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Molecular and polymeric donors for bulk heterojunction organic solar cells

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Molecular and polymeric donors for bulk heterojunction organic solar cells

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MOLECULAR AND POLYMERIC DONORS FOR BULK HETEROJUNCTION
ORGANIC SOLAR CELLS

By

Alejandra E. Beier

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)

December 2013

Copyright 2013 Alejandra E. Beier

ii

DEDICATION

To my wonderful family and all the people that believed in me...

iii

ACKNOWLEDGEMENTS
As this challenging journey comes to an end, I would like to acknowledge the
many people that have made this work possible and have guided me throughout the last
five years. First of all, I would like to thank my advisor Prof. Barry Thompson for taking
me in to his research group and teaching me about synthesis, photovoltaics, and life
lessons.
I would like to thank all the people in the Thompson group, especially Dr. Beate
Burkhart and Dr. Petr Khlyabich for their friendship, kindness when I needed it, good
advice, and for always believing in me not only as a person, but also as a scientist. Many
projects that I worked on have benefited from the abundant help and collaboration from
Sangtaik Noh and the help and support of Joahanna Macaranas. I would like to thank
them both for all their contributions and their friendship. Additionally, I would like to
acknowledge Andrey Rudenko for our great conversations and for the great, clean, and
orderly lab we shared and the friendship we developed, as well as Alia Latif for her
support and friendship.
Over the years many people have lent me a hand with instrumentation and for that
I would like to thank the Gupta Lab, especially Scott Seidel for his help with the
goniometer, the Hogen-Esch group for letting me use their SEC, the Armani group for
their help with the optical microscope, and the Brutchey and Williams groups for
allowing me to use their gloveboxes. I would also like to thank Dr. Brian Conley, Dr.
iv

Ginger Shultz, and Dr. Somesh Kumar for the useful discussions, friendships, and
mentorships.
I would like to take the time to thank my committee members, Prof. G. Surya
Prakash, Prof. Andrea Armani, and my advisor for their mentorship and support. I would
like to extend my gratitude to Prof. Thieo E. Hogen-Esch and Prof. Steve E. Bradforth for
being members of my qualifying exam committee.
Furthermore, I would like to thank the many people in the Chemistry department
who have helped me during my time at USC, namely Michele Dea, David Hunter, Carole
Phillips, Jessy May, Robert Aniszfeld, Ross Lewis, Frank Devlin, Allan Kershaw, Darrell
Karrfalt from the stockroom, and many others.
Last but not least I would like to thank my family, particularly my husband Dr.
Christopher Beier for always supporting me, encouraging me, and having faith in me. I
would like to thank my parents Elizabeth and Simon for teaching me many lessons that
prepared me for the challenges in life, I would like to thank my brothers for always
believing in me and my parents-in-law, Esther and Kim for all their support. I would also
like to thank all my extended family, especially my uncle Jose for all his wise advice.
Also, I want to thank all my colleagues and close friends who have supported me through
the ups and downs of life, especially Dana Mustafa, Dr. Anna Dawsey, and Janet Olsen,
as well as all my friends that are all over the world and my Silverlake community, who
have given me strength.

v

TABLE OF CONTENTS
DEDICATION ii
ACKNOWLEDGEMENTS
iii
LIST OF TABLES
vii
LIST OF FIGURES
ix
ABSTRACT xviii
Chapter 1: Small Molecules in Solution Processed Binary and Ternary
Blend Bulk Heterojunction Solar Cells

1
1.1 General Introduction: Organic Photovoltaics 1
1.2 Device Architecture: Polymer-Fullerene Bulk Heterojunction 3
1.2.1 Device Operation: Polymer-Fullerene Bulk
Heterojunction

6
1.3 Beyond Binary Blend Photovoltaics 12
1.4 Ternary Blend Photovoltaics 13
1.4.1 Small Molecule Overview
17
1.5 Small Molecules as Donors in Binary Blend Bulk Heterojunction 19
1.5.1 Small Molecules Dyes 20
1.5.1.1 Diketopyrrolopyrrole-Based Small Molecules 20
1.5.1.2 Other Noteworthy Small Molecule Dyes 28
1.5.2 Oligothiophene of D/A Small Molecules 30
1.6 Small Molecules as Donors in Ternary Blend Bulk
Heterojunction

40
1.6.1 Oligothiophenes and Diketopyrrolopyrrole Dyes 41
1.6.2 Porphyrins, Phthalocyanines, and Squaraines 45
1.7 Small Molecule Ternary Blend BHJ Overview 50
1.8 Optimization of Donor Materials for Ternary Blend BHJ Solar
Cells

52
1.9 General Conclusions for Ternary Blend BHJ Solar Cells 52
1.10 References 58
Chapter 2: Polymer, Dye, Fullerene Ternary Blend Organic Photovoltaics:
Exploring the Influence of Blend Composition on the Open-circuit Voltage

66
2.1 Introduction 66
2.2 Synthesis of DPP Dye 70
2.3 Characterization and Photovoltaic Performance 73
2.4 Summary of Small Molecule Ternary Blends and Outlook 80
2.5 Extension to Covalent Dye-Fullerene Dyad 81
2.6 References 87
vi

Chapter 3: Polythiophene Side-chain Functionalization for Surface Energy
Modification in Conjugated Polymer-Based Photovoltaics

91
3.1 Introduction 91
3.2 Synthesis of Random Copolymers 95
3.3 Characterization and Photovoltaic Performance 98
3.4 Conclusions and Outlook 107
3.5 References 108
Chapter 4. Thiophene-Based Diketopyrrolopyrrole Semi-Random Polymer
Analogues for Enhancing the V
oc

110
4.1 Introduction 110
4.2 Synthesis 113
4.3 Characterization and Photovoltaic Performance 119
4.4 Conclusion and Outlook for Diketopyrrolopyrrole Semi-
Random Polymer Analogues

124
4.5 References 125
BIBLIOGRAPHY
128
APPENDIX 1 Polymer, Dye, Fullerene Ternary Blend Organic
Photovoltaics: Exploring the Influence of Blend Composition on the Open-
circuit Voltage

139
A1.1 Materials and Methods 139
A1. 2 Synthesis of Small Molecules and Dyads 141
A1.3 Small Molecule, Fullerene Dyad and Polymer Characterization 148
A1.4 Device Characterization and Fabrication 155
A1.5 References for Appendix 1 157
APPENDIX 2 Polythiophene Side-chain Functionalization for Surface
Energy Modification in Conjugated Polymer-Based Photovoltaics

158
A2.1 Materials and Methods 158
A2.2 Synthesis of Polymer Family 160
A2.3 Polymer Characterization 164
A2.4 Device Characterization and Fabrication 172
A2.5 References for Appendix 2 172
APPENDIX 3 Thiophene-Based Diketopyrrolopyrrole Semi-Random
Polymer Analogues for Enhancing the V
oc

173
A3.1 Synthesis of Semi-Random Polymers 173
A3.2 Polymer Characterization 178
A3.3 Device Characterization and Fabrication 185
A3.4 References for Appendix 3 187

vii

LIST OF TABLES
Table 1.1 Solar cell performance of polymer:SM:fullerene ternary
blends at various SM compositions and measured at AM1.5G
(100 mA cm
-2
).

43-44

Table 1.2 Photovoltaic performance of ternary blend devices measured
at AM1.5G (100 mA cm
-2
).

45

Table 1.3 Photovoltaic performance of ternary blend devices measured
at AM1.5G (100 mA cm
-2
).

47-48

Table 2.1 Optical and electronic properties of P3HT, PDPP, and
PCBM.

74

Table 2.2 Photovoltaic performances of devices made of P3HT and
PDPP as donor with PC
61
BM as acceptor.

75

Table 2.3 Optimized photovoltaic performance of P3HT and PDPP as
donor with PC
61
BM as acceptor.

78

Table 3.1 Optical properties for oxyethylene-containing copolymers
and corresponding poly (3-hexylthiophene) and poly (3-
dioxaheptathiophene).

98

Table 3.2 Surface energy table for every ratio of PDHT starting at
100% followed by 78%, 53%, 37%, 23%, and 0%.

100

Table 3.3 Photovoltaic properties of P3HT, P3HT
77
-co-P3DHT
23
,
P3HT
63
-co-P3DHT
37
, P3HT
50
-co-P3HT
50
, P3HT
22
-co-
P3DHT
78
, and P3DHT polymers with PC
61
BM.

106

Table 4.1 Optical and electronic properties table of P3HT, P3HTT-
DPP, P3HTT-PDPP(Hex), P3HTT-PDPP(EH), and P3HTT-
DPP-PDPP(EH) two-acceptor polymer in the solid state.

121
viii

Table 4.2 Photovoltaic performances of P3HTT-PDPP(Hex), P3HTT-
PDPP(EH), P3HTT-DPP-PDPP(EH), P3HTT-DPP, and
P3HT as donors with PC
61
BM as acceptor.

123

Table A2.1 Surface energy table for every ratio of P3DHT un-annealed,
as-cast films starting at 100% followed by P3HT
27
-co-
P3DHT
73
, P3HT
47
-co-P3DHT
53,
P3HT
77
-co-P3DHT
23
, P3HT,
and PCBM.

169

ix

LIST OF FIGURES
Figure 1.1 Illustration of donor/acceptor device architectures.
Bilayer donor/acceptor solar cell (left) and bulk
heterojunction donor/acceptor solar cell (right).
Magnification of the bilayer (bottom left) and
bicontinuous morphology of the active layer in
BHJ (bottom right).

3

Figure 1.2 Pathway to generation of free charges by
photoexcitation. a) Photon is absorbed by the
donor material and an exciton is formed. b) The
exciton diffuses to the interface. c) An electron is
transferred from the donor to the acceptor. d)
Coulombically bound hole-electron pair is
dissociated into free charge carriers. e) Free
carriers are transported to their respective phases
towards the electrodes. f) Free charge carriers are
collected at the electrodes. Reproduced with
permission from Kroon, R. et al. Polym. Rev.
2008, 48, 531–582. Copyright (2008) Taylor and
Francis.

7

Figure 1.3 Current-voltage (J-V) curve depiction and
performance parameters definitions. The typical
current-voltage curves for dark and light current in
solar cells are shown on the left and illustrate the
important parameters such as J
sc
, V
oc
, FF, and J
m

as well as V
m
(current and voltage at the maximum
power point). The power conversion efficiency (η
in %) is defined as the ratio of power out (P
out
) to
power in (P
in
). The FF is the ratio of maximum
power divided by J
sc
x V
oc
.

8

Figure 1.4 Photon flux from the sun as a function of
wavelength and absorbance, in black in red at
AM1.5G. Absorption profile of P3HT in purple.
Reproduced with permission from Thompson, B.
C. and Fréchet, J. M. J. Angew. Chem. Int. Ed.
2008, 47, 58–77. Copyright (2008) John Wiley and
Sons.

9

Figure 1.5 Representation of energetic band level diagrams of 10
x

P3HT:PCBM and current-voltage relationship.
Illustrating HOMO and LUMO energies of a
donor/acceptor pair, the band gap of the donor
(E
g
), and the V
oc
which is empirically related to the
HOMO
D
– LUMO
A
offset.

Figure 1.6 Hypothetical representation of light absorption of
donor1/donor2/acceptor ternary blend system
assuming the fullerene is PC
71
BM.

14

Figure 1.7 Hypothetical scheme of band structure in a ternary
blend comprised of two donors and an acceptor.
V
oc
represents the HOMO
D
-LUMO
A
relationship
for each donor. D1-A for the V
oc
of donor 1 and
acceptor while D2-A the V
oc
of donor 2 and
acceptor. D1-A>D2-A.

15
Figure 1.8

V
oc
for the ternary blend BHJ solar cells as a
function of the amount of ICBA in the blend.
Reproduced with permission from Khlyabich, P. P.
et al. J. Am. Chem. Soc. 2011, 133, 14534–14537.
Copyright (2011) American Chemical Society.

17
Figure 1.9 Synthetic scheme of 2-acetinone.

21

Figure 1.10 Illustration of the a) Chemical structures of
DPP(TBFu)
2
and PC
71
BM. b) Device architecture
small molecule DPP(TBFu)
2
:PC
71
BM devices.
Reproduced with permission from Walker, B. et al.
Adv. Funct. Mater. 2009, 19, 3063–3069.
Copyright (2009) John Wiley and Sons.

22

Figure 1.11 UV/Vis absorption spectra of a) DPP(TBFu)
2
in
solution and film and pristine film of PC
71
BM. c)
Different composition of donor/acceptor blends as-
cast in the solid state. Reproduced with permission
from Walker, B. et al. Adv. Funct. Mater. 2009,
19, 3063–3069. Copyright (2009) John Wiley and
Sons.

23

Figure 1.12 Chemical structure of DPP(CT)
2
.

25

xi

Figure 1.13 Illustration of a) chemical structure of
NDT(TDPP)
2
and b) External quantum efficiency
and absorption spectra of NDT(TDPP)
2
:PC
61
BM
(1.5:1.0) under AM1.5G (100 mW cm
-2
)
illumination. Reproduced with permission from
Loser, S. et al. J. Am. Chem. Soc. 2011, 133,
8142–8145. Copyright (2011) American Chemical
Society.

26

Figure 1.14 Chemical structure of Ph(TDPP)
2
.

27

Figure 1.15 Donor-acceptor-donor (D-A-D) structures
containing DPP core moieties flaked by electron-
rich end-groups. Group: 1=triphenyl amine,
2=benzodithiophene, 3= C1-pyrene substituent,
and 4= C2-pyrene substituent. Reproduced with
permission from Lee, O. P. et al. Adv. Mater.
2011, 23, 5359–5363. Copyright (2011) John
Wiley and Sons.

28

Figure 1.16 Molecular structure of merocyanine HB366.

29

Figure 1.17 Chemical structure of diisobutylamino-2,6-
dihydroxyphenyl squaraine (SQ).

30

Figure 1.18 Chemical structure of fluorinated[bisDMFA-Th]-
BT-HxTh
3
derivatives. Reproduced with
permission from Paek, S. et al. J. Phys. Chem. C
2012, 116, 23205–23213. Copyright (2012)
American Chemical Society.

31

Figure 1.19
Chemical structure of d-DTS(PTTh
2
)
2
. Notice the
orientation of the nitrogen atoms is away from the
interior core. Reproduced with permission from
Garcia, A. et al. Adv. Mater. 2012, 24, 5368–5373.
Copyright (2012) John Wiley and Sons.

33

Figure 1.20 Chemical structures of p-DTS(PTTh
2
)
2
and p-
DTS(FBTTh
2
)
2
, where R
1
= n-hexyl and R
2
= 2-

34
xii

ethylhexyl. Reproduced with permission from Van
der Poll, T. S. et al. Adv. Mater. 2012, 24, 3646–
3649. Copyright (2012) John Wiley and Sons.

Figure 1.21 a) Plot of PCE and FF vs. ITO-resistances. Squares
represent the power conversion efficiency and
circles represent the fill factor. Reproduced with
permission from Wang, D. H. et al. Adv. Energy
Mater. 2013, DOI: 10.1002/aenm.201300277.
Copyright (2013) John Wiley and Sons.

35

Figure 1.22 Chemical structure of acceptor-donor-acceptor (A-
D-A) BDT-DPP small molecule.

36

Figure 1.23 Molecular structures of DCAO7T and
DCAO3(BDT)3T where R=hexyl.

37

Figure 1.24 Chemical structure of benzodithiophene based
small molecules with ethylrhodanine end-
groups.
117
R-groups also represent 2-ethylhexoxy
groups in the case of DR3TBDT
116
and octyl
groups in the case of DERHD7T.
118
Reproduced
with permission from Zhou, J. et al. J. Am. Chem.
Soc. 2013, 135, 8484-8487. Copyright (2013)
American Chemical Society.

39

Figure 1.25 Chemical structures of the small molecules
illustrated in ternary blend solar cells. Examples
shown include DPP-based SMs, oligothiophenes,
anthracene-based SMs, phthalocyanines,
porphyrins, and squaraine molecules.

42

Figure 2.1 TQTFA weight % vs. V
oc
plot for ternary blend
BHJ solar cells of P3HT:PC
71
BM:TQTFA as a
function of dye loading in the blend. Dye content
was 20, 25,30, and 35%, while the V
oc
went from
0.60 V for binary P3HT:PCBM reference to 0.65,
0.69, 0.71, and 0.73 V.

69

Figure 2.2 Synthetic scheme of diphenyl-
diketopyrrolopyrrole. Formation of DPP core by

71
xiii

ring-closing reaction followed by alkylation, di-
bromination, and Suzuki coupling between
brominated precursor and phenyl-boronic acid.

Figure 2.3 Structure of donor and acceptors molecules.
HOMO and LUMO energy levels diagram of each
component in ternary blend devices
(P3HT:PDPP:PCBM).

72

Figure 2.4 Absorption spectra of P3HT (red line), PDPP (blue
line), and PCBM (orange line) films. All spin-
coated from chlorobenzene and PCBM, spin-
coated from chloroform.

73

Figure 2.6 Correlation between V
oc
and small molecule
composition in ternary blends devices. PDPP
weight % vs. V
oc
plot for ternary blend BHJ solar
cells of P3HT:PDPP:PC
61
BM as a function of dye
loading in the blend. Dye content went from 0-
100%. The V
oc
went from 0.61 V for binary
P3HT:PCBM reference to 0.87 V for
PDPP:PCBM.

77

Figure 2.7 Plot depicting the correlation between V
oc
and SM
composition in ternary blends. This plot includes
all data points (0-100 wt% dye) to illustrate the
quadratic relationship that was not achieved when
FF values were individually optimized.

79

Figure 2.8 Plot of best FF values and small molecule
composition in ternary blends after optimization.

79

Figure 2.9 Synthetic scheme of the first fullerene dyad target
for the study of small molecule ternary blend
devices. Hydrolysis of PCBM resulted in PCBA
followed by DCC-mediated esterification to
covalently attach dye to fullerene, PCB-NCS.

82

Figure 2.10 Synthetic scheme of the second fullerene dyad
target for the study of small molecule ternary blend
devices. Stille coupling of mono-BrDPP and
protected alcohol-thiophene followed by
bromination and Suzuki coupling to arrive at

84
xiv

PDPP-Th-THP. The last two steps consist of acid-
catalyzed deprotection of the alcohol group and
esterification by DCC mediation to yield PCB-
PDPP dyad.

Figure 2.11 Absorption spectra of PCBM (orange line), as-cast
PCB-PDPP (green line), and annealed PCB-PDPP
(red line) films. All spin-coated from
chlorobenzene and PCBM, spin-coated from
chloroform. PCB-PDPP annealed at 120°C for 30
min.

85

Figure 3.1 Surface energy and chemical structures of
polymers PIDTDPP, PIDTDPP2, and PIDTDPP3.
Reproduced with permission from Sun, Y.et al. J.
Mater. Chem. 2012, 22, 5587. Copyright (2012)
The Royal Society of Chemistry.

93

Figure 3.2 Family of monomers for surface energy
modification. 1) 2-bromo-3-(2-ethoxyethanol)-5-
trimethylstannylthiophene. 2) 2-bromo-3-(2-
dioxahepta)-5-trimethylstannylthiophene. 3) 2-
bromo-3hexylthiophene-5-
trimethylstannylthiophene. 4) 2-bromo-3-(4-
nonafluorohepta)-5-trimethylstannylthiophene.

94

Figure 3.3 Synthetic scheme of monomer and general
procedure for polymerization of homopolymer and
copolymers via Stille coupling polymerization.

97

Figure 3.4 Absorption spectra of polymer films spin-cast from
chloroform for P3DHT and chlorobenzene for all
copolymers and P3HT.

99

Figure 3.5
Plot of P3DHT content in random copolymers vs.
surface energy.

102

Figure 3.6 GIXRD of as-cast thin films spin-coated from CB
solutions at 5 mg/mL polymer concentration.

104

Figure 3.7 J-V curve for copolymers compositions of 23%
P3DHT, 37% P3DHT, 50% P3DHT and P3HT
under 100 mW/cm
2
(AM1.5G) illumination.

105

xv

Figure 4.1 Representation of the different aryl groups used in
this chapter. From left: thiophene, benzene,
pyridine.

113

Figure 4.2 Monomer synthetic schemes.

114

Figure 4.3 Chemical structure of control monomer 3. 115

Figure 4.4
General polymer synthetic scheme. Semi-random
polymerization by palladium catalyzed Stille
coupling. m= 2-bromo-3-hexyl-5-trimethyltin
thiophene, n= bis-2,5-(trimethyltin) thiophene, o=
bis(bromo-aryl-alkyl) diketopyrrolopyrrole.

116
Figure 4.5 Chemical structures of semi-random copolymers:
P3HTT-PDPP(Hex), P3HTT-PDPP(EH), and
P3HTT-DPP.

117

Figure 4.6 Control reaction using 2-bromopyridine. 117

Figure 4.7 Chemical structure of 2-acceptor semi-random
polymer P3HTT-DPP-PDPP(EH).

118

Figure 4.8 Absorption spectra of P3HT (dark green line as-
cast, light green line annealed), P3HTT-DPP (black
line as-cast, gray line annealed), P3HTT-
PDPP(Hex) (red line as-cast, dark red line
annealed), P3HTT-PDPP(EH) (dark purple line as-
cast, light purple line annealed), and two-acceptor
polymer P3HTT-DPP-PDPP(EH) (orange line as-
cast) in the solid state.

120

Figure A1.1 Synthesis of 3-(5-bromothiophen-2-yl)-2,5-bis(2-
ethylhexyl)-6-(thiophen-2-yl)-3,3a-dihydro-
pyrrolo[3,4-c]pyrrole-1,4-dione (mono-BrDPP).

141

Figure A1.2 Synthesis of 2,5-bis(2-ethylhexyl)-3,6-bis(5-
phenylthiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4-
dione (PDPP).

142

Figure A1.3 Synthesis of 2-(tetrahydro-2H-pyran-2-oxyethyl)-5-
trimethyltin-thiophene.

143

xvi

Figure A1.4
1
H NMR of PDPP in CDCl
3
. 148

Figure A1.5
13
C NMR of PDPP in CDCl
3
. 149

Figure A1.6
1
H NMR of PCB-NCS in CDCl
3
. 150

Figure A1.7
1
H NMR PCB-PDPP in CDCl
3.
151

Figure A1.8
13
C NMR PCB-PDPP in CDCl
3.
152

Figure A1.9 Molar absorptivity spectra of PDPP in solution
(blue line).

153
Figure A1.10 Absorption spectra of PDPP as-cast (black line) and
annealed (red line) in the solid state (top). Molar
absorptivity of PDPP and in solution (bottom).

154

Figure A1.11 High-resolution electrospray ionization mass
spectrometry (HR-ESI-MS) of PCB-PDPP with m/z
value of 1605.

155

Figure A2.1
1
H NMR of P3HT in CDCl
3
. 164

Figure A2.2
1
H NMR of P3HT
77
-co-P3DHT
23
in CDCl
3
. 165

Figure A2.3
1
H NMR of P3HT
50
-co-P3DHT
50
in CDCl
3
166

Figure A2.4
1
H NMR of P3HT
27
-co-P3DHT
73
in CDCl
3
. 167

Figure A2.5
1
H NMR of P3DHT in CDCl
3
. 168

Figure A2.6 GIXRD of annealed thin films of P3DHT (black
line), P3HT
77
-co-P3DHT
23
, (blue line), P3HT
47
-co-
P3DHT
53
(red line), P3HT
27
-co-P3DHT
73
(green
line), P3HT (purple line). All were spin-coated
from CB and annealed at 156 ºC for 30 min under
N
2
.

170

Figure A2.7 TEM images at different copolymer compositions
for as-cast films. TEM images of P3DHT:PC
61
BM,
P3HT
77
-co-P3DHT
23
:PC
61
BM, P3HT:PC
61
BM,
P3HT
47
-co-P3HT
53
:PC
61
BM, and P3HT
27
-co-
P3DHT
73
:PC
61
BM all prepared in 1:0.8 ratios
corresponding to solar cells conditions. Blends of
P3HT
77
-co-P3DHT
23
:PC
61
BM and P3HT
27
-co-

171
xvii

P3DHT
73
:PC
61
BM require purification to remove
palladium particles.

Figure A3.1 Synthetic scheme of 3 (dibromo-thieno-
diktetopyrrolopyrrole).

173

Figure A3.2
1
H NMR of 1 in DMSO-d
6
. 178

Figure A3.3
1
H NMR of 1a in CDCl
3
. 179

Figure A3.4
1
H NMR of 1b in CDCl
3
. 179

Figure A3.5
1
H NMR of 2 in DMSO-d
6
. 180

Figure A3.6
1
H NMR of 2a in CDCl
3
. 181

Figure A3.7
13
C NMR of 2a in CDCl
3
. 181

Figure A3.8
1
H NMR of 2b in CDCl
3
. 182

Figure A3.9
13
C NMR of 2b in CDCl
3
. 182

Figure A3.10
1
H NMR of P3HTT-PDPP (Hex) in CDCl
3
. 183

Figure A3.11
1
H NMR of P3HTT-PDPP (EH) in CDCl
3
. 184

Figure A3.12
1
H NMR of P3HTT-DPP in CDCl
3
. 185

Figure A3.13
J-V curve for semi-random copolymers: P3HTT-PDPP
(Hex), P3HTT-PDPP (EH), P3HTT-DPP-PDPP (EH),
P3HTT-DPP, and P3HT under 100 mW/cm
2
(AM1.5G)
illumination.

186

xviii

ABSTRACT
The use of organic photovoltaics (OPVs) is the focus of this dissertation because
of its low-cost, flexibility, and lightweight. The goal is to explore fundamental chemistry
with the ultimate purpose of improving device efficiency. Applying new chemistry in the
context of binary and ternary blends platforms is demonstrated. Ternary blend bulk
heterojunction (BHJ) is an emerging platform for great potential to exceed efficiency of
binary blend BHJs.
In this dissertation, three types of materials are examined: small organic
molecules, random poly(thiophene)-based copolymers, and semi-random polymers,
materials which are relevant to ternary blends. In Chapter 1, binary blends consisting of a
small molecule donor and a fullerene acceptor are introduced to encompass the classes of
materials showing power conversion efficiencies (PCE) higher that 4%. Small molecules
(SMs) are attractive materials for photovoltaics because their syntheses is reproducible
and facile, purification is simpler than for polymers, and are well-defined molecular
structures or monodisperse. Structural variations in SMs lead to changes in the properties,
particularly low-lying HOMO levels which contribute to high open-circuit voltage (V
oc
)
values, which could be higher performing for ternary blends than the respective limiting
binary blends. Currently, either current or voltage is controlled, but in many instances at
the expense of fill factor (FF), this is a limitation of SM ternary blend solar cells that is
further studied in Chapter 2.
xix

The following chapters of this dissertation discuss concepts pertaining to ternary
blend BHJ solar cells and each chapter highlights fundamental underlying principles of
ternary blends. The core aspects that are addressed relate to how structural variations
influence the overall material properties, thus their performance in devices.
A new ternary blend system is introduced in Chapter 2 with the goal to ascertain
how the compositional dependence of the system leads to a tunable V
oc
in devices made
of polymer/dye/fullerene. Diketopyrrolopyrrole (DPP) based SM is the choice of small
molecule for this purpose for its frontier orbital energy levels, absorption profile, and
high V
oc
in binary blends with PCBM (0.9 V). This SM is reported for the first time as a
donor in binary blends. In this work, small molecules can work in ternary blend BHJ
solar cells by contributing to V
oc
tunability, although the FF values are low and there is no
clear trend when they are optimized. These devices show that SMs are extremely
sensitive to processing conditions as evidenced by the optimization of voltage and the
lower FF values, additionally the SM shows lower degree of reproducibility.
A different approach for developing basic principles in ternary blends, consists of
tuning the surface energy of a novel family of random copolymers through composition.
These copolymers contain oxyethylene chains and by varying its content, surface energy
is found to change and ultimately influence the morphology. Surface energy is closely
associated with miscibility between polymers and potentially applied to ternary blends. It
is proposed in the alloy model that two polymers can have surface energies close in value
and lead to V
oc
tuning in ternary blends. The copolymers of interest in Chapter 3
demonstrate surface energy is tuned from high to low oxyethylene content. The
xx

composition-dependence surface energy allowed for tuning the
hydrophobicity/hydrophobicity of polymer films, without influencing the electronic
properties of the materials. Devices made of these copolymers and PCBM demonstrated
that the resulting active layer blends can function without detrimental changes to the
polymer with higher oxyethylene content. Also, surface energy tunability can be used to
predict how polymers can mix with one another and can be extrapolated to ternary
system.
Lastly, exploring the application of semi-random copolymers containing different
aryl groups in the backbone in Chapter 4, leads to the possibility for tuning the HOMO
energy level to enhance the V
oc
in binary blends and use the polymers for developing
materials that are relevant to ternary blends in the future. This application addresses the
issue of limiting voltage for semi-random polymers and expands the scope of polymers
for ternary blends. Here, the synthetic efforts are discussed, as well as the
characterization of the polymers, and their use in binary blend devices.

1

Chapter 1: Small Molecules in Solution Processed Binary and
Ternary Blend Bulk Heterojunction Solar Cells
1.1 General Introduction: Organic Photovoltaics
With the increasing awareness of global warming growing over the past decade,
new ways to transform solar energy into electricity have been developed. The sun is the
most abundant potential source of energy. Having a limited supply of fossil fuels, among
them natural gas, coal and oil underlies the pressing need for this type of efforts.
Currently, only less than 1% of all energy is solar, however the sun can supply the
world’s needs for a whole year in an hour.
1,2
Based on the statistics, the power of energy
resources in the world is estimated to be only 20 TW while the power from the sun is
approximately 10
5
TW.
3
One approach has been the introduction of photovoltaic cells,
PV for short, as they provide a cost-effective alternative for renewable energy sources
coming from the sun.
4

The predominant technology in this field is based on inorganic semiconducting
materials, such as single or multi crystalline silicon solar cells. Silicon-based solar cells
have been known for over 50 years. One of the first reported solar cells in this class dates
back to 1954 when Chapin and coworkers developed a silicon single p-n junction solar
cell achieving a power conversion efficiency (PCE) of 6%.
5
More recently, silicon solar
cells have achieved power conversion efficiencies of ~25%.
6
The major limitations for
their application have been the high cost of fabrication and the size of the solar panels as
2

they are heavy and bulky. Given this fact, thin film solar cells have emerged as an
alternative,
7
where inorganic materials such as amorphous silicon, group II-VI
semiconductors, CdTe and other chalcogenides like CuInSe
2
(CIS) or CuInGaSe
2
(CIGS),
have been known to be more absorptive than crystalline silicon and their efficiencies in
devices are comparable and competitive.
8
Another type of thin films includes organic
materials, which have been gaining more interest for being cost-effective, as well as
having simpler technologies that go into their fabrication.
While inorganic thin film solar cells have been thriving, organic thin film solar
cells are not far behind with efficiencies as high as 12%.
9
Organic solar cells have the
potential of producing 1 kW/h of electricity for only 49¢ to 85¢.
10
Another property that
makes these cells suitable for converting sunlight into electricity is that organic materials
have higher absorption coefficients allowing for less material to be used and still be in the
order of 10
5
cm
-1
in films up to 200 nm in thickness.
4
In addition, organic materials are
less dense than their counterparts, which combined with the small amount of material
needed renders this type of solar cells lightweight and therefore low cost.
11,12

The next aspect to consider is how inorganic semiconductors operate. In
traditional inorganic p-n junctions photon absorption leads to free electrons and holes
which can generate electric current. Whereas for organic materials the downside is that,
an exciton is formed after optical absorption solely as a bound electron-hole pair. Due to
a higher binding energy of organic materials and lower dielectric constant than its
inorganic counterparts,
13
separation of free electrons and hole carriers occurs only at

higher energy than just thermal and only after diffusing to the interface,
inorganic materials where free electrons and holes are generated more readily.
1.2 Device Architecture: Polymer
Ongoing work in the field o
late 1970s and 1980s, particularly the use of high vacuum deposition of small molecules.
In 1986, Tang reported a vacuum
copper phthalocyanine donor
a bilayer cell. He demonstrated a working device with almost 1% power conversion
efficiency (PCE) and fill factor (FF) of 65%. This work illustrated the idea of the
heterojunction, which is currently defined as either bilay
heterojunction (BHJ) solar cells as depicted in

Figure 1.1 Illustration of donor/acceptor device architectures. Bilayer donor/acceptor solar
cell (left) and bulk heterojunction
bilayer (bottom left) and
right).
higher energy than just thermal and only after diffusing to the interface,
inorganic materials where free electrons and holes are generated more readily.
Device Architecture: Polymer-Fullerene Bulk Heterojunction
Ongoing work in the field of OPV has been propelled by the advancements in the
late 1970s and 1980s, particularly the use of high vacuum deposition of small molecules.
In 1986, Tang reported a vacuum-deposited perylene tetracarboxylic acid acceptor and
copper phthalocyanine donor
15
“two-layer organic photovoltaic cell” considered today as
a bilayer cell. He demonstrated a working device with almost 1% power conversion
efficiency (PCE) and fill factor (FF) of 65%. This work illustrated the idea of the
heterojunction, which is currently defined as either bilayer/planar heterojunction
heterojunction (BHJ) solar cells as depicted in Figure 1.1.
16,17

of donor/acceptor device architectures. Bilayer donor/acceptor solar
bulk heterojunction donor/acceptor solar cell (right). Magnification of the
bilayer (bottom left) and bicontinuous morphology of the active layer in BHJ (bottom
3
higher energy than just thermal and only after diffusing to the interface, unlike for
inorganic materials where free electrons and holes are generated more readily.
14

Fullerene Bulk Heterojunction
f OPV has been propelled by the advancements in the
late 1970s and 1980s, particularly the use of high vacuum deposition of small molecules.
deposited perylene tetracarboxylic acid acceptor and
oltaic cell” considered today as
a bilayer cell. He demonstrated a working device with almost 1% power conversion
efficiency (PCE) and fill factor (FF) of 65%. This work illustrated the idea of the
er/planar heterojunction
15
or bulk

of donor/acceptor device architectures. Bilayer donor/acceptor solar
agnification of the
morphology of the active layer in BHJ (bottom
4

In an organic heterojunction, excitons must find enough driving force to
dissociate into free charges at the interface. For this reason the two materials need to have
different electron affinities and ionization potentials. Excitons diffuse through the
interface of a donor with a lower ionization potential to reach an electron acceptor site
with high electron affinity where charges can then be generated and transported to their
respective phases (holes to the donor and electrons to the acceptor). This difference in
potential energy between the two interacting species and the larger exciton binding
energy required lead to exciton dissociation. In bilayer heterojunction devices, excitons
must be formed within no more than 10 nm of the interface, otherwise they would decay
to the ground state.
18
Given that exciton diffusion lengths in organic materials are much
lower (1-10 nm) than the thickness of the films, only photons absorbed within this
distance from the interface will contribute to overall light-harvesting.
19
Exciton diffusion
length depends on both exciton mobility and exciton lifetime, which need to be increased
in order to have higher photocurrents in BHJ solar cells.
20
To overcome the limitations of
bilayer heterojunction, bulk heterojunction solar cells came about almost two decades
ago.
16,17

This new technology involved the blending of the donor and acceptor materials to
form an interpenetrating network to maximize the interfacial area for more efficient
exciton dissociation into free charge carriers which may occur everywhere in the active
layer.
16,21
The photon-to-electron conversion efficiency is therefore increased as the
absorbed photons are in closer contact with the interface and contribute to photocurrent
generation. Another positive attribute to BHJ solar cells is that the active layer,
5

comprised of donor and acceptor, is solution-processable and can be fabricated in one
step by a variety of methods and utilizing minimal amount of energy, while easily
adapted to large area devices.
22,23

BHJ is the predominant architecture for organic solar cells and the combination of
polymer donor and fullerene acceptor as a single active layer has been the most
successfully studied.
24
Fullerenes are widely used in BHJs for several reasons: high
electron affinity (low-lying LUMO), high electron mobility demonstrated in FETs (1 cm
2

V
-1
s
-1
)
25
, the ability to pack into ordered crystalline phases leading to effective charge
transport , and producing ultrafast photoinduced electron transfer (from a donor).
3

Commonly a C
60
derivative is used as a result of functionalization with
solubilizing groups, more specifically [6,6]-phenyl-C
61
butyric acid methyl ester (PCBM)
developed by Wudl and coworkers,
3,26
while other functional groups are used and
maintain similar electronic properties to C
60
. The fullerene component remains constant
in solar cell literature whereas the polymer donor varies depending on its electronic band
levels. Poly(3-hexylthiophene) (P3HT) has been the most commonly used donor
27
and
much of the work in the field is based on its derivatives as absorbing materials.
28,29

While a continuous development on organic photovoltaics (OPVs) has been
taking place, the idea of small band gap materials also was explored over the last two
decades. These materials have a characteristic bandgap of less than 2eV, and they work
as the donors. Regioregular P3HT (rr-P3HT) makes up the benchmark polymeric solar
cell with an efficiency that reached 5% and a band gap of 1.9 eV. While widely used, it
6

suffers an absorbance limitation as only photons below a wavelength of 650 nm in the
solar spectrum are absorbed.
30
By decreasing the band gap, the number of photons
harvested increases. Ideally a material whose band gap is 1.4 eV is desired and this is
determined by several factors like charge separation, how absorbing the donor is and
what the acceptor’s properties are.
31

While efforts for finding suitable materials to optimize the performance of polymer-based
organic solar cells will be addressed here, it is also important to understand the
mechanism of operation. This understanding will allow for better materials design.

1.2.1 Device Operation: Polymer-Fullerene Bulk Heterojunction
In order to optimize the performance of organic solar cells, the fundamental
processes by which sunlight is converted to electricity must be understood.
32
These are
shown on Figure 1.2 and consist of: the absorption of a photon by the donor material,
which generates an exciton (bound electron-hole pair); the exciton diffuses to the
interface and the electron is transferred to the acceptor material while the hole is in the
donor. These two species are Coulombically bound (geminate pair), but still located on
their respective material. Following is the dissociation of the geminate pair into free
charge carriers, then the transport of such carriers to the electrodes (electrons to the
cathode and holes to the anode) and finally charge collection at the electrodes. Within
some steps in this sequence are loss mechanisms in which energy is lost. Specifically if 1)
photons are partially

Figure 1.2 Pathway to generation of free charges by photoexcitation
absorbed by the donor material and an exciton is formed. b) The ex
interface. c) An electron is transferred from the donor to the acceptor. d) Coulombically
bound hole-electron pair is dissociated into free charge carriers. e) Free carriers are
transported to their respective phases towards the elect
collected at the electrodes. Reproduced with permission from Kroon, R.
Rev. 2008, 48, 531–582.

absorbed either because the active layer is too thin or the band gap is t
excitons decay to the ground state as when they are generated far from the interface, 3)
geminate pair recombination after electron transfer and 4) bimolecular recombination at
the transport stage.
29
Any of these losses affects the performance of the device and
therefore need to be considered when designing new materials.
Performance in organic solar cells is characterized by different parameters seen in
Figure 1.3, which relate back to the
to the ratio of power out over power in as a percentage, and is determined more
specifically by the open-
Pathway to generation of free charges by photoexcitation
absorbed by the donor material and an exciton is formed. b) The exciton diffuses to the
interface. c) An electron is transferred from the donor to the acceptor. d) Coulombically
electron pair is dissociated into free charge carriers. e) Free carriers are
transported to their respective phases towards the electrodes. f) Free charge carriers are
collected at the electrodes. Reproduced with permission from Kroon, R.
582. Copyright (2008) Taylor and Francis.
absorbed either because the active layer is too thin or the band gap is t
excitons decay to the ground state as when they are generated far from the interface, 3)
geminate pair recombination after electron transfer and 4) bimolecular recombination at
Any of these losses affects the performance of the device and
need to be considered when designing new materials.
Performance in organic solar cells is characterized by different parameters seen in
, which relate back to the power conversion efficiency (PCE). PCE or
to the ratio of power out over power in as a percentage, and is determined more
-circuit voltage (V
oc
), the short-circuit current (
7

Pathway to generation of free charges by photoexcitation. a) Photon is
citon diffuses to the
interface. c) An electron is transferred from the donor to the acceptor. d) Coulombically
electron pair is dissociated into free charge carriers. e) Free carriers are
rodes. f) Free charge carriers are
collected at the electrodes. Reproduced with permission from Kroon, R. et al. Polym.
absorbed either because the active layer is too thin or the band gap is to wide, 2) when
excitons decay to the ground state as when they are generated far from the interface, 3)
geminate pair recombination after electron transfer and 4) bimolecular recombination at
Any of these losses affects the performance of the device and
Performance in organic solar cells is characterized by different parameters seen in
power conversion efficiency (PCE). PCE or η equals
to the ratio of power out over power in as a percentage, and is determined more
circuit current (J
sc
), and the fill

factor (FF) which corresponds
V
oc
as shown below. The current
such it has a profound influence on the PCE and will be accordingly explained.
Figure 1.3 Current
parameters definitions.
current in solar cells are shown on the
parameters such as
the maximum power point
defined as the ratio of power out (P
maximum power divided by

The efficiency of solar cells is related directly to
conventional standard solar spectrum AM 1.5G (spectrum corresponding to the sun at 45°
above the horizon) and 100 mW/cm
with the J
sc
as being related to the absorption breadth and intensity of the a
The broader the absorption achieved, the greater the number of photons that can be
harvested and lead to charge carriers. The current is limited by the band gap (
donor material, the smaller the E
factor (FF) which corresponds to the ratio of maximum power over the product of
as shown below. The current-voltage relationship is defined by opposing forces; as
such it has a profound influence on the PCE and will be accordingly explained.
Current-voltage (I-V) curve depiction and performance
parameters definitions. The typical current-voltage curves for dark and light
ent in solar cells are shown on the left and illustrate the
parameters such as J
sc
, V
oc
, FF, and J
m
as well as V
m
(current and voltage at
the maximum power point). The power conversion efficiency (η
as the ratio of power out (P
out
) to power in (P
in
). The FF is the ratio of
maximum power divided by J
sc
x V
oc
.
The efficiency of solar cells is related directly to the product of J
conventional standard solar spectrum AM 1.5G (spectrum corresponding to the sun at 45°
above the horizon) and 100 mW/cm
2
light intensity.
29
Each component is interconnected,
as being related to the absorption breadth and intensity of the a
The broader the absorption achieved, the greater the number of photons that can be
harvested and lead to charge carriers. The current is limited by the band gap (
donor material, the smaller the E
g
, the higher the J
sc
. A conventional way of representing
8
to the ratio of maximum power over the product of J
sc
and
voltage relationship is defined by opposing forces; as
such it has a profound influence on the PCE and will be accordingly explained.

) curve depiction and performance
voltage curves for dark and light
the important
nd voltage at
power conversion efficiency (η in %) is
The FF is the ratio of
J
sc
, V
oc
, and FF at
conventional standard solar spectrum AM 1.5G (spectrum corresponding to the sun at 45°
Each component is interconnected,
as being related to the absorption breadth and intensity of the active layer.
The broader the absorption achieved, the greater the number of photons that can be
harvested and lead to charge carriers. The current is limited by the band gap (E
g
) of the
way of representing

the solar spectrum is by correlating the onset of absorption to how many photons can be
harvested into electrons or the highest photon flux, which as seen below occurs at higher
wavelengths, therefore lower energy.
device. Hence the interest for low band
the case of P3HT shown in purple in

Conversely, the V
related to the energetic difference between the HOMO of the donor (HOMO
LUMO of the acceptor (LUMO
level, while increasing the
compromise illustrated in
the solar spectrum is by correlating the onset of absorption to how many photons can be
harvested into electrons or the highest photon flux, which as seen below occurs at higher
wavelengths, therefore lower energy.
28
This in turn lowers the voltage produced in the
device. Hence the interest for low band gap polymers which absorb beyond 650 nm as is
the case of P3HT shown in purple in Figure 1.4.

Figure 1.4 Photon flux from the sun
as a function of wavelength and
absorbance, in black in red at
AM1.5G. Absorption profile of
P3HT in purple. Reproduced with
permission from Thompson, B. C.
and Fréchet, J. M. J. Angew. Chem.
Int. Ed. 2008, 47, 58–77. Copyright
(2008) John Wiley and Sons.
V
oc
suffers with a smaller band gap since it is proportionally
related to the energetic difference between the HOMO of the donor (HOMO
LUMO of the acceptor (LUMO
A
). Increasing the V
oc
means having a lower
hile increasing the J
sc
requires high-lying HOMO. This is called the
compromise illustrated in Figure 1.5 and the way to overcome it is by balancing both
9
the solar spectrum is by correlating the onset of absorption to how many photons can be
harvested into electrons or the highest photon flux, which as seen below occurs at higher
This in turn lowers the voltage produced in the
gap polymers which absorb beyond 650 nm as is
gap since it is proportionally
related to the energetic difference between the HOMO of the donor (HOMO
D
) and the
means having a lower-lying HOMO
lying HOMO. This is called the J
sc
-V
oc

and the way to overcome it is by balancing both

parameters and tuning the energetic levels of the materials in order to optimize the
performance of binary blend single
has a limiting theoretical efficiency around 11
Figure
level
voltage relationship. Illustrating
LUMO energies of a donor/
band gap of the donor (
empirically related to the HOMO
offset

Another way to enhance the
as using bis-indene-C
60
as the acceptor with P3HT as the donor, achieving
for instance. Further considerations for enhancing this parameter are the energy offset
between the LUMO of the donor and the LUMO of the acceptor
parameters and tuning the energetic levels of the materials in order to optimize the
ce of binary blend single-layer bulk heterojunction solar cells. This platform
has a limiting theoretical efficiency around 11-12%.
12,33,34

Figure 1.5 Representation of energetic band
level diagrams of P3HT:PCBM and current-
voltage relationship. Illustrating HOMO and
LUMO energies of a donor/acceptor pair, the
band gap of the donor (E
g
), and the V
oc
which is
empirically related to the HOMO
D
– LUMO
A

offset.
Another way to enhance the V
oc
is by using a higher-lying LUMO acceptor
as the acceptor with P3HT as the donor, achieving
for instance. Further considerations for enhancing this parameter are the energy offset
between the LUMO of the donor and the LUMO of the acceptor, which is required to be
10
parameters and tuning the energetic levels of the materials in order to optimize the
layer bulk heterojunction solar cells. This platform
lying LUMO acceptor
35
, such
as the acceptor with P3HT as the donor, achieving V
oc
of 0.84 V,
for instance. Further considerations for enhancing this parameter are the energy offset
, which is required to be
11

at least 0.3 eV to enable electron transfer. This offset need not be significantly larger than
that value in order to allow an increase in device performance and not waste energy in the
process.
Besides these considerations, balanced transport of electrons in the acceptor and
holes in the donor are important as they influence the FF. Indirectly, FF depends on
processes like charge dissociation, charge carrier transport, and recombination. Charge
transport of holes and electrons must be balanced to avoid space-charge limited current
(SCLC) accumulation, which ultimately leads to low FF even when they differ by an
order or magnitude. This further affects the active layer by limiting the thickness at which
recombination can be prevented.
36
Hole mobility in the polymer also influences the FF,
which in turn affects the morphology. Polymer:fullerene level of miscibility in the blend
needs to be tuned in order to have an ideal morphology and to overcome macro-phase
segregation.
Aspects such as aggregation, crystallinity and phase-separation are closely related
to the FF. The fabrication of the active layer (and device as a whole), the processing
(spin-coating, solvent), post-processing treatment (solvent, solvent vapor or thermal
annealing), and the optimization (composition, use of additives) determine if the FF will
be greater than 0.6, considering optimal FF of 0.70-0.75 for literature reports.
37,38

Ternary blends have emerged as a potential route to increase η by overcoming the
J
sc
-V
oc
compromise.
39,40
The focus of this work has been on finding ways to overcome
this limiting relationship between voltage and current by means of a new platform,
12

ternary blend BHJ SCs, thereby maximizing the product of the two as will be thoroughly
discussed in the next few pages.

1.3 Beyond Binary Blend Photovoltaics
With power conversion efficiencies greater than 9%, single-layer BHJ solar cells,
made of a polymer and a fullerene, have dominated the literature over the past two
years.
38,41–44
Even as this platform continues to move forward, the maximum predicted
efficiency is just 11% as reported by Scharber and coworkers.
33
The need for improving
the efficiency while maintaining the simplicity of solution-processable organic solar cells
is key. One approach to increase the PCE is by use of tandem cells, which employ two
absorber materials and whose upper limit efficiency is in the 14-15% range.
45
The
downside of this type of device is the complexity and cost of fabrication, which leaves
the need for yet another approach.
Ternary blends have emerged as a route to increase PCE or η by simultaneously
tuning the J
sc
and V
oc
. The idea of having two complementary absorbers as donor
materials allows for the broadening of the absorption envelope and by careful selection of
frontier orbital levels of the components, a higher V
oc
than can be attained in the limiting
binary blends. This new platform combines the simplicity of solution processable active
layers and the potential to surpass the efficiency of tandem cells. The active layer consists
of two donors and an acceptor
46–53
or one donor and two acceptors.
54–57
Other examples
13

found in the literature include a polymer, small molecule, and fullerene acceptor
39,58–76
or
polymer, nanoparticle, and fullerene acceptor.
77–81

1.4 Ternary Blend Photovoltaics
The ternary blend approach, proposed over the last few years, has emerged as an
alternative to tandem solar cells as mentioned previously. The clear objective is to
improve the spectral response of the three-component blend compared to the
corresponding binary blends, hence lead to increased J
sc
as shown on Figure 1.6.
Improving photon harvesting is an understandable consequence of multiple component
ternary blends, but the effect on the V
oc
parameter is more difficult to describe. In some
reports, the V
oc
has been proposed to be pinned to smallest V
oc
found in the respective
donor/acceptor (D/A) binary blend,
52,53,82
which limits the potential of simultaneously
increasing the J
sc
and V
oc
.

Figure 1.6
absorption of donor1/donor2/acceptor
blend system assuming the fullerene is PC

Since the HOMO
1.7, hole transport and collection would occur through the donor with the highest
HOMO while electron transport and collection occur through the lowest
However, there are some cases in which this simplistic mechanism does not apply and the
composition-dependent V
higher content of the third component in some cases.
tunability in ternary blends, where the FF remained high were presented by Thompson
al. A two-polymer and one acceptor system (D
system (D/A
1
/A
2
)
54
showed a
components, thereby demonstrating that tunability at high FF is a key parameter.

Figure 1.6 Hypothetical representation of light
absorption of donor1/donor2/acceptor ternary
blend system assuming the fullerene is PC
71
BM.
Since the HOMO
D
-LUMO
A
relationship governs the V
oc
as illustrated in
, hole transport and collection would occur through the donor with the highest
HOMO while electron transport and collection occur through the lowest
However, there are some cases in which this simplistic mechanism does not apply and the
V
oc
is tunable, although the FF has been seen to decrease with
higher content of the third component in some cases.
55,60
The first examples of
tunability in ternary blends, where the FF remained high were presented by Thompson
polymer and one acceptor system (D
1
/D
2
/A)
50
and a donor and two acceptor
showed a V
oc
tunable across the full composition range between
components, thereby demonstrating that tunability at high FF is a key parameter.
14
as illustrated in Figure
, hole transport and collection would occur through the donor with the highest-lying
HOMO while electron transport and collection occur through the lowest-lying LUMO.
40

However, there are some cases in which this simplistic mechanism does not apply and the
has been seen to decrease with
The first examples of V
oc

tunability in ternary blends, where the FF remained high were presented by Thompson et.
and a donor and two acceptor
tion range between
components, thereby demonstrating that tunability at high FF is a key parameter.

Figure 1.7
structure in a ternary blend comprised of two
donors and an acceptor.
HOMO
D1-
while D2
D1-

In a broader context of ternary blends, the two examples mentioned above
demonstrated that tuning the electronic properties of polymeric and
possible. The concept of an organic alloy emerged in which a combination of material
properties is targeted. As is the case in inorganic semiconductors, electronic properties
can be predicted with compositional changes of the two mater
proposed in this model that complementary components form a single
which the V
oc
changes continuously with composition without affecting the FF. More
specifically, the acceptor LUMO (comprising complementary a
HOMO (comprising complementary donors) change energy with varying composition.
In light of this development, ternary blend systems consisting of P3HT:indene

Figure 1.7 Hypothetical scheme of band
structure in a ternary blend comprised of two
donors and an acceptor. V
oc
represents the
HOMO
D
-LUMO
A
relationship for each donor.
-A for the V
oc
of donor 1 and acceptor
while D2-A the V
oc
of donor 2 and acceptor.
-A>D2-A.
In a broader context of ternary blends, the two examples mentioned above
demonstrated that tuning the electronic properties of polymeric and molecular systems is
possible. The concept of an organic alloy emerged in which a combination of material
properties is targeted. As is the case in inorganic semiconductors, electronic properties
can be predicted with compositional changes of the two materials forming the alloy.
proposed in this model that complementary components form a single-
changes continuously with composition without affecting the FF. More
specifically, the acceptor LUMO (comprising complementary acceptors) or the donor
HOMO (comprising complementary donors) change energy with varying composition.
In light of this development, ternary blend systems consisting of P3HT:indene
15
In a broader context of ternary blends, the two examples mentioned above
molecular systems is
possible. The concept of an organic alloy emerged in which a combination of material
properties is targeted. As is the case in inorganic semiconductors, electronic properties
ials forming the alloy. It is
-phase mixture in
changes continuously with composition without affecting the FF. More
cceptors) or the donor
HOMO (comprising complementary donors) change energy with varying composition.
83

In light of this development, ternary blend systems consisting of P3HT:indene-C
60
-
16

bisadduct (ICBA):PCBM (as D:A
1
:A
2
)
54
or poly(3-hexylthiophene-thiophene-
diketopyrrolopyrrole):poly(3-hexylthiophene-co-3-(2-ethylhexylthiophene)): PCBM
(P3HTT-DPP-10%:P3HT
75
-co-EHT
25
:PCBM as D
1
:D
2
:A)
50
, full-range composition
tunability of the V
oc
, without adversely affecting the FF or the J
sc
, was established for the
first time. Both V
oc
and J
sc
increased because there is a composition dependence of the
band energies analogous to the description of an alloy by the quadratic dependence
shown below and because the optical absorption of polymer blends is the weighed sum of
the peaks of the pure materials, respectively.
83

Thinking about a two acceptor system, such as (P3HT:ICBA:PCBM), when the
V
oc
and the acceptor weight fraction of ICBA variation is plotted as is on Figure 1.8, a
variation in LUMO
A
is suggested. The variation of the acceptor LUMO can be fitted to a
quadratic relationship where (E
LUMO
= a+bx+cx
2
) in which x represents the acceptor
composition as ICBA
x
and PCBM
1-x
. The composition dependence of the band energies
in an inorganic alloy with a quadratic dependence is depicted by the extension of
Vegard's law E(x)=E
A
+(E
B
-E
A
-1)x+bx
2
where b is the bowing parameter.
84
This effect is
seen in inorganic binary alloys and had not previously been seen for organic molecular
species for which the distinct energy levels in each component of the blend would be
preserved.
84
Hence, both current and voltage are composition-dependent and their values
are not limited by the smaller of their respective binary blend devices.
17

Figure 1.8 V
oc
for the ternary blend BHJ solar
cells as a function of the amount of ICBA in the
blend. Reproduced with permission from
Khlyabich, P. P. et al. J. Am. Chem. Soc. 2011,
133, 14534–14537. Copyright (2011) American
Chemical Society.

The idea of an alloy formation is central to the ability to overcome the J
sc
-V
oc
compromise and makes the ternary blend platform attractive to pursue, as observed for
two polymer/acceptor and polymer/two-acceptor systems.

1.4.1 Small Molecule Overview
The advancement brought by ternary blends has only been observed for two
polymers and a fullerene or a polymer and two fullerenes and has yet to be demonstrated
for polymer, small molecule, and fullerene systems. Analysis of literature relevant to
18

polymer:dye:fullerene (D
1
:D
SM
:A) blends will be the focus of this chapter. Tunable V
oc

with high FF throughout the full composition range of the components, for this type of
ternary blend, has not been demonstrated previously and is of great interest to develop for
the ternary platform because small molecules are interesting. Small molecules (SMs)
have the potential for ease of synthetic reproducibility as well as simpler purification and
continue to be explored. They also are known for having well-defined structures and
being monodisperse.
85
In order to give context to this platform it is essential to
understand how small molecule BHJ binary blend solar cells perform and demonstrate
that SMs have great potential to contribute to ternary blend solar cell systems due to their
ease to handle.
The following section focuses exclusively on small molecule (SM) binary blend
BHJ devices in the form of SM:fullerene. BHJ solar cells have been predominantly based
on polymeric donors as they provide good film-forming capabilities, ideal film
morphology and are solution processable. While these positive attributes are important in
device fabrication, synthetic reproducibility between batches and ease of purification are
lacking and therefore hold back the prospect of commercial applications. SMs offer facile
syntheses, purification, reproducibility between batches, etc. The challenge is in
controlling the film-forming ability of small molecules given that solution processable
small molecules are harder to process than polymeric materials and vapor deposited
SMs.
86
This difficulty lies at the molecular level where intermolecular forces between
conjugated SMs are strong and therefore induce crystallization, which leads to
aggregation and poor film quality.
87
Ideally a SM with good film-forming ability would
19

be used and its morphology could be controlled by means of post-processing treatment.
SMs usually tend to self-assemble into ordered domains and consequently possess high
charge carrier mobility.
88

1.5 Small Molecules as Donors in Binary Blend Bulk Heterojunction
Here, the best performing solution processable small molecule bulk
heterojunction examples found in the literature up to date will be presented. The criteria
for the selection of the SMs that will be showcased includes efficiencies of 4% and
higher only. PCE of 4% serves as a good reference point for SMs since it is equivalent to
the typical η of P3HT:PCBM, which serves as benchmark for polymers. Additionally,
many important class of SMs perform around 4% η. These encompass the most recent
advancements in the optimization of small molecule binary BHJ solar cells. There are
many different types of solution-processable small molecules used in BHJ and they have
been commonly and broadly divided by others
40,85,87,89
into dyes, oligothiophenes,
triphenylamine-based molecules, and fused acenes. The latter two will not be covered in
much detail here as they do not follow the criteria established and are predominantly used
in organic field-effect transistors (OFETs) and bilayer heterojunction photovoltaics.
90

20

1.5.1 Small Molecule Dyes
Among the dyes there are metal-containing complexes such as subphthalocyanine
(SubPc), phthalocyanine (Pc), merocyanine, squaraine, diketopyrrolopyrrole,
borondipyrromethene (BODIPY), isoindigo, perylene diimide, and quinacridone. These
are all solution processable and among them diketopyrrolopyrroles (DPPs) have shown
the best results for SM BHJ along with oligothiophenes (OT) in section 1.5.2. These two
main types will be the focus of this section on small molecule binary BHJ solar cells,
although a few notable examples pertaining to the other classes of dyes will be
recognized here as well. The first sub-section to follow overlaps with the subsequent as
the division of DPPs and OT class of SM overlap.
1.5.1.1 Diketopyrrolopyrrole-Based Small Molecules
Diketopyrrolopyrrole-based organic materials have been used in industry as
pigments for paints, inks and plastics.
91
More specifically, 1,4-diketo-3,6-
diarylpyrrolo[3,4c]pyrroles (DPPs) consist of an electron accepting core flanked by an
aromatic ring which induces planarity. The first example of such a dye was introduced by
Farum and coworkers in 1974, when they intended to synthesize 2-acetinone (a lactam
derivative of cyclobutadiene) and accidentally made a lactam derivative of pentalene,
1,4-diketo-3,6-diphenylpyrrolo[3,4c]pyrrole,
92
as shown below on Figure 1.9. This
synthesis was low-yielding and conditions were optimized by Ciba-Geigy Company.
They have since developed a more efficient route to commercialize DPP at the industrial
level by reacting a cyano-substituted aromatic ring starting material with a dialkyl

succinate in the presence of base in good yields. These procedures have been patented
and widely used to make various derivatives.

The conjugation of aromatic rings attached to DPP dyes allow tuning of the
frontier orbital energy levels in different derivatives and to th
absorption that these already have. Other aspects such as their high thermal stability and
large scale synthesis make them promising materials for solar cell applications and have
been gaining more attention over the last two d
to make DPP soluble can occur at the
groups can be substituted on positions 2 and 5 of the lactam ring to control
making this core versatile and easy to handle in solution in a wide range of organic
solvents. Nguyen and coworkers have demonstrated that the different su
DPP influence the solid-state packing and the optical properties of a material.
DPP offers a high optical density a
have an electron donating quality as well as high hole mobility, allow for efficient
succinate in the presence of base in good yields. These procedures have been patented
widely used to make various derivatives.
91

Figure 1.9 Synthetic scheme of 2-acetinone.
The conjugation of aromatic rings attached to DPP dyes allow tuning of the
frontier orbital energy levels in different derivatives and to the influence the strong light
absorption that these already have. Other aspects such as their high thermal stability and
large scale synthesis make them promising materials for solar cell applications and have
been gaining more attention over the last two decades.
93
Interestingly, alkyl substitution
to make DPP soluble can occur at the N,N-positions, and as previously not
groups can be substituted on positions 2 and 5 of the lactam ring to control
making this core versatile and easy to handle in solution in a wide range of organic
solvents. Nguyen and coworkers have demonstrated that the different su
state packing and the optical properties of a material.
DPP offers a high optical density and when combined with oligothiophenes which
have an electron donating quality as well as high hole mobility, allow for efficient
21
succinate in the presence of base in good yields. These procedures have been patented
The conjugation of aromatic rings attached to DPP dyes allow tuning of the
e influence the strong light
absorption that these already have. Other aspects such as their high thermal stability and
large scale synthesis make them promising materials for solar cell applications and have
Interestingly, alkyl substitution
positions, and as previously noted, aromatic
groups can be substituted on positions 2 and 5 of the lactam ring to control π-stacking
making this core versatile and easy to handle in solution in a wide range of organic
solvents. Nguyen and coworkers have demonstrated that the different substitutions on
state packing and the optical properties of a material.
94

nd when combined with oligothiophenes which
have an electron donating quality as well as high hole mobility, allow for efficient

fabrication of BHJ solar cells. A target approach to achieve this has been by increasing
the V
oc
as in the case of 3,6
ethylhexyl)pyrrolo[3,4-c]pyrrole
1.10). Comprised of a fused benzofuran to maintain conjugation and having more
electronegativity coming for the oxygen atom stabilizes
material.
88
This dye has a strong absorption up to 710 nm (refer to
HOMO level of -5.2 eV and a LUMO of
solar cells using DPP(TBFu)
Walker et al. in which solvents such as thiophene, chloroform, trichloroethylene, and
carbon disulfide were tested. From this study they were able to determine solubility
parameters which lead to optimal phase
annealing a correlation between solvent and morphology was found.
Figure 1.10 Illustration of the a) Chemical structures of DPP(TBFu)
PC
71
BM. b) Device architecture small molecule DPP(TBFu)
devices. Reproduced with permission from Walker, B.
Mater. 2009, 19, 3063

fabrication of BHJ solar cells. A target approach to achieve this has been by increasing
as in the case of 3,6-bis(5-benzofuran-2-yl)thiophen
]pyrrole-1,4-dione (DPP(TBFu)2) by Walker et al
). Comprised of a fused benzofuran to maintain conjugation and having more
electronegativity coming for the oxygen atom stabilizes the HOMO level of the
This dye has a strong absorption up to 710 nm (refer to Figure 1.
5.2 eV and a LUMO of -3.4 eV. Before optimal conditions for BHJ
solar cells using DPP(TBFu)
2
:PC
71
BM were found, a solvent study was performed by
in which solvents such as thiophene, chloroform, trichloroethylene, and
on disulfide were tested. From this study they were able to determine solubility
parameters which lead to optimal phase-separation and in combination to thermal
annealing a correlation between solvent and morphology was found.
95

Illustration of the a) Chemical structures of DPP(TBFu)
BM. b) Device architecture small molecule DPP(TBFu)
devices. Reproduced with permission from Walker, B. et al. Adv. Funct.
, 3063–3069. Copyright (2009) John Wiley and Sons.
22
fabrication of BHJ solar cells. A target approach to achieve this has been by increasing
yl)thiophen-2-yl-2,5-bis(2-
et al (see Figure
). Comprised of a fused benzofuran to maintain conjugation and having more
the HOMO level of the
Figure 1.11) and a
3.4 eV. Before optimal conditions for BHJ
BM were found, a solvent study was performed by
in which solvents such as thiophene, chloroform, trichloroethylene, and
on disulfide were tested. From this study they were able to determine solubility
separation and in combination to thermal

Illustration of the a) Chemical structures of DPP(TBFu)
2
and
BM. b) Device architecture small molecule DPP(TBFu)
2
:PC
71
BM
Adv. Funct.
Copyright (2009) John Wiley and Sons.

Mixing DPP(TBFu)
with J
sc
of 10.0 mA/cm
-2

also a result of a high V
(HOMO
dye
-LUMO
fullerene
)=1.2 eV as predicted by Scharber and coworkers where a
relationship between band gap and the LUMO of the donor can help predict the
other parameters.
33

Figure 1.11
DPP(TBFu)
film of PC
donor/acceptor blends as
Reproduced with permission from Walker, B.
al. Adv. Funct. Mater.
Copyright (2009) John Wiley and Sons

DPP(TBFu)
2
and PC
71
BM in a 1:0.67 ratio gave a high performing device
and PCE of 4.4% when annealed at 110°C. This improvement is
V
oc
of 0.92 compared to an estimated value of 0.9 V based on
)=1.2 eV as predicted by Scharber and coworkers where a
relationship between band gap and the LUMO of the donor can help predict the

Figure 1.11 UV/Vis absorption spectra of a)
DPP(TBFu)
2
in solution and film and pristine
film of PC
71
BM. c) Different composition of
donor/acceptor blends as-cast in the solid state.
Reproduced with permission from Walker, B. et
Adv. Funct. Mater. 2009, 19, 3063–3069.
Copyright (2009) John Wiley and Sons.
23
BM in a 1:0.67 ratio gave a high performing device
and PCE of 4.4% when annealed at 110°C. This improvement is
of 0.92 compared to an estimated value of 0.9 V based on
)=1.2 eV as predicted by Scharber and coworkers where a
relationship between band gap and the LUMO of the donor can help predict the V
oc
and
24

The absorption intensity is also seen to increase with annealing and with increasing ratios
of dye to fullerene as observed in the absorption profiles below. A higher degree of
crystallinity is also observed from the intensity in the XRD pattern. This also correlates to
the surface morphology in which the domain size increases to hundreds of nanometers
when annealing occurs >100°C and still maintaining small domain sized morphology.
An ester-functionalized DPP derivative was designed by Chen et al. assigned as
DPP(CT)
2
seen in Figure 1.12. The donor attached to an electron-withdrawing group
results in a low-lying HOMO level (-5.33 eV) leading to a V
oc
of 0.94 V.
Simultaneously, the onset of absorption, almost approaching 750 nm, grants a low band
gap of 1.65 eV as compared to DPP(TBFu)
2
where benzofuran affects the HOMO level,
but to a lesser extent. Devices of DPP(CT)
2
:PC
71
BM (1:0.66) followed by thermal
annealing at 90°C for only 10 minutes result in a three-fold improvement in efficiency
compared to as-cast devices (1.14 to 4.02%). The FF is noticeably larger (0.50) for the
annealed device compared to 0.34 for the as-cast device, the current J
sc
=8.55 mA cm
-2

was also almost high, almost three times higher. Furthermore, mobility measurements
illustrate that when annealing occurs balanced charge transport is inferred as electron and
hole mobility ratios approach unity (10
-6
cm
2
V
-1
s
-1
range for SCLC method).
96

Figure 1.12
Other examples in the DPP class include an electron
attached to electron-poor diketopyrro
Figure 1.13a. This is another example in which DPP is used to lower the band gap of the
material, as it does in the case of copolymers.
observed (J
sc
=11.27 mA cm
the high molar absorption, low band gap and low
material. Highly ordered molecules lead to high hole mobilities (
with μ
h
=7.18 X 10
-3
cm
2

by X-ray diffraction. Another remarkable feature of this system is the high external
quantum efficiency (EQE) of 68% at 550 nm observed in
ratio of electrons flowing out (current produced) of device to the number of incident
photons. All of these characteristics combined lead to a 4.06% efficiency.

Figure 1.12 Chemical structure of DPP(CT)
2
.

Other examples in the DPP class include an electron-rich naphthodithiophene
poor diketopyrrolopyrrole unit flanked with thiophenes seen in
. This is another example in which DPP is used to lower the band gap of the
material, as it does in the case of copolymers.
97,98
Both large currents and volta
=11.27 mA cm
-2
, V
oc
=0.84) for blends of NDT(TDPP)
2
and PC
the high molar absorption, low band gap and low-lying HOMO level of the donor
material. Highly ordered molecules lead to high hole mobilities (μ
h
) as evidently shown
V
-1
s
-1
in OFET

and the retained order and crystallinity observed
ray diffraction. Another remarkable feature of this system is the high external
quantum efficiency (EQE) of 68% at 550 nm observed in Figure 1.13b
ratio of electrons flowing out (current produced) of device to the number of incident
photons. All of these characteristics combined lead to a 4.06% efficiency.
25
rich naphthodithiophene
lopyrrole unit flanked with thiophenes seen in
. This is another example in which DPP is used to lower the band gap of the
Both large currents and voltages are
and PC
61
BM due to
lying HOMO level of the donor
) as evidently shown
and the retained order and crystallinity observed
ray diffraction. Another remarkable feature of this system is the high external
Figure 1.13b, relating to the
ratio of electrons flowing out (current produced) of device to the number of incident
photons. All of these characteristics combined lead to a 4.06% efficiency.
99

Figure 1.13
NDT(TDPP)
absorption spectra of NDT(TDPP)
AM1.5G (100 mW cm
permission from
8142–8145.

Another DPP with an acceptor
conjunction with PC
71
BM
of DPP-phenyl-DPP flanked by thiophene rings
Figure 1.14. Lee and coworkers observed that by changing the donor component in the
molecule from thiophene to phenyl the HOMO level was spatially lowered due to the
weaker electron-donating ability of phenyl versus thio
Figure 1.13 Illustration of a) chemical structure of
NDT(TDPP)
2
and b) External quantum efficiency and
absorption spectra of NDT(TDPP)
2
:PC
61
BM (1.5:1.0) under
AM1.5G (100 mW cm
-2
) illumination. Reproduced with
permission from Loser, S. et al. J. Am. Chem. Soc. 2011, 133
8145. Copyright (2011) American Chemical Society.
Another DPP with an acceptor-donor-acceptor (A-D-A) configuration in
BM shows a high voltage at 0.93 V. This small molecule consists
DPP flanked by thiophene rings to either side (Ph(TDPP)
. Lee and coworkers observed that by changing the donor component in the
molecule from thiophene to phenyl the HOMO level was spatially lowered due to the
donating ability of phenyl versus thiophene. In addition, a more ideal
26

ustration of a) chemical structure of
and b) External quantum efficiency and
BM (1.5:1.0) under
) illumination. Reproduced with
133,
Copyright (2011) American Chemical Society.
A) configuration in
a high voltage at 0.93 V. This small molecule consists
to either side (Ph(TDPP)
2
) shown on
. Lee and coworkers observed that by changing the donor component in the
molecule from thiophene to phenyl the HOMO level was spatially lowered due to the
phene. In addition, a more ideal

and bicontinuous morphology is observed with the phenyl containing SM. Overall this
lead to a 4.01% device with 9.09 mA cm
Figure 1.14

A substantially higher FF is found for a pyrene
in devices with PC
71
BM by Fréchet and coworkers yielding 4.1%. A famil
molecules on Figure 1.1
(TPA), benzodithiophene (BDT), and pyrene as donors and DPP as acceptor were
synthesized to study the effects of planarity and symmetry. Also, different size bran
alkyl chains were attached to the DPP core, with 2
solubility, molecular packing and crystallinity of the materials as seen by X
diffraction (XRD). Out of all four molecules, pyrene attached to DPP by carbon n
(C2) showed the best results indicative of high charge carrier mobility, low
recombination losses and leakages plus balanced charge transport. The
are favorable in this system and lead to a favorable morphology and high FF equal to
0.58, and PCE of 4.1%.
101
and bicontinuous morphology is observed with the phenyl containing SM. Overall this
lead to a 4.01% device with 9.09 mA cm
-2
J
sc
and a FF of 0.47.
100

Figure 1.14 Chemical structure of Ph(TDPP)
2
.
A substantially higher FF is found for a pyrene-flanked DPP small molecule used
BM by Fréchet and coworkers yielding 4.1%. A famil
Figure 1.15 possessing self-assembly capabilities with triphenylamine
(TPA), benzodithiophene (BDT), and pyrene as donors and DPP as acceptor were
synthesized to study the effects of planarity and symmetry. Also, different size bran
alkyl chains were attached to the DPP core, with 2-hexyldecyl being the best in terms of
solubility, molecular packing and crystallinity of the materials as seen by X
diffraction (XRD). Out of all four molecules, pyrene attached to DPP by carbon n
(C2) showed the best results indicative of high charge carrier mobility, low
recombination losses and leakages plus balanced charge transport. The π
are favorable in this system and lead to a favorable morphology and high FF equal to
101

27
and bicontinuous morphology is observed with the phenyl containing SM. Overall this

flanked DPP small molecule used
BM by Fréchet and coworkers yielding 4.1%. A family of D-A-D
assembly capabilities with triphenylamine
(TPA), benzodithiophene (BDT), and pyrene as donors and DPP as acceptor were
synthesized to study the effects of planarity and symmetry. Also, different size branched
hexyldecyl being the best in terms of
solubility, molecular packing and crystallinity of the materials as seen by X-ray
diffraction (XRD). Out of all four molecules, pyrene attached to DPP by carbon number 2
(C2) showed the best results indicative of high charge carrier mobility, low
recombination losses and leakages plus balanced charge transport. The π-π interactions
are favorable in this system and lead to a favorable morphology and high FF equal to

Figure 1.15 Donor
moieties flaked by electron
2=benzodithiophene,
substituent. Reproduced with permission from
2011, 23, 5359–5363.

1.5.1.2 Other Noteworthy Small Molecule Dyes
Merocyanine (MC) dyes are small molecules with high absorption coefficients in
the 10
5
cm
-1
range, and have tunable frontier orbital energy levels. They make up a class
of traditional colorants used in colori
moments, and polarizability these dyes have found applications in non
photorefractive materials. One of the first BHJ solar cells containing MC dye, processed
by solution, was developed by
Donor-acceptor-donor (D-A-D) structures containing DPP core
laked by electron-rich end-groups. Group: 1=triphenyl amine,
=benzodithiophene, 3= C1-pyrene substituent, and 4= C2
Reproduced with permission from Lee, O. P. et al. Adv. Mater.
5363. Copyright (2011) John Wiley and Sons.
Other Noteworthy Small Molecule Dyes
Merocyanine (MC) dyes are small molecules with high absorption coefficients in
range, and have tunable frontier orbital energy levels. They make up a class
of traditional colorants used in coloring textiles. Due to their high absorption, dipole
moments, and polarizability these dyes have found applications in non-
photorefractive materials. One of the first BHJ solar cells containing MC dye, processed
by solution, was developed by the groups of Meerholz and Würthner in 2008.
28

D) structures containing DPP core
=triphenyl amine,
= C2-pyrene
Adv. Mater.
Merocyanine (MC) dyes are small molecules with high absorption coefficients in
range, and have tunable frontier orbital energy levels. They make up a class
ng textiles. Due to their high absorption, dipole
-linear optics and
photorefractive materials. One of the first BHJ solar cells containing MC dye, processed
the groups of Meerholz and Würthner in 2008.
102

Since then, modification to the structure has been achieved by having a highly
dipolar donor-acceptor (D
dimers after which the dipole moments cancel in the solid state.
Solution processa
AM1.5, 100 mW cm
-2
. Dye HB366 on
study.
103
Its optimization lead to replacing PCBM with PC
(hence J
sc
=10.2 mA cm
mentioned later in Olson and
electrodes. Most striking is the
0.44, although this system did not require post
progress for merocyanine

Since then, modification to the structure has been achieved by having a highly
acceptor (D-A) planar π-system that self-assembles into centrosymmetric
dimers after which the dipole moments cancel in the solid state.
Solution processable devices of HB366:PC
61
BM gave efficiencies of 4.5 at
. Dye HB366 on Figure 1.16 was the best performing donor in this
Its optimization lead to replacing PCBM with PC
71
BM for enhanced absorption
.2 mA cm
-2
), PEDOT:PSS as hole-transport layer replaced (HTL,
mentioned later in Olson and Bazan work) with MoO
3
, and lastly using Ba/Ag
king is the V
oc
of 1 V, while the FF remained on the low end (<0.6) at
0.44, although this system did not require post-processing treatment is showed great
progress for merocyanine dyes, predominantly vapor-deposited in the past.

Figure 1.16 Molecular
structure of merocyanine
HB366.
29
Since then, modification to the structure has been achieved by having a highly
assembles into centrosymmetric
BM gave efficiencies of 4.5 at
was the best performing donor in this
BM for enhanced absorption
transport layer replaced (HTL,
, and lastly using Ba/Ag as metal
of 1 V, while the FF remained on the low end (<0.6) at
processing treatment is showed great
deposited in the past.

The next class of noteworthy dyes is squaraines (SQ) on
displaying high absorption coefficients, broad absorption in the 500
chemical and thermal stabilities, and last but not least, good film
by Thompson, Forrest and coworkers. The squaraine used was previously a component in
bilayer devices and was vacuum deposited. On the other hand, th
post-processing annealing by exposing the active layer (formed by spin
dichloromethane (DCM) vapors under a nitrogen environment gave control of the
nanoscale phase-separation, consequently enhancing the morphology. This
to 5.5% SQ:PC
71
BM (1:6) devices thanks to a matching of the exciton diffusion length
and the average crystallite size of SQ demonstrate by the TEM images and an ideal
morphology by a reduced series resistance (R
Figure 1.17
diisobutylamino
squaraine (SQ).
1.5.2 Oligothiophene
A small molecule semiconductor with unsymmetrical triarylamine and
terthiophene donors and fluorine
The next class of noteworthy dyes is squaraines (SQ) on Figure 1.1
displaying high absorption coefficients, broad absorption in the 500-900 n
chemical and thermal stabilities, and last but not least, good film-forming abilities as seen
by Thompson, Forrest and coworkers. The squaraine used was previously a component in
bilayer devices and was vacuum deposited. On the other hand, they have discovered that
processing annealing by exposing the active layer (formed by spin
dichloromethane (DCM) vapors under a nitrogen environment gave control of the
separation, consequently enhancing the morphology. This
BM (1:6) devices thanks to a matching of the exciton diffusion length
and the average crystallite size of SQ demonstrate by the TEM images and an ideal
morphology by a reduced series resistance (R
SA
).
104

Figure 1.17 Chemical structure of
diisobutylamino-2,6-dihydroxyphenyl
squaraine (SQ).

Oligothiophene or D/A Small Molecules
A small molecule semiconductor with unsymmetrical triarylamine and
terthiophene donors and fluorine-substituted benzothiadiazole (BTD) acceptor is studied
30
Figure 1.17, also
900 nm range, good
forming abilities as seen
by Thompson, Forrest and coworkers. The squaraine used was previously a component in
ey have discovered that
processing annealing by exposing the active layer (formed by spin-coating) to
dichloromethane (DCM) vapors under a nitrogen environment gave control of the
separation, consequently enhancing the morphology. This process lead
BM (1:6) devices thanks to a matching of the exciton diffusion length
and the average crystallite size of SQ demonstrate by the TEM images and an ideal
A small molecule semiconductor with unsymmetrical triarylamine and
substituted benzothiadiazole (BTD) acceptor is studied

by Paek and coworkers
provide strong intramolecular charge
possess a low band gap.
105
HOMO level of the molecule (from
with PC
71
BM gave a modest 4.24% PCE.

Figure 1.18
fluorinated[bisDMFA
Reproduced with permis
Phys. Chem. C
(2012) American Chemical Society.

Alternatively, acceptor
yet another core unit, 3,6
acrylonitrile groups. Devices were made with this SM and PC
optimization with –CN additive, although no details about it have been reported by
Sharma and coworkers, supposedly giving high efficiencies of 3.76% and 4.96%,
as seen on Figure 1.18. So called push-pull D
provide strong intramolecular charge-transfer (ICT) , hence higher molar absorptivity and
105
BTZ substituent's increased the V
oc
(0.89 v) by lowering the
HOMO level of the molecule (from -5.09 eV to -5.12 eV). This solution processable SM
BM gave a modest 4.24% PCE.
Figure 1.18 Chemical structure of
fluorinated[bisDMFA-Th]-BT-HxTh
3
derivatives.
Reproduced with permission from Paek, S. et al. J.
Phys. Chem. C 2012, 116, 23205–23213. Copyright
(2012) American Chemical Society.
Alternatively, acceptor-donor-acceptor (A-D-A) structures were developed, with
yet another core unit, 3,6-dithienylcarbazole (DTC) end-capped w
acrylonitrile groups. Devices were made with this SM and PC
61
BM or PC
CN additive, although no details about it have been reported by
Sharma and coworkers, supposedly giving high efficiencies of 3.76% and 4.96%,
31
pull D-A-D structures
transfer (ICT) , hence higher molar absorptivity and
(0.89 v) by lowering the
5.12 eV). This solution processable SM

Chemical structure of
derivatives.
.
Copyright
A) structures were developed, with
capped with nitro-phenyl
BM or PC
71
BM and
CN additive, although no details about it have been reported by
Sharma and coworkers, supposedly giving high efficiencies of 3.76% and 4.96%,
32

respectively. For both systems significant photoluminescence quenching was observed
(90%), inferring photo-induced charge transfer between donor and acceptor.
106

Additionally, a broadening in the absorption and shift from solution to film was observed,
indicative of planarization of the chromophore and strong π-π interaction as well as a
slight vibronic feature around 656 nm leading to a high J
sc
=11.23 mA cm
-2
and a low
band gap of 1.74 eV.
107

A variation to this central core was presented by Zhou and coworkers using a
dithienosilole unit as donor flanked with cyanoacetate-terthiophene acceptor groups
(DCAO3TSi).
108
This SM, having electron-withdrawing end groups in combination with
a planar structure, allows having high mobility, low band gap, and good film-forming
capabilities. Devices consisting of 1:0.8 donor to PC
61
BM in the active layer gave a PCE
of 5.84%, with V
oc
of 0.80 V, J
sc
of 11.52 mA cm
-2
, and FF of 0.54. More efforts to
improve device performance, by this group in particular, are presented later in the chapter
showcasing some of the highest-performing small molecule devices in the literature.
Analogously, the groups of Dana Olson and Guillermo Bazan studied the dependence of
hole-transfer layers (HTL) on the performance of SM BHJ solar cells. For this, a system
seen on Figure 1.19 and developed by Bazan and coworkers, a dithienosilole (DTS)
attached to bis(pyridinethiadiazolo-thiophene-hexylthiophene) d-DTS(PTTh
2
)
2
was
further investigated. An oxygen plasma treated NiO
x
layer replaced PEDOT:PSS HTL
and lead to a 5.1% efficiency due to a better alignment of the work functions and the

elimination of interfacial
sulfonic acid groups from the PSS component.

Figure 1.19 Chemical structure of
orientation of the nitrogen atoms is away from the interior core.
Reproduced wit
24, 5368–5373.

As shown in one of their later studies, Bazan and Heeger expand on the use of
additive co-solvents allowed for higher performance as morphological i
arrive from the higher degree of crystallization to give a 6.7% device with
DTS(PTTh
2
)
2
and PC
71
BM, but with MoO
specifically the orientation of the heteroatom in the pyridal thiadiazole group was deemed
critical to keep pushing
nitrogen atoms are located proximal to the interior fragment as seen on
which lead to high performance. In this report, Nguyen and Bazan took a step further and
employed a 5-fluorobenzo[c] [1,2,5]thiadiazole (FBT) as the acceptor unit and proved
that the new SM, p-DTS(FBTTh
therefore PEDOT:PSS can be used in a device with it. This last variation slightly
elimination of interfacial chemistry occurring between the donor and the acidity of the
sulfonic acid groups from the PSS component.
109

Chemical structure of d-DTS(PTTh
2
)
2
. Notice the
orientation of the nitrogen atoms is away from the interior core.
Reproduced with permission from Garcia, A. et al. Adv. Mater.
Copyright (2012) John Wiley and Sons.
As shown in one of their later studies, Bazan and Heeger expand on the use of
solvents allowed for higher performance as morphological i
arrive from the higher degree of crystallization to give a 6.7% device with
BM, but with MoO
x
HTL.
110
A closer look at the structure, more
specifically the orientation of the heteroatom in the pyridal thiadiazole group was deemed
critical to keep pushing the efficiency to higher values. In the previous report, the
nitrogen atoms are located proximal to the interior fragment as seen on
which lead to high performance. In this report, Nguyen and Bazan took a step further and
enzo[c] [1,2,5]thiadiazole (FBT) as the acceptor unit and proved
DTS(FBTTh
2
)
2
is less prone to interacting with the acid in PSS and
therefore PEDOT:PSS can be used in a device with it. This last variation slightly
33
chemistry occurring between the donor and the acidity of the

. Notice the
orientation of the nitrogen atoms is away from the interior core.
Adv. Mater. 2012,
As shown in one of their later studies, Bazan and Heeger expand on the use of
solvents allowed for higher performance as morphological improvements
arrive from the higher degree of crystallization to give a 6.7% device with d-
A closer look at the structure, more
specifically the orientation of the heteroatom in the pyridal thiadiazole group was deemed
the efficiency to higher values. In the previous report, the
nitrogen atoms are located proximal to the interior fragment as seen on Figure 1.20,
which lead to high performance. In this report, Nguyen and Bazan took a step further and
enzo[c] [1,2,5]thiadiazole (FBT) as the acceptor unit and proved
is less prone to interacting with the acid in PSS and
therefore PEDOT:PSS can be used in a device with it. This last variation slightly

improved the efficiency
0.68 FF,
111
which is high
Figure 1.
DTS(PTTh
R
1
=
with permission from
Adv. Mater.
(2012) John Wiley and Sons.

Most recently the efficiency parameters for small molecule solution processable
BHJ have become the new state
varying the resistance of indium tin oxide (ITO) anode.
Heeger have shown the highest recorded PCE of 8.24% when using 5
14.74 mA cm
-2
, V
oc
of 0.77, and FF of 0.72 seen in
giving a 7.0% device with a 0.89 V V
oc
, 12.8 mA cm
which is high and resembling more polymer FF values.

Figure 1.20 Chemical structures of p-
DTS(PTTh
2
)
2
and p-DTS(FBTTh
2
)
2
, where
= n-hexyl and R
2
= 2-ethylhexyl. Reproduced
with permission from Van der Poll, T. S. et al.
Adv. Mater. 2012, 24, 3646–3649. Copyright
012) John Wiley and Sons.
Most recently the efficiency parameters for small molecule solution processable
BHJ have become the new state-of-the-art system with p-DTS(FBTTh
varying the resistance of indium tin oxide (ITO) anode.
112
The groups of Bazan and
he highest recorded PCE of 8.24% when using 5
of 0.77, and FF of 0.72 seen in Figure 1.21. Once again in this
34
, 12.8 mA cm
-2
J
sc
, and
Most recently the efficiency parameters for small molecule solution processable
DTS(FBTTh
2
)
2
:PC
71
BM by
The groups of Bazan and
he highest recorded PCE of 8.24% when using 5Ω/□ ITO, J
sc
of
. Once again in this

section, a “polymer-like” FF is observed. This overall improvement is attributed to the
enhanced EQE (80% at 600
series resistance (R
s
) of ITO layer and FF, although the mechanism for this improvement
is not fully understood yet.
Figure 1.21
Squares represent
circles represent the fill factor. Reproduced with permission
from Wang, D. H.
10.1002/aenm.201300277
and Sons.

Benzodithiophene (BDT) groups are widely
cells and have achieved among the highest efficiencies.
is an acceptor-donor-acceptor (A
into the 800 nm range, low band gap (E
like” FF is observed. This overall improvement is attributed to the
enhanced EQE (80% at 600-650 nm region) as well as the influence the thickness and the
) of ITO layer and FF, although the mechanism for this improvement
is not fully understood yet.

Figure 1.21 a) Plot of PCE and FF vs. ITO-resistances.
Squares represent the power conversion efficiency and
circles represent the fill factor. Reproduced with permission
Wang, D. H. et al. Adv. Energy Mater. 2013, DOI:
10.1002/aenm.201300277. Copyright (2013) John Wiley

Benzodithiophene (BDT) groups are widely used in the field of SM BHJ solar
cells and have achieved among the highest efficiencies. BDT-DPP, shown in
acceptor (A-D-A) molecule that has shown enhanced absorption
into the 800 nm range, low band gap (E
g
=1.71 eV) and has demonstrated an enhanced
35
like” FF is observed. This overall improvement is attributed to the
650 nm region) as well as the influence the thickness and the
) of ITO layer and FF, although the mechanism for this improvement

resistances.
the power conversion efficiency and
circles represent the fill factor. Reproduced with permission
, DOI:
Copyright (2013) John Wiley
used in the field of SM BHJ solar
DPP, shown in Figure 1.22
A) molecule that has shown enhanced absorption
as demonstrated an enhanced

phase-separation and highly ordered molecular stacking in the solid state. When a 1:1
blend of BDT-DPP:PC
71
(DIO) as additive, FF=0.62,
importantly the efficiency is 5.29%.
of nitrogen atoms in DPP with 2
further functionalized with EH chains.
Figure 1.22
donor
molecule.

BDT flanked by terthiophenes and end
known as DCAO3T(BDT)3T seen in
its precursor DCAO7T (5.08%).
mobility (4.5 x 10
-4
cm
2

high FF of 0.60. They suggested that since the mobilities of SM and PC
to each other in value that they had a better miscibility which further contributed to a
percolated pathway morphology as evidenced from the TEM and AFM images of the
blend.
115

separation and highly ordered molecular stacking in the solid state. When a 1:1
71
BM is processed from o-dichlorobenzene and 1,8
(DIO) as additive, FF=0.62, V
oc
is 0.72 V, current approaches 12 mA/cm
importantly the efficiency is 5.29%.
113
This small molecule is made soluble by alkylation
of nitrogen atoms in DPP with 2-ethylhexyl (EH) groups and BDT core with thiophenes
further functionalized with EH chains.

Figure 1.22 Chemical structure of acceptor-
onor-acceptor (A-D-A) BDT-DPP small
molecule.
BDT flanked by terthiophenes and end-capped with alkyl cyanoacetate groups,
known as DCAO3T(BDT)3T seen in Figure 1.23 has improved efficiency of 5.44% over
its precursor DCAO7T (5.08%).
114
The planarity of this molecule contributed to its high
V
-1
s
-1
), which combined with its optimized morphology gave
high FF of 0.60. They suggested that since the mobilities of SM and PC
o each other in value that they had a better miscibility which further contributed to a
percolated pathway morphology as evidenced from the TEM and AFM images of the
36
separation and highly ordered molecular stacking in the solid state. When a 1:1
dichlorobenzene and 1,8-diiodooctane
s 12 mA/cm
2
, and more
This small molecule is made soluble by alkylation
ethylhexyl (EH) groups and BDT core with thiophenes
capped with alkyl cyanoacetate groups,
has improved efficiency of 5.44% over
The planarity of this molecule contributed to its high
), which combined with its optimized morphology gave
high FF of 0.60. They suggested that since the mobilities of SM and PC
61
BM were close
o each other in value that they had a better miscibility which further contributed to a
percolated pathway morphology as evidenced from the TEM and AFM images of the

Figure 1.23
DCAO3(BDT
While benzodithiophenes have proven to be good donor materials for OPVs
further modifications to its structure have been necessary for pushing the efficiency even
higher. One particular way has been to end
example of this is di-3-ethylrhodanine septithiophene (DERHD7T) seen in
with high absorption coefficient (7.2 x 10
shoulder in the film absorption profile, and low band gap of 1.72 eV, bu
importantly good film-forming abilities. Devices with PCBM and DERHD7T give 6.1%
efficiency, 0.47 FF, and high
absorption and high ordering /crystallinity leading to efficient charge carriers and
transport.
In an attempt to fine tune the structure and get even greater performance, Chen
and coworkers took the broadly used BDT core acceptor unit and functionalized it with
Figure 1.23 Molecular structures of DCAO7T and
DCAO3(BDT)3T where R=hexyl.

While benzodithiophenes have proven to be good donor materials for OPVs
further modifications to its structure have been necessary for pushing the efficiency even
higher. One particular way has been to end-functionalize with rhodanine g
ethylrhodanine septithiophene (DERHD7T) seen in
with high absorption coefficient (7.2 x 10
4
cm
-1
), strong π-π stacking from the vibronic
shoulder in the film absorption profile, and low band gap of 1.72 eV, bu
forming abilities. Devices with PCBM and DERHD7T give 6.1%
efficiency, 0.47 FF, and high J
sc
of 13.98 mA cm
-2
, which originate from the strong
absorption and high ordering /crystallinity leading to efficient charge carriers and
In an attempt to fine tune the structure and get even greater performance, Chen
and coworkers took the broadly used BDT core acceptor unit and functionalized it with
37

Molecular structures of DCAO7T and
While benzodithiophenes have proven to be good donor materials for OPVs
further modifications to its structure have been necessary for pushing the efficiency even
functionalize with rhodanine groups. An
ethylrhodanine septithiophene (DERHD7T) seen in Figure 1.24,
stacking from the vibronic
shoulder in the film absorption profile, and low band gap of 1.72 eV, but most
forming abilities. Devices with PCBM and DERHD7T give 6.1%
, which originate from the strong
absorption and high ordering /crystallinity leading to efficient charge carriers and
In an attempt to fine tune the structure and get even greater performance, Chen
and coworkers took the broadly used BDT core acceptor unit and functionalized it with
38

ethylhexyloxy groups. To this core terthiophenes end-functionalized with 3-
ethylrhodanines were attached giving small molecule DR3TBDT seen in Figure 1.24.
This molecule has a high absorption coefficient (6.3 x 10
4
cm
-1
) due to the rhodanine dye
presence which also renders the low band gap of the molecule. With the use of
polydimethylsiloxane (PDMS) as an additive helped make a more even bicontinuous
interpenetrating network in the active layer between DR3TBDT donor and PC
71
BM
acceptor believed to be due to the high miscibility of the two. PCE is among the highest
ever reported at 7.38% (certified at 7.10%) with high currents and FF (12.21 mA cm
-2

and 0.65, respectively).
116

More recently, to extend the conjugation of the small molecule, the same research
group developed a similar compound as the previous, but incorporated thiophene
substituents at the 4- and 8- position of BDT giving strong intramolecular charge transfer
and broad absorption. This new molecule, DR3TBDTT seen in Figure 1.24, shows a red-
shift in absorption both in solution and film compared to DR3TBDT and also broad
absorption leading to a J
sc
of 13.17 mA cm
-2
. A high V
oc
=0.96 V and FF of 0.66 is also
obtained and believed to be caused by the weakened intermolecular interactions of the
bulky long alkyl chains. DR3TBDTT:PC
71
BM (1:0.8) with PDMS additive was
optimized to give one of the highest power conversion efficiency for a small molecule
BHJ reported to date at 8.12%.
117

Figure 1.24 Chemical structure of benzodithiophene based small
molecules with ethylrhodanine end
2-ethylhexoxy groups in the cas
the case of DERHD7T.
al. J. Am. Chem. Soc.
American Chemical Society.

This overview accounts for the best performing solution processable small
molecule BHJ solar cells present in the literature. From these examples predominantly of
DPP and oligothiophenes, several characteristics can be highl
molecules can perform well with PCE ranging from 7
the state-of-the-art polymer BHJs (0.60
which is also similar to polymer currents. Lastly and most im
V) in these systems shows how the energetic levels of the SMs can be tuned by changing
the chemical structure and lowering the HOMO level of a give
using D/A units and various side chains.
Chemical structure of benzodithiophene based small
molecules with ethylrhodanine end-groups.
117
R-groups also represent
ethylhexoxy groups in the case of DR3TBDT
116
and octyl groups in
the case of DERHD7T.
118
Reproduced with permission from Zhou, J.
J. Am. Chem. Soc. 2013, 135, 8484-8487. Copyright (201
American Chemical Society.
This overview accounts for the best performing solution processable small
molecule BHJ solar cells present in the literature. From these examples predominantly of
DPP and oligothiophenes, several characteristics can be highlighted. First, small
molecules can perform well with PCE ranging from 7-8.2%; FF values are as high as for
art polymer BHJs (0.60-0.72) and J
sc
ranges between 13 and 15 mA cm
which is also similar to polymer currents. Lastly and most importantly, the
V) in these systems shows how the energetic levels of the SMs can be tuned by changing
the chemical structure and lowering the HOMO level of a given molecule, particularly
using D/A units and various side chains.
39

Chemical structure of benzodithiophene based small
groups also represent
and octyl groups in
Reproduced with permission from Zhou, J. et
8487. Copyright (2013)
This overview accounts for the best performing solution processable small
molecule BHJ solar cells present in the literature. From these examples predominantly of
ighted. First, small
8.2%; FF values are as high as for
ranges between 13 and 15 mA cm
-2
,
portantly, the V
oc
(0.70-1.0
V) in these systems shows how the energetic levels of the SMs can be tuned by changing
molecule, particularly
40

A range of structurally different dyes and oligothiophenes has been covered thus
far and their properties shown in this section, specially the high V
oc
demonstrate the
potential SMs have as donors. This will now lead the discussion to ternary blend BHJ
solar cells where small molecules are incorporated as the third component in
polymer:fullerene blends.

1.6 Small Molecules as Donors in Ternary Blend Bulk Heterojunction
A wide range of materials were presented in the previous section, while this
section focuses on how composition of the SM affects the V
oc
of ternary blend devices,
more specifically, examines previous examples and device polymer evidence about how
V
oc
relates to composition. This understanding is an important focus of this work. Among
the classes of SM that will be covered here are: oligothiophenes and DPP, squaraines,
phthalocyanines, porphyrins dyes. This overview will start the discussion for chapter two
in which one particular type of small molecule in ternary blends with P3HT:PCBM will
be the focus.

1.6.1 Oligothiophenes and Diketopyrrolopyrrole Dyes
Low band gap DPP-based oligothiophene has been incorporated to P3HT:PCBM
and due to the increased absorption, compared to the binary blend, a rise in
41

photogenerated carriers lead to a higher J
sc
of 8.6 mA cm
-2
(from 7.7 mA cm
-2
). The use
of SMD1 molecule seen in Figure 1.25, allowed to tune the V
oc
of the system to some
extent with composition as seen on Table 1. The PCE reached 3.21% when 2 mg/mL of
small molecule was present and its contribution was also seen in the 17% EQE around
the 700 nm. This early example demonstrated a way to extend the absorption breadth of
OPVs.
74

Dimethylphenylamine (DMPA) groups were attached to a DPP core and used in
ternary blend devices with P3HT:PCBM by Lee and coworkers.
76
The dithieno-
diketopyrrolopyrrole core contained decyltetradecyl chains to avoid aggregation of the
SM. The vibronic feature observed for the binary polymer:fullerene was kept when 5 and
10% DMPA-DTDPP (structure seen in Figure 1.25) was added indicating there was no
negative effect on the crystallinity of the active layer. Additionally, the absorption
spectrum was enhanced around 650 nm and was attributed to the SM presence and
corroborated by the EQE (a 10% increase). This lead to a slightly higher J
sc
of 9.87 and
9.14 mA cm
-2
at 5% and 10% SM loading, respectively. Overall, a modest 12% increase
in efficiency was observed as seen on Table 1.1. DMPA-DTDPP:PCBM data was not
available for comparison as an end-point reference.
A dibenzo[f, h]thieno[3,4-b]quinoxaline-based SM (TQTFA) seen in Figure 1.25
was incorporated into a P3HT:PC
71
BM system at varying compositions and the V
oc
was
42

accordingly tuned surpassing that of P3HT:PC
71
BM (0.60 V). The downside was that the
FF decreased when more TQTFA was added.
60
This is a noteworthy example of
tunability in which the V
oc
is not pinned to the smaller value between the two binary
blends approaching that of TQTFA (0.90 V) as seen in Table 1.1. Other ratios that were

Figure 1.25 Chemical structures of the small molecules illustrated in ternary blend solar
cells. Examples shown include DPP-based SMs, oligothiophenes, anthracene-based SMs,
phthalocyanines, porphyrins, and squaraine molecules.
43

explored in this paper are shown and a noticeable V
oc
tunability is observed. The SM also
contributed to enhanced EQE response in the 340-500 region where it absorbs strongly.
This is the first example of composition-dependent V
oc
for a small molecule ternary blend
solar cell and leads the way to overcoming the low FF limitations of many ternary blends.
The best performing device for this system gave 4.5% η, a 15% increase compared to
P3HT:PC
71
BM.
Table 1.2 Solar cell performance of polymer:SM:fullerene ternary blends at various SM
compositions and measured at AM1.5G (100 mA cm
-2
).
Donors Acceptors Composition
(D:D:A)
J
sc

(mA
cm
-2
)
V
oc

(V)
FF PCE(%) Reference
P3HT:SMD1 PC
71
BM 1:0:1 7.7 0.60 0.63 2.93 (74)
1:0.2:1 8.6 0.63 0.59 3.21 (74)
P3HT:DMPA-
DTDPP
PC
61
BM 1:0:0.8 8.27 0.60 0.61 3.02 (76)
1:0.05:0.8 9.84 0.60 0.58 3.37
1:0.1:0.8 9.17 0.60 0.57 3.13
P3HT:TQTFA PC
71
BM 1:0:1 9.74 0.60 0.67 3.90 (60)
1:.0.2:1 9.96 0.65 0.63 4.10
1:0.25:1 10.62 0.69 0.61 4.50
1:0.30:1 9.10 0.71 0.52 3.32
1:0.35:1 8.52 0.73 0.45 2.79
0:1:1 4.60 0.90 0.31 1.30
44

The next type of ternary blend device includes thieno-benzothiadiazole end-
capped by cyanovinylene-4-nitrophenyl groups (BTD-TNP) first developed by Sharma
and Mikroyannidis. In their initial report they use a vinylene copolymer (P) as the donor,
BTD-TNP as small molecule, and PCBM as the acceptor. They observed a slight increase
in V
oc
(0.81 V) and J
sc
compared to the binary blends and contribution from both donors
in light absorption to yield modest PCE of 2.6% after annealing.
63
The same system was
followed up with the use of P3HT as the donor instead of vinylene copolymer to give an
improved ternary blend, although the V
oc
was lower (0.70 V) and there was no clear
indication of how much SM content affected the performance seen in Table 1.1. The
efficiency reached 4.10% attributed mainly to the broaden spectral breadth.
64
Neither of
these two examples contributes to a highly tunable voltage, but they simply show
optimization by means of thermal annealing.
Table 1.1 Continued
Donors Acceptors Composition
(D:D:A)
J
sc

(mA
cm
-2
)
V
oc

(V)
FF PCE(%) Reference
P:BTD-TNP PC
61
BM 1:1:1 3.7 0.85 0.52 1.64 (63)
1:1:1 5.80 0.81 0.55 2.60
Annealed
100°C/10min
P3HT:SM PC
61
BM

1:1:1 9.2 0.73 0.55 3.69 (64)
1:1:1 10.3 0.70 0.57 4.10
Annealed
120°C/2min
45

A naphthalene-based polymer donor material (PBTADN) and PC
71
BM were
blended with an anthracene-based star-shaped molecule shown in Figure 1.25. Either a 4-
hexylphenyl-substituted anthracene bithiophene (HBantHBT) or a phenyl-substituted
anthracene bithiophene (BantHBT) were used as the small molecules. They are known to
self-assemble into good crystalline structures and have a high field effect mobility in
(organic field-effect transistors) OFETs.
58
The use of HBantHBT and BantHBT in
ternary blends has significantly increased the performance of polymer and SM binary
blends. These systems have V
oc
of 0.89 and 0.91 V, J
sc
of 8.7 and 11.0 mA cm
-2
, and 4.1
and 5.6%, respectively. They have even surpassed the performance of ternary blends
with P3HT instead of PBTADN, although the FF values are lower, but the V
oc
is 0.2 V
higher and an EQE that increased 10% due to the polymer contribution. The best results
were seen with BantHBT SM at 0.2 wt % loading.
62

1.6.2 Porphyrins, Phthalocyanines, and Squaraines
Table 1.2 Photovoltaic performance of ternary blend devices measured at AM1.5G
(100 mA cm
-2
).
Donors Acceptors Composition
(D:D:A)
J
sc

(mA
cm
-2
)
V
oc

(V)
FF PCE(%) Reference
P3HT:3 PC
61
BM 1:0.01:1 2.4 0.58 0.46 1.06 (72)
1:0.03:1 2.9 0.56 0.42 0.72
1:0.05:1 2.6 0.53 0.39 0.43
MEH-PPV:tBuPP PC
61
BM 1:0:4 5.34 0.83 0.40 1.77 (59)
1:1:8 3.90 0.64 0.33 0.83
46

Lyons and coworkers included a Ni(II) porphyrin-oligothiophene (3) in a ternary
blend system reaching 1.06% efficiency and 10% increased EQE.
72
Their goal was to
obtain better performance and in incorporating a porphyrin to favorably influence
donor/acceptor interactions for better mixing and controlled morphology. A family of
five porphyrin compatibilizers was prepared, with varying thiophene ring length and
composition. Increasing this SM (Figure 1.25) loading possibly lead to aggregation of
the porphyrin and a need for optimization was identified. The results were consistent with
the idea that at high porphyrins loading aggregation hinders performance as evidenced on
Table 1.2 where not much improvement is neither seen nor high V
oc
values achieved.
Alternatively, a metal-free porphyrin ternary blend was presented by Cooling et
al. with poly(methoxyethylhexyloxy-phenylenevinylene) (MEH-PPV) as the polymer
donor. Different bulky substituents were present at the meta position on the meso phenyl
groups to decrease the aggregation of porphyrins as seen in optical micrographs of
MEH-PPV:porphyrin:PCBM (1:1:8). Both the current and voltage increased with bulkier
groups on the porphyrin, but did not improve over those of binary MEH-PPV:PCBM. It
was hypothesized that the cause of it were either electronic or morphological effects. The
porphyrin (either pTP, XyP, or tBuPP) (Figure 1.25) acted as a site for charge
recombination and the bulkier substituents hindered this process by increasing spatial
separation, hence increasing the V
oc
. Electronic effects were observed to dictate the
performance giving V
oc
of 0.64 V and PCE of 0.83% for MEH-PPV:tBuPP:PCBM
59
as
seen on Table 1.2, while binary MEH-PPV:PCBM showed V
oc
of 0.83 V.
47

Another kind of dye includes silicon phthalocyanine applied to ternary blends by
Honda and coworkers. The careful selection of silicon phthalocyanine bis(trihexylsilyl
oxide) (SiPc) over zinc-phthalocyanine (ZnPc) in ternary blend with P3HT and PCBM
showed an increase in J
sc
from 6.5 to 7.9 mA cm
-2
and PCE improved from 2.2 to 2.7%.
This study showed that SiPc is a better dye for ternary blends due to the reduced dye
aggregation as a result of having bulky substituents that allow effective π-π stacking.
Additionally, light harvesting is more efficient due to the absorption into the near-
IR in as-cast and annealed devices. The presence of two peaks in the EQE suggests that
SiPc definitely contributes to the generating photocurrent and even more so after
annealing (20% EQE). The low-wavelength region increase represents an indirect effect
coming from the dye. From this Honda and coworkers determined that the dye must be
located at the interface.
65
The same group later developed a 4-component system with the
only the addition of silicon naphthalocyanine (SiNc) to give 4.3% efficiency. They
claimed doubling the photocurrent to 20% in the NIR region while the V
oc
increased
slightly (0.55 to 0.57 V)
67
and the FF was 0.69 as seen on Table 1.3.
Table 1.3 Photovoltaic performance of ternary blend devices measured at AM1.5G (100 mA
cm
-2
).
Donors Acceptors Composition
(D:D:A)
J
sc

(mA
cm
-2
)
V
oc

(V)
FF PCE(%) Reference
P3HT:ZnPc PC
61
BM 1:0.07:1 6.2 0.48 0.37 1.10 (65)
P3HT:SiPc PC
61
BM 1:0.07:1 7.9 0.58 0.59 2.70
48

Moreover, the composition of SiPc seen on Table 1.3, was explored and
optimized at 4.8 wt% loading giving a current of 11.1 mA cm
-2
, V
oc
of 0.58, FF=0.65 and
PCE of 4.2%. While increasing the dye loading, both voltage and FF remained the same
(as-cast and annealed) until 17 wt% SiPc when both parameters decreased to some extent.
At the optimal composition ~20% higher J
sc
is achieved due to the selective localization
of the dye at the interface. Also, the mechanism of the EQE enhancement, which
Table 1.3 Continued
Donors Acceptors Composition
(D:D:A)
J
sc

(mA
cm
-2
)
V
oc

(V)
FF PCE(%) Reference
P3HT:SiPc:SiNc PC
61
BM 1:0.048:0.015:1 10.90 0.57 0.69 4.3 (67)
1:0.048:0:1 10.30 0.57 0.69 4.10
1:0:0.015:1 9.94 0.55 0.68 3.70
P3HT:SiPc PC
61
BM 1:0:1 9.69 0.55 0.66 3.5 (73)
1:0.05:1 11.10 0.58 0.65 4.2
P3HT:DPSQ PC
61
BM 1:0.05:0.7 8.80 0.61 0.64 3.38 (70)
1:0.1:0.7 8.90 0.61 0.53 2.88
P3HT:DPSQ PC
61
BM 1:0:1 10.30 0.59 0.53 3.27 (71)
1:0.005:1 11.00 0.59 0.54 3.48
1:0.01:1 11.60 0.60 0.65 4.51
1:0.025:1 11.90 0.56 0.59 3.92
1:0.01:1 12.20 0.54 0.60 3.93
49

indicates efficient exciton harvesting is studied by this group further.
66
This conclusion of
dye localization is backed by the difference in surface energy of the components and the
crystallization of the blend. The lower values for SiPc surface energy (γ) are indicative of
phase segregation at the interface and is determined to be the main cause for this
phenomenon.
73
Surface energy will be the focus of chapter 3 and will be described in
further detail as it pertains to surface energy modification of copolymer side-chain in
ternary blend BHJ.
Last, but not least is the squaraine (SQ) small molecule-based ternary blends.
Forrest and coworkers investigated the effect of SQ (Figure 1.25) in ternary blends with
P3HT and PCBM. Diisobutylamino-2,6-dihydroxyphenyl squaraine (DPSQ) mixed at 5
wt% improved the efficiency (3.4%) as well as the J
sc
(8.9 mA cm
-2
) by about 20%,
compared to the binary polymer device, due to the strong absorption coming from DPSQ
between 650 and 750 nm and an EQE of 55%. The optimal ratio was found at 5 wt% dye,
the V
oc
did not change substantially and the FF was 0.64. This result is not surprising
given that the V
oc
is close to that of P3HT:PCBM and possibly DPSQ:PCBM, although
this value is not reported. Photoluminescence (PL) measurements showed quenching
indicating that energy transfer from P3HT to DPSQ was occurring at the optimized
composition seen on Table 1.3.
70

More recently, Taylor and coworkers experimentally confirmed energy transfer
from polymer to DPSQ in the same system by ultrafast spectroscopy.
71
A strong
absorption is also observed to peak at 675 nm as SQ loading is increased and the
50

characteristic P3HT vibronic shoulder does not change, but becomes more defined
indicating the presence of SQ does not disrupt the crystallinity. Photoluminescence at
various SQ compositions is compared (0-5 wt%) leading to the same conclusion as the
previous group. There studies go further to understand the photophysical effects of
energy transfer of their devices. Their best performing ternary blend device consists of a
1 wt % dye loading yielding 4.51% PCE, same V
oc
as the binary device (0.60 V),
enhanced J
sc
of 11.6 mA cm
-2
, and high FF of 0.65 (Table 1.3). Overall this report shows
the progressive improvement of a SQ-based ternary blend system by optimal composition
and solvent choice, replacing chlorobenzene with o-dichlorobenzene.

1.7 Small Molecule Ternary Blend BHJ Overview
Small molecules in ternary blend BHJs have been studied more broadly in recent
years. There are some SM-based ternary blends which show modest improvements in
terms of V
oc
as compared to P3HT:PCBM limiting binary blend, for instance in the case
of SMD1.
74
In a particular example, a composition dependent V
oc
is observed at the
expense of decreased FF in TQTFA ternary blends. This tunability was achieved over a
broad range of compositions.
60

In some cases, the V
oc
does not change when a SM is introduced to P3HT:PCBM
as for DMPA-DTDPP
76
and SiPc,
65
which may be due to the fact that the V
oc
values for
the end points (P3HT:PCBM or SM:PCBM) are close to one another. For ternary blends
of either DMPA-DTDPP or SiPc, the J
sc
is improved since the SM contributes positively
51

toward the photocurrent. In yet another case, there was only one composition attempted
in ternary blends (second part of Table 1.1) and the only variation is between as-cast and
annealed devices.
53,63

Another interesting aspect in the case of porphyrins and phthalocyanines is that
these are limited by aggregation at high SM loading which hinders device performance
and V
oc
as seen on Table 1.2. The voltage does not improve, but in the case of SiPc, other
aspects such as surface energy are calculated and use as an indicator of where the dye is
located and how well it mixes in the blend. Additionally, SiPc shows an enhancement in
EQE and absorption, hence leading to improved J
sc
compared to P3HT:PCBM.
73

Table 1.3 shows ternary blend results which do not demonstrate V
oc
tunability,
but other parameters are affected, such working FF values and it mainly illustrates the
various ternary blends examples reported in the literature.
65–67
Squaraines, on the other
hand have a strong absorption and demonstrate much higher FF than in binary blends.
70,71

While there is not a general trend for all of the systems shown here, a wide range
of SMs have been highlighted and they all contribute to different parameters of
performance. In some cases the V
oc
can be tuned by changing the composition without
detriment to P3HT:PCBM performance, however in other cases a voltage enhancement is
not observed. Based on this overview it is not clear of the proposed alloy model can
operate in polymer:SM:fullerene ternary blends.

52

1.8 Optimization of Donor Materials for Ternary Blend BHJ Solar Cells
Chapter 1 presents the advancements in the field of both solution processable
small molecule organic photovoltaics and ternary blend organic photovoltaics. While this
review is not comprehensive, it showcases the most efficient systems reported to date.
Small molecule solar cells have reached >8% efficiencies, rendering them as attractive
donor materials for broadening light absorption and tuning the open-circuit voltage.
Different structural features are highlighted, such as how well a molecule packs in the
solid-state, feasibility of solution processability, and film-forming ability, among others.
Small molecules in ternary blends represent an important class of components for
expanding the scope of this platform. They have potential to achieve higher J
sc
x V
oc

product. For this purposes, tuning the V
oc
with compositional changes while maintaining
a high FF is essential. Many reports in the literature show tunability of the V
oc
, but the FF
drops or does not follow an increase or decrease linearly. Even though there are examples
in which both of these goals are achieved, SM ternary blends have yet to demonstrate
such feat. The simplicity of fabrication, reproducibility and ease of purification make
SMs attractive donor materials, which continue to be explored.

1.9 General Conclusions for Ternary Blend BHJ Solar Cells
While a continuous development on organic photovoltaics (OPVs) has been
taking place, the idea of small band gap materials has been explored over the last two
53

decades. These materials have a characteristic bandgap of less than 2eV, and they work
as the donors. Regioregular P3HT (rr-P3HT) makes up the benchmark polymeric solar
cell with an efficiency that reached 5% and a band gap of 1.9 eV. While widely used, it
suffers an absorbance limitation as only photons below a wavelength of 650 nm in the
solar spectrum are absorbed.
33
Addressing some of the limitations for this benchmark
system are at the core of the work presented in this dissertation. Two opposing
parameters, the V
oc
and the J
sc
referred to as the J
sc
-V
oc
compromise need to be balanced
in order to optimize the performance of binary blend single-layer bulk heterojunction
solar cells. This platform has a limiting theoretical efficiency around 11-12%,
12,33,34

which is why ternary blends have emerged as a potential route to increase η by
overcoming the J
sc
-V
oc
compromise.
39,40
The focus of this work has been on using new
materials for understanding the underlying principles of ternary blend BHJ SCs.
Ternary blends combine the idea of having two complementary absorbers as
donor materials allowing for broadening of the absorption envelope and by careful
selection of frontier orbital levels of the components, a higher V
oc
than can be attained in
the limiting binary blends. This new platform combines the simplicity of solution
processable active layers and the potential to surpass the efficiency of tandem cells. The
active layer of interest in Chapter 1 and 2 is polymer:small molecule:fullerene.
Improving photon harvesting is an understandable consequence of multiple
component ternary blends, but the effect on the V
oc
parameter is more difficult to
describe. In some reports, the V
oc
has been proposed to be pinned to smallest V
oc
found in
54

the respective donor/acceptor (D/A) binary blend,
52,53,82
which limits the potential of
simultaneously increasing the J
sc
and V
oc.
In a broader context of ternary blends, the
concept of an organic alloy emerged in which a combination of material properties is
targeted. As is the case in inorganic semiconductors, electronic properties can be
predicted with compositional changes of the two materials forming the alloy. These
constitute a single-phase mixture in which the V
oc
changes continuously with composition
without affecting the FF.
The chosen criteria for analyzing SM in binary blends (section 1.5) and building
up the connection to solution processable SM ternary blend BHJ was reported. This
section demonstrated which systems are the best performing (4-8.2%) in the literature.
Two major classes are DPP and oligothiophenes and several conclusions should be
mentioned: first, small molecules can perform well with PCE ranging from 7-8.2%; FF
values are as high as for the state-of-the-art polymer BHJs (0.60-0.72) and J
sc
ranges
between 13 and 15 mA cm
-2
, which is also similar to polymer currents. Most importantly,
a V
oc
between 0.70 and 1.0 V was shown indicating how tuning the chemical structure
and lowering the HOMO level of a given molecule, particularly using D/A units and
various side chains. The high V
oc
SMs have shown is essential to the discussion of
ternary blend BHJ solar cells where small molecules are incorporated as the third
component in polymer:fullerene blends.
Previously reported two polymer and one acceptor
50
and one polymer and two
acceptor
54
systems demonstrated full-range composition tunability of the V
oc
, without
55

adversely affecting the FF or the J
sc
. Both V
oc
and J
sc
increased because there is a
composition dependence of the band gap analogous to the description of an alloy by the
quadratic dependence shown below and because the optical absorption of polymer blends
is the weighed sum of the peaks of the pure materials, respectively.
83
This has yet to be
established for SM-based ternary blends. Small molecules in ternary blend BHJs show
modest improvements in terms of V
oc
as compared to P3HT:PCBM limiting binary blend,
and in one particular example, a composition dependent V
oc
is observed at the expense of
decreased FF in TQTFA ternary blends.
For the most part V
oc
does not change when a SM is introduced to P3HT:PCBM
although other parameters change, such as the J
sc
in SiPc ternary blends,
65
improve since
the SM contributes positively toward the photocurrent. Examples such as porphyrins and
phthalocyanines aggregate at high loadings which hinders device performance and V
oc
.
On the other hand, squaraines, on have a strong absorption and demonstrate much higher
FF than in binary blends.
70,71
A general trend cannot be drawn from this section, simply a
wide range of SMs have been highlighted and they all contribute to different parameters
of performance. In some cases the V
oc
can be tuned by changing the composition without
detriment to P3HT:PCBM performance, however in other cases a voltage averaging is
not observed.
A specific example of ternary blend BHJ studied on Chapter 2 is PDPP, which
shows composition-dependent V
oc
in P3HT:PDPP:PCBM devices. The synthesis of this
dye was developed here and PDPP was used in solution processable binary blend BHJ for
56

the first time. V
oc
of 0.90 V was demonstrated for this system and 0.61-0.87 V for ternary
blends at the full composition range between limiting binary blends (P3HT:PCBM and
PDPP:PCBM), although the FF was not optimal. The FF was individually optimized and
the V
oc
did not follow the same linear trend. An interesting observation for this system is
that it is extremely sensitive to processing and annealing conditions making the devices
less reproducible. In future work, dye-fullerene dyads that were presented in this chapter
will be studied for analyzing the effect of covalent attachment to the dye and restraining
the morphology to further understand PDPP ternary blend systems.
An alternative approach to ternary blend BHJ solar cells is by using a newly
synthesized and developed family of random copolymers that have different contents of
polar oxyethylene side-chains (Chapter 3). Varying the monomer composition the surface
energy of the materials was tuned from 33.48 mJ m
-2
for P3DHT to 18.94 mJ m
-2
. This
tunability is demonstrated for the first time for oxyethylene substituted random
polythiophenes. Tuning of the surface energy follows a regular trend with varying
composition and based on the band energies measured, the electronic properties are
unchanged. The surface energy modifications tune the hydrophilicity or hydrophobicity
of the polymers. In ternary blends this will be a useful way to determine how well two
polymers mix or demix and use as a predictor tool for working devices and to determine
whether the V
oc
is pinned to the lesser value of its limiting binary blends or not. In future
the control these polymers will be used for studying the effects of surface energy on
polymer-polymer mixing in terms of the morphology of ternary blends.
57

Other polymers which can be applied to the ternary blend platform are semi-
random polymers, particularly DPP analogues with flanking thiophene and benzene rings.
The latter two polymers are developed in Chapter 4 which have tunable HOMO levels
which lead to high V
oc
=0.76-0.83 V. These systems highlight the importance of tuning
electronic properties for efficient device performance and the application of these
concepts to ternary blend systems, which has the potential of positively affecting the J
sc

and V
oc
simultaneously.

58

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66

Chapter 2: Polymer, Dye, Fullerene Ternary Blend Organic
Photovoltaics: Exploring the Influence of Blend
Composition on the Open-circuit Voltage

2.1 Introduction
Over the past decade, single-layer bulk heterojunction (BHJ) solar cells,
composed of a polymer donor and a fullerene acceptor, have been thoroughly studied.
These systems have achieved power conversion efficiencies (PCE) of 9% and higher.
1–6
While the advancements are encouraging, this platform has limited theoretical
efficiencies (η) of approximately 11-12%.
7-9
Ternary blends have emerged as a potential
route to increase the power conversion efficiency (η) by overcoming what is called the
short-circuit current-open-circuit voltage (J
sc
-V
oc
)

compromise.
10,11
By having two donors
and an acceptor or alternatively a donor and two acceptors, it is possible to enhance the
absorption breadth of a single-layer solar cell, hence the J
sc
.
12–17
On the other hand, the
V
oc
, has been thought to be pinned to the smaller V
oc
of the respective binary blends of the
components
12,13,18
, which limits the potential ternary blends have to impact the efficiency.
Therefore, the tunability of the V
oc
in this platform is of significant importance in order to
attain the highest J
sc
x V
oc
product in solution processable BHJ solar cells. Thus, the V
oc

needs to be tunable with composition in a three-component system. There are some cases
in which the V
oc
is pinned to the lower value of the corresponding binary blend (either
D
1
:A or D
2
:PCBM), therefore limiting the potential for higher efficiencies in ternary
67

blends.
12,13
Specifically, thiophene-based polymers have been used in ternary blend
devices made up of P3HT:PC
61
BM. The use of 25 weight% functionalized
poly(thiophenes), such as pentafluorophenyl ether side-chain poly(thiophene), allowed
for an increase in J
sc
and decrease in series resistance in BHJs, however the V
oc
remained
at 0.60 V.
19
Poly(cyclopentadithiophene-(2,1,3-benzothiadiazole)) was incorporated in
P3HT:PCBM to give an enhanced absorption in the NIR region, which resulted in an
increase in J
sc
when 20-25% weight PCPDTBT was added, while FF was 0.31.
14,20
This
modest improvement in the photovoltaic performance further lead to a silicon-containing
analog (Si-PCPDTBT), which allowed even higher loadings of polymer (40%). This
resulted in improved spectral absorption, hence J
sc
values, while maintaining the V
oc
at
~0.6 V, but slightly reduced FF values at higher loadings.
21
When the acceptor was
changed to an indene-C
60
bisadduct (ICBA) by Ameri and coworkers, the V
oc
was 0.79 V,
which was roughly intermediate between that of Si-PCPDTBT:ICBA and P3HT:ICBA
when 20 wt% Si-PCPDTBT was added, with a FF of ~0.65.
22
In this work an
enhancement in both current and voltage was observed, although not all compositions
were explored and the FF was low for Si-PCPDTBT:ICBA (0.32-0.40). Similarly, when
anthracene-based phenylene vinylene semi-crystalline and amorphous polymers are
blended with PCBM, the FF drastically decreases, although a wide range of compositions
was tested, the tunability of V
oc
is only observed up to 50% loading of amorphous
polymer.
23

Despite of the decreases in FF, the observed polymer ternary blend examples
above, show that the J
sc
is composition dependent, although the V
oc
is limited to the
68

minor value of the corresponding binary blend. A limited number of cases have
showcased this composition-dependent tunability of the V
oc
even though the FF is
steadily reduced.
24–27
As a major step forward, the Thompson group has developed two
systems which show tunable V
oc
in polymer/polymer/fullerene
28
and
polymer/fullerene/fullerene
29
systems in which a high FF is retained at all compositions
as the lower band gap component was increased. Also, spectral broadening has been
observed in these ternary blend devices leading to higher PCE values by composition
tunability of both J
sc
and V
oc
.
In an analogous path to explore this approach, small molecules are used in ternary
blend solar cells. Small molecules offer the potential advantages of reproducible
synthesis
30
and purification simplicity, solution processability, and good film-forming
capabilities.
31
However, small molecules in ternary blends have primarily been focused
on as "sensitizers" in which the range of compositions is quite narrow, except in rare
cases the full range of composition has been explored. Namely, the work of Huang et al.
notably shows a composition dependent V
oc
as seen on Figure 2.1, however a drastic FF
reduction is observed with higher loading of thieno[quinoxaline]-bis(thiophene-diphenyl-
fluorene-2-amine) (TQTFA) dye when incorporated to P3HT:PC
71
BM.
24

It is of interest here to ascertain how compositional changes influence the V
oc
in
ternary blend solar cells made up of polymer/dye/fullerene. The purpose of this strategy
is to demonstrate device improvement by means of ternary blend devices. Ternary blends
are attractive architectures in that they require the same solution processability as do
69

binary blends, facilitating the fabrication while compounding the effects of more
commonly used binary systems. One potential benefit of its use is the increase in
attainable J
sc
x V
oc
product taking into account the composition of the parts.
More specifically, the addition of a third component into a donor: acceptor blend
allows for tuning of the V
oc
while simultaneously increasing the J
sc
by increasing the
absorption breadth of the blend. The understanding of these effects is of great interest in
the studies mentioned here and the ability to identify the causes is imperative for the
expansion of this strategy. Here, the findings of a resulting ternary blend system based on
the perturbation of the P3HT:PCBM BHJ solar cell are presented.
The chosen system consists of P3HT:PDPP:PCBM, where PDPP stands for
phenyl-diketopyrrolopyrrole. Analogues of this molecule were previously shown to

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40
0.58
0.60
0.62
0.64
0.66
0.68
0.70
0.72
0.74
V
oc
(V)
TQTFA loading (wt. %)

Figure 2.1 TQTFA weight % vs. V
oc
plot
for ternary blend BHJ solar cells of
P3HT:PC
71
BM:TQTFA as a function of
dye loading in the blend. Dye content was
20, 25,30, and 35%, while the V
oc
went
from 0.60 V for binary P3HT:PCBM
reference to 0.65, 0.69, 0.71, and 0.73 V.

70

perform as donor or acceptor in the literature.
32
The criteria for this choice was centered
around the film forming ability of the SM, its synthetic simplicity, potential for high V
oc

by measurement of the HOMO level, proof of working in binary blends, etc. By initially
showing a working binary blend solar cell made up of PDPP:PC
61
BM, many of its
positive attributes can be drawn (good film-forming ability, high V
oc
, and reproducibility)
and applied in the fabrication of ternary blends and further expand the scope of this
platform. To do so, the composition dependence and factors that influence it were
explored. This system was chosen for its potential for full compositional tunability of the
V
oc.

2.2 Synthesis of DPP Dye
For this system phenyl bis-substituted diketopyrrolopyrrole (PDPP) dye was
chosen based on an analogous molecule precedent in the literature, see Figure 2.2.
32
In
general, DPP units with flanking thiophenes are used to minimize steric hindrance and
enhance planarity to improve chain packing and π-π interaction, which further improves
hole mobility.
33,34
Furthermore, the energy levels of low band gap polymers in general
have been observed to be influenced by DPP by lowering the HOMO and LUMO levels,
which ultimately influences the V
oc
to reach values as high as 0.9 V.
33
Based on literature
precedence, DPP dyes with structural similarities, but containing either trifluoromethyl or
phenylene vinylene substituents have been used as donors for PCBM and acceptors for
P3HT.
32,35,36
These DPP derivatives have shown that when used as donors, they are
71

capable of high V
oc
and good device performance. In this study diphenyl-
diketopyrrolopyrrole (PDPP) was chosen as the small molecule donor due to its solubility
and good film forming ability, as well as its frontier orbital energy levels. Below in
Figure 2.3, the energetic level differences are portrayed for each component in the
ternary blend (P3HT, PDPP, and PC
61
BM).

Figure 2.2 Synthetic scheme of diphenyl-diketopyrrolopyrrole.
Formation of DPP core by ring-closing reaction followed by
alkylation, di-bromination, and Suzuki coupling between brominated
precursor and phenyl-boronic acid.

2,5-bis(2-ethylhexyl)
dione (PDPP) was synthesized via Suzuki coupling of dibromo
diketopyrrolopyrrole with bromo
mixture of toluene and ethanol as solvents. The same small molecule has been
synthesized via direct arylation of C
version from the syntheses of analogous trifluoromethyl
Sonar et al particularly, that of trifluoromethane substituted DPP.

Figure 2.3
HOMO and LUMO energy levels diagram of each
component in ternary blend devices (P3HT:PDPP:PCBM).
ethylhexyl)-3,6-bis(5-phenylthiophen-2-yl)pyrrolo[3,4-c]pyrrole
(PDPP) was synthesized via Suzuki coupling of dibromo
diketopyrrolopyrrole with bromo-phenylboronic acid and catalyzed by Pd(PPh
mixture of toluene and ethanol as solvents. The same small molecule has been
ed via direct arylation of C-H bonds.
37
The procedure used here is a modified
version from the syntheses of analogous trifluoromethyl-vinyl-substituted DPP dyes by
particularly, that of trifluoromethane substituted DPP.
32

Figure 2.3 Structure of donor and acceptors molecules
HOMO and LUMO energy levels diagram of each
component in ternary blend devices (P3HT:PDPP:PCBM).

72
c]pyrrole-1,4-
(PDPP) was synthesized via Suzuki coupling of dibromo-bisthiophene-
phenylboronic acid and catalyzed by Pd(PPh
3
)
4
in a
mixture of toluene and ethanol as solvents. The same small molecule has been
The procedure used here is a modified
tuted DPP dyes by

acceptors molecules.
HOMO and LUMO energy levels diagram of each
component in ternary blend devices (P3HT:PDPP:PCBM).

The effect of the small molecule
both its properties and photovoltaic performance are described
chapter.

2.3 Characterization and Photovoltaic Performance
The optical properties observed were obtained by UV/Vis
solution in chlorobenzene as seen on
and spin-coated from the same solvent for PDPP and P3HT, while PCBM from
chloroform.

Figure 2.4
line), and PCBM (orange line) films. All spin
chlorobenzene and PCBM, spin
The effect of the small molecule donor content in ternary blends has been studied and
both its properties and photovoltaic performance are described in the remainder of the
Characterization and Photovoltaic Performance
The optical properties observed were obtained by UV/Vis
in chlorobenzene as seen on Appendix 1 and in thin-films as seen on
coated from the same solvent for PDPP and P3HT, while PCBM from
Figure 2.4 Absorption spectra of P3HT (red line), PDPP (blue
line), and PCBM (orange line) films. All spin-coated from
nzene and PCBM, spin-coated from chloroform.
73
nor content in ternary blends has been studied and
in the remainder of the
The optical properties observed were obtained by UV/Vis spectroscopy in
films as seen on Figure 2.4
coated from the same solvent for PDPP and P3HT, while PCBM from

Absorption spectra of P3HT (red line), PDPP (blue
coated from
coated from chloroform.
74

The absorption spectra of PDPP dye was measured and a broad absorption
breadth was observed (up to ~ 720 nm ) in thin films with a λ
max
at 584 nm in the dual
band transition attributed to the π-π
*
transition. P3HT shows a vibronic shoulder around
600 nm and a high absorption coefficient (85,541 cm
-1
) while PDPP is lower at 60,324
cm
-1
. PCBM has a narrow absorbance profile predominant in the UV region with a λ
max
at
336 nm and an absorption coefficient of 94,303 cm
-1
.
The data seen in Table 2.1 summarizes the optical and electronic data of all the
components used in ternary blends in this chapter. Ternary blend systems benefit from
the addition of PDPP given it is a low band gap donor compared to P3HT and from its
lower-lying HOMO energy level, hence both J
sc
and V
oc
could be influenced.

The HOMO energy level values were obtained from cyclic voltammetry of the
films and the optical band gap values were obtained from the absorption spectra band
edge in the thin film spectra and are summarized in Table 2.1 as well. In the case of
Table 2.1 Optical and electronic properties of P3HT, PDPP, and PCBM.
Sample λ
max, abs

(nm)
Absorption
Coefficient
(cm
-1
)
HOMO
(eV)
E
g
(nm/eV)
Optical
LUMO
(eV)
P3HT 554 85,541 -5.25 659/1.90 -3.35
PDPP 584 60,324 -5.47 718/1.72 -3.75
PCBM 336 94,303 -5.92 667/1.82 -4.10
Films were spin-coated from chlorobenzene for PDPP and P3HT and chloroform for
PCBM. Concentrations were 10 mg/mL for all solutions and films were used as-cast
after spin-coating.
75

PCBM, the LUMO was measured and the HOMO was estimated based on the optical
band gap, which is 1.86 eV.
The photovoltaic properties of small molecule-based ternary blends were studied
in BHJ solar cells using PC
61
BM as the acceptor in a conventional device configuration
of ITO/PEDOT:PSS/P3HT:PDPP:PC
61
BM/Al. Detailed device fabrication procedures are
described in Appendix 1. An optimal condition for these ternary blends was spin-coating
from CB solution followed by solvent vapor annealing from o-DCB vapors. As a
reference P3HT:PC
61
BM was also processed under the above conditions, although
solvent vapor annealing is not the optimal processing condition for this binary system,
which has shown FF of 0.68 when thermally annealed
38
and shows a FF of 0.46 in Table
2.2. The full range of dye compositions was explored at different acceptor compositions
showing optimal performance while demonstrating the tunability of V
oc
. A regular
evolution of the V
oc
was observed in Table 2.2, consistent with the alloy model explained
in Chapter 1. The dependence of dye composition on the V
oc
can be described by the
Table 2.2 Photovoltaic performances of devices made of P3HT and PDPP as donor
with PC
61
BM as acceptor.
P3HT:PDPP:PC
61
BM
J
sc
(mA cm
-2
)
V
oc
(V)
FF PCE %
1:0:0.8 6.12 0.61 0.46 1.74
0.8:0.2:1.04 4.45 0.61 0.42 1.15
0.6:0.4:1.28 2.15 0.63 0.34 0.46
0.4:0.6:1.52 1.48 0.69 0.30 0.30
0.2:0.8:1.76 2.33 0.81 0.35 0.65
0:1:2 1.68 0.87 0.56 0.81
Solutions were spin-coated from chlorobenzene (10 mg/mL in polymer and SM) and
solvent vapor annealed with o-dichlorobenzene for 10 minutes.

76

quadratic dependence between the two donors as seen in alloys.
39,40
The effect of this
relationship matches the description of an organic alloy as observed by the Thompson
group where a two donor and one acceptor system and a donor and two acceptor system
were studied.
28,29

Simultaneously, as the dye content increases in the ternary blends in this chapter
and the voltage is tuned the FF is reduced, specifically unlike D
1
/D
2
/A, or D/A
1
/A
2
. The
two end points consisting of P3HT:PCBM and PDPP:PCBM showed baseline FF values
of 0.46 and 0.56 and V
oc
values of 0.61 and 0.87, respectively. The V
oc
is not to be
pinned to the lesser value when looking at the rest of the compositions in the ternary
blend region of the table, as well as the dye weight percent and open-circuit voltage
relationship conveyed when plotted against each other as in Figure 2.6.
77

Given the steady decrease in FF, individual optimization at each composition was
pursued and conditions to optimize this parameter were carefully selected as seen on
Table 2.3 below, solvent vapor annealing times and acceptor compositions were
individually optimized. As the amount of dye became the same as that of P3HT, the V
oc

only gradually increased and seemed to plateau until reaching 60 % dye and 40% P3HT.
Past this ratio, there is roughly a linear increase in V
oc
when 90% dye to 10% P3HT ratio
is approached as seen on Table 2.3. On the other hand, the FF does not reveal a
consistent pattern. These results can be rooted in the inherent nature of the small
molecule (limited charge transport due to grain boundaries segregation in highly
0.0 0.2 0.4 0.6 0.8 1.0
0.60
0.65
0.70
0.75
0.80
0.85
0.90
V
oc
(V)
PDPP loading (wt. %)

Figure 2.6 Correlation between V
oc
and
small molecule composition in ternary
blends devices. PDPP weight % vs. V
oc
plot
for ternary blend BHJ solar cells of
P3HT:PDPP:PC
61
BM as a function of dye
loading in the blend. Dye content went from
0-100%. The V
oc
went from 0.61 V for
binary P3HT:PCBM reference to 0.87 V for
PDPP:PCBM.

78

crystalline SM)
41
by which, despite the high V
oc
, currents and FF values are potentially
affected by the higher charge trapping
42
and recombination losses.
31
Also, the difficulty
of controlling the morphology in solution-processable SM BHJ seems to be the cause as
it is extremely influenced by solvent choice, post-processing treatment, and use of
additives.
30

Several devices were fabricated displaying this pattern of lowering the FF with
increased dye loading. While FF was optimized, the V
oc
was not fully tunable at all ratios
as shown in the plot on Figure 2.7, which shows the potential this optimization had.
Table 2.3 shows the best FF for each composition. The trend is not linear and this is
displayed in Figure 2.8. Particularly at 60-80 weight % PDPP, where the FF is drastically
Table 2.3 Optimized photovoltaic performance of P3HT and PDPP as donor with
PC
61
BM as acceptor.
P3HT:PDPP:PC
61
BM
Annealing
Time
(min.)
J
sc
(mA
cm
-2
)
V
oc
(V)
FF PCE %
1:0:1 20 7.12 0.62 0.55 2.41
0.9:0.1:0.90 20 6.07 0.64 0.51 1.98
0.8:0.2:1.08 20 1.91 0.65 0.42 0.53
0.7:0.3:1.39 20 4.83 0.64 0.46 1.44
0.6:0.4:1.20 10 3.60 0.66 0.42 1.00
0.5:0.5:1.28 20 3.28 0.66 0.45 0.96
0.4:0.6:1.52 10 2.26 0.70 0.32 0.50
0.3:0.7:1.60 15 2.17 0.70 0.30 0.46
0.2:0.8:1.70 15 2.38 0.69 0.32 0.48
0.1:0.9:1.80 15 1.96 0.77 0.40 0.61
0:1:1.6 13 2.90 0.90 0.51 1.33
a) Chlorobenzene solutions for spin-coating and b) o-dichlorobenzene for solvent vapor
annealing.

79

low. A comparable trend is observed for TQTFA in which Huang and coworkers
explored compositions up to 75 weight% SM in blends and saw the concomitant drop in
FF.
24
Furthermore, the binary blend of TQTFA:PC
71
BM (1:1.25) is consistent with the
FF decrease displaying an even lower value (FF=0.32) unlike the observation made in
this work where binary blend PDPP:PCBM (1:2) and even (1:1.6) showed FF of 0.56 and
0.51, respectively.

0.0 0.2 0.4 0.6 0.8 1.0
0.60
0.65
0.70
0.75
0.80
0.85
0.90
Voc (V)
PDPP loading (wt.%)
0.0 0.2 0.4 0.6 0.8 1.0
0.30
0.35
0.40
0.45
0.50
0.55
FF
PDPP loading wt%

Figure 2.7 Plot depicting the correlation between V
oc
and SM composition in
ternary blends. This plot includes all data points (0-100 wt% dye) to illustrate
the quadratic relationship that was not achieved when FF values were
individually optimized. Figure 2.8 Plot of best FF values and small molecule
composition in ternary blends after optimization.

80

2.4 Summary of Small Molecule Ternary Blends and Outlook
A small molecule system derived from a diketopyrrolopyrrole was presented and
used in binary blends (PDPP:PCBM). These, have initially shown they can reach a V
oc
of
~0.90 V while maintaining a high FF= 0.56. The use of this small molecule as a donor in
binary blends has been demonstrated for the first time. The main focus of this chapter
was on the use of PDPP in ternary blend devices with P3HT and PCBM.
Tunability of the V
oc
has been demonstrated at various compositions, although not
every step in the composition gradient varied significantly. Furthermore, the V
oc
started to
a plateau between 10-50 wt % PDPP and slowly increased linearly when more than 60 wt
% of dye was loaded. While there was not a direct trend throughout the composition
range in terms of V
oc
, individual optimal ratios have shown moderate FF values, while
simultaneously maintaining high V
oc
values.
Small molecules can work in ternary blend BHJ solar cells by contributing to V
oc

tunability as has been observed for the system highlighted here as well as several systems
in Chapter 1. A common interesting observation for all these systems has been that SMs
are more sensitive to processing conditions and post-processing treatment and to a greater
extent than polymers. Concurrently, the disadvantages for SMs have been the low degree
of reproducibility of the devices seen here and the low FF. The FF has been affected by
the active layer formation step in that the morphology has yet to be understood or much
less controlled.
81

Future studies include the incorporation of covalently attached dye to fullerene
and its use in ternary blend devices with P3HT. In the interest of explaining the
composition dependence on the V
oc
, PCB-PDPP dyad will be presented in the following
section. Interactions in this underlying system are speculated to give insight into interface
structure and qualitatively support the alloy model from an interface perspective. The
objective of such a task would be to link morphology and electronic processes in this
system and obtain a better understanding of the underlying effects of constraining the
morphology via chemical modifications.

2.5 Extension to Covalent Dye-Fullerene Dyad
The influence that having two components covalently attached in a ternary blend
may have on device performance has started to be examined. This would serve as a
method for constraining the morphology as a way to begin to understand the relationship
between morphology and electronic processes in ternary blends. For this purpose, a dye-
fullerene dyad was synthesized and the characterization of this molecule will be shown in
this section. Many examples of dye-fullerene dyads or triad have been reported in the
literature and showed enhancement of the absorption and have attained higher V
oc

values.
43–47

Initially a nitro-hydro-cyanostilbene (NHCS) small molecule was targeted and it
was synthesized in one step with modifications from know procedure, by mixing
equimolar amounts of 4-hydroxybenzaldehyde and 4-nitrobenzylcyanide in the presence
82

of base.
48
The dye-fullerene dyad containing NHCS was synthesized in two steps as seen
in Figure 2.9. First, hydrolysis of PCBM to give the carboxylic acid precursor [6,6]-
phenyl-C
61
-butyric acid (PCBA) almost quantitatively. The next step was achieved via
N,N’dicyclohexylcarbodiimide (DCC) mediated esterification between PCBA and NHCS
as seen on Figure 2.9 this synthetic step was inspired by the work of Søndergaard and
coworkers for oxyethylene-substituted fullerenes.
49
Evidence that devices made of the
resulting modified fullerene and P3HT showed better performance, by using the dye to

Figure 2.9 Synthetic scheme of the first fullerene dyad target for the study of small
molecule ternary blend devices. Hydrolysis of PCBM resulted in PCBA followed by
DCC-mediated esterification to covalently attach dye to fullerene, PCB-NCS.

83

enhance both the absorption breath and lower the band gap of the acceptor, as opposed to
P3HT:PCBM was shown by Sharma and coworkers.
50
In their work, they have a three-
step synthesis to arrive at the desired dyad and in this section it was shown that the
synthesis was optimized. This account did not fully explore the potential influence this
dye had on the absorption spectrum in the solid state and the V
oc
by means of thorough
characterization of the SM, hence became of much interest to study. Interestingly, when
dye-fullerene dyad (PCB-NCS) was used with P3HT in 1:1 ratio devices the V
oc
was 0.86
V. This increase from 0.68 V in P3HT:PCBM could be attributed to a “sensitization”
effect by which the dye extends the absorption primarily in the UV and slightly in the
visible region and has a low HOMO level.
When first using NHCS in binary blend devices, solubilizing groups were
attached to the hydroxyl end group and good film quality was not achieved due to
aggregation, even after overcoming the solubility limitation of NHCS. This limitation
carried over to the fullerene-small molecule dyad; hence device fabrication could not be
optimized for the purposes of this study.
In view of the need for a more soluble and better capable of film formation dyad,
a novel modified fullerene with covalent attachments to a DPP-based dye was
synthesized for the first time. This complex molecule was made by a multi-step synthesis
starting with the selective bromination of DPP following the procedure of Loser et al
yielding mono-BrDPP, see Appendix 1. The next step seen on Figure 2.10, consisted of
palladium catalyzed Stille coupling between mono-BrDPP and a trimethyltin substituted
84

thiophene also functionalized with a protected alcohol forming tetrahydropyran-
diketopyrrolopyrrole (THP-DPP). This molecule was subsequently brominated using
NBS followed by coupling to phenyl-boronic acid via Suzuki coupling in the presence of

Figure 2.10 Synthetic scheme of the second fullerene dyad target for the study of small
molecule ternary blend devices. Stille coupling of mono-BrDPP and protected alcohol-
thiophene followed by bromination and Suzuki coupling to arrive at PDPP-Th-THP.
The last two steps consist of acid-catalyzed deprotection of the alcohol group and
esterification by DCC mediation to yield PCB-PDPP dyad.

Pd(PPh
3
)
4
. The next step consisted on the acidic deprotection of the alcohol in
THP by heating with aqueous acetic acid. Lastly, a DCC mediated esterification between
protected alcohol PDPP
shown in the scheme on
alternative since the film properties of PDPP were substantially better than those of
NHCS. The structures were verified by
via mass spectrometry, as all seen in
characterized by UV/Vis spectroscopy see in
voltammetry.

Figure 2.11
PCB-PDPP (green line), and annealed PCB
All spin-coated from chlorobenzene and PCBM,
chloroform. PCB
. The next step consisted on the acidic deprotection of the alcohol in
by heating with aqueous acetic acid. Lastly, a DCC mediated esterification between
PDPP-Th-THP precursor and PCBA took place in 60% yield as
shown in the scheme on Fig. 2.10. DPP was chosen as the dye for this dyad as an
e the film properties of PDPP were substantially better than those of
NHCS. The structures were verified by
1
H and
13
C NMR and the fullerene additionally
via mass spectrometry, as all seen in Appendix 1. The newly formed dyad was further
V/Vis spectroscopy see in Figure 2.11, X-ray diffraction, and cyclic

11 Absorption spectra of PCBM (orange line), as
PDPP (green line), and annealed PCB-PDPP (red line) films.
coated from chlorobenzene and PCBM, spin-coated from
chloroform. PCB-PDPP annealed at 120°C for 30 min.
85
. The next step consisted on the acidic deprotection of the alcohol in PDPP-Th-
by heating with aqueous acetic acid. Lastly, a DCC mediated esterification between
took place in 60% yield as
. DPP was chosen as the dye for this dyad as an
e the film properties of PDPP were substantially better than those of
C NMR and the fullerene additionally
. The newly formed dyad was further
ray diffraction, and cyclic

Absorption spectra of PCBM (orange line), as-cast
PDPP (red line) films.
coated from
86

PCB-PDPP shows a much stronger absorption in the 500-700 nm region than PCBM,
although PCBM absorbs more intensely in the UV region. It also has higher absorption
coefficient 94,303 vs. 61,560 cm
-1
for PCB-PDPP. PCB-PDPP films undergo no changes
when annealed.
Further studies and the use of this dyad in solar cells will be explored. The
preliminary studies shown in this section demonstrate that the dye-fullerene dyad
approach could be a useful indicator for dye interactions in ternary blends and possibly
aid in locating the dye, although more investigation in this topic is required in the future.

87

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91

Chapter 3: Polythiophene Side-chain Functionalization for
Surface Energy Modification in Conjugated Polymer-Based
Photovoltaics
3.1 Introduction
In this chapter the use of a family of new polymers to study the fundamental
underlying principles of ternary blend BHJ solar cells will be presented. Mainly focusing
on a basic model for relating morphological and electronic properties in
polymer1/polymer2/fullerene ternary blends and how the degree of polymer mixing can
be tuned and how the resulting morphology will influence the properties of ternary blend
BHJ SCs.
While the advancements are encouraging in the binary blend platform, its limited
theoretical efficiency (η) approximates only 11-12%.
1-3
Ternary blends have already been
established in this report as a potential route to increase the power conversion efficiency
(η) by overcoming the so-called J
sc
-V
oc
compromise.
4,5
The idea of having two
complementary absorbers as donor materials as in the alloy model allows for the
broadening of the absorption breadth and by carefully selecting the frontier orbital levels
of the components, a higher V
oc
can be attained than in limiting binary blends. With an
alloy donor phase in ternary blends, ideally two polymers will be miscible and lead to a
tunable V
oc
not pinned to the lesser value. In the opposite case, where the two polymers
are immiscible, the V
oc
would be expected to be pinned to the lesser value of its limiting
92

binary blends. This new platform combines the simplicity of solution processable active
layers and the potential to surpass the efficiency of tandem cells.
Chapter 3 focuses on how to modify the surface energy properties of a new family
of polymers and thereby how these influence and control the morphology. Certain
variables, which are intrinsic to the polymer and fullerene, affect the morphology in a
device, such as the crystallinity and the miscibility of the materials. More specifically, the
interaction parameter (χ) is used to control the morphology in these blends. χ is closely
related to the surface energy (γ) of the components involved and it is calculated from the
physical measurement of the contact angle in thin films.
6
Ideally, according to the alloy
model,
7
the donor phase will consist of fully miscible donor 1 (D
1
) and donor 2 (D
2
),
hence their surface energies would be close to each other in value (γD
1
≈ γD
2
). There is
evidence that changes in surface energy influence morphology even though solar cells are
not at thermodynamic equilibrium.
8,9
Changes in the structure of a polymer by side-
chain
10
as seen on Figure 3.1 or end-group
11
modifications have lead to changes in
surface energy as demonstrated by Sun and coworkers.

93

Figure 3.1 Surface energy and chemical structures of polymers PIDTDPP, PIDTDPP2,
and PIDTDPP3. Reproduced with permission from Sun, Y.et al. J. Mater. Chem. 2012,
22, 5587. Copyright (2012) The Royal Society of Chemistry.

The main focus of this chapter is on testing if surface energy can be tuned without
impacting electronic properties of a novel family of random copolymers based on the
monomers shown in table Figure 3.2, particularly focusing on monomer 2. Ultimately
the long-term goal is to study how polymer chemical structure influences the degree of
mixing between polymers in ternary blend solar cells composed of
donor1/donor2/acceptor (D
1
/D
2
/A). The effects that structural changes have on the
morphology and ultimately on the photovoltaic performance are of great interest initially
in a binary blend device configuration. This will serve to develop an understanding for
the role morphology plays in controlling electronic interactions and the factors involved
in finding the optimal relationship between the two.
94

Figure 3.2 Family of monomers for surface energy modification. 1) 2-bromo-3-(2-
ethoxyethanol)-5-trimethylstannylthiophene. 2) 2-bromo-3-(2-dioxahepta)-5-
trimethylstannylthiophene. 3) 2-bromo-3hexylthiophene-5-trimethylstannylthiophene. 4)
2-bromo-3-(4-nonafluorohepta)-5-trimethylstannylthiophene.

While the goals of this project encompass many aspects relating to morphology,
the core of this chapter is solely on surface energy tuning. The reason for this is that the
interaction parameter between components in a ternary blend plays a key role in
controlling the morphology. More specifically, the synthesis of alkylthiophene
copolymers was designed to control the surface energy by tuning the hydrophobicity and
hydrophilicity of the oxyethylene side-chains. This design includes maintaining the
electronic properties of the backbone.
Ternary platforms would be composed of two polymers and a fullerene. In it,
tuning miscibility between D1 and D2 with attractive electronic relationships would be
achieved by surface energy modification leading to controlled morphology attained by
side-chain functionalization of random copolymers. The family of polymers proposed for
this purposes is based on four different monomers which range from most hydrophobic to
most hydrophilic: 4-semifluorohexyl thiophene (4), 3-hexylthiophene (3), 3-(2-
dioxahepta)thiophene (2), and 3-(2-ethoxyethanol)thiophene (1). The monomer of
95

interest in this chapter will be 3-(2-dioxahepta)thiophene (2). With it, random copolymers
were synthesized to show the potential for tuning of the hydrophobic/hydrophilic nature
by means of monomer feed ratio variation as demonstrated by the surface energy values
calculated. The full range of compositions starting with each homopolymer and the
copolymers in between was explored and the surface energy was tuned from 33.50 mJ/m
2

for poly(3-oxyethylene thiophene) all the way down to 18.94 mJ/m
2
for poly(3-
hexylthiophene), in other words from hydrophilic to hydrophobic.
Additional work on this topic includes the optimization of binary blend devices of
each polymer in the poly (3-(2-dioxahepta)thiophene) family and PC
61
BM.
Consequently, the use of this framework will be employed to verify photovoltaic
performance and morphology in ternary blend solar cells, although this is outside of the
scope of this chapter.

3.2 Synthesis of Random Copolymers
The idea of this project is to give access to a family of random copolymers
designed with a broad range of precision-tunable surface energy. The synthesis of these
new polymers will be presented in this section in detail. The monomers were chosen to
generate random copolymers via Stille coupling. Homopolymers of 2
12,13
and 3
14
have
been synthesized previously via Grignard Metathesis Polymerization and 3 also by
McCullough
15
or Stille
16
coupling polymerization methods.

Analogous monomers to
hydroxyl-terminated monomer 1 and semi-fluoro monomer 4 have been reported and
96

previously polymerized.
17–19
Varying the polarity of the side chain allows for tunability
of the surface energy as evidenced in entries for both the contact angle and surface
energy trends on table 3.2.
A family of random copolymers with varying ratios of 3-hexyl-thiophene and 3-
(2-dioxahepta)thiophene was synthesized to give polymers in the form of P3HT
m
-co-
P3DHT
n
, where m= 3-hexylthiophene and n=3-(2-dioxahepta)thiophene percentages.
Purposely, there is a two-carbon spacer between the thiophene ring and the oxyethylene
side groups to maintain the electronic properties the same as those of P3HT and only
influence the surface interactions in thin-film devices (refer to Table 3.1). From the data
obtained, the goal is to correlate surface energies to morphology in binary and ternary
blend solar cells.
Monomer synthesis, seen in Figure 3.3 took place in three steps, first bromination
of 3-(2-ethanol) thiophene with N-bromosuccinimide (NBS) in good yield
20
followed by
reacting with 2-chloroethylmethyl ether in a neat reaction to give the desired substituted
thiophene following literature procedures.
12
The last step consisted of stannylation of the
5- position by basic trans-metallation, adapted from the synthesis of P3HT monomer
analogue.
Polymer synthesis, also seen in Figure 3.3 below was done by Stille coupling
polymerization in the presence of 5-10 mol% palladium-tetrakis (triphenylphosphine)
((PdPPh
3
)
4
) catalyst in dimethylformamide (DMF) at high temperatures. Both
homopolymers and copolymers were synthesized following this general procedure and
97

the content of catalyst varied with composition in order to drive the reaction to higher
molecular weight polymers. The values measured by GPC in THF were the number
average molecular weight (M
n
) and the polydispersity index (PDI). The range for M
n
(in
g/mol)/PDI= 30,050/3.44 for P3HT, 21,448/2.49 for P3HT
77
-co-P3DHT
23
, 16,811/1.68
for P3HT
47
-co-P3DHT
53
,

10,857/1.43 for P3HT
27
-co-P3DHT
73
, and

11,055/1.77 for
P3DHT. For the copolymers, as the content of P3DHT increases the M
n
decreases
gradually. These novel copolymers completed the new family of materials of interest.

Figure 3.3 Synthetic scheme of monomer and general procedure for polymerization of
homopolymer and copolymers via Stille coupling polymerization.

98

3.3 Characterization and Photovoltaic Performance
Polymer absorption spectra was measured and absorption up to 658 nm was
observed in thin films and λ
max
values ranging from 518 to 554 nm seen in Table 3.1
attributed to the π-π
*
transition. Optical and electronic data is summarized for P3HT,
PDHT, and random copolymers at different compositions. The optical properties
observed were obtained by UV/Vis spectroscopy on thin films spin-coated from
chlorobenzene. The band energy levels as reflected in the HOMO and LUMO values are
all roughly the same, as expected by the two-carbon spacer presence. These values were
obtained from cyclic voltammetry and the optical band gap values were obtained from
the band edge in the absorption spectra in Figure 3.4. The absorption spectra show that
the vibronic shoulder characteristic of P3HT is maintained when oxyethylene side chains
are introduced in the copolymers. This feature remains pronounced even at 73% loading.
Table 3.1 Optical properties for oxyethylene-containing copolymers and corresponding
poly (3-hexylthiophene) and poly (3-dioxaheptathiophene).
Sample
M
n

g/mol
(PDI)
λ
max,
abs.

(nm)
Absorption
Coefficient
(cm
-1
)
HOMO
(eV)
E
g

(nm/eV)
Optical
LUMO
(eV)
Hole
mobility
(cm
2
V
-1
s
-1
)
P3HT
30,050
(3.08)
554 85,530 -5.25 656/1.89 -3.36
1.8 x 10
-4

P3HT
77
-co-P3DHT
23

21,448
(2.49)
547 81,756 -5.21 657/1.89 -3.32
1.7 x 10
-4

P3HT
47
-co-P3DHT
53

16,811
(1.68)
520 77,416 -5.19 657/1.89 -3.30
6.8 x 10
-5

P3HT
27
-co-P3DHT
73

10,857
(1.43)
547 68,314 -5.20 658/1.88 -3.32
2.7 x 10
-7*

P3DHT
11,055
(1.77)
518 79,357 -5.16 658/1.88 -3.28
5.8 x 10
-7

CV (versus Fc/Fc
+
) in acetonitrile and 0.1 M TBAPF
6
. Optical band gaps from onset of
absorption in UV/Vis spectra films. Films spin-coated from chlorobenzene.*Mobility for
P3HT
22
-co-P3DHT
78
is shown.

The onset of absorption is roughly the same for each homopolymer and copolymer and is
further indication that the band gap values for every polymer are preserved. An
interesting trend seems to develop for
decreases when P3HT is diluted with increasing content of P3DHT. However, PDHT
homopolymer shows a more intermediate absorption coefficient value (79,357 cm
resembling that of the copolymer containing 53% P3DH
with the molecular weights and based on previous studies done of rr
also be attributed to the regioregularity (RR), that is, the percentage of head
linkages. When RR is increased, absorpti
also been observed to increase.
Figure 3.4 Absorption spectra of polymer films spin
for P3DHT and chlorobenzene for all copolymers and P3HT.
The onset of absorption is roughly the same for each homopolymer and copolymer and is
further indication that the band gap values for every polymer are preserved. An
interesting trend seems to develop for the absorption coefficient, which gradually
decreases when P3HT is diluted with increasing content of P3DHT. However, PDHT
homopolymer shows a more intermediate absorption coefficient value (79,357 cm
resembling that of the copolymer containing 53% P3DHT. This effect seems to correlate
with the molecular weights and based on previous studies done of rr-P3HT films, it could
also be attributed to the regioregularity (RR), that is, the percentage of head
linkages. When RR is increased, absorption coefficients also go up, and mobilities have
also been observed to increase.
21
For instance, in the polymers established h
Absorption spectra of polymer films spin-cast from chloroform
for P3DHT and chlorobenzene for all copolymers and P3HT.
99
The onset of absorption is roughly the same for each homopolymer and copolymer and is
further indication that the band gap values for every polymer are preserved. An
the absorption coefficient, which gradually
decreases when P3HT is diluted with increasing content of P3DHT. However, PDHT
homopolymer shows a more intermediate absorption coefficient value (79,357 cm
-1
)
T. This effect seems to correlate
P3HT films, it could
also be attributed to the regioregularity (RR), that is, the percentage of head-to-tail (HT)
on coefficients also go up, and mobilities have
For instance, in the polymers established here, RR is

cast from chloroform
100

quantified by the
1
H NMR integration of the α-methylene protons on hexyl chains which
corresponds to the HT versus head-to-head (HH) linkages
22
to give 91% RR for P3HT,
89% RR for P3HT
77
-co-P3DHT
23
, and 85% for P3DHT. These three polymers show the
same descending trend in absorption coefficient values. Although the RR values are
merely estimation as RR of oxyethylene-based polymers has not been calculated
previously given the complexity of identifying the α-methylene protons.
Furthermore, surface energy (γ) measurements were performed and a trend was
observed as seen on Table 3.2. First, it is important to briefly talk about what surface

Table 3.2 Surface energy table for every ratio of PDHT starting at 100%
followed by 78%, 53%, 37%, 23%, and 0%.
Polymer
Contact
Angle, θ
c
(deg.)
Surface
Energy, γ
(mJ m
-2
)
Solvent
Harmonic
mean
(mJ m
-2
)
P3DHT
a

77.94 36.77 Water
33.48
84.30 26.58 Glycerol
P3HT
22
-co-P3DHT
78

a

80.06 35.43 Water
30.39
81.00 28.46 Glycerol
P3HT
47
-co-P3DHT
53

a

83.89 33.05 Water
28.26
85.71 25.79 Glycerol
P3HT
63
-co-P3DHT
37

a

91.38 28.36 Water
23.50
89.80 23.49 Glycerol
P3HT
77
-co-P3DHT
23

a

97.06 24.84 Water
20.73
92.00 22.26 Glycerol
P3HT
a

103.76 20.77 Water
18.94
94.67 20.80 Glycerol
a) Solutions spin-coated from chlorobenzene. Thermally annealed at 150°C
for 30 minutes. Contact angles measured by goniometer in both pure water
and glycerol.
101

energy is: an imbalance of attractive forces between molecules at the surface. It is defined
as the total free surface energy of a material per unit area.
23

The way to measure surface energy is by first obtaining the contact angle (θ
γ
)
reading from a goniometer after depositing a droplet of a given solvent onto a film and
using the angle for calculations. In this chapter, the solvents of choice were water and
glycerol, more detailed procedures can be found in Appendix 2. The values on the far-
right column on Table 3.2 show a harmonic mean value, which was calculated using the
Wu method
24
employing a modified version of the Young equation and measuring
contact angles using one solvent at a time. The range of tunability is evident as γ
P3HT
~19
mJ m
-2
and its extreme, more hydrophilic counterpart γ
P3DHT
~ 33.5 mJ m
-2
demonstrate
it. Looking at Table 3.2, it is evident that with thermal annealing there is a linear
relationship between contact angle and surface energy. Contact angles increase from
hydrophilic to hydrophobic while surface energy decreases. This kind of tunability was
expected for this family of polymers and shows that small compositional changes can
influence the physical properties, such as surface energy without affecting electronic
properties.
As seen in the plot shown on Figure 3.5, surface energy values can be plotted
against the content of P3DHT almost displaying a linear trend. This plot can be used to
predict the surface energy of other side-chain modified polymers, particularly ones using
the monomers mentioned in Figure 3.2 based on the hydrophilicity in this case as the
higher the surface energy the more hydrophilic the polymer is.
102

0 20 40 60 80 100
18
20
22
24
26
28
30
32
34
Surface Energy (mJ m
-2
)
P3DHT %
Surface Energy

Figure 3.5 Plot of P3DHT content in random
copolymers vs. surface energy.

The tunability achieved here would serve as a predictor to better select which
polymers to blend together in the case of ternary blends. In this platform having D
1
and
D
2
a few scenarios may occur. For one thing, it is expected that a fully miscible phase
will be obtained, as in the alloy model, which would consist of almost equal surface
energies (γD
1
≈ γD
2
) and form a bicontinuous nanopenetrating network blend with
PCBM. It is why in this report the limiting binary blends for all the polymers were
explored and devices fabricated as seen in Figure 3.7. The structural changes applied to
the polymer, by means of composition ratio variation, allow tuning of the surface energy
and will serve to determine the degree to which polymers will mix across the gamut (of
oxyethylene content) from an alloy with highly similar structures, hence mixing
103

effectively and homogeneously to an immiscible polymer blend demixing with dissimilar
chemical structures and forming a heterogeneous two-phase system.
In contrast to a likely kinetically trapped morphology
8
for polythiophenes, the
surface energy properties for the polymers seem to be annealing dependent based on the
measurements taken when the films are annealed. When the contact angle is measured on
as-cast films the values are scattered and no real trends are observed seen on Table A2.1,
showcasing the potential of having a wide range of compositions of oxyethylene-based
polymers to use in the future in ternary blends and serve as predictors of morphology.
Morphology is commonly characterized by grazing incidence X-ray diffraction
(GIXRD), transmission electron microscopy (TEM), and atomic force microscopy
(AFM). Here, pristine polymers were characterized in the solid-state by XRD as-cast and
annealed at 150°C shown on Figures 3.6 and A2.6, respectively and by TEM on
FigureA2.7. To gain a deeper insight into the morphology, diffraction data was taken of
as-cast pristine films showing they were semicrystalline with (100) interchain distances
between 15 and 16 Å. The interchain distances for P3HT and P3HT
47
-co-P3DHT
53
are
lower than for the rest of the copolymers and the intensities higher, while P3DHT
homopolymer could be considered almost featureless in that region suggesting it is
amorphous. The presence of the vibronic shoulders in the UV/Vis spectra corroborate the
semicrystallinity of the rest of the films as supported by Figure 3.4. The TEM data of
binary blends in Figure A2.7 shows preliminary results of the bicontinuous morphology
in the case of P3DHT, P3HT, and P3HT
47
-co-P3DHT
53
.

Figure 3.6 GIXRD of as
solutions at 5 mg/mL polymer concentration

This somewhat optimal morphology can be observed for blends of the given
polymer and PCBM (1:0.8) analyzed as
nanostructure, it is less well
accumulation may have occurred. This is true to a larger extent for blends of P3HT
P3DHT
23
:PCBM and P3HT
to be analyzed for annealed samples to drive the phase segregation to an optimal scale
and more clearly observe the bicontinuous pathways from
were used in device work for these polymer:fullerene blends seen in
detailed in Table 3.3.
GIXRD of as-cast thin films spin-coated from CB
solutions at 5 mg/mL polymer concentration.
This somewhat optimal morphology can be observed for blends of the given
polymer and PCBM (1:0.8) analyzed as-cast. While P3HT
47
-co-P3DHT
53
nanostructure, it is less well-defined displaying small regions where catalyst
accumulation may have occurred. This is true to a larger extent for blends of P3HT
:PCBM and P3HT
24
-co-P3DHT
76
:PCBM shown in the images. More data needs
yzed for annealed samples to drive the phase segregation to an optimal scale
and more clearly observe the bicontinuous pathways from Figure A2.7. The same ratios
were used in device work for these polymer:fullerene blends seen in
104

coated from CB
This somewhat optimal morphology can be observed for blends of the given
53
shows a similar
defined displaying small regions where catalyst
accumulation may have occurred. This is true to a larger extent for blends of P3HT
77
-co-
:PCBM shown in the images. More data needs
yzed for annealed samples to drive the phase segregation to an optimal scale
. The same ratios
were used in device work for these polymer:fullerene blends seen in Figure 3.7 and

The photovoltaic properties of
were studied in BHJ solar cells using PC
configuration of ITO/PEDOT:PSS/polymer:PC
procedures are described in
J-V curves (Figure 3.7) followed by the discussion about the performance of this family
of polymers, thus far. Four different compositions were tested in binary b
well as P3DHT and P3HT as reference. Below, in
23%, 37%, 50%, and 78% P3DHT
Figure 3.7 J-V curve for copolymers compositions of 23% P3DHT, 37%
P3DHT, 50% P3DHT and P3HT under
illumination.
The photovoltaic properties of this family of polymers as donors in binary
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. Detailed device fabricat
procedures are described in Appendix 1. Preliminary results are presented here with their
) followed by the discussion about the performance of this family
of polymers, thus far. Four different compositions were tested in binary b
well as P3DHT and P3HT as reference. Below, in Table 3.3 is the optimized data for
23%, 37%, 50%, and 78% P3DHT-containing polymers shown with clearly varying FF
curve for copolymers compositions of 23% P3DHT, 37%
P3DHT, 50% P3DHT and P3HT under 100 mW/cm
2
(AM1.5G)
105
this family of polymers as donors in binary blends
BM as the acceptor in a conventional device
BM/Al. Detailed device fabrication
Preliminary results are presented here with their
) followed by the discussion about the performance of this family
of polymers, thus far. Four different compositions were tested in binary blend devices, as
is the optimized data for
containing polymers shown with clearly varying FF

curve for copolymers compositions of 23% P3DHT, 37%
(AM1.5G)
106

values, modest J
sc
with V
oc
values that are close to P3HT, as another indication that the
electronic properties were not perturbed. In the case of P3DHT and P3HT
22
-co-P3DHT
78

all parameters are low, the active layer is thin and no current is flowing through the
device, there are many leakages and therefore the FF is in the 0.30 range. Surprisingly,
the V
oc
is close to 0.60 for both polymers, although P3DHT is the lowest possibly
indicating that is amorphous and the annealing time needs to be optimized. Additionally,
these two devices were spin-coated from chloroform solutions and either a solvent
mixture or a different polar solvent may need to be employed.

For the rest of the devices, the currents are low and the active layers could be made
thicker to improve upon these values. The FF for the first four polymers are showing
working devices and have been optimized. The performance can also be linked to the M
n

Table 3.3 Photovoltaic properties of P3HT, P3HT
77
-co-P3DHT
23
, P3HT
63
-co-P3DHT
37
,
P3HT
50
-co-P3HT
50
, P3HT
22
-co-P3DHT
78
, and P3DHT polymers with PC
61
BM.
Polymer
Annealing
Time
(min./temp.)
J
sc
(mA cm
-2
)
V
oc
(V)
FF PCE %
P3HT 45/145°C 7.87 0.62 0.51 2.47
P3HT
77
-co-P3DHT
23
60/120°C 4.73 0.60 0.45 1.26
P3HT
63
-co-P3DHT
37
60/120°C 6.47 0.58 0.46 1.74
P3HT
50
-co-P3DHT
50
60/120°C 4.24 0.56 0.50 1.20
P3HT
22
-co-P3DHT
78
30/r.t. 0.50 0.61 0.33 0.10
P3DHT 30/r.t. 0.1 0.54 0.32 0.10
All devices were spin-coated from CB and annealed at the specified temperatures under
N
2
after aluminun deposition, except for P3HT
22
-co-P3DHT
78
and P3DHT, which were
spin-coated from chloroform solutions and solvent vapor annealed in o-DCB vapors.
107

values, which decrease with higher P3DHT content, although devices are functioning
with the presence of hydrophilic side-chains up to ~75% content.

3.4 Conclusion and Outlook
In this chapter, a new family of side-chain modified random copolymers
containing oxyethylene side-chains was presented. By changing the monomer
composition, specifically the poly(3-(2dioxahepta)thiophene) content, the surface energy
was modified. The composition-dependence surface energy allowed for tuning the
hydrophobicity/hydrophobicity of polymer films, hence controlling it without influencing
the electronic properties of the materials, as expected.
Devices made of these copolymers and PCBM demonstrated that the resulting
active layer blends could still function without detrimental changes to the polymer with
higher P3DHT content. This effect can also be seen and inferred from the TEM images
where a bicontinuous nanopenetrating network morphology is maintained.
Surface energy tunability can act as a way to predict how polymers can mix with
one another and extrapolated to ternary systems with D
1
and D
2
. The potential for tuning
the hydrophobic/hydrophilic nature of these polymers was shown here and it will be used
in the future for ternary blends to control the morphology of the donor phase and achieve
an alloy phase. This will ultimately lead to higher performing devices. Additionally, this
model can be extended to other structures with different side-chains.
108

3.5 References
(1) Scharber, M. C.; Mühlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A. J.;
Brabec, C. J. Adv. Mater. 2006, 18, 789–794.
(2) 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.
(3) Kotlarski, J. D.; Blom, P. W. M. Appl. Phys. Lett. 2011, 98, 053301.
(4) Chen, Y.-C.; Hsu, C.-Y.; Lin, R. Y.-Y.; Ho, K.-C.; Lin, J. T. ChemSusChem 2013,
20–35.
(5) Ameri, T.; Khoram, P.; Min, J.; Brabec, C. J. Adv. Mater. 2013, DOI:
10.1002/adma.201300623.
(6) Nilsson, S.; Bernasik, A.; Budkowski, A.; Moons, E. Macromolecules 2007, 40,
8291–8301.
(7) Street, R. A.; Davies, D.; Khlyabich, P. P.; Burkhart, B.; Thompson, B. C. J. Am.
Chem. Soc. 2013, 135, 986.
(8) Kim, B. J.; Miyamoto, Y.; Ma, B.; Fréchet, J. M. J. Adv. Funct. Mater. 2009, 19,
2273–2281.
(9) McNeill, C. R. Energy Environ. Sci. 2012, 5, 5653.
(10) Sun, Y.; Chien, S.-C.; Yip, H.-L.; Chen, K.-S.; Zhang, Y.; Davies, J. A.; Chen, F.-
C.; Lin, B.; Jen, A. K.-Y. J. Mater. Chem. 2012, 22, 5587.
(11) Kim, J. S.; Lee, Y.; Lee, J. H.; Park, J. H.; Kim, J. K.; Cho, K. Adv. Mater. 2010,
22, 1355–1360.
(12) Bricaud, Q.; Cravino, A.; Leriche, P.; Roncali, J. Sol. Energy Mater. Sol. Cells
2009, 93, 1624–1629.
(13) Kim, J.; Song, I. Y.; Park, T. Chem. Commun. 2011, 47, 4697.
(14) Loewe, R. S.; Ewbank, P. C.; Liu, J.; Zhai, L.; McCullough, R. D. Macromolecules
2001, 34, 4324–4333.
(15) Woo, C. H.; Thompson, B. C.; Kim, B. J.; Toney, M. F.; Fréchet, J. M. J. J. Am.
Chem. Soc. 2008, 130, 16324–16329.
109

(16) Burkhart, B.; Khlyabich, P. P.; Cakir Canak, T.; LaJoie, T. W.; Thompson, B. C.
Macromolecules 2011, 44, 1242–1246.
(17) Brusso, J. L.; Lilliedal, M. R.; Holdcroft, S. Polym. Chem. 2011, 2, 175-180.
(18) Mouffouk, F.; Higgins, S. J.; Brown, S. J.; Sedghi, N.; Eccleston, B.; Reeman, S.
Chem. Commun. 2004, 2314.
(19) Wang, B.; Watt, S.; Hong, M.; Domercq, B.; Sun, R.; Kippelen, B.; Collard, D. M.
Macromolecules 2008, 41, 5156–5165.
(20) Costanzo, P. J.; Stokes, K. K. Macromolecules 2002, 35, 6804–6810.
(21) Thompson, B. C.; Fréchet, J. M. J. Angew. Chem. Int. Ed. 2008, 47, 58–77.
(22) Sivula, K.; Luscombe, C. K.; Thompson, B. C.; Fréchet, J. M. J. J. Am. Chem. Soc.
2006, 128, 13988–13989.
(23) Owen, M. J. Comprehensive desk reference of polymer characterization and
analysis; Brady, R. F., Ed.; Surface Energy; American Chemical Society ; Oxford
University Press: Washington, D.C. : Oxford ; New York, 2003.
(24) Bulliard, X.; Ihn, S.-G.; Yun, S.; Kim, Y.; Choi, D.; Choi, J.-Y.; Kim, M.; Sim, M.;
Park, J.-H.; Choi, W.; Cho, K. Adv. Funct. Mater. 2010, 20, 4381–4387.

110

Chapter 4: Thiophene-Based Diketopyrrolopyrrole Semi-
Random Polymer Analogues for Enhancing the V
oc

4.1 Introduction
Semi-random polymers based on regioregular P3HT (rr-P3HT) are conjugated
polymers which can be used in efficient polymer:fullerene BHJ solar cells. They have
emerged as a new class of polymers over the past few years.
1–3
These combine a P3HT-
like character and a randomized sequence distribution leading to a mixture of good
properties for BHJ solar cells. Among them, is the multichromophoric nature of the
polymers broadening the absorption spectra by introducing an acceptor monomer unit in
small compositions relative to the other monomers. Additionally, the characteristics
coming from P3HT such as: high hole mobility and semicrystallinity, as well as optimal
mixing at low fullerene ratios. This class of polymers has shown low band gaps (1.27
eV)
1
, record high short-circuit currents of ~16 mA cm
-2
and impressive EQE of 40% at
800 nm region for a two-acceptor semi-random polymer containing 10%
diketopyrrolopyrrole (DPP) and 5% thienopyrrole-dione (TPD).
4
Semi-random
copolymers have reached efficiencies approaching 6%
4
and continue to show great
promise for improving upon the absorption breadth, reducing fullerene loading for
optimal device performance, high hole mobilities.
2,3
Additionally, the donor-acceptor
effect
8
that semi-random polymers have and the interactions between an electron-poor
and an electron-rich unit directly affects the electronic properties by lowering the band
111

gap, therefore tuning polymer HOMO energies. It is expected that these combined effects
would ultimately lead to high V
oc
. These polymers have also been proven useful as
tunable materials in ternary blends, for instance tuning the V
oc
.
29

Semi-random structures presented here consist of random polymerization of
specific linkages determined by the functional groups by which the donors and
acceptor(s) monomers are substituted with. The pattern of the linkages is restricted to
certain combinations, bromo substituents will only react with alkyl-tin substituents for
instance and not with other bromo substituents (considering Stille coupling
polymerization). This regiospecific arrangement is expected to lead to highly ordered
structures and therefore high hole mobilities.
5
Each repeat unit can be made up of
different monomer permutations, hence lead to multiple chromophores and ultimately
broader absorption.
6

In semi-random polymers the extension of the absorption breadth is achieved,
hence influencing the performance. These polymers provide a route for generating P3HT-
analogues with broadly diverse electronic structures for use in probing the effects of
electronic interactions. Doing so by influencing the HOMO energy levels to improve the
overall V
oc
of semi-random copolymers is necessary and of great interest for application
in ternary blend BHJ solar cells. The goal of using these materials is to combine a large
J
sc
and by means of changing the chemical structure of the backbone achieve a high V
oc

and move to higher η of semi-random polymers in general.
112

Semi-random polymers utilize easy-to-prepare monomers which can be
polymerized by a simple and reliable method, Stille coupling polymerization shown to be
reproducibly used for a wide range of monomers. The attractive features of this type of
materials include having broad absorption profiles, high hole mobilities,
semicrystallinity, and good miscibility with PC
61
BM. On the other hand, the V
oc
values
are limited and could be further enhanced. Other semi-random polymers bearing
benzothiadizole (BTD)
1
or thienopyrazine (TP)
7
as acceptor units resulted in polymers
with very promising optical and electronic properties but only moderate solar cell
efficiencies reported, so with the polymers presented here the idea is to overcome this
limitation as well.
Here, a new family of semi-random copolymers made up of known monomers
will be presented. These monomers consist of DPP analogues by which the HOMO level
can be tuned. DPP cores with different flanking aryl rings will be presented, as well as
their application in solar cells. Changing the electronic properties of the polymers will
allow for another avenue to tackle a parameter (V
oc
) that has been limiting current semi-
random copolymer binary blend device performance. The new set of polymers, which
will be presented in the next section, allows exploring structural changes and their
twofold effect: namely, changing the electronic properties and modifying the surface
energy by changing co-monomer composition. These polymers are rich in 3-
hexylthiophene monomer unit making up 80% of the backbone in semi-random
polymers. This structure similarity between each polymer compared to P3HT should
allow for homogeneous mixing between two polymers and contribute this way to ternary
113

blends. As observed in Chapter 2, it is believed that compositional tuning of the V
oc
is
possible while having a donor phase congruent with the alloy model and that intimate
mixing between two polymers is possible as hypothesized in Chapter 3.

4.2 Synthesis
In order to broaden the scope of semi-random polymers, by means of tuning the
HOMO energy levels, a family of new copolymers has been synthesized. These are
composed of 3-hexylthiophene, thiophene, and aryl-diketopyrrolopyrrole-aryl (Ar-DPP-
Ar) monomers. The chosen aromatic groups shown in Figure 4.1, start with thiophene,
benzene, and pyridine, becoming more electron deficient from left to right. DPP-based
monomers containing thiophene
3
and benzene
9–11
flanking groups have been reported
previously in the literature and synthesized here with slight modifications, whereas
pyridine flanked DPP monomer derivatives have only been found in patents and in
reports where their procedures are not known.
12,13
The monomers chosen to generate
semi-random polymers are shown in Figure 4.2. The first step in the synthesis occurs via

Figure 4.1 Representation of the different aryl
groups used in this chapter. From left: thiophene,
benzene, pyridine.
114

Figure 4.2 Monomer synthetic schemes.
115

a condensation reaction with an aromatic nitrile in the presence of a strong base followed
by ring closing of the DPP core.
14
In the first part in order to make monomer precursors 1
and 2 (Figure 4.2) the solvent used was t-amyl oxide, which is formed by the reaction of
sodium metal and iron(III)chloride with dry t-amyl alcohol.
10
The resulting oxide is more
reactive than the alcohol and therefore allows for the reaction to occur in decent yields
for both 1 and 2.
The limited solubility of these precursors, particularly in common organic
solvents at low concentrations required alkylation with either n-hexyl groups (1a, 2a) or
2-ethylhexyl groups (1b, 2b). Known synthetic procedures for phenyl-DPP (1a, 1b) were
followed with slight temperature and solvent dryness modifications. It proved to be a
modular and versatile procedure, as it was possible to apply it to the synthesis of pyridyl-
DPP monomers (2a, 2b). Another positive attribute to this synthesis is the simplification
of steps having built-in bromine groups as compared to that of control DPP monomer
shown in Figure 4.3, flanked by thiophene rings (3) see Appendix 3.

Figure 4.3 Chemical
structure of control
monomer 3.
116

Polymer synthesis took place as indicated in the general scheme (Figure 4.4) in
which the three co-monomers were reacted at 80% 3-hexylthiophene, 10% thiophene,
and 10% ArDPP via Stille coupling polymerization catalyzed by Pd(PPh
3
)
4
. The feed
ratio was verified by
1
H NMR integration seen in Appendix 3. P3HTT-ArDPP is the
general name given to the polymers and it stands for: poly(3-hexylthiophene)thiophene-
phenyldiketopyrrolopyrrole, where Ar for thiophene is implicit and only specified for

Figure 4.4 General polymer synthetic scheme. Semi-random polymerization by
palladium catalyzed Stille coupling. m= 2-bromo-3-hexyl-5-trimethyltin thiophene, n=
bis-2,5-(trimethyltin) thiophene, o= bis(bromo-aryl-alkyl) diketopyrrolopyrrole.
117

phenyl substituted DPP units. The synthesis of P3HTT-PDPP(Hex), P3HTT-PDPP(EH),
and P3HTT-DPP seen in Figure 4.5, was achieved in high yields and following the
procedure above, conversely, monomers 2a and 2b, which were reported elsewhere,
13

were not polymerized by the same conditions. Seo reported making similar monomer 2b,
but the procedure was missing in the report. The reason for this failed reaction for 2b was
the inherent reactivity of the pyridine isomer used to attach to the DPP core as observed
by control reaction in Figure 4.6, which shows a low yield and may offer insight as to
why polymerization does not occur.

Figure 4.6 Control reaction using 2-bromopyridine.

Figure 4.5 Chemical structures of semi-random copolymers: P3HTT-PDPP(Hex),
P3HTT-PDPP(EH), and P3HTT-DPP.

118

The reaction scheme in Figure 4.6 shows the reaction conditions to test the reactivity of
pyridil units in molecules 2a and 2b. 2-bromopyridine was used as a control and obtained
low conversion, which corroborated the inefficiency of the polymerization of 2a or 2b.
Another avenue to fully take advantage of the polymerizable monomers is a two-
acceptor copolymer seen in Figure 4.7, was synthesized to further observe the influence
on the HOMO level. This polymer, P3HTT-DPP-PDPP(EH) contains 75% 3-
hexylthiophene, 10% thiophene, and 7.5% each phenyl-DPP and thiophene-DPP and it
was synthesized under the same conditions shown on Figure 4.4. This polymer
resembles P3HTT-DPP in appearance both as a solid and in solution, while
distinguishable by its electronic properties, which will be shown in the following section.

Figure 4.7 Chemical structure of 2-acceptor semi-random
polymer P3HTT-DPP-PDPP(EH).

119

4.3 Characterization and Photovoltaic Performance
The optical properties of DPP-based semi-random polymers were obtained using
UV/Vis spectroscopy in the solid state as shown from the spectra in Figure 4.8. All films
were spin coated from o-DCB and P3HT was used as a reference in all measurements.
Introducing aryl-DPP units proves to significantly decrease the absorption coefficient as
well as the band gap of the semi-random polymers compared to P3HT. It also leads to a
dual band absorption expanding to longer wavelengths in the case of P3HTT-DPP and
P3HTT-DPP-PDPP(EH). Such a band absorption profile is evidence of π-π
*
transition at
short wavelengths and intramolecular charge transfer (ICT) at longer wavelengths.
15

Absorption coefficients for all the semi-random polymers, compared to P3HT are
about half as low. P3HTT-PDPP(Hex) has a higher absorption coefficient and it is
slightly red-shifted (by about 15-20 nm) than P3HTT-PDPP(EH), values shown on Table
4.1, which can be attributed to better stacking of the side-chains (which could lead to
higher ordering) with linear side-chains versus branched, inhibiting backbone planarity as
well as a dilution effect from the chromophore by having bulkier side-chains.
16

Additionally, these profiles do not show a vibronic feature like P3HT and their
onset of absorption is > 650 nm while for both P3HTT-DPP and P3HTT-DPP-
PDPP(EH), the onset is 831 amd 841 nm, respectively and they do show vibronic features
indicating higher ordering and strong intermolecular π-π interactions.
17

It was also observed that thermal annealing actually decreased thin film
absorption for all polymers except for P3HT changing
achieving the opposite effect of increasing the absorption coefficient.
interesting aspect about these spectra is that the
polymer P3HTT-DPP-PDPP(EH) seems to dominate the absorption profile and resembles
it while absorbing more broadly at 400 and ~600 nm.
Figure 4.8 Absorption spectra of P3HT
line annealed), P3HTT
PDPP(Hex) (red line as
(dark purple line as
polymer P3HTT-DPP

It was also observed that thermal annealing actually decreased thin film
absorption for all polymers except for P3HT changing the absorption profile slightly and
achieving the opposite effect of increasing the absorption coefficient.
interesting aspect about these spectra is that the thiophene-DPP unit in the two
PDPP(EH) seems to dominate the absorption profile and resembles
it while absorbing more broadly at 400 and ~600 nm.
Absorption spectra of P3HT (dark green line as-cast, light green
line annealed), P3HTT-DPP (black line as-cast, gray line annealed), P3HTT
PDPP(Hex) (red line as-cast, dark red line annealed), P3HTT-
(dark purple line as-cast, light purple line annealed), and two
DPP-PDPP(EH) (orange line as-cast) in the solid state.
120
It was also observed that thermal annealing actually decreased thin film
the absorption profile slightly and
achieving the opposite effect of increasing the absorption coefficient.
18
Another
DPP unit in the two-acceptor
PDPP(EH) seems to dominate the absorption profile and resembles

cast, light green
cast, gray line annealed), P3HTT-
-PDPP(EH)
cast, light purple line annealed), and two-acceptor
in the solid state.
121

The HOMO energy levels for P3HT and the P3HTT-DPP-based polymers were
measured by CV with ferrocene as a reference. These values were converted to the
vacuum scale using the approximation of the ferrocene redox couple being 5.1 eV
relative to vacuum.
19
Table 4.1 shows decreasing HOMO levels (spatially) starting with
P3HTT-DPP going to P3HT, P3HTT-PDPP(Hex), P3HTT-DPP-PDPP(EH), and P3HTT-
PDPP(EH), this means that the goal of tuning the HOMO level was accomplished. It is
important to note that for the two-acceptor polymer, while many characteristics resemble
P3HTT-DPP, the HOMO level certainly is tuned by the influence of phenyl-DPP unit as
well (-5.35 eV HOMO). Additionally, its value was expected to be intermediate between
P3HTT-DPP and P3HTT-PDPP(EH), given that both polymers containing one of the
DPP-based units that make up the two-acceptor polymer. HOMO level values for both
Table 4.1 Optical and electronic properties table of P3HT, P3HTT-DPP, P3HTT-
PDPP(Hex), P3HTT-PDPP(EH), and P3HTT-DPP-PDPP(EH) two-acceptor polymer in
the solid state.
Polymer
M
n
(g/mol)/
PDI
λ
max, abs

(nm)
Absorption
Coefficient
(cm
-1
)
HOMO
(eV)
E
g

(nm/eV)
Optical
P3HT 24,300/2.90 558 114,072 -5.22 665/1.86
P3HTT-DPP 17,605/3.18 685 63,103 -5.20 831/1.50
P3HTT-PDPP(Hex) 27,265/3.23 572 64,401 -5.32 710/1.75
P3HTT-PDPP(EH) 28,267/2.97 537 57,280 -5.40 697/1.78
P3HTT-DPP-
PDPP(EH)
24,441/3.23 682 50,616 -5.35 841/1.47
Absorption coefficient for as-cast films spin-coated from o-dichlorobenzene (5 mg/mL).
Optical band gaps obtained from onset of absorption in UV/Vis spectra in films.
Electrochemical HOMO values obtained from CV (vs. Fc/Fc
+
) in acetonitrile containing
0.1M TBAPF
6
.

122

P3HT and P3HTT-DPP are in agreement with literature precedence
3
and for all practical
purposes are the same (<0.5 eV difference).
This family of polymers has demonstrated having high molecular weights ranging
from 17,000 to 28,000 g/mol shown in Table 4.1, lower band gaps than P3HT, lower-
lying HOMO levels, and lower absorption coefficients. After all the characterization was
performed, device fabrication was undertaken with a configuration of:
ITO/PEDOT:PSS/polymer:PC
61
BM/Al. For fabrication details refer to Appendix 3. The
optimized polymer:PC
61
BM weight ratios for P3HTT-PDPP(Hex), P3HTT-PDPP(EH),
P3HTT-DPP-PDPP(EH), P3HTT-DPP, and P3HT were found to be 1:2, 1:1.5, 1:3, 1:1.3,
and 1:0.9, respectively. Optimal processing conditions included solvent vapor annealing
of the composites for individually determined times (Table 4.2) in a N
2
cabinet after
spin-coating and prior to aluminum deposition. V
oc
values are tuned by replacing
thiophene flanking groups with phenyl groups in the semi-random polymers showcased
here. P3HTT-PDPP(Hex) showed the highest V
oc
(0.83 V), while 0.76 V for P3HTT-
PDPP(EH), 0.60 V for P3HTT-DPP-PDPP(EH), and 0.58 V for P3HTT-DPP. This trend
is not correlated with HOMO energy data, from which the two-acceptor polymer would
be expected to have a V
oc
somewhere between parent polymers P3HTT-DPP and P3HTT-
PDPP(EH) values (0.58 and 0.76 V, respectively). The low V
oc
, in spite of the low
HOMO level for the 2-acceptor polymer has been observed in other reports for P3HTT-
TP-TDP in which having an intermediate HOMO energy between parent one-acceptor
polymers leads to a V
oc
closer to that of the higher HOMO level parent polymer.
2

Additionally, the band gap for P3HTT-DPP-PDPP(EH) is close to that of P3HTT-DPP,
123

therefore their V
oc
values are almost identical. As references, when compared to P3HTT-
DPP and P3HT, the other polymers achieved the goal of lowering the HOMO. On the
other hand, the FF (0.61 for P3HT vs. 0.34 for P3HTT-PDPP(Hex)) decreased
substantially (Table 4.2). For P3HTT-DPP-PDPP(EH) there is still room for
improvement in terms of FF as more ratios could be explored. In terms of J
sc
, the lower
values for the first three polymers can be attributed to the non-optimal annealing time,
whereas for the reference polymers optimal conditions have been found and high η values
obtained.

Table 4.2 Photovoltaic performances of P3HTT-PDPP(Hex), P3HTT-PDPP(EH),
P3HTT-DPP-PDPP(EH), P3HTT-DPP, and P3HT as donors with PC
61
BM as acceptor.
Ratio
J
sc

(mA/cm
-2
)
V
oc
(V) FF
PCE
%
Annealing
(min.)
P3HTT-
PDPP(Hex):PCBM
1:2 3.75 0.83 0.34 1.07 40
P3HTT-
PDPP(EH):PCBM
1:1.5 2.42 0.76 0.34 0.62 20
P3HTT-DPP-
PDPP(EH):PCBM
1:3 4.88 0.60 0.40 1.18 45
P3HTT-DPP:PCBM 1:1.3 9.65 0.58 0.61 3.41 30
P3HT:PCBM 1:0.8 8.51 0.58 0.57 2.79 30
Solutions were spin-coated from o-dichlorobenzene (12.5 mg/mL for all polymers
except P3HT, 15 mg/mL) and solvent annealed under N
2
for different times prior to
aluminum deposition.

124

4.4 Conclusion and Outlook for Diketopyrrolopyrrole Semi-Random
Polymer Analogues
In this chapter a new family of semi-random polymers was presented to highlight
the importance tuning electronic properties in the context of binary blend solar cells. By
varying the aryl groups that flank the DPP core unit, low-lying HOMO monomers were
synthesized, characterized and polymerized. The polymers were made by Stille coupling
and employed in devices where the V
oc
has been efficiently tuned. This tunability along
with the structural changes to the backbone allowed for improvements in device
performance as seen in binary blends and their expansion into ternary blend BHJs.
Lowering the HOMO energy levels of the polymers used the V
oc
can be tuned,
which could lead to higher performing devices in ternary blend systems that would
surpass η of limiting binary blends. An aspect of interest for the use of these polymers in
ternary blends could be their surface energy, which was explored in Chapter 3, as it could
be tuned and the overall morphology controlled while the voltage is enhanced by
lowering the HOMO level of the materials.

125

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139

APPENDIX 1 Polymer, Dye, Fullerene Ternary Blend Organic
Photovoltaics: Exploring the Influence of Blend
Composition on the Open-circuit Voltage
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 dry reactions were performed with glassware that was oven dried and flamed
under high vacuum and backfilled with N
2
. Flash chromatography was performed using a
Teledyne CombiFlash Rf instrument in combination with RediSep Rf 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 or Varian 500. For polymer molecular weight determination,
polymer samples were dissolved in HPLC grade o-DCB at a concentration of 0.05
mg/ml, heated until dissolved and filtered at room temperature using a 0.2 μm PTFE
filter. Size-exclusion chromatography (SEC) 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
140

Viscotek TDA 305 RI detector. The instrument was calibrated vs. polystyrene standards
(1,050 – 3,800,000 g/mol) and data was analyzed using OmniSec 4.6.0 software.
Cyclic voltammetry was collected using an EG&G instruments Model 263A
potentiostat under the control of PowerSuite 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
and a Pt wire counter electrode was
purged with nitrogen and maintained under nitrogen atmosphere during all
measurements. Acetonitrile was distilled over CaH
2
prior to use and tetrabutyl
ammonium hexafluorophosphate (0.1 M) was used as the supporting electrolyte. Polymer
and small molecule films were made by repeatedly dipping the Pt wire in a 1% (w/w)
polymer solution in chloroform and dried under nitrogen prior to measurement.
For thin film measurements polymers and small molecules were spin coated onto
pre-cleaned glass slides from chlorobenzene solutions (10 mg/ml). UV-vis absorption
spectra were obtained on a Perkin-Elmer Lambda 950 spectrophotometer. The thickness
of the thin films and GIXRD measurements were obtained using Rigaku Diffractometer
Ultima IV using Cu Kα radiation source (λ = 1.54 Å) in the reflectivity and grazing
incidence X-Ray diffraction mode, respectively.

141

A1. 2 Synthesis of Small Molecules and Dyads
Synthetic procedures for the synthesis of brominated DPP precursors: 3-(5-
bromothiophen-2-yl)-2,5-bis(2-ethylhexyl)-6-(thiophen-2-yl)-3,3a-dihydropyrrolo[3,4-
c]pyrrole-1,4-dione (mono-BrDPP) as seen on Figure A1.1 and 2,5-Diethylhexyl-3,6-
bis(5-bromothiophene-2-yl)pyrrolo[3,4-c]-pyrrole-1,4-dione (di-BrDPP), which is the
starting material for PDPP as seen on Figure A1.2, were followed without modifications
from reported literature.
1,2
Also, the synthesis of PCBA was followed from known
procedures.
3

Figure A1.1 Synthesis of 3-(5-bromothiophen-2-yl)-
2,5-bis(2-ethylhexyl)-6-(thiophen-2-yl)-3,3a-dihydro-
pyrrolo[3,4-c]pyrrole-1,4-dione (mono-BrDPP).

2,5-bis(2-ethylhexyl)-3,6-bis(5-phenylthiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4-dione
(PDPP): In a 3-neck flask dissolved 0.12 g (0.20 mmol) dibromo-DPP, 0.061 mg (0.50
mmol) phenyl boronic acid, and 0.016 g (0.013 mmol) Pd(PPh
3
)
4
in 8 mL toluene. To
that mixture added 3.85 mL of 2 M K
2
CO
3
and 1.70 mL ethanol. Degassed mixture
thoroughly for 20-30 minutes and stirred at 80°C for 24 hrs. Removed solvents and
142

purified via column chromatography in silica with chloroform as the eluent to yield 115
mg (85%) dark purple solid.
1
H NMR (400 MHz, CDCl
3
) δ 8.97 (dd, 2H), 7.68 (dd, 4H),
7.47 (d, 2H), 7.44 (t, 4H), 7.37 (d, 2H), 4.09 (m, 4H), 1.96 (m, 2H), 1.28 (m, 16H), 0.88
(m, 12H).
13
C NMR (400 MHz, CDCl
3
) δ 161.83, 149.74, 139.99, 136.86, 133.29,
129.25, 126.23, 124.58, 108.30, 46.09, 39.34, 30.46, 28.67, 23.80, 23.21, 14.16, 10.69.

Figure A1.2 Synthesis of 2,5-bis(2-ethylhexyl)-3,6-bis(5-
phenylthiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4-dione (PDPP).

The novel dye-fullerene dyad PCB-PDPP synthesis mentioned in section 2.5 was
accomplished in five steps starting with mono-BrDPP. This synthetic route was develop
in the Thompson group with the utilization of known organic modifications. Figure A1.3
shows the synthesis of protected 2-bromoethanol (1), 2-(2-ethoxy2-tetrahydro-2H-
pyran)thiophene (2), and 2-(2-ethoxy2-tetrahydro-2H-pyran)-5-trimethyltin thiophene
(3).

143

Figure A1.3 Synthesis of 2-(tetrahydro-2H-pyran-2-oxyethyl)-5-trimethyltin-thiophene.

2-(2-bromoethoxy)tetrahydro-2H-pyran (1): 1.48 g (17.56 mmol) 3,4-Dihydro-2H-
pyran dissolved in 30 mL dichloromethane (DCM) and cooled to 0°C. To this solution
added 1.76 g (14.10 mmol) 2-bromoethanol and catalytic amount ~2mg (0.01 mmol) p-
toluene sulfonic acid. Stirred at 0°C for an additional hour and room temperature
overnight. Quenched with 10 g ( 119 mmol) NaHCO
3
and stirred for an hour, followed
by filtration and removed solvent from filtrate in vacuo. Purified by vacuum distillation
(75°C, 70 mTorr) to yield 3.0 g (99%) colorless oil.
1
H NMR (400 MHz, CDCl
3
) δ 4.67
(1H, t), 4.01 (1H, m), 3.89 (1H, tt), 3.75 (1H, m), 3.51 (3H, m), 1.83 (1H, m), 1.72 (1H,
tt), 1.57 (4H, m).
2-(2-ethoxy2-tetrahydro-2H-pyran)thiophene (2): 3 g (14.35 mmol) of 1 were
dissolved in 9 mL of dry THF. In a separate flask, 1.40 mL (17.22 mmol) previously
dried thiophene were dissolved in 9 mL dry THF. This flask was cooled to -40°C and
10.8 mL (17.22 mmol) n-BuLi were added and stirred for 30 minutes. Simultaneously
cooled first solution to -40°C then transferred to second reaction mixture while at -40 to -
50°C for another 10 minutes and room temperature overnight. Resulting clear orange
mixture was transferred to water and extracted with ether three times, dried over MgSO
4

and removed solvents under vacuo to give a dark orange oil. Purified via column
144

chromatography with hexanes:DCM (1:1) as eluents. Lastly, distilled to dryness under
vacuum and ended with 1.33 g (50%) colorless oil.
1
H NMR (400 MHz, CDCl
3
) δ 7.13
(1H, dd), 6.93 (1H, td), 6.85 (1H, dd), 4.64 (1H, t), 3.97 (1H, qd), 3.82 (1H, tt), 3.63 (1H,
qd), 3.48 (1H, m), 3.13 (2H, td), 1.86 (1H, m), 1.72 (1H, m), 1.56 (4H, m).
2-(2-ethoxy2-tetrahydro-2H-pyran)-5-trimethyltin thiophene (3): 1.28 g (6.04 mmol)
of 2 were dissolved in 11 mL dry THF. In a separate flask, 1.03 mL (7.37 mmol) freshly
distilled DIA and dissolved in 4.90 mL dry THF. Cooled basic solution at -78°C for 15
minutes before adding 3.90 mL (6.22 mmol) n-BuLi. Once LDA was formed transferred
based to first solution at the same temperature for 50 minutes. Added 7.36 mL
(7.36mmol) Me
3
SnCl and stirred at cryogenic temperature for 30 minutes and at room
temperature overnight. Quenched with water and extracted organic layer with ether
followed by drying over MgSO
4
and removal of solvents to yield 2.04 g (91%) yellow oil
and used without further purification.
1
H NMR (400 MHz, CDCl
3
) δ 7.02 (1H, dd), 6.97
(1H, dd), 4.64 (1H, t), 3.97 (1H, m), 3.83 (1H, m), 3.64 (1H, m), 3.49 (1H, m), 3.17 (3H,
td), 1.86 (1H, m), 1.73 (1H, m), 1.62 (2H, m), 1.55 (4H, m), 0.34 (9H, t).
The following detailed procedures pertain to the syntheses of dyes and dyad-fullerene
precursors shown in Figure 2.9 and Figure 2.10.
4-nitro-4'-hydro-cyanostilbene (NHCS): 3.77 g (30,8 mmol) 4-hydroxybenzaldehyde
and 5 g (30.8 mmol) 4-nitrobenzylcyanide was heated in ethanol (385 mL) and heated to
reflux. 1.53 mL (15,42 mmol) piperidine was added and the reaction was further refluxed
for 2 h. Treated reaction with diluted HCl and cooled it to room temperature. This
145

resulted in precipitation of a clear yellow solid that was filtered off with cold methanol
and dried to give 5.5g (90%).
For the synthesis in Figure 2.10: step one consisted of the synthesis of THP-
DPP, followed by bromination to give Br-THP-DPP. Suzuki coupling yielded between
the previous molecule and a phenyl ring gave PDPP-Th-THP. Next, the deprotection of
the alcohol took place in acidic conditions to give PDPP-Th-EtOH and lastly a DCC-
mediated esterification to covalently attach to PCBA and give PCB-PDPP.
2,5-bis(2-ethylhexyl)-3-(5'-(2-((tetrahydro-2H-pyran-2-yl)oxy)ethyl)-[2,2'bithiophen]
-5-yl)-6-(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4-dione (THP-PDPP): In a flask,
delivered 357.6 mg (0.59 mmol) mono-BrDPP and dissolved in 59 mL toluene. Added
354 mg (0.94 mmol) 3 and degassed mixture thoroughly prior to adding 34 mg (0.03
mmol) Pd(PPh
3
)
4
. Degassed again and stirred at 85°C overnight under N
2
. Removed
solvent and subjected solid product to column chromatography with chloroform as eluent
to give 405 mg (93%) purple/green solid.
1
H NMR (400 MHz, CDCl
3
) δ 8.93 (1H, dd),
8.86 (1H, dd), 7.61 (1H, dd), 7.26 (1H, dd), 7.25 (1H, dd), 7.15 (1H, dd), 6.83 (1H, dd),
4.66 (1H, t), 4.03 (4H, m), 3.98 (1H, m), 3.83 (1H, tt), 3.65 (1H, m), 3.50 (1H, m), 3.12
(2H, td), 1.88 (3H, m), 1.30 (16H, m), 0.88 (11H, m).
Bromination of THP-PDPP: Cooled to 0°C a solution of 0.41 mg (0.55 mmol) THP-
PDPP dissolved in 27 mL chloroform and added 0.11 g (0.61 mmol) NBS in one portion.
Continued stirring at room temperature for several hours. Removed solvent and subjected
to column with chloroform as eluent to yield 0.29 g (65%) dark green/purple solid.
1
H
146

NMR (400 MHz, CDCl
3
) δ 9.04 (1H, dd), 8.94 (1H, dd), 8.61 (1H, dd), 7.48 (1H, dd),
7.25 (1H, d), 7.22 (1H, m), 6.85 (1H, s), 4.66 (1H, t), 4.03 (4H, m), 3.95 (4H, m), 3.83
(2H, t), 3.64 (2H, t), 3.52 (2H, m), 3.10 (3H, m), 1.84 (3H, m), 1.30 (14H, m), 0.89 (13H,
m).
2,5-bis(2-ethylhexyl)-3-(5-phenylthiophen-2-yl)-6-(5'-(2-((tetrahydro-2H-pyran-2-yl)
oxy)ethyl)-[2,2'-bithiophen]-5-yl)pyrrolo[3,4-c]pyrrole-1,4-dione (PDPP-Th-THP):
In a 3-neck flask mixed 0.29 g (0.36 mmol) Br-THP-PDPP, 66 mg (0.59 mmol)
phenylboronic acid, and 28 mg (0.02 mmol) Pd(PPh
3
)
4
and dissolved solids with 14 mL
toluene, a 6.91 mL 2M K
2
CO
3
solution, and 3.10 mL EtOH. Degassed mixture for 20
minutes before stirring at 80°C overnight. Removed solvents in vacuo and purified via
column chromatography with chloroform to give 0.24 g (83%) green solid.
1
H NMR (400
MHz, CDCl
3
) δ 8.94 (2H, s), 7.66 (3H, m), 7.44 (3H, m), 7.24 (0.8H, dd), 7.06 (0.8H,
dd), 6.86 (1H, d), 4.67 (1.6H, t), 4.04 (5H, m), 3.84 (2H, m), 3.64 (2H, m), 3.51 (2H, m),
3.12 (3H, t), 1.92 (4H, m), 1.75 (2H, m), 1.57 (7H, m), 1.30 (8H, m), 0.88 (13H, m).
Noticeable over integration indicative of two species present at this step which are non-
separable. This is overcome in the following steps.
Deprotection of PDPP-Th-THP to give (PDPP-Th-EtOH): Dissolved 0.18 g (0.23
mmol) PDPP-Th-THP in 24 mL of THF and to it added a mixture of 4:1 HOAc/H
2
O and
stirred overnight. Quenched the mixture with water and extracted the organic layer with
DCM. Dried over MgSO
4
, filtered and dried green solid. Subjected product to column
chromatography with DCM as eluent to yield 105 mg (68%).
1
H NMR (400 MHz,
147

CDCl
3
) δ 8.94 (2H, dd), 7.66 (2H, dd), 7.47 (1H, dd), 7.43 (2H, t), 7.36 (1H, dd), 7.24
(0.8H, dd), 7.16 (0.9H, dd), 6.84 (1H, dd), 4.06 (4H, m), 3.91 (2H, t), 3.09 (2H, t), 1.92
(2H, m), 1.30 (18H, m), 0.88 (12H, m).
Procedure for the synthesis of [6, 6]-phenyl-C
61
-butyric acid [3-(5-bromothiophen-2-
yl)-2,5-bis(2-ethylhexyl)-6-(thiophen-2-yl)-3,3a-dihydropyrrolo[3,4-c]pyrrole-1,4-
dione] ester (PCB-PDPP): Dispersed 0.075 g (0.084 mmol) PCBA (prepared as reported
in the literature) in 25.6 mL carbon disulfide. To this solution added 0.10 g (0.14 mmol)
PDPP-Th-EtOH dissolved with 2.30 mL chloroform and 1.75 mL carbon disulfide.
Lastly, added 0.0164 g dimethylaminopyridine (DMAP) and 0.045 g (0.21 mmol)
dicyclohexylcarbodiimide (DCC) and stirred to reflux temperature under N
2
. After 48
hrs., removed solvents and purified by column chromatography in silica with DCM as
eluent to yield 80 mg (60%) blue metallic solid.
1
H NMR (500 MHz, CDCl
3
) δ 8.92 (dd,
2H), 7.90 (dd, 2H), 7.68 (dd, 2H), 7.54 (t, 2H),7.45 (m, 4H), 7.37 (d, 2H), 7.24 (d, 1H),
7.15 (d, 1H), 6.82 (d, 1H), 4.34 (t, 2H), 4.01 (m, 4H), 3.16 (t, 2H), 2.88 (m, 2H), 2,18 (m,
2H), 1.92 (m, 2H), 1.26 (m, 16H), 0.87 (m, 11H).
13
C NMR (500 MHz, CDCl
3
) δ
172.80, 148. 70, 145.73, 145.14, 144.96, 144.62, 143.96, 143.71, 142.93, 142.86, 142.13,
142.06, 140.92, 140.69, 140.64, 138.27, 138.14, 137.96, 137.53, 136.72, 132.10, 132.07,
129.17, 128.42, 126.16, 51.76, 39.23, 29.70, 29.45, 28.25, 23.13, 14.77, 14.12, 14.09,
10.59.

A1.3 Small Molecule, Fullerene Dyad and Polymer

Small Molecule, Fullerene Dyad and Polymer Characterization
Figure A1.4
1
H NMR of PDPP in CDCl
3
.

148
Characterization

Figure A1.5
13
C NMR of PDPP in CDCl
3
.

149

Figure A1.6 Figure A1.6
1
H NMR of PCB-NCS in CDCl
3
.
150

Figure A1.7
1
H NMR PCB-PDPP in CDCl
3
.

151

Figure A1.8
13
C NMR PCB-PDPP in CDCl
3
.

152

Figure A1.9
solution (blue line).
Figure A1.9 Molar absorptivity spectra of PDPP in
solution (blue line).

153

of PDPP in
154

300 400 500 600 700 800
0.0
2.0x10
3
4.0x10
3
6.0x10
3
8.0x10
3
1.0x10
4
1.2x10
4
1.4x10
4
1.6x10
4
1.8x10
4
2.0x10
4
2.2x10
4
Absorption Coefficient (cm-1)
Wavelength (nm)
1 PDPP as-cast
2 PDPP annealed

300 400 500 600 700 800
0.0
5.0x10
4
1.0x10
5
1.5x10
5
2.0x10
5
2.5x10
5
Molar Absorptivity (M-1*cm-1)
Wavelength (nm)
PDPP

Figure A1.10 Absorption spectra of PDPP as-cast (black
line) and annealed (red line) in the solid state (top). Molar
absorptivity of PDPP and in solution (bottom).

Figure A1.11 High-resolution electrospray ionization mass spectrometry
of PCB-PDPP with m/z value of 1605.0.

A1.4 Device Characterization
Device fabrication consisted of the following steps: first
substrates (20 Ω/ , from
detergent (Tergitol
®
NP
acetone, and 3-propyl alcohol, and dried in a
PEDOT:PSS (Baytron
®
P VP AI 4083, filtered at 0.45
clean ITO-coated glass substrates and then
resolution electrospray ionization mass spectrometry
with m/z value of 1605.0.
racterization and Fabrication
Device fabrication consisted of the following steps: first ITO
from Thin Film Devices Inc.) were cleaned orderly
NP-9 aqueous solution), deionized water, tetrachloroethylene,
propyl alcohol, and dried in a pressurized air stream. A thin layer of
P VP AI 4083, filtered at 0.45 μm) was first spin
coated glass substrates and then annealed to remove water at 12
155

resolution electrospray ionization mass spectrometry (HR-ESI-MS)
ITO-coated glass
orderly by sonication in
ionized water, tetrachloroethylene,
stream. A thin layer of
m) was first spin-coated on the
annealed to remove water at 120 °C for 60
156

minutes under vacuum. Separate solutions of polymer and PC
61
BM were prepared in CB
or o-DCB solvents. The solutions were stirred for 4-6 hrs before they were mixed at the
desired ratios and stirred for overnight to form a homogeneous mixture. Subsequently,
the polymer:PC
61
BM or polymer:SM:fullerene active layer was spin-coated on top of the
PEDOT:PSS layer at speed ranging from 700-900 RPM. The P3HT:PDPP:PC
61
BM layer
was spin cast from a solution in CB containing 8 mg/mL P3HT and PDPP, and 12
mg/mL PC
61
BM. The solution of P3HT:PC
61
BM was prepared by dissolving the polymer
(10 mg/ml) and PCBM (8 mg/ml) in CB or o-DCB. Substrates of P3HT:PDPP:PC
61
BM
were placed over a surface submerging partially in o-DCB in a Petri dish to solvent vapor
anneal at 15-30 minutes depending on the composition of the blend. 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
. The substrates with P3HT:PC
61
BM blend were thermally annealed
at 156°C for 30 minutes under N
2
in the vacuum oven and cooled to room temperature
subsequent to electrode deposition. After annealing, either thermally or by solvent vapor,
the photovoltaic measurements were carried out.
The current-voltage (J-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
157

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.

A1.5 References for Appendix 1
(1) Khlyabich, P. P.; Burkhart, B.; Ng, C. F.; Thompson, B. C. Macromolecules 2011,
44, 5079–5084.
(2) Loser, S.; Bruns, C. J.; Miyauchi, H.; Ortiz, R. P.; Facchetti, A.; Stupp, S. I.; Marks,
T. J. Journal of the American Chemical Society 2011, 133, 8142–8145.
(3) Hummelen, J. C.; Knight, B. W.; LePeq, F.; Wudl, F.; Yao, J.; Wilkins, C. L. J.
Org. Chem. 1995, 60, 532–538.

158

APPENDIX 2 Polythiophene Side-chain Functionalization for
Surface Energy Modification in Conjugated Polymer-Based
Photovoltaics
A2.1 Materials and Methods
For general materials and methods section and molecular weight determination
details see appendix 1. Average-number molecular weight (M
n
) determined by GPC with
polystyrene standard and THF as eluent. Polymer contact angle measurements were
performed on Ramé-Hart automated goniometer/tensiometer with DROPimage Advanced
Model 290 equipped with an automated dispensing system. Surface energy calculated
based on single-solvent coefficients. Also, they were calculated by the Harmonic mean
method (Wu method)
1
using dispersive (γ
d
) and polar (γ
p
) coefficients for each solvent
and determine the overall surface energies using a derived form of the Young equation.
2

p d total
p p
G
d p
G
d d
G
d d
G
G G
p p
W
d p
W
d d
W
d d
W
W W
4 4
) cos 1 (
4 4
) cos 1 (
γ + γ = γ
γ + γ
γ γ
+
γ + γ
γ γ
= θ + γ
γ + γ
γ γ
+
γ + γ
γ γ
= θ + γ

159

According to this formula, W refers to deionized water and G refers to glycerol. The γ
d

and γ
p
components were obtained for each solvent from reported works by Fowkes
3
and
Owens and Wendt.
4

Transmission electron microscopy (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 measurements were prepared from the chlorobenzene
solutions of P3HT
m
-co-P3DHT
n
polymers at various ratios and optimized 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 P3HT
m
-co-P3DHT
n

films were picked up with the 600 hex mesh copper grids (Electron Microscopy
Sciences).
Polymer mobility was measured using a hole-only device configuration of
ITO/PEDOT:PSS/ P3DHT
n
-co-P3HT
m
/Al in the space charge limited current (SCLC)
regime. The devices preparations were the same as described in Appendix 1 for solar
cells. The dark current was measured under ambient conditions. At sufficient potential,
the mobilities in the device can be determined by fitting the dark current to the model of
SCLC as described by the following equation:
3
2
0
8
9
L
V
J
R SCLC
μ ε ε =
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
160

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 (16 – 20 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
between the ohmic region and the SCL region transition and is found to be about 0.6 V.
Polymer film thicknesses were measured using GIXRD in the reflectivity mode.

A2.2 Synthesis of Polymer Family
Synthetic procedures for the synthesis of 2-bromo-3-hexylthiophene,
5
2-bromo-5-
trimethyltin-3-hexylthiophene and P3HT were followed without modifications from
reported literature.
6
The following procedures are referring to Figure 3.3.
2-bromo-3-(2-ethanol) thiophene (1a): In a 100 mL round bottom flask delivered 5.06 g
(39.52 mmol) 3-(2-ethanol) thiophene and dissolved in 49 mL THF. To this solution
added 7.60 g (42.68 mmol) NBS at 0°C. Stirred at this temperature for 30 minutes and
warmed to room temperature and stirred for 2 hours to obtain a homogeneous yellow
reaction mixture once completion of the reaction took place. Transferred mixture to 150
mL water and 150 mL diethyl ether and the aqueous layer was extracted. Rinsed the
organic layer with NaOH 2M solution five times. Dried over MgSO
4
and removed
solvent. Purified via vacuum distillation to obtain 7.22 g (88%) of yellow oil as the
161

product.
1
H NMR (400 MHz, CDCl
3
) δ 7.23 (1H, d), 6.87 (1H, d), 3.84 (2H, t), 2.85
(2H,t), 1.47 (1H, s).
2-bromo-3-(2-(2-methoxyethoxy) ethyl) thiophene (1b): To a small 3-neck flask added
7.22 g (34.88 mmol) 2-bromo-(3-(2-ethanol)) thiophene followed by addition of 3.00 g
(52.32 mmol) crushed KOH. To this mixture added 8.00 mL (87.20 mmol) 2-chloroethyl
methyl ether and lastly 1.60 mL (3.49 mmol) Aliquat 336. Heated neat orange mixture to
65°C for 24 hours. Poured brown reaction mixture into 150 mL water and 150 mL diethyl
ether and extracted the organic layer. Washed aqueous layer with diethyl ether a few
times and dried over MgSO
4
followed by removal of solvent by rotary evaporation and
column chromatography in silica with petroleum ether: diethyl ether mixture (2:3). To
ensure dryness distilled light yellow oil under vacuum to obtain 5.74 g (62%) colorless
oil.
1
H NMR (400 MHz, CDCl
3
) δ 7.20 (1H, d), 6.88 (1H, d), 3.63 (4H, m), 3.55 (2H, m),
3.39 (3H, s), 2.89 (2H, d).
2-bromo-5-trimethyltin-3-(2-(2-methoxyethoxy) ethyl) thiophene (2): 5.60 g (21.13
mmol) 2-bromo-3-(2-(2-methoxyethoxy) ethyl) thiophene was dissolved in 42 mL dry
THF. Separately, dissolved freshly distilled 3.61 mL (25.78 mmol) diisopropylamine
(DIA) in 18 mL THF. Cooled this solution to -78°C, 8.70 mL (21.76 mmol) n-BuLi was
added dropwise and the mixture was stirred for 30 minutes at to -78°C, followed by an
ice bath for 15 minutes and again 30 minutes at to -78°C. To this basic mixture monomer
solution was transferred via cannula and stirred for 45 minutes at to -78°C. Then, added
25.8 mL (25.78 mmol) trimethyltin chloride dropwise and continued stirring for about 20
162

minutes. The mixture was allowed to warm to room temperature and stirred overnight.
Water was added to quench the reaction and extracted the organic phase with Et
2
O
several times. The resulting organic layer was dried over MgSO
4
and the solvent
evaporated in vacuo. Vacuum distillation gave the product as a faint yellow oil (6.97 g,
65% yield).
1
H NMR (400 MHz, CDCl
3
) δ 6.92 (1H, t), 3.63 (4H, m), 3.56 (2H, m), 3.39
(3H, s), 2.91 (2H, t), 0.35 (9H, t).
General Stille Polymerization for P3DHT and P3HT-co-P3DHT copolymers: All
monomers were dissolved in dry DMF to give a 0.04 M and degassed for 15 minutes.
Added 5-8 mol% Pd(PPh
3
)
4
in one portion and degassed again thoroughly and then heat
the mixture at 95°C for 48 hours. Cooled the reaction mixture to room temperature and
carefully precipitated after redissolving all formed solids and adding ammonium
hydroxide base. Filtered and purified via Soxhlet extraction using MeOH, hexanes and
chloroform. The polymer solution was then reduced under vacuum and reprecipitated,
filtered, and dried.
P3DHT: 88% yield, M
n
=11,055, PDI=1.77.
1
H NMR (400 MHz, CDCl
3
) δ 7.08 (1H, s),
3.77 (2H, t), 3.66 (2H, t), 3.58 (2H, t), 3.40 (3H, s), 3.12 (2H, t).
P3HT
77
-co-P3DHT
23
: 81% yield, M
n
=21,448, PDI=2.49.
1
H NMR (400 MHz, CDCl
3
) δ
6.98 (1H, s), 3.78-3.54 (1.70H, t), 3.41 (0.60H, s), 3.13 (0.4H, s), 2.80 (1.80H, t), 2.56
(0.22H, m), 1.71 (2H, t), 1.35 (6H, m), 0.91 (3H, t).
163

P3HT
50
-co-P3DHT
50
: 56% yield, M
n
=16,811, PDI=1.68.
1
H NMR (400 MHz, CDCl
3
) δ
7.03 (1H, q), 3.79-3.57 (6H, t), 3.40 (3H, s), 3.13 (2H, s), 2.79 (1.80H, t), 2.56 (0.40H,
m), 1.70 (2H, t), 1.35 (6H, m), 0.91 (3H, t).
P3HT
27
-co-P3DHT
73
: 43% yield, M
n
=10,857, PDI=1.43.
1
H NMR (400 MHz, CDCl
3
) δ
7.04 (1H, m), 3.78-3.58 (3H, t), 3.40 (1H, s), 3.13 (0.6H, s), 2.80 (0.80H, t), 2.58 (0.2H,
m), 1.87 (1H, t), 1.35 (3H, m), 0.91 (1H, t).
P3HT: 95% yield, M
n
=30,050, PDI=3.08.
1
H NMR (400 MHz, CDCl
3
) δ 6.97 (1H, s),
2.80 (2H, t), 2.55 (0.20H, s), 1.70 (2H, s), 1.40 (6H, m), 0.90 (3H, m).

A2.3 Polymer Characterization
Figure A2.1
Characterization

Figure A2.1
1
H NMR of P3HT in CDCl
3
.
164

Figure A2.2 Figure A2.2
1
H NMR of P3HT
77
-co-P3DHT
23
in CDCl
3

165
3
.

Figure A2.3

Figure A2.3
1
H NMR of P3HT
50
-co-P3DHT
50
in CDCl
3
166
3

Figure A2.4

Figure A2.4
1
H NMR of P3HT
27
-co-P3DHT
73
in CDCl
3
167

3

Figure A2.5

Figure A2.5
1
H NMR of P3DHT in CDCl
3

168

169

Table A2.1 Surface energy table for every ratio of P3DHT unannealed, as-cast films
starting at 100% followed by P3HT
27
-co-P3DHT
73
, P3HT
47
-co-P3DHT
53,
P3HT
77
-
co-P3DHT
23
, P3HT, and PCBM.
Polymer
Contact
Angle, θ
c
(deg.)
Surface
Energy, γ
(mJ m
-2
)
Solvent
Harmonic
mean
(mJ m
-2
)
PDHT
a

91.61 28.22 Water
25.27
93.14 21.64 Glycerol
P3HT
27
-co-P3DHT
73

b

94.53 26.41 Water
22.18
91.28 22.67 Glycerol
P3HT
47
-co-P3DHT
53

b

90.97 28.62 Water
33.39
88.02 24.48 Glycerol
P3HT
77
-co-P3DHT
23

b

97.44 24.62 Water
21.93
88.40 24.28 Glycerol
P3HT
b

105.08 19.98 Water
19.00
94.91 20.67 Glycerol
PCBM
a

90.22 29.09 Water
24.62
84.39 26.44 Glycerol
Contact angles measured by goniometer in both pure water and glycerol of as-cast
films. a) Spin-coated from chloroform solution. b) Spin-coated from CB solution.

Figure A2.6 GIXRD
P3HT
77
-co-P3DHT
P3HT
27
-co-P3DHT
coated from CB
GIXRD of annealed thin films of P3DHT (black line),
P3DHT
23
, (blue line), P3HT
47
-co-P3DHT
53
(red line),
P3DHT
73
(green line), P3HT (purple line). All were
coated from CB and annealed at 156 ºC for 30 min under N
2
.

170

(black line),
(red line),
All were spin-

Figure A2.7 TEM images at different copolymer compositions for as
TEM images of P3DHT
P3HT
47
-co-P3HT
53
:PC
1:0.8 ratios corresponding to
P3DHT
23
:PC
61
BM and P3HT
palladium particles.

TEM images at different copolymer compositions for as
TEM images of P3DHT:PC
61
BM, P3HT
77
-co-P3DHT
23
:PC
61
BM, P3HT:PC
:PC
61
BM, and P3HT
27
-co-P3DHT
73
:PC
61
BM all
1:0.8 ratios corresponding to solar cells conditions. Blends of P3HT
and P3HT
27
-co-P3DHT
73
:PC
61
BM require purification to remove
171
TEM images at different copolymer compositions for as-cast films.
, P3HT:PC
61
BM,
BM all prepared in
. Blends of P3HT
77
-co-
BM require purification to remove
172

A2.4 Device Characterization and Fabrication
Device fabrication procedures and measurement of J-V curve of photovoltaic
devices were presented in Appendix 1. For mobility measurements refer to section A2.1.
Polymer:PC
61
BM films were spin-coated from CB solution (10 mg/ml in P3HT) to which
aluminum layer was vacuum deposited. Prior to deposition, the substrates were pumped
down to high vacuum (< 7×10
-7
Torr) followed by aluminum (100 nm) deposition
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 4.6
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 120 or 145 °C. After annealing, the
devices were cooled down to room temperature and measurements were carried out.

A2.5 References for Appendix 2
(1) Wu, S. Polymer interface and adhesion; M. Dekker: New York, 1982.
(1) Owen, M. J. Comprehensive desk reference of polymer characterization and
analysis; Brady, R. F., Ed.; Surface Energy; American Chemical Society ; Oxford
University Press: Washington, D.C. : Oxford ; New York, 2003; pp 361-374.
(3) Fowkes, F. M. Industrial & Engineering Chemistry 1964, 56, 40–52.
(4) Owens, D. K.; Wendt, R. C. Journal of Applied Polymer Science 1969, 13, 1741–
1747.
(5) Woo, C. H.; Thompson, B. C.; Kim, B. J.; Toney, M. F.; Fréchet, J. M. J. Journal of
the American Chemical Society 2008, 130, 16324–16329.
(6) Burkhart, B.; Khlyabich, P. P.; Cakir Canak, T.; LaJoie, T. W.; Thompson, B. C.
Macromolecules 2011, 44, 1242–1246.
173

APPENDIX 3 Thiophene-Based Diketopyrrolopyrrole Semi-
Random Polymer Analogues for Enhancing the V
oc

A3.1 Synthesis of Semi-Random Polymers
For general materials and methods section see Appendix 1. Synthetic procedures
for the synthesis of EH-Br
2
DPP were followed without modifications from reported
literature.
1
The preparation of monomers 1a and 1b in Figure 4.2, was based on reported
literature and modified according to observed changes required.
2–6

Figure A3.1 Synthetic scheme of 3 (dibromo-thieno-diktetopyrrolopyrrole).

174

The solvent chosen for the formation of analogous DPP-cores with other aryl groups was
sodium t-amyl oxide. For its preparation, t-amyl alcohol was dried under CaH
2
overnight,
distilled and reacted with sodium metal at reflux temperature and reacted in the presence
of iron (III) chloride. This procedure was slightly modified from the literature, primarily
in the use of drying agent prior to reaction and bypassing the distillation step after
reaction was over and using solution as in on the following steps.
3,6-bis(4-bromophenyl)pyrrolo[3,4-c]pyrrole-1,4-dione (1): In a 100 mL 3-neck round
bottom flask delivered 5.0 g (27.48 mmol) 1-bromo-4-cyanobenzene and dissolved in 48
mL sodium t-amyl oxide. To this solution added 1.80 mL (13.61 mmol) dimethyl
succinate dropwise while stirring at 110°C. Stirred at this temperature overnight and
cooled to room temperature and quenched with ~20 mL equal parts HCl and MeOH.
Filtered precipitate and rinsed with cold MeOH and dried. Obtained 2.66 g (44%) off-
white red solid.
1
H NMR (400 MHz, DMSO-d
6
) δ 7.80 (6H, dd), 7.75 (1H, d), 7.10 (1H,
d).
General procedure for 3,6-bis(4-bromophenyl)-2,5-dialkylpyrrolo[3,4-c]pyrrole-1,4-
dione (1a, 1b) in Figure 4.2: In a 50 mL 3-neck round bottom flask delivered 1 and
dissolved in N-methyl-2-pyrrolidone (NMP) to give a 0.13 M solution. To this solution,
added 2.2 molar equivalents of t-BuOK and stirred mixture at 70°C. Delivered 6 molar
equivalents of n-hexyl (for 1a) or 2-ethylhexyl (for 1b) and stirred for 24 hours. Filtered
mixture and added water and extracted organic layer with dichloromethane (DCM)
several times. The resulting organic layer was dried over MgSO
4
and the solvent
175

evaporated in vacuo. Dried the solid product and removed residual NMP. Purified via
column chromatography in silica with hexanes/DCM gradient as eluent.
1a: 11% yield red solid,
1
H NMR (400 MHz, CDCl
3
) δ 7.68 (8H, dd), 3.72 (4H, m), 1.58
(4H, m), 1.22 (12H, m), 0.83 (6H, t).
1b: 14% yield reddish orange solid,
1
H NMR (400 MHz, CDCl
3
) δ 7.65 (6H, dd), 3.70
(4H, m), 1.26 (4H, m), 1.09 (11H, m), 0.80 (8H, t), 0.71 (7H, t).
3,6-bis(6-bromopyridin-3-yl)pyrrolo[3,4-c]pyrrole-1,4-dione (2): To a small 3-neck
flask added 3.01 g (16.39 mmol) 2-bromo-5cyanopyridine and dissolved with 33 mL t-
amyl oxide and heated to reflux at 110°C. Added 1.10 mL (8.20 mmol) dimethyl
succinate dropwise and continued heating at the same temperature overnight and
quenched with ~20 mL equal parts HCl and MeOH. Filtered mixture and dried to obtain
solids 2.31 g (63%) pink/red solids.
1
H NMR (400 MHz, DMSO-d
6
) δ 12.51 (2H, d), 8.22
(2H, dd), 7.64 (2H, dd), 6.40 (2H, dd).
General procedure for 3,6-bis(6-bromopyridin-3-yl)-2,5-dialkylpyrrolo[3,4-
c]pyrrole-1,4-dione (2a, 2b): In a 50 mL 3-neck round bottom flask delivered 2 and
dissolved in N-methyl-2-pyrrolidone (NMP) to give a 0.13 M solution. To this solution,
added 2.2 molar equivalents of t-BuOK and stirred mixture at 70°C (for 2a) and 90°C
(for 2b). Delivered 6 molar equivalents of n-hexyl (for 2a) or 2-ethylhexyl (for 2b) and
stirred for 24 hours. Added water to the mixture and extracted organic layer with
dichloromethane (DCM) several times. The resulting organic layer was dried over
MgSO
4
and the solvent evaporated in vacuo. Dried the solid product and removed
176

residual NMP. Purified via column chromatography in silica with 30 % MeOH/DCM
gradient as eluent. Recrystallized 2a with MeOH and small amount of hexanes.
Resubjected 2b to column chromatography with DCM as eluent.
2a: 15% yield needle-like pink/orange solid,
1
H NMR (400 MHz, CDCl
3
) δ 7.78 (2H, d),
7.37 (2H, dd), 6.56 (2H, dd), 3.92 (4H, t), 1.73 (4H, m), 1.31 (12H, m), 0.88 (6H, m).
13
C
NMR (400 MHz, CDCl
3
) δ 160.33, 144.30, 138.07, 121.27, 115.79, 90.65, 50.24, 30.75,
28.57, 25.65, 21.92, 13.43.
2b: 23% yield off-white yellow solid,
1
H NMR (400 MHz, CDCl
3
) δ 7.72 (2H, dd), 7.37
(2H, dd), 6.56 (2H, dd), 3.83 (4H, m), 1.83 (2H, m), 1.30 (16H, m), 0.90 (12H, m).
13
C
NMR (400 MHz, CDCl
3
) δ 161.07, 145.27, 138.44, 121.75, 116.34, 90.87, 54.17, 38.47,
30.03, 28.32, 23.37, 22.85, 13.95, 10.30.
General Stille Polymerization for semi-random copolymers: All monomers were
dissolved in dry DMF to give a 0.04 M and degassed for 15 minutes. Added 5 mol%
Pd(PPh
3
)
4
in one portion and degassed again thoroughly to then heat the mixture at 95°C
for 48 hours. Cooled the reaction mixture to room temperature and carefully precipitated
after redissolving all formed solids and adding ammonium hydroxide base. Filtered and
purified via Soxhlet extraction using MeOH, hexanes, and chloroform. The polymer
solution was then reduced under vacuum and reprecipitated, filtered, and dried.
P3HTT-PDPP(Hex): Yield: 95%, M
n
= 27,265, PDI = 3.23.
1
H NMR (400 MHz, CDCl
3
)
δ 7.87 (0.8H, dd), 7.73 (0.7H, dd), 7.02 (1H, d), 3.81 (1H, m), 2.82 (2H, t), 2.58 (0.6H,
m), 1.71 (3H, t), 1.35 (11H, tt), 0.92 (5H, m).
177

P3HTT-PDP (EH): Yield: 70%, M
n
= 28,267, PDI = 2.97.
1
H NMR (400 MHz, CDCl
3
)
δ 7.83 (0.4H, dd), 7.71 (0.6H, dd), 6.99 (1H, dd), 3.81 (0.4H, m), 2.81 (2H, t), 2.56
(0.3H, m), 1.71 (2H, t), 1.39 (6H, m), 0.92 (3H, t), 0.76 (1H, m).
P3HTT-DPP: Yield: 75%, M
n
= 17,605, PDI = 3.18.
1
H NMR (400 MHz, CDCl
3
) δ 8.93
(0.4H, s), 7.75 (0.7H, dd), 7.40 (0.6H, dd), 7.13 (0.7H, dd), 6.98 (1H, s), 4.05 (0.85H, m),
2.80 (2H, t), 2.58 (0.6H, m), 1.94 (0.6H, m), 1.71 (2H, t), 1.35 (8H, m), 0.92 (5H, t).
P3HTT-DPP-PDPP(EH): Yield: 81%, M
n
= 24,441, PDI = 3.23.
1
H NMR (400 MHz,
CDCl
3
) δ 8.93 (0.5H, s), 7.77 (1H, dd), 7.10 (0.7H, m), 6.80 (1H, s), 4.06 (0.7H, m), 3.81
(0.8H, m), 2.79 (2H, t), 2.64 (0.6H, m), 1.70 (2H, t), 1.36 (10H, m), 0.92 (6H, t).

A3.2 Polymer Characterization
Figure A3.2
Characterization
Figure A3.2
1
H NMR of 1 in DMSO-d
6
.
178

Figure A3.3
1
H NMR of 1a in CDCl
3
.
Figure A3.4
1
H NMR of 1b in CDCl
3
.
179

Figure A3. Figure A3.5
1
H NMR of 2 in DMSO-d
6
.

180

Figure A3.6
1
H NMR of 2a in CDCl
3
.
Figure A3.7
13
C NMR of 2a in CDCl
3
.
181

Figure A3.8
1
H NMR of 2b in CDCl
3
.
Figure A3.9
13
C NMR of 2b in CDCl
3
.
182

Figure A3. Figure A3.10
1
H NMR of P3HTT-PDPP (Hex) in CDCl

183

in CDCl
3
.

Figure A3. Figure A3.11
1
H NMR of P3HTT-PDPP (EH) in CDCl

184

3
.

Figure A3.
A3.3 Device Characterization
Device fabrication
devices were presented in
DCB solution. The films were placed in a
anneal at room temperature before being transferred to the vacuum chamber for
aluminum deposition. P3HT:PC
(EH):PC
61
BM, and P3HTT

Figure A3.12
1
H NMR of P3HTT-DPP in CDCl
3
.
erization and Fabrication
evice fabrication procedures and measurement of J-V curve
were presented in Appendix 1. Polymer:PC
61
BM films were spin
solution. The films were placed in a nitrogen cabinet for 20-40 min
anneal at room temperature before being transferred to the vacuum chamber for
aluminum deposition. P3HT:PC
61
BM, P3HTT-PDPP (Hex):PC
61
BM, P3HTT
BM, and P3HTT-DPP layers were spin cast from a solution in
185

curve of photovoltaic
s were spin-coated from o-
40 min to solvent
anneal at room temperature before being transferred to the vacuum chamber for
BM, P3HTT-PDPP
DPP layers were spin cast from a solution in o-DCB

containing 12.5 mg/mL i
PDPP (EH) was spin cast from a solution containing 15 mg/mL polymer and 15 mg/mL
PC
61
BM. At the final stage, the substrates were pumped down to high vacuum (< 7×10
Torr) and aluminum (100 nm) was thermal
Benchtop Turbo IV Coating System onto the active layer through shadow masks to
define the active area of the devices as
solar cell was tested without thermal treatme
current-voltage measurements.

Figure A3.13
P3HTT-
DPP-PDPP (EH), P3HTT
mW/cm
2

containing 12.5 mg/mL in polymer and 12.5 mg/mL PC
61
BM. Whereas P3HTT
PDPP (EH) was spin cast from a solution containing 15 mg/mL polymer and 15 mg/mL
BM. At the final stage, the substrates were pumped down to high vacuum (< 7×10
Torr) and aluminum (100 nm) was thermally evaporated at 3 – 5 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 4.6 mm
2
. All semi-random polymer:fullerene BHJ
solar cell was tested without thermal treatment. Refer to appendix 1 for details on
voltage measurements.
Figure A3.13 J-V curve for semi-random copolymers:
-PDPP (Hex), P3HTT-PDPP (EH), P3HTT-
PDPP (EH), P3HTT-DPP, and P3HT under 100
2
(AM1.5G) illumination.
186
BM. Whereas P3HTT-DPP-
PDPP (EH) was spin cast from a solution containing 15 mg/mL polymer and 15 mg/mL
BM. At the final stage, the substrates were pumped down to high vacuum (< 7×10
-7

5 Å/sec using a Denton
Benchtop Turbo IV Coating System onto the active layer through shadow masks to
random polymer:fullerene BHJ
nt. Refer to appendix 1 for details on

random copolymers:
-
100
187

A3.4 References for Appendix 3
(1) Khlyabich, P. P.; Burkhart, B.; Ng, C. F.; Thompson, B. C. Macromolecules 2011,
44, 5079–5084.
(2) Celik, S.; Ergun, Y.; Alp, S. Journal of Fluorescence 2009, 19, 829–835.
(3) Morton, C. J. .; Gilmour, R.; Smith, D. M.; Lightfoot, P.; Slawin, A. M. .; MacLean,
E. J. Tetrahedron 2002, 58, 5547–5565.
(4) Guo, E. Q.; Ren, P. H.; Zhang, Y. L.; Zhang, H. C.; Yang, W. J. Chemical
Communications 2009, 5859-5861.
(5) Rabindranath, A. R.; Zhu, Y.; Heim, I.; Tieke, B. Macromolecules 2006, 39, 8250–
8256.
(6) Zhang, G.; Liu, K.; Li, Y.; Yang, M. Polymer International 2009, 58, 665–673.

Abstract (if available)

Abstract The use of organic photovoltaics (OPVs) is the focus of this dissertation because of its low-cost, flexibility, and lightweight. The goal is to explore fundamental chemistry with the ultimate purpose of improving device efficiency. Applying new chemistry in the context of binary and ternary blends platforms is demonstrated. Ternary blend bulk heterojunction (BHJ) is an emerging platform for great potential to exceed efficiency of binary blend BHJs. ❧ In this dissertation, three types of materials are examined: small organic molecules, random poly(thiophene)-based copolymers, and semi-random polymers, materials which are relevant to ternary blends. In Chapter 1, binary blends consisting of a small molecule donor and a fullerene acceptor are introduced to encompass the classes of materials showing power conversion efficiencies (PCE) higher that 4%. Small molecules (SMs) are attractive materials for photovoltaics because their syntheses is reproducible and facile, purification is simpler than for polymers, and are well-defined molecular structures or monodisperse. Structural variations in SMs lead to changes in the properties, particularly low-lying HOMO levels which contribute to high open-circuit voltage (Voc) values, which could be higher performing for ternary blends than the respective limiting binary blends. Currently, either current or voltage is controlled, but in many instances at the expense of fill factor (FF), this is a limitation of SM ternary blend solar cells that is further studied in Chapter 2. ❧ The following chapters of this dissertation discuss concepts pertaining to ternary blend BHJ solar cells and each chapter highlights fundamental underlying principles of ternary blends. The core aspects that are addressed relate to how structural variations influence the overall material properties, thus their performance in devices. ❧ A new ternary blend system is introduced in Chapter 2 with the goal to ascertain how the compositional dependence of the system leads to a tunable Voc in devices made of polymer/dye/fullerene. Diketopyrrolopyrrole (DPP) based SM is the choice of small molecule for this purpose for its frontier orbital energy levels, absorption profile, and high Voc in binary blends with PCBM (0.9 V). This SM is reported for the first time as a donor in binary blends. In this work, small molecules can work in ternary blend BHJ solar cells by contributing to Voc tunability, although the FF values are low and there is no clear trend when they are optimized. These devices show that SMs are extremely sensitive to processing conditions as evidenced by the optimization of voltage and the lower FF values, additionally the SM shows lower degree of reproducibility. ❧ A different approach for developing basic principles in ternary blends, consists of tuning the surface energy of a novel family of random copolymers through composition. These copolymers contain oxyethylene chains and by varying its content, surface energy is found to change and ultimately influence the morphology. Surface energy is closely associated with miscibility between polymers and potentially applied to ternary blends. It is proposed in the alloy model that two polymers can have surface energies close in value and lead to Voc tuning in ternary blends. The copolymers of interest in Chapter 3 demonstrate surface energy is tuned from high to low oxyethylene content. The composition-dependence surface energy allowed for tuning the hydrophobicity/hydrophobicity of polymer films, without influencing the electronic properties of the materials. Devices made of these copolymers and PCBM demonstrated that the resulting active layer blends can function without detrimental changes to the polymer with higher oxyethylene content. Also, surface energy tunability can be used to predict how polymers can mix with one another and can be extrapolated to ternary system. ❧ Lastly, exploring the application of semi-random copolymers containing different aryl groups in the backbone in Chapter 4, leads to the possibility for tuning the HOMO energy level to enhance the Voc in binary blends and use the polymers for developing materials that are relevant to ternary blends in the future. This application addresses the issue of limiting voltage for semi-random polymers and expands the scope of polymers for ternary blends. Here, the synthetic efforts are discussed, as well as the characterization of the polymers, and their use in binary blend devices.

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Asset Metadata

Creator Beier, Alejandra E. (author)

Core Title Molecular and polymeric donors for bulk heterojunction organic solar cells

School College of Letters, Arts and Sciences

Degree Doctor of Philosophy

Degree Program Chemistry

Publication Date 08/19/2015

Defense Date 08/06/2013

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

Tag OAI-PMH Harvest,open-circuit voltage tuning,random copolymers,semi-random polymers,small molecule bulk heterojunction,surface energy,ternary blend bulk heterojunction organic solar cells

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Thompson, Barry C. (committee chair), Armani, Andrea M. (committee member), Prakash, G. K. Surya (committee member)

Creator Email alejandra.aviles.bo@gmail.com

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c3-322694

Unique identifier UC11288401

Identifier etd-BeierAleja-2007.pdf (filename),usctheses-c3-322694 (legacy record id)

Legacy Identifier etd-BeierAleja-2007.pdf

Dmrecord 322694

Document Type Dissertation

Rights Beier, Alejandra E.

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