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

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

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SYNTHESIS AND CHARACTERIZATION OF 3-HEXYLESTERTHIOPHENE
BASED RANDOM AND SEMI-RANDOM POLYMERS AND THEIR USE IN
TERNARY BLEND SOLAR CELLS

by

Sangtaik Noh

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

May 2016

Copyright 2016 Sangtaik Noh

ii

ACKNOWLEDGEMENTS

The times that I spent for this program cannot be finished without many people
who have been truly supportive and helpful. My short but sincere acknowledgement
should be dedicated to ones that I always find warmth and encouragement.

I would like to thank Prof. Barry C. Thompson for not only being a great advisor but
also a mentor who have patiently guided me through the program. Dr. Thompson has
always taught me how to face research problems and never gave up on the challenges in
and out of the lab. He has always been a great teacher, positive researcher, and a kind
person with sarcastic humors. It is very fortunate of me to work with him and cannot
thank enough for his understanding and believing. I hope he remains a same great mentor
as we continue our relationship.

I also would like to thank our group members who have walked every steps and
worked side by side. Dr. Petr Khlyabich, Dr. Beate Burkhart, and Dr. Alejandra Beier, and
have taught me many things. It was great experience to work with Dr. Andrey Rudenko
and Dr. Bing Xu. I also want to thank other group members Alia Latif, Jenna Howard,
Seyma Ekiz, Betsy Melebrink and especially Nemal Gobalasinham for helpful
discussions and fun times together.

iii

I would like to thank Prof. Mark E. Thompson, Prof. Richard L. Brutchey, Prof.
Alexander V . Benderskii, and Jongseung Yoon for agreeing to serve on my qualifying
exam committee and their support.

I am grateful to professors in USC for kindly let me use the instruments. Prof. G.
K. Surya Prakash, Prof. Stephen E. Bradforth, Prof. Thieo E. Hogen-Esch, Prof. Travis J.
Williams, Prof. Malancha Gupta, Prof. Andrea Armani.

Personal thanks to all my friends, staff members in USC to spend great times
together.

Best love to my father, mother and brother who have always supported me with
unconditional love.

Last but not least, great accomplishment of meeting my wife was done during
this program. I would like to say thank you to my true love for being there on my side.

iv

TABLE OF CONTENTS

ACKNOWLEDGEMENTS ................................................................................................ ii
LIST OF TABLES ........................................................................................................... viii
LIST OF FIGURES ............................................................................................................ x
ABSTRACT ..................................................................................................................... xiii
Chapter 1. Polymer-based Ternary Blend Solar Cells ........................................................ 1
1.1 Introduction ............................................................................................................... 1
1.1.1 Introduction: What is polymer-based solar cell? ................................................ 1
1.1.2 Principle: Binary blend BHJ solar cells .............................................................. 3
1.1.3 Efficiency of polymer-based solar cells ............................................................. 5
1.2. Beyond binary blend solar cells ............................................................................. 12
1.2.1 Efficiency limit of binary blend polymer-based solar cells .............................. 12
1.2.2 Tandem cell structure ....................................................................................... 13
1.2.3 Other approaches .............................................................................................. 16
1.3. Ternary blend BHJ solar cell .................................................................................. 17
1.3.1 Concept ............................................................................................................. 17
1.3.2 Enhancement of absorption and V
oc
in ternary blend solar cells ...................... 18

v

1.3.3 State of the art ternary blend solar cells ........................................................... 22
1.3.4 Mechanism of ternary blend solar cells ............................................................ 25
1.3.5 Relationship between two donor pairs for ternary blend solar cell .................. 31
1.4 Conclusion ............................................................................................................... 37
1.5 References ............................................................................................................... 38
Chapter 2. Facile Enhancement of Open Circuit Voltage of
Random and Semi-Random P3HT-Analogs in Solar Cells via
Incorporation of Hexyl Thiophene-3-Carboxylate ........................................................... 43
2.1. Introduction ............................................................................................................ 43
2.2 Experimental Section .............................................................................................. 47
2.2.1 Materials and Methods. .................................................................................... 47
2.2.2 Synthetic Procedures. ....................................................................................... 49
2.2.3 Device fabrication and characterization ........................................................... 53
2.2.4 Mobility Measurements. ................................................................................... 54
2.3 Results and discussions ........................................................................................... 55
2.3.1 Polymer characterizations. ................................................................................ 58
2.4 Conclusion ............................................................................................................... 65
2.5 References ............................................................................................................... 66

vi

Chapter 3. Surface Energy Modification for Controlling Organic
Alloy Formation of Two Polymer Donors in Ternary Blend Solar Cells ......................... 71
3.1 Introduction ............................................................................................................. 71
3.2 Results and discussions ........................................................................................... 76
3.3 Conclusion ............................................................................................................... 86
3.4 References ............................................................................................................... 88
Chapter 4. Exploration of 3-hexylesterthiophene based
Semi-random Polymers for Ternary Blend Solar Cell Devices ........................................ 90
4.1 Introduction ............................................................................................................. 90
4.2 Experimental section ............................................................................................... 96
4.2.1 Materials and methods ...................................................................................... 96
4.2.2 Device fabrication and characterization ........................................................... 98
4.2.3 Results and discussions .................................................................................. 100
4.3 Conclusion ............................................................................................................. 110
4.4 References ............................................................................................................. 111
BIBLIOGRAPHY ........................................................................................................... 113
Appendix 1. Facile Enhancement of Open-Circuit Voltage of
Random and Semi-Random P3HT-Analogs in Solar Cells via
Incorporation of Hexyl Thiophene-3-Carboxylate ......................................................... 123

vii

A1 .1 Structure Verification of Small Molecules and Polymers ................................. 123
Appendix 2. Surface Energy Modification for Controlling Organic
Alloy Formation of Two Polymer Donors in Ternary Blend Solar Cells ....................... 139
Appendix 3. Exploration of 3-hexylesterthiophene based
semi-random polymers for ternary solar cell devices ..................................................... 141
A3. 1. Structure Verification of Polymers................................................................... 141

viii

LIST OF TABLES

Table 1.1. Ternary blend system of small molecule
sensitizer added to the P3HT: PC
61
BM binary blend
and change of photovoltaic parameters..................................................................... 19

Table 1.2. Summary of device performance parameters
of ternary blend and corresponding binary blend references. .................................... 24

Table 1.3. Photovoltaic parameters of ternary blend device and its sub-cells .................. 27

Table 2.1. Characterization of random copolymers
P3HT-co-3HETs and semi-random copolymers
P3HTT-DPP-3HETs. ................................................................................................. 58

Table 2.2. Average photovoltaic parameters and SCLC hole
mobilities of random copolymers P3HT-co-3HETs and
semi-random copolymers P3HTT-DPP-3HETs. ........................................................ 62

Table 3.1. The material combination of ternary blend model system
studied in this chapter. ................................................................................................ 75

Table 3.2. Optical and electrical properties of polymers .................................................. 77

Table 3.3. Photovoltaic device parameters of ternary blend
A1(P3HTT-DPP:P3HT
75
-co-EHT
25
:PC
61
BM) and
A2(P3HTT-DPP-40%MEO:P3HT
75
-co-EHT
25
:PC
61
BM)
with several different composition of D1:D2. ............................................................ 79

Table 3.4. Photovoltaic device parameters of ternary blend
B1 (P3HTT-DPP:P3HT
50
-co-HET
50
:PC
61
BM) and
B2 (P3HTT-DPP-40%MEO:P3HT
50
-co-HET
50
:PC
61
BM)
with several different composition of D1:D2. ............................................................ 82

Table 4.1. Molecular and electronic properties of all polymers ..................................... 101

Table 4.2. Average photovoltaic parameters and SCLC hole
mobilities of 3HET random and semi-random polymers ......................................... 103

Table 4.3. Average photovoltaic parameters of ternary blend devices. .......................... 108

Table A1.1. Summary of raw short-circuit current densities (J
sc,raw
),
spectral-mismatch factor (M), spectral mismatch-corrected

ix

short-circuit current densities (J
sc,corr
) and integrated
short-circuit current densities (J
sc,EQE
) for BHJ solar cells
based on semi-random copolymers ......................................................................... 136

Table A2.1. Summary of raw short-circuit current densities (J
sc,raw
),
spectral-mismatch factor (M), spectral mismatch-corrected
short-circuit current densities (J
sc,corr
) and integrated
short-circuit current densities (J
sc,EQE
) for BHJ solar cells
based on semi-random copolymers for ternary blend
solar cells in system A1,A2,B1,and B2................................................................... 139

x

LIST OF FIGURES

Figure 1.1. Electronic structure of binary blend BHJ device. ............................................ 4

Figure 1.2. J-V curve of a solar cell device and photovoltaic parameters. ........................ 6

Figure 1.3. Chemical structures of representative donor and acceptor molecules. ............ 8

Figure 1.4. Contour plot of PCE with PC
61
BM (LUMO = 4.3eV)
as acceptor material. ................................................................................................... 13

Figure 1.5. An example of tandem cell structure connected in series. ............................. 16

Figure 1.6. Complementary absorption of ternary blend solar cells ................................ 18

Figure 1.7. Various structures of small molecule dyes added to the
P3HT:PC
61
BM biary blend solar cells listed in Table 1.1. ........................................ 20

Figure 1.8. Energy level diagram of ternary blend consist of D1, D2 and A. .................. 22

Figure 1.9. Mechanism of ternary blend solar cells based on
(a) cascade model, (b) parallel model, and (c) organic alloy model. ......................... 30

Figure 1.10. PSR study of ternary blend system consist of
P3HT:PC
61
BM:ICBA and P3HTT-DPP-10%:P3HT
75
-co-EHT
25
:PC
61
BM. .............. 30

Figure 2.1. Synthesis of (a) Monomer 2-bromo-5-trimethyltin-
3-hexylesterthiophene, (b) Stille polymerization for
random copolymers P3HT-co-3HETs and (c) Stille polymerization
for semi-random copolymers P3HTT-DPP-3HETs.
(i) 1-hexanol, DCC/DMAP, DCM, 48 hrs
(ii) -78 °C, LDA, CBr
4
, THF(iii) TMPMgCl∙LiCl, tMeSnCl, THF
(iv) Pd(PPh
4
)
3
, DMF, 48 hrs...................................................................................... 56

Figure 2.2. UV−vis absorption and GIXRD data of
(a), (c) random copolymers P3HT-co-3HETs and
(b), (d) semi-random copolymers P3HTT-DPP-3HETs in thin films......................... 60

Figure 2.3. EQE of (a) random copolymer P3HT-co-3HET:PC
61
BM
devices and (b) semi-random copolymer P3HTT-DPP-3HET:PC
61
BM devices. ...... 64

xi

Figure 3.1. Structures of the polymers used in model system. ........................................ 75

Figure 3.2. (a) Comparison of V
oc
trend in system A1 and A2.
(b) Comparison of V
oc
trend in P3HT/OXCMA/OXCBA
and P3HT/OXCMA/OXCTA ternary blend system from ref.20. ............................... 79

Figure 3.3. GIXRD result of ternary blend system (a) A1 and
(c) A2 and corresponding plot of maximum peak position
depending on the composition for ternary system (b) A1 and (d) A2. ....................... 81

Figure 3.4. GIXRD result of ternary blend system (a) B1 and
(c) B2 and corresponding plot of maximum peak position
depending on the composition for ternary system (b) B1 and (d) B2. ....................... 84

Figure 3.5. (a) EQE, (b) UV-Vis data of the ternary blend system
A1 and (c) EQE, (d) UV-Vis data of the ternary blend system A2. ........................... 85

Figure 3.6. (a) EQE, (b) UV-Vis data of the ternary blend system
B1 and (c) EQE, (d) UV-Vis data of the ternary blend system B2. ............................ 86

Figure 4.1. Structure and composition of semi-random polymers
P3HTT-BTD-HETs, P3HTT-TPD-HETs, and P3HTT-Tz-HETs. ............................. 95

Figure 4.2. UV-Vis absorption of semi-random HET polymer films:
(i) P3HTT-BTD, (ii) P3HTT-BTD-HET40%, (iii) P3HTT-BTD-HET80%,
(iv) P3HTT-TPD, (v) P3HTT-TPD-HET40%, (vi) P3HTT-TPD-HET80%,
(vii) P3HTT-Tz, (viii) P3HTT-Tz-HET40%, and (ix) P3HTT-Tz-HET80%. .......... 102

Figure 4.3. GIXRD data of P3HTT-BTD-HET and P3HTT-Tz-HET
semi-random polymer films: (i) P3HTT-BTD, (ii) P3HTT-BTD-HET40%,
(iii) P3HTT-BTD-HET80%, (iv) P3HTT-Tz, (v) P3HTT-Tz-HET40%,
and (vi) P3HTT-Tz-HET80%. .................................................................................. 105

Figure 4.4. EQE data of (a) P3HTT-BTD-HETs (b) P3HTT-TPD-HETs,
and (c) P3HTT-Tz-HETs : (i) 0%, (ii) 40%, (iii) 80%
HET composition respectively. ................................................................................ 106

Figure A1.1.
1
H NMR of 3-hexylesterthiophene. .......................................................... 123

Figure A1.2.
1
H NMR of 2-bromo-3-hexylesterthiophene. ........................................... 124

Figure A1.3.
1
H NMR of 2-bromo-5-trimethyltin-3-hexylesterthiophene. .................... 125

Figure A1.4.
1
H NMR of P3HT. ..................................................................................... 126

xii

Figure A1.5.
1
H NMR of P3HT
75
-co-3HET
25
. ............................................................... 127

Figure A1.6.
1
H NMR of P3HT
50
-co-3HET
50
................................................................ 128

Figure A1.7.
1
H NMR of P3HT
25
-co-3HET
75
................................................................ 129

Figure A1.8.
1
H NMR of P3HET ................................................................................... 130

Figure A1.9.
1
H NMR of P3HTT-DPP ........................................................................... 131

Figure A1.10.
1
H NMR of P3HTT-DPP-HET10%. ....................................................... 132

Figure A1.11.
1
H NMR of P3HTT-DPP-HET20%. ....................................................... 133

Figure A1.12.
1
H NMR of P3HTT-DPP-HET40%. ....................................................... 134

Figure A1.13.
1
H NMR of P3HTT-DPP-HET80%. ....................................................... 135

Figure A1.14. Cyclic V oltametry of P3HT-co-3HETs. .................................................. 137

Figure A1.15. Cyclic V oltametry of P3HT-DPP-HETs. ................................................. 138

Figure A3.1.
1
H NMR of P3HTT-BTD. ......................................................................... 141

Figure A3.2.
1
H NMR of P3HTT-BTD-HET40%.......................................................... 142

Figure A3.3.
1
H NMR of P3HTT-BTD-HET80%.......................................................... 143

Figure A3.4.
1
H NMR of P3HTT-TPD. ......................................................................... 144

Figure A3.5.
1
H NMR of P3HTT-TPD-HET40%. ......................................................... 145

Figure A3.6.
1
H NMR of P3HTT-TPD-HET80%. ......................................................... 146

Figure A3.7.
1
H NMR of P3HTT-Tz. ............................................................................. 147

Figure A3.8.
1
H NMR of P3HTT-Tz-HET40%. ............................................................ 148

Figure A3.9.
1
H NMR of P3HTT-Tz-HET80%. ............................................................ 149

xiii

ABSTRACT

Bulk-heterojunction (BHJ) polymer:fullerene solar cells have drawn vast interest
for low-cost renewable energy source. Among efforts to improve power conversion
efficiency (PCE) of the solar cells ternary blend has been recently proven to be a
successful strategy to overcome efficiency limitation of traditional single layer binary
blend device while keeping the simple device structure. Unlike binary blend solar cells
which have been studied thoroughly over the decades, the mechanism of ternary blend is
still in debate, especially the compositional open circuit voltage (V
oc
) trend of the ternary
blend system composed of two donor polymers and one acceptor. The systematic study is
required to understand the ternary system in more detail. Contribution of this thesis is
directed to design a set of electroactive polymers that are synthesized to act as a donor in
ternary blend devices.

Chapter 1 reviews the concept and current status of ternary blend bulk
heterojucntion research. The advantage of ternary blend solar cells are explained over
tandem solar cells which is another successful strategy to overcome ultimate efficiency
limit of single layer binary blend solar cells.

Chapter 2 describes synthesis of random and semi-random poly(3-hexylthiophene)
(P3HT) based copolymer containing varying composition of electron withdrawing 3-
hexylesterthiophene (3HET) units. The semi-random polymers synthesized here are

xiv

diketopyrrolopyrrole (DPP) containing polymers. The effect of 3HET unit and content on
optical and electronical properties of the polymer such as UV-Vis absorptions, highest
occupied molecular orbital (HOMO) energy levels, polymer crystallinities, hole
mobilities, and photovoltaic properties are inveistigated in detail. Especially significant
V
oc
enhancement without sacrificing other properties is highlighted.

Chapter 3 discuss how surface energy differences of the two donor polymers are
affecting the V
oc
trend of the ternary blend system. The importance of compatibility
between the two donors has been stressed for compositional tuning of V
oc
in the device.
In order to discuss the effect of surface energy on the V
oc
trend, random and semi-random
polymers with different surface energy has been used to model study designed to
specifically observe effect of surface energy for the first time.

Chapter 4 continues to investigate the effect of 3HET unit on three other semi-
random polymers which contain benzothiadiazole (BTD), thienopyrroledione (TPD), and
thiazolothiazole (Tz). Depending on the composition of 3HET units, some of this
polymers exhibited high V
oc
over 0.8 V which is highest for semi-random polymers
known so far. Detailed characterization revealed that these polymers shows suitable
properties for ternary blend system consist of poly(3-hexylthiophene-thiophene-
diketopyrrolopyrrole) (P3HTT-DPP) and Phenyl-C
61
-butyric acid methyl ester (PC
61
BM).

1

Chapter 1. Polymer-based Ternary Blend Solar Cells

1.1 Introduction

1.1.1 Introduction: What is polymer-based solar cell?

Nature offers many forms of inexhaustible energy sources such as wind, tide,
solar, geothermal. Among them, most promising candidate in terms of the amount of total
energy readily available is definitely solar energy. A rough estimation is that one hour of
solar radiation falls on earth (~14 TW) is more than energy used by the entire population
in one year. However, the consumption of solar energy is only a small portion, less than
half percent of the total energy consumed in the United States.
1
Hence, study of solar
energy conversion is not only attractive research field but also potential energy solution
for human society.
Different types of solar cells have been developed depending on material type
used for their active layer. Silicon solar cells made of single crystal silicon are currently
dominating the market with high efficiency exceeding 20%.
2
However, due to the high
cost of processing single crystal silicon and also the bulkiness and heavy weight of the
solar panels, expensive price of the solar panel is acting as a barrier to thrive in the
market. As attempts to lower the cost of the panel, amorphous silicon or III-V group

2

semiconductors, CdTe, Copper Indium Gallium Selenide (CIGS) solar cells were
developed in recent years. These inorganic thin film solar cells offer somewhat cheaper
cost of fabrication compared to the single crystal silicon solar cell and are catching up the
efficiency of the single crystal silicon solar cell. However, although these inorganic solar
cells show great efficiency and stability, the technologies are not free from ultimate
scarcity of the inorganic materials (silicon or indium) as well as high temperature
manufacturing cost.
Other types of solar cells, such as Dye-Sensitized Solar Cells (DSSC), Organic
Photovoltaics (OPVs), hybrid-PV , and Pervoskites utilize organic materials as an active
layer or a part of active layer in order to lower cost of solar cells and fabrication process.
Organic materials shows higher absorption coefficient than inorganic materials so that the
fabrication of the device with thinner film (using less material) is possible. Also, most of
organic materials are solution-proccessible which can further reduce the cost of cell
fabrication steps compared to high vacuum deposition for inorganic materials. Synthetic
viability of organic material is another merit enabling fine tune of the photovoltaic
properties. Although it should be noted that there are several issues to be addressed, such
as low efficiency and stability issues, solar cells using organic material has been an active
field of research and are thought to be the promising way of lowering fabrication cost of
solar cells.
Among organic solar cells, polymer-based OPVs have a unique strategy for
lowering the cost of fabrication. First of all, it is unrivalled from other types of solar cells
which show difficulty in offering roll-to-roll processibility with all-solution method.
3,4

3

Especially, polymer blends are readily available in the form of inks or sprays and also
have flexibility when casted into film. If the OPV is competing with inorganic solar cell
this solution based roll-to-roll process is essential for reducing the cost of the devices and
panels.
Another advantage over traditional inorganic solar cells is the fact that the
resulting solar cell device or panel is light-weighted and flexible. Solar panels are likely
to be installed on the roof-like places where there is no shade made by other
constructions. Lightness and flexibility of polymer solar cells not only paves way to
significantly reduce transport/installation cost of the solar panel but also introduces
possibility of portable, wearable solar device easily. Growing interest in realization of
flexible electronic devices in recent years is also boosting more focus in the field of OPV .
Hence, the focus of this dissertation will be mainly on the polymer-based OPVs,
or polymer solar cells.

1.1.2 Principle: Binary blend BHJ solar cells

Typical polymer solar cells consist of binary blend of polymer donor and fullerene
derivative acceptor sandwiched by two electrodes as shown in Figure 1.1. The cathode
and anode material is carefully chosen to meet the specific energy level and transparency.
Generally, the blend solution is spin-coated on top of poly(3,4-
ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) coated indium tin oxide
(ITO) glass substrate. The morphology of the active layer is a result of kinetically trapped
morphology resulting from evaporation of solvent. The thickness of the active layer is

4

usually around 100nm because of high absorption coefficient and low carrier mobility.
The interpenetrating network of BHJ morphology maximizes the interface between donor
and acceptor enabling exciton to reach the interface easily.

Figure 1.1. Electronic structure of binary blend BHJ device.

The mechanism of the device can be described as four steps; i) Absorption, ii)
Exciton migration, iii) Charge transfer, iv) Charge transport, and v) Charge collection at
the electrodes. Absorption of the photon occurs in either polymer donor phase or acceptor
phase. The absorption of the photon is limited by the bandgap of the material. Only
photons with larger energy than the band gap of the polymer can be absorbed. This gives
ground limitation of the short circuit current density (J
sc
) at the end so that using
optimized bandgap is clearly important for high performance solar cells. Moreover, the
absorption of photon is also depending on the absorption coefficient. For most of polymer
BHJ solar cells, acceptor components are fullerene derivatives which have low absorption

5

coefficient. Therefore, most of the photons are absorbed by polymer donor phase and low
bandgap polymers are dominant in polymer BHJ solar cells. Result of photon absorption,
an exciton, which is strongly bound electron-hole pair, is generated. The excitons in the
organic semiconductor are too tightly bound that the dissociation does not happen by
thermal energy. Therefore, the excitons have to migrate to the donor-acceptor interface in
order to be separated in to holes and electrons. The diffusion length of the exciton is very
limited (~10 nm), which is why only excitons near the interface can reach the interface.
In this regard, BHJ structure is found to be better than bilayer sturucture to maximize the
interface area and form nanometer size domains. Once excitons migrate and arrive at the
donor-acceptor interface, charge separation occurs if the energy offset is sufficient to
dissociate strongly bound excitons. Experimentally, ~0.3 eV energy offset is known to be
required for this process. Finally, separated electron and hole should be transported and
collected to each electrode.

1.1.3 Efficiency of polymer-based solar cells

Power conversion efficiency (PCE) of a photovoltaic device is commonly
assessed by the equation:
PCE = P
out
/ P
in
= (J
sc
ⅹ V
oc
ⅹ FF) / P
in
= (J
m
ⅹ V
m
) / P
in

where P
in
the input power and P
out
is the maximum possible output power. P
out
is easily
determined by current density – voltage (J-V) characteristic shown in Figure 1.2. It is a
product of J
sc
and V
oc
and fill factor (FF) or a product of J
m
and V
m
(current density and
voltage at maximum power point).

6

Figure 1.2. J-V curve of a solar cell device and photovoltaic parameters.

P
in
is more difficult to determine in laboratory setup unless very sophisticated
measurements and calibrations are performed. Generally, J-V characteristics are
conducted under standard solar spectrum AM 1.5G (solar spectrum corresponding to 45
degree and 100 mW/cm
2
light intensity).
5
However, because organic solar cells have
different varying spectral response and there is some discrepancy between Xenon lamp
radiance and actual solar radiance, often Incident Photon-to-Current Efficiecny (IPCE)
measurement are followed. IPCE or External quantum efficiency (EQE) measurements
determine incident power to current efficiency by measuring current at each wavelength
using monochromator. The mismatch factor can be calculated by the equation:

7

where E
ref
(λ) is the reference spectral irradiance; E
S
(λ) is the source spectral irradiance;
S
R
(λ) is the spectral responsivity; and S
T
(λ) is the spectral responsivity of the test cell,
each as a function of wavelength (λ). Spectral responsivities S (λ) for the tested devices
were calculated based on the EQE values, according to the equation:

where the constant term q/hc equals 8.0655 x 10
5
for wavelength in units of meters and
S(λ) in units of AW
-1
. Based on the spectral responsivities S (λ) obtained using the
equation, integrated short-circuit current densities (J
sc,EQE
) can be obtained:

In order to mismatch-correct the efficiencies of the BHJ solar cells, J
sc
were
divided by the M, as defined in equation:
.
Since Tang et al. have first demonstrated 1% efficiency OPV in 1980s
6
, various
strategies to improve efficiency of the polymer solar cells have been widely investigated.
For example, design and synthesis of novel materials for active layers as well as
optimization of device architecture and controlling morphology of the active layers. As a
result, correlation of solar cell parameters and improvement was possible.
In order to improve J
sc
parameter of the efficiency, low bandgap polymers were
synthesized. At first, homopolymers such as P3HT and Poly[2-methoxy-5-(3,7-

8

dimethyloctyloxy)-1,4-phenylene-vinylene)] (MDMO-PPV) were used as donor
polymers at first as shown on Figure 1.3. Later on, majority of polymer donors are made
of alternating copolymer of donor unit and acceptor unit. For example, poly(2,7-
carbazole-alt-4,7-dithienyl-2,1,3-benzothiadiazole) (PCDTBT) is made of carbazole
based electron donating unit and dithenyl benzothiadiazole electron withdrawing unit.
With push-pull driving forces between donor and acceptor unit also stabilizes quinoidal
structure and electron delocalized structure which leads to narrower bandgap by
hybridization of orbitals.

Figure 1.3. Chemical structures of representative donor and acceptor molecules.

Semi-random polymers
7
utilize similar method of copolymerizing electron donor
and acceptor unit but with small portion of acceptor unit incorporation. A randomized

9

sequence distribution of acceptor unit leads to multichromophoric nature of the polymer
broadening the absorption spectrum not just red-shifting the absorption peak position.
Using acceptor molecules with higher absorption coefficient is also a great way to
improve absorption. For example, the substitution of PC
61
BM with PC
71
BM generally
gives higher J
sc
due to improved light absorption
8
in the visible region of solar spectrum.
Also, there are many on-going research on non-fullerene acceptors.
9
However,
controlling absorption properties are not necessarily enough to enhance J
sc
. Other
parameters, such as charge carrier mobility, molecular packing, and packing orientation
affect J
sc
values.

V
oc
of the OPV is mainly governed by energy difference between highest
occupied molecular orbital (HOMO) energy level of the donor and lowerst unoccupied
molecular orbital (LUMO) energy level of the acceptor. Since fullerene derivatives are
generally used as acceptor materials, lowering HOMO level of the polymer donor will
give higher V
oc
. Tuning of the HOMO level can be accomplished by tuning the electronic
properties of the conjugated backbone. For most well studied polymer P3HT, HOMO
level of about -5.0 eV in combination with LUMO level of about -4.3 eV of PC
61
BM
corresponds to V
oc
value at around 0.6 V and sets the reference point for polymer design.
Utilizing copolymer with less electron rich unit with strong acceptor unit give higher V
oc

like PCDTBT (0.89 V)
10
or PFO-DPT (0.95 V)
11
. It should be noted that V
oc
value
above 0.6 V is a distinct number because V
oc
of single crystal silicon solar cell is limited
at about 0.6 V . High V
oc
of OPVs the only parameter which could easily surpass the

10

parameter of the silicon-based inorganic solar cells. To sum up, V
oc
enhancement stems
from selection of appropriate energy levels of donor and acceptor materials. However, it
should be noted that V
oc
value is more accurately determined by energy of charge transfer
(CT) state
12
which contains information on the structures at the donor/acceptor interface
and bimolecular recombination.

FF is the ratio between the maximum possible power and the product of V
oc
and
J
sc
. FF is least understood parameter among the three and is affected by many factors
such as bimolecular recombination, balance of charge carrier mobility, active layer
morphology, and resistances of the devices. In general, utilizing proper interlayers and
optimized morphology of the active layer often leads to high FF and currently surpassing
70% FF value is often reported in many state of the art device cases.

Three key parameters of PCE are important to understand/enhance the efficiency
of the polymer-based solar cells. However, these parameters are closely correlated and
enhancing three values at the same time is difficult.

State of the art efficiency of the single layer binary BHJ solar cell is currently
over 10%
2,13–15
Liu et al. have demonstrated series of polymers (PffBT4T-2OD,
PBTff4T-2OD, PNT4T-2OD) in combination with many kinds of fullerene-based
acceptors and for all of them showed the efficiency of 9-10.8%. By carefully controlling
aggregation behavior of the three donor polymers, nearly ideal morphology was achieved

11

and high FF of up to 77% led the top device performances. PTB7 based polymers
14,16–18

also have been well known to performing high PCE of over 10%. Fine control of
molecular structure of PTB7 polymers and optimization of device architectures and
interfacial layers reduced recombination and very high FF of over 70% is realized. All in
all, there has been tremendous improvement of polymer-based solar cells reaching
efficiency of double digit.

12

1.2. Beyond binary blend solar cells

1.2.1 Efficiency limit of binary blend polymer-based solar cells

Rapid increase of efficiency from 1% to 10% in a few decades endows promising
potential of polymer-based solar cells. However, for the successful commercialization of
the polymer solar cell devices, it is agreed that even higher efficiency is required to
compete with the market dominating inorganic solar cells.
19
A question naturally arises
assessing the estimation of current technology in terms of theoretical limitation of the
devices. The ultimate efficiency limit of the single junction binary blend solar cells has
been predicted from different research groups. Many types of empirical models have been
derived based on the variation of HOMO-LUMO energy levels of donor and acceptor and
practical FF, EQE value gives the ultimate efficiency of 11-15%,
20–22
although more
theoretical model taking into the ideal case brings 20-24%
22,23
of efficiency potentially
achievable. For example, as shown in Figure 1.4, Scharber et al. plotted the PCE of
single layer solar cell as function of absorber bandgap and the LUMO offset in
assumption of 80% EQE and 75% FF. This gave ultimate efficiency limit of 15% if the
bandgap of ~1.5 eV donor polymer was used. The efficiency limit of empirical model
mainly originates from so called “J
sc
- V
oc
compromise” which means that if you would
lower the bandgap of the polymer to enhance J
sc
value of the device by broadening
spectral absorption, the V
oc
of the device would be decreased because of the HOMO
D
-
LUMO
A
gap is decreased. On the contrary, if you would lower the HOMO energy of the

13

polymer to get the maximized V
oc
, the bandgap of the polymer will be larger so that the
low energy light would not be absorbed.

Figure 1.4. Contour plot of PCE with PC
61
BM (LUMO = 4.3eV) as acceptor material.

Therefore, there is not much space left for improving the PCE considering that the
efficiency of the state of the art polymer device is already over 10%. Other novel
approaches are critical to overcome inherent limitation of binary blend single layer
polymer-based solar cells.

1.2.2 Tandem cell structure

Tandem cell structure with two or more absorbing layers is a promising approach
to overcome an efficiency limit of single layer polymer solar cells. By stacking two or
more subcells that have complimentary absorptions, more photons can be harvested. If

14

the subcells are connected in series V
oc
of the total cell is stacked while J
sc
is limited to
the highest sub-cells and if they are connected in parallel
24
, J
sc
approaches to the sum of
subcell J
sc
, while V
oc
is limited to minimum of the subcells. However, in vast majority of
cases, tandem cells refer to stacking of subcells in series as can be seen in Figure 1.5. In
tandem cell sturucture, unlike single junction solar cells, a thin recombination interlayer
should be present between the two subcells where an electron from one subcell and a hole
from the other subcell recombine to cancel out. Also, the interlayer must be optically
transparent so that a photon of low energy which cannot be absorbed by a large bandgap
polymer can pass through and absorbed by the other subcell which have low bandgap
polymer.
Tandem cell structure enables circumvent the problem of two major losses in solar
cells: sub-bandgap transmission and thermalization of the hot charge carriers. Therefore,
the theoretical efficiency limit of the tandem solar cells is higher than single junction
devices. Similar to previous practical modeling anticipates the ultimate efficiency of
tandem cells to be ~30% higher efficiency as compare to single junction solar cells.
21,25

State of the art polymer tandem solar cells are reported recently by Prof. Yang Yang’s
group: 10.6%
26
with double junction and 11.55%
27
with triple junction and over 12% was
reported by Heliatek company.
28

However, tandem structure has some major disadvantages. Materials used for
intermediate layer should meet the high requirements in many kinds of aspects such as
high optical transparency, ohmic contact between the layers, appropriate energy level
alignment, and efficient charge recombination.
29
Also, the material property of interlayer

15

should be robust enough to survive processing of the top subcells. If the whole tandem
structure is manufactured in solution processing, it is extremely difficult to protect
interlayer and the bottom layer from solvents. For many years, different types of
interlayers were developed for polymer tandem cells. However, such high technological
demands would raise the cost of the intermediate layer itself, which is against the goal of
the organic solar cell pursuing for the low cost and higher efficiency at the same time.
Moreover, the optical spacer effect has to be considered for the optimization of
the device performance. Maximum appropriate optical density distribution should be
designed to be focused on each active layer. This varies a lot depending on the active
materials of the subcells as well as the thickness of the relevant layers, which requires
extensive simulation and optimization steps. High requirements for recombination
interlayer cause reproducibility issue as well. Since multiple components and layers are
involved in stacking tandem structure, larger fluctuation of photovoltaic performances is
inevitable.
In sum, tandem structure is a great way to improve efficiency but the complexity
of stacking layers and interlayer lead to higher cost of fabrication of the device.
Considering the goal of polymer solar cells, comprehensive evaluation of cost per
efficiency enhancement is required.

16

Figure 1.5. An example of tandem cell structure connected in series.

1.2.3 Other approaches

A few novel approaches are suggested for efficiency enhancement beyond
limitation of binary blend organic solar cells. Plasmonic enhanced solar cells utilize metal
nanoparticles incorporated into additional layer or active layers to improve light
harvesting.
30,31
An upconversion devices
32
utilize unused sub-threshold photons behind
solar cells creating one high energy photon and radiate back to the active layer so that the
photon can be absorbed in the active layer. However, these strategies often involve
additional high-cost specific metal nanostructure or upconverting dye materials, which
require accurate cost-effectiveness for the efficiency improvement.

17

1.3. Ternary blend BHJ solar cell

1.3.1 Concept

Ternary blend strategy is an approach to overcome efficiency limit of the binary
blend polymer solar cells without sacrificing simplicity of the device structure and
fabrication processing. By blending three components which have three different
absorption range of solar spectrum as shown in Figure 1.6, this approach aim for
maximizing absorption breadth similar to tandem structure. However, device structure of
the ternary blend solar cell is exactly same as binary blend single junction device, only
different in their active layer that it is a mixture of three components consists of two
donors and an acceptor or a donor and two acceptors. Although using three components
in a single layer inevitably adds an additional interaction between the components, there
are significant interests attracted in this type of device recently. Upon using three
components, higher efficiency than that of corresponding binary blend combination was
observed in many cases, which gives great potential to achieve higher than efficiency of
10-12%, theoretical limit estimated by binary blend single junction BHJ solar cells.

18

Figure 1.6. Complementary absorption of ternary blend solar cells

Because a single conjugated polymer donors generally have limited absorption
breadth compared to solar spectrum ranging from UV to VIS-IR and far IR, only one
component of the polymer (D1) is often not sufficiently harvesting wide wavelength
range of photons. As UV region is mainly absorbed by PC
61
BM acceptor (A) phase, by
adding additional complementary absorbing component (D2) to the binary blend system,
whole ternary components cover wide range of solar spectrum leading to possibility of J
sc

improvement.

1.3.2 Enhancement of absorption and V
oc
in ternary blend solar cells

This idea of broadening absorption was originated from adding dye sensitizers to
the binary BHJ system. By adding small amount of near-IR absorbing dyes to the system,

19

J
sc
of the ternary blend was increased significantly compared to the corresponding binary
blend references. Table 1.1. shows representative reports of P3HT:PC
61
BM system with
addition of various small molecule sensitizers (Figure 1.7) as a third component.
Comparing photovoltaic parameters before and after addition of the dye molecule, it can
be well observed that the J
sc
of the ternary device is significantly increased compared to
the control P3HT:PC
61
BM device primarily due to the broadened absorptions.
Interestingly, the optimized dye concentration of the ternary blend device
appeared at very low dye concentration value, less than 10% for most of the cases. It was
also commonly observed that as more and more of the dye molecules added to the system,
the morphology of the host binary system starts to be deteriorated resulting in low FF
values.

Table 1.1. Ternary blend system of small molecule sensitizer added to the P3HT:
PC
61
BM binary blend and change of photovoltaic parameters.
Binary
System
dye dye
conc.
(wt%)
J
sc

change
(
mA/cm
2
)
V
oc
change
(V)
FF
change
PCE
change
(%)
re
f
Yea
r
P3HT:PC
71
BM t-
BuSiNC
8 9.4→11.4 0.60→0.62 0.67→0.63 3.80→4.46
33
2014
P3HT:PC
71
BM SiNC 6 9.4→10.9 0.60→0.59 0.67→0.67 3.80→4.26
33
2014
P3HT:PC
71
BM SiPC 4.8 8.96→10.3 0.55→0.57 0.71→0.69 3.5→4.1
34
2010
P3HT:PC
71
BM SiNC 1.5 8.96→9.94 0.55→0.55 0.71→0.68 3.5→3.7
34
2010
P3HT:PC
71
BM SQ 1 10.3→11.6 0.59→0.60 0.53→0.65 3.27→4.51
35
2013
P3HT:PC
71
BM DPP-CN 8 9.4→12.29 0.66→0.66 0.52→0.58 3.23→4.70
36
2012

20

P3HT:PC
71
BM DIB-SQ 1.2 8.4→9.7 0.57→0.58 0.56→0.66 2.69→3.60
37
2014
P3HT:PC
71
BM TBU-SQ 2.5 9.4→12.6 0.66→0.66 0.56→0.62 3.47→5.15
38
2015
P3HT:PC
71
BM TQTFA 0.5 9.74→10.6
2
0.60→0.69 0.67→0.61 3.90→4.50
39
2010
P3HT:PC
61
BM DMPA-
DTDPP
5 8.27→9.84 0.60→0.60 0.61→0.58 3.02→3.37
40
2012
P3HT:PC
61
BM CPA 10 8.95→9.33 0.58→0.70 0.66→0.65 3.4→4.3
41
2014
P3HT:PC
61
BM DPSQ 5 7.3→8.8 0.59→0.60 0.64→0.64 2.75→3.38
42
2013

Figure 1.7. Various structures of small molecule dyes added to the P3HT:PC
61
BM biary
blend solar cells listed in Table 1.1.

21

Large number and various types of ternary blend systems, either composed of a
polymer donor and two near-IR sensitizer dyes,
33
a polymer and a near-IR sensitizer
polymer,
43
a polymer and a nanoparticle sensitizer,
44
and two polymer donors and a
nanoparticle acceptor
45
have been reported in pursue of increasing absorption breadth and
J
sc
value. In many cases, J
sc
of the ternary blend was increased compared to the
corresponding binary blend references due to added photo-response. However, the
increase of the current seems strongly dependant on specific ratio of the components. In
some cases, the overall amount of dye addition is limited to small amount (less than 10%).
In other cases, content of third component can span the whole composition (0-100%)
without suffering low FF, being a true ternary blend system.

Until not many years ago, it was believed the V
oc
of the ternary blend system is
thought to be pinned to the lowest possible energy associated with HOMO of a donor and
LUMO of an acceptor. As can be seen from energy level diagram in Figure 1.8, V
oc
was
thought to be determined by HOMO
D2
-LUMO
A.
Hence, it was believed that addition of
sensitizer molecule would only enhance J
sc
parameter. Recently, tuning of the V
oc
value
according to the composition of specific blending ratio is continuously reported by
Thompson et al.
46,47
Intermediate V
oc
value openes up possibility of V
oc
enhancement
compared to the corresponding binary blend reference. As increment of J
sc
and V
oc
at the
same time in some cases is a unique phenomenon observed in ternary blend solar cells.

22

Figure 1.8. Energy level diagram of ternary blend consist of D1, D2 and A.

1.3.3 State of the art ternary blend solar cells

State of the art efficiency of ternary blend solar cell is over 10% at the moment
which is close to record efficiency of the binary blend solar cells. As shown in first row
of Table 1.2, Liu et al. have reported avg. 9.9% (10.2% max efficiency) for two polymer
donor ternary blend device.
48
They utilized two high-performance polymers in
expectation of even higher efficiency with the blend. Even though the V
oc
was pinned to
the lowest possible V
oc
, efficiency of the device was improved due to increased
photocurrent and FF. This result demonstrated that even for two highly efficient polymer
donors in binary blend can be further improved by using ternary blend device.

23

Yang et al. demonstrated as high as 8.7% efficiency for ternary blend device.
49
In
their work, they also chose combination of two or more donor polymers from their high-
performance polymer pool and examined the efficiency of the ternary blend solar cell
based on the structural similarity of the donors. The best results came up when the
preferable packing of each polymer is conserved after blending of the two donor
polymers, PTB7 and PBDTT-SeDPP; hence the electronic properties are preserved
contrary to blending of two donor polymers, P3HT and PBDTT-SeDPP, which have
different packing preference. The implication of the report is that the molecular
compatibility of the donor molecule is important for designing successful ternary blend
system. Yu et al. also have demonstrated 8.22% efficiency device with two polymer
donors and PC
71
BM acceptor.
50
In this case, by adding PID2 to the PTB7:PC
71
BM
binary blend system, enhancement efficiency was achieved. The general mechanism of
efficiency enhancement in ternary blend is increasing the absorption of light by adding
the third component. But in addition to this effect, charges were transported better
through polymers and devices. By addition of 10% PID2 to the system, more favorable
fibrillar structure and smaller domain size was induced. Later on, the same group have
reported similar polymer pairs (PTB7-Th and PID2) to achieve even higher efficiency of
9.2% as shown in fourth row of the Table 2. In the case of ternary blend system based on
polymer/small molecule/PC
71
BM, Zhang et al, have reported 8.4% efficiency with
PBDTTPD-HT:BDT-3T-CNCOO:PC
71
BM. At 40% loading of small molecule, the
formation of favorable nanostructures led high FF of 0.71.

24

Table 1.2. Summary of device performance parameters of ternary blend and
corresponding binary blend references.
PTB7-Th:PDBT-T1:PC
71
BM J
sc,
(mA/cm
2
)
V
oc
(V) FF (%) PCE (%)
100:0:110 16.1 0.8 67.9 8.7
80:20:110 17.8 0.81 69.5 9.9
0:100:110 13.8 0.91 72.3 9.2
PTB7:PBTTT-SeDPP:PC
71
BM J
sc,
(mA/cm
2
)
V
oc
(V) FF (%) PCE (%)
1:0:2 15.1 0.72 66.3 7.2
0.5:0.5:2 18.7 0.69 67.4 8.7
0:1:2 16.9 0.68 62.9 7.2
PTB7:PID2:PC
71
BM J
sc,
(mA/cm
2
)
V
oc
(V) FF (%) PCE (%)
1:0:1.5 15 0.72 67.1 7.25
0.9:0.1:1.5 16.8 0.72 68.7 8.22
0:1:1.5 5.27 0.86 44.4 2.01
PTB7-Th:PID2:PC
71
BM J
sc,
(mA/cm
2
)
V
oc
(V) FF (%) PCE (%)
1:0:1.5 14.92 0.75 70.3 7.88
0.9:0.1:1.5 16.68 0.78 70.8 9.20
0:1:1.5 5.29 0.86 44.3 2.01

25

PBDTTPD-HT:BDT-3T-
CNCOO:PC
71
BM
J
sc,
(mA/cm
2
)
V
oc
(V) FF (%) PCE (%)
1:0:1 11.79 0.990 58 6.85
0.6:0.4:1 12.17 0.969 71 8.40
0:1:0.6667 10.11 0.968 73 7.48

From these results, it is demonstrated that careful selection of components could
lead the enhancement of efficiency compared to the corresponding binary blend systems.
Therefore, it opens up the possibility of achieving higher efficiency than the efficiency
limit of the binary blend solar cell system.

1.3.4 Mechanism of ternary blend solar cells

Working mechanism of the ternary blend solar cell device is different from binary
blend due to additional interfaces raised from third component. Until now, the mechanism
is not fully understood so there is not one definitive model explains varying cases of
different blend combinations. However, three plausible mechanisms have been suggested
so far.

(a) Cascade model
This mechanism is suggested by Koppe et al. in 2010 from the study of ternary
blend system consist of P3HT, PCPDTBT, and PC
61
BM.
51
In this model, three

26

components form cascade-like energy level as shown in Figure 1.9(a). such that the
HOMO/LUMO energy level of PCPDTBT is in between the HOMO/LUMO levels of
P3HT and PC
61
BM respectively. The mechanism of the system is described as a third
component sensitizer added to a binary blend solar cell system and position at the
interface between donor and acceptor. Excitons generated at the low bandgap PCPDTBT
are separated at the interface with either P3HT or PC
61
BM. Main characteristic of this
model is that the generated charges are transported through highest or lowest energy level
material. In this case, holes flow toward P3HT domain, the highest HOMO among the
components and electrons flow toward PC
61
BM, the lowest LUMO of the components.
Intuitively, the optimal structural morphology is depicted so that the sensitizer is
positioned in between P3HT and PC
61
BM, facing both domains at the same time. Excess
sensitizers that are present in the middle of donor or acceptor domain may function as
exciton traps, leading to low FF of the ternary blend device.
In this model, V
oc
of the ternary blend solar cell device is expected to remain same
regardless of sensitizer amount because the V
oc
is determined by the difference between
LUMO level of PC
61
BM and highest HOMO energy level of the available polymer. So-
called “pinned” V
oc
is expected even at high composition of sensitizer ratio because the
charges are still collected eventually through polymer with highest HOMO energy level.
However, as previously mentioned, the maximum content of sensitizer without
sacrificing FF is very limited in many cases so observation of V
oc
increase at high
sensitizer content is rare.

27

(b) Parallel model
In this mechanism, electronic states of two polymer donors exist separately and
the excitons generated at each polymer domain migrate to donor/acceptor interface and
charge separation occurs. Through respective donor phase, holes generated at each donor
phase are transported through two percolated pathways formed by each donor polymer
and collected at the anode while electron is collected through common acceptor as
described in Figure 1.9(b). The working principle of this model resembles two separate
binary blend subcells connected in parallel. The important aspect of this model is the
morphology of the ternary blend is that there are individual transporting pathways of each
polymer donors is conserved.

Table 1.3. Photovoltaic parameters of ternary blend device and its sub-cells
Cells Thickness (nm) J
sc
(mA/cm
2
) V
oc
(V) FF PCE (%)
TAZ BHJ cell ~50 6.53 0.73 0.69 3.30
DTBT BHJ cell ~50 5.68 0.81 0.49 2.27
Ternary(0.5:0.5:1) ~100 11.17 0.77 0.62 5.34
Cells Thickness (nm) J
sc
(mA/cm
2
) V
oc
(V) FF PCE (%)
DTffBT BHJ cell ~50 7.86 0.89 0.57 3.97
DTPyT BHJ cell ~50 6.99 0.83 0.56 3.28
Ternary(0.5:0.5:1) ~100 13.47 0.87 0.58 6.83

Yang et al. have demonstrated the parallel-like phenomenon with ternary blend
BHJ solar cell composed of TAZ:DTBT:PC
61
BM and DTffBT:DTPyT:PC
61
BM.
52
They

28

fabricated two binary blend sub-cells with half the thickness of the ternary blend device
and compared to the ternary blend solar cell with same donor ratio as can be seen on
Table 1.3. Addition of photocurrent of each subcell was well matched with the ternary
blend solar cell implying each polymer has contributed separately on the current
generation process. They have also demonstrated that EQE of the ternary blend system
was similar to the sum of those for two sub-cells.
V
oc
value of the Parallel model is expected to be in between of the V
oc
value of the
two binary blend references. This is different from the expectation of abovementioned
cascade model where the V
oc
is pinned to the smallest V
oc
of corresponding binary blend.
Since there is no charge transfer between the two polymers, it is often compared to
parallel circuit model
53,54
and good agreement was observed experimental value.
However, V
oc
difference between the corresponding binary blend device is often small
(<0.2~0.3 V) so that accurate V
oc
trend measurement is often not easy. Also, as V
oc
value
of the ternary blend system is often affected by many other parameters, careful
interpretation should be made.

(c) Organic Alloy model
Organic Alloy model is that two of the components in ternary blend system form
intimate mixture so that the mixture is essentially acting like one alloy component often
observed in the inorganic alloys where the continuous energy levels depending on the
composition is common.
55
Alloy model was first suggested by Thompson et al. from the
study of P3HT: PC
61
BM:Indene-C
60
bis-adduct (ICBA) ternary blend system which have

29

one polymer donor and two fullerene derivative acceptors.
56
Two fullerene-based
acceptors have different LUMO levels by about 0.2 V . As the composition of the two
acceptors (PC
61
BM:ICBA) were continuously changed from 1:0 to 0:1, while overall
ratio of P3HT to acceptor is kept 1:1, the V
oc
of the ternary blend device was increased.
As two acceptor molecules are structurally similar and they are easily intermixable in
molecular level, the model describes that the origin of the tunability in V
oc
is based on the
formation of new LUMO level from organic alloy of acceptors. Similar case was also
reported with two polymer donors and one acceptor.
57
As composition of two structurally
similar polymer donors but different in HOMO energy level changes, the V
oc
of the
ternary blend device was changed accordingly as can be seen in Figure 1.9(c). Street et al.
have measured photocurrent spectral response (PSR) to study the CT state of the ternary
blend to find consistency with formation of an organic alloy.
58
As can be seen in Figure
1.10 the energy of CT state increases continuously in both cases, as amount of low
HOMO material increases.

30

Figure 1.9. Mechanism of ternary blend solar cells based on (a) cascade model, (b)
parallel model, and (c) organic alloy model.

Figure 1.10. PSR study of ternary blend system consist of P3HT:PC
61
BM:ICBA and
P3HTT-DPP-10%:P3HT
75
-co-EHT
25
:PC
61
BM.

Recently, there also has been an attempt to explain V
oc
trend of the ternary blend
in terms of local morphology effect.
59
In the paper, they hypothesized that V
oc
is

31

determined by the local E
CT
state formed by aggregations induced by addition of third
component. Therefore they suggest V
oc
tuning on the order of ~100 meV is induced from
modification of lowest-lying CT states. However, accurate and vast information of CT-
state energy information is required to support the hypothesis.
All in all, complete and detailed understanding of the mechanism for ternary
blend solar cell is still developing. As there are many variables affecting V
oc
of the ternary
blend system, close investigation on the relevant components will help understanding the
complexity of mechanism.

1.3.5 Relationship between two donor pairs for ternary blend solar cell

Not all combination of materials leads to successful ternary blend devices.
J
sc
/PCE enhancement of the ternary blend device is possible when accompanied by
judicious selection of materials, blend ratio, and thickness of the active layer as well as
the appropriate interaction between the components. Generally, an acceptor is fixed to
PC
61
BM so relationship between two polymer donor molecules is important for overall
efficiency enhancement of ternary blend system. Unfavorable interaction between the two
polymers may lead to morphological traps reducing photovoltaic performance. However,
the relationship between two polymer donors is not studied intensively. Recently,
compatibility of the two polymer donors is speculated to be the key factor for the
successful ternary blend system. This compatibility of the two polymers is thought to be
strongly related to structural similarity of the two polymers in terms of molecular
orientation, crystallinity, degree of mixing, surface energy, and so on.

32

(1) Surface energy
Surface energy of the material can be used to define miscibility of the components.
The surface energy is usually measured by contact angle measurements with water and
other solvent drops on the spin-coated films.
Thompson et al. have conducted comparative study on ternary blend BHJ solar
cells of P3HTT-DPP-10%:PCDTBT:PC
61
BM and P3HTT-DPP-10%:P3HT
75
-co-
EHT
25
:PC
61
BM.
60
While the latter showed successful power PCE enhancement and V
oc

tuning depending on the polymer mixing composition, the former showed pinning of the
V
oc
to the lowest possible V
oc
. Here, the hypothesis was suggested that the big difference
in surface energy between the polymers originated from less structural similarity in
P3HTT-DPP-10% and PCDTBT case has affected the result. Surface of PCDTBT
(29.5mN/m) was found to be relatively more different to P3HT
75
-co-EHT
25
(22.1mN/m)
relative to P3HTT-DPP-10% (19.9mN/m). Structure of the polymer PCDTBT was also
more different to P3HT
75
-co-EHT
25
which has 75% 3-hexylthiophene unit in the
backbone. While P3HTT-DPP-10% has semi-random structure with 80% 3-
hexylthiophene in the main chain of PCDTBT has completely different structure of
alternating polymer with no thiophene in the backbone.
Similar comparison study was conducted by Ito et al.
61
on ternary blend of
P3HT:SiPc:PC
61
BM/ICBA system and P3HT:PC
61
BM:ICBA system. While it was found
that the V
oc
of the P3HT:PC
61
BM:ICBA is tuned depending on the composition of
PC
61
BM/ICBA, the V
oc
of the P3HT:SiPc: PC
61
BM was kept to the lowest possible V
oc
up

33

to 45% of SiPc loadings. Because SiPc molecules have an intermediate surface energy
between that of P3HT and fullerenes, SiPc molecules are less likely to mix with PC
61
BM
or ICBA but rather preferentially contact with P3HT.
Brabec et al. also used surface energy parameter to explain performance
difference between ternary blend of P3HT:Si-PCPDTBT:PC
61
BM/ICBA and P3HT:C-
PCPDTBT:PC
61
BM/ICBA.
62
Although two PCPDTBT molecules are very similar in
structure, their surface energy difference was huge (γ
Si-PCPDTBT
= 26.4 ± 2.1 mN m
-1
and
γ
C-PCPDTBT
= 40.5 ± 1.5 mN m
-1
) so that the C-PCPDTBT is more likely to be located near
fullerene domains while Si-PCPDTBT is more mixed to amorphous domain of P3HT.
This study suggests that even a small structural modification of a polymer structure has a
huge impact on the morphology of the ternary blend systems.
Although surface energy is bulk property which is affected by many conditions
such as cystallinity, surface state, and roughness of the film, these results suggest that the
surface energy could be a possible indicator for structural assessment of polymer mixing
compatibility.

(2) Crystallinity
Intimate mixing of two polymers in molecular level is often well indicated by the
conservation of crystallinity/packing of the polymers.

(i) Two crystalline polymers
As P3HT is the most well studied donor polymer, most ternary blend studies

34

include P3HT as one donor. Regioregular P3HT is one of the most crystalline conjugated
polymer and the structure of the P3HT packing structure is well understood. Grazing-
incidence X-ray diffraction (GIXRD) is a great tool to investigate crystallinity. Loo and
Klyabich have recently demonstrated structural evidence of formatting organic alloy
model ternary blend system P3HTT-DPP-10%:P3HT
75
-co-EHT
25
:PC
61
BM by intensive
GIXRD study.
63
They observed d-spacing of the polymer donor is continuously changing
indicating continuous change in nanocrystalline structure possibly from formation of
intimate mixing of the two semi-crystalline polymers in molecular level. Similar results
were found in recent result of ternary blend consist of PTB7-Th:PDBT-T1:PC
71
BM.
48
In
their GIWAXS result, merge of the two π-π stacking peaks and shifting of lamellar (100)
peak depending on the composition is observed. The result suggests that in order to form
successful ternary blend system from two crystalline polymers, it should be accompanied
by the two polymers with highly compatibility.

(ii) Two amorphous polymers
Yao et al. have demonstrated successful ternary blend polymer systems based on
their ladder-type polymers.
64
Due to the out-of-plane side chain structure of the ladder-
type polymers inhibiting packed structure, the polymers demonstrated amorphous
properties. While ternary blend system of three combinations of their polymers
(LP1:LP3, LP3:LP2,: LP2:LP4) all showed increased solar cell efficiency than their
binary blend control devices, P3HT:LP4 combination showed decreased performance
compared with both of binary devices. By studying AFM images of the ternary blend film,

35

it was attributed that the roughness of P3HT:LP4 film was significantly larger than
blending of other ladder-type polymers. The inhomogeneous mixing of the polymer is
thought to be incompatibility of the highly crystalline P3HT and amorphous ladder-type
polymers.

(iii) An amorphous and a crystalline polymer
Hoppe et al have executed a study on ternary blend system consist of an
amorphous and a semi-crystalline polymer with PC
61
BM.
65
Interestingly, two copolymers
that were used for this study have identical backbone structure but only different in side
chains leading to either amorphous or semi-crystalline properties. Due to this similarity in
structure, it is well suited for observing effect of crystallinity on the ternary blend system.
The result was found that performance enhancement of ternary blend solar cell device
was observed by 10% addition of amorphous polymer (AnE-PVba) to the semi-
crystalline polymer (AnE-PV) and PC
61
BM binary system. The increase of the efficiency
was due to the increase of the V
oc
, despite that the fact that AnE-PVba:PC
61
BM did not
showed higher V
oc
than the semi-crystalline counterpart. Thus, it was suggested in this
paper that certain amount of the amorphous polymer somehow leaded to the optimal
morphology of the ternary blend system.
Another study with ternary blend system of P3HT:PCPDTBT:PC
61
BM was
executed by Russell et al.
66
XRD peak showed that the P3HT crystals do not change
significantly with the addition of a small amount of PCPDTBT up to 25% of weight
fraction. They found that small amount of amorphous PCPDTBT led the formation of

36

P3HT fibrillar network imbedded in amorphous mixture of PC
61
BM, PCPDTBT, and
amorphous region of P3HT resulting in enhanced charge transfer. However, further
studies are required for deeper understanding on the morphology of ternary blend system.

(3) Glass transition
Another criterion to evaluate the degree of mixing of the polymer blends is the
glass transition temperature (T
g
) that can be measured with different scanning calorimetry
(DSC) methods. T
g
of the polymer indicates physical phase transition of amorphous
region of the polymer originates from inherent structure of the polymer chain hence each
polymer has different T
g
. In binary or ternary blend that is well-mixed to molecular level,
polymer chains are distributed statistically depending on the composition resulting in
only one T
g
occurs for the system. In partially mixed system, two T
g
shows up but shifted
to each other compared to the two T
g
s from the pure material. In completely immiscible
polymer blends, T
g
will show up in their original position. Although it should be noted
that only T
g
itself cannot be a direct measure of the miscibility of the blend system
because the T
g
peak is resulted only from amorphous domains of the polymers. However,
it is an easy method to comprehend the interaction between the polymers. Unluckily, not
many information on T
g
of conjugated polymers is accumulated.

37

1.4 Conclusion

To summarize, compatibility of the two polymer donors are important in
achieving successful ternary blend system. The study of relationship between the
polymers is actively executed in this field. Lack of comprehension on the mechanism
may be overcome by stacking individual study of polymer donor pairs of ternary blend.
However, more systematic approach dealing with relevant factors of compatibility
between the two polymers is essential. Structural similarity of the two polymers are
important but even a slight structural difference could unexpectedly make huge impact on
the morphology of the system. By reviewing recent results, similar surface energy and
preservation of crystalline structure/packing orientation is suggested as an initial design
rules for organic ternary solar cells. However, further studies with different energy levels,
morphologies, and transport properties need to be examined. A comprehensive
understanding of the relationship between the two polymers in ternary blend systems can
pave the way to successful ternary solar cells of high efficiency.

38

1.5 References

(1) http://www.nrel.gov

(2) Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D. Prog.
Photovolt. Res. Appl. 2013, 21 (1), 1–11.

(3) Krebs, F. C.; Gevorgyan, S. A.; Alstrup, J. J. Mater. Chem. 2009, 19 (30), 5442.

(4) Krebs, F. C.; Tromholt, T.; Jørgensen, M. Nanoscale 2010, 2 (6), 873.

(5) Kroon, R.; Lenes, M.; Hummelen, J. C.; Blom, P. W. M.; de Boer, B. Polym. Rev.
2008, 48 (3), 531–582.

(6) Tang, C. W. Appl. Phys. Lett. 1986, 48 (2), 183.

(7) Burkhart, B.; Khlyabich, P. P.; Cakir Canak, T.; LaJoie, T. W.; Thompson, B. C.
Macromolecules 2011, 44 (6), 1242–1246.

(8) Wienk, M. M.; Kroon, J. M.; Verhees, W. J. H.; Knol, J.; Hummelen, J. C.; van
Hal, P. A.; Janssen, R. A. J. Angew. Chem. Int. Ed. 2003, 42 (29), 3371–3375.

(9) Zhou, E.; Cong, J.; Wei, Q.; Tajima, K.; Yang, C.; Hashimoto, K. Angew. Chem.
2011, 123 (12), 2851–2855.

(10) Blouin, N.; Michaud, A.; Leclerc, M. Adv. Mater. 2007, 19 (17), 2295–2300.

(11) Zhou, Q.; Hou, Q.; Zheng, L.; Deng, X.; Yu, G.; Cao, Y. Appl. Phys. Lett. 2004, 84
(10), 1653.

(12) Graham, K.; Amassian, A.; Lee, R.; Ndjawa, G. O. N.; Erwin, P.; Thompson, M.;
Nordlund, D.; Vandewal, K.; Hoke, E.; Salleo, A.; McGehee, M. SPIE Newsroom
2013.

(13) Liu, Y.; Zhao, J.; Li, Z.; Mu, C.; Ma, W.; Hu, H.; Jiang, K.; Lin, H.; Ade, H.; Yan,
H. Nat. Commun. 2014, 5, 5293.

(14) Liao, S.-H.; Jhuo, H.-J.; Yeh, P.-N.; Cheng, Y.-S.; Li, Y.-L.; Lee, Y.-H.; Sharma,
S.; Chen, S.-A. Sci. Rep. 2014, 4, 6813.

(15) Hu, H.; Jiang, K.; Yang, G.; Liu, J.; Li, Z.; Lin, H.; Liu, Y.; Zhao, J.; Zhang, J.;
Huang, F.; Qu, Y.; Ma, W.; Yan, H. J. Am. Chem. Soc. 2015, 137 (44), 14149–
14157.

39

(16) He, Z.; Zhong, C.; Su, S.; Xu, M.; Wu, H.; Cao, Y. Nat. Photonics 2012, 6 (9),
593–597.

(17) Liao, S.-H.; Jhuo, H.-J.; Cheng, Y.-S.; Chen, S.-A. Adv. Mater. 2013, 25 (34),
4766–4771.

(18) Ye, L.; Zhang, S.; Zhao, W.; Yao, H.; Hou, J. Chem. Mater. 2014, 26 (12), 3603–
3605.

(19) Kalowekamo, J.; Baker, E. Sol. Energy 2009, 83 (8), 1224–1231.

(20) Kotlarski, J. D.; Blom, P. W. M. Appl. Phys. Lett. 2011, 98 (5), 053301.

(21) Scharber, M. C.; Sariciftci, N. S. Prog. Polym. Sci. 2013, 38 (12), 1929–1940.

(22) Janssen, R. A. J.; Nelson, J. Adv. Mater. 2013, 25 (13), 1847–1858.

(23) Koster, L. J. A.; Shaheen, S. E.; Hummelen, J. C. Adv. Energy Mater. 2012, 2 (10),
1246–1253.

(24) Guo, F.; Kubis, P.; Li, N.; Przybilla, T.; Matt, G.; Stubhan, T.; Ameri, T.; Butz, B.;
Spiecker, E.; Forberich, K.; Brabec, C. J. ACS Nano 2014, 8 (12), 12632–12640.

(25) Dennler, G.; Scharber, M. C.; Brabec, C. J. Adv. Mater. 2009, 21 (13), 1323–1338.

(26) You, J.; Dou, L.; Yoshimura, K.; Kato, T.; Ohya, K.; Moriarty, T.; Emery, K.;
Chen, C.-C.; Gao, J.; Li, G.; Yang, Y. Nat. Commun. 2013, 4, 1446.

(27) Chen, C.-C.; Chang, W.-H.; Yoshimura, K.; Ohya, K.; You, J.; Gao, J.; Hong, Z.;
Yang, Y. Adv. Mater. 2014, 26 (32), 5670–5677.

(28) http://www.heliatek.com/

(29) Yuan, Y.; Huang, J.; Li, G. Green 2011, 1 (1).

(30) Gan, Q.; Bartoli, F. J.; Kafafi, Z. H. Adv. Mater. 2013, 25 (17), 2385–2396.

(31) Chuang, M.-K.; Chen, F.-C. ACS Appl. Mater. Interfaces 2015, 7 (13), 7397–7405.

(32) Schulze, T.; Lips, K.; Schmidt, T. W. SPIE Newsroom 2014.

(33) Lim, B.; Bloking, J. T.; Ponec, A.; McGehee, M. D.; Sellinger, A. ACS Appl.
Mater. Interfaces 2014, 6 (9), 6905–6913.

40

(34) Honda, S.; Ohkita, H.; Benten, H.; Ito, S. Chem. Commun. 2010, 46 (35), 6596.

(35) Huang, J.-S.; Goh, T.; Li, X.; Sfeir, M. Y.; Bielinski, E. A.; Tomasulo, S.; Lee, M.
L.; Hazari, N.; Taylor, A. D. Nat. Photonics 2013, 7 (6), 479–485.

(36) Sharma, G. D.; Singh, S. P.; Roy, M. S.; Mikroyannidis, J. A. Org. Electron. 2012,
13 (9), 1756–1762.

(37) An, Q.; Zhang, F.; Li, L.; Wang, J.; Zhang, J.; Zhou, L.; Tang, W. ACS Appl.
Mater. Interfaces 2014, 6 (9), 6537–6544.

(38) Ananda Rao, B.; Sasi Kumar, M.; Sivakumar, G.; Singh, S. P.; Bhanuprakash, K.;
Rao, V. J.; Sharma, G. D. ACS Sustain. Chem. Eng. 2014, 2 (7), 1743–1751.

(39) Huang, J.-H.; Velusamy, M.; Ho, K.-C.; Lin, J.-T.; Chu, C.-W. J. Mater. Chem.
2010, 20 (14), 2820.

(40) Lee, J.; Yun, M. H.; Kim, J.; Kim, J. Y.; Yang, C. Macromol. Rapid Commun.
2012, 33 (2), 140–145.

(41) Ye, L.; Xia, H.; Xiao, Y.; Xu, J.; Miao, Q. RSC Adv 2014, 4 (3), 1087–1092.

(42) Cho, Y. J.; Lee, J. Y.; Chin, B. D.; Forrest, S. R. Org. Electron. 2013, 14 (4),
1081–1085.

(43) Ameri, T.; Min, J.; Li, N.; Machui, F.; Baran, D.; Forster, M.; Schottler, K. J.;
Dolfen, D.; Scherf, U.; Brabec, C. J. Adv. Energy Mater. 2012, 2 (10), 1198–1202.

(44) Yoon, S.; Heo, S. J.; Kim, H. J. Phys. Status Solidi RRL - Rapid Res. Lett. 2013, 7
(8), 534–537.

(45) Kim, H. D.; Ohkita, H.; Benten, H.; Ito, S. ACS Appl. Mater. Interfaces 2014, 6
(20), 17551–17555.

(46) Khlyabich, P. P.; Burkhart, B.; Thompson, B. C. J. Am. Chem. Soc. 2011, 133 (37),
14534–14537.

(47) Khlyabich, P. P.; Burkhart, B.; Thompson, B. C. J. Am. Chem. Soc. 2012, 134 (22),
9074–9077.

(48) Liu, T.; Huo, L.; Sun, X.; Fan, B.; Cai, Y.; Kim, T.; Kim, J. Y.; Choi, H.; Sun, Y.
Adv. Energy Mater. 2015, n/a – n/a.

41

(49) Yang, Y. (Michael); Chen, W.; Dou, L.; Chang, W.-H.; Duan, H.-S.; Bob, B.; Li,
G.; Yang, Y. Nat. Photonics 2015, 9 (3), 190–198.

(50) Lu, L.; Xu, T.; Chen, W.; Landry, E. S.; Yu, L. Nat. Photonics 2014, 8 (9), 716–
722.

(51) Koppe, M.; Egelhaaf, H.-J.; Dennler, G.; Scharber, M. C.; Brabec, C. J.; Schilinsky,
P.; Hoth, C. N. Adv. Funct. Mater. 2010, 20 (2), 338–346.

(52) Yang, L.; Zhou, H.; Price, S. C.; You, W. J. Am. Chem. Soc. 2012, 134 (12), 5432–
5435.

(53) Duck, B. C.; Vaughan, B.; Cooling, N.; Zhou, X.; Holdsworth, J. L.; Wen, L. L.;
Rasmussen, S. C.; Dastoor, P. C.; Belcher, W. J. Sol. Energy Mater. Sol. Cells
2013, 114, 65–70.

(54) Kouijzer, S.; Li, W.; Wienk, M. M.; Janssen, R. A. J. J. Photonics Energy 2014, 5
(1), 057203.

(55) Moses, P. G.; Van de Walle, C. G. Appl. Phys. Lett. 2010, 96 (2), 021908.

(56) Khlyabich, P. P.; Burkhart, B.; Thompson, B. C. J. Am. Chem. Soc. 2011, 133 (37),
14534–14537.

(57) Khlyabich, P. P.; Burkhart, B.; Thompson, B. C. J. Am. Chem. Soc. 2012, 134 (22),
9074–9077.

(58) Street, R. A.; Davies, D.; Khlyabich, P. P.; Burkhart, B.; Thompson, B. C. J. Am.
Chem. Soc. 2013, 135 (3), 986–989.

(59) Mollinger, S. A.; Vandewal, K.; Salleo, A. Adv. Energy Mater. 2015, 5 (23), n/a –
n/a.

(60) Khlyabich, P. P.; Rudenko, A. E.; Street, R. A.; Thompson, B. C. ACS Appl. Mater.
Interfaces 2014, 6 (13), 9913–9919.

(61) Xu, H.; Ohkita, H.; Benten, H.; Ito, S. Jpn. J. Appl. Phys. 2014, 53 (1S), 01AB10.

(62) Ameri, T.; Khoram, P.; Heumüller, T.; Baran, D.; Machui, F.; Troeger, A.; Sgobba,
V.; Guldi, D. M.; Halik, M.; Rathgeber, S.; Scherf, U.; Brabec, C. J. J Mater Chem
A 2014, 2 (45), 19461–19472.

(63) Loo, Y.-L.; Khlyabich, P. P. SPIE Newsroom 2015.

42

(64) Yao, K.; Xu, Y.-X.; Li, F.; Wang, X.; Zhou, L. Adv. Opt. Mater. 2015, 3 (3), 321–
327.

(65) Kästner, C.; Rathgeber, S.; Egbe, D. A. M.; Hoppe, H. J. Mater. Chem. A 2013, 1
(12), 3961.

(66) Gu, Y.; Wang, C.; Liu, F.; Chen, J.; Dyck, O. E.; Duscher, G.; Russell, T. P.
Energy Env. Sci 2014, 7 (11), 3782–3790.

43

Chapter 2. Facile Enhancement of Open Circuit
Voltage of Random and Semi-Random P3HT-Analogs
in Solar Cells via Incorporation of Hexyl Thiophene-3-
Carboxylate

2.1. Introduction

The past fifteen years have witnessed the emergence of numerous conjugated
polymers in pursuit of high performance materials for organic photovoltaics,
1
field-effect
transistors,
2
and electrochromic devices.
3
Specifically, the promise of organic
photovoltaics as a flexible, lightweight, low-cost, and easily processed material has
motivated the steady improvement of these materials.
4,5
Recently, polymer-based single-
junction BHJ solar cells have regularly exhibited PCEs of 9-10%.
6-9
The PCE of an
organic solar cell is defined by η = (J
sc
∙ V
oc
∙ FF)/P
in
where J
sc
is short-circuit current
density,
10
V
oc
is open-circuit voltage,
11
FF is fill factor,
12
and P
in
is solar input power.
Efforts to improve the efficiency typically include reducing the electronic band
gap of the polymer for increased J
sc
or lowering the polymer HOMO for increased V
oc
(or

44

some combination of both), since the V
oc
correlates to the HOMO
DONOR
–
LUMO
ACCEPTOR
offset.
13
As a result, much research has focused on perfectly alternating
donor-acceptor (D/A) copolymers, which allow for very finely-tuned electron-poor
moieties and electron-rich moieties to achieve precise control over the HOMO and
LUMO and thus, V
oc
. While D/A copolymers are a viable strategy, the pursuit of such
finely-controlled structures has led to a number of drawbacks. Achieving each monomer
often requires numerous synthetic steps, which will increase the cost of production.
14
D/A
copolymers also often require additives for optimal performance,
15
which can hasten
degradation and are detrimental to solar cell stability.
16
Furthermore, they often require
high fullerene ratios (even as high as 1:4) for working devices, which limit polymer light
absorption in active layers with confined thicknesses.
4

Despite the increasing complexity of structural design in D/A polymers, the
thiophene unit has steadfastly remained the signature motif of conjugated polymers.
4,17,18

Indeed, the most studied conjugated polymer, regioregular poly(3-hexylthiophene) (rr-
P3HT), overcomes many of these drawbacks typically associated with D/A copolymers.
It is one of the easiest and cheapest conjugated polymers to synthesize (even more so
now with the emergence of direct arylation polymerization (DArP)
19-22
) and is compatible
with roll-to-roll processing.
23
Furthermore, it is semi-crystalline and exhibits excellent
morphology with PC
61
BM at favorable mixing ratios (1:0.8),
24
without the need for
additives or complex device architectures. This combined with its high peak absorption
coefficient, high hole mobility, and impressive charge transfer rate with PC
61
BM,
25
make
P3HT highly attractive; however, these traits do not overshadow its most significant

45

shortcoming. Due to unfavorable positioning of the frontier energy levels, as well as its
wide band gap (E
g
), P3HT’s maximum J
sc
and V
oc
in fullerene blends is restricted, and
cap the achievable efficiency of P3HT solar cells.
26

To combat these drawbacks, our group developed a new polymer architecture
based on rr-P3HT, semi-random rr-P3HT analogs.
27-32
These copolymers feature small
amounts (typically 5-15%) of an acceptor monomer that is dispersed in the polymer chain
via Stille polycondensation. Due to the rational utilization of functional groups, these
polymers exhibit restricted linkage patterns, where discreet acceptor units are distributed
along rr-P3HT backbone. As a result, the absorption is broadened but, importantly, the
most favorable features of rr-P3HT are preserved (high hole mobility, semi-crystallinity,
and miscibility with fullerenes).
28
Despite the considerable array of narrow band gap
polymers that have been explored, there is still a need for wide band gap polymers with a
low lying HOMO.
Until fairly recently, when it came to the design of conjugated polymers, it was
widely believed that the backbone primarily determines the electronic properties.
Attention to the side-chains, which were thought to be necessary only for improved
solubility and processability, was limited.
33,34
Nonetheless, it was acknowledged that side
chains could play an important role in morphology, the most obvious example being the
distinct differences between rr-P3HT and regiorandom P3HT, the latter being
amorphous and inefficient in solar cells.
35,36
The prevelance of alkoxy chains also hinted
at the greater role side-chains could play in electronic properties.
34,37-39
Steadily, the
pronounced influence of side-chains has been increasingly realized as a critically

46

important design strategy for controlling the electronic properties of polymers.
30,33,40,42
In
addition to their ability to influence crystallinity, lamellar stacking distance, glass
transition and melting temperatures, as well as miscibility with fullerenes, side-chains can
also modify the HOMO of the polymer.
40
For example, we reported that the incorporation
of 2-ethylhexyl side chains into rr-P3HT were shown to lower the HOMO of the
resulting polymer but maintain its E
g
.
30
Similarly, incorporation of 3-cyanothiophene into
rr-P3HT successfully lowered the HOMO without sacrificing important properties of rr-
P3HT,
43,44
though the percentage of incorporation was limited to 20% due to reduced
solubility. Recently, we showed polymer properties could be finely tuned by
incorporating 3-hexyloxythiophene units in semi-random polymers.
45

A goal of the present work is to expand the toolkit of side-chains for P3HT
analogs, as side-chains are a modular and facile route to elicit specific HOMOs in
polymers. The alkyl ester functional group has the advantage of being an electron
withdrawing group, lowering the HOMO in the same vein as cyano- and fluoroalkyl side-
chains, but with better solubility in common organic solvents. Here we report the
synthesis of 3-substituted thiophene-based random and semi-random copolymers
containing hexyl thiophene-3-carboxylate, or 3-hexylesterthiophene (3HET), which is
electron withdrawing but can be functionalized with a long alkyl chain for sustained
solubility even at high monomer incorporation. Polymers containing alkylesterthiophene
units have been reported previously, which have higher V
oc
than P3HT in solar cells, but
the reported polymers were perfectly alternating due to synthetic challenges of
regiospecific monomers.
46,47
Pomerantz, et al. have prepared low molecular weight (6

47

kDA) poly(3-alkylthiophenes) via Ullman and Kumada methods.
48-51
For the first time,
we report the preparation of the P3HET Stille monomer, 2-bromo-5-trimethyltin-3-
hexylesterthiophene, which was used as a precursor for Stille polymerizations in random
and semi-random copolymers.

2.2 Experimental Section

2.2.1 Materials and Methods.

The monomers 2-bromo-5-trimethyltin-3-hexylthiophene, 2,5-
bis(trimethyltin)thiophene and 2,5-Diethylhexyl-3,6-bis(5-bromothiophene-2-
yl)pyrrolo[3,4-c]-pyrrole-1,4-dione were synthesized following published procedures.
27,28

All reagents from commercial sources were used as received unless otherwise noted. N-
Bromosuccinimide (NBS) was recrystallized from hot water. Solvents were purchased
from VWR and used without further purification except for THF, which was dried over
sodium/benzophenone before being distilled. All reactions were performed under dry N
2

in glassware that was pre-dried in oven, unless otherwise noted. Flash chromatography
was performed on a Teledyne CombiFlash Rf instrument with RediSep Rf normal phase
disposable columns.
1
H NMR spectra were recorded in CDCl
3
on a Varian Mercury 400
NMR Spectrometer.

48

Number average molecular weight (M
n
) and polydispersity (PDI) were
determined by size exclusion chromatography (SEC) using a Viscotek GPC Max VE
2001 separation module and a Viscotek Model 2501 UV detector, with 70 °C HPLC
grade 1,2-dichlorobenzene (o-DCB) as eluent at a flow rate of 1 mL/min on one 300 ×
7.8 mm TSK-Gel GMHHR-H column (Tosoh Corp). The instrument was calibrated vs
polystyrene standards (1050−3,800,000 g/mol), and data were analyzed using OmniSec
4.6.0 software. Polymer samples for SEC measurements were prepared by dissolving a
polymer in HPLC grade o-DCB at a concentration of 1 mg/mL and then dissolved at
40 °C prior to filtering through a 0.2 μm PTFE filter.
Cyclic voltammetry (CV) was performed on Princeton Applied Research
VersaStat3 potentiostat under the control of VersaStudio Software. A standard three-
electrode cell based on a Pt wire working electrode, a silver wire pseudo reference
electrode (calibrated vs Fc/Fc
+
which is taken as 5.1 eV vs vacuum), and a Pt wire
counter electrode was purged with nitrogen and maintained under a nitrogen atmosphere
during all measurements. Polymer films were made by drop-casting an o-DCB solution
of polymer (10 mg/mL) and tetrabutylammonium hexafluorophosphate (TBAPF
6
) (30
mg/mL) onto the Pt wire and dried under nitrogen prior to measurement. Acetonitrile was
distilled over CaH
2
prior to use, and TBAPF
6
(0.1 M) was used as the supporting
electrolyte.
UV−vis absorption spectra were obtained on a Perkin-Elmer Lambda 950
spectrophotometer. For thin film measurements, polymers were spin-coated onto pre-
cleaned glass slides from 7 mg/mL o-DCB solutions. Thickness of the samples and

49

GIXRD measurements were obtained using Rigaku diffractometer Ultima IV using a Cu
-ray
diffraction mode, respectively.

2.2.2 Synthetic Procedures.

3-hexylesterthiophene (1). Modified from previous literature.
52
To a solution of
thiophene-3-carboxylic acid (1.545 g, 12.06 mmol) in 36 mL dichloromethane (DCM) is
added 4-dimethylaminopyridine (DMAP) (515.7 mg, 4.22 mmol, 0.35 eq.) and 1-hexanol
(2.464g, 24.12 mmol, 2eq.). After about 5-10 min, N,N'-Dicyclohexylcarbodiimide (DCC)
(2.737g, 13.27 mmol, 1.1eq.) was added. The mixture was allowed to stir at room
temperature for 2 days. Precipitated urea is filtered off and the filtrate is subjected to flash
column chromatography with hexane:DCM = 1:1. After vacuum distillation gave the
product as a colorless liquid (2.21g, 10.4 mmol, 86.4%). 1H NMR (400 MHz, CDCl
3
8.10 (dd, 1H), 7.53 (dd, 1H), 7.30 (dd, 1H), 4.27 (t, 2H), 1.74 (m, 2H), 1.42 (m, 2H), 1.34
(m, 4H), 0.90 (t, 3H).
2-bromo-3-hexylesterthiophene (2). Similar to previous literature.
53
Two 3-neck
flasks were flame dried, which connected to dry N
2
line through a drying tube filled with
molecular sieves. To a flask 1, fresh lithium diisopopylamine (LDA) was synthesized by
following steps. Diisopropylamine (DIA) (1.71 mL, 12.203mmol 1.2 eq.) was added and
dissolved in 12 mL THF and the solution was cooled down to -78 °C before adding n-
butyl lithium (7 mL, 11.19 mmol, 1.1 eq.) dropwise. After 5 min, the mixture was heated

50

to 0 °C for 20 min and cooled down backed to -78 °C. In a flask 2, 3-hexylesterthiophene
(2.159 g, 10.17 mmol) is dissolved in 6 mL THF and cooled down to -78 °C. LDA was
transferred from flask 1 to flask 2 through cannula and reacted for about 2 hours at -78 °C.
Carbon tetrabromide (CBr
4
) (3.541 g, 10.68 mmol, 1.05 eq.) dissolved in 5 mL of THF
was added rapidly to the reaction mixture. After about an hour reaction mixture was
heated up to room temperature stirred overnight. The solvent was evaporated under
reduced pressure, the residue was purified by column chromatography with 1:1
hexanes/DCM and vacuum distillation to afford the product as a colorless liquid (1.367
g, 4.69 mmol, 46.2%).
1
H NMR (400 MHz, CDCl
3
): δ 7.37 (d, 1H), 7.22 (d, 1H), 4.28 (t,
2H), 1.75 (m, 2H), 1.44 (m, 2H), 1.34 (m, 4H), 0.90 (t, 3H).
2-bromo-5-trimethyltin-3-hexyesterthiophene (3). To a flame-dried 3-neck
flask, 2-bromo-3-hexylesterthiophene (1.367 g, 4.696 mmol) was dissolved in 2.8 mL
THF and cooled down to -78 °C. 0.61 M solution of 2,2,6,6-
tetramethylpiperidinylmagnesium chloride lithium chloride complex (TMPMgCl• LiCl)
in THF (9.24 mL, 5.635 mmol, 1.2 eq) was added dropwise under N
2
. The mixture was
kept at -78 °C for 3 hr before 1.0 M solution of trimethyltin chloride in hexane (5.635 mL,
5.635 mmol, 1.20 eq) was added slowly. The mixture was allowed to warm up to room
temperature and stirred for overnight. After extraction with diethyl ether and water, the
organic layer was dried over MgSO
4
. The solvent was evaporated under reduced pressure
and the mixture was subjected to column chromatography with 1:1 hexane/DCM and the
product was obtained with trace amount of impurity (starting material). The product
mixture was put in high vacuum and heated at 60-70 °C overnight to remove starting

51

material impurity. Purified product was achieved as a yellowish oil (0.783.1 g, 1.725
mmol, 36.7 %).
1
H NMR (400 MHz, CDCl3): δ 7.41 (s, 1H), 4.28 (t, 2H), 1.76 (m, 2H),
1.45 (m, 2H), 1.34 (m, 4H), 0.90 (t, 3H), 0.39 (s, 9H).
General procedures for Stille Polymerization. All monomers were dissolved in
dry DMF to afford a 0.04 M solution. The solution was then degassed by purging N
2
for
15 min before 0.04 eq of Pd(PPh
3
)
4
(relative to the total moles of all comonomers) was
added in one portion. The solution was degassed for 15 more minutes and then allowed to
stir at 95 °C for 48 h. Then the reaction mixture was cooled briefly and precipitated into
methanol. Purification was achieved through Soxhlet extractions with a sequence of
solvents (methanol, hexane, and chloroform). The last fraction was concentrated under
reduced pressure, precipitated in methanol, vacuum filtered and then dried overnight
under high vacuum.
P3HT
75
-co-3HET
25
. Soxhlet extracted with methanol, hexanes and finally
chloroform. Yield 56%.
1
H NMR (400 MHz, CDCl
3
): δ 7.80 (m, 0.01H), 7.49 (m, 0.13H),
7.36 (m, 0.13H), 6.98 (m, 0.27H), 4.30 (t, 0.25H), 2.80 (t, 0.78H), 1.71 (m, 1.11H), 1.44,
1.34 (m, 3.34H), 0.91 (m, 1.53H).
P3HT
50
-co-3HET
50
. Soxhlet extracted with methanol, hexanes and finally
chloroform. Yield 59%.
1
H NMR (400 MHz, CDCl
3
): δ 7.82 (m, 0.10H), 7.53 (m, 0.17H),
7.37 (m, 0.17H), 7.04 (m, 0.06H), 4.30 (t, 0.50H), 2.80 (t, 0.43H), 1.72, 1.57 (m, 1.46H),
1.40, 1.33 (m, 3.03H), 0.89 (m, 1.38H).
P3HT
25
-co-3HET
75
. Soxhlet extracted with methanol, hexanes and finally
chloroform. Yield 50%.
1
H NMR (400 MHz, CDCl
3
): δ 7.86 (m, 0.23H), 7.56 (m, 0.09H),

52

7.41 (m, 0.09H), 7.00 (m, 0.01H), 4.30 (t, 0.75H), 2.80 (t, 0.18H), 1.74, 1.57 (m, 1.21H),
1.33 (m, 2.09H), 0.89 (m, 1.36H).
P3HET. Soxhlet extracted with methanol, hexanes and finally chloroform. Yield
69%.
1
H NMR (400 MHz, CDCl
3
): δ 7.86 (s, 0.42H), 4.30 (t, 1H), 1.75 (m, 1.02H), 1.39
(m,0.97H), 1.32 (m, 1.96H), 0.89 (m, 1.41H).
P3HTT-3HET-DPP 10%. Soxhlet extracted with methanol, hexanes and finally
chloroform. Yield 98%.
1
H NMR (400 MHz, CDCl
3
): δ 8.92 (m, 0.13H), 7.48 (m, 0.07H),
7.35 (m, 0.07H), 7.12 (m, 0.18H), 6.99 (m, 0.29H), 4.29 (t, 0.12H), 4.05 (m, 0.21H), 2.79
(t, 0.90H), 1.93 (m, 0.14H), 1.70 (m, 1.17H), 1.34 (m, 4.72H), 0.90 (m, 2.50H).
P3HTT-3HET-DPP 20%. Soxhlet extracted with methanol, hexanes and finally
chloroform. Yield 95%.
1
H NMR (400 MHz, CDCl
3
): δ 8.93 (m, 0.12H), 7.80 (m, 0.03H),
7.52 (m, 0.13H), 7.37 (m, 0.12H), 7.15 (m, 0.22H), 7.00 (m, 0.20H), 4.30 (t, 0.25H), 4.06
(m, 0.23H), 2.80 (t, 0.73H), 1.94 (m, 0.14H), 1.71 (m, 1.38H), 1.35 (m, 4.69H), 0.91 (m,
2.41H).
P3HTT-3HET-DPP 40%. Soxhlet extracted with methanol, hexanes and finally
chloroform. Yield 91%.
1
H NMR (400 MHz, CDCl
3
): δ 8.93 (m, 0.11H), 7.82 (m, 0.08H),
7.52 (m, 0.16H), 7.40 (m, 0.16H), 7.16 (m, 0.12H), 4.30 (t, 0.50H), 4.06 (m, 0.22H), 2.80
(t, 0.49H), 1.94 (m, 0.22H), 1.73 (m, 1.23H), 1.34 (m, 4.44H), 0.91 (m, 2.23H).
P3HTT-3HET-DPP 80%. Soxhlet extracted with methanol, hexanes and finally
chloroform. Yield 93%.
1
H NMR (400 MHz, CDCl
3
): δ 8.92 (m, 0.13H), 7.86 (m, 0.29H),
7.68 (m, 0.07H), 7.52 (m, 0.12H), 7.36 (m, 0.11H), 4.30 (t, 1H), 4.06 (m, 0.24H), 1.92 (m,
0.20H), 1.74 (m, 1.07H), 1.32 (m, 4.55H), 0.89 (m, 2.42H).

53

2.2.3 Device fabrication and characterization

All steps of device fabrication and characterization were performed in air. ITO-
coated glass substrates (10 Ω/sq, Thin Film Devices Inc.) were sequentially cleaned by
sonication in detergent, deionized water, tetrachloroethylene, acetone, and isopropyl
alcohol, and dried in a nitrogen stream. About 40 nm of PEDOT:PSS (Baytron R P VP
AI 4083, filtered with a 0.45 μm PVDF syringe filter—Pall Life Sciences) was first spin-
coated on the pre-cleaned ITO-coated glass substrates and annealed at 120 °C for 60 min
under vacuum. Separate solutions of the polymers and PC
61
BM were prepared in o-DCB.
The solutions were stirred for 6 h before they were mixed at the desired ratios and stirred
overnight to form a homogeneous solution. Subsequently, the polymer:PC
61
BM active
layer was spin-coated (filtered with a 0.45 μm PVDF syringe filter—Pall Life Sciences)
on top of the PEDOT:PSS layer. The optimized ratio for random copolymers:PC
61
BM
were 1:0.8 except 1:0.9 for P3HT
50
-co-3HET
50
. For semi-random copolymers with
DPP:PC
61
BM were all 1:1. The concentrations of the blends were 10 mg/ml in polymer
(except 11 mg/ml for P3HTT-DPP, P3HTT-DPP-3HET 10%, P3HTT-DPP-3HET 80%
and 12 mg/ml for P3HTT-DPP-3HET 20%). For optimized conditions, devices of all
the polymers were stored under nitrogen for 20 min after spin-coating and then placed in
the vacuum chamber for aluminum deposition. The substrates were pumped down to high
vacuum (<9×10
-7
Torr) and aluminum (100 nm) was thermally evaporated at 3–6 Å /s
using a Denton Benchtop Turbo IV Coating System onto the active layer through shadow

54

masks to define the active area of the devices as 5.18 mm
2
. The current density–voltage
(J–V) characteristics of the photovoltaic devices were measured under ambient conditions
using a Keithley 2400 source-measurement unit. An Oriel® Sol3A class AAA S11 solar
simulator with a Xenon lamp (450 W) and an AM 1.5G filter was used as the solar
simulator. An Oriel PV reference cell system 91 150 V was used as the reference cell to
calibrate the light intensity of the solar simulator (to 100 mW/cm
2
).
External quantum efficiency measurements were performed using a 300 W Xenon
arc lamp (Newport Oriel), chopped and filtered monochromatic light (250 Hz, 10 nm
FWHM) from a Conerstone 260 1/4 M double grating monochromator (Newport 74125)
together with a light bias lock-in amplifier. A silicon photodiode calibrated at Newport
was utilized as the reference cell.

2.2.4 Mobility Measurements.

Mobility of the polymer was measured using a hole-only device configuration of
ITO/PEDOT:PSS/Polymer/Al in the space charge limited current regime. The devices
preparations were the same as described above for solar cells. The dark current was
measured under ambient conditions. At sufficient potential the mobilities of charges in
the device can be determined by fitting the dark current to the model of SCL current and
described by:

55

where J
SCLC
is the current density, ε
0
is the permittivity of space, ε
R
is the
dielectric constant of the polymer (assumed to be 3), μ is the zero-field mobility of the
majority charge carriers, V is the effective voltage across the device (V = V
applied
– V
bi
–
V
r
), and L is the thickness of the polymer film. The series and contact resistance of the
device (20 – 30 Ω) was measured using a blank (ITO/PEDOT:PSS/Al) configuration and
the voltage drop due to this resistance (V
r
) was subtracted from the applied voltage. The
built-in voltage (V
bi
), which is based on the relative work function difference of the two
electrodes, was also subtracted from the applied voltage. The built-in voltage can be
determined from the transition between the ohmic region and the SCL region and is
found to be about 1 V. Polymer film thicknesses were measured using XRD in the
reflectivity mode.

2.3 Results and discussions

Facile synthesis of 3HET unit is described on Figure 2.1(a).1-Hexanol was
chosen for esterification reaction with thiophene 3-carboxylic acid in order to observe
characteristics of polymers originates only from the ester group not from the length of the
alkyl chain. However, various other kinds of alkyl chains can be easily adopted via
esterifiacaion in high yield. Bromination of 3HET on 2-position is critical for regio-

56

specific monomer in accordance with traditional 3HT monomer brominated at 2-position.
Electron-withdrawing nature of ester group makes traditional bromination using NBS
favoring 5-position bromination. Adopting a method using lithiated diisopropylamine
(LDA) and carbon tetrabromide (CBr
4
) from previous literature, desired 2-brominated
product can be isolated.
54

Figure 2.1. Synthesis of (a) Monomer 2-bromo-5-trimethyltin-3-hexylesterthiophene, (b)
Stille polymerization for random copolymers P3HT-co-3HETs and (c) Stille
polymerization for semi-random copolymers P3HTT-DPP-3HETs. (i) 1-hexanol,
DCC/DMAP, DCM, 48 hrs (ii) -78 °C, LDA, CBr
4
, THF(iii) TMPMgCl∙LiCl, tMeSnCl,
THF (iv) Pd(PPh
4
)
3
, DMF, 48 hrs

57

For stannylation reaction, we empoyed a commercially available solution of
2,2,6,6-tetramethylpiperidinylmagnesium chloride lithium chloride complex
(TMPMgCl∙LiCl) instead of LDA because standard conditions of stannylating the 5-
position of 2-bromo-3-hexylthiophene using LDA fails to translate according to previous
case.
43
Purification of final product could not be executed via vacuum distillation due to
the high boiling point of the material. Instead, the purification was executed through
regular column chromatography, which gave trace amount of starting material identified
by
1
H NMR. We put the product mixture in the high vacuum with heating to evaporate
off the starting material to get pure monomer for polymerizations. It should be noted that
the novel monomer 2-bromo-5-trimethyltin-3-hexylesterthiophene makes possible to
accommodate 3HET unit into conjugated polymers with Stille polymerization which is
known to be the most widely used polymerization method.
Stille polymerization of random and semi-random polymers was executed as
shown in Figure 2.1(b) and (c). The ratio of monomers were held constant as our model
polymers previously reported,
28
except for the 3HT monomer and 3HET which were
exchanged in whole range from all 3HT to all 3HET. The monomer compositions of
random and semi-random copolymers were fairly consistent with the feeding ratio which
can be easily identified by
1
H NMR characteristic peaks at 4.30 and 2.80 ppm.(Figure
A1.4 –A1.13) Multiple peaks at aromatic region may denote complex arrangement and
twisting of backbone arising from the bulky ester group.

58

Table 2.1. Characterization of random copolymers P3HT-co-3HETs and semi-random
copolymers P3HTT-DPP-3HETs.
Polymer Mn (kg/mol) /PDI
a
E
g
(nm/eV)
b
HOMO (eV)
c

P3HT 22.8/2.86 1.92 -5.21
P3HT
75
-co-3HET
25
36.2/2.56 1.90 -5.39
P3HT
50
-co-3HET
50
73.3/2.01 1.90 -5.45
P3HT
25
-co-3HET
75
25.5/2.63 1.94 -5.61
P3HET 11.3/2.72 2.18 -5.95
P3HTT-DPP 10.7/3.36 1.49 -5.26
P3HTT-DPP-HET 10% 12.2/3.78 1.50 -5.27
P3HTT-DPP-HET 20% 15.1/3.52 1.50 -5.31
P3HTT-DPP-HET 40% 17.7/3.76 1.51 -5.35
P3HTT-DPP-HET 80% 21/3.3 1.53 -5.49
a.
Determined by GPC with polystyrene as standard and o-DCB as eluent.
b
. Optical band
gaps from onset of absorption in UV−vis spectra in annealed films.
c.
Determined by
cyclic voltammetry (vs Fc/Fc
+
) in acetonitrile containing 0.1 M TBAPF
6
.

2.3.1 Polymer characterizations.

Molecular weights of the copolymers were analyzed by GPC in o-DCB against
polystyrene standards. Most copolymers have decent molecular weights (above 10,000
g/mol) as shown in Table 2.1. Preservation of monomer reactivity regardless of
functional groups on thiophene-3-position is great advantage in copolymerization which

59

is hardly found in other types of polymerizations such as GRIM polymerization method.
High regioregularity can be retained for all copolymers as evidenced by the integrated
peak areas corresponding to H-T linkages of 3HT at 2.81 ppm and H-H linkages at 2.60
ppm.
UV-vis absorption profiles of P3HT-co-3HET and P3HTT-DPP-3HET
copolymers were obtained in thin films. As shown in Figure 2.2(a) and (b), all random
copolymers upto 75% 3HET content shows similar absorption coefficient and bandgap
compared to their parent polymer P3HT. However, blue shift in absorption onset and
maximum of P3HET homopolymer was observed possibly due to backbone distortion
stems from sterics between bulky ester groups. Interestingly, the vibronic shoulder
55

associated with semicrystallinity and ordering in solid states became more pronounced
for P3HT
75
-co-3HET
25
, P3HT
50
-co-3HET
50
compared to parent polymer P3HT. This
vibronic peak indicates highly ordered structure of π-stacking in the solid states also
observed in similar polymer structure.
56
When 3HET units are incorporated into semi-
random copolymers P3HTT-DPP-3HET, bandgap and the onset of absorption was
similar to their reference polymer, P3HTT-DPP, throughout the whole range of
composition. Interestingly, similar to random copolymer cases, characteristic peak at
around 750 nm were more pronounced as content of 3HET increased indicating ordered
stacking in solid state.

60

Figure 2.2. UV−vis absorption and GIXRD data of (a), (c) random copolymers P3HT-
co-3HETs and (b), (d) semi-random copolymers P3HTT-DPP-3HETs in thin films.

GIXRD data of the polymer films were obtained to study the effect of 3HET unit
on the solid state packing structure. As can be seen on Figure 2.2(c), random polymers
showed strong (100) planes of semi-crystalline peak at around 4.5-5° shifted from P3HT
peak showing at 5.5°. The crystallinity of the polymers was similar to that of P3HT film
as indicated by the similar intensity of the diffraction peak. As more 3HET unit contents
get high, the peak is shifted to low angle, meaning lamellar distance of the polymer is
increased, probably due to the length of the hexylester group being longer than hexyl

61

group. As for semi-random polymers shown on Figure 2.2(d), all the peaks were in
similar intensity and as more 3HET units are incorporated, the diffraction peak shifted to
low angle, meaning the (100) lamellar distance has been increased. Due to the structural
similarity of 3HT and 3HET unit, replacement of 3HT unit with 3HET unit had minimal
effect on the packing of random and semi-random polymers.
HOMO levels of all polymers were measured by CV with ferrocene as reference
and results are summarized in Table 2.1. As strong electron acceptor 3HET unit content
increases, both random and semi-random polymers exhibits lower HOMO level. The
homopolymer P3HET exhibits almost as 600 mV lower than that of P3HT. Similar to
electron donor 3HOT
45
and electron acceptor 3-cyanothiophene
43
unit in our previous
works, 3HET unit had impact on HOMO level of semi-random polymers as well. It
should be emphasized that unlike incorporation of 3-cyanothiophene unit into random
polymers where the content of the unit is limited to 20% due to solubility issue, 3HET
units contain long alkyl chain enabling the whole range of composition possible.
Combined with our previous results of 3HOT random polymer system, HOMO level of
P3HT can be widely tuned from -4.73 eV to -5.93 eV depending on composition of easily
synthesized 3HOT or 3HET monomer units.

62

2.3.2 Device performance

The photovoltaic properties of the random polymers and the semi-random
polymers in BHJ OPV devices were studied in a conventional device configuration of
ITO/PEDOT:PSS/polymer:PC
61
BM/Al. The average parameter of the devices, J
sc
, V
oc
,
FF, and PCE are summarized in Table 2.2. All of the devices are individually
optimized.

Table 2.2. Average photovoltaic parameters and SCLC hole mobilities of random
copolymers P3HT-co-3HETs and semi-random copolymers P3HTT-DPP-3HETs.
Polymer:PC
61
BM J
sc
a

(mA/cm
2
)
V
oc
(V)
FF PCE
a

(%)
Hole Mobility
e

(cm
2
/V ∙s)
P3HT
b
8.67 0.580 0.584 2.94 4.4×10
-4

P3HT
75
-co-3HET
25
b
8.24 0.665 0.508 2.78 2.5×10
-4

P3HT
50
-co-3HET
50
c
9.06 0.736 0.529 3.53 3.4×10
-4

P3HT
25
-co-3HET
75
b
4.14 0.844 0.417 1.46 2.7×10
-5

P3HET
b
2.45 0.922 0.486 1.10 2.8×10
-5

P3HTT-DPP
d
11.88 0.611 0.603 4.38 3.6×10
-4

P3HTT-DPP-3HET 10%
d
11.06 0.617 0.585 3.99 3.6×10
-4

P3HTT-DPP-3HET 20%
d
10.25 0.636 0.566 3.69 2.8×10
-4

P3HTT-DPP-3HET 40%
d
9.61 0.688 0.571 3.78 1.5×10
-4

P3HTT-DPP-3HET 80%
d
7.85 0.758 0.466 2.77 3.2×10
-5

a
Mismatch corrected. The optimized ratio of polymer:PC
61
BM is
b
1:0.8,
c
1:0.9, and
d

1:1.
e
Measured from pristine polymer films in their optimized condition.

Most importantly, V
oc
of the random polymers and semi-random polymers are
linearly increased as component of the strong electron-withdrawing 3HET monomer

63

increases. The increase of charge transfer state energy directed by gap between HOMO
level of polymers and LUMO level of PC
61
BM matched well with tendency of CV
measurement. Clear linear correlation between them confirms that the composition of
3HET monomer in the polymer chain is reflected in the electronic property of the
polymer.
Moreover, the structural similarity between hexyl group and hexylester group
minimizes detrimental effect on the electrical properties of conjugated polymer caused by
replacing 3HT unit by 3HET unit. All polymer devices maintained high J
sc
and FF upto
50% 3HET unit incorporation and decent result for even higher composition while J
sc
and
FF drops steeply in the case of 3-cyanothiophene copolymer devices at higher than 15%
content. Interchangeability of 3HET unit and 3HT unit stems from their long hexyl chains.
Whole compositions of copolymers are well dissolved in o-DCB at room temperature,
which was not observed in 3-cyanothiophene copolymers.

64

Figure 2.3. EQE of (a) random copolymer P3HT-co-3HET:PC
61
BM devices and (b)
semi-random copolymer P3HTT-DPP-3HET:PC
61
BM devices.

EQE measurement was conducted in order to study the photocurrent response of
the solar cell fabricated with random and semi-random copolymers and shown in Figure
2.3. All the polymers showed broad and strong photoresponse which is comparable to
that of the P3HT:PC
61
BM and P3HTT-DPP:PC
61
BM reference devices especially with
low loading (up to 50%) of 3HET unit.
Hole mobilities of the polymers were determined using space-charge limited
current (SCLC) method. The results shown in Table 2.2 again indicates that the hole
mobility of the random and semi-random copolymers are comparable to their parent
polymers up to 50% content, which matches well with the trend in photovoltaic
properties.

65

2.4 Conclusion

Incorporation of 3HET units successfully lowered HOMO level of the random
and semi-random copolymers synthesized from Stille polymerization. As a result, solar
cell device made from the series of polymers and PC
61
BM showed enhanced V
oc
from
0.58 of control polymer without 3HET unit to 0.92 when all 3HT monomers were
replaced with 3HET unit. Structural similarity between 3HT and 3HET enabled whole
range of composition interchangeable without sacrificing other electronic properties. This
result demonstrates that 3HET monomer could be widely applicable to other types of
thiophene-based conjugated polymers when modulation of HOMO level is required for
the polymer-based organic electronics applications.

66

2.5 References

(1) Lu, L.; Zheng, T.; Wu, Q.; Schneider, A. M.; Zhao, D.; Yu, L. Chem. Rev. 2015,
115 (23), 12666–12731.
(2) Sirringhaus, H. Advanced Materials 2014, 26 (9), 1319–1335.
(3) Kerszulis, J. A.; Amb, C. M.; Dyer, A. L.; Reynolds, J. R. Macromolecules 2014,
47 (16), 5462–5469.
(4) Khlyabich, P. P.; Burkhart, B.; Rudenko, A. E.; Thompson, B. C. Polymer 2013, 54
(20), 5267–5298.
(5) Gendron, D.; Leclerc, M. Energy Environ. Sci. 2011, 4 (4), 1225–1237.
(6) Yu, W.; Huang, L.; Yang, D.; Fu, P.; Zhou, L.; Zhang, J.; Li, C. J. Mater. Chem. A
2015, 3 (20), 10660–10665.
(7) Huo, L.; Liu, T.; Sun, X.; Cai, Y .; Heeger, A. J.; Sun, Y . Advanced Materials 2015,
27 (18), 2938–2944.
(8) Liu, Y .; Zhao, J.; Li, Z.; Mu, C.; Ma, W.; Hu, H.; Jiang, K.; Lin, H.; Ade, H.; Yan,
H. Nature Communications 2014, 5, 5293.
(9) Hu, H.; Jiang, K.; Yang, G.; Liu, J.; Li, Z.; Lin, H.; Liu, Y .; Zhao, J.; Zhang, J.;
Huang, F.; Qu, Y .; Ma, W.; Yan, H. Journal of the American Chemical Society 2015, 137
(44), 14149–14157.
(10) Monestier, F.; Simon, J.-J.; Torchio, P.; Escoubas, L.; Flory, F.; Bailly, S.; de
Bettignies, R.; Guillerez, S.; Defranoux, C. Solar Energy Materials and Solar Cells 2007,
91 (5), 405–410.

67

(11) Qi, B.; Wang, J. J. Mater. Chem. 2012, 22 (46), 24315–24325.
(12) Qi, B.; Wang, J. Phys. Chem. Chem. Phys. 2013, 15 (23), 8972–8982.
(13) Clarke, T. M.; Durrant, J. R. Chem. Rev. 2010, 110 (11), 6736–6767.
(14) Liang, Y .; Feng, D.; Wu, Y .; Tsai, S.-T.; Li, G.; Ray, C.; Yu, L. Journal of the
American Chemical Society 2009, 131 (22), 7792–7799.
(15) Pivrikas, A.; Neugebauer, H.; Sariciftci, N. S. Solar Energy 2011, 85 (6), 1226–
1237.
(16) Wang, D. H.; Pron, A.; Leclerc, M.; Heeger, A. J. Adv. Funct. Mater. 2013, 23 (10),
1297–1304.
(17) Cinar, M. E.; Ozturk, T. Chem. Rev. 2015, 115 (9), 3036–3140.
(18) Bian, L.; Zhu, E.; Tang, J.; Tang, W.; Zhang, F. Progress in Polymer Science 2012,
37 (9), 1292–1331.
(19) Rudenko, A. E.; Wiley, C. A.; Tannaci, J. F.; Thompson, B. C. Journal of Polymer
Science Part A: Polymer Chemistry 2013, 51 (12), 2660–2668.
(20) Rudenko, A. E.; Thompson, B. C. J. Polym. Sci. Part A: Polym. Chem. 2015, 53
(2), 135–147.
(21) Rudenko, A. E.; Latif, A. A.; Thompson, B. C. J. Polym. Sci. Part A: Polym. Chem.
2015, 53 (12), 1492–1499.
(22) Wang, Q.; Takita, R.; Kikuzaki, Y .; Ozawa, F. J. Am. Chem. Soc. 2010, 132 (33),
11420–11421.
(23) Krebs, F. C. Solar Energy Materials and Solar Cells 2009, 93 (4), 394–412.
(24) Dang, M. T.; Hirsch, L.; Wantz, G.; Wuest, J. D. Chem. Rev. 2013, 113 (5), 3734–

68

3765.
(25) Bakulin, A. A.; Hummelen, J. C.; Pshenichnikov, M. S.; van Loosdrecht, P. H. M.
Adv. Funct. Mater. 2010, 20 (10), 1653–1660.
(26) Bundgaard, E.; Krebs, F. C. Solar Energy Materials and Solar Cells 2007, 91 (11),
954–985.
(27) Burkhart, B.; Khlyabich, P. P.; Cakir Canak, T.; LaJoie, T. W.; Thompson, B. C.
Macromolecules 2011, 44 (6), 1242–1246.
(28) Khlyabich, P. P.; Burkhart, B.; Ng, C. F.; Thompson, B. C. Macromolecules 2011,
44 (13), 5079–5084.
(29) Burkhart, B.; Khlyabich, P. P.; Thompson, B. C. J. Photon. Energy 2012, 2 (1),
021002–1.
(30) Burkhart, B.; Khlyabich, P. P.; Thompson, B. C. Macromolecules 2012, 45 (9),
3740–3748.
(31) Burkhart, B.; Khlyabich, P. P.; Thompson, B. C. ACS Macro Lett. 2012, 1 (6), 660–
666.
(32) Burkhart, B.; Khlyabich, P. P.; Thompson, B. C. Macromol. Chem. Phys. 2013, 214
(6), 681–690.
(33) Thompson, B. C.; Kim, B. J.; Kavulak, D. F.; Sivula, K.; Mauldin, C.; Fréchet, J.
M. J. Macromolecules 2007, 40 (21), 7425–7428.
(34) He, J.; Su, Z.; Yan, B.; Xiang, L.; Wang, Y . Journal of Macromolecular Science,
Part A 2007, 44 (9), 989–993.
(35) Mauer, R.; Kastler, M.; Laquai, F. Adv. Funct. Mater. 2010, 20 (13), 2085–2092.

69

(36) Stevens, D. M.; Qin, Y .; Hillmyer, M. A.; Frisbie, C. D. J. Phys. Chem. C 2009,
113 (26), 11408–11415.
(37) Koeckelberghs, G.; Vangheluwe, M.; Samyn, C.; Persoons, A.; Verbiest, T.
Macromolecules 2005, 38 (13), 5554–5559.
(38) Koeckelberghs, G.; Vangheluwe, M.; Doorsselaere, K. V .; Robijns, E.; Persoons, A.;
Verbiest, T. Macromol. Rapid Commun. 2006, 27 (22), 1920–1925.
(39) Guo, X.; Ortiz, R. P.; Zheng, Y .; Kim, M.-G.; Zhang, S.; Hu, Y .; Lu, G.; Facchetti,
A.; Marks, T. J. J. Am. Chem. Soc. 2011, 133 (34), 13685–13697.
(40) Mei, J.; Bao, Z. Chem. Mater. 2014, 26 (1), 604–615.
(41) Livi, F.; Zawacka, N. K.; Angmo, D.; Jørgensen, M.; Krebs, F. C.; Bundgaard, E.
Macromolecules 2015, 48 (11), 3481–3492.
(42) Zhang, H.; Ye, L.; Hou, J. Polymer International 2015, 64 (8), 957–962.
(43) Khlyabich, P. P.; Rudenko, A. E.; Thompson, B. C. J. Polym. Sci. Part A: Polym.
Chem. 2014, 52 (8), 1055–1058.
(44) Roth, B.; Rudenko, A. E.; Thompson, B. C.; Krebs, F. C. J. Photon. Energy 2014, 5
(1), 0572051–0572059.
(45) Xu, B.; Noh, S.; Thompson, B. C. Macromolecules 2014, 47 (15), 5029–5039.
(46) Zhang, M.; Guo, X.; Yang, Y .; Zhang, J.; Zhang, Z.-G.; Li, Y . Polymer Chemistry
2011, 2 (12), 2900.
(47) Zhang, M.; Guo, X.; Ma, W.; Ade, H.; Hou, J. Advanced Materials 2014, 26 (33),
5880–5885.
(48) Amarasekara, A. S.; Pomerantz, M. Synthesis 2003, No. 14, 2255–2258.

70

(49) Pomerantz, M.; Cheng, Y .; Kasim, R. K.; Elsenbaumer, R. L. Synthetic Metals
1999, 101 (1–3), 162–165.
(50) Pomerantz, M.; Cheng, Y .; Kasim, R. K.; Elsenbaumer, R. L. Synthetic Metals
1997, 85 (1–3), 1235–1236.
(51) Pomerantz, M.; Yang, H.; Cheng, Y . Macromolecules 1995, 28 (17), 5706–5708.
(52) Neises, B.; Steglich, W. Angew. Chem. Int. Ed. Engl. 1978, 17 (7), 522–524.
(53) Guo, X.; Watson, M. D. Org. Lett. 2008, 10 (23), 5333–5336.
(54) Khlyabich, P. P.; Rudenko, A. E.; Thompson, B. C. Journal of Polymer Science
Part A: Polymer Chemistry 2014, 52 (8), 1055–1058.
(55) Kim, Y .; Cook, S.; Tuladhar, S. M.; Choulis, S. A.; Nelson, J.; Durrant, J. R.;
Bradley, D. D. C.; Giles, M.; McCulloch, I.; Ha, C.-S.; Ree, M. Nature Materials 2006, 5
(3), 197–203.
(56) Murphy, A. R.; Liu, J.; Luscombe, C.; Kavulak, D.; Fréchet, J. M. J.; Kline, R. J.;
McGehee, M. D. Chemistry of Materials 2005, 17 (20), 4892–4899.

71

Chapter 3. Surface Energy Modification for Controlling
Organic Alloy Formation of Two Polymer Donors in
Ternary Blend Solar Cells

3.1 Introduction

As described in Chapter 1, ternary blend solar cells consist of D1:D2:A is an
efficient approach to improve PCE without adding complexity to the structure of the
device. Especially, the tuning of V
oc
between that of D1:A and D2:A endows the
possibility of increase in J
sc
and V
oc
simultaneously which is rarely possible in tandem
cells
1
or other device structures.
2
However, the understanding of ternary blend system has
been premature since the additional factor of interaction between D1 and D2 complicates
the BHJ morphology and electrical state of the ternary blend system. The morphological
studies of ternary blend to date have been limited to individual system which performs
high efficiency in the devices and have been focusing on giving explanation to the
increased PCE.
3,4
Therefore, the two donor polymer combinations (D1-D2) were selected
merely based on their known PCE in binary blend device but not on the structural
relationship between the two polymers. Also, the composition of the third component (D2)
is usually limited to less than 25% of the blend because the higher composition frequently
resulted in low FF and efficiency.
5-7
Although these studies observed that favorable

72

morphology is induced from addition of the third component, explaining the origin for
such change was not sufficient.
3,8

Especially, they lack detailed morphological description of the devices and the
case-by-case V
oc
trend which was pinned to the lowest V
oc
in some cases and was tuned to
the composition of the two donors in other cases. Since V
oc
is closely related to the
HOMO level of the donor and the LUMO level of the fullerene acceptor, compositional
dependence of V
oc
of the device gives critical information on the electrical state of the
D1-D2 blend and thereby the blend morphology of the two donors. Although there are
some models suggested recently
9-11
, clear explanation is not possible due to limited
number of successful ternary blend cases with V
oc
tuning
125
and lack of rigorous model
study.
A leap toward the understanding the mechanism is accomplished by Thompson
and coworkers who introduced “organic alloy model” which suggested that similar
surface energy of two polymers is the origin of tuning of V
oc
. By comparing surface
energy difference of the two polymer donors in ternary blend films consist of P3HTT-
DPP:P3HT
75
-co-EHT
25
:PC
61
BM which are previously known show tuning of V
oc
and
P3HTT-DPP:PCDTBT:PC
61
BM which are known that V
oc
tuning is not observed, it is
suggested that similar surface energies of the two donors ensures intimate mixing of the
two polymers resulting in formation of “organic alloy”. HOMO level of this alloy donor
is determined by the mixing composition which commonly found in inorganic alloy
cases.
12
Later on, it was found that co-crystallization was observed in ternary blend
P3HTT-DPP:P3HT
75
-co-EHT
25
:PC
61
BM case when the two surface energies of the

73

donor are close.
13,14
Continuous shifting of semicrystalline lamellar stacking peak
observed by GIXRD is a strong evidence of co-crystallization and hence mixing of the
two polymer donors to the molecular level. Although co-crystallization is strong evidence
of intimate mixing and hence V
oc
tuning,
15
the observation of V
oc
tuning did not always
coincide with full co-crystallization. Even though ternary blend case of P3HTT-
TPD:P3HTT-DPP:PC
61
BM showed V
oc
tuning, co-crystallization was not observed.
14

The fact that similar surface energy does not always guarantee full co-crystallization
requires deeper explanation of morphological correlation between alloy formation and
co-crystallization.
On the other hand, non-alloying pair P3HT
75
-co-EHT
25
and PCDTBT has very
different nature, one being semi-crystalline, high HOMO energy level, rr-thiophene-
based polymer and the other being carbazole-based, deep HOMO energy level, and
amorphous polymer. Therefore, other than surface energy difference between the two
polymers, many factors could play a role in V
oc
tuning phenomenon. More systematic
comparison is required to confirm the surface energy is the key factor to the tuning of the
V
oc
. Unfortunately, there were few available combinations of polymer donors to study the
effect of surface energy in ternary blend solar cells exhibiting tuned V
oc
so far.
Recently in 2015, our group has introduced a series of 3-hexylthiophene-based
polymers which have surface energy tuned by modifying alkyl side chain of thiophene
with either oligoehter or semifluoroalkyl chains.
16
Depending of the comonomer
composition, surface energy of the 3-substituted thiophene copolymer can be tuned from
14 to 27 mN/m. Since the modification of alkyl chain was limited to the decoupled

74

position from the backbone conjugation by at least two carbon spacers, optical and
electronic properties such as absorption profiles and HOMO energy levels were preserved.
Surface energy tuning without perturbing other properties enables controlling other
variables that could have effect on the compatibility of the two polymers.
Herein, effect of surface energy on the V
oc
trend of ternary blend system is studied
by utilizing surface energy modified P3HTT-DPP-40%MEO. P3HTT-DPP-40%MEO
is same as P3HTT-DPP but has 40% of the 3HT monomer substituted with thiophene
with oligoether chain in order to make the polymer more hydrophilic. Figure 3.1 shows
chemical structures of P3HTT-DPP and P3HTT-DPP-40%MEO chosen as our model
polymers for our study. Although 40% of the side chains the only difference between the
two polymers, existence of hydrophilic oligoether chains endows significantly higher
surface energy (25 vs 21 mN/m) while exhibiting very similar electronic and optical
properties as expected. We will use these polymers as donor 1 (D1) for our ternary blend
model system to understand whether the surface energy difference is the origin of the
tuning of V
oc
in ternary blend devices. As for donor 2 (D2), P3HT
75
-co-EHT
25
and
P3HT
50
-co-HET
50
whose structure is shown in Figure 3.1 were chosen because the
former is already known, and the latter is newly discovered to have V
oc
tuning property in
the ternary device paired with P3HTT-DPP. Also these polymers show very similar
surface energy as P3HTT-DPP, semicrystallinity, good photovoltaic properties, and yet
deeper HOMO level energy.

75

S
S
S
N
N
S
O
O
k
m
l
k=80%
l=10%
m=10%
S
S S
S
N
N
S
O
O
O
O
k l
m
n
k=40%
l=40%
m=10%
n=10%

P3HTT-DPP P3HTT-DPP-40%MEO
S
S
m n
m=75%
n=25%
P3HT
75
-co-EHT
25
S
S
m n
O
m=50%
n=50%
O
P3HT
50
-co-HET
50

Figure 3.1. Structures of the polymers used in model system.

Table 3.1. The material combination of ternary blend model system studied in this
chapter.

Table 3.1. shows the material combination of ternary blend model system A1, A2,
B1, and B2 that will be used for comparison study in this chapter. System A1 and A2, B1
and B2 only differs in donor 1 which have either regular P3HTT-DPP or more
System D1 D2 A
A1 P3HTT-DPP P3HT
75
-co-EHT
25
PC
61
BM
A2 P3HTT-DPP-40%MEO P3HT
75
-co-EHT
25
PC
61
BM
B1 P3HTT-DPP P3HT
50
-co-3HET
50
PC
61
BM
B2 P3HTT-DPP-40%MEO P3HT
50
-co-3HET
50
PC
61
BM

76

hydrophilic P3HTT-DPP-40%MEO. Through comparing A1 and A2, B1 and B2, we
investigate the effect on V
oc
trend when one of the donors, P3HTT-DPP is replaced with
more hydrophilic P3HTT-DPP-MEO40%. This will control other structural factors of
the polymer, giving clearer picture on effect of surface energy on ternary blend system.

3.2 Results and discussions

In Table 3.2, optical and electrical properties of each polymers used in the model
system is shown. Synthesis and other properties can be found in previous literatures.
17,18

First thing to notice is that most of the properties of P3HTT-DPP is similar as P3HTT-
DPP-40%MEO except surface energy. This is again expected because the only
difference is the 40% of monomer has more polar side chain with ethylene oxide moiety.
These characteristics make good control of other factors that might affect the V
oc
of the
device and validates our comparison study. P3HT
75
-co-EHT
25
and P3HT
50
-co-HET
50

polymers have deeper HOMO energy level than the DPP polymers, complimentary
absorption peak, and similar surface energy which makes good candidate as secondary
donor (D2) in ternary blend system.

77

Table 3.2. Optical and electrical properties of polymers.
P3HTT-
DPP
P3HTT-DPP-
40%MEO
P3HT
75
-co-
EHT
25

P3HT
50
-co-
HET
50

HOMO energy level -5.26 eV -5.14 eV -5.43 eV -5.61 eV
E
g
(optical) 1.49 eV 1.49 eV 1.9 eV 1.94 eV
Hole mobility 3.6×10
-4
cm
2
V
-1
s
-1

2.04×10
-4
cm
2
V
-1
s
-1

1.4×10
-4
cm
2
V
-1
s
-1

3.4×10
-4
cm
2
V
-1
s
-1

V
oc
of BHJ device
vs. PC
61
BM
0.61 V 0.60 V 0.69 V 0.73 V
Surface energy 21 mN/m 25 mN/m 22 mN/m 22 mN/m

Table 3.3. shows photovoltaic device parameters of two ternary blend system A1:
(P3HTT-DPP:P3HT
75
-co-EHT
25
:PC
61
BM) and A2: (P3HTT-DPP-MEO40%:P3HT
75
-
co-EHT
25
:PC
61
BM) with five different composition of the two donor polymers varying
from 100:0 to 0:100. Overall donor to acceptor ratios are optimized individually to get
the highest efficiency and reasonable FF. The V
oc
of the end points are essentially same
because P3HTT-DPP and P3HTT-DPP-MEO40% has same HOMO level energy.
Major difference is that the V
oc
trend of the two systems that while A1 shows
compositional dependent V
oc
, A2 shows pinning of the V
oc
(or turning off tuning voltage)
to the smaller possible value which is similar to the V
oc
value from P3HTT-DPP-
MEO40%:PC
61
BM binary blend device. This pinning V
oc
phenomenon can be explained
by the two separate CT states and hole transport is dominated by higher HOMO energy
level of the two polymer donors. Thus, severe phase separation of the two polymer
donors can be inferred from this pinning V
oc
trend.
The impact of surface energy on the location of third component has been
systematically studied by Ito et al. for the case of dye molecule in the polymer blend of

78

P3HT and Polystyrene (PS).
19
By modifying substituents of silicon phthalocyanine dyes,
location of three dyes with different surface energies were traced within the blend of
regiorandom(rra)-P3HT (surface energy = 21 mN/m) and Polystyrene (PS) (surface
energy = 26 mN/m) which has comparable surface energy of PC
61
BM (surface energy =
29 mN/m). As a result, a dye molecule with low surface energy turned out to be located
within the low surface energy rra-P3HT phase while a dye with a high surface energy
belonged to high surface energy PS medium. Although behavior of the polymer may
different from the small molecule covered in this paper, similar behavior of polymers can
be implicated for the overall morphology of the ternary blend system.
Interesting V
oc
value can be observed for the case of P3HTT-DPP-
40%MEO:P3HT
75
-co-EHT
25
:PC
61
BM=0.2:0.8:0.8, where the V
oc
of the ternary blend
device is lower than both end points. This low V
oc
point may be induced from
unoptimized device condition or so-called ‘trapping effect’ which was previously found
in P3HT/OXCMA/OXCTA ternary blend case.
20
Bumjoon Kim et al. have studied effect
of fullerene tris-adducts on the performance of P3HT/OXCMA/OXCTA ternary blend
system and found out the V
oc
trend was bent as composition of OXCTA was increased in
the acceptor blend and some of the V
oc
values at high OXCTA fraction (from 0.8 to 0.9)
was lower than that of P3HT/OXCMA binary blend. They have concluded that there
exist unfavorable morphology that OXCMA prevents efficient charge transfer and acting
as electron trapping site. Similar V
oc
trend is can be observed in Figure 3.2 below.

79

0.0 0.2 0.4 0.6 0.8 1.0
0.56
0.58
0.60
0.62
0.64
0.66
0.68

Voc, V
Fraction of P3HT
75
-co-EHT
25
in donor blend
A1
A2
(a)

Figure 3.2. (a) Comparison of V
oc
trend in system A1 and A2. (b) Comparison of V
oc

trend in P3HT/OXCMA/OXCBA and P3HT/OXCMA/OXCTA ternary blend system
from ref.20.

Table 3.3. Photovoltaic device parameters of ternary blend A1(P3HTT-DPP:P3HT
75
-co-
EHT
25
:PC
61
BM) and A2(P3HTT-DPP-40%MEO:P3HT
75
-co-EHT
25
:PC
61
BM) with
several different composition of D1:D2.
A1: P3HTT-DPP:P3HT
75
-co-
EHT
25
:PC
61
BM
J
sc
a

(mA/cm
2
)
V
oc
(V) FF PCE
a

(%)
1:0:1 12.24 0.617 0.580 4.38
0.8:0.2:0.8

9.58 0.642 0.573 3.57
0.5:0.5:0.8

9.26 0.647 0.580 3.47
0.2:0.8:0.8

8.61 0.652 0.523 2.94
0:1:0.8

7.04 0.680 0.516 2.47
A2: P3HTT-DPP-40%:P3HT
75
-
co-EHT
25
:PC
61
BM

J
sc
a

(mA/cm
2
)
V
oc
(V) FF PCE
a

(%)
1:0:1 9.42 0.596 0.481 2.70
0.8:0.2:0.8

9.12 0.606 0.488 2.70
0.5:0.5:0.8

8.27 0.603 0.441 2.20
0.2:0.8:0.8

7.94 0.576 0.479 2.19
0:1:0.8

7.04 0.680 0.516 2.47
a
Mismatch corrected

GIXRD measurement data of the ternary blend film comparing ternary blend
system A1 and A2 is shown in Figure 3.3(a) and (c). These films were processed in
exact same condition as for fabricating solar cell devices. The peak shown in the graph

80

corresponds to the (100) direction of the polymer crystal lattice, proving all the polymer
blends retain some level of crystallinity even blended with PC
61
BM. The appearance of
single peak from two different polymers could be the overlap of two close peaks from
each polymer or the evidence of co-crystal formation. Figure 3.3(b) plots the maximum
peak position according to the composition of P3HT
75
-co-EHT
25
. On ternary blend
system A1, it is clearly observed that the peak positions are gradually shifted with
composition, which is good evidence of the two polymer donors interact well to form co-
crystals, resulting in changes in d-spacing of the polymer crystallites. This result matches
well with previous study of more detailed study of GIXRD measurement in ternary blend
system.

81

0 20 40 60 80 100
5.1
5.2
5.3
5.4
5.5
5.6
b)
Two theta, degree
P3HTT-DPP composition, %

0 20 40 60 80 100
5.1
5.2
5.3
5.4
5.5
5.6
d)
P3HTT-DPP-40%MEO composition, %
Two theta, degree

Figure 3.3. GIXRD result of ternary blend system (a) A1 and (c) A2 and corresponding
plot of maximum peak position depending on the composition for ternary system (b) A1
and (d) A2.

Interestingly, however, it is observed that the peak positions remain same even
with increasing composition of secondary donor on ternary blend system A2, as shown in
Figure 3.3(d). Although the peak intensity of P3HTT-DPP-MEO40% is low and may
not be identified at a low level of composition, the peak position of P3HTT-DPP-
MEO40% is clearly different from P3HT
75
-co-EHT
25
. This suggests that the (100)
lattice is not perturbed by addition of the other donor component meaning little
interaction exist between the two donor polymers.
3 4 5 6 7 8
0
1000
2000
3000
4000
5000
Intensity, a.u.
Two theta, degree
P3HTT-DPP:P3HT
75
-co-EHT
25
:PCBM
100:0:100
80:20:80
50:50:80
20:80:80
0:100:80
a)
3 4 5 6 7 8
0
1000
2000
3000
4000
5000
6000
c)
P3HT
75
-co-EHT
25
:P3HTT-DPP-40% MEO:PCBM
100:0:100
80:20:80
50:50:80
20:80:80
0:100:80
Two theta, degree
Intensity, a.u.

82

To strengthen our view, same comparison study was conducted on ternary blend
system B1 and B2. As anticipated from the properties of Table 3.2, P3HT
50
-co-HET
50

has very similar properties with P3HT
75
-co-EHT
25
used in model system A1 and A2.
Table 3.4 summarizes photovoltaic device results of the two ternary blend systems.
Again, the V
oc
of the system B1 is identified to be tuned according to the composition of
the two donor polymers while B2 is pinned to the lowest possible V
oc
. Unlike A2 system
which showed unusual trapping effect of V
oc
trend, B2 system showed clear pinning of
the V
oc
throughout the composition. Similar reasoning can be suggested to the V
oc

behavior of the B1/B2 contrast. The modification of side chain changed the surface
energy of the P3HTT-DPP and hence phase separation between P3HT
50
-co-HET
50
and
P3HTT-DPP-40%MEO resulted in pinned V
oc
determined by HOMO of P3HTT-DPP-
40%MEO and PC
61
BM.

Table 3.4. Photovoltaic device parameters of ternary blend B1 (P3HTT-DPP:P3HT
50
-
co-HET
50
:PC
61
BM) and B2 (P3HTT-DPP-40%MEO:P3HT
50
-co-HET
50
:PC
61
BM)
with several different composition of D1:D2.
P3HTT-DPP:P3HT
50
-co-
HET
50
:PC
61
BM
J
sc
a

(mA/cm
2
)
V
oc
(V) FF PCE
a

(%)
1:0:1 12.24 0.617 0.580 4.38
0.8:0.2:0.9

9.87 0.640 0.600 3.79
0.5:0.5:0.9

9.37 0.653 0.608 3.72
0.2:0.8:0.9

7.84 0.664 0.545 2.78
0.1:0.9:0.9 7.25 0.676 0.494 2.42
0:1:0.9

9.06 0.736 0.529 3.53
P3HTT-DPP-40%MEO:P3HT
50
-
co-HET
50
:PC
61
BM

J
sc
a

(mA/cm
2
)
V
oc
(V) FF PCE
a

(%)
1:0:1 9.42 0.596 0.481 2.70
0.8:0.2:0.9

8.93 0.601 0.467 2.51
0.5:0.5:0.9

8.45 0.600 0.501 2.54
0.2:0.8:0.9

7.67 0.609 0.483 2.25
0:1:0.9

9.06 0.736 0.529 3.53
a
Mismatch corrected

83

GIXRD measurement data of the ternary blend film comparing ternary blend
system B1 and B2 is shown in Figure 3.4(a) and (c). Here, the (100) reflection peak
from both P3HT
50
-co-HET
50
and P3HTT-DPP/P3HTT-DPP-40%MEO can be
identified throughout the composition unlike the case of A1 or A2. Separate peaks
suggest that there is lack of full co-crystallization. Tracking the maximum position of
each peaks are plotted in Figure 3.4(b), 4(d) that the peaks are somewhat shifting
towards each other may indicate minor degree of co-crystallization but compared to the
A1 system, clear difference can be observed. Although peak positions are hard to identify
due to low intensity on system B2, shifting of P3HTT-DPP40%MEO peak position is
even smaller than that of P3HTT-DPP in B1 which indicates none of co-crystallization
occurs. Although full co-crystallization was not found in both of the system B1 and B2,
surface energy difference played a critical role in turning on and off of V
oc
tuning in the
ternary blend device. Lack of full co-crystallization even with organic alloy pair was
already found in previous study.
14
Since mixing phase (amorphous region) plays an
important role in V
oc
of the polymer:fullerene binary blend,
21,22
more rigorous study
focusing on the amorphous region of polymer blend is also important.

84

0 20 40 60 80 100
4.4
4.6
4.8
5.0
5.2
5.4
5.6
P3HTT-DPP composition, %
Two theta, degree
Max peak P3HT
50
-co-HET
50
Max peak P3HTT-DPP
4.4
4.6
4.8
5.0
5.2
5.4
5.6
b)
Two theta, degree

0 20 40 60 80 100
4.4
4.6
4.8
5.0
5.2
5.4
5.6
Max peak P3HT
50
-co-HET
50
Max peak P3HTT-DPP-40%MEO
Two theta, degree
Two theta, degree
P3HTT-DPP-40%MEO composition, %
4.4
4.6
4.8
5.0
5.2
5.4
5.6
d)

Figure 3.4. GIXRD result of ternary blend system (a) B1 and (c) B2 and corresponding
plot of maximum peak position depending on the composition for ternary system (b) B1
and (d) B2.

Figure 3.5(a) and (c), 3.6(a), and 3.6(c) shows the external quantum efficiency
results from ternary blend system A1, A2, B1, and B2. For all of the ternary blend system,
photocurrent response beyond 700 nm are from corresponding low band gap DPP-based
polymer (D1) and is clearly showing that the response matches well with the composition
of the two donors. The results indicate that whether the V
oc
of ternary system is tuned or
not, exciton generated from each polymer phase efficiently contributes to the current.
3 4 5 6 7 8
0
1000
2000
3000
4000
5000
6000
7000
8000
a)
Intensity, a.u.
P3HTT-DPP:P3HT
50
-co-HET
50
:PCBM
100:0:100
80:20:80
50:50:80
20:80:80
0:100:80
Two theta, degree
3 4 5 6 7 8
0
1000
2000
3000
4000
5000
6000
7000
c)
Intensity, a.u.
Two theta, degree
P3HTT-DPP-40EO:P3HT
50
-co-HET
50
:PCBM
100:0:100
80:20:80
50:50:80
20:80:80
0:100:80

85

UV-Vis results for ternary blend system A1, A2, B1, and B2 are also shown in Figure
3.5(b) and (d), 3.6(b), and 6(d). The films were fabricated exact same conditions as the
solar cell device. No distinct change has been noticed with or without V
oc
tuning,
indicating that absorption of the films is not affected by phase separation and is still a
linear combination of the absorption of the two materials.

400 500 600 700 800 900 1000
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
b)
Intensity, a.u.
Wavelength, nm
P3HTT-DPP:P3HT75-co-EHT25:PCBM
1:0:1
0.8:0.2:0.8
0.5:0.5:0.8
0.2:0.8:0.8
0:1:0.8

400 500 600 700 800 900 1000
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
d)
Intensity, a.u.
Wavelength, nm
P3HTT-DPP40EO:P3HT75-co-EHT25:PCBM
100:0:100
80:20:80
50:50:80
20:80:80
0:100:80

Figure 3.5. (a) EQE, (b) UV-Vis data of the ternary blend system A1 and (c) EQE, (d)
UV-Vis data of the ternary blend system A2.

400 500 600 700 800 900 1000
0
10
20
30
40
50
60
70
80
a)
External quatumefficiency, %
Wavelength, nm
P3HTT-DPP:P3HT75-co-EHT25:PCBM
100:0:100
80:20:80
50:50:80
20:80:80
0:100:80
400 500 600 700 800 900 1000
0
10
20
30
40
50
60
70
80
c)
P3HTT-DPP-40% MEO:P3HT75-co-EHT25:PCBM
100:0:100
80:20:80
50:50:80
20:80:80
0:100:80
Wavelength, nm
External quatumefficiency, %

86

400 500 600 700 800 900 1000
0
10
20
30
40
50
60
70
80
a)
External quatum efficiency, %
Wavelength, nm
P3HT
50
-co-HET
50
:P3HTT-DPP:PCBM
100:0:100
80:20:80
50:50:80
20:80:80
0:100:80

400 500 600 700 800 900 1000
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
b)
Intensity, a.u.
Wavelength, nm
P3HTT-DPP:P3HT
50
-co-HET
50
:PCBM
1:0:1
0.8:0.2:0.8
0.5:0.5:0.8
0.2:0.8:0.8
0:1:0.8

400 500 600 700 800 900 1000
0
10
20
30
40
50
60
70
80
c)
Wavelength, nm
External quatum efficiency, %
P3HT50-co-HET50:P3HTT-DPP40% MEO:PCBM
100:0:100
80:20:80
50:50:80
20:80:80
0:100:80
400 500 600 700 800 900 1000
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
d)
Intensity, a.u.
Wavelength, nm
P3HTT-DPP40% MEO:P3HT
50
-co-HET
50
:PCBM
1:0:1
0.8:0.2:0.8
0.5:0.5:0.8
0.2:0.8:0.8
0:1:0.8

Figure 3.6. (a) EQE, (b) UV-Vis data of the ternary blend system B1 and (c) EQE, (d)
UV-Vis data of the ternary blend system B2.

3.3 Conclusion

By utilizing semi-random polymer P3HTT-DPP and P3HTT-DPP-40%MEO
which are only different in more hydrophilic ethylene oxide moiety in the side chain, we
could turn on/off the tuning of the V
oc
in the ternary blend systems. By comparing with
the two ternary system cases of A1, A2 and B1, B2, it can be generalized that the surface

87

energy is one parameter that controls compatibility hence V
oc
tuning of the donor polymer
blend in the active layer. In A1 and A2 case, co-crystallization coincide with V
oc
tuning
supported by shifting peak position of lamellar lattice peak but B1 and B2 did not show
co-crystallization effect. About 3 mN/m of surface energy difference seems enough to
induce molecular scale phase separation. Hence, it is suggested that careful selection of
the two donors with very close surface energy is required for observing V
oc
tuning in
ternary blend solar cell devices.

88

3.4 References

(1) You, J.; Dou, L.; Hong, Z.; Li, G.; Yang, Y. Prog. Polym. Sci. 2013, 38 (12),
1909–1928.

(2) Etxebarria, I.; Furlan, A.; Ajuria, J.; Fecher, F. W.; Voigt, M.; Brabec, C. J.; Wienk,
M. M.; Slooff, L.; Veenstra, S.; Gilot, J.; Pacios, R. Sol. Energy Mater. Sol. Cells 2014,
130, 495–504.

(3) Lu, L.; Xu, T.; Chen, W.; Landry, E. S.; Yu, L. Nat. Photonics 2014, 8 (9), 716–
722.

(4) Lu, L.; Chen, W.; Xu, T.; Yu, L. Nat. Commun. 2015, 6, 7327.

(5) Gu, Y.; Wang, C.; Liu, F.; Chen, J.; Dyck, O. E.; Duscher, G.; Russell, T. P.
Energy Env. Sci 2014, 7 (11), 3782–3790.

(6) Benten, H.; Nishida, T.; Mori, D.; Xu, H.; Ohkita, H.; Ito, S. Energy Env. Sci 2016,
9 (1), 135–140.

(7) Lin, R.; Wright, M.; Puthen Veettil, B.; Uddin, A. Synth. Met. 2014, 192, 113–118.

(8) Yang, Y. (Michael); Chen, W.; Dou, L.; Chang, W.-H.; Duan, H.-S.; Bob, B.; Li,
G.; Yang, Y. Nat. Photonics 2015, 9 (3), 190–198.

(9) Yang, L.; Yan, L.; You, W. J. Phys. Chem. Lett. 2013, 4 (11), 1802–1810.

(10) Mollinger, S. A.; Vandewal, K.; Salleo, A. Adv. Energy Mater. 2015, 5 (23), n/a –
n/a.

(11) Wang, Z.; Zhang, Y.; Zhang, J.; Wei, Z.; Ma, W. Adv. Energy Mater. 2016, n/a –
n/a.

(12) Street, R. A.; Davies, D.; Khlyabich, P. P.; Burkhart, B.; Thompson, B. C. J. Am.
Chem. Soc. 2013, 135 (3), 986–989.

(13) Loo, Y.-L.; Khlyabich, P. P. SPIE Newsroom 2015.

(14) Khlyabich, P. P.; Rudenko, A. E.; Thompson, B. C.; Loo, Y.-L. Adv. Funct. Mater.
2015, 25 (34), 5557–5563.

(15) Zhang, J.; Zhang, Y.; Fang, J.; Lu, K.; Wang, Z.; Ma, W.; Wei, Z. J. Am. Chem.
Soc. 2015, 137 (25), 8176–8183.

89

(16) Howard, J. B.; Noh, S.; Beier, A. E.; Thompson, B. C. ACS Macro Lett. 2015,
725–730.

(17) Khlyabich, P. P.; Burkhart, B.; Ng, C. F.; Thompson, B. C. Macromolecules 2011,
44 (13), 5079–5084.

(18) Burkhart, B.; Khlyabich, P. P.; Thompson, B. C. Macromolecules 2012, 45 (9),
3740–3748.

(19) Xu, H.; Wada, T.; Ohkita, H.; Benten, H.; Ito, S. Sci. Rep. 2015, 5, 9321.

(20) Kang, H.; Kim, K.-H.; Kang, T. E.; Cho, C.-H.; Park, S.; Yoon, S. C.; Kim, B. J.
ACS Appl. Mater. Interfaces 2013, 130506090125002.

(21) Bartelt, J. A.; Beiley, Z. M.; Hoke, E. T.; Mateker, W. R.; Douglas, J. D.; Collins,
B. A.; Tumbleston, J. R.; Graham, K. R.; Amassian, A.; Ade, H.; Fréchet, J. M. J.; Toney,
M. F.; McGehee, M. D. Adv. Energy Mater. 2013, 3 (3), 364–374.

(22) Sweetnam, S.; Graham, K. R.; Ngongang Ndjawa, G. O.; Heumüller, T.; Bartelt, J.
A.; Burke, T. M.; Li, W.; You, W.; Amassian, A.; McGehee, M. D. J. Am. Chem. Soc.
2014, 136 (40), 14078–14088.

90

Chapter 4. Exploration of 3-hexylesterthiophene based
Semi-random Polymers for Ternary Blend Solar Cell
Devices

4.1 Introduction

One of the most critical issues for increasing efficiency of is improving J
sc
and V
oc

at the same time and minimizing trade-off between the two parameters.
1,2
The trade-off
between J
sc
and V
oc
is directly related to match of the molecular orbital energy level of
the polymer donor and fullerene acceptor. As absorption onset of the conjugated polymer
is determined by bandgap (E
g
), narrow bandgap is essential for harvesting broad solar
spectrum from Ultraviolet (UV) to Infrared (IR) where most solar energy is concentrated
in. For example, conjugated polymer with a bandgap of 1.1 eV (equivalent to bandgap of
silicon semiconductor) can absorb 77% of the incident photon at most.
3
However, too low
of a bandgap is limited by both the energy offset
4,5
required for charge transfer between
lowest unoccupied molecular orbital (LUMO) energies between the donor polymer and
fullerene acceptor and deep highest occupied molecular orbital (HOMO) energy level
required for high V
oc
.
6
Considering these opposing parameters, theoretical estimation of
optimal bandgap for efficiency exceeding 10% is predicted to be 1.3-1.75 eV and HOMO
level of the polymer between -5.4 and -5.7 eV .
7
Designing conjugated polymer donor thus
meet challenge to modify energy levels in regard to appropriate value.

91

One concept that has been proven to be successful is synthesizing alternating
copolymer comprise of donor-acceptor (D-A) unit.
8,9
D-A alternating copolymers
demonstrated quinoidal effect which increases double bond character to the polymer
backbone and hence lowering the bandgap of the polymer. Furthermore, HOMO level of
the copolymer is mainly dependent on the donor unit and their LUMO level is related to
acceptor unit. Therefore, by careful selection of D-A units, energy level of the resulting
copolymer can be modified.
10
Although successful, fine tuning of the energy level
requires whole new set of D-A combination and/or multistep synthesis of modifying
energetic of monomer units.
11
Another critical issue of D-A alternating copolymers is
narrow absorption range. Low bandgap D-A copolymers show merely red-shift of the
absorption maxima to the near-IR direction instead of broadening of the spectral coverage.
As a result, weak absorption region appears at 450-550 nm region which does not belong
to either absorption peak of the phenyl-C
61
-butyric acid methyl ester (PC
61
BM) or
absorption maxima of the D-A polymer. For example, a low band gap polymer
poly[(4,40-bis(2- ethylhexyl)dithieno[3,2-b:20,30-d]silole)-2,6-diyl-alt-(2,1,3-
benzothiadiazole)-4,7-diyl] (PSBTBT) (E
g
= 1.5 eV) covers only a fraction of the solar
spectrum, leaving more than 60% of the solar spectrum remain unabsorbed.
12

Another strategy that Thompson and coworkers have demonstrated is a series of
D-A copolymer called semi-random copolymers to address the issue of broadening
instead of shifting of the absorption peak.
13-17
Semi-random polymers are mainly
consisted of regioregular poly(3-hexylthiophene) (rr-P3HT) with small amount (usually
less than 15%) of acceptor monomer(s) randomly distributed in the main chain. This

92

randomized distribution of acceptor unit generates diversification of chromophores such
as all-3HT region, 3HT with low acceptor region, and 3HT with high acceptor region,
resulting in broadening of the absorption instead of red shifting the absorption peak.
Moreover, as P3HT based semi-random polymers have mainly rr-3HT sequence in the
chain, the polymers maintain attractive properties of P3HT such as semicrystallinity and
high hole mobility, which is distinct from other semi-random polymers often have low
mobility and crystallinity.
18

Many kinds of acceptor monomers which can be synthesized without complex
multistep reaction have been adopted to P3HT based semi-random polymer fashion.
19,20

Up to this point, poly(3-hexylthiophene-thiophene-diketopyrrolopyrrole) (P3HTT-
DPP)
17
, poly(3-hexylthiophene-thiophene-thienopyrroledione) (P3HTT-TPD)
15
, poly(3-
hexylthiophene-thiophene-benzothiadiazole) (P3HTT-BTD)
13,21
, and poly(3-
hexylthiophene-thiophene-thienopyrazine) (P3HTT-TP)
13
which all showed broadening
of the absorption while not sacrificing hole mobility and/or semicrystallinity. Among
those, P3HTT-DPP-10% showed low bandgap (1.6 eV), high PCE (~5%) compared to
P3HT being a benchmark polymer for semi-random P3HT polymers.
Moreover, properties of the semi-random polymers can be finely tuned by
adopting combination of monomers which is relatively easy to synthesize. Not only the
acceptor content can be adjusted but also two or more types of acceptors
15
or a donor and
an acceptor unit
14
can also be adopted to the chain as far as the ratio of dibromo species
and distannylated thiophenes are matched. For example, as more of the DPP unit is
introduced into the backbone of semi-random P3HTT-DPP from 5% to 15%, steady

93

increase of intramolecular charge transfer (ICT) band in the absorption profile and
decrease of bandgap has been observed.
However, V
oc
of the binary blend device made from P3HT based semi-random
polymers:PC
61
BM shows relatively low values between 0.5 V and 0.7 V . The reason is
not clear at the moment but it is suggested that since majority (70-80%) of the monomers
in the semi-random polymer chain is 3-hexylthiophene (3HT), the HOMO level is not
altered drastically from the HOMO level of P3HT homopolymer despite substantial
loading of the acceptor monomers. For example, even 15% loading of strong acceptor
DPP unit to the polymer chain, HOMO level of the P3HTT-DPP-15% did not change
compared to that of P3HT. Interestingly, systematic modification of the V
oc
of P3HT-
based semi-random polymer:PC
61
BM device was previously found possible if 3HT
monomers were replaced with hexyloxythiophene (3HOT)
22
or hexylesterthiophene
(3HET) monomer (discussed in chapter 2). As electron-withdrawing hexylesterthiophene
brings the HOMO level of polymer down, the V
oc
of the solar cell device can be increased
as content of the 3HET increases up to 0.92 V for P3HET composed of all 3HET
monomers and up to 0.76 V for P3HTT-DPP-HET80% where 80% of monomers are
composed of 3HET.
Possibility of getting high V
oc
with semi-random polymer opens up important
opportunity with ternary blend solar cell device. Regarding ternary blend solar cell device
composed of D1:D2:A, semi-random 3HET polymers with deep HOMO energy is an
ideal third component (D2) for ternary blend device if we fix P3HTT-DPP, a benchmark
semi-random polymer, as a donor (D1) and PC
61
BM as an acceptor (A). We have

94

discussed that on previous chapter that the similar surface energy (degree of mixing)
between the two donors is an important factor for observing tuning of the V
oc
in ternary
blend devices. Since majority of the monomers in semi-random 3HET polymers are made
of either 3HT or 3HET, structural similarity is maintained while V
oc
is higher than that of
P3HTT-DPP:PC
61
BM. Although series of P3HTT-DPP-HET was synthesized in
chapter 2, high bandgap semi-random polymer is required for potential increase of J
sc
in
the ternary blend device resulting from complementary absorption of the two donor
polymers.
Herein, we focus on the synthesis and characterization of nine semi-random
polymers to study the effect of 3HET monomer on semi-random polymer:PC
61
BM binary
devices and their potential use on the ternary blend device in pair with P3HTT-DPP. For
this study, three kinds of acceptors BTD, TPD, and thiazolothiazole (Tz)
23
semi-random
polymers were chosen and fixed to 10% of the monomer composition. The two existing
semi-random polymers, BTD and TPD were chosen because of relatively high V
oc
of the
P3HT semi-random polymers:PC
61
BM device that is previously known to be around 0.7
V.
155,163
Novel acceptor Tz is a thiazolothiazole based acceptor unit that has been
synthesized by Frederik C. Krebs’ laboratory. Each series of copolymers composed of 0%,
40%, and 80% of 3HET monomer are synthesized to study the effect of 3HET monomer
replacing 3HT monomer. Composition of 40% was chosen because the electrical
properties of the P3HT polymers are maintain at around half of the 3HT monomers were
replaced with 3HET monomers. Structures and the composition of all the polymers are
described in Figure 4.1.

95

Figure 4.1. Structure and composition of semi-random polymers P3HTT-BTD-HETs,
P3HTT-TPD-HETs, and P3HTT-Tz-HETs.

S
S S
C
6
H
13
O
k
l
m
n
P3HTT-BTD k=80% l=0% m=10% n=10%
P3HTT-BTD-HET40% k=40% l=40% m=10% n=10%
P3HTT-BTD-HET80% k=0% l=80% m=10% n=10%
N
S
N O
S
S S
S
C
6
H
13
O
k
l
m
n
N
O O
C
8
H
17
O
P3HTT-TPD k=80% l=0% m=10% n=10%
P3HTT-TPD-HET40% k=40% l=40% m=10% n=10%
P3HTT-TPD-HET80% k=0% l=80% m=10% n=10%
S
S S
S
C
6
H
13
O
k
l
m
N
S
S
N
S
C
6
H
13
C
6
H
13
n
O
P3HTT-Tz k=80% l=0% m=10% n=10%
P3HTT-Tz-HET40% k=40% l=40% m=10% n=10%
P3HTT-Tz-HET80% k=0% l=80% m=10% n=10%

96

4.2 Experimental section

4.2.1 Materials and methods

Solvents were purchased from VWR and used without further purification.
Syntheses of all the monomers were described in previous publications
13,15
or in Chapter
2. Polymerization was conducted via Stille type polymerization. First, all the monomers
were put into flame-dried 3-neck round bottom flask. Dri-solv DMF was added to afford
0.04M solution. The solution was degassed by purging N
2
for 15 min before 0.04 eq of
Pd(PPh
3
)
4
(relative to the total moles of all comonomers) was added in one portion. The
solution was degassed for 15 more minutes and then allowed to stir at 95 °C for 48 h.
Then the reaction mixture was cooled briefly and precipitated into methanol. Purification
was achieved through Soxhlet extractions with a sequence of solvents (methanol, hexane,
and chloroform). The last fraction was concentrated under reduced pressure, precipitated
in methanol, vacuum filtered and then dried overnight under high vacuum.

P3HTT-BTD Yield 69.3%. P3HTT-BTD-HET40% Yield 77.0%. P3HTT-BTD-
HET80% Yield 89.5%. P3HTT-TPD Yield 74.5%. P3HTT-TPD-HET40% Yield
87.3%. P3HTT-TPD-HET80% Yield 91.1%. P3HTT-Tz Yield 66.2%. P3HTT-Tz-
HET40% Yield 60.6%. P3HTT-Tz-HET80% Yield 93.0%.

97

1
H NMR spectra were recorded in CDCl
3
on a Varian Mercury 400 NMR
Spectrometer and can be found on Appendix A4, from Figure A4.1. to Figure A4.9.
Number average molecular weight (M
n
) and polydispersity (PDI) were determined by
size exclusion chromatography (SEC) using a Viscotek GPC Max VE 2001 separation
module and a Viscotek Model 2501 UV detector, with 70 °C HPLC grade 1,2-
dichlorobenzene (o-DCB) as eluent at a flow rate of 1 mL/min on one 300 × 7.8 mm
TSK-Gel GMH
HR
-H column (Tosoh Corp). The instrument was calibrated vs polystyrene
standards (1050−3,800,000 g/mol), and data were analyzed using OmniSec 4.6.0
software. Polymer samples for SEC measurements were prepared by dissolving a
polymer in HPLC grade o-DCB at a concentration of 1 mg/mL and then dissolved at
40 °C prior to filtering through a 0.2 μm PTFE filter.
Cyclic voltammetry (CV) was performed on Princeton Applied Research
VersaStat3 potentiostat under the control of VersaStudio Software. A standard three-
electrode cell based on a Pt wire working electrode, a silver wire pseudo reference
electrode (calibrated vs Fc/Fc
+
which is taken as 5.1 eV vs vacuum), and a Pt wire
counter electrode was purged with nitrogen and maintained under a nitrogen atmosphere
during all measurements. Polymer films were made by drop-casting an o-DCB solution
of polymer (10 mg/mL) and tetrabutylammonium hexafluorophosphate (TBAPF
6
) (30
mg/mL) onto the Pt wire and dried under nitrogen prior to measurement. Acetonitrile was
distilled over CaH
2
prior to use, and TBAPF
6
(0.1 M) was used as the supporting
electrolyte.

98

UV−vis absorption spectra were obtained on a Perkin-Elmer Lambda 950
spectrophotometer. For thin film measurements, polymers were spin-coated onto pre-
cleaned glass slides from 7 mg/mL o-DCB solutions. Thickness of the samples and
GIXRD measurements were obtained using Rigaku diffractometer Ultima IV using a Cu
K  radiation source (λ = 1.54 Å) in the reflectivity and grazing incidence X-ray
diffraction mode, respectively.

4.2.2 Device fabrication and characterization

All steps of device fabrication and characterization were performed in air. ITO-
coated glass substrates (10 Ω/sq, Thin Film Devices Inc.) were sequentially cleaned by
sonication in detergent, deionized water, tetrachloroethylene, acetone, and isopropyl
alcohol, and dried in a nitrogen stream. About 40 nm of PEDOT:PSS (Baytron R P VP
AI 4083, filtered with a 0.45 μm PVDF syringe filter—Pall Life Sciences) was first spin-
coated on the pre-cleaned ITO-coated glass substrates and annealed at 120 °C for 60 min
under vacuum. Separate solutions of the polymers and PC
61
BM were prepared in o-DCB.
The solutions were stirred for 6 h before they were mixed at the desired ratios and stirred
overnight to form a homogeneous solution. Subsequently, the polymer:PC
61
BM active
layer was spin-coated (filtered with a 0.45 μm PVDF syringe filter—Pall Life Sciences)
on top of the PEDOT:PSS layer. The optimized ratio for random copolymers:PC
61
BM
were 1:1 except 1:5 for P3HTT-BTD and 1:1.3 for P3HTT-TPD, P3HTT-TPD-
HET40%, P3HTT-TPD-HET80%. The concentrations of the blends were 10 mg/ml in

99

polymer (except 11 mg/ml for P3HTT-BTD, P3HTT-BTD-HET40%, P3HTT-BTD-
HET80%, P3HTT-TPD, and P3HTT-TPD-HET80%). The overall concentration
polymer donors of ternary blend P3HTT-DPP:P3HTT-TPD-HET40%:PC
61
BM and
P3HTT-DPP:P3HTT-Tz-HET40%:PC
61
BM was 10mg/ml. For optimized conditions,
devices of all the polymers

were stored under nitrogen for 20 min after spin-coating and
then placed in the vacuum chamber for aluminum deposition. The substrates were
pumped down to high vacuum (<9x10
-7
Torr) and aluminum (100 nm) was thermally
evaporated at 3–6 Å /s using a Denton Benchtop Turbo IV Coating System onto the active
layer through shadow masks to define the active area of the devices as 5.18 mm
2
. The
current density–voltage (J–V) characteristics of the photovoltaic devices were measured
under ambient conditions using a Keithley 2400 source-measurement unit. An Oriel
®

Sol3A class AAA S11 solar simulator with a Xenon lamp (450 W) and an AM 1.5G filter
was used as the solar simulator. An Oriel PV reference cell system 91 150 V was used as
the reference cell to calibrate the light intensity of the solar simulator (to 100 mW cm
-2
).
External quantum efficiency measurements were performed using a 300 W Xenon
arc lamp (Newport Oriel), chopped and filtered monochromatic light (250 Hz, 10 nm
FWHM) from a Conerstone 260 1/4 M double grating monochromator (Newport 74125)
together with a light bias lock-in amplifier. A silicon photodiode calibrated at Newport
was utilized as the reference cell.
Mobility of the polymer was measured using a hole-only device configuration of
ITO/PEDOT:PSS/Polymer/Al in the space charge limited current regime. The devices
preparations were the same as described above for solar cells. The dark current was

100

measured under ambient conditions. At sufficient potential the mobilities of charges in
the device can be determined by fitting the dark current to the model of SCL current and
described by:

where J
SCLC
is the current density, ε
0
is the permittivity of space, ε
R
is the
dielectric constant of the polymer (assumed to be 3), μ is the zero-field mobility of the
majority charge carriers, V is the effective voltage across the device (V = V
applied
– V
bi
–
V
r
), and L is the thickness of the polymer film. The series and contact resistance of the
device (20 – 30 Ω) was measured using a blank (ITO/PEDOT:PSS/Al) configuration and
the voltage drop due to this resistance (V
r
) was subtracted from the applied voltage. The
built-in voltage (V
bi
), which is based on the relative work function difference of the two
electrodes, was also subtracted from the applied voltage. The built-in voltage can be
determined from the transition between the ohmic region and the SCL region and is
found to be about 1 V. Polymer film thicknesses were measured using XRD in the
reflectivity mode.

4.2.3 Results and discussions

Purpose of replacing 40% and 80% of 3HT with 3HET was to observe distinct
HOMO level change compared to 0% all 3HT control polymer. Stille polymerization
method was used to avoid introducing other structural variations. The monomer
compositions of semi-random copolymers were fairly consistent with the feeding ratio

101

which can be easily identified by
1
H NMR characteristic peaks at 4.30 and 2.80 ppm.
Molecular and electronic properties are summarized in in Table 4.1. Molecular weight of
the polymers was all over 10,000 g/mol. Most importantly, HOMO level of the polymers
measured by CV was decreased as more of the HET monomers is adopted in the polymer
chain. This was consistent with P3HTT-DPP-HETs case discussed in Chapter 2,
however, the extent of change of HOMO level was more pronounced than P3HTT-DPP-
HETs case. For example, even with 40% of 3HET replacement, HOMO level was
lowered by 0.3-0.4 eV whereas only 0.1eV was changed for the DPP case.

Table 4.1. Molecular and electronic properties of all polymers

Polymer M
n
(kg/mol) /PDI
a
E
g, optical
(eV)
b
HOMO (eV)
c
P3HTT-BTD 22.8/3.3 1.60 -5.4
P3HTT-BTD-HET40% 14.5/3.9 1.66 -5.8
P3HTT-BTD-HET80% 13.2/3.3 1.77 -6.0
P3HTT-TPD 20.9/2.9 1.80 -5.5
P3HTT-TPD-HET40% 24.8/3.2 1.81 -5.8
P3HTT-BTD-HET80% 54.0/5.9 1.86 -6.0
P3HTT-Tz 12.8/2.9 1.85 -5.5
P3HTT-Tz-HET40% 47.0/3.7 1.86 -5.8
P3HTT-Tz-HET80% 39.5/4.2 1.91 -6.0
a. Determined by GPC with polystyrene as standard and o-DCB as eluent. b. Optical
band gaps from onset of absorption in UV−Vis spectra in annealed films. c. Determined
by cyclic voltammetry (vs Fc/Fc
+
) in acetonitrile containing 0.1 M TBAPF
6
.

102

400 500 600 700 800 900 1000
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
iii
ii
Absorption coefficient *10
-5
, cm
-1
Wavelength, nm
i

400 500 600 700 800 900 1000
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
vi
v
Absorption coefficient *10
-5
, cm
-1
Wavelength, nm
iv

400 500 600 700 800 900 1000
0.0
0.2
0.4
0.6
0.8
1.0
1.2
ix
viii
Absorption coefficient *10
-5
, cm
-1
Wavelength, nm
vii

Figure 4.2. UV-Vis absorption of semi-random HET polymer films: (i) P3HTT-BTD, (ii)
P3HTT-BTD-HET40%, (iii) P3HTT-BTD-HET80%, (iv) P3HTT-TPD, (v) P3HTT-
TPD-HET40%, (vi) P3HTT-TPD-HET80%, (vii) P3HTT-Tz, (viii) P3HTT-Tz-
HET40%, and (ix) P3HTT-Tz-HET80%.

103

UV-vis absorption profiles of P3HTT-BTD-HETs, P3HTT-TPD-HETs, and
P3HTT-Tz-HETs were obtained in thin films. As shown in Figure 4.2, all HET
copolymers shows similar absorption coefficient compared to their parent polymer P3HT.
However, blue shift in absorption onset was observed especially the case of P3HTT-
BTD-HETs. Optical Bandgap of the polymers were slightly increased as more of the
3HET monomer was incorporated, which suggests that HOMO level is more influenced
by electron withdrawing 3HET monomer than LLUMO level. Absorption maxima of
P3HTT-TPD-HETs and P3HTT-Tz-HETs, is located at 500-600 nm, which shows good
complementary absorption profile to that of P3HTT-DPP.
The photovoltaic properties of the semi-random polymers in BHJ devices were
studied in a conventional device configuration of ITO/PEDOT:PSS/polymer:PC
61
BM/Al.
The average parameter of the devices, J
sc
, V
oc
, FF, and PCE are summarized in Table 4.2.
All of the devices are individually optimized.

Table 4.2. Average photovoltaic parameters and SCLC hole mobilities of 3HET random
and semi-random polymers
Polymer:PC
61
BM
J
sc
a

(mA/cm
2
)
V
oc
(V) FF
PCE
a

(%)
Hole Mobility
e

(cm
2
/V∙ s )
P3HTT-BTD
b
2.71 0.619 0.406 0.68 2.07x10
-4

P3HTT-BTD-HET40%
c
4.73 0.698 0.534 1.76 0.64x10
-4

P3HTT-BTD-HET80%
c
5.26 0.846 0.493 2.19 0.55x10
-4

P3HTT-TPD
d
4.64 0.698 0.621 2.02 4.26x10
-4

104

P3HTT-TPD-HET40%
d
6.42 0.791 0.514 2.61 0.14x10
-4

P3HTT-BTD-HET80%
d
4.93 0.815 0.483 1.94 0.19x10
-4

P3HTT-Tz
c
4.79 0.638 0.509 1.56 0.65x10
-4

P3HTT-Tz-HET40%
c
6.37 0.788 0.534 2.68 0.20x10
-4

P3HTT-Tz-HET80%
c
6.00 0.881 0.493 2.61 0.29x10
-4

a
Mismatch corrected. The optimized ratio of polymer:PC
61
BM is
b
1:5,
c
1:1, and
d
1:1.3.
e

Measured from pristine polymer films in their optimized condition.

Most importantly, V
oc
of the polymer:PC
61
BM devices are increased as 3HET
monomer composition increases from 0% to 40% and 80%. As a result high V
oc
over 0.8
V was observed for the first time in semi-random polymers and the highest V
oc
of 0.881 is
the highest V
oc
observed so far for the thiophene based semi-random polymers. Structural
similarity between 3HT and 3HET gives good interchangability between the two
monomers so that J
sc
and FF values of the devices were maintained within the series and
for 40% and 80% HET semi-random polymers. Therefore, overall performance of
devices were better for 40% and/or 80% HET semi-random polymers which is different
from the case of P3HTT-DPP-HET semi-random polymers discussed in Chapter 2. Hole
mobility of the polymers were determined using SCLC method. The results shown in
Table 4.2 again indicates that the hole mobility of semi-random copolymers are
comparable to their parent polymers.

105

4 5 6 7 8
0
500
1000
1500
2000
2500
3000
iii
ii
Intensity, a.u.
Two theta, degrees
i
4 5 6 7 8
0
500
1000
1500
2000
2500
3000
3500
4000
vi
v
Intensity, a.u.
Two theta, degrees
iv

Figure 4.3. GIXRD data of P3HTT-BTD-HET and P3HTT-Tz-HET semi-random
polymer films: (i) P3HTT-BTD, (ii) P3HTT-BTD-HET40%, (iii) P3HTT-BTD-
HET80%, (iv) P3HTT-Tz, (v) P3HTT-Tz-HET40%, and (vi) P3HTT-Tz-HET80%.

GIXRD data of the polymer films were obtained in order to study the effect of
3HET unit on the solid state packing structure. As can be seen on Figure 4.3, semi-
random polymers showed strong (100) lattice peak at around 4-6° except the P3HTT-
TPD-HETs which are all amorphous. The crystallinity of the 3HET semi-random
polymers was comparable to that of semi-random polymer with all 3HT polymers. As
more 3HET unit contents get high, the peak is shifted to low two theta degrees, indicating
lamellar distance of the polymer is increased, probably due to the length of the hexylester
group being longer than hexyl group. Calculated (100) direction lamellar stacking
distance was increased from 15.9 Å to 17.8 Å and 19.0 Å for BTD series and from
14.6 Å to 15.9 Å and 16.8 Å for the Tz series. (P3HT: 16.4 Å, P3HET: 19.2 Å
for comparison)

106

400 500 600 700 800 900
0
10
20
30
40
50
60
70
Wavelength, nm
External Quantum Efficiency, %
iii
ii
i
a
400 500 600 700 800 900
0
10
20
30
40
50
60
70
iii
ii
Wavelength, nm
External Quantum Efficiency, %
i
b

400 500 600 700 800 900
0
10
20
30
40
50
60
70
External Quantum Efficiency, %
Wavelength, nm
iii
ii
i
c

Figure 4.4. EQE data of (a) P3HTT-BTD-HETs (b) P3HTT-TPD-HETs, and (c)
P3HTT-Tz-HETs : (i) 0%, (ii) 40%, (iii) 80% HET composition respectively.

107

EQE measurement was conducted in order to study photocurrent response of the
solar cell. EQE data shown in Figure 4.4 shows strong photoresponse at around 500-600
nm range. For 40% and 80% content of 3HET semi-random copolymers show stronger
photoresponse than 0% copolymers which makes good agreement with the device data.
The trend is different from the case of 40% and 80% of P3HTT-DPP-HET semi-random
polymer.

4.2.4 Ternary blend devices

High V
oc
P3HT-based semi-random polymers are a good candidate for third
component in ternary blend device with P3HTT-DPP:PC
61
BM. Regardless of acceptor
monomer type in the polymer backbone, P3HT-based semi-random polymers are
structurally similar in terms of majority of the monomer is 3HT or 3HET. Hence good
compatibility with the other donor P3HTT-DPP is expected. As discussed earlier, similar
surface energy is important factor for compatibility of the two donor components.
25,26
The
surface energy measured by contact angle of water and glycerol droplet on nine polymer
films were all similar values (20-22 mN/m) to that of P3HTT-DPPs which is 20-21mN/m
except P3HTT-Tz and P3HTT-Tz-HET40% which was 23.7 mN/m and 22.70 mN/m
respectively. Here, two P3HT based semi-random polymers P3HTT-TPD-HET40% and
P3HTT-Tz-HET40% was selected for testing ternary blend device in pursuit of studying
V
oc
trend and other photovoltaic parameters.
Ternary blend device of P3HTT-TPD-HET40%:P3HTT-DPP:PC
61
BM and

108

P3HTT-Tz-HET40%:P3HTT-DPP:PC
61
BM was tested and the photovoltaic parameters
are summarized in Table 4.3. Five different compositions (1:0, 0.8:0.2, 0.5:0.5, 0.2:0.8,
0:1) of the two donors were tested to observe V
oc
trend.

Table 4.3. Average photovoltaic parameters of ternary blend devices.
P3HTT-Tz-HET40%:P3HTT-
DPP:PC
61
BM
J
sc
a

(mA/cm
2
)
V
oc
(V)
FF
PCE
a

(%)
1:0:1 6.38 0.798 0.517 2.63
0.8:0.2:1 6.59 0.662 0.494 2.16
0.5:0.5:1 8.39 0.648 0.566 3.08
0.2:0.8:1 11.08 0.633 0.578 4.05
0:1:1 12.24 0.617 0.580 4.38
P3HTT-TPD-HET40%:P3HTT-
DPP:PC
61
BM
J
sc
a

(mA/cm
2
)
V
oc
(V)
FF
PCE
a

(%)
1:0:1.3 6.42 0.791 0.514 2.61
0.8:0.2:1.3 5.47 0.617 0.459 1.55
0.5:0.5:1.2 7.41 0.615 0.544 2.48
0.2:0.8:1 9.75 0.621 0.626 3.79
0:1:1 12.24 0.617 0.580 4.38
a
Mismatch corrected.
24

As shown in the Table 4.3, all of the FF values of the ternary blend devices of
P3HTT-Tz-HET40%:P3HTT-DPP:PC
61
BM and P3HTT-Tz-HET40%:P3HTT-

109

DPP:PC
61
BM were close to 0.5 which suggests that the device is close to optimized
condition. Interestingly, V
oc
trend of the P3HTT-Tz-HET40%:P3HTT-DPP:PC
61
BM
blend is increased depending on the increasing P3HTT-Tz-HET40% component.
However, the trend was not linearly dependent and the extent of changing is low
compared to P3HT
75
-co-3EHT
25
:P3HTT-DPP:PC
61
BM case. This trend maybe
originates from lower degree of mixing between the two polymers due to larger
difference of surface energy. (P3HTT-DPP: ~20 mN/m, P3HTT-Tz-HET40%: ~23
mN/m)
In comparison, V
oc
trend of P3HTT-TPD-HET40%:P3HTT-DPP:PC
61
BM
ternary blend device is more clearly pinned to the lower value upto 80% of P3HTT-
TPD-HET40% content. This trend was surprising because surface energy value of the
P3HTT-TPD-HET40% (~21 mN/m) and P3HTT-DPP (~20 mN/m) are closer than the
prior combination and tuning of the V
oc
depending on the composition of the two
polymers were expected. However, one thing should be noted that the P3HTT-TPD-
HET40% is an amorphous polymer unlike P3HTT-Tz-HET40%. There are few cases of
ternary blend systems of one donor being semicrystalline polymer and another being
amorphous polymer, the phenomenon has to be studied further.

110

4.3 Conclusion

The contribution of this study is to expand the library of polymers that could
potentially used to be used for a donor material in semi-random ternary blend (D1:D2:A)
device. Three different compositions of HET-based semi-random polymers containing
BTD, TPD, and Tz were synthesized and characterized. As more of the HET monomers
were adopted in the polymer chain, the V
oc
of the binary blend devices were increased to
over 0.8 V which is one of the highest V
oc
observed in thiophene based semi-random
polymers without too much deviation of surface energy from P3HTT-DPP. As a result, a
couple of polymers, P3HTT-TPD-HET40% and P3HTT-Tz-HET40%, were qualified
to be a good candidate in terms of optical and electrical property for the study of ternary
blend device and their voltage tuning.
Two of the semi-random polymers, P3HTT-TPD-HET40% and P3HTT-Tz-
HET40%, were utilized in ternary blend device with P3HTT-DPP and PC
61
BM.
Intermediate V
oc
was observed for P3HTT-Tz-HET40%:P3HTT-DPP:PC
61
BM ternary
blend device while pinning of V
oc
trend was observed for P3HTT-TTPD-
HET40%:P3HTT-DPP:PC
61
BM was observed despite similar surface energy of the two
donors. Since these results are not in accordance with the result found in chapter 3, more
detailed study is required to obtain complete picture of the morphology and voltage
tunability. However, the methodology discussed in this chapter to synthesize deep
HOMO semi-random polymer is proved to be an effective tool for future ternary blend
study.

111

4.4 References

(1) Chen, H.-Y.; Hou, J.; Zhang, S.; Liang, Y.; Yang, G.; Yang, Y.; Yu, L.; Wu, Y.; Li,
G. Nat. Photonics 2009, 3 (11), 649–653.

(2) An, Q.; Zhang, F.; Li, L.; Wang, J.; Sun, Q.; Zhang, J.; Tang, W.; Deng, Z. ACS
Appl. Mater. Interfaces 2015, 7 (6), 3691–3698.

(3) Nunzi, J.-M. Comptes Rendus Phys. 2002, 3 (4), 523–542.

(4) Kawashima, K.; Tamai, Y.; Ohkita, H.; Osaka, I.; Takimiya, K. Nat. Commun.
2015, 6, 10085.

(5) Dimitrov, S. D.; Bakulin, A. A.; Nielsen, C. B.; Schroeder, B. C.; Du, J.; Bronstein,
H.; McCulloch, I.; Friend, R. H.; Durrant, J. R. J. Am. Chem. Soc. 2012, 134 (44),
18189–18192.

(6) Thompson, B. C.; Fréchet, J. M. J. Angew. Chem. Int. Ed. 2008, 47 (1), 58–77.

(7) Scharber, M. C.; Mühlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A. J.;
Brabec, C. J. Adv. Mater. 2006, 18 (6), 789–794.

(8) Kularatne, R. S.; Magurudeniya, H. D.; Sista, P.; Biewer, M. C.; Stefan, M. C. J.
Polym. Sci. Part Polym. Chem. 2013, 51 (4), 743–768.

(9) Lu, L.; Zheng, T.; Wu, Q.; Schneider, A. M.; Zhao, D.; Yu, L. Chem. Rev. 2015,
115 (23), 12666–12731.

(10) Li, Y. Acc. Chem. Res. 2012, 45 (5), 723–733.

(11) Kim, H. G.; Kim, M.; Clement, J. A.; Lee, J.; Shin, J.; Hwang, H.; Sin, D. H.; Cho,
K. Chem. Mater. 2015, 27 (19), 6858–6868.

(12) Sista, S.; Hong, Z.; Chen, L.-M.; Yang, Y. Energy Environ. Sci. 2011, 4 (5), 1606.

(13) Burkhart, B.; Khlyabich, P. P.; Cakir Canak, T.; LaJoie, T. W.; Thompson, B. C.
Macromolecules 2011, 44 (6), 1242–1246.

(14) Burkhart, B.; Khlyabich, P. P.; Thompson, B. C. J. Photonics Energy 2012, 2 (1),
021002–1.

112

(15) Burkhart, B.; Khlyabich, P. P.; Thompson, B. C. ACS Macro Lett. 2012, 1 (6),
660–666.

(16) Burkhart, B.; Khlyabich, P. P.; Thompson, B. C. Macromol. Chem. Phys. 2013,
214 (6), 681–690.

(17) Khlyabich, P. P.; Burkhart, B.; Ng, C. F.; Thompson, B. C. Macromolecules 2011,
44 (13), 5079–5084.

(18) Braunecker, W. A.; Oosterhout, S. D.; Owczarczyk, Z. R.; Kopidakis, N.; Ratcliff,
E. L.; Ginley, D. S.; Olson, D. C. ACS Macro Lett. 2014, 3 (7), 622–627.

(19) Najari, A.; Beaupré, S.; Berrouard, P.; Zou, Y.; Pouliot, J.-R.; Lepage-Pérusse, C.;
Leclerc, M. Adv. Funct. Mater. 2011, 21 (4), 718–728.

(20) Neto, B. A. D.; Lapis, A. A. M.; da Silva Júnior, E. N.; Dupont, J. Eur. J. Org.
Chem. 2013, 2013 (2), 228–255.

(21) Lee, J.-Y.; Choi, M.-H.; Song, H.-J.; Moon, D.-K. J. Polym. Sci. Part Polym.
Chem. 2010, 48 (21), 4875–4883.

(22) Xu, B.; Noh, S.; Thompson, B. C. Macromolecules 2014.

(23) Zhang, Z.-G.; Min, J.; Zhang, S.; Zhang, J.; Zhang, M.; Li, Y. Chem. Commun.
2011, 47 (33), 9474.

(24) Rudenko, A. E.; Noh, S.; Thompson, B. C. Nanotechnology 2013, 24 (48), 484002.

(25) Khlyabich, P. P.; Rudenko, A. E.; Street, R. A.; Thompson, B. C. ACS Appl. Mater.
Interfaces 2014, 6 (13), 9913–9919.

(26) Khlyabich, P. P.; Rudenko, A. E.; Thompson, B. C.; Loo, Y.-L. Adv. Funct. Mater.
2015, 25 (34), 5557–5563.

113

BIBLIOGRAPHY

Amarasekara, A. S.; Pomerantz, M. Synthesis 2003, No. 14, 2255–2258.

Ameri, T.; Khoram, P.; Heumüller, T.; Baran, D.; Machui, F.; Troeger, A.; Sgobba, V.;
Guldi, D. M.; Halik, M.; Rathgeber, S.; Scherf, U.; Brabec, C. J. J Mater Chem
A 2014, 2 (45), 19461–19472.

Ameri, T.; Min, J.; Li, N.; Machui, F.; Baran, D.; Forster, M.; Schottler, K. J.; Dolfen, D.;
Scherf, U.; Brabec, C. J. Adv. Energy Mater. 2012, 2 (10), 1198–1202.

An, Q.; Zhang, F.; Li, L.; Wang, J.; Sun, Q.; Zhang, J.; Tang, W.; Deng, Z. ACS Appl.
Mater. Interfaces 2015, 7 (6), 3691–3698.

An, Q.; Zhang, F.; Li, L.; Wang, J.; Zhang, J.; Zhou, L.; Tang, W. ACS Appl. Mater.
Interfaces 2014, 6 (9), 6537–6544.

Ananda Rao, B.; Sasi Kumar, M.; Sivakumar, G.; Singh, S. P.; Bhanuprakash, K.; Rao, V.
J.; Sharma, G. D. ACS Sustain. Chem. Eng. 2014, 2 (7), 1743–1751.

Bakulin, A. A.; Hummelen, J. C.; Pshenichnikov, M. S.; van Loosdrecht, P. H. M. Adv.
Funct. Mater. 2010, 20 (10), 1653–1660.

Bartelt, J. A.; Beiley, Z. M.; Hoke, E. T.; Mateker, W. R.; Douglas, J. D.; Collins, B. A.;
Tumbleston, J. R.; Graham, K. R.; Amassian, A.; Ade, H.; Fréchet, J. M. J.;
Toney, M. F.; McGehee, M. D. Adv. Energy Mater. 2013, 3 (3), 364–374.

Benten, H.; Nishida, T.; Mori, D.; Xu, H.; Ohkita, H.; Ito, S. Energy Env. Sci 2016, 9 (1),
135–140.

Bian, L.; Zhu, E.; Tang, J.; Tang, W.; Zhang, F. Progress in Polymer Science 2012, 37
(9), 1292–1331.

Blouin, N.; Michaud, A.; Leclerc, M. Adv. Mater. 2007, 19 (17), 2295–2300.

Braunecker, W. A.; Oosterhout, S. D.; Owczarczyk, Z. R.; Kopidakis, N.; Ratcliff, E. L.;
Ginley, D. S.; Olson, D. C. ACS Macro Lett. 2014, 3 (7), 622–627.

Bundgaard, E.; Krebs, F. C. Solar Energy Materials and Solar Cells 2007, 91 (11), 954–
985.

Burkhart, B.; Khlyabich, P. P.; Cakir Canak, T.; LaJoie, T. W.; Thompson, B. C.

114

Macromolecules 2011, 44 (6), 1242–1246.

Burkhart, B.; Khlyabich, P. P.; Thompson, B. C. ACS Macro Lett. 2012, 1 (6), 660–666.

Burkhart, B.; Khlyabich, P. P.; Thompson, B. C. J. Photon. Energy 2012, 2 (1), 021002–
1.

Burkhart, B.; Khlyabich, P. P.; Thompson, B. C. Macromol. Chem. Phys. 2013, 214 (6),
681–690.

Burkhart, B.; Khlyabich, P. P.; Thompson, B. C. Macromolecules 2012, 45 (9), 3740–
3748.

Chen, C.-C.; Chang, W.-H.; Yoshimura, K.; Ohya, K.; You, J.; Gao, J.; Hong, Z.; Yang,
Y. Adv. Mater. 2014, 26 (32), 5670–5677.

Chen, H.-Y.; Hou, J.; Zhang, S.; Liang, Y.; Yang, G.; Yang, Y.; Yu, L.; Wu, Y.; Li, G.
Nat. Photonics 2009, 3 (11), 649–653.

Cho, Y. J.; Lee, J. Y.; Chin, B. D.; Forrest, S. R. Org. Electron. 2013, 14 (4), 1081–1085.

Chuang, M.-K.; Chen, F.-C. ACS Appl. Mater. Interfaces 2015, 7 (13), 7397–7405.

Cinar, M. E.; Ozturk, T. Chem. Rev. 2015, 115 (9), 3036–3140.

Clarke, T. M.; Durrant, J. R. Chem. Rev. 2010, 110 (11), 6736–6767.

Dang, M. T.; Hirsch, L.; Wantz, G.; Wuest, J. D. Chem. Rev. 2013, 113 (5), 3734–3765.

Dennler, G.; Scharber, M. C.; Brabec, C. J. Adv. Mater. 2009, 21 (13), 1323–1338.

Dimitrov, S. D.; Bakulin, A. A.; Nielsen, C. B.; Schroeder, B. C.; Du, J.; Bronstein, H.;
McCulloch, I.; Friend, R. H.; Durrant, J. R. J. Am. Chem. Soc. 2012, 134 (44),
18189–18192.

Duck, B. C.; Vaughan, B.; Cooling, N.; Zhou, X.; Holdsworth, J. L.; Wen, L. L.;
Rasmussen, S. C.; Dastoor, P. C.; Belcher, W. J. Sol. Energy Mater. Sol. Cells
2013, 114, 65–70.

Etxebarria, I.; Furlan, A.; Ajuria, J.; Fecher, F. W.; Voigt, M.; Brabec, C. J.; Wienk, M.
M.; Slooff, L.; Veenstra, S.; Gilot, J.; Pacios, R. Sol. Energy Mater. Sol. Cells
2014, 130, 495–504.

Gan, Q.; Bartoli, F. J.; Kafafi, Z. H. Adv. Mater. 2013, 25 (17), 2385–2396.

115

Gendron, D.; Leclerc, M. Energy Environ. Sci. 2011, 4 (4), 1225–1237.

Graham, K.; Amassian, A.; Lee, R.; Ndjawa, G. O. N.; Erwin, P.; Thompson, M.;
Nordlund, D.; Vandewal, K.; Hoke, E.; Salleo, A.; McGehee, M. SPIE
Newsroom 2013.

Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D. Prog. Photovolt. Res.
Appl. 2013, 21 (1), 1–11.

Gu, Y.; Wang, C.; Liu, F.; Chen, J.; Dyck, O. E.; Duscher, G.; Russell, T. P. Energy Env.
Sci 2014, 7 (11), 3782–3790.

Guo, F.; Kubis, P.; Li, N.; Przybilla, T.; Matt, G.; Stubhan, T.; Ameri, T.; Butz, B.;
Spiecker, E.; Forberich, K.; Brabec, C. J. ACS Nano 2014, 8 (12), 12632–
12640.

Guo, X.; Ortiz, R. P.; Zheng, Y.; Kim, M.-G.; Zhang, S.; Hu, Y.; Lu, G.; Facchetti, A.;
Marks, T. J. J. Am. Chem. Soc. 2011, 133 (34), 13685–13697.

Guo, X.; Watson, M. D. Org. Lett. 2008, 10 (23), 5333–5336.

He, J.; Su, Z.; Yan, B.; Xiang, L.; Wang, Y. Journal of Macromolecular Science, Part A
2007, 44 (9), 989–993.

He, Z.; Zhong, C.; Su, S.; Xu, M.; Wu, H.; Cao, Y. Nat. Photonics 2012, 6 (9), 593–597.

Honda, S.; Ohkita, H.; Benten, H.; Ito, S. Chem. Commun. 2010, 46 (35), 6596.

Howard, J. B.; Noh, S.; Beier, A. E.; Thompson, B. C. ACS Macro Lett. 2015, 725–730.

Hu, H.; Jiang, K.; Yang, G.; Liu, J.; Li, Z.; Lin, H.; Liu, Y.; Zhao, J.; Zhang, J.; Huang,
F.; Qu, Y.; Ma, W.; Yan, H. J. Am. Chem. Soc. 2015, 137 (44), 14149–14157.

Huang, J.-H.; Velusamy, M.; Ho, K.-C.; Lin, J.-T.; Chu, C.-W. J. Mater. Chem. 2010, 20
(14), 2820.

Huang, J.-S.; Goh, T.; Li, X.; Sfeir, M. Y.; Bielinski, E. A.; Tomasulo, S.; Lee, M. L.;
Hazari, N.; Taylor, A. D. Nat. Photonics 2013, 7 (6), 479–485.

Huo, L.; Liu, T.; Sun, X.; Cai, Y.; Heeger, A. J.; Sun, Y. Advanced Materials 2015, 27
(18), 2938–2944.

Janssen, R. A. J.; Nelson, J. Adv. Mater. 2013, 25 (13), 1847–1858.

116

Kalowekamo, J.; Baker, E. Sol. Energy 2009, 83 (8), 1224–1231.

Kang, H.; Kim, K.-H.; Kang, T. E.; Cho, C.-H.; Park, S.; Yoon, S. C.; Kim, B. J. ACS
Appl. Mater. Interfaces 2013, 130506090125002.

Kästner, C.; Rathgeber, S.; Egbe, D. A. M.; Hoppe, H. J. Mater. Chem. A 2013, 1 (12),
3961.

Kawashima, K.; Tamai, Y.; Ohkita, H.; Osaka, I.; Takimiya, K. Nat. Commun. 2015, 6,
10085.

Kerszulis, J. A.; Amb, C. M.; Dyer, A. L.; Reynolds, J. R. Macromolecules 2014, 47 (16),
5462–5469.

Khlyabich, P. P.; Burkhart, B.; Ng, C. F.; Thompson, B. C. Macromolecules 2011, 44
(13), 5079–5084.

Khlyabich, P. P.; Burkhart, B.; Rudenko, A. E.; Thompson, B. C. Polymer 2013, 54 (20),
5267–5298.

Khlyabich, P. P.; Burkhart, B.; Thompson, B. C. J. Am. Chem. Soc. 2011, 133 (37),
14534–14537.

Khlyabich, P. P.; Burkhart, B.; Thompson, B. C. J. Am. Chem. Soc. 2012, 134 (22),
9074–9077.

Khlyabich, P. P.; Rudenko, A. E.; Street, R. A.; Thompson, B. C. ACS Appl. Mater.
Interfaces 2014, 6 (13), 9913–9919.

Khlyabich, P. P.; Rudenko, A. E.; Thompson, B. C. J. Polym. Sci. Part A: Polym. Chem.
2014, 52 (8), 1055–1058.

Khlyabich, P. P.; Rudenko, A. E.; Thompson, B. C. Journal of Polymer Science Part A:
Polymer Chemistry 2014, 52 (8), 1055–1058.

Khlyabich, P. P.; Rudenko, A. E.; Thompson, B. C.; Loo, Y.-L. Adv. Funct. Mater. 2015,
25 (34), 5557–5563.

Kim, H. D.; Ohkita, H.; Benten, H.; Ito, S. ACS Appl. Mater. Interfaces 2014, 6 (20),
17551–17555.

Kim, H. G.; Kim, M.; Clement, J. A.; Lee, J.; Shin, J.; Hwang, H.; Sin, D. H.; Cho, K.
Chem. Mater. 2015, 27 (19), 6858–6868.

117

Kim, Y.; Cook, S.; Tuladhar, S. M.; Choulis, S. A.; Nelson, J.; Durrant, J. R.; Bradley, D.
D. C.; Giles, M.; McCulloch, I.; Ha, C.-S.; Ree, M. Nature Materials 2006, 5
(3), 197–203.

Koeckelberghs, G.; Vangheluwe, M.; Doorsselaere, K. V.; Robijns, E.; Persoons, A.;
Verbiest, T. Macromol. Rapid Commun. 2006, 27 (22), 1920–1925.

Koeckelberghs, G.; Vangheluwe, M.; Samyn, C.; Persoons, A.; Verbiest, T.
Macromolecules 2005, 38 (13), 5554–5559.

Koppe, M.; Egelhaaf, H.-J.; Dennler, G.; Scharber, M. C.; Brabec, C. J.; Schilinsky, P.;
Hoth, C. N. Adv. Funct. Mater. 2010, 20 (2), 338–346.

Koster, L. J. A.; Shaheen, S. E.; Hummelen, J. C. Adv. Energy Mater. 2012, 2 (10),
1246–1253.

Kotlarski, J. D.; Blom, P. W. M. Appl. Phys. Lett. 2011, 98 (5), 053301.

Kouijzer, S.; Li, W.; Wienk, M. M.; Janssen, R. A. J. J. Photonics Energy 2014, 5 (1),
057203.

Krebs, F. C. Solar Energy Materials and Solar Cells 2009, 93 (4), 394–412.

Krebs, F. C.; Gevorgyan, S. A.; Alstrup, J. J. Mater. Chem. 2009, 19 (30), 5442.

Krebs, F. C.; Tromholt, T.; Jørgensen, M. Nanoscale 2010, 2 (6), 873.

Kroon, R.; Lenes, M.; Hummelen, J. C.; Blom, P. W. M.; de Boer, B. Polym. Rev. 2008,
48 (3), 531–582.

Kularatne, R. S.; Magurudeniya, H. D.; Sista, P.; Biewer, M. C.; Stefan, M. C. J. Polym.
Sci. Part Polym. Chem. 2013, 51 (4), 743–768.

Lee, J.; Yun, M. H.; Kim, J.; Kim, J. Y.; Yang, C. Macromol. Rapid Commun. 2012, 33
(2), 140–145.

Lee, J.-Y.; Choi, M.-H.; Song, H.-J.; Moon, D.-K. J. Polym. Sci. Part Polym. Chem. 2010,
48 (21), 4875–4883.

Li, Y. Acc. Chem. Res. 2012, 45 (5), 723–733.

Liang, Y.; Feng, D.; Wu, Y.; Tsai, S.-T.; Li, G.; Ray, C.; Yu, L. Journal of the American
Chemical Society 2009, 131 (22), 7792–7799.

118

Liao, S.-H.; Jhuo, H.-J.; Cheng, Y.-S.; Chen, S.-A. Adv. Mater. 2013, 25 (34), 4766–4771.

Liao, S.-H.; Jhuo, H.-J.; Yeh, P.-N.; Cheng, Y.-S.; Li, Y.-L.; Lee, Y.-H.; Sharma, S.;
Chen, S.-A. Sci. Rep. 2014, 4, 6813.

Lim, B.; Bloking, J. T.; Ponec, A.; McGehee, M. D.; Sellinger, A. ACS Appl. Mater.
Interfaces 2014, 6 (9), 6905–6913.

Lin, R.; Wright, M.; Puthen Veettil, B.; Uddin, A. Synth. Met. 2014, 192, 113–118.

Liu, T.; Huo, L.; Sun, X.; Fan, B.; Cai, Y.; Kim, T.; Kim, J. Y.; Choi, H.; Sun, Y. Adv.
Energy Mater. 2015, n/a – n/a.

Liu, Y.; Zhao, J.; Li, Z.; Mu, C.; Ma, W.; Hu, H.; Jiang, K.; Lin, H.; Ade, H.; Yan, H. Nat.
Commun. 2014, 5, 5293.

Liu, Y.; Zhao, J.; Li, Z.; Mu, C.; Ma, W.; Hu, H.; Jiang, K.; Lin, H.; Ade, H.; Yan, H.
Nature Communications 2014, 5, 5293.

Livi, F.; Zawacka, N. K.; Angmo, D.; Jørgensen, M.; Krebs, F. C.; Bundgaard, E.
Macromolecules 2015, 48 (11), 3481–3492.

Loo, Y.-L.; Khlyabich, P. P. SPIE Newsroom 2015.

Lu, L.; Chen, W.; Xu, T.; Yu, L. Nat. Commun. 2015, 6, 7327.

Lu, L.; Xu, T.; Chen, W.; Landry, E. S.; Yu, L. Nat. Photonics 2014, 8 (9), 716–722.

Lu, L.; Zheng, T.; Wu, Q.; Schneider, A. M.; Zhao, D.; Yu, L. Chem. Rev. 2015, 115 (23),
12666–12731.

Mauer, R.; Kastler, M.; Laquai, F. Adv. Funct. Mater. 2010, 20 (13), 2085–2092.

Mei, J.; Bao, Z. Chem. Mater. 2014, 26 (1), 604–615.

Mollinger, S. A.; Vandewal, K.; Salleo, A. Adv. Energy Mater. 2015, 5 (23), n/a – n/a.

Monestier, F.; Simon, J.-J.; Torchio, P.; Escoubas, L.; Flory, F.; Bailly, S.; de Bettignies,
R.; Guillerez, S.; Defranoux, C. Solar Energy Materials and Solar Cells 2007,
91 (5), 405–410.

Moses, P. G.; Van de Walle, C. G. Appl. Phys. Lett. 2010, 96 (2), 021908.

119

Murphy, A. R.; Liu, J.; Luscombe, C.; Kavulak, D.; Fréchet, J. M. J.; Kline, R. J.;
McGehee, M. D. Chemistry of Materials 2005, 17 (20), 4892–4899.

Najari, A.; Beaupré, S.; Berrouard, P.; Zou, Y.; Pouliot, J.-R.; Lepage-Pérusse, C.;
Leclerc, M. Adv. Funct. Mater. 2011, 21 (4), 718–728.

Neises, B.; Steglich, W. Angew. Chem. Int. Ed. Engl. 1978, 17 (7), 522–524.

Neto, B. A. D.; Lapis, A. A. M.; da Silva Júnior, E. N.; Dupont, J. Eur. J. Org. Chem.
2013, 2013 (2), 228–255.

Nunzi, J.-M. Comptes Rendus Phys. 2002, 3 (4), 523–542.

Pivrikas, A.; Neugebauer, H.; Sariciftci, N. S. Solar Energy 2011, 85 (6), 1226–1237.

Pomerantz, M.; Cheng, Y.; Kasim, R. K.; Elsenbaumer, R. L. Synthetic Metals 1997, 85
(1–3), 1235–1236.

Pomerantz, M.; Cheng, Y.; Kasim, R. K.; Elsenbaumer, R. L. Synthetic Metals 1999, 101
(1–3), 162–165.

Pomerantz, M.; Yang, H.; Cheng, Y. Macromolecules 1995, 28 (17), 5706–5708.

Qi, B.; Wang, J. J. Mater. Chem. 2012, 22 (46), 24315–24325.

Qi, B.; Wang, J. Phys. Chem. Chem. Phys. 2013, 15 (23), 8972–8982.

Roth, B.; Rudenko, A. E.; Thompson, B. C.; Krebs, F. C. J. Photon. Energy 2014, 5 (1),
0572051–0572059.

Rudenko, A. E.; Latif, A. A.; Thompson, B. C. J. Polym. Sci. Part A: Polym. Chem. 2015,
53 (12), 1492–1499.

Rudenko, A. E.; Noh, S.; Thompson, B. C. Nanotechnology 2013, 24 (48), 484002.

Rudenko, A. E.; Thompson, B. C. J. Polym. Sci. Part A: Polym. Chem. 2015, 53 (2),
135–147.

Rudenko, A. E.; Wiley, C. A.; Tannaci, J. F.; Thompson, B. C. Journal of Polymer
Science Part A: Polymer Chemistry 2013, 51 (12), 2660–2668.

Scharber, M. C.; Mühlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A. J.;
Brabec, C. J. Adv. Mater. 2006, 18 (6), 789–794.

120

Scharber, M. C.; Sariciftci, N. S. Prog. Polym. Sci. 2013, 38 (12), 1929–1940.

Schulze, T.; Lips, K.; Schmidt, T. W. SPIE Newsroom 2014.

Sharma, G. D.; Singh, S. P.; Roy, M. S.; Mikroyannidis, J. A. Org. Electron. 2012, 13 (9),
1756–1762.

Sirringhaus, H. Advanced Materials 2014, 26 (9), 1319–1335.

Sista, S.; Hong, Z.; Chen, L.-M.; Yang, Y. Energy Environ. Sci. 2011, 4 (5), 1606.

Stevens, D. M.; Qin, Y.; Hillmyer, M. A.; Frisbie, C. D. J. Phys. Chem. C 2009, 113 (26),
11408–11415.

Street, R. A.; Davies, D.; Khlyabich, P. P.; Burkhart, B.; Thompson, B. C. J. Am. Chem.
Soc. 2013, 135 (3), 986–989.

Sweetnam, S.; Graham, K. R.; Ngongang Ndjawa, G. O.; Heumüller, T.; Bartelt, J. A.;
Burke, T. M.; Li, W.; You, W.; Amassian, A.; McGehee, M. D. J. Am. Chem.
Soc. 2014, 136 (40), 14078–14088.

Tang, C. W. Appl. Phys. Lett. 1986, 48 (2), 183.

Thompson, B. C.; Fréchet, J. M. J. Angew. Chem. Int. Ed. 2008, 47 (1), 58–77.

Thompson, B. C.; Kim, B. J.; Kavulak, D. F.; Sivula, K.; Mauldin, C.; Fréchet, J. M. J.
Macromolecules 2007, 40 (21), 7425–7428.

Wang, D. H.; Pron, A.; Leclerc, M.; Heeger, A. J. Adv. Funct. Mater. 2013, 23 (10),
1297–1304.

Wang, Q.; Takita, R.; Kikuzaki, Y.; Ozawa, F. J. Am. Chem. Soc. 2010, 132 (33), 11420–
11421.

Wang, Z.; Zhang, Y.; Zhang, J.; Wei, Z.; Ma, W. Adv. Energy Mater. 2016, n/a – n/a.

Wienk, M. M.; Kroon, J. M.; Verhees, W. J. H.; Knol, J.; Hummelen, J. C.; van Hal, P.
A.; Janssen, R. A. J. Angew. Chem. Int. Ed. 2003, 42 (29), 3371–3375.

Xu, B.; Noh, S.; Thompson, B. C. Macromolecules 2014, 47 (15), 5029–5039.

Xu, H.; Ohkita, H.; Benten, H.; Ito, S. Jpn. J. Appl. Phys. 2014, 53 (1S), 01AB10.

Xu, H.; Wada, T.; Ohkita, H.; Benten, H.; Ito, S. Sci. Rep. 2015, 5, 9321.

121

Yang, L.; Yan, L.; You, W. J. Phys. Chem. Lett. 2013, 4 (11), 1802–1810.

Yang, L.; Zhou, H.; Price, S. C.; You, W. J. Am. Chem. Soc. 2012, 134 (12), 5432–5435.

Yang, Y. (Michael); Chen, W.; Dou, L.; Chang, W.-H.; Duan, H.-S.; Bob, B.; Li, G.;
Yang, Y. Nat. Photonics 2015, 9 (3), 190–198.

Yao, K.; Xu, Y.-X.; Li, F.; Wang, X.; Zhou, L. Adv. Opt. Mater. 2015, 3 (3), 321–327.

Ye, L.; Xia, H.; Xiao, Y.; Xu, J.; Miao, Q. RSC Adv 2014, 4 (3), 1087–1092.

Ye, L.; Zhang, S.; Zhao, W.; Yao, H.; Hou, J. Chem. Mater. 2014, 26 (12), 3603–3605.

Yoon, S.; Heo, S. J.; Kim, H. J. Phys. Status Solidi RRL - Rapid Res. Lett. 2013, 7 (8),
534–537.

You, J.; Dou, L.; Hong, Z.; Li, G.; Yang, Y. Prog. Polym. Sci. 2013, 38 (12), 1909–1928.

You, J.; Dou, L.; Yoshimura, K.; Kato, T.; Ohya, K.; Moriarty, T.; Emery, K.; Chen, C.-
C.; Gao, J.; Li, G.; Yang, Y. Nat. Commun. 2013, 4, 1446.

Yu, W.; Huang, L.; Yang, D.; Fu, P.; Zhou, L.; Zhang, J.; Li, C. J. Mater. Chem. A 2015,
3 (20), 10660–10665.

Yuan, Y.; Huang, J.; Li, G. Green 2011, 1 (1).

Zhang, H.; Ye, L.; Hou, J. Polymer International 2015, 64 (8), 957–962.

Zhang, J.; Zhang, Y.; Fang, J.; Lu, K.; Wang, Z.; Ma, W.; Wei, Z. J. Am. Chem. Soc.
2015, 137 (25), 8176–8183.

Zhang, M.; Guo, X.; Ma, W.; Ade, H.; Hou, J. Advanced Materials 2014, 26 (33), 5880–
5885.

Zhang, M.; Guo, X.; Yang, Y.; Zhang, J.; Zhang, Z.-G.; Li, Y. Polymer Chemistry 2011,
2 (12), 2900.

Zhang, Z.-G.; Min, J.; Zhang, S.; Zhang, J.; Zhang, M.; Li, Y. Chem. Commun. 2011, 47
(33), 9474.

Zhou, E.; Cong, J.; Wei, Q.; Tajima, K.; Yang, C.; Hashimoto, K. Angew. Chem. 2011,
123 (12), 2851–2855.

122

Zhou, Q.; Hou, Q.; Zheng, L.; Deng, X.; Yu, G.; Cao, Y. Appl. Phys. Lett. 2004, 84 (10),
1653.

123

Appendix 1. Facile Enhancement of Open-Circuit Voltage of
Random and Semi-Random P3HT-Analogs in Solar Cells via
Incorporation of Hexyl Thiophene-3-Carboxylate

A1 .1 Structure Verification of Small Molecules and Polymers

Figure A1.1.
1
H NMR of 3-hexylesterthiophene.

124

Figure A1.2.
1
H NMR of 2-bromo-3-hexylesterthiophene.

125

Figure A1.3.
1
H NMR of 2-bromo-5-trimethyltin-3-hexylesterthiophene.

126

Figure A1.4.
1
H NMR of P3HT.

127

Figure A1.5.
1
H NMR of P3HT
75
-co-3HET
25
.

128

Figure A1.6.
1
H NMR of P3HT
50
-co-3HET
50

129

Figure A1.7.
1
H NMR of P3HT
25
-co-3HET
75

130

Figure A1.8.
1
H NMR of P3HET

131

Figure A1.9.
1
H NMR of P3HTT-DPP

132

Figure A1.10.
1
H NMR of P3HTT-DPP-HET10%.

133

Figure A1.11.
1
H NMR of P3HTT-DPP-HET20%.

134

Figure A1.12.
1
H NMR of P3HTT-DPP-HET40%.

135

Figure A1.13.
1
H NMR of P3HTT-DPP-HET80%.

136

Table A1.1. Summary of raw short-circuit current densities (J
sc,raw
), spectral-mismatch
factor (M), spectral mismatch-corrected short-circuit current densities (J
sc,corr
) and
integrated shortcircuit current densities (J
sc,EQE
) for BHJ solar cells based on semi-
random copolymers
Polymer:PC
61
BM
J
sc,raw
(mA/cm
2
)
M
J
sc,corr

(mA/cm
2
)
J
sc,EQE
(mA/cm
2
)
J
sc
error
(%)
P3HT
8.80 1.02 8.67 8.78 1.3
P3HT
75
-co-3HET
25

8.44 1.03 8.24 8.46 2.8
P3HT
50
-co-3HET
50

9.24 1.02 9.06 8.97 1.0
P3HT
25
-co-3HET
75

3.76 0.91 4.14 4.31 4.6
P3HET

2.25 0.92 2.45 2.32 4.9
Polymer:PC
61
BM
J
sc,raw
(mA/cm
2
)
M
J
sc,corr

(mA/cm
2
)
J
sc,EQE
(mA/cm
2
)
J
sc
error
(%)
P3HTT-DPP

9.96 0.84 11.88 11.53 2.9
P3HTT-DPP-3HET 10%

9.46 0.85 11.06 11.39 2.9
P3HTT-DPP-3HET 20% 8.76 0.85 10.25 10.12 1.3
P3HTT-DPP-3HET 40% 8.28 0.86 9.61 9.62 0.1
P3HTT-DPP-3HET 80% 6.60 0.84 7.85 7.89 0.6

137

Figure A1.14. Cyclic V oltametry of P3HT-co-3HETs.
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8
-12
-8
-4
0
4
8
12
16
Voltage, V vs Fc/Fc
+
Current *10
5
, A
P3HT
75
-co-3HET
25
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
-20
-15
-10
-5
0
5
10
15
20
25
Current *10
5
, A
Voltage, V vs Fc/Fc
+
P3HT
25
-co-3HET
75

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
-25
-20
-15
-10
-5
0
5
10
15
20
25
30
35
40
45
P3HET
Current *10
5
, A
Voltage, V vs Fc/Fc
+

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8
-20
-10
0
10
20
30
40
P3HT
Voltage, V vs Fc/Fc
+
Current *10
5
, A
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8
-20
-15
-10
-5
0
5
10
15
20
25
Voltage, V vs Fc/Fc
+
Current *10
5
, A
P3HT
50
-co-3HET
50

138

Figure A1.15. Cyclic V oltametry of P3HT-DPP-HETs.

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6
-25
-20
-15
-10
-5
0
5
10
15
20
25
30
Current *10
5
, A
Voltage, V vs Fc/Fc
+
P3HTT-DPP-HET10%

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
-40
-30
-20
-10
0
10
20
30
40
50
60
Current *10
5
, A
Voltage, V vs Fc/Fc
+
P3HTT-DPP-HET40%

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
-20
-15
-10
-5
0
5
10
15
20
25
30
Current *10
5
, A
Voltage, V vs Fc/Fc
+
P3HTT-DPP-HET80%

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6
-10
-5
0
5
10
15
Current *10
5
, A
Voltage, V vs Fc/Fc
+
P3HTT-DPP
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6
-40
-30
-20
-10
0
10
20
30
40
50
60
Current *10
5
, A
Voltage, V vs Fc/Fc
+
P3HTT-DPP-HET20%

139

Appendix 2. Surface Energy Modification for Controlling
Organic Alloy Formation of Two Polymer Donors in Ternary
Blend Solar Cells

Table A2.1. Summary of raw short-circuit current densities (J
sc,raw
), spectral-mismatch
factor (M), spectral mismatch-corrected short-circuit current densities (J
sc,corr
) and
integrated shortcircuit current densities (J
sc,EQE
) for BHJ solar cells based on semi-
random copolymers for ternary blend solar cells in system A1,A2,B1,and B2.
A1
J
sc,raw
(mA/cm
2
)
M
J
sc,corr

(mA/cm
2
)
J
sc,EQE
(mA/cm
2
)
J
sc
error
(%)
P3HTT-DPP:P3HT
75
-co-
EHT
25
:PC
61
BM=1:0:1
10.18 0.83 12.24 12.21 0.2
P3HTT-DPP:P3HT
75
-co-
EHT
25
:PC
61
BM=0.8:0.2:0.8

8.11 0.85 9.58 9.26 3.5
P3HTT-DPP:P3HT
75
-co-
EHT
25
:PC
61
BM=0.5:0.5:0.8

8.34 0.90 9.26 9.32 0.6
P3HTT-DPP:P3HT
75
-co-
EHT
25
:PC
61
BM=0.2:0.8:0.8

8.39 0.97 8.61 8.67 0.7
P3HTT-DPP:P3HT
75
-co-
EHT
25
:PC
61
BM=0:1:0.8

7.13 1.01 7.04 7.38 4.6
A2
J
sc,raw
(mA/cm
2
)
M
J
sc,corr

(mA/cm
2
)
J
sc,EQE
(mA/cm
2
)
J
sc
error
(%)
P3HTT-DPP-40%MEO:
P3HT
75
-co-
EHT
25
:PC
61
BM=1:0:1

7.84 0.83 9.42 9.16 2.8
P3HTT-DPP-40%MEO:
P3HT
75
-co-
EHT
25
:PC
61
BM=0.8:0.2:0.8
7.83 0.86 9.12 9.04 0.9
P3HTT-DPP-40%MEO:
P3HT
75
-co-
EHT
25
:PC
61
BM=0.5:0.5:0.8
7.50 0.91 8.27 8.31 0.5
P3HTT-DPP-40%MEO:
P3HT
75
-co-
EHT
25
:PC
61
BM=0.2:0.5:0.8
7.76 0.98 7.94 8.04 1.2
P3HTT-DPP-
40%MEO:P3HT
75
-co-
EHT
25
:PC
61
BM=0:1:0.8
7.13 1.01 7.04 7.38 4.6
B1
J
sc,raw
(mA/cm
2
)
M
J
sc,corr

(mA/cm
2
)
J
sc,EQE
(mA/cm
2
)
J
sc
error
(%)
P3HTT-DPP:P3HT
50
-co- 10.18 0.83 12.24 12.21 0.2

140

HET
50
:PC
61
BM=1:0:1
P3HTT-DPP:P3HT
50
-co-
HET
50
:PC
61
BM=0.8:0.2:0.9

8.46 0.86 9.84 9.67 4.1
P3HTT-DPP:P3HT
50
-co-
HET
50
:PC
61
BM=0.5:0.5:0.9

8.50 0.91 9.37 9.40 1.0
P3HTT-DPP:P3HT
50
-co-
HET
50
:PC
61
BM=0.2:0.8:0.9

7.72 0.98 7.88 7.85 0.4
P3HTT-DPP:P3HT
50
-co-
HET
50
:PC
61
BM=0:1:0.9

9.24 1.02 9.06 8.98 0.9
B2
J
sc,raw
(mA/cm
2
)
M
J
sc,corr

(mA/cm
2
)
J
sc,EQE
(mA/cm
2
)
J
sc
error
(%)
P3HTT-DPP-
40%MEO:P3HT
50
-co-
HET
50
:PC
61
BM=1:0:1
7.84 0.83 9.42 9.16 2.8
P3HTT-DPP-
40%MEO:P3HT
50
-co-
HET
50
:PC
61
BM=0.8:0.2:0.9

7.76 0.87 8.93 8.61 3.7
P3HTT-DPP-
40%MEO:P3HT
50
-co-
HET
50
:PC
61
BM=0.5:0.5:0.9

7.79 0.92 8.45 8.76 3.5
P3HTT-DPP-
40%MEO:P3HT
50
-co-
HET
50
:PC
61
BM=0.2:0.8:0.9

7.67 1.00 7.67 7.69 0.3
P3HTT-DPP-
40%MEO:P3HT
50
-co-
HET
50
:PC
61
BM=0:1:0.9

9.24 1.02 9.06 8.98 0.9

141

Appendix 3. Exploration of 3-hexylesterthiophene based semi-
random polymers for ternary solar cell devices

A3. 1. Structure Verification of Polymers.

Figure A3.1.
1
H NMR of P3HTT-BTD.

142

Figure A3.2.
1
H NMR of P3HTT-BTD-HET40%.

143

Figure A3.3.
1
H NMR of P3HTT-BTD-HET80%.

144

Figure A3.4.
1
H NMR of P3HTT-TPD.

145

Figure A3.5.
1
H NMR of P3HTT-TPD-HET40%.

146

Figure A3.6.
1
H NMR of P3HTT-TPD-HET80%.

147

Figure A3.7.
1
H NMR of P3HTT-Tz.

148

Figure A3.8.
1
H NMR of P3HTT-Tz-HET40%.

149

Figure A3.9.
1
H NMR of P3HTT-Tz-HET80%.

Abstract (if available)

Abstract Bulk-heterojunction (BHJ) polymer:fullerene solar cells have drawn vast interest for low-cost renewable energy source. Among efforts to improve power conversion efficiency (PCE) of the solar cells ternary blend has been recently proven to be a successful strategy to overcome efficiency limitation of traditional single layer binary blend device while keeping the simple device structure. Unlike binary blend solar cells which have been studied thoroughly over the decades, the mechanism of ternary blend is still in debate, especially the compositional open circuit voltage (Voc) trend of the ternary blend system composed of two donor polymers and one acceptor. The systematic study is required to understand the ternary system in more detail. Contribution of this thesis is directed to design a set of electroactive polymers that are synthesized to act as a donor in ternary blend devices. ❧ Chapter 1 reviews the concept and current status of ternary blend bulk heterojucntion research. The advantage of ternary blend solar cells are explained over tandem solar cells which is another successful strategy to overcome ultimate efficiency limit of single layer binary blend solar cells. ❧ Chapter 2 describes synthesis of random and semi-random poly(3-hexylthiophene) (P3HT) based copolymer containing varying composition of electron withdrawing 3-hexylesterthiophene (3HET) units. The semi-random polymers synthesized here are diketopyrrolopyrrole (DPP) containing polymers. The effect of 3HET unit and content on optical and electronical properties of the polymer such as UV-Vis absorptions, highest occupied molecular orbital (HOMO) energy levels, polymer crystallinities, hole mobilities, and photovoltaic properties are inveistigated in detail. Especially significant Voc enhancement without sacrificing other properties is highlighted. ❧ Chapter 3 discuss how surface energy differences of the two donor polymers are affecting the Voc trend of the ternary blend system. The importance of compatibility between the two donors has been stressed for compositional tuning of Voc in the device. In order to discuss the effect of surface energy on the Voc trend, random and semi-random polymers with different surface energy has been used to model study designed to specifically observe effect of surface energy for the first time. ❧ Chapter 4 continues to investigate the effect of 3HET unit on three other semi-random polymers which contain benzothiadiazole (BTD), thienopyrroledione (TPD), and thiazolothiazole (Tz). Depending on the composition of 3HET units, some of this polymers exhibited high Voc over 0.8 V which is highest for semi-random polymers known so far. Detailed characterization revealed that these polymers shows suitable properties for ternary blend system consist of poly(3-hexylthiophene-thiophene-diketopyrrolopyrrole) (P3HTT-DPP) and Phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM).

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Asset Metadata

Creator Noh, Sangtaik (author)

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

School College of Letters, Arts and Sciences

Degree Doctor of Philosophy

Degree Program Chemistry

Publication Date 04/01/2016

Defense Date 03/09/2016

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

Tag 3-hexylesterthiophene,OAI-PMH Harvest,P3HT,polymer,semi-random,solar cell,surface energy,ternary blend

Format application/pdf (imt)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Thompson, Barry C. (committee chair), Brutchey, Richard L. (committee member), Yoon, Jongseung (committee member)

Creator Email notank@gmail.com,sangtain@usc.edu

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c40-224118

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