3
)
4
was added in one portion. The
solution was degassed for 10 additional minutes and then heated for 44 hours at 95
o
C.
Then the reaction mixture was cooled to room temperature and precipitated into
methanol. Purification was achieved via soxhlet extraction using methanol followed by
hexanes, CH
2
Cl
2
and finally CHCl
3
. The polymer was then reprecipitated into methanol,
vacuum filtered and dried. Yield: 50% (158 mg), M
n
= 24,570, PDI = 2.3.
1
H NMR (500
MHz, CDCl
3
) δ 8.93 (s, 0.24H), 7.13-6.97 (m, 1.42H), 4.05 (s br, 0.43H), 2.79 (s, 2H),
2.56 (m, 0.10H), 1.94 (s, 0.21H), 1.70 (m, 2.15H), 1.44, 1.35 (m, 9.60H), 0.91 (s, 5.28H).
P3HTT-DPP-15% 371 mg (0.90 mmol) 2-bromo-5-trimethyltin-3-
hexylthiophene, 79 mg (0.19 mmol) 2,5-bis(trimethyltin)thiophene and 132 mg (0.19
mmol) 2,5-diethylhexyl-3,6-bis(5-bromothiophene-2-yl)pyrrolo[3,4-c]-pyrrole-1,4-dione
were dissolved in dry 32 ml DMF to give a 0.04 M solution. The solution was then
degassed for 20 min before 65 mg (4 mol%) Pd(PPh
3
)
4
was added in one portion. The
238
solution was degassed for 10 additional minutes and then heated for 44 hours at 95
o
C.
Then the reaction mixture was cooled to room temperature and precipitated into
methanol. Purification was achieved via soxhlet extraction using methanol followed by
hexanes, toluene and finally chlorobenzene. The polymer was then reprecipitated into
methanol, vacuum filtered and dried. Yield: 77% (210 mg), M
n
= 17,570, PDI = 2.9.
1
H
NMR (500 MHz, Tetrachloroethane-d
2
, 100
o
C) δ 8.89 (s, 0.43H), 7.33 (m, 0.49H), 7.20
(m, 0.9H), 7.04 (m, 0.67H), 4.07 (s br, 0.81H), 2.85 (s, 2H), 2.73 (m, 0.07H), 1.97 (s,
0.46H), 1.75 (s, 2.19H), 1.40 (m br, 12.13H), 0.95 (s, 6.75H).

239
A2.2 Polymer Characterization

Figure A2.3
1
H NMR of P3HTT-DPP-5% in CDCl
3
.
240

Figure A2.4
1
H NMR of P3HTT-DPP-10% in CDCl
3
.
241

Figure A2.5
1
H NMR of P3HTT-DPP-15% in tetrachloroethane-d2.

Monomer Feed Ratio vs. Monomer Composition in Polymers:
Example of P3HTT-DPP-5%:
In order to calculate the monomer composition in the polymer the protons marked
A and B in the
1
H NMR spectra can be used. A-protons are aromatic protons from bis-
thiophene-diketopyrrolopyrrole and B-protons are the benzylic protons in hexylthiophene
(B-protons have two distinct peaks marked with arrows because of the regioregularity of
the polymer <100%). Protons of the unsubstituted thiophene unit cannot be used for the
242
calculation as they are indistinguishable from other aromatic protons in the region around
7 ppm, but they will be used to check consistency of the calculations.
In P3HTT-DPP-5% hexylthiophene and bis-thiophene-diketopyrrolopyrrole make
up 95% of the polymer based on the monomer feed ratio. As we for now only consider
the integrations of these two monomers the 95% will be set as 100%. This means that A-
protons should be 5.26% of the total integration of A and B-peaks ((5/95)*100). As such,
B-peaks should be 94.74% of the total integration of A and B, if the monomer feed ratio
is equivalent to the polymer composition. Looking at Figure A2.3, the integration for A is
0.11 and for combined B 2.07. This gives an actual value of 5.05% for A
((0.11/2.18)*100) and 94.95% for B. Dividing 5.05% by the predicted value of 5.26% we
get 0.96. Multiplication of the 5% feed ratio of bis-thiophene-diketopyrrolopyrrole with
0.96 gives the actual monomer ratio as 4.8%. Considering the accuracy of NMR these
values confirm that the feed ratio is in fact the monomer ratio in the polymer.
To double check our assumption that bis-thiophene-diketopyrrolopyrrole and
unsubstituted thiophene are present in the polymer in equal amounts (allowing us to use
the above described calculation method) the number of aromatic protons in the region
around 7 ppm are calculated and compared with the actual integration value. The
aromatic proton of hexylthiophene (which should integrate for 1.04 considering that the
two benzylic protons integrate as 2.07), the other two aromatic protons of bis-thiophene-
diketopyrrolopyrrole (integrating for 0.11) as well as two aromatic protons of
unsubstituted thiophene (theoretically also integrating for 0.11 if the assumption of equal
amounts of the two monomers incorporated in the polymer is correct) are all situated
243
around 7 ppm. This adds up to a predicted integration of 1.26 for all aromatic protons
other than the A protons. The actual integration of this region is 1.22 as can be seen in
Figure A2.3, confirming the assumption that equal ratios of bis-thiophene-
diketopyrrolopyrrole and unsubstituted thiophene are in the polymer and that the
monomer feed ratio is equivalent (within the limits of NMR) to the polymer composition.

Table A2.1 Optical properties of P3HT, P3HTT-DPP-5%, P3HTT-DPP-10% and
P3HTT-DPP-15% in o-DCB and thin films, spin-coated from o-DCB.


max,abs
(nm)
solution
Absorptivity
(L/cm*g)

max,abs

(nm)
film
Absorption
coefficient (cm
-1
)
E
g

(nm/eV)
c

P3HT 463 40.5 559 122068
a
648/1.91
P3HTT-DPP-5%
449 25.3 532 56503
b
814/1.52
670 17.8 682 43528
b

743 36547
b

P3HTT-DPP-10%
438 25.6 502 42107
b
821/1.51
671 33.7 685 71460
b

746 61924
b

P3HTT-DPP-15%
452 14.6 472 36400
b
848/1.46
696 40.5 703 97210
b

767 93677
b

a
Annealed at 150

ºC for 30 min under N
2
.
b
As-cast.
c
Calculated from the absorption
band edge in thin films, E
g
= 1240/ 
edge
.

244

Figure A2.6 CV traces for the oxidation of a. P3HTT-DPP-5%, b. P3HTT-DPP-10%
and c. P3HTT-DPP-15%.
245

Figure A2.7 GIXRD of thin films of (i) P3HT
(spin-coated from CB and annealed at 150 ºC for
30 min under N
2
) (black line), (ii) P3HTT-DPP-
5% (spin-coated from o-DCB and annealed at
150 ºC for 30 min under N
2
) (red line), (iii)
P3HTT-DPP-10% (spin-coated from o-DCB and
tested as-cast) (green line), (iv) P3HTT-DPP-
10% (spin-coated from o-DCB and annealed at
150

ºC for 30 min under N
2
) (purple line) and (v)
P3HTT-DPP-15% (spin-coated from o-DCB and
annealed at 150

ºC for 30 min under N
2
) (blue
line).
246

A2.3 Device Fabrication and Characterization
For general information on device fabrication, measurement of I-V characteristics
of photovoltaic devices and mobility measurements see appendix 1. The P3HT:PC
61
BM
film was spin-coated from CB solution (10 mg/ml in P3HT), forming 95 nm thick layer
and directly placed in the vacuum chamber for aluminum deposition. The P3HTT-DPP-
5%:PC
61
BM and P3HTT-DPP-10%:PC
61
BM layers were spin cast from a solution in o-
DCB containing 10 mg/ml P3HTT-DPP-5% or P3HTT-DPP-10% and 10 mg/ml PC
61
BM
(P3HTT-DPP-5%) or 13.5 mg/ml PC
61
BM (P3HTT-DPP-10%). Films were placed in a

Figure A2.8 TEM images of (a) P3HT:PC
61
BM, (b) P3HTT-DPP-5%:PC
61
BM, (c)
P3HTT-DPP-10%:PC
61
BM and (d) P3HTT-DPP-15%:PC
61
BM prepared under optimal
solar cells conditions.
247
nitrogen cabinet for 20 min, before being transferred to the vacuum chamber. Due to
limited solubility, the solution of P3HTT-DPP-15%:PC
61
BM was prepared by dissolving
the polymer (8 mg/ml) and PC
61
BM (20.8 mg/ml) in a o-DCB solvent. As in case of
P3HTT-DPP-5% and P3HTT-DPP-10% devices were placed for 20 min in a nitrogen
cabinet before loading in the vacuum chamber. At the final stage, the substrates were
pumped down to high vacuum (< 7×10
-7
Torr) and aluminum (100 nm) was thermally
evaporated at 3 – 4 Å/sec using a Denton Benchtop Turbo IV Coating System onto the
active layer through shadow masks to define the active area of the devices as 4.9 mm
2
.
Thermal annealing of P3HT:PC
61
BM blends was carried out by directly placing the
completed devices in the nitrogen oven for 60 min at 145 °C. After annealing, the devices
were cooled down to room temperature before measurements were carried out. All
P3HTT-DPP:PC
61
BM BHJ solar cell was tested without thermal treatment. See appendix
1 for details on current-voltage measurements.
EQE measurements were performed using a 300 W Xenon arc lamp (Newport
Oriel), chopped and filtered monochromatic light (250 Hz, 10 nm FWHM) from a
Conerstone 260 1/4 M double grating monochromator (Newport 74125) together with an
EG&G 7220 lock-in amplifier. A silicon photodiode (Hamamatsu S1787-04, 8RA filter)
calibrated at the National Renewable Energy Laboratory (NREL) was utilized as the
reference cell.
The spectral mismatch correction (spectral-mismatch factor (M)) was performed
according to Shrotriya et al.,
4
where mismatch factor is defined as:
248
22
11
22
11
( ) ( ) ( ) ( )
( ) ( ) ( ) ( )
Ref R S T
Ref T S R
E S d E S d
M
E S d E S d




     
     



(1),
where E
Ref
( λ) is the reference spectral irradiance; E
S
( λ) is the source spectral irradiance;
S
R
( λ) is the spectral responsivity; and S
T
( λ) is the spectral responsivity of the test cell,
each as a function of wavelength ( λ). Spectral responsivities S( λ) for the tested devices
were calculated based on the external quantum efficiency (EQE) values, according to
equation 2:
( ) ( )
q
S EQE
hc

  (2),
where the constant term q/hc equals 8.0655 x 10
5
for wavelength in units of meters and
S( λ) in units of AW
-1
. Based on the spectral responsivities S( λ) obtained using equation 2,
integrated short-circuit current densities (J
sc,EQE
) can be obtained:
2
1
,
( ) ( )
sc EQE Ref T
J E S d


   

(3).
In order to mismatch-correct the efficiencies of the BHJ solar cells, short-circuit
current densities (J
sc
) were divided by the M, as defined in equation 4. The raw data (J
sc
),
spectral-mismatch factor (M) and the spectral mismatch-corrected short-circuit current
densities (J
sc,corr
) are summarized in Table A2.2.
,
sc
sc corr
J
J
M

(4).

249
Table A2.2 Raw short-circuit current densities (Jsc), spectral-mismatch factor (M),
spectral mismatch-corrected short-circuit current densities (Jsc,corr) and integrated short-
circuit current densities (Jsc,EQE).
Polymer:PC
61
BM
(ratio)
J
sc

(mA/cm
2
)
M
J
sc,corr
(mA/cm
2
)
J
sc,EQE
(mA/cm
2
)
(J
sc,corr
/ J
sc,EQE
)
x 100
(%)
P3HT (1:1)
a
9.90 1.05 9.49 9.02 4.9
P3HTT-DPP-5% (1:1)
b
8.33 0.87 9.57 9.07 5.2
P3HTT-DPP-10% (1:1.3)
b
10.54 0.76 13.87 13.24 4.5
P3HTT-DPP-5% (1:2.6)
b
9.54 0.71 13.44 12.78 4.9
a
Spin-coated from CB and annealed at 150

ºC for 30 min under N
2
after aluminum
deposition.
b
Spin-coated from o-DCB and tested after 20 minutes of slow solvent
evaporation in a N
2
cabinet.

Figure A2.9 J-V curves of the BHJ solar cells
based on P3HT (black line), P3HTT-DPP-5%
(red line), P3HTT-DPP-10% (green line) and
P3HTT-DPP-15% (blue line) with PC
61
BM as
the acceptor under AM 1.5G illumination (100
mW/cm
2
) at the optimal conditions for solar cell
performance.
250
A2.4 References Appendix 2
(1) Burkhart, B.; Khlyabich, P. P.; Cakir Canak, T.; LaJoie, T. W.; Thompson, B. C.
Macromolecules 2011, 44, 1242-1246.
(2) Huo, L.; Hou, J.; Chen, H.-Y.; Zhang, S.; Jiang, Y.; Chen, T. L.; Yang, Y.
Macromolecules 2009, 42, 6564-6571.
(3) Tamayo, A. B.; Tantiwiwat, M.; Walker, B.; Nguyen, T.-Q. J. Phys. Chem. C 2008,
112, 15543-15552.
(4) Shrotriya, V.; Li, G.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. Adv. Funct. Mater.
2006, 16, 2016-2023.

251
APPENDIX 3 Semi-Random Two Acceptor Copolymers:
Influence of the Acceptor Composition on Physical
Properties and Solar Cell Performance
A3.1 Synthesis
For a general materials and methods section see appendix 1. The synthetic
procedures for 2-bromo-5-trimethyltin-3-hexylthiophene, 2,5-bis(trimethyltin)thiophene,
P3HT, P3HTT-DPP-10% and P3HTT-DPP-15% have been described in appendix 1 and
2. 1,3-dibromo-5-octylthieno[3,4-c]pyrrole-4,6-dione was synthesized according to
reported procedures.
1,2

Stille Polymerization for P3HTT-TPD-10%, P3HTT-TPD-15%, P3HTT-
TPD-DPP (1:1), P3HTT-TPD-DPP (2:1) and P3HTT-TPD-DPP (1:2):
P3HTT-TPD-10% 396 mg (0.96 mmol) 2-bromo-5-trimethyltin-3-
hexylthiopehene, 49 mg (0.12 mmol) 2,5-bis(trimethyltin)thiophene and 51 mg (0.12
mmol) 1,3-dibromo-5-octylthieno[3,4-c]pyrrole-4,6-dione were dissolved in 30 ml dry
DMF to give a 0.04 M solution. The solution was then degassed for 20 min before 60 mg
(4 mol%) Pd(PPh
3
)
4
was added in one portion. The solution was degassed for 10
additional minutes and then heated for 44 hours at 95
o
C. Then the reaction mixture was
cooled to room temperature and precipitated into methanol. Purification was achieved via
soxhlet extraction using methanol followed by hexanes, CH
2
Cl
2
and finally CHCl
3
. The
252
polymer was then reprecipitated into methanol, vacuum filtered and dried. Yield: 66%
(136 mg), M
n
= 22,630, PDI = 1.98.
1
H NMR (500 MHz, tetrachloroethane-d2) δ 7.87,
7.84 (m, 0.28H), 7.16-7.03 (1.06H), 3.69 (s br, 0.25H), 2.85 (s, 2H), 2.62 (m, 0.14H),
1.76 (m, 2.55H), 1.48-1.33 (m, 9.02H), 0.96 (s, 3.67H).
P3HTT-TPD-15% 462 mg (1.13 mmol) 2-bromo-5-trimethyltin-3-
hexylthiopehene, 99 mg (0.24 mmol) 2,5-bis(trimethyltin)thiophene and 102 mg (0.24
mmol) 1,3-dibromo-5-octylthieno[3,4-c]pyrrole-4,6-dione were dissolved in 40 ml dry
DMF to give a 0.04 M solution. The solution was then degassed for 20 min before 80 mg
(4 mol%) Pd(PPh
3
)
4
was added in one portion. The solution was degassed for 10
additional minutes and then heated for 44 hours at 95
o
C. Then the reaction mixture was
cooled to room temperature and precipitated into methanol. Purification was achieved via
soxhlet extraction using methanol followed by hexanes, CH
2
Cl
2
and finally
chlorobenzene. The polymer was then reprecipitated into methanol, vacuum filtered and
dried. Yield: 58% (159 mg), M
n
= 12,090, PDI = 2.05.
1
H NMR (500 MHz,
tetrachloroethane-d2) δ 7.87, 7.84 (m, 0.45H), 7.17 -7.05 (m, 1.12H), 3.7069 (s br,
0.48H), 2.85 (s, 2H), 2.61 (m, 0.21H), 1.75 (m, 2.97H), 1.48-1.33 (m, 11.00H), 0.96 (s,
4.37H).
P3HTT-TPD-DPP (1:1) 373 mg (0.91 mmol) 2-bromo-5-trimethyltin-3-
hexylthiopehene, 80 mg (0.19 mmol) 2,5-bis(trimethyltin)thiophene, 41 mg (0.09 mmol)
1,3-dibromo-5-octylthieno[3,4-c]pyrrole-4,6-dione and 66 mg (0.09 mmol) of 2,5-
diethylhexyl-3,6-bis(5-bromothiophene-2-yl)pyrrolo[3,4-c]-pyrrole-1,4-dione were
dissolved in 32 ml dry DMF to give a 0.04 M solution. The solution was then degassed
253
for 20 min before 65 mg (4 mol%) Pd(PPh
3
)
4
was added in one portion. The solution was
degassed for 10 additional minutes and then heated for 44 hours at 95
o
C. Then the
reaction mixture was cooled to room temperature and precipitated into methanol.
Purification was achieved via soxhlet extraction using methanol followed by hexanes,
CH
2
Cl
2
and finally CB. The polymer was then reprecipitated into methanol, vacuum
filtered and dried. Yield: 69% (171 mg), M
n
= 11,730, PDI = 2.98.
1
H NMR (500 MHz,
tetrachloroethane-d2) δ 8.88 (s, 0.21H), 7.88 -7.85 (m, 0.21H), 7.38-7.05 (m, 1.54H),
4.08 (s br, 0.43H), 3.71 (s br, 0.21H), 2.86 (s, 2H), 2.66 (m, 0.10H), 1.97 (s, 0.25H), 1.77
(m, 2.34H), 1.49-1.35 (m, 10.38H), 0.96 (s, 4.90H).
P3HTT-TPD-DPP (2:1) 479 mg (1.17 mmol) 2-bromo-5-trimethyltin-3-
hexylthiopehene, 103 mg (0.25 mmol) 2,5-bis(trimethyltin)thiophene, 71 mg (0.17 mmol)
1,3-dibromo-5-octylthieno[3,4-c]pyrrole-4,6-dione and 57 mg (0.08 mmol) of 2,5-
diethylhexyl-3,6-bis(5-bromothiophene-2-yl)pyrrolo[3,4-c]-pyrrole-1,4-dione were
dissolved in 42 ml dry DMF to give a 0.04 M solution. The solution was then degassed
for 20 min before 83 mg (4 mol%) Pd(PPh
3
)
4
was added in one portion. The solution was
degassed for 10 additional minutes and then heated for 44 hours at 95
o
C. Then the
reaction mixture was cooled to room temperature and precipitated into methanol.
Purification was achieved via soxhlet extraction using methanol followed by hexanes,
CH
2
Cl
2
, chloroform and finally CB. The polymer was then reprecipitated into methanol,
vacuum filtered and dried. Yield: 65% (198 mg), M
n
= 12,650, PDI = 3.36.
1
H NMR (500
MHz, tetrachloroethane-d2) δ 8.88 (s, 0.14H), 7.87 -7.85 (m, 0.28H), 7.35-7.05 (m,
254
1.39H), 4.08 (s br, 0.28H), 3.70 (s br, 0.29H), 2.86 (s, 2H), 2.64 (m, 0.13H), 1.98 (s,
0.15H), 1.76 (m, 2.42H), 1.51-1.41 (m, 11.42H), 0.96 (s, 4.65H).
P3HTT-TPD-DPP (1:2) 476 mg (1.16 mmol) 2-bromo-5-trimethyltin-3-
hexylthiopehene, 102 mg (0.25 mmol) 2,5-bis(trimethyltin)thiophene, 35 mg (0.08 mmol)
1,3-dibromo-5-octylthieno[3,4-c]pyrrole-4,6-dione and 113 mg (0.16 mmol) of 2,5-
diethylhexyl-3,6-bis(5-bromothiophene-2-yl)pyrrolo[3,4-c]-pyrrole-1,4-dione were
dissolved in 41 ml dry DMF to give a 0.04 M solution. The solution was then degassed
for 20 min before 82 mg (4 mol%) Pd(PPh
3
)
4
was added in one portion. The solution was
degassed for 10 additional minutes and then heated for 44 hours at 95
o
C. Then the
reaction mixture was cooled to room temperature and precipitated into methanol.
Purification was achieved via soxhlet extraction using methanol followed by hexanes,
chloroform, toluene and finally CB. The polymer was then reprecipitated into methanol,
vacuum filtered and dried. Yield: 53% (171 mg), M
n
= 19,700, PDI = 3.07.
1
H NMR (500
MHz, tetrachloroethane-d2) δ 8.87 (s, 0.30H), 7.87 -7.85 (m, 0.15H), 7.34-7.96 (m,
1.76H), 4.07 (s br, 0.62H), 3.69 (s br, 0.15H), 2.86 (s, 2H), 2.66 (m, 0.13H), 1.98 (s,
0.36H), 1.76 (m, 2.48H), 1.49-1.35 (m, 11.91H), 0.95 (s, 5.65H).

255
A3.2 Polymer Characterization

Figure A3.1
1
H NMR of P3HTT-TPD-10% in tetrachloroethane-d2 at 60 °C.

Figure A3.2
1
H NMR of P3HTT-TPD-15% in tetrachloroethane-d2 at 60 °C.

256

Figure A3.3
1
H NMR of P3HTT-TPD-DPP (1:1) in tetrachloroethane-d2 at 60 °C.

Figure A3.4
1
H NMR of P3HTT-TPD-DPP (2:1) in tetrachloroethane-d2 at 60 °C.

257

Figure A3.5
1
H NMR of P3HTT-TPD-DPP (1:2) in tetrachloroethane-d2 at 60 °C.

Figure A3.6 UV/vis spectra of polymer solutions in o-DCB
with P3HTT-DPP-10% and P3HTT-DPP-15% for
reference.
258

Figure A3.7 CV traces for the oxidation of thin
films (as cast) in acetonitrile with ferrocene as
the reference (Fc/Fc
+
= 5.1 eV relative to
vacuum).
3,4

Figure A3.8 GIXRD of thin films spin coated
from o-DCB. P3HT was solvent annealed for 20
minutes under N
2
and P3HTT-TPD-10%,
P3HTT-TPD-DPP (1:1), P3HTT-TPD-DPP
(2:1), P3HTT-TPD-DPP (1:2) were thermally
annealed at 150 °C for 45 minutes under N
2
.
P3HTT-TPD-15% did not show any crystalline
features.
259

Figure A3.9 GIXRD of thin films spin coated from o-DCB
and solvent annealed for 20 minutes under N
2
. Only
P3HTT-TPD-DPP (1:1) and P3HTT-TPD-DPP (1:2) show
crystalline features whereas P3HTT-TPD-10%, P3HTT-
TPD-15% and P3HTT-TPD-DPP (2:1) are completely
amorphous.
260

Figure A3.10 TEM images of a) P3HT:PC
61
BM, b) P3HTT-DPP-10%: PC
61
BM,
c) P3HTT-DPP-15%: PC
61
BM, d) P3HTT-TPD-10%: PC
61
BM, e) P3HTT-TPD-
15%:PC
61
BM, f) P3HTT-TPD-DPP (1:1):PC
61
BM, g) P3HTT-TPD-DPP (2:1):
PC
61
BM, h) P3HTT-TPD-DPP (1:2):PC
61
BM prepared under optimal solar cell
conditions.
261
A3.3 Device Fabrication and Characterization
For general device fabrication, measurement of I-V characteristics of photovoltaic
devices and mobility measurement methods see appendix 1. Procedures for EQE
measurements and mismatch correction are described in appendix 2. Separate solutions of
P3HTT-TPD-10% and P3HTT-TPD-15% in CB and P3HT, P3HTT-TPD-DPP (1:1),
P3HTT-TPD-DPP (2:1), P3HTT-TPD-DPP (1:2), P3HTT-DPP-10%, P3HTT-DPP-15%
in o-DCB and PC
61
BM in CB or in o-DCB were prepared. The solutions were stirred for
24 hrs before they were mixed at the desired ratios and stirred for 24 hrs to form a
homogeneous mixture. Subsequently, the polymer:PC
61
BM active layer was spin-coated
(with a 0.45 m PTFE syringe filter - Pall Life Sciences) on top of the PEDOT:PSS
layer. Concentrations of the polymer:PC
61
BM solutions for P3HTT-TPD-10%, P3HTT-
TPD-15%, P3HT, P3HTT-TPD-DPP (1:1), P3HTT-TPD-DPP (2:1), P3HTT-DPP-10%,
were 10 mg/ml in polymer. Concentrations were of 8 mg/ml for P3HTT-TPD-DPP (1:2)
and P3HTT-DPP-15% for polymer:PC
61
BM solutions. Upon spin-coating of
polymer:PC
61
BM solutions based on P3HT, P3HTT-TPD-DPP (1:1), P3HTT-TPD-DPP
(2:1), P3HTT-TPD-DPP (1:2), P3HTT-DPP-10%, P3HTT-DPP-15%, films were first
placed to the N
2
cabinet for: 30 min in case of P3HT, P3HTT-DPP-10%, P3HTT-DPP-
15%; 20 min in case of P3HTT-TPD-DPP (1:1), P3HTT-TPD-DPP (2:1), P3HTT-TPD-
DPP (1:2) and then placed in the vacuum chamber for aluminum deposition. At the final
stage, the substrates were pumped down to high vacuum (< 9×10
-7
Torr) and aluminum
(100 nm) was thermally evaporated at 3 – 4 Å/sec using a Denton Benchtop Turbo IV
Coating System onto the active layer through shadow masks to define the active area of
262
the devices as 4.4 mm
2
. P3HTT-TPD-10% and P3HTT-TPD-15% based solar cells were
tested as-cast.

Table A3.1 Raw short-circuit current densities (J
sc
), spectral-mismatch factor (M),
spectral mismatch-corrected short-circuit current densities (J
sc,corr
) and integrated short-
circuit current densities (J
sc,EQE
).
Polymer:PC
61
BM
J
sc

(mA/cm
2
)
M
J
sc,corr
(mA/cm
2
)
J
sc,EQE
(mA/cm
2
)
(J
sc,corr
/
J
sc,EQE
) × 100
(%)
P3HT
a,b
9.88 1.00 9.87 9.64 2.31
P3HTT-TPD-10%
c
4.75 0.88 5.38 5.17 3.94
P3HTT-TPD-15%
c
4.85 0.91 5.33 5.16 3.31
P3HTT-DPP-10%
a,b
11.53 0.79 14.62 13.89 4.78
P3HTT-DPP-15%
a,b
11.03 0.77 14.28 13.72 3.68
P3HTT-TPD-DPP (1:1)
a,d
12.35 0.81 15.26 14.74 3.40
P3HTT-TPD-DPP (2:1)
a,d
9.93 0.85 11.67 11.24 3.86
P3HTT-TPD-DPP (1:2)
a,d
12.75 0.78 16.37 16.15 1.09
a
Spin-coated from o-DCB and placed to the N
2
cabinet before aluminum deposition for
b
30 min and
d
20 min.
c
Spin-coated CB and tested as-cast.

263

A3.4 References Appendix 3
(1) Y.; Leclerc,
M. J. Am. Chem. Soc. 2010, 132, 5330–5331.
(2)
J. Am. Chem. Soc. 2010, 132, 7595–7597.
(3) Cardona, C. M.; Li, W.; Kaifer, A. E.; Stockdale, D.; Bazan, G. C. Adv. Mater.
2011, 23, 2367–2371.
(4) Thompson, B. C.; Kim, Y.-G.; McCarley, T. D.; Reynolds, J. R. J. Am. Chem.
Soc. 2006, 128, 12714–12725.

Figure A3.11 J-V curves of all polymer:PC
61
BM BHJ solar
cells under AM 1.5G illumination (100 mW/cm
2
) at
optimized conditions for solar cell performance. P3HT is
shown for reference.
264
APPENDIX 4 Semi-Random Two-Acceptor Copolymers:
Elucidating Electronic Trends Through Multiple Acceptor
Combinations
A4.1 Synthesis
For general materials and methods section see appendix 1. The synthetic
procedures for 2-bromo-5-trimethyltin-3-hexylthiophene, 2,5-bis(trimethyltin)thiophene,
4,7-dibromo-2,1,3-benzothiadiazole, 5,7-dibromo-thieno[3,4-b]pyrazine and 2,5-
diethylhexyl-3,6-bis(5-bromothiophene-2-yl)pyrrolo[3,4-c]-pyrrole-1,4-dione have been
described in appendix 1 and 2. 1,3-dibromo-5-octylthieno[3,4-c]pyrrole-4,6-dione was
synthesized according to the literature and references are given in appendix 3.
Stille Polymerization for P3HTT-BTD-DPP, P3HTT-TP-DPP, P3HTT-BTD-
TPD and P3HTT-TP-TPD:
P3HTT-BTD-DPP 348 mg (0.85 mmol) 2-bromo-5-trimethyltin-3-
hexylthiopehene, 75 mg (0.18 mmol) 2,5-bis(trimethyltin)thiophene, 27 mg (0.09 mmol)
4,7-dibromo-2,1,3-benzothiadiazole and 62 mg (0.09 mmol) of 2,5-diethylhexyl-3,6-
bis(5-bromothiophene-2-yl)pyrrolo[3,4-c]-pyrrole-1,4-dione were dissolved in 30 ml dry
DMF to give a 0.04 M solution. The solution was then degassed for 20 min before 60 mg
(4 mol%) Pd(PPh
3
)
4
was added in one portion. The solution was degassed for 10
additional minutes and then heated for 44 hours at 95
o
C. Then the reaction mixture was
cooled to room temperature and precipitated into methanol. Purification was achieved via
265
soxhlet extraction using methanol followed by hexanes, toluene and finally
chlorobenzene. The polymer was then reprecipitated into methanol, vacuum filtered and
dried. Yield: 57% (124 mg), M
n
= 15,350, PDI = 3.4.
1
H NMR (500 MHz,
tetrachloroethane-d2) δ 8.88 (s, 0.2 3H), 8.03 (s, 0.22H), 7.89 (s, 0.22H), 7.35-7.05 (m,
1.73H), 4.08 (s, 0.46H), 2.94-2.86 (m, 2H), 2.65 (m, 0.11H), 1.98 (s, 0.26H), 1.76 (m,
2.07H), 1.51-1.41 (m, 9.74H), 0.96 (s, 4.46H).
P3HTT-TP-DPP 333 mg (0.81 mmol) 2-bromo-5-trimethyltin-3-
hexylthiopehene, 72 mg (0.17 mmol) 2,5-bis(trimethyltin)thiophene, 26 mg (0.09 mmol)
5,7-dibromo-thieno[3,4-b]pyrazine and 60 mg (0.09 mmol) of 2,5-diethylhexyl-3,6-bis(5-
bromothiophene-2-yl)pyrrolo[3,4-c]-pyrrole-1,4-dione were dissolved in 29 ml dry DMF
to give a 0.04 M solution. The solution was then degassed for 20 min before 60 mg (4
mol%) Pd(PPh
3
)
4
was added in one portion. The solution was degassed for 10 additional
minutes and then heated for 44 hours at 95
o
C. Then the reaction mixture was cooled to
room temperature and precipitated into methanol. Purification was achieved via soxhlet
extraction using methanol followed by hexanes, toluene and finally CB. The polymer was
then reprecipitated into methanol, vacuum filtered and dried. Yield: 65% (136 mg), M
n
=
22,770, PDI = 3.3.
1
H NMR (500 MHz, tetrachloroethane-d2) δ 8.8 7 (s, 0.23H), 8.56 (s,
0.22H), 7.55 (s, 0.23H), 7.34-7.05 (m, 1.64H), 4.08 (s, 0.46H), 2.86 (s, 2H), 2.63 (m,
0.14H), 1.98 (s, 0.41H), 1.76 (m, 2.29H), 1.51-1.41 (m, 9.19H), 0.97 (s, 4.33H).
P3HTT-BTD-TPD 447 mg (1.09 mmol) 2-bromo-5-trimethyltin-3-
hexylthiopehene, 96 mg (0.23 mmol) 2,5-bis(trimethyltin)thiophene, 34 mg (0.12 mmol)
4,7-dibromo-2,1,3-benzothiadiazole and 49 mg (0.12 mmol) of 1,3-dibromo-5-
266
octylthieno[3,4-c]pyrrole-4,6-dione were dissolved in 39 ml dry DMF to give a 0.04 M
solution. The solution was then degassed for 20 min before 78 mg (4 mol%) Pd(PPh
3
)
4

was added in one portion. The solution was degassed for 10 additional minutes and then
heated for 44 hours at 95
o
C. Then the reaction mixture was cooled to room temperature
and precipitated into methanol. Purification was achieved via soxhlet extraction using
methanol followed by hexanes, toluene and finally CB. The polymer was then
reprecipitated into methanol, vacuum filtered and dried. Yield: 66% (167 mg), M
n
=
16,770, PDI = 3.0.
1
H NMR (500 MHz, tetrachloroethane-d2) δ 8. 03 (s, 0.23H), 7.88-
7.85 (m, 0.47H), 7.27-7.05 (m, 1.11H), 3.70 (s, 0.23H), 2.93-2.86 (m, 2H), 2.63 (m,
0.15H), 1.76 (m, 2.33H), 1.49-1.35 (m, 9.07H), 0.96 (s, 3.28H).
P3HTT-TP-TPD 511 mg (1.25 mmol) 2-bromo-5-trimethyltin-3-
hexylthiopehene, 110 mg (0.26 mmol) 2,5-bis(trimethyltin)thiophene, 39 mg (0.13 mmol)
5,7-dibromo-thieno[3,4-b]pyrazine and 57 mg (0.13 mmol) of 1,3-dibromo-5-
octylthieno[3,4-c]pyrrole-4,6-dione were dissolved in 45 ml dry DMF to give a 0.04 M
solution. The solution was then degassed for 20 min before 89 mg (4 mol%) Pd(PPh
3
)
4

was added in one portion. The solution was degassed for 10 additional minutes and then
heated for 44 hours at 95
o
C. Then the reaction mixture was cooled to room temperature
and precipitated into methanol. Purification was achieved via soxhlet extraction using
methanol followed by hexanes, chloroform, toluene and finally CB. The polymer was
then reprecipitated into methanol, vacuum filtered and dried. Yield: 35% (98 mg), M
n
=
12,580, PDI = 3.3.
1
H NMR (500 MHz, tetrachloroethane-d2) δ 8. 56 (s, 0.23H), 7.85 (s,
267
0.23H),7.54 (s, 0.24H), 7.25-7.05 (m, 1.08H), 3.70 (s, 0.22H), 2.86 (s, 2H), 2.64 (m,
0.16H), 1.77 (m, 2.39H), 1.49-1.35 (m, 9.85H), 0.96 (s, 3.56H).

A4.2 Polymer Characterization

Figure A4.1
1
H NMR of P3HTT-BTD-DPP in tetrachloroethane-d2 at
60 °C.

268

Figure A4.2
1
H NMR of P3HTT-TP-DPP in tetrachloroethane-d2 at
60 °C.
269

Figure A4.3
1
H NMR of P3HTT-BTD-TPD in tetrachloroethane-d2 at
60 °C.
270

Figure A4.4
1
H NMR of P3HTT-TP-TPD in tetrachloroethane-d2 at
60 °C.
271

Figure A4.5 UV/vis absorption spectra of two-acceptor
copolymers in o-DCB solution. (i) is P3HT (black), (ii) is
P3HTT-BTD-DPP (blue), (iii) is P3HTT-TP-DPP (orange),
(iv) is P3HTT-BTD-TPD (green) and (v) is P3HTT-TP-
TPD (red).

Figure A4.6 GIXRD of thin films spin coated from o-DCB
and solvent annealed for 20 min under N
2
. (i) is P3HT
(black), (ii) is P3HTT-BTD-DPP (blue) and (iii) is P3HTT-
TP-DPP (orange).
272
A4.3 Device Fabrication and Characterization
For general device fabrication, measurement of I-V characteristics of photovoltaic
devices and mobility measurement methods see appendix 1. Procedures for EQE
measurements and mismatch correction are described in appendix 2. Separate solutions of
P3HT, P3HTT-BTD-TPD, P3HTT-TP-TPD, P3HTT-BTD-DPP, P3HTT-TP-DPP, and
PC
61
BM in o-DCB were prepared. The solutions were stirred for 24 hrs before they were
mixed at the desired ratios and stirred for 24 hrs to form a homogeneous mixture.
Subsequently, the polymer:PC
61
BM active layer was spin-coated (with a 0.45 m PTFE
syringe filter - Pall Life Sciences) on top of the PEDOT:PSS layer. Concentrations of the
polymer:PC
61
BM solutions in all cases were 10 mg/ml in polymer. Upon spin-coating of
polymer:PC
61
BM solutions based on P3HT, P3HTT-BTD-TPD, P3HTT-TP-TPD,
P3HTT-BTD-DPP, P3HTT-TP-DPP, films were first placed to the N
2
cabinet for: 30 min
in case of P3HT; 20 min in case of P3HTT-BTD-TPD, P3HTT-TP-TPD, P3HTT-BTD-
DPP, P3HTT-TP-DPP and then placed in the vacuum chamber for aluminum deposition.
At the final stage, the substrates were pumped down to high vacuum (< 9×10
-7
Torr) and
aluminum (100 nm) was thermally evaporated at 3 – 4 Å/sec using a Denton Benchtop
Turbo IV Coating System onto the active layer through shadow masks to define the
active area of the devices as 4.4 mm
2
. P3HTT-TPD-10% and P3HTT-TPD-15% based
solar cells were tested as-cast.

273
Table A4.1 Raw short-circuit current densities (J
sc
), spectral-mismatch factor (M),
spectral mismatch-corrected short-circuit current densities (J
sc,corr
) and integrated short-
circuit current densities (J
sc,EQE
).
Polymer:PC
61
BM
a

J
sc

(mA/cm
2
)
M
J
sc,corr
(mA/cm
2
)
J
sc,EQE
(mA/cm
2
)
(J
sc,corr
/ J
sc,EQE
)
× 100 (%)
P3HT
b
9.88 1.00 9.87 9.64 2.31
P3HTT-BTD-DPP
c
9.47 0.87 10.91 11.14 2.08
P3HTT-TP-DPP
c
6.57 0.83 7.94 7.71 2.96
P3HTT-BTD-TPD
c
6.18 0.90 6.89 6.59 4.49
P3HTT-TP-TPD%
c
4.23 0.93 4.56 4.56 0.01
a
Spin-coated from o-DCB and placed to the N
2
cabinet before aluminum deposition for
b

30 min and
c
20 min.

Figure A4.7 Linear correlation between HOMO energies and
V
oc
of semi-random two-acceptor polymers (P3HTT-TP-BTD,
P3HTT-TPD-DPP (1:1), P3HTT-TPD-DPP (1:2), P3HTT-TPD-
DPP (2:1), P3HTT-BTD-DPP, P3HTT-TP-DPP, P3HTT-BTD-
TPD and P3HTT-TP-TPD. The data point at 5.29 eV/ 0.36 V
corresponding to P3HTT-TP-TPD is excluded when plotting the
linear correlation (red line, R = 0.877).
274
APPENDIX 5 Solar Cells Based on Semi-Random P3HT
Analogues Containing Dithienopyrrole: Influence of
Incorporating a Strong Donor
A5.1 Synthesis
For general materials and methods section see appendix 1. The synthetic
procedures for 5,7-dibromo-thieno[3,4-b]thienopyrazine, 4,7-dibromo-2,1,3-
benzothiadiazole, 2-bromo-5-trimethyltin-3-hexylthiophene and 2,5-
bis(trimethyltin)thiophene as well as synthesis of 2,5-diethylhexyl-3,6-bis(5-
bromothiophene-2-yl)pyrrolo[3,4-c]-pyrrole-1,4-dione are described in appendix 1 and 2.
3,3’-Dibromo-2,2’-bithienyl was synthesized according to the literature.
1

5-Bromononane (1) 6.67 ml (38.0 mmol) 5-nonanol was added to a flame-dried
3-neck-flask and dissolved in 65 ml CH
2
Cl
2
. 13.23 g (39.9 mmol) carbon tetrabromide
was added and the mixture was stirred until the solid dissolved. It was then cooled to 0
°C and 11.96 g (45.6 mmol) triphenylphosphine was added portion wise. The solution
was stirred at 0 C for 30 min, warmed to r.t. and stirred overnight. The slightly yellow
solution was then poured into hexanes and filtered to remove triphenylphosphineoxide.
The solvent was evaporated in vacuo and the product was purified via flash
chromatography (hexanes). The colorless, liquid product was isolated with a yield of 79%
(6.24 g, 30 mmol).
1
H NMR (400 MHz, CDCl
3
) δ 4.03 (quint., 1H), 1.80 (m, 4H), 1.52
275
(m, 2H), 1.34 (m, 6H), 0.91 (t, 6H);
13
C NMR (100 MHz, CDCl
3
) δ 58.81, 38.88, 29.68,
22.16, 13.95.
Nonane-5-amine (2) 4.00 g (19.20 mmol) 5-bromononane was dissolved in 130
ml of dry DMF. 5.36 g (28.96 mmol) potassium phthalimide was added and the reaction
mixture was heated to 80 C and left stirring overnight. After cooling to r.t. the mixture
was poured into saturated ammonium chloride solution and EtOAc was added to dissolve
the white precipitate. After separation of the phases the organic phase was washed with
H
2
O, dried with MgSO
4
and the solvent was evaporated in vacuo. 4.449 g (16.27 mmol)
of the obtained crude N-[1-butylpentyl]phthalimide was dissolved in 52 ml of EtOH. 1.5
ml (32.5 mmol) hydrazine was added and the reaction mixture was heated to 85 °C. After
three days the mixture was cooled to r.t. and the solvent was evaporated in vacuo. The
residual solid was dissolved in 10 % NaOH and Et
2
O. The phases were separated and the
aqueous phase was extracted several times with Et
2
O. The combined organic phases were
dried with MgSO
4
and the solvent was evaporated in vacuo. The resulting yellow-brown
liquid was distilled under vacuum to give 1.286 g (47%) of a clear liquid product.
1
H
NMR (400 MHz, CDCl
3
) δ 2.60 (m, 1H), 1.25 (m, 12H), 0.83 (t, 6H);
13
C NMR (100
MHz, CDCl
3
) δ 51.05, 37.76, 28.28, 22.75, 13.95.
N-(1-Butylpentyl)dithieno[3,2-b:2’ ,3’-d]pyrrole (3) 720 mg (2.22 mmol) 3,3’-
dibromo-2,2’-bithienyl, 514 mg (5.33 mmol) NatBuO, 275 mg (0.44 mmol) BINAP and
3 ml dry toluene were added to a 3-neck round bottom flask. The solution was degassed
for ten minutes, 350 mg (2.44 mmol) nonane-5-amine was added and the solution was
degassed five more minutes. 104 mg (0.11 mmol) Pd
2
dba
3
was added and the reaction
276
mixture was heated to 110 C for 3 days. It was then cooled to r.t. and water was added.
The aqueous phase was extracted several times using Et
2
O, the combined organic phases
were dried with MgSO
4
and the solvent was evaporated in vacuo. The resulting solid was
purified using flash chromatography (PET ether/Et
2
O (1.5%)) to give a white solid (457
mg, 67%).
1
H NMR (400 MHz, CDCl
3
) δ 7.10 (d, 2H), 7.01 (d, 2H), 4.20 (m, 1H), 2.01
(m, 2H), 1.83 (m, 2H), 1.21 (m, 6H), 1.04 (m, 2H), 0.78 (t, 6H);
13
C NMR (100 MHz,
CDCl
3
) δ 144.09, 122.35, 114.73, 111.78, 59.78, 34.82, 28.71, 22.33, 13.86.
2,6-Dibromo-N-(1-butylpentyl)dithieno[3,2-b:2’ ,3’-d]pyrrole (4) To a flame
dried 3-neck flask 1.962 g (6.43 mmol) of N-(1-butylpentyl)dithieno[3,2-b:2’,3’-
d]pyrrole and 300 ml of dry DMF was added. The solution was cooled to -20 °C and
2.394 g (13.50 mmol) N-bromosuccinimide was added in one portion. The solution was
stirred at -20 °C for three hours and then warmed to r.t. Water was added to quench the
reaction followed by Et
2
O to dissolve the white precipitate. The aqueous phase was
extracted twice with Et
2
O and the combined organic phases were washed with H
2
O to
remove residual DMF. The organic phase was then dried with MgSO
4
and the solvent
was removed in vacuo. The resulting solid was recrystallized from methanol to give
shiny, beige needles (2.265 g, 4.89 mmol, 76%).
1
H NMR (400 MHz, CDCl
3
) δ 7.03 (s,
2H), 4.06 (m, 1H), 1.91 (m, 2H), 1.82 (m, 2H), 1.25 (m, 4H), 1.14 (m, 2H), 1.02 (m, 2H),
0.80 (t, 6H);
13
C NMR (100 MHz, CDCl
3
) δ 140.57, 114.98, 114.85, 109.67, 60.17,
34.77, 28.67, 22.32, 13.86.
General Stille Polymerization: All monomers were dissolved in dry DMF to
give a 0.04 M solution. The solution was then degassed for 20 min. 4 mol% Pd(PPh
3
)
4

277
was added in one portion, the solution was degassed for 10 additional minutes and then
heated for 48 hours at 95 C. Then the reaction mixture was cooled to r.t. and precipitated
in MeOH. Purification was achieved via soxhlet extraction using MeOH and hexanes for
all polymers. For P3HTT-DTP the final solvent was CHCl
3
. For P3HTT-BTD-DTP and
P3HTT-TP-DTP an additional extraction with CH
2
Cl
2
was used prior to CB. P3HTT-
DPP-DTP was also extracted with CH
2
Cl
2
and then dissolved in CHCl
3
. The polymers
were then reprecipitated in MeOH, vacuum filtered and dried. P3HTT-DTP: Yield: 94%,
M
n
= 20,150, PDI = 3.5.
1
H NMR (500 MHz, CDCl
3
) δ 7.12-6.98 (m, 2.21H), 4.18 (s,
0.27H), 2.80 (s, 2H), 2.57 (m, 0.03H), 2.03 (m, 0.59H), 1.88 (s, 0.57H). 1.71 (s, 2.13H),
1.45-1.35 (m, 9.1H), 1.11 (s, 0.7H), 0.91-0.82 (m, 5.24H). P3HTT-BTD-DTP: Yield:
78%, M
n
= 20,030, PDI = 2.8.
1
H NMR (500 MHz, tetrachloroethane-d2) δ 8.04 (s,
0.25H), 7.89 (s, 0.28H), 7.28-7.03 (m, 1.98H), 4.22 (s, 0.1H), 2.94-2.85 (m, 2H), 2.64 (m,
0.01H), 2.07 (s, 0.29H), 1.96 (s, 1H), 1.76 (m, 2.2H), 1.49-1.40 (m, 11.2H), 1.21 (s,
0.29H), 0.96-0.89 (m, 4.15H). P3HTT-TP-DTP: Yield: 65%, M
n
= 21,200, PDI = 3.0.
1
H NMR (500 MHz, tetrachloroethane-d2) δ 8.56 (s, 0.25H), 7.54 (s, 0.26H), 7.17-7.04
(m, 1.83H), 4.23 (s, 0.12H), 2.86 (s, 2H), 2.63 (m, 0.04H), 2.06 (m, 0.21H), 1.94 (m,
0.22H), 1.77 (s, 2.02H), 1.49-1.40 (m, 10.7H), 1.20 (s, 0.38H), 0.96-0.89 (m, 4.16H).
P3HTT-DPP-DTP: Yield: 71%, M
n
= 19,620, PDI = 3.6.
1
H NMR (500 MHz, CDCl
3
) δ
8.93 (s, 0.3H), 7.10-6.98 (m, 2.17H), 4.18-4.05 (m, 0.42H), 2.79-2.59 (m, 2H), 2.03 (m,
0.32H), 1.92 (m, 0.67H), 1.70 (s, 2.34H), 1.44-1.35 (m, 11.9H), 1.11 (s, 0.45H), 0.92-
0.84 (m, 7.12H).
278
A5.2 Device Fabrication and Characterization
For general device fabrication, measurement of I-V characteristics of photovoltaic
devices and mobility measurement methods see appendix 1. Procedures for EQE
measurements and mismatch correction are described in appendix 2. The P3HT:PC
61
BM
and P3HTT-DTP:PC
61
BM film was spin-coated from CB solution (10 mg/ml in P3HT
and P3HTT-DTP) and directly placed in the vacuum chamber for aluminum deposition.
The P3HTT-TP-DTP:PC
61
BM, P3HTT-BTD-DTP:PC
61
BM and P3HTT-DPP-
DTP:PC
61
BM layers were spin cast from a solution in o-DCB containing 10 mg/ml
P3HTT-TP-DTP and P3HTT-BTD-DTP and 10 mg/ml PC
61
BM. Films were placed in a
nitrogen cabinet for 20 min, before being transferred to the vacuum chamber. P3HTT-
DPP-DTP:PC
61
BM layers were spin cast from a solution in o-DCB containing 10 mg/ml
P3HTT-DPP-DTP and 1 vol% of 1,8-octanedithiol and were then placed for 20 min in a
nitrogen cabinet before loading in the vacuum chamber. At the final stage, the substrates
were pumped down to high vacuum (< 2×10
-6
Torr) and aluminum (100 nm) was
thermally evaporated at 3 – 4 Å/sec using a Denton Benchtop Turbo IV Coating System
onto the active layer through shadow masks to define the active area of the devices as 4.4
mm
2
. Thermal annealing of P3HT:PC
61
BM blends was carried out by directly placing the
completed devices in the nitrogen oven for 30 min at 145 °C. After annealing, the devices
were cooled down to room temperature before measurements were carried out. All
P3HTT-DTP-containing:PC
61
BM BHJ solar cells were tested without thermal treatment.

279
A5.3 References Appendix 5
(1) Gronowitz, S. Acta. Chem. Scand. 1961, 15, 1393–1395.

280
APPENDIX 6 Influence of the Ethylhexyl Side-Chain
Content on the Open-Circuit Voltage in rr-Poly(3-
hexylthiophene-co-3-(2-ethylhexyl)thiophene) Copolymers
A6.1 Synthesis
For general materials and methods section see appendix 1. 3-(2-
Ethylhexyl)thiophene (1) was synthesized according to the literature.
1

2-Bromo-3-(2-ethylhexyl)thiophene (2) 1.622 g (8.27 mmol) of 3-(2-
ethylhexyl)thiophene was dissolved in 13 ml glacial acetic acid. 1.472 g (8.27 mmol) of
N-bromosuccinimide was added in portion and the reaction was stirred for 30 min. It was
then poured in H
2
O and extracted with ethyl ether. The organic phase was then washed
with 10% NaOH-solution multiple times, dried with MgSO
4
and the solvent was removed
in vacuo. The product was purified using flash chromatography (hexanes) to give a clear
oil (2.071 g, 90%).
1
H NMR (400MHz, CDCl
3
) δ 7 18 ( 2 ) 6 76 ( 2 ) 2 50 ( 2 )
1.59 (m, 1H), 1.27 (m, 8H), 0.88 (t, 6H).
2-Bromo-5-trimethyltin-3-(2-ethylhexyl)thiophene (3) Two 3-neck flasks were
flame dried. In flask number one a LDA solution was prepared by dissolving 0.45 ml
freshly distilled diisopropylamine in 6.5 ml dry THF. The solution was cooled to -78 ºC,
1.66 ml (2.66 mmol) n-BuLi was added dropwise and the mixture was stirred for 30 min
at -78 ºC. In flask number two 705 mg (2.56 mmol) 2-bromo-3-(2-ethylhexyl)thiophene
was dissolved in 6.5 ml dry THF and cooled to -78 ºC. Then the LDA solution from flask
281
number one was transferred via cannula into flask number two and the mixture was left
stirring for 45 min. 3.15 ml (3.15 mmol) trimethyltin chloride was added via syringe and
the reaction was allowed to warm to r.t. and left stirring over night. Water was added and
the aqueous phase was extracted several times with ethyl ether. The combined organic
phases were dried over MgSO
4
and the solvent was removed in vacuo. Vacuum
distillation gave a clear oil (900 mg, 80%).
1
H NMR (400MHz, CDCl
3
) δ 6 81 ( 1 )
2.49 (d, 2H), 1.61 (m, 1H), 1.28 (m, 8H), 0.88 (t, 6H), 0.35 (s, 9H).
13
C NMR (100 MHz,
CDCl
3
) δ 142 24 137 59 136 85 114 03 39 99, 33.40, 32.46, 28.74, 25.71, 23.04, 14.11,
10.83, -8.22.
Stille Polymerization for P3HT, P3HT
90
-co-EHT
10
, P3HT
75
-co-EHT
25
,
P3HT
50
-co-EHT
50
, P3EHT:
P3HT 578 mg (1.41 mmol) 2-bromo-5-trimethyltin-3-hexylthiophene was
dissolved in 35 ml dry DMF to give a 0.04 M solution. The solution was the degassed for
20 min before 70 mg (4 mol%) Pd(PPh
3
)
4
was added in one portion. The solution was
degassed for 10 additional minutes and then heated for 44 hours at 95 ºC. Then the
reaction mixture was cooled to r.t. and precipitated into methanol. Purification was
achieved via soxhlet purification using methanol, hexanes and CHCl
3
. The polymer was
then reprecipitated into methanol, vacuum filtered and dried. Yield: 76% (182 mg), M
n
=
24,240, PDI = 2.7.
1
H NMR (500MHz, CDCl
3
) δ 6.98 (s, 1.03H), 2.80 (t, 2H), 2.57 (m,
0.l0H), 1.71 (m, 2.07H), 1.54-1.44 (m, 7.13H), 0.91 (t, 3.29H).
P3HT
90
-co-EHT
10
406 mg (0.99 mmol) 2-bromo-5-trimethyltin-3-
hexylthiophene and 48 mg (0.11 mmol) 2-bromo-5-trimethyltin-3-(2-
282
ethylhexyl)thiophene were dissolved in 28 ml dry DMF to give a 0.04 M solution. The
solution was the degassed for 20 min before 55 mg (4 mol%) Pd(PPh
3
)
4
was added in one
portion. The solution was degassed for 10 additional minutes and then heated for 44
hours at 95 ºC. Then the reaction mixture was cooled to r.t. and precipitated into
methanol. Purification was achieved via soxhlet purification using methanol, hexanes and
CHCl
3
. The polymer was then reprecipitated into methanol, vacuum filtered and dried.
Yield: 79% (149 mg), M
n
= 21,330, PDI = 2.5.
1
H NMR (500MHz, CDCl
3
) δ 6 98 (
0.87H), 6.94 (s, 0.10H), 2.80-2.75 (m, 2H), 1.57 (m, 0.03H) 1.71 (m, 2H), 1.44-1.35 (m,
7H), 0.91 (m, 3.42H).
P3HT
75
-co-EHT
25
425 mg (1.04 mmol) 2-bromo-5-trimethyltin-3-
hexylthiophene and 151 mg (0.35 mmol) 2-bromo-5-trimethyltin-3-(2-
ethylhexyl)thiophene were dissolved in 35 ml dry DMF to give a 0.04 M solution. The
solution was the degassed for 20 min before 69 mg (4 mol%) Pd(PPh
3
)
4
was added in one
portion. The solution was degassed for 10 additional minutes and then heated for 44
hours at 95 ºC. Then the reaction mixture was cooled to r.t. and precipitated into
methanol. Purification was achieved via soxhlet purification using methanol, hexanes and
CHCl
3
. The polymer was then reprecipitated into methanol, vacuum filtered and dried.
Yield: 68% (165 mg), M
n
= 26,120, PDI = 2.5.
1
H NMR (500MHz, CDCl
3
) δ 6 98 (
0.75H), 6.94 (s, 0.25H), 2.80-2.75 (m, 2H), 2.56 (m, 0.06H), 1.71 (m, 1.74H), 1.44-1.29
(m, 7H), 0.91 (m, 3.86H).
P3HT
50
-co-EHT
50
259 mg (0.63 mmol) 2-bromo-5-trimethyltin-3-
hexylthiophene and 277 mg (0.63 mmol) 2-bromo-5-trimethyltin-3-(2-
283
ethylhexyl)thiophene were dissolved in 32 ml dry DMF to give a 0.04 M solution. The
solution was the degassed for 20 min before 64 mg (4 mol%) Pd(PPh
3
)
4
was added in one
portion. The solution was degassed for 10 additional minutes and then heated for 44
hours at 95 ºC. Then the reaction mixture was cooled to r.t. and precipitated into
methanol. Purification was achieved via soxhlet purification using methanol, methanol:
CHCl
3
(1:1) and CHCl
3
. The polymer was then reprecipitated into methanol, vacuum
filtered and dried. Yield: 78% (180 mg), M
n
= 40,130, PDI = 2.0.
1
H NMR (500MHz,
CDCl
3
) δ 6 98 ( 0 5 ) 6 94 ( 0 5 ) 2 8 -2.73 (m, 2H), 2.53 (m, 0.04H), 1.70 (m,
1.56H), 1.43-1.30 (m, 7.88H), 0.91-0.89 (m, 4.84H).
P3EHT 413 mg (0.94 mmol) 2-bromo-5-trimethyltin-3-(2-ethylhexyl)thiophene
was dissolved in 24 ml dry DMF to give a 0.04 M solution. The solution was the
degassed for 20 min before 47 mg (4 mol%) Pd(PPh
3
)
4
was added in one portion. The
solution was degassed for 10 additional minutes and then heated for 44 hours at 95 ºC.
Then the reaction mixture was cooled to r.t. and precipitated into methanol. Purification
was achieved via soxhlet purification using methanol, methanol: CHCl
3
(1:1) and CHCl
3
.
The polymer was then reprecipitated into methanol, vacuum filtered and dried. Yield:
60% (111 mg), M
n
= 22,180, PDI = 2.9.
1
H NMR (500MHz, CDCl
3
) δ 6 94 ( 0 86 )
2.72 (m, 2H). 1.71 (m, 1.14H), 1.37-1.28 (m, 8.71H), 0.88 (t, 6.13H).

284
A6.2 Polymer Characterization

Figure A6.1
1
H NMR of P3HT in CDCl
3
.

285

Figure A6.2
1
H NMR of P3HT
90
-co-EHT
10
in CDCl
3
.

286

Figure A6.3
1
H NMR of P3HT
75
-co-EHT
25
in CDCl
3
.

287

Figure A6.4
1
H NMR of P3HT
50
-co-EHT
50
in CDCl
3
.
288

Figure A6.5
1
H NMR of P3EHT in CDCl
3
.

289

Figure A6.6 UV/vis absorption of all five polymers in as
cast thin films (spin coated from CB) where P3HT is purple
line, P3HT
90
-co-EHT
10
is green line, P3HT
75
-co-EHT
25
is
blue line, P3HT
50
-co-EHT
50
is red line and P3EHT is
orange line.
290

Figure A6.7 DSC curves where a) is P3HT, b) is P3HT
90
-co-EHT
10
, c) is P3HT
75
-co-
EHT
25
and d) is P3HT
50
-co-EHT
50
.
291

Figure A6.8 CV traces for the oxidation of thin films (as cast) where a) is P3HT, b) is
P3HT
90
-co-EHT
10
, c) is P3HT
75
-co-EHT
25
, d) is P3HT
50
-co-EHT
50
and e) is P3EHT.
Ferrocene was used as a reference and values were converted to the vacuum scale using
the approximation that the ferrocene redox couple is 5.1 eV relative to vacuum.
292

Figure A6.9 CV traces for the oxidation of polymers in solution (CHCl
3
with
tetrabutylammonium tetrafluoroborate as supporting electrolyte) where a) is P3HT, b) is
P3HT
90
-co-EHT
10
, c) is P3HT
75
-co-EHT
25
, d) is P3HT
50
-co-EHT
50
and e) is P3EHT.
Ferrocene was used as a reference and values were converted to the vacuum scale using
the approximation that the ferrocene redox couple is 5.1 eV relative to vacuum.

293
A6.3 Device Fabrication and Characterization
For general device fabrication, measurement of I-V characteristics of photovoltaic
devices and mobility measurement methods see appendix 1. Procedures for EQE
measurements are described in appendix 2. Ratios of polymer:fullerene as well as
solvents used see chapter 7. Thermal annealing of P3HT:PC
61
BM blends was carried out
by directly placing the completed devices in the nitrogen oven for 60 min at 145 °C,
P3HT
90
-co-EHT
10
:PC
61
BM blends for 60 min at 110 °C, P3HT
75
-co-EHT
25
:PC
61
BM
blends for 30 min at 110 °C, P3HT
50
-co-EHT
50
:PC
61
BM blends for 10 min at 110 °C.
After annealing, the devices were cooled down to room temperature before measurements
were carried out. P3EHT:PC
61
BM BHJ solar cells were tested without thermal treatment.

A6.4 References Appendix 6
(1) Zhang, Y.; Tajima, K.; Hirota, K.; Hashimoto, K. J. Am. Chem. Soc. 2008, 130,
7812–7813.

Abstract (if available)

Abstract The energy demand of the world population is predicted to increase up to 40% by 2035 and one of the biggest tasks in recent history is to find technologies that can meet this challenge. The use of solar energy is a very promising research direction because enough photons reach the earth within one hour to meet the energy demand of the human population for an entire year. The vision of organic photovoltaics, as opposed to silicon solar cells, is that of low cost, flexible and lightweight solar cells which are easily integrated in existing infrastructure. ❧ The main focus of this dissertation are semi-random regioregular poly(3-hexylthiophene) (rr-P3HT) based polymers, which are a novel class of broadly absorbing, conjugated polymers for efficient polymer:fullerene bulk heterojunction solar cells. Semi-random polymers combine a low band gap characteristic of the donor/acceptor (D/A) approach with the broad absorption profile of random copolymers all while retaining high hole mobilities and favorable mixing with fullerene acceptors. This new family of polymers is able to overcome drawbacks of both perfectly alternating D/A and random copolymers. ❧ The concept of semi-random copolymers is introduced in chapter 2 and shows that the combination of randomized sequence distribution and P3HT-like character allows these polymers to have an unprecedented set of properties important for efficient polymer:fullerene solar cells. Their multichromophoric nature enables broad absorption of the solar photon flux using only small amounts (5% to 17.5%) of acceptor monomers while important qualities of P3HT such as semicrystallinity, high hole mobility and mixing at favorable ratios with fullerenes are retained. ❧ Subsequently, chapter 3 focuses on the influence the amount of acceptor, in this case diketopyrrolopyrrole (DPP), has on the properties of semi-random copolymers. Importantly a broad photocurrent response, representative of the polymer absorption profile confirms that semi-random D/A copolymers are an effective platform for improving light harvesting in BHJ solar cells and results in efficient polymer:fullerene solar cells with P3HTT-DPP-10% reaching an efficiency of 5.7%. ❧ Of special interest among semi-random copolymers are those containing multiple distinct acceptor monomers. Consequently, in chapter 4 a family of semi-random two-acceptor copolymers containing thienopyrroledione (TPD) and DPP is introduced which illustrates that the observed broad absorption profiles and high absorption coefficients can translate into a strong and broad photocurrent response. External quantum efficiency values of up to 40% at 800 nm for P3HTT-TPD-DPP(1:2) and current densities of over 16 mA/cm2 are achieved, which are among the highest currents ever reported with PC61BM as the acceptor. Further work on two-acceptor polymers (chapter 5) focuses on additional acceptor combinations in order to gain insight on general trends of polymer properties such as absorption profiles and frontier energy levels. ❧ The strong donor dithienopyrrole was incorporated into semi-random copolymers (chapter 6), but no broadening of the absorption profile was observed. Instead this study emphasizes that monomer combinations must be carefully selected in order to retain the semicrystalline nature of P3HT and to maintain a HOMO energy suitable for targeting reasonable values of the open circuit voltage (Voc) in bulk heterojunction solar cells. ❧ Finally, chapter 7 focuses on a simple poly-3-alkylthiophene based model system in order to study the influence of branched alkyl side chains on the Voc. Results from this study suggest that the introduction of small amounts of 2-ethylhexyl chains is a viable way of tuning the Voc in polymers with almost no change in all other properties, which could be an especially promising strategy for semi-random copolymers.

Linked assets

University of Southern California Dissertations and Theses

Conceptually similar

PDF

Influence of polymer structure on electronic properties and performance as donor materials in bulk heterojunction solar cells

PDF

Synthesis and characterization of 3-hexylesterthiophene based random and semi-random polymers and their use in ternary blend solar cells

PDF

Structure-function studies of random and semi-random poly(3-hexyl thiophene) copolymers

PDF

Efficient ternary blend bulk heterojunction solar cells with tunable open-circuit voltage

PDF

Molecular and polymeric donors for bulk heterojunction organic solar cells

PDF

Improving the efficiency and stability of organic solar cells through ternary strategies

PDF

Direct C−H arylation for the synthesis of conjugated polymers

PDF

From perfectly alternating to random multichromophoric conjugated polymers for ternary blend bulk heterojunction solar cells

PDF

Direct arylation polymerization (DARP) for the synthesis of conjugated polymers

PDF

Improving the sustainability of conjugated polymer synthesis via direct arylation polymerization

PDF

Organic photovoltaics: from materials development to device application

PDF

The influence of CdSe surface ligands on hybrid inorganic/organic solar cell performance

PDF

Porphyrin based near infrared‐absorbing materials for organic photovoltaics

PDF

Non-traditional architectures for semiconducting polymers

PDF

Properties and applications of dipyrrin-based molecules in organic photovoltaics

PDF

Excited state dynamics of organic photovoltaics probed using ultrafast spectroscopy

PDF

Development of polystyrene sulfonic acid-polyvinylidene fluoride (PSSA-PVDF) blends for direct methanol fuel cells (DMFCS)

PDF

Molecular and morphological effects on the operational parameters of organic solar cells

PDF

PSSA-PVDF semi-IPN blends for direct methanol fuel cells

PDF

Squaraines and their applications to organic photovoltaics

Asset Metadata

Creator Burkhart, Beate (author)

Core Title Development of a family of semi-random multichromophoric polymers for application in organic solar cells

School College of Letters, Arts and Sciences

Degree Doctor of Philosophy

Degree Program Chemistry

Publication Date 01/05/2015

Defense Date 11/05/2012

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag broad absorption profile,conjugated polymers,OAI-PMH Harvest,organic solar cells,P3HT,polymer:fulleren bulk heterojuchtion,semi-random polymers,two-acceptor polymers

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Thompson, Barry C. (committee chair), Prakash, G. K. Surya (committee member), Thompson, Mark E. (committee member)

Creator Email bburkhar@usc.edu,bburkhart@gmx.de

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c3-127600

Unique identifier UC11292535

Identifier usctheses-c3-127600 (legacy record id)

Legacy Identifier etd-BurkhartBe-1387.pdf

Dmrecord 127600

Document Type Dissertation

Rights Burkhart, Beate

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the a...

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus MC 2810, 3434 South Grand Avenue, 2nd Floor, Los Angeles, California 90089-2810, USA