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

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

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Content IMPROVING THE EFFICIENCY AND STABILITY OF ORGANIC SOLAR CELLS
THROUGH TERNARY STRATEGIES
by
Negar Kazerouni
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 2023
Copyright 2022 Negar Kazerouni
ii

Dedication

To my family and friends

iii

Acknowledgments
Foremost, I would like to express my sincere gratitude to my advisor, Prof. Barry C. Thompson
for his consistent support and guidance during my Ph.D. Without his assistance and dedicated
involvement in every step throughout the process, this work would have never been accomplished.
I would like to thank you very much for your support and understanding over these past five years.
I would also like to thank my parents for their sincere love and support throughout my life.
Without them, this day would not have been possible and I am forever grateful. I would also like
to thank my sister and brother (Nina and Navid) for always being there for me and for not allowing
me to give up in difficult situations.
Furthermore, I would like to acknowledge the members of my PhD committee Prof. Sri
Narayan and Prof. Wei Wu, for their insightful comments and encouragement. My thanks also go
out to the support I received from the collaborative work I undertook, special thanks to Prof. Sri
Narayan, Prof. Mark E. Thompson and Prof. Xiaodan Gu. I would also greatly thank the rest of
the research team for their collaborative effort during my graduate studies. To Dr. Melenbrink, Dr.
Pankow, Dr. Samal, Dr. Ye and Dr. Das thanks for your patience guiding me throughout my
research when we were colleagues. And thanks for showing me that I can still count on you to
always provide helpful suggestions and help with instrumentation and general lab questions even
after. I also thank all the present members of the research group. Qinpei Wan, Alexander Schmitt,
Grace Castillo and Timothy Bennett thanks for being the most knowledgable and supportive
colleagues and friends. And special thanks to Tanin Hooshmand who is not only a great colleague,
but also she became my best friend.
Besides the academic support, I have many friends and acquaintances to thank for listening to
me over the past five years. I cannot begin to express my gratitude and appreciation for their
iv

friendship and support. To Dr. Roshandel, who is my most wonderful and generous friend, thanks
for providing me extensive personal and professional guidance.
The work in this dissertation would not have been possible without the assistance of the
following individuals: Dr. Melenbrink (polymer synthesis and characterizations in Chapters 2),
Dr. Das (monomer synthesis, polymer synthesis in Chapter 2, 3), Qingpei Wan (monomer
synthesis, polymer synthesis in Chapter 2,3,4), Alexander Schmitt (monomer synthesis, polymer
synthesis in Chapter 3,4), Yunfei Wang (mechanical properties measurments in Chapter 3).

v

Table of Contents

Dedication…..………………………………………………………………………………..........ii
Acknowledgments…..…………………………………………………………………………… iii
Table of Contents………..…………………………………………………………………........... v
List of Tables………………..…………………………………………………………….......... viii
List of Figures…………………..………………………………………………………………… x
List of Schemes……………………..……………………………………………………..……xvii
Abstract…………………………………..…………………………………………………….xviii
Chapter 1: The Role of The Third Component in the Stability of Ternary Organic Solar
Cells………………………………………………………………………………. ……………..22
1.1 Introduction…………………………………...……………………………………………… ……….22
1.1.1 Ternary strategy……………………………………...……………………………………………... ………….23
1.2 Device Stability……………………………………...………………………………………... ………25
1.2.1 Inherent Instability of Morphology………………………………………………………………….. …………25
1.2.2 Light Instability……………………………………………………...……………………………… ………….26
1.2.3 Thermal Instability………………………………………………………...………………………... ………….26
1.2.4 Air Instability…………………………………………………………………...…………………... ………….26
1.2.5 Burn-in Instability…………………………………………………………………...……………… ………….26
1.2.6 Mechanical Instability…………………………………………………………………...………….. ………….27
1.3 Role of the Third Material in Improving Stability and Mechanisms………………...…… …………..27
1.3.1 Retaining optimum morphology………………………………………………………………...….. ………….27
vi

1.3.2 Improving Thermal Stability……………………………………………………………………….. ………….38
1.3.3 Improving Photostability…………………………………………………………………………… ………….48
1.3.4 Improving Mechanical properties…………………………………………………………………... ………….49
1.4 Conclusion and Outlook……………………………………………………………………………….58
1.5 References……………………………………………………………………………………... ……...61
Chapter 2: Ternary Blend Organic Solar Cells Incorporating Ductile Conjugated Polymers
with Conjugation Break Spacers………………………………………….……………………...81
2.1 Introduction……………………………………………………………………...………….. ………...81
2.2 Results and Discussion……………………………………………………………..………. ………...88
2.2.1 Photovoltaic Performance……………………………………………………………………...….. …………...88
2.2.2 Morphology and Charge Carrier Mobility………………………………………………………… …………...94
2.3 Conclusions………………………………………………………………………………….. ……….99
2.4 References……………………………………………………………………………….. …...……...101
Chapter 3: A Universal Strategy to Improve the Mechanical Properties of Polymer blends
by Incorporating Ductile Conjugated Polymers with Conjugation Break
Spacers………………………………………………………………………………… ……….108
3.1 Introduction…………………………………………………………………..….………….. ……….108
3.2 Results and Discussion…………………………………………………………..…………. ……….111
3.3 Conclusion…………………………………………………………………………..………. ………115
3.4 References………………………………………………………………………………...….. ……...115
Chapter 4: Improving the Performance of All-polymer Solar Cells by Blending with
Syndiotactic and Atactic Poly((N-carbazolylethylthio)propyl methacrylate)…….. …………...118
vii

4.1 Introduction…………………………………………………………………………………. ……….118
4.2 Results and Discussion……………………………………………………………………………….122
4.3 Conclusion…………………………………………………………………………………... ………135
4.4 References…………………………………………………………………………………… ………136
Biographical Sketch…………………………………………………………………......... ……141
Appendix A………………………………………………………………………………... …...142
A.1 Materials and Methods……………………………………………………………………... ……….142
A.2 Device Fabrication…………………………………………………………………...…...... ……….144
A.3 SCLC Device Fabrication and Characterization………………………………………… ….………145
A.4 Thin film measurements…………………………………………………………………….……….146
A.5 GIXRD…………………………………………………………………..………………….. ………160
A.6 References………………………………………………………………………………. …...……...174
Appendix B………………………………………………………………………..……………175
B.1 Materials and Methods…………………………………………………………………..………….. 175
B.2 SCLC Device Fabrication and Characterization……………………………………………..……... 176
B.3 Device Fabrication and Characterization………………………………………………………..….. 178
B.3 References…………………………………………………………………………………………... 182

viii

List of Tables
Table 1.1 Photovoltaic performance of the ternary devices and the corresponding
mechanisms for morphological stability enhancement. ................................................................ 37
Table 1.2 Photovoltaic performance of the ternary devices and the corresponding
mechanisms for thermal stability enhancement. ........................................................................... 47
Table 1.3 Photovoltaic performance of the ternary devices and the corresponding
mechanisms for photo stability enhancement. .............................................................................. 49
Table 1.4 Photovoltaic performance of the ternary devices and the corresponding
mechanisms for mechanical stability enhancement. ..................................................................... 58
Table 2.1 Photovoltaic properties of ternary P3HTT-ehDPP-10%:CBS:PC61BM Solar Cells. ... 90
Table 3.1 SEC and mechanical properties for pristine P4, P5, PTQ10 and their blends. ........... 112
Table 4.1 Photovoltaic characteristics of the best-performing photovoltaic devices based on
various PM7:Pendant:PZ1 blend films, under the illumination of AM 1.5G 100 mW cm
−2
. .... 125
Table 4.2 Measured contact angles 𝞱 of water and Glycerol for polymers. Surface energies
were calculated according to Wu model. .................................................................................... 130
Table 4.3 Charge mobility determined from dark J-V curves for hole and electron
dominated carrier devices for binary and ternary blend films. ................................................... 132
Table A-1 SEC and HOMO/Eg data for the P3HTT-ehDPP-10% and CBS semi-random
polymer family ............................................................................................................................ 168
ix

Table A-2 Raw Photovoltaic data for ternary solar cells and binary controls without MM
correction. ................................................................................................................................... 169
Table A-3 Photovoltaic properties of binary reference and ternary solar cells. ......................... 170
Table A-4 2θ, interchain distances (100), GIXRD intensities, full-width at half maximum
(FWHM) values, and crystallite size (nm) calculated from Scherrer’s equation for binary
and ternary as-cast films. ............................................................................................................ 171
Table A-5 Hole mobility data for ternary P3HTT-ehDPP-10%:CBS:PC61BM devices............. 173
Table B-1 SEC and HOMO/Eg data for the P3HTT-ehDPP-10% and CBS semi-random
polymer family ............................................................................................................................ 176
Table B-2 Photovoltaic properties of binary reference and ternary solar cells........................... 179

x

List of Figures
Figure 1.1 (a) The evolution in the efficiency of ternary organic solar cell started from
1.9% in 2009 to 19.3% in 2022. Reproduced with permission [
44
] (b) Complementary
absorption spectra of the components, the ternary device structure and the morphology
and proposed possible locations of the third component in the photoactive layer. ...................... 24
Figure 1.2 The schematic of the crystallization mechanism in (a) binary blend of
PM6:Y6 and (b) ternary blend of PM6: 20% BTO:Y6 from casting to film formation
(the solvent (CB) evaporation stage occurs at 0–28 s for binary and 0–30 s for ternary). ............ 29
Figure 1.3 (a) DSC profiles for pristine PTB7, PC61BM and PDI-DPP-PDI, PTB7:PC61BM
binary blend and 10% PDI-DPP-PDI-based ternary blend. The blends are without DIO
additive treatment. (b) The schematic of morphology optimization after the introduction of
PDI-DPP-PDI. ............................................................................................................................... 31
Figure 1.4 (a) Device stability of binary (PM6:Y6) and ternary (PM6: BTP-MCA:Y6).
Inset: corresponding AFM height images (2*2 μm
2
) of active layers. (b) Contact angles
of water and ethylene glycol on pristine PM6, Y6, and BTP-MCA films.................................... 32
Figure 1.5 The schematic of the crystallization mechanism of PEK-DZ matrices in
(a) pristine form, (b) in ternary PM6:Y6:PEK-DZ blend BHJ film. (c) FRET process steps:
1: generation of a singlet exciton; 2: energy transfer from PEK-DZ to PM6;
3: charge transfer from the PM6; 4: recombination to the ground state. ...................................... 33
Figure 1.6 (a) J–V and (b) the EQE spectra of PM6:Y6 and PM6:Y6:S-Fuller-PMI devices
before and after 500 h of light soaking. Normalized parameters of c) PCE, d) FF, e) VOC,
xi

and f) JSC of binary and ternary devices over the course of 500 h continuous illumination.
(g) Chemical structure of PM6, Y6, and S-Fuller-PMI. ............................................................... 35
Figure 1.7 (a) The Schematic of morphology of the binary, ternary and oligomer-assisted
active later. (b) Chemical structure of acceptor Y6 and BTP-eC9. (c) Chemical structures of
polymer donor PM6, oligomer DA, ADA, DAD and DADA…………………………………...36
Figure 1.8 The schematic of the morphology evolution of blend films: binary all-PSC,
ternary-30%, ternary-50%, and binary PCBM-PSC before and after thermal annealing. ............ 39
Figure 1.9 (a) GIXRD profiles of PTB7-Th : ITIC binary and PTB7-Th : C7 : ITIC ternary
film. (b) XPS profiles of the top surfaces of the PTB7-Th:ITIC and
PTB7-Th:10 wt% C7:ITIC blend layers. (c) The schematic of the proposed morphology
and the distributions of molecules before and after adding C7. (d) Chemical structure of
PTB7-Th, ITIC, and Coumarin 7 (C7).......................................................................................... 40
Figure 1.10 The schematic morphology of the proposed effect of crosslinking: under
thermal stress, the initial morphology degrades due to migration of small molecules.
Crosslinking can lock the morphology and suppress the molecule migration.............................. 41
Figure 1.11 The AFM images of the blends before and after annealing at 150 °C for 24 h.
(a) PM6:Y6, (b) PM6:Y6:DTODF-4F, (c) aged PM6:Y6, and (d) aged PM6:Y6:DTODF-4F.
The schematic mechanism for morphological stability (e) with and (f) without C-DTODF-4F.
(f) Molecular structure of PM6, Y6, and DTODF-4F, and its crosslinking process. ................... 42
xii

Figure 1.12 (a) The device structure and chemical structures of FTAZ, PBDB-T, and IT-M.
(b) Normalized UV–Vis absorption spectra and (c) energy levels of FTAZ, PBDB-T,
and IT-M. d) J–V curves of devices with different PBDB-T ratios.............................................. 43
Figure 1.13 Chemical structure of PM6, Y6 and PM6-b-PTY6 block copolymer and the
schematic of the morphology of blend film of PM6:Y6 with (right) and without (left) block
copolymer additive........................................................................................................................ 44
Figure 1.14 Chemical structures of PTB7-Th, Si-BDT, and DCNBT-TPIC polymers. ............... 46
Figure 1.15 The schematic of the morphology for blends with different polymer acceptor
content. .......................................................................................................................................... 51
Figure 1.16 (a) The schematic of the blend morphology in the binary all-PSCs and
ternary-PSCs. (b) The proposed strain-evolved deformation mechanism of the blend
films under tensile strain with low and high PC71BM ratios. ....................................................... 52
Figure 1.17 The schematic morphology of the blend without (a) and with (b-f) SEBS
ratios (0–30%). (g) Chemical structures of PM6, N3 and SEBS. ................................................. 53
Figure 1.18 The schematic morphology of the proposed mechanisms when poly(aryl ether)
is introduced into the BHJ layer to enhance (a) thermal and (b) mechanical stability. ................ 55
Figure 1.19 (a) The chemical structures of PD, SMA, and DACC. (b) The schematic of the
morphological evolution under thermal stress without DACC. (c) The schematic effect of
DACC on the morphology by improving the interaction between PD and SMA and stabilize
the PD–SMA interfaces. ................................................................................................................ 56
xiii

Figure 2.1 (a) J–V curve and (b) EQE spectra of
ehDPP-10%:30% T-8-T/10% ehDPP:PC61BM. P3HTT-ehDPP-10%: PC61BM J-V curve
and EQE spectrum provided for fully conjugated reference. ....................................................... 93
Figure 2.2 Grazing-incidence X-ray diffraction patterns of
P3HTT-ehDPP-10%:30% T-8-T/10% ehDPP:PC61BM thin films spin-cast from o-
dichlorobenzene (o-DCB) and dried under N2 for 30 min. P3HTT-ehDPP-10%: PC61BM
data provided for fully conjugated reference ................................................................................ 96
Figure 2.3 Hole mobility trends in P3HTT-ehDPP-10%:CBS:PC61BM ternary blends at
different donor ratios..................................................................................................................... 99
Figure 3.1 Overview of the film-on-water methodology used to measure the mechanical
response, which includes a floating film of polymer, linear actuator, clamp attached to a
load cell, and a trough filled with water. .................................................................................... 113
Figure 3.2 Representative stress-strain graphs were obtained through film-on-water
methodology with pristine P4, P5, PTQ10 and their blends. ...................................................... 114
Figure 4.1 (a) Polymer structures and device architecture. (b) Energy levels of related
materials and HUMO and LUMO of PAMAs.
13
........................................................................ 123
Figure 4.2 Photocurrent–voltage curves of (a) PM7:PAMA1:PZ1, (b) PM7:PAMA2:PZ1
and (c) PM7:PAMA3:PZ1. ......................................................................................................... 124
Figure 4.3 Corresponding external quantum efficiency (EQE) spectra of PM7:PAMA1:PZ1
devices......................................................................................................................................... 126
xiv

Figure 4.4 Variation of device parameters as a function of PAMAs content. ............................ 127
Figure 4.5 (a) Absorption coefficient spectra. (b) Photoluminescence spectra of the
photoactive material films (excited at 530 nm). ......................................................................... 128
Figure 4.6 The hole and electron mobilities of the binary ternary blends with different
PAMAs contents. ........................................................................................................................ 135
Figure A-1 J-V curve of P3HTT-ehDPP-10%:P1:PC61BM ternary solar cells. ......................... 146
Figure A-2 J-V curve of P3HTT-ehDPP-10%:P2:PC61BM ternary solar cells. ......................... 147
Figure A-3 J-V curve of P3HTT-ehDPP-10%:P3:PC61BM ternary solar cells. ......................... 148
Figure A-4 J-V curve of P3HTT-ehDPP-10%:P4:PC61BM ternary solar cells. ......................... 149
Figure A-5 J-V curve of P3HTT-ehDPP-10%:P5:PC61BM ternary solar cells. ......................... 150
Figure A-6 J-V curve of P3HTT-ehDPP-10%:P6:PC61BM ternary solar cells. ......................... 151
Figure A-7 J-V curve of P3HTT-ehDPP-10%:P7:PC61BM ternary solar cells. ......................... 152
Figure A-8 EQE spectra of P3HTT-ehDPP-10%:P1:PC61BM ternary solar cells. ..................... 153
Figure A-9 EQE spectra of P3HTT-ehDPP-10%:P2:PC61BM ternary solar cells. ..................... 154
Figure A-10 EQE spectra of P3HTT-ehDPP-10%:P3:PC61BM ternary solar cells. ................... 155
Figure A-11 EQE spectra of P3HTT-ehDPP-10%:P4:PC61BM ternary solar cells. ................... 156
Figure A-12 EQE spectra of P3HTT-ehDPP-10%:P5:PC61BM ternary solar cells. ................... 157
xv

Figure A-13 EQE spectra of P3HTT-ehDPP-10%:P6:PC61BM ternary solar cells. ................... 158
Figure A-14 EQE spectra of P3HTT-ehDPP-10%:P7:PC61BM ternary solar cells. ................... 159
Figure A-15 Grazing-incidence X-ray diffraction patterns of
P3HTT-ehDPP-10%:P1:PC61BM thin films spin-cast from o-dichlorobenzene (o-DCB)
and placed in a N2 cabinet for 30 min. P3HTT-ehDPP-10%:PC61BM absorption spectrum
provided for fully conjugated reference. ..................................................................................... 161
Figure A-16 Grazing-incidence X-ray diffraction patterns of
P3HTT-ehDPP-10%:P2:PC61BM thin films spin-cast from o-dichlorobenzene (o-DCB)
and placed in a N2 cabinet for 30 min. P3HTT-ehDPP-10%:PC61BM absorption spectrum
provided for fully conjugated reference. ..................................................................................... 162
Figure A-17 Grazing-incidence X-ray diffraction patterns of
P3HTT-ehDPP-10%:P3:PC61BM thin films spin-cast from o-dichlorobenzene (o-DCB)
and placed in a N2 cabinet for 30 min. P3HTT-ehDPP-10%:PC61BM absorption spectrum
provided for fully conjugated reference. ..................................................................................... 163
Figure A-18 Grazing-incidence X-ray diffraction patterns of
P3HTT-ehDPP-10%:P4:PC61BM thin films spin-cast from o-dichlorobenzene (o-DCB)
and placed in a N2 cabinet for 30 min. P3HTT-ehDPP-10%:PC61BM absorption spectrum
provided for fully conjugated reference. ..................................................................................... 164
Figure A-19 Grazing-incidence X-ray diffraction patterns of
P3HTT-ehDPP-10%:P5:PC61BM thin films spin-cast from o-dichlorobenzene (o-DCB)
xvi

and placed in a N2 cabinet for 30 min. P3HTT-ehDPP-10%:PC61BM absorption spectrum
provided for fully conjugated reference. ..................................................................................... 165
Figure A-20 Grazing-incidence X-ray diffraction patterns of
P3HTT-ehDPP-10%:P6:PC61BM thin films spin-cast from o-dichlorobenzene (o-DCB)
and placed in a N2 cabinet for 30 min. P3HTT-ehDPP-10%:PC61BM absorption spectrum
provided for fully conjugated reference. ..................................................................................... 166
Figure A-21 Grazing-incidence X-ray diffraction patterns of
P3HTT-ehDPP-10%:P7:PC61BM thin films spin-cast from o-dichlorobenzene (o-DCB)
and placed in a N2 cabinet for 30 min. P3HTT-ehDPP-10%:PC61BM absorption spectrum
provided for fully conjugated reference. ..................................................................................... 167
Figure B-1 J-V curve of PM7:PAMA1:PZ1 ternary solar cells. ................................................ 179
Figure B-2 J-V curve of PM7:PAMA2:PZ1 ternary solar cells. ................................................ 180
Figure B-3 J-V curve of PM7:PAMA3:PZ1 ternary solar cells. ................................................ 181
Figure B-4 Contact angle images of glycerol and water droplets on the neat films
PM7, PZ1 and PAMAs. .............................................................................................................. 181

xvii

List of Schemes
Scheme 2.1 Chemical structures of P3HTT-ehDPP-10% (parent donor polymer), PC61BM
as the acceptor and CBS donor polymers used in this study. ....................................................... 88
Scheme 3.1 The structure of P4, P5 (ductile polymers) and PTQ10, P3HTT-ehDPP-10%
(fully conjugated polymers) ........................................................................................................ 111
Scheme 4.1 (a) The synthetic route of Poly(acrylate) with syndiotactic or atactic backbone.
(b) The structure of PM7 as the host polymer donor and PZ1 as the polymer host acceptor. .... 121
Scheme A.1 Semi-random polymers with conjugation-break spacers synthesized for this
study. ........................................................................................................................................... 143

xviii

Abstract
The past three years have witnessed rapid growth in the field of organic solar cells with
intensive efforts being devoted to material development, device engineering, and understanding of
device physics. The power conversion efficiency of single-junction organic solar cells has now
reached high values of over 19%. In addition to efficiency, stability, which is still the main barrier
to the commercial application of organic solar cells, needs to be investigated.
Developing a strategy which includes non-halogenated green solvent processability, uniform
large-area module fabrication, annealing-free production, high efficiency, and excellent stability,
are among the critical requirements for industrial development of organic solar cells.
The ternary strategy is a promising technology to overcome these limits towards high power
conversion efficiency for practical application. Although the efficiency for ternary organic solar
cells has surpassed 19%, the stability, degradation mechanisms and mechanical properties in the
ternary strategy are still not well investigated. Therefore, the scientific community is changing the
focus to the stability constraints of organic solar cells, largely driven by the recent critical need to
understand and optimize the stability and reliability of organic photovoltaic based products. In
some cases, the third component in ternary organic solar cells is chosen such that although it
exhibits complementary absorption in the solar spectrum with the host blend, it does not show
great photovoltaic performance in binary blends. However, at low ratios in ternary blends, it has a
synergistic benefit on performance and stability, which demonstrates a significant advantage for
developing highly efficient and stable organic solar cells.
In this dissertation, ternary strategies are presented with the aim to provide useful insights of
the role of the third component not only on the photovoltaic performances but also on improvement
of the device stability.
xix

Chapter 1 emphasizes the crucial role of the third component in improving the efficiency
and specifically the stability. Introducing the third element in the photoactive layer of organic
photovoltaic (OPV) devices is a promising strategy towards improving the efficiency and stability
of this technology while maintaining relatively low costs. The fundamental understanding of this
efficient and sustainable strategy motivates us to design and apply novel polymers with specific
features as the third component to enable more effective and stable solar cells, described in details
in Chapters 2-4.
In Chapter 2, a broad family of ductile semi-random donor-acceptor (D-A) copolymers with
8-carbon alkyl conjugation break spacer (CBS) units were incorporated into ternary blend organic
solar cells in order to determine their impact on the electronic metrics of solar cell performance.
The goal of this study was to shine light on rational co-optimization strategies for photovoltaic and
mechanical properties in flexible and stretchable organic solar cells. The ternary blended active
layers were based on two polymer donors and the acceptor [6,6]-Phenyl-C61-butyric acid methyl
ester (PC61BM). In all cases, the majority polymer donor component was the previously reported
fully conjugated semi-random polymer P3HTT-ehDPP-10%, comprised of 80% 3-hexylthiophene,
10% diketopyrrolopyrrole (DPP) with 2-ethylhexyl (eh) side chains, and 10% thiophene. As the
second donor, three different classes of CBS polymers were used, where the spacer length was
kept constant at 8 methylene units. In the first class, the CBS unit was incorporated at 10% and
20% with eh side chains on DPP at matching contents of 10% and 20%. In the second class, the
CBS unit and DPP unit were incorporated at matching contents of 20%, 30% and 40%, where the
side chains of DPP was replaced with 2-decyltetradecyl (dtd). In the third class, with CBS
incorporated at 20% and 30%, the DPP monomer content was fixed at 10% and eh side chains
were again used. The mechanical properties of these polymers are quite notable with elastic moduli
xx

as low as 8.54 MPa and fracture strains as high as 432%. However, it was found that as elasticity
increased, hole mobility decreased. In this study, we observed that the hole mobilities of the ternary
active layers generally increased upon increasing the content of the CBS polymer up to 15% of the
overall donor fraction. The higher carrier mobilities likely contribute to the higher JSC observed in
many of the ternary devices. The as-cast ternary solar cells made in ambient environment without
any pre/post treatment gave strong performance up to 25% of CBS polymer loading. This work
demonstrates that introducing highly stretchable CBS polymers with poor charge mobility does
not adversely affect solar cell performance, offering insights into the development of ternary
strategies for flexible/stretchable organic solar cells.
In Chapter 3, two semi-random polymers were synthesized via Stille polycondensation.
Conjugation-break spacers (CBS) of 8-carbon alkyl spacers were incorporated into the polymer
backbone at the content of 30% and 40% (P4 and P5, respectively). Compared to a fully conjugated
DPP polymer, incorporation of CBS units has been shown to enhance the fracture strain from 68%
to 325% and 398%, for P4 and P5 respectively. Although these polymers displayed extraordinary
mechanical properties, with elastic moduli as low as 14.8 MPa (for P4), they have low hole
mobilities. However, our previous study (chapter 2) shows that in ternary blends with up to 25%
CBS polymer in the donor fraction, PCE values similar to or exceeding the binary reference
(DPP:PCBM) were observed in several cases. In this study, we are motivated to investigate the
effect of CBS polymers specifically on mechanical properties in a blend with a well-known
polymer donor, PTQ10. The goal of this study is to explore the impact of CBS polymers on the
ductility of blends with polymers that are not structural analogs. This study proposes that semi-
random polymers with conjugation-break spacers are promising candidates for further study in
flexible electronics.
xxi

In Chapter 4, the focus is on all-polymer solar cells. On one hand, efficient all-polymer solar
cells (all-PSCs) can be fabricated from polymerized small-molecule acceptors (PSMAs) with high
optical absorption and electron mobilities. On the other hand, the ternary strategy can be applied
as an effective method for improving the blend film morphology, absorption ability, and device
performance. However, the ternary strategy has had very limited success in all-polymer solar cells
(all-PSCs) because of the scarcity of new polymers and the challenges faced during third
component optimization. Here, we designed nonconjugated pendant electroactive polymers
containing electroactive units in the side chain with different tacticity (syndiotactic and atactic)
and molecular weight. Incorporation of the pendant polymers into binary blends of PM7:PZ1
results in over 19% improvement in the photovoltaic performance of all-polymer ternary devices.
As a result, all-PSCs featuring pendant polymers achieve a high power conversion efficiency (PCE
= 8%), outperforming binary devices without the pendant polymer (i.e., PM7:PZ1, PCE = 6.91%).
Importantly, these all-PSCs are fabricated in air, without any thermal or solvent treatment.
22

Chapter 1: The Role of The Third Component in the Stability of Ternary Organic Solar
Cells
1.1 Introduction
Organic solar cells (OSCs) based upon organic semi-conducting materials promise low-cost,
printable, and flexible photovoltaics, have attracted significant attention in the last 30 years.
1–5
The
intrinsic flexibility of organic semiconductors has promoted their adoption in virtually every area
of organic electronics, including next-generation wearable and stretchable electronics and
sensors
6–8
, transistors,
9
light emitting diodes (LEDs)
10,11
, bioelectronics
12,13
, thermoelectrics
14
,
photodetectors
15
and many more.
16–19

Thanks to the breakthrough progress of non-fullerene acceptors materials, the power
conversion efficiency (PCE) has reached over 19% for conventional OSCs.
20–23
Industrialization
value of OSCs depends on the PCE, which is the vital concern in the OSC field.
24,25
OSCs
performance metrics are now comparable with the traditional inorganic semiconductor
photovoltaics. Therefore, great interests have been aroused to drive the technology from lab to
commercial applications.
To maximize the PCE, a holistic strategy that can simultaneously improve open circuit voltage
(VOC), short circuit current (JSC), and fill factor (FF) is required. Significant efforts have been
devoted to design and optimize synthesizing new molecular structures,
26
blend film morphology,
27

ternary strategy,
28,29
transport layers, or electrodes,
30,31
and device architectures and film
deposition methods.
32
However, strong degradation during operation and even shelf storage have
been reported even for very stable OSCs.
33
It is apparent that improving the stability of OSCs is a
central challenge, where we define stability to encompass both long-term performance (lifetime)
and robust performance (durability toward mechanical deformation). Emergence of non-fullerene
23

acceptor could increase OSC lifetime approaching 10 years.
34
Furthermore, designing specific
fabrication techniques could increase outdoor lifetime up to 27000 years.
35
With this regard, deep
understanding of the degradation mechanism is an acknowledged prerequisite to develop
mechanisms and strategies for highly stable OSCs.
Incorporation of additional components is a facile strategy to preserve the optimized
morphology of the active layers.
36
Here, we explore the effect of the third component on the overall
stability of the bulk-heterojunction (BHJ) organic solar cell.
1.1.1 Ternary strategy
In recent years, ternary strategy has been the focus of many studied to improve the PCE.
20,37–
39
Since the first report about three decades ago,
40
organic photovoltaic (OPV) devices have been
widely studied as a promising third-generation solar technology.
In a conventional OPV film a binary system made of a donor (D) and an acceptor (A). The D
and the A can be blend in a form of bulk heterojunction (BHJ) or as a bilayer device to make the
photoactive layer. However, since materials of the active layer typically have low absorption
profiles, this system displays inherent limitations in terms of PCE. This limitation is mainly
observed in fullerene-based OPV with PCE hardly attaining 10%.
41
Moreover, the stability of the
BHJ binary system under operational conditions is poor.
42
They degrade very rapidly in the
presence of moisture, heat and light.
43,44
While several literature studies have reported PCEs over
18%,
45,46
many of them are achieved via the ternary strategy.
29,38,47,48
In these systems, a third
component is introduced to the photoactive layer to either increase light absorption or regulate the
morphology to enhance the PCE and stability (Figure 1.1). Although, our theoretical
understanding of the influence of the third component is still limited, this strategy might just be
the sweet spot for enhancing the performance of the solar cells while minimizing manufacturing
24

complexities. It has been shown that the third component will help to optimize the short-circuit
current (Jsc) and Voc
49,50
trade-off by increasing spectral range, reducing energy loss and at the
same time tune the morphology to enhance charge mobility and transport,
51
alleviating the
shortcomings of binary BHJ and tandem BHJ.
29,52,53

Figure 1.1 (a) The evolution in the efficiency of ternary organic solar cell started from 1.9% in
2009 to 19.3% in 2022. Reproduced with permission [
44
] (b) Complementary absorption spectra of
the components, the ternary device structure and the morphology and proposed possible locations of
the third component in the photoactive layer. Reproduced with permission [
54
]
25

Despite the notable progress in both the efficiency and stability, the future of OSCs is uncertain
because of several stability challenges. This review article discusses fundamental concepts of
stability in ternary OSCs, with a focus on factors limiting the device stability and strategies to
improve the stability and the progress achieved in the stability of OSCs.
Firstly, we briefly described the degradation factors that limit the device lifetime, such as
morphology of photoactive layer, oxygen and water ingress, photo degradation and mechanical
degradation. Then, we surveyed and analyzed the role of the third component in increasing the
stability, such as, morphology control and electrical enhancement.
1.2 Device Stability
High performance and excellent long-term stability are both very critical for commercializing
OSCs. For example, chemical degradation is inevitable for most organic photovoltaic
semiconductor materials when exposed to water, oxygen, and irradiation. Moreover, the optimal
morphology of BHJ active layer is in fact not the thermodynamical steady case at equilibrium.
Therefore, under the real operation condition the photoactive layer will slowly evolve toward the
equilibrium case and result in changes in the morphology.
55
In this section, several factors and
mechanisms behind the instability of the active layer are discussed.
56–58

1.2.1 Inherent Instability of Morphology
BHJ photoactive layer should possess an optimized nanoscale bi-continuous morphology.
57,59

The optimum mixed D and A phases are required to satisfy an efficient exciton dissociation and
charge carriers transport process and limit the charge recombination process, simultaneously.
60
As
mentioned earlier, this ideal morphology usually is not thermodynamically steady and will slowly
26

evolve toward the equilibrium.
61,62
In many cases, the high mobility of D and A materials and their
structural incompatibilities result in phase separation of donor and acceptor.
63

1.2.2 Light Instability
Photodegradation is an inevitable barrier to fabricate real stable devices.
64
This phenomenon
can be elucidated under two main mechanisms: photochemical and photophysical degradation.
57

The former changes the structures of D and A materials when the active layer is under
illumination.
65
The latter is found when photodegradation induces charge accumulation in the
active layer which relates to the decrease in device performance.
66,67

1.2.3 Thermal Instability
Thermal oxidation of the active layer during annealing of the device is another factor that needs
to be addressed.
68–70
The morphology of the active layer undergoes critical changes from
metastable state formed during the relatively high rate of solvent evaporation toward thermally
stable state upon heating.
71

1.2.4 Air Instability
Diffusion of the ambient oxygen and water can be detrimental to the device performance. This
phenomenon can cause severe physical and chemical degradation in electrodes, carrier transport
layers, and active layers of OSCs. For example, oxygen and water permeation can oxidize metal
electrodes with low function and create an electrically insulating metal oxide layer to obstacle the
charge transport and extraction.
56,72

1.2.5 Burn-in Instability
The initial significant loss in device performance is called “burn-in” degradation. Although the
origin of burn-in degradation is a controversial research topic, the fullerene dimerization, the
27

increased trap mediated charge recombination, the broad polydispersity of polymer donors, and
organic or inorganic impurities in the film are among the main reasons.
56,73–75
Morphology changes
on the other hand, seems to be the real root reason for the burn-in loss.
76

1.2.6 Mechanical Instability
In practical usage, maintenance the high initial efficiency is very critical. Photovoltaic
performance of OSCs could deteriorate upon exposure to complex mechanical stresses from
manufacture to operation. For example, squeeze by the rolls and thermal expansion caused by
coefficient mismatch between active layer and substrate can cause fracture in the active layer for
strain-initiated efficiency degradation.
77

1.3 Role of the Third Material in Improving Stability and Mechanisms
The long-term stability of ternary OSCs has been studied extensively in recent years. Although
oxygen or moisture-activated degradation of OPVs can be circumvented by encapsulation, the
other factors, would always occur inevitably as OPVs are exposed to the irradiation and stress.
78

In this review, ternary solar cell is highlighted as an effective approach to mitigate these effects.
Most device performance degrading have been referred to the morphology instabilities. In the next
section, morphology degradation in ternary solar cells have been discussed. However, it is worth
noting that sometimes morphological instability and other forms of instabilities are not
independent and the former causes the latter.
79
In this review, we tried our best to distinguish
different mechanisms to enhance the stability and discuss them separately.
1.3.1 Retaining optimum morphology
The phase separation in OSCs usually denotes the segmentation of the donor phase, acceptor
phase, and donor/acceptor mixed phase in the formation of BHJ films. It is also of critical
28

importance to control the degree of crystallization and aggregation to achieve and maintain the
optimized morphology in OSCs.
80,81
Ternary strategy can be rationally employed to induce and
stabilize the refined phase separation,
82,83
better dispersity and crystallization,
84
resulting in
improved morphological stability of the device.
83,85–87

Recently, several studies show that the aggregation of the well-known NFA acceptor (Y6) is
tunable by addition of a small molecule.
86,88,89
The results demonstrate that homogeneous material
distribution has been achieved rather than the aggregated blocks of the acceptor.
90
The bi-
continuous interpenetrating network structures in the ternary blends provide more D/A contact
interface and constructs efficient charge transport channels. This morphology enhancement also
improves device stability. For example, using highly ordered small molecular donor guest is an
effective strategy for constructing efficient ternary systems.
91
Chen et al. incorporated a crystalline
small molecule (G19) into host D18-Cl:Y6 systems to finely tune film morphology and
demonstrated a synergistic benefit for both photovoltaic performance and storage lifetime.
92
G19
exhibits an extremely ordered edge-on orientation which could enhance charge transfer and
suitable phase separation in the host. Similar effect has been reported by introducing a small-
molecule donor BPR-SCl, with deep-lying HOMO energy level and strong crystallinity, into
PM6:BTP-eC9 host binary blend. The doping of BPR-SCl is found to enhance the crystallinity of
the photoactive layers but slightly reduce the donor/acceptor phase separation scale with positive
effect on improving the photoinduced stability of devices.
29

The miscibility is supposed to be a key factor in influencing the crystallization and phase
separation of the donor and acceptor in the active layer and the corresponding performance of solar
cells.
93,94
It has been shown that the solubility and molecular packing can be simultaneously
improved in ternary systems with robust operating stability when the guest has compatibility with
29

the host.
95
A small molecule, BTO involving an amphiphilic oligo(ethylene glycol) (OEG) chain,
has outstanding compatibility with the acceptor component Y6; which is helpful for the crystalline
organization of Y6. The role of BTO in the solution state is improving the solubility of Y6 by
inhibiting excessive molecular aggregation (Figure 1.2). Furthermore, during the crystallization
process the molecular interaction between BTO and Y6 provides sufficient time for molecular
assembly of Y6, facilitating the ordered structure.

Figure 1.2 The schematic of the crystallization mechanism in (a) binary blend of PM6:Y6 and (b)
ternary blend of PM6: 20% BTO:Y6 from casting to film formation (the solvent (CB) evaporation
stage occurs at 0–28 s for binary and 0–30 s for ternary). Reproduced with permission [
95
]

Similar mechanism has been observed using other Y6 derivatives. It has been proposed that
similar structures and good compatibility of Y6 derivative acceptors can form an alloy-like state.
For example, Y6-1O could enhance crystalline packing and well-aligned face-on molecular
30

orientation in the binary blends of PM6:Y7-BO.
96
The well-adjusted morphology of the ternary
devices have better stability compared with binary OSCs.
The control of molecular packing and restrain the aggregation of NFAs can also be achieved
through designing device processing.
97
Several fabrication techniques, such as sequential slot-die
coating (SSD)
98
and Layer-by-layer (LBL) deposition
99,100
, have been proposed for attaining
optimized phase separation and suppress over aggregation for better stability of devices.
72,80

Recently, Zhang et al. compared photovoltaic stability between devices fabricated by BHJ and
LbL techniques.
101
In this strategy, IDIC and DRTB-T-C4 were used as the third components and
PM6:BTP-eC9 was the control binary device. To clarify the underlying reasons for the difference
of device stability dominated by FF loss, the hole mobility (μh) and electron mobility (µe) of
BHJ/LbL-type multicomponent devices under continuous heating were determined by utilizing the
space charge limited current (SCLC) method. It was shown that electron mobility degradation was
slowed down in BHJ/LbL-type quaternary devices, which led to improved thermal stability of
photoactive layer morphology in these multicomponent systems. Morphology studies also showed
that the BHJ/LbL-type quaternary systems before and after thermal aging treatment present
relatively stable molecular packing behaviors in the crystalline regions. Zhang et al. proposed that
the synergistic regulation of donor and acceptor materials can act as a feasible strategy to improve
molecular packing and restrain the aggregation of NFAs, thereby stabilizing the intermolecular
electron–phonon coupling in quaternary OPV devices. The resultant stable intermolecular
electron–phonon coupling could effectively suppress photocarrier and simultaneously increase
efficiency and stability in BHJ/LbL-type quaternary devices.
Reducing the effect of aggregation in ternary blends has been also reported for many
fullerene:non-fullerene (NFA) blends.
102–104
For these systems, fullerene can attenuate the
31

aggregation of NFAs and improve device stability.
102
For example, Li et al. observed that addition
of fullerene to a NFA, can specifically enhance NFA phase separation without any significant
changes for the donor component.
102
Yu et al. used differential scanning calorimetry (DSC)
measurements to show that when an air stable non-fullerene acceptor, perylenediimide (PDI)-
diketopyrrolopyrrole (DPP)-PDI, is added to PTB7:PC61BM, the endothermic peaks of the host
blend is diminished. It was proposed that the role of the third component is defined as a compatible
structure “stabilizer” which helps increase the device stability. (Figure 1.3).
105

Figure 1.3 (a) DSC profiles for pristine PTB7, PC61BM and PDI-DPP-PDI, PTB7:PC 61BM
binary blend and 10% PDI-DPP-PDI-based ternary blend. The blends are without DIO additive
treatment. (b) The schematic of morphology optimization after the introduction of PDI-DPP-PDI.
Reproduced with permission [
105
]
Another mechanism for locking the optimum morphology has been proposed by Ni et al.
106
A
guest small molecule (BTP-MCA) which has weak tendency of aggregation has been introduced
to PM6:Y6 binary blends. The wetting coefficient of BTP-MCA (0.372) suggests that the guest
molecule is distributed at the interface between PM6 and Y6. Therefore, it can act as a framework
to better maintain the initial morphology of the film, slowing down the morphological change with
time (Figure 1.4).
32

Figure 1.4 (a) Device stability of binary (PM6:Y6) and ternary (PM6: BTP-MCA:Y6). Inset:
corresponding AFM height images (2*2 μm
2
) of active layers. (b) Contact angles of water and
ethylene glycol on pristine PM6, Y6, and BTP-MCA films. Reproduced with permission [
106
]
Polymer additives is a useful strategy to improve the performance of solar cells due to the
formation of interconnected fibrils and optimize and control the morphology of the active
layer.
93,107
Feng et al. demonstrated that addition of a third component in all-polymer blend lead
to a positive effect on the storage stability. The ternary all-PSC solar cells (PBDB-T:PTB7-
Th:DCNBT-TPC) retained ≈92% of the initial PCE after 400 h, while the PBDB-T:DCNBT-TPC
and PTB7-Th:DCNBT-TPC binary cells yielded a lower stability with ≈85% of their initial
PCEs.
108
Polymer acceptors specifically, such as N2200, have been found to act as a morphology
stabilizer which can create a stable fibrillar structure in the blend film.
109,110
So et al. demonstrated
the superior light stability of N2200 added ternary devices based on both PBDB-T-2F:IT-4F and
PBDB-T:ITIC-M based binary system.
111
The origin of enhanced stability of the N2200 added
ternary device is attributed to the more stable morphology in the active layer. Moreover, it has
been proposed that the degree of the crystallinity and aggregation properties as well as the
miscibility of the polymer guest is crucial for forming the ideal morphology. Bao et al. synthesized
a series of poly(aryl ether ketone)s, named as PEK-DZx (x = 0, 25, 50, 75, and 100) with varied
aggregation and photoluminescence behaviors. These polymers with varied amount of DHPZ
segments in PEK-DZ backbones showed different crystallinity, aggregation and miscibility
33

properties as well as Fö rster resonance energy transfer (FRET) effect between PEK-DZ and
photovoltaic molecules (Figure 1.5). It was shown that compared to the PM6:Y6 binary blend,
molecular packing was enhanced in PM6:PEK-DZ25:Y6 ternary blend (5% wt.). Furthermore,
more favorable phase separation and a strong FRET effect in BHJ layers boosted the device
performance and stability.
112

Figure 1.5 The schematic of the crystallization mechanism of PEK-DZ matrices in (a) pristine
form, (b) in ternary PM6:Y6:PEK-DZ blend BHJ film. (c) FRET process steps: 1: generation of a
singlet exciton; 2: energy transfer from PEK-DZ to PM6; 3: charge transfer from the PM6; 4:
recombination to the ground state. Reproduced with permission [
112
]

Formation of an alloy-like composite in ternary systems, in which the third component acts as
energy level regulator to optimize the HOMO offset, is another effective strategy to boost OSCs
performance.
48,113,114
Even further, it has been shown that storage stability can simultaneously
34

improve in these systems, due to the stabilizing function of the alloy states in the photoactive
layer.
115–117
Hao et al. demonstrated that the alloy states in the photoactive layer has stabilizing
function which controls the aggregation characteristics of NFAs. They adopted alloy-like model
separately into host donor and acceptor materials of the state-of-the-art binary PM6:BTP-4Cl blend
with the self-stable polymer acceptor PDI-2T and small molecule donor DRCN5T as the third
components, delivering the simultaneously enhanced photovoltaic efficiency and storage stability.
In such ternary systems, two separate arguments can rationalize their operating principles: (1) the
acceptor alloys strengthen the conformational rigidity of BTP-4Cl molecules to restrain the
intramolecular vibrations for rapid relaxation of high-energy excited states to stabilize BTP-4Cl
acceptor. (2) The donor alloys optimize the fibril network microstructure of PM6 polymer to
restrict the kinetic diffusion and aggregation of BTP-4Cl molecules. According to the superior
morphological stability, non-radiative defect trapping coefficients can be drastically reduced
without forming the long-lived, trapped charge species in ternary blends. The results highlight the
novel protective mechanisms of engineering the alloy-like composites for reinforcing the long-
term stability of NFA-based ternary OSCs.
116

Similarly, Wang et al. reported that formation of Y6:S-Fuller-PMI alloys in PM6:Y6:S-Fuller-
PMI enhanced their compatibility with donor PM6 (Figure 1.6). The rylene-fullerene hybrid small
molecule, S-Fuller-PMI, was able to preserve the favorable phase separation between PM6 and Y6
domains. It was shown that the entropy increasing trend by the additional components promoted
more balanced molecular arrangement and morphological stability in blends.
36,118

In consideration of the enlarged entropy between Y6 and S-Fuller-PMI interfaces (the weight
ratio for Y6 and S-Fuller-PMI was controlled at 12:1 for the optimal ternary blend), the impact of
the alloy formation on the morphological stability in ternary devices were measured. The binary
35

and ternary devices were tested for light-induced degradation experiments. The results suggested
that the construction of dual-acceptor alloys effectively ensured the long-term stability of such
optimal ternary morphology, as a result of the boosted compatibility between these alloys and PM6
domains.
36

Figure 1.6 (a) J–V and (b) the EQE spectra of PM6:Y6 and PM6:Y6:S-Fuller-PMI devices before
and after 500 h of light soaking. Normalized parameters of c) PCE, d) FF, e) V OC, and f) J SC of binary
and ternary devices over the course of 500 h continuous illumination. (g) Chemical structure of PM6,
Y6, and S-Fuller-PMI. Reproduced with permission [
36
]

Zhang et al. compared the effect of alloy formation for two operating principles on
morphological stability. Two different ternary blends (the acceptor alloys (PM6:PDI-2T:BTP-4Cl)
and the donor alloys (PM6:DRCN5T:BTP-4Cl)) were studied. It was concluded that the former
could strengthen the conformational rigidity of host acceptor molecules to restrain the
36

intramolecular vibrations for rapid relaxation of high-energy excited states to stabilize BTP-4Cl
acceptor. While the latter could optimize the fibril network microstructure of PM6 polymer donor
to restrict the kinetic diffusion and aggregation of BTP-4Cl molecules.
116

Despite the intensive investigation on alloy-like ternary systems, research groups are still
designing novel systems to induce superior properties to OSCs. For example, Cheng et al.
introduced a new concept of oligomer-assisted high-performance ternary OSCs.
119
The formed
alloy-like phase of the oligomer:host polymer blend enabled the oligomer-assisted OSCs to fuse
the advantages of both binary and ternary devices, exhibiting substantially enhanced performance
and stability compared to the control devices. The improvement in device stability was attributed
to the optimal morphology after the addition of A-D-A oligomers (Figure 1.7).

Figure 1.7 (a) The Schematic of morphology of the binary, ternary and oligomer-assisted active
later. (b) Chemical structure of acceptor Y6 and BTP-eC9. (c) Chemical structures of polymer donor
PM6, oligomer DA, ADA, DAD and DADA. Reproduced with permission [
119
]
37

Table 1.1 summarizes the performance and the operating mechanism for morphological
stability for some of the discussed ternary systems in the context.
Table 1.1 Photovoltaic performance of the ternary devices and the corresponding mechanisms
for morphological stability enhancement.
Content
of
additive
Ternary blend VOC (V)
JSC
(mA cm
-2
)
FF
PCE
(%)
Mechanism
20% PM6:MQ5:M36 0.894 25.36 76.02 17.24 Morphology modifier
120

3% PM6:BTP-4Cl:PDI-2T 0.874 25.7 72.66 16.32 Alloy formation
116

3% PM6:DRCN5T:BTP-4Cl 0.875 25.85 74.41 16.83 Alloy formation
116

2% PM6:DMBI-BDZC:Y6 0.853 26.79 80.2 18.33 Morphology modifier
121

30% PM6:Y6:AQx-3 0.87 26.82 77.2 18.01 Alloy formation
48

30%
PM6-Ir1:BTP-
eC9:PC71BM
0.863 27.8 76.31 18.31
Improved/balanced charge transport
properties
122

14% PM6:BTP-MCA:Y6 0.87 26.74 73.0 17 Morphology stabilizer
106

20% PM6:BTO:Y6 0.85 26.32 74.32 16.59 Morphology modifier
95

0.02% PM6:DMBI-BDZC:Y6 0.853 26.79 80.20 18.33 Morphology modifier
121

0.8%
PTB7-Th:P(NDI2OD-
T2:PC71BM
0.78 20.3 69 11.2 Morphology stabilizer
110

10% PM6:Y6-1O:Y7-BO 0.867 26.35 79.27 18.11 Morphology modifier
96

10% ZnP-TBO:4TIC:6TIC 0.8 24.58 74.93 14.73 Morphology modifier
123

10%
PBDB-TF:APDC-
TPDA:Y6
0.84 25.98 77.5 16.96
Morphology /charge thermodynamic
optimization
88

20% PM6:BTTzR:Y6 0.87 26.2 77.7 17.7 Energy transfer
86

10% PBDB-T:N2200:ITIC 0.928 16.86 72.9 11.41 Morphology modifier
109

14% PTQ10:Y6:PC71BM 0.85 25.32 74.69 16.07 Morphology modifier
102

38

1.3.2 Improving Thermal Stability
As mentioned, the behavior of OSCs after thermal degradation is generally correlated to
morphological changes occurring in the active layer. Therefore, the development of strategies to
improve morphological stabilities will result in realizing thermally stable and highly efficient
OSCs.
101
For example, in addition to the effect of fullerenes on maintaining optimum morphology,
some studies show the beneficial effect of them on thermal stability.
124,125
Kim et al. studied the
mixing behavior of PCBM in the ternary blends and fullerene’s role in the thermal stability of
ternary devices with different PCBM contents.
126
It was shown that below the critical loading of
PCBM (<30 wt % of the acceptor mixture), PCBM was well-mixed in the active layer, forming an
intermixed phase without separate PCBM domains. The intermixed phase remained stable under
thermal stress, providing high thermal stability to the PSC while the presence of fullerene
molecules in the intermixed phase promoted efficient charge generation. Accordingly, the
PBDTTTPD:P(NDI2HD-T):PCBM ternary- PSC with 10 wt % PCBM had a higher PCE (7.12%)
than both of the binary active layers, namely, the all-PSC (6.67%) and PCBM-PSC (6.12%)
systems. Thus, this systematic study demonstrated that ternary blends with controlled ratios of
polymer and PCBM acceptors represent excellent candidate active layer materials, imparting both
high efficiency and thermal stability to devices, which are crucial requirements for portable,
flexible solar cells. It was shown that as PCBM content increased, the number density of PCBM
crystals increased gradually with annealing time. Thus, it is apparent that the large drop in
performance of the ternary- PSCs with high PCBM concentrations is associated with the formation
of PCBM crystallites (Figure 1.8).
39

Figure 1.8 The schematic of the morphology evolution of blend films: binary all-PSC, ternary-
30%, ternary-50%, and binary PCBM-PSC before and after thermal annealing. Reproduced with
permission [
126
]
Activating noncovalent interactions or crosslinking is another strategy to freeze the
morphology and improve the stability of the OSCs.
127–132
For example, incorporation of hydrogen
bond moieties into the backbone or side chain of conjugated polymers could enhance mechanical
performance, facilitate morphological organization, and promote self-healing ability.
133
Li et al.
showed that the carbonyl end groups of NFA can react with a guest small dye molecule (SR197)
to form the N−H…O noncovalent interaction.
127
As a result, ternary devices exhibited better
thermal stability which was attributed to the existence of H-bonding suppressed the destruction of
electron transport channels under the heating process. Similarly, Kong et al. used hydrogen bond
interactions in ternary strategy for increasing the JSC, FF and thermal/photo stability. Ternary
devices were developed by incorporating the small organic molecule coumarin7 (C7) into the
PTB7-Th:ITIC binary system (Figure 1.9). After doping with 10 wt% C7, a sharper and stronger
diffraction peak was observed compared with the binary blend film, which suggested that C7 can
promote the crystallization and ordered packing of polymer donor PTB7-Th, resulting in a more
40

homogeneous distribution. The improved BHJ film morphology was correlated to the hydrogen
bonds between C7 and ITIC detected by FT-IR spectra.
134

Figure 1.9 (a) GIXRD profiles of PTB7-Th : ITIC binary and PTB7-Th : C7 : ITIC ternary film.
(b) XPS profiles of the top surfaces of the PTB7-Th:ITIC and PTB7-Th:10 wt% C7:ITIC blend
layers. (c) The schematic of the proposed morphology and the distributions of molecules before and
after adding C7. (d) Chemical structure of PTB7-Th, ITIC, and Coumarin 7 (C7). Reproduced with
permission [
134
]
Crosslinking has been used also to prevent lateral crystallization and aggregation related to
morphological degradation (Figure 1.10).
132
Park et al. investigated the burn-in loss in ternary
41

blended OPVs prepared from a UV-crosslinkable semiconducting polymer (P2FBTT-Br) and a
non-fullerene acceptor (IEICO-4F) via a green solvent process. The synthesized P2FBTT-Br can
be crosslinked by UV irradiation for 150 s and dissolved in 2-methylanisole due to its asymmetric
structure. In OPV performance and burn-in loss tests performed at 75 °C or AM 1.5G Sun
illumination for 90 h, UV-crosslinked devices with PC71BM show 9.2% power conversion
efficiency and better stability against burn-in loss than binary devices.

Figure 1.10 The schematic morphology of the proposed effect of crosslinking: under thermal
stress, the initial morphology degrades due to migration of small molecules. Crosslinking can lock
the morphology and suppress the molecule migration. Reproduced with permission [
132
]
Similarly, Ma et al. designed and synthesized an effective crosslinker (namely, DTODF-4F),
with conjugated fluorene-based backbone and crosslinkable epoxy side chains (Figure 1.11). This
crosslinker was introduced into PM6:Y6-based binary devices to stabilize the morphology of the
blend. DTODF-4F with two epoxy groups can be in situ crosslinked into a stable network structure
under ultraviolet radiation. It was demonstrated that modified binary devices exhibit better stability
under continuous heating owing to the morphology fixation of the bulk heterojunction.
79

42

Figure 1.11 The AFM images of the blends before and after annealing at 150 °C for 24 h. (a)
PM6:Y6, (b) PM6:Y6:DTODF-4F, (c) aged PM6:Y6, and (d) aged PM6:Y6:DTODF-4F. The
schematic mechanism for morphological stability (e) with and (f) without C-DTODF-4F. (f)
Molecular structure of PM6, Y6, and DTODF-4F, and its crosslinking process. Reproduced with
permission [
79
]
Often, thermally induced small molecule aggregation or crystallization has been identified as
a key mechanism for the PCE loss.
135
Several studies show that polymer additive can also act as
an inhibitor of ternary film crystallization and consequently enhance thermal stability.
136,137
Hu et
43

al. proposed that when a brittle polymer (PBDB-T), which has low miscibility with the host NFA,
is added to the binary FTAZ:IT-M blend ( Figure 1.12), NFA crystallization is limited and
consequently thermal stability increases.
138

Figure 1.12 (a) The device structure and chemical structures of FTAZ, PBDB-T, and IT-M. (b)
Normalized UV–Vis absorption spectra and (c) energy levels of FTAZ, PBDB-T, and IT-M. d) J–V
curves of devices with different PBDB-T ratios. Reproduced with permission [
138
]
Recently, Cui et al. proposed a strategy to improve the photovoltaic efficiency and thermal
stability simultaneously in all-polymer-hosted OSCs.
139
A small-molecule-guest can act as a filler
to the nanovoids in all-polymer binary blend of PBDB-TF:B1:PY-IT. It has been proposed that
this strategy can manipulate the electron–phonon interactions and improve morphology stability.
In other words, the guest small molecule can lock the so called “vertical distribution gradient”
morphology of all-PSCs under thermal stress.
Polymer acceptors specifically have been recently found as a promising third component in
the ternary blend to improve the stability.
109,140
Li et al. incorporated polymer acceptor N2200
into the PTB7-Th:PC71BM fullerene based binary system as the third component.
141
The results
44

indicate that the addition of N2200 could also enhance the thermal stability of the device. The
fabricated PTB7-Th:N2200:PC71BM ternary device maintained 80% of its initial PCE after the
heat treatment at 100 °C for 1100 h which enhanced by 10% when compared to PTB7-
Th:PC71BM based binary devices, which should be attributed to the more stable morphology of
ternary active layers. Li et al. improved the thermal/environmental stability of PM6:Y6 by
addition of only 5 wt% PM6-b-PTY6 (Figure 1.13). The results showed that the addition of
structurally compatible block copolymer additive can tune both surface and interface, and further
effectively

Figure 1.13 Chemical structure of PM6, Y6 and PM6-b-PTY6 block copolymer and the schematic
of the morphology of blend film of PM6:Y6 with (right) and without (left) block copolymer additive.
Reproduced with permission [
142
]
improve the long-term stability of devices against thermal stress, moisture and oxygen.
142

Similar to alloy formation between two small molecules, Park et al. reported an alloy formation
between a small molecule acceptor (N3) and a polymer acceptor (PY-P2).
143
It has been proposed
that introduction of PY-P2 polymer into PM6:N3 BHJ improved device thermal stability. Mixing
polymer acceptor into the small molecular N3 domain would be favorable for improving thermal
stability of the blend morphology because the polymer acceptor have a relatively high thermal
45

transition temperature compared with N3.
81
This stability enhancement has been attributed to
significant suppression of domain growth and phase separation between the components via
kinetic trapping effect.
144
In other words, the alloyed polymer chains of PY-P2 might effectively
decrease the movement freedom of N3 by trapping N3 crystallites in the intertwined network
structure of PY-P2.
143

Last but not least, rational designing and functionalizing polymers with specific properties is
another strategy to overcome thermal instability. For example, terpolymerization and non-
conjugated backbone are synthetic strategies to rationally regulate the molecular aggregation. This
strategy enables fine-tuning the blend morphology, which is associated with the more efficient
charge generation, and more balanced charge transport properties in the devices.
145

Gokulnath et al. introduced siloxane-terminated side chain on the BDT moiety on a polymer
guest donor which improved the hydrophobicity and the stability of all-PSC devices. Ternary
devices were made of a wide band gap Si-BDT, middle band gap PTB7-Th, and a polymer acceptor
DCNBT-TPIC (Figure 1.14). It was shown that introducing the siloxane-terminated side chain on
the BDT moiety improves the hydrophobicity, which could improve the stability of all-PSC
devices. Moreover, a good atmosphere and thermal stabilities were confirmed from the optimized
efficient all-PSC devices (0.6:0.4:0.6).
146

46

Table 1.2 summarizes the performance and the operating mechanism for thermal stability for
some of the discussed ternary systems in the context.

Figure 1.14 Chemical structures of PTB7-Th, Si-BDT, and DCNBT-TPIC polymers. Reproduced
with permission [
146
]

47

Table 1.2 Photovoltaic performance of the ternary devices and the corresponding mechanisms
for thermal stability enhancement.
Content
of
additive
Ternary blend VOC (V)
JSC
(mA cm
-2
)
FF PCE (%) Mechanism
20% PM6:PY-P2:N3 0.85 25.2 71.2 15.2 Alloy formation
143

10% PTB7-Th:C7:ITIC 0.81 18.36 67.66 10.01 Hydrogen bonds
134

5% PM6:PM6-b-PTY6:Y6 0.86 25.55 75 16.48 Morphology compatibilizer
142

5% PM6:PAE:Y6 0.86 26.82 70.12 16.13 Morphology compatibilizer
147

10%
PBDTTTPD:PCBM:P(NDI2
HD-T)
1.04 12.93 53 7.12 Morphology stabilizer
126

10% PM6:PEK-DZ:Y6 0.86 26.5 71.6 16.19
Crystallinity/aggregation modifier
and Fö rster resonance energy
transfer (FRET)
112

0.5% PM6:DTODF-4F:Y6 0.846 25.5 74 16.1
Morphology stabilizer
(crosslinking)
79

30% PBDB-T-2F:FOIC:IT-2F 0.905 21.9 70.4 13.95 Morphology regulator
148

40%
PTB7-Th:Si-BDT:DCNBT-
TPIC
0.86 22.32 68.20 13.45 Improving hydrophobicity
146

20% PBDB-T:IBC-F:IE4F-S 0.887 22.83 74.4 15.06
Crystallinity/aggregation modifier
and Fö rster resonance energy
transfer (FRET)
149

10% PM6:TIT-2Cl:Y6 0.876 26.63 77.93 18.18 Morphology regulator
80

5% PTB7-Th:SR197:ITIC 0.875 18.59 67.79 11.03 Hydrogen bonds
127

8% PM6:Y6:SR197:PC71BM 0.841 27.11 76.62 17.48 Hydrogen bonds
129

10% PM6:PITIC-Ph:Y6 0.87 26.5 74 17 Suppressing phase segregation
136

15% PM7:SiCl-BDT:Y7 0.86 27.73 70.43 17.23
Energy transfer/balanced charge
carrier
150

- PM6:PYT:PY2F-T 0.9 25.2 76 17.2 Morphology modifier
82

10% PTB7-Th:N2200:PC71BM 0.79 15.1 64.3 7.5 Morphology modifier
141

10% PBDB-TF:B1:PY-IT 24.15 0.947 71.41 16.38 Nanovoid filler
139

-
PM6:BTP-eC9:L8-BO-
F(LBL)
28.36 0.836 73.0 17.31
Optimized exciton diffusion
length/vertical phase separation
151

48

1.3.3 Improving Photostability

The role of the third component on enhancing π-π stacking, improving miscibility between D
and A and increasing intermolecular interactions could also be significant.
149,152
Efficient π-π
overlap between molecules could increase the hole and electron mobilities while simultaneously
suppressing the trap-assisted and bimolecular recombination rates.
153
As a result, device
performance and more importantly, thermal and photostability are improved.
154

Several studies have proposed that addition of low concentration dopant into the active layer
is a successful strategy to achieve high performance accompanied with excellent stability in
device.
155–157
For example, Li et al. recently showed that an air-stable small molecule (DMBI-
BDZC) can be used as n-type dopant for the acceptor Y6 and modifies the morphology
simultaneously. It was proposed that this process significantly enhances and balances charge
carrier mobilities, improves exciton dissociation, increases carrier lifetime, and suppresses charge
recombination with reduced energy loss at the heterojunction.
121

For thick active layers, LBL processing is a promising strategy to optimize morphology and
enhance thermal stability. Cai et al. employed LBL approach, and it was shown that for two NFAs,
the exciton diffusion length in the mixed phase has been enlarged. Moreover, this strategy is useful
to induce vertical phase separation.
158
These two features are beneficial for exciton generation and
dissociation (high FF of 73%) which leads to higher photostability.
151

Lastly, halogenation has been introduced as a very efficient strategy to improve the
photovoltaic performance
159,160
and photophysical property of NFAs.
161
Recently halogenation of
small molecule acceptors has shown to be effectively useful for ambient stability of the
devices.
162,163
For example, chlorinated polymers can induce stronger aggregates and exhibit
enhanced noncovalent interaction in comparison with the nonhalogenated analogue in ternary
49

blends.
164
This could be beneficial to the charge transfer in the vertical direction and promote the
performance and stability of the OSCs.
165

Table 1.3 summarizes the performance and the operating mechanism for photo stability for
some of the discussed ternary systems in the context.
Table 1.3 Photovoltaic performance of the ternary devices and the corresponding mechanisms
for photo stability enhancement.
Content of
additive
Ternary blend VOC (V)
JSC
(mA cm
-2
)
FF
PCE
(%)
Mechanism
20% PBDB-T:IBC-F:IE4F-S 0.887 22.83 74.4 15.06
Enhanced π–π stacking coherence
length
149

-
PM6:DRTB-T-C4 + BTP-
eC9:IDIC
0.85 26.43 77.25 17.35
Electron–phonon coupling
mechanisms
101

10% PBDB-T:N2200:ITIC-M 0.902 14.8 69.7 9.3 Morphology stabilizer
111

20% PM6:MQ5:M36 0.894 25.36 76.02 17.24 Morphology stabilizer
120

10%
PTB7:PDI-DPP-
PDI:PC61BM
0.78 15.95 70.02 8.71 Morphology stabilizer
105

40%
PBDB-T:PTB7-
Th:DCNBT-TPC
0.81 21.9 68.3 12.1 Morphology regulator
108

50% PM6:BTP-eC9:BTP-S14 0.852 27.51 78.28 18.4 Optimized phase separation
87

10% PM6:ADA:BTP-eC9 0.859 27.58 77.34 18.32 Alloy formation
119

10% PM6:PC71BM:PY-IT 0.836 26.87 77.88 17.49 Morphology stabilizer
125

20% PM6:BPR-SCl:BTP-eC9 0.856 27.13 77 18.02 Morphology regulator
29

10% PM6:PY-IT:BN-T 0.955 22.65 74.3 16.09 Morphology modifier
83

5% PM6:ITIC-M:Y6 0.859 26.35 80.1 18.13 Morphology regulator
84

5%
PBT(E)BTz: PBDB-
TF:BTP-4Cl
0.866 25.55 73.48 16.26
Balanced charge transport/energy
transfer
75

1.3.4 Improving Mechanical properties
Generally, small molecular acceptors exhibit high PCEs, while having relative poor flexibility
(stretchability) due to brittle crystalline features.
166
This limitation could be overcome in ternary
50

strategy by introducing a guest polymer. Several studies have shown the positive effect of polymer
additives on mechanical properties in many non-fullerene systems.
138,167
The role of the guest
polymer in many NFA blends is reported as an inhibitor for NFA crystallization, which could
result in mechanical ductility.
75,138
Another strategy for enhancing the mechanical robustness and
PCE is incorporating high-molecular-weight (MW) polymers to the active layer.
168
It has been
proposed that enhancement in the mechanical ductility of the blends is correlated to the tie
molecules and chain entanglements formulated by the long chains of guest polymer. These
crosslinking points produced in the polymer chain, or in between polymer chains, could
dramatically improve the dissipation of external stress. The proposed mechanism is illustrated in
(Figure 1.15). In regime I, without polymer acceptor (PA) content, Y7 domains were isolated in
the PM6:Y7 blend. This regime showed poor mechanical properties since the stress could be
concentrated along sharp and fragile PM6−Y7 interfaces; therefore, cracks could propagate easily
even under small strains. In regime II, the three components formulate properly intermixed
domains with moderate domain purities. Tie molecules formed by long PA chains bridge and
interconnect the isolated Y7 domains, which could effectively dissipate external mechanical stress,
making the blend films more resistant to the crack formation and mechanical failure. In regime III,
although excessive PA could increase mechanical ductility, formation of segregated and pure
domains of the three components with comparatively smaller donor−acceptor interfacial areas
could fail the device performance.
51

Figure 1.15 The schematic of the morphology for blends with different polymer acceptor
content.
168

The effect of inhibiting NFA crystallization has been also observed by research groups using
fullerenes (PCBM
169
or PC71BM
170
) as the third component in OSCs. The role of fullerenes in
enhancing the mechanical properties is proposed as a crystallization inhibitor which disrupts
crystalline structure of host polymers and forms well-mixed amorphous phase. Lee et al. further
introduced a critical value of 30 wt% concentration of the PC71BM where above this ratio, the
formation of fullerene aggregates has a reverse effect on mechanical properties due to the presence
of weak interfaces between the brittle PC71BM and polymer domains (Figure 1.16).
170

52

Figure 1.16 (a) The schematic of the blend morphology in the binary all-PSCs and ternary-PSCs.
(b) The proposed strain-evolved deformation mechanism of the blend films under tensile strain with
low and high PC 71BM ratios. Reproduced with permission [
170
]
The excessive aggregation of NFAs under thermal heating or light illumination by a photo-
crosslinkable small molecule as the third component has been proposed by Wang et al.
171
Reduced
NFA crystallites can enhance ductility and mechanical properties. It has been proposed that by
controlling the crosslinking density, including crosslinker concentration and crosslinking time,
mechanical toughness of active layer film has been improved. The enhanced device stability is
attributed to the suppressed crystallization of the polymer donor chains and inhibited excessive
aggregation of NFA.
53

Interestingly, introducing insulating and ductile polymers at low ratios as the third component
can enhance power conversion efficiencies, stabilize morphology, and improve stretchability of
devices.
172,173
Although these polymers usually do not show excellent electronic properties,
applying them as the third component can offer better mechanical characteristics and stability to
the active layer.

Figure 1.17 The schematic morphology of the blend without (a) and with (b-f) SEBS ratios (0–
30%). (g) Chemical structures of PM6, N3 and SEBS. Reproduced with permission [
174
]
Peng et al. studied the relationship between morphology and mechanical/photovoltaic
performance of a novel ternary system comprised of a polymer, a non-fullerene acceptor, and a
thermoplastic elastomer (PM6:SEBS:N3) (Figure 1.17).
174

Due to the soft/ductile properties of SEBS and the positive correlation between COS and SEBS
domain size, it was found that the deformation of SEBS and the size, shape, and distribution of
SEBS in the ternary blend films play a critical role in mechanical stretching. Besides, the variation
of PM6/N3 aggregate structure induced by adding SEBS may also affect the stretchable properties.
54

For low content of SEBS (≈2%), its domains are isolated and small, which contributes to the
improvement of stretchability. This improvement in mechanical stretchability was attributed to the
soft/ductile aid of SEBS phases, better out-of-plane π–π stacking, and higher face-on fraction.
175,176

Han et al. introduced PAE matrices strategy by adding a resin as supporting matrices into the
PM6:Y6 blend (Figure 1.18). Poly(aryl ether) (PAE) resins have highly twisted-stiff backbones
without any side chains, which possess excellent mechanical stability, thermal stability, and good
compatibility with organic photovoltaic materials. It was shown that addition of 5% PAE into
PM6:Y6 could serve as supporting matrices with a tunneling effect without sacrificing
photovoltaic performance and simultaneously improve the flexibility and stability of devices by
restraining the migration of molecular chain and fasten morphology of the host.
147,177

55

Figure 1.18 The schematic morphology of the proposed mechanisms when poly(aryl ether) is
introduced into the BHJ layer to enhance (a) thermal and (b) mechanical stability. Reproduced with
permission [
147
]
A comparison study by Dauzon et al. between addition of a crosslinker or an elastomer suggests
that the elastomer method seems to be considerably more universal by acting as a mechanical
softener in modern fullerene-free blend systems.
178

Lee et al. demonstrated that the use of donor-acceptor alternating copolymer-type
compatibilizers (DACCs) in high-performance SMA-based OSCs could enhance the PCE, thermal
and mechanical stability, simultaneously (Figure 1.19).
179
It was shown that DACC could stabilize
56

the blend morphology and improve the D:A interfacial properties while acting as an electroactive
constituent that directly contributes to charge generation/transport in the blends. Therefore, the
thermal stability and mechanical robustness of the OSCs can be increased by suppressing the
amalgation of the domains and improving the interfacial adhesion/cohesion properties.

Figure 1.19 (a) The chemical structures of P D, SMA, and DACC. (b) The schematic of the
morphological evolution under thermal stress without DACC. (c) The schematic effect of DACC on
the morphology by improving the interaction between P D and SMA and stabilize the P D–SMA
interfaces. Reproduced with permission [
179
]
Introducing a flexible non-conjugated spacer into backbones of polymers has been
demonstrated as an efficient approach to enhance the photovoltaic and mechanical properties.
180

Thompson et al. determined potential co-optimization strategies for photovoltaic and
mechanical properties in OSCs. A broad family of ductile semirandom donor–acceptor (D–A)
copolymers with 8-carbon alkyl conjugation break spacer (CBS) units were incorporated into a
57

brittle binary host of P3HTT-ehDPP-10%:PCBM. A CBS, such as a flexible alkyl chain, can
reduce the elastic modulus and increases ductility.
181
It was observed that the hole mobilities of
the ternary active layers generally increased upon increasing the content of the CBS polymer up
to 15% of the overall donor fraction. In other words, introducing highly stretchable CBS polymers
with poor charge mobility does not adversely affect solar cell performance, offering insights into
the development of ternary strategies for flexible/stretchable organic solar cells.
182,183

Recently, all-PSCs have attracted enormous attention as suitable candidates for power
generation in portable and wearable photoelectric devices owe to their high mechanical ductility
and robustness.
184–186
Owing to the rapid developments of polymer acceptors,
166,187–189
the record
PCE of all-PSCs based on polymer acceptors have now surpassed 17%,
82
which is close to the top
PCE of polymer solar cells based on NFAs.
20,37,47
Compared with polymer:small molecule solar
cells, all-PSCs often exhibit excellent morphological and thermal stability and mechanical
robustness properties, which are favorable to manufacture large-scale solar cell modules.
126,190–195

The miscibility of the host and the guest polymer could be an effective factor to achieve high
performance and excellent stability.
196
For example, Ma et al. presented a ternary system with two
well miscible polymer donors, PM6 and J71, and a polymer acceptor PY-IT that could outperform
the binary reference. This improvement is attributed to effective energy transfer, tuned
crystallinity, and improved phase separation. Furthermore, the good miscibility between two
donors enabled the active layer to have well-maintained film morphology without damaging the
initial nanoscale network, accompanied with film ductility.
197

58

Table 1.4 summarizes the performance and the operating mechanism for mechanical stability
for some of the discussed ternary systems in the context

59

Table 1.4 Photovoltaic performance of the ternary devices and the corresponding mechanisms
for mechanical stability enhancement.
Content
of
additive
Ternary blend VOC (V)
JSC
(mA cm
-2
)
FF PCE (%) Mechanism
2% PM6:SEBS:N3 0.839 25.8 72.4 15.69 Increase of soft/ductile component
174

20% FTAZ:PBDB-T:IT-M 0.95 18.1 73.6 12.7
Suppression of SMA
crystallization
138

10% PM6:P(NDI2ODT2):Y7 0.87 25.31 69 15.19
Interconnect domains/tie
molecules
168

20% PM6:J71:PY-IT 0.942 23.29 75.2 16.52
Optimized crystallinity/phase
separation
197

10% D18-Cl:G19:Y6 0.871 27.36 77.72 18.53 Morphology modifier
92

20% PM6:PCBM:PT-IT 0.956 22.93 73.6 15.92 Morphology regulator
169

30%
PTB7-Th:PC71BM:P(NDI2HD-
T2)
0.81 15.99 53 6.8 Morphology modifier
170

20% PBDB-T:DACC:Y10-Br 0.92 24.11 77 17.08
Improving the interfacial
adhesion/cohesion properties
179

1.4 Conclusion and Outlook
After a decade of rapid development, the best PCEs of OSCs have surpassed 19%, suggesting
that OSCs are on the right path to catching up with commercialization. Although great progress
has been made, there are still challenges and unanswered questions for the further development of
OPV technology. Here, we propose possible optimization pathways to further improve the
performance and operational stability of OSCs.
1. Investigation into the formulation of polymer acceptors
Currently, most of the NIR organic materials for OPV devices are based on the small-molecule
NFAs. Since usually the polymeric molecules can provide stronger absorption and better phase
stability than the small molecules, polymer acceptors are therefore of great interests. In this case,
it is still unclear what type of donor materials should be used to pair with the polymeric acceptors.
60

PolymerD:polymerA combination is an appealing pair because of the enhanced phase stability and
improved mechanical properties. Furthermore, the judicious and rational selection of the third
component has a high impact on all-PSCs. The guest polymer can significantly suppress the phase
separation and suitably stabilize the blend microstructure; however, controlling the morphology
and charge transport in ternary all-PSCs is very challenging. Therefore, more investigations need
to be done to fully unveil the potential of the polymer acceptors.
2. Understanding the synergistic effects of structure-morphology-processing techniques
Morphology control has a significant impact on OSCs stabilities. Processing treatments and
techniques, such as solvent engineering, thermal annealing, solvent vapor annealing, additive
engineering and sequential deposition can effectively control crystallization and phase separation
and optimize the morphology of the blend films. However, the method that works for one system
is not necessarily effective for another system. Thus, these optimization methods often need to be
carefully and even fully explored. In addition, one can adjust the side chains and regioregularity
of the polymer to effectively control the conformation, orientation, and stacking of the molecule.
For example, high regioregularity in conjugated polymers can enhance optical and electrical
properties while reducing mechanical thoughness.
198,199
Therefore, precise control of these
strategies in a synergistic and elegant way to achieve a win–win situation needs to be addressed.
3. Future commercialization of OPVs
OPV technologies have a scope in vast diverse applications in bioelectronics, transistors, and
sensors. Therefore, the device stability issue should be investigated more carefully toward future
commercial applications. For example, for use as the power source in wearable devices such as
implantable body sensors or transdermal biosensors, ultra-flexible OPVs must satisfy some
requirements, such as transparent electrode with high transparency, conductivity, and the
61

fabricated devices must exhibit remarkable mechanical stability.
200
Furthermore, beyond high
photovoltaic efficiency and stability, low cost is another essential prerequisites for commercial
application. For example, One gram of the common donor material PM6 and acceptor material Y6
cost more than $4000. Therefore, for industrial production and commercial applications the
developments of low-cost manufacturing in materials or manufacturing methods are demanded for
mass production.
To sum up, we strongly believe that ternary strategy would provide insights for OPVs
developments and pave roads leading to rapid commercialization of highly efficient solar cells
with large area, high stability, flexibility, and efficiency.

62

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Chapter 2: Ternary Blend Organic Solar Cells Incorporating Ductile Conjugated
Polymers with Conjugation Break Spacers
2.1 Introduction
Organic semiconducting polymers, in contrast to bulk inorganic semiconductors, are
solution processable and offer the potential for cost-effective printing on flexible, large area
substrates to generate flexible/stretchable electronics (such as wearable devices).
1
Both high
charge carrier mobility and mechanical compliance are required for such applications.
2
Charge
transport mechanisms have been intensively studied in conjugated polymers (CPs) and great
improvements have made in the last three decades.
3,4
However, a deep understanding and
optimization of the mechanical performance of semiconducting polymers is still in its infancy.
5

Compared to high molecular weight commodity polymers, such as polyethylene or nylon, organic
semiconducting polymers do not exhibit high strength and toughness.
6,7
For flexible/stretchable
electronics applications, materials with low elastic modulus are required. It was shown that many
conjugated polymers have considerable stiffness (elastic modulus varies widely in the range of
0.1 to 8 GPa).
8–12
In other words, compared to conventional elastomers such as
poly(dimethylsiloxane) (PDMS) (E = 0.6−2.5 MPa)
13
or polyisoprene (E = 0.36 MPa),
13,14
most
fully conjugated polymers have poor mechanical properties, are brittle, and tend to fracture at low
strains (<10%).
10,15,16
Thus, designing new organic semiconducting polymers and developing new
strategies for multi-component systems that can effectively improve the electronic and
mechanical properties of flexible, stretchable, and long-lasting organic electronics is of
paramount importance. These materials should possess high elasticity (low modulus), elastic
limit (yield point), strain at fracture, and toughness.
17–19

83

Several approaches have been explored toward this end. For example, it was shown that
long and extended side chains can decrease the volume fraction of the stiff conjugated units and
open up the space between polymer chains. Consequently, the elastic modulus is decreased which
results in better mechanical properties of the semiconducting polymer.
20,21
Other than side chain
engineering,
22–24
there are a few other approaches have been studied to increase the elasticity of
semiconducting polymers, including introducing dynamic non-covalent (hydrogen bonding) or
soft crosslinkers,
9,25
nanoconfinement,
26,27
elastomeric matrices,
28,29
and double-crystalline block
copolymers.
30

Recently, conjugation break spacers (CBS) were introduced to the field of organic
electronics.
9,31–37
A CBS unit is a non-conjugated segment, such as a flexible alkyl chain that
disrupts the conjugation along the polymer backbone. Initially, conjugation breakers were
designed to improve processability of polymer semiconductors. Several studies have shown that
incorporating flexible CBS units into conjugated backbones is an effective technique to modulate
solubility,
38
solution processability
39
and even offers melt-processable semiconducting
polymers,
40,41
which eliminates the need for toxic organic solvent for thin film formation. Another
focus of CBS research is to improve mechanical properties and specifically, stretchability of
conjugated polymers. Flexible linkages cause numerous degrees of conformational freedom and
energetic disorder in polymer chains. Savagatrup et al. reported an improvement in ductility and
modulus by increasing proportions of the nonconjugated unit in diketopyrrolopyrrole ‐based
polymers.
37
The optoelectronic properties of multiple conjugated rigid segments that are linked by
non-conjugated soft chains can be molecular weight independent. Li et al. showed that compared
with a small molecule reference, rigid conjugated segments linked by non-conjugated soft
segments had better photovoltaic performance by a factor of 29–73%.
42
Overall, it is clear that
84

incorporation of CBS units into conjugated polymers has greatly impacted mechanical properties,
where CBS polymers have consistently shown elastic moduli of less than 1 GPa. However,
balancing this improvement and charge carrier mobility is nontrivial, and in many cases still
remains a challenge.
9,37

In the past two decades, organic photovoltaics (OPVs) have been considered as one of the best
strategies for the sustainable production of electricity due to their advantages of solution
processability, light weight and flexibility.
43–46
One aspect of OPV technology that needs to receive
more attention is the mechanical properties of conjugated polymers and the changes in
photovoltaic performance of devices under mechanical strain. Achieving high efficiency and
robust mechanical properties simultaneously is quite challenging due to the fact that crystalline
semiconducting polymers with high charge mobilities tend to be brittle.
21,47
In the first reported
stretchable organic solar cells, an active layer was spin coated on a pre ‐strained elastomeric
membrane (P3HT:PC61BM on PDMS substrate). The microscale wrinkles that formed upon
release of the strain imparted elasticity to the device under tensile strain (up to 27%) with very
little change in the photovoltaic properties.
48
To date, the highest PCE of flexible organic solar
cells has reached above 12%.
49
However, there are few examples in the literature of intrinsically
stretchable materials used for organic solar cells.
37,50,51
Savagatrup et al. studied the effects of CBS
units on the mechanical and photovoltaic properties of a series of diketopyrrolopyrrole ‐based
polymers. It was reported that the photovoltaic properties of DPP ‐based CBS polymers with
PC61BM decreased with increasing ratio of the CBS units. The major challenge in utilizing CBS
polymers in OPV arises directly from the competition between their electronic and mechanical
properties where CBS content has been observed to have a negative impact on the charge carrier
mobility of the polymer.
37

85

Constructing ternary organic solar cells is a potentially convenient strategy to further enhance
performance with CBS polymers. The concept of ternary organic solar cells initially was proposed
as an effective strategy to boost the device performance by broadening the light absorption.
52,53

This was achieved by incorporating a third component with complementary absorption into a
binary donor-acceptor host.
54
Later, many studies reported that a third component could also
enable modulation of energy levels and film-morphology of the active layer in solar cells and
demonstrated that the third component in ternary blends plays versatile functions.
55
In other words,
the advantages of the third component can be reflected in the device performance and enhance
short-circuit current-density (JSC), open-circuit voltage (VOC), and fill-factor (FF).
56–59
The
performance of ternary devices considerably depends on the materials combinations that can form
matched electronic structure and proper blend morphology for charge generation and transport.
60,61

Baran et al. reported that employing a third component into a low-bandgap:fullerene binary solar
cell not only enhances the photovoltaic performance but also synergistically improves both storage
lifetime and photo-stability.
62
Recently, Huang et al. reported that the mechanical properties of a
ternary blend of PBDTTT-OFT:IEICO-4F:PC71BM were superior to the corresponding binary
devices.
63
This is typically due to the fact that compared with conventional polymer-fullerene solar
cells, non-fullerene acceptors (NFA) and especially polymer acceptors are intrinsically more
ductile than fullerenes.
64,65

Based on the current deficiencies in stretchable OPV, we were motivated to investigate a class
of highly elastic semi-random CBS polymers previously developed in our group
21,47
as
components in ternary blend OPV. Our focus is to elucidate the impact of the CBS polymers on
the electronic device characteristics of the solar cells to validate whether such an approach has the
potential to lead to high performance solar cells. Here we have focused on ternary blends based on
86

a fully conjugated polymer, a CBS polymer, and PC61BM, with the understanding that fullerene-
based OPV will likely not have strong mechanical properties,
17,64,66,67
but rather with a focus on
demonstrating if adding CBS polymers (with inferior electronic properties) can be tolerated with
retention of device performance and thus could potentially provide a route to solar cells with
intrinsically stretchable active layers. The idea of introducing these CBS polymers into a fully
conjugated polymer:fullerene binary system is to provide a pathway to active layers with enhanced
mechanical deformability for stretchable OPV. In order to design our ternary systems, we have
considered to employ donor polymers with good chemical structure compatibility, as
incompatibility between the host and the guest components could disturb the optimized
morphology of the donor/acceptor binary blend active layer.
68,69

In our previous studies we synthesized polymers with CBS units in 3-hexylthiophene-DPP
based semi-random polymers and we examined several different classes. Members of these classes
selected for this study are shown in Scheme 2.1.
21,47
Here we focus only on CBS polymers with
8-carbon alkyl spacers. In class I, the CBS content was varied concurrently with the DPP monomer
content (with 2-ethylhexyl (eh) side chains) at 10% and 20% with a corresponding 3-
hexylthiophene content of 80% and 60%. For this class of polymers, it was shown that with 10%
of 8-carbon alkyl spacer, the elastic modulus was decreased from 0.32 MPa with the fully
conjugated analogue (P3HTT-ehDPP-10%, Scheme 1) to 0.14 GPa and crack-onset strains (COS)
increased from 10% to >80% (using the film-on-elastomer technique). Compared to the fully
conjugated analogue, the hole mobility decreased from 9.29 x 10
-4
cm
2
V
-1
s
-1
to 2.08 x 10
-5
cm
2

V
-1
s
-1
. By increasing the content of spacer to 20%, the elastic moduli increased to 0.65 GPa, while
COS remained >80% and mobility dropped one more order to 6.49 x 10
-6
cm
2
V
-1
s
-1
. Overall, the
mode of failure for this class of polymers is ductile (compared to the brittle conjugated parent
87

polymer) and more notably, the beneficial properties of P3HT such as absorption and electronic
energy levels were largely retained.
47
The highest occupied molecular orbitals (HOMO), compared
to the fully conjugated polymer, were slightly upshifted and the band gap (Eg) saw similar minor
shifts to higher energy (Table A-1 includes HOMO and Eg data for all polymers).
When the side chain on the DPP monomer was replaced with 2-decyltetradecyl (dtd) (Class II,
Scheme 2.1, the solubility was improved, and higher molecular weights were obtained (molecular
weight data for all polymers in Scheme 2.1 are shown in Supplementary Table A-1). These
polymers achieved remarkable mechanical properties. For example, with 20% of CBS monomer
in the polymer chain, an elastic modulus of 15.07 MPa was measured along with a fracture strain
of 185% (using the film-on-water technique). By increasing the CBS ratio to 30% and 40%, elastic
moduli of 14.84 and 27.39 MPa were obtained, respectively, while fracture strains increased to a
remarkable 325% and 398%, respectively. Predictably, these structures with outstanding
mechanical properties resulted in diminished electronic properties. Compared to the fully
conjugated dtdDPP parent polymer (analogous to P3HTT-ehDPP-10%), the hole mobility
decreased more than one order from 4.24 x 10
-4
cm
2
V
-1
s
-1
to 1.01 x 10
-5
, 6.22 x 10
-6
and 4.92 x
10
-6
cm
2
V
-1
s
-1
for 20%, 30% and 40% of CBS loading, respectively.
21

Finally, in the third class of polymers, the ehDPP monomer content was fixed at 10 mol %,
and the CBS monomer was incorporated at 20% and 30% while corresponding 3-hexylthiophene
content was at 70% and 60%, respectively. Extremely high ductility was achieved in this class of
polymers at the cost of electronic properties. With 20% and 30% of CBS in these polymers, elastic
moduli of 52.7 and 8.54 MPa were obtained, respectively, while fracture strains were measured at
200% and 432%, respectively (film-on-water).
21
The hole mobility of this class of polymer
dropped from 9.29 x 10
-4
cm
2
V
-1
s
-1
to 7.05 x 10
-6
cm
2
V
-1
s
-1
and 2.06 x 10
-6
cm
2
V
-1
s
-1
when
88

20% and 30% of CBS were incorporated into the polymer, respectively.
21
Evaluation of these
series of polymers leads to the conclusion that there is a trade-off between mechanical and
electronic properties and that the more elastic these polymers become, the worse the charge carrier
mobility becomes.
All of the CBS polymers in Scheme 2.1 were initially tested in in binary solar cells with
PC61BM as the acceptor. Likely due to their deficient hole mobility, the binary device
performances were extremely poor. For example, for an optimized 10% T-8-T/10%
ehDPP:PC61BM binary system (Class I), the highest PCE achieved was 0.7% (JSC=3.2 mA cm
−2
,
VOC=0.65 V, FF=34, Table A-2). As such, we sought to study the unique functionalities of these
polymers in ternary organic solar cells and investigate the device performances by incorporating
the CBS polymers into a fully conjugated polymer host system (Scheme 2.1) based on P3HTT-
ehDPP-10% and PC61BM. To the best of our knowledge, this is the first ternary solar cell study
incorporating intrinsically elastic CBS polymers.

89

Scheme 2.1 Chemical structures of P3HTT-ehDPP-10% (parent donor polymer), PC 61BM as the
acceptor and CBS donor polymers used in this study.

2.2 Results and Discussion
2.2.1 Photovoltaic Performance
To evaluate the potential of these polymers in OPV, ternary bulk heterojunction solar cells
were fabricated with an ITO/PEDOT:PSS/conjugated polymer:CBS polymer:PC61BM/Al
conventional device architecture. All of the devices were fabricated and characterized in air. The
J-V characteristics of the devices were measured at an active area of 5.18 mm
2
. O-dichlorobenzene
(o-DCB) was used as the solvent with the overall polymer concentration of 10 mg/mL. For all
devices the weight ratio of donors:PC61BM was kept constant at 1:1.3. For ternary devices, CBS
polymers were added at 10%, 15% and 25% of the total donor polymer fraction. Higher ratios of
90

CBS (up to 50%) were tested for some CBS polymers (Table A-2). However, adding more than
25% of CBS in the ternary blend was observed to deteriorate the device performances sharply.
Therefore, here we focus on the photovoltaic properties of ternary devices with up to 25% CBS
polymer in the donor fraction. The photovoltaic performance data including VOC, JSC, FF, and PCEs
for host binary and ternary devices are summarized in Table 2.1. The JSC values were well matched
(within 5% error) with the integrated JSC values obtained from the EQE spectra (see Table A-3 for
the mismatch factor and JEQE). The average PCE of the fully conjugated P3HTT-ehDPP10%:
PC61BM reference was 4.35% over 25 pixels (VOC=0.6 V; JSC=10.8 mA cm
−2
; FF=55.1). Upon
introducing the CBS polymers of Class I, 10% T-8-T/10% ehDPP and 20% T-8-T/10% ehDPP,
the efficiency was observed to decrease in both cases as the content of the CBS polymer increased
in the system. Predictably, we attribute losses in efficiency primarily to the fact that the
optoelectronic properties of these CBS polymers are inferior to the fully conjugated parent, which
leads to decreases in the JSC and FF, especially at high contents.
47
Since the hole mobility of 20%
T-8-T/10% ehDPP is lower than 10% T-8-T/10% ehDPP, we rationalize this as a reason why
devices containing the former experienced a greater decrease in efficiency than the latter.

91

Table 2.1 Photovoltaic properties of ternary P3HTT-ehDPP-10%:CBS:PC 61BM Solar Cells.
CBS polymer
Composition
P3HTT-ehDPP-10%::CBS:PC61BM
a
(% CBS in polymer fraction)
JSC
(mA/cm
2
)
b,c

VOC
(V)
FF
PCE
(%)
Crack-onset
strain
(COS)
(%)
21,47

P3HTT-ehDPP-10%
(reference)
1:0:1.3 10.8 0.60 55.1 4.35 10
d

10% T-8-
T/10%ehDPP
(ClassI)
0.9:0.1:1.3 (10%) 9.56 0.61 52.4 3.92
>80
d, e
0.85:0.15:1.3 (15%) 9.56 0.59 48.9 3.55
0.75:0.25:1.3 (25%) 8.25 0.60 48.4 3.09

20% T-8-
T/20%ehDPP
(ClassI)
0.9:0.1:1.3 (10%) 9.47 0.59 50.8 3.66
>80
d, e
0.85:0.15:1.3 (15%) 9.55 0.57 46.4 3.15
0.75:0.25:1.3 (25%) 8.27 0.58 44.3 2.68

20% T-8-
T/20%dtdDPP
(ClassII)
0.9:0.1:1.3 (10%) 10.00 0.59 52.4 3.94
185 ± 26
f
0.85:0.15:1.3 (15%) 10.98 0.59 52.7 4.36
0.75:0.25:1.3 (25%) 8.44 0.59 48.1 3.12

30% T-8-
T/30%dtdDPP
(ClassII)
0.9:0.1:1.3 (10%) 10.60 0.59 54.3 4.32
325 ± 44
f
0.85:0.15:1.3 (15%) 10.30 0.59 51.2 4.00
0.75:0.25:1.3 (25%) 10.30 0.60 52.7 4.21

40% T-8-
T/40%dtdDPP
(ClassII)
0.9:0.1:1.3 (10%) 10.70 0.60 58.1 4.82
398 ± 32
f
0.85:0.15:1.3 (15%) 9.30 0.59 55.1 4.05
0.75:0.25:1.3 (25%) 9.83 0.59 49.7 3.85

20% T-8-
T/10%ehDPP
(ClassIII)
0.9:0.1:1.3 (10%) 9.27 0.60 53.2 3.70
200 ± 5
f
0.85:0.15:1.3 (15%) 9.54 0.60 53.7 3.89
0.75:0.25:1.3 (25%) 8.90 0.60 49.2 3.33

30% T-8-
T/10%ehDPP
(ClassIII)
0.9:0.1:1.3 (10%) 7.39 0.59 45.9 2.56
432 ± 38
f
0.85:0.15:1.3 (15%) 8.87 0.61 51.8 3.52
0.75:0.25:1.3 (25%) 11.04 0.60 54.9 4.60

a
All devices were spin-coated from o-dichlorobenzene (o-DCB) and dried under N2 for 30 min
before aluminum deposition. b Mismatch corrected. c Standard deviations of less than 0.5
mA/cm
2
were observed in all cases averaged over six to twenty-five pixels. d Obtained from
optical micrographs, film-on-elastomer measurements from neat polymers. e Test terminated at
92

80% strain due to potential for PDMS substrate breakage. f Obtained from strain at failure, film-
on-water measurements from neat polymers

With class II CBS polymers, the 2-ethylhexyl side chains on DPP are replaced by 2-
decyltetradecyl. In the ternary blends, the addition of the Class II CBS polymers does not decrease
the Jsc as much as the Class I polymers and the average current in this set of ternary devices is
higher, especially at lower CBS contents. For example, by addition of 15% of 20% T-8-T/20%
dtdDPP, the current is slightly higher than the binary reference system. The higher current in this
set could possibly be attributed to the higher molecular weight of this family of polymers (see
Table A-1). With 30% T-8-T/20% dtdDPP even up to 25% CBS polymer incorporation resulted
in almost the same performance compared to the fully conjugated reference binary system. This
means that incorporation of a CBS polymer into the host did not inhibit the system, despite the
poorer electronic properties of the CBS, even up to 25% composition. As previously noted, the
charge mobility in this set of polymers is poor and the mobility decreases as the CBS content is
increased. However, for 40% T-8-T/40% dtdDPP (with the lowest hole mobility in Class II of 4.92
x 10
-6
cm
2
V
-1
s
-1
) we could still observe that at 10% incorporation of this CBS polymer into the
binary system, the device efficiency (4.82%) actually exceeds the reference binary (4.35%). As
such, adding 10% of a highly elastic polymer with an inadequate hole mobility could actually make
a binary device work better. However, higher content of this polymer resulted in decreasing current
and efficiency.
Lastly, with the Class III CBS polymers, addition of 20% T-8-T/10% ehDPP follows a similar
trend observed in the previous CBS Classes. Although the overall current, FF and efficiency of the
ternary devices are lower than the binary, the ternary devices are still functional up to 25% content
93

of the CBS polymer. All J-V (Figure A-1 to A-7) and EQE (Figure A-8 to A-14) curves are shown
in appendix. Surprisingly, addition of 30% T-8-T/10% ehDPP showed the opposite trend
compared to the previous CBS polymers, where increasing the content of the CBS polymer
resulted in an increasing PCE and an efficiency exceeding the binary reference (4.60 vs. 4.35%)
was achieved when incorporating 25% of this polymer. Figure 2.1a shows the current density
versus voltage (J–V) curves with a flux of 100 mW/cm
2
under simulated AM 1.5G conditions and
Figure 2.1b shows the external quantum efficiency (EQE) of the optimized ternary solar cells with
30% T-8-T/10% ehDPP at different donor ratios. At the ratio of 25% CBS, the EQE values of
ternary devices are higher than the binary reference in the low energy region of 600-800 nm, but
lower in the high energy region of 400-600 nm. As indicated in Table 2.1, this polymer showed
the highest fracture strain (over 430%) which is among the highest reported for conjugated
polymers.
Although comparable values of JSC were obtained for most of the ternary devices, our results
demonstrate that CBS polymers with dtd side chains on DPP generally showed higher JSC and FF.
One hypothesis is based on the side chains on the CBS polymers being longer than the side chains
on the host polymer. This could result in modulating the morphology to generate an
interpenetrating mesoscale polymer domain based on similar results reported by Chang et. al.
56
In
their work, a ternary blend with two polymer donors with nearly identical absorption spectra and
similar energy levels were used. However, the side chains on their D1 and D2 were oriented
differently. They proposed that introducing the second donor to the binary system improved the
morphology from nanoscale (10–20 nm) fine fibrils to a meso scaled morphology, in which the
donor domain volume swelled after adding D2, coupled with reduced donor phase crystallinity in
the ternary blend. The better charge collection and higher JSC they observed were attributed to the
94

resulting interpenetrating network of the donor and acceptor phases. Overall, though, the ternary
solar cell results presented here demonstrate that addition of up to 25% CBS polymer in the donor
fraction leads to functioning devices with similar and sometimes improved performance. This
bodes well for the future use of this strategy in flexible/stretchable OPV.

Figure 2.1 (a) J–V curve and (b) EQE spectra of ehDPP-10%:30% T-8-T/10% ehDPP:PC 61BM.
P3HTT-ehDPP-10%: PC 61BM J-V curve and EQE spectrum provided for fully conjugated reference.

95

2.2.2 Morphology and Charge Carrier Mobility
Crystallinity in as-cast ternary blend films was examined by grazing incidence x-ray diffraction
(GIXRD). GIXRD data for all neat CBS polymers was previously reported.
21,47
The GIXRD data
showing the 100 diffraction peaks corresponding to the polymer lamellar packing for all ternary
active layers are shown in Figure A-15 to A-21 and Table A-4. Compared to the fully conjugated
binary reference system, all ternaries showed an increase in the lamellar packing distance.
Surprisingly, in ternary blends constructed with Class I CBS polymers, for all ratios, a higher
intensity peak was observed compared to the fully conjugated DPP:PC61BM blend, indicating
enhanced crystallinity relative to the binary reference. In our previous studies, we showed that
this class of polymer exhibited a lower intensity diffraction peak compared to P3HTT-ehDPP-
10%. Although, introducing the third component into a binary system may disturb the crystallinity
of both donor and acceptor phases,
63
many studies reported that the optimized morphology, in
terms of molecular crystallite orientation and aggregation, can be maintained
69
or enhanced
70
in
ternary blends. This could be due to the structural similarity and compatibility between the third
component with the host donor or acceptor.
68,71
However, it is quite rarely reported that the
crystallinity of the ternary active layer is improved by a less crystalline component.
72
The intensity
of the peak is highest for the 10% blend and decreased at 15% and 25% ratios.
In contrast, with Class II CBS polymers, except for the cases of 10% of 20% T-8-T/20%
dtdDPP and 30% T-8-T/30% dtdDPP, the diffraction peak for all ternary blends was decreased
compared to the fully conjugated binary reference. However, the lamellar packing distance
increased even more compared to Class I. It is likely that the longer alkyl side chains on the DPP
units of the CBS have a more disruptive impact on the morphology. The highest JSC in Class II
was achieved at 15% of 20% T-8-T/20% dtdDPP (10.98 mA cm
−2
which shows 1.7% improvement
96

compared to the binary reference) with crystallinity slightly lower than the reference. In ternary
systems with 40% T-8-T/40% dtdDPP, although all peak intensities are lower than the reference,
at 10% conent the current is almost as high as the reference binary cell (10.7 mA cm
−2
). The more
significant changes in morphology induced by Class II polymers, coupled with improved device
performance relative to Class I, suggests that the switch from eh to dtd side chains may have a
similar impact as in the work reported by Chang et. al
56
noted earlier.
In the third class of CBS polymers, except for the 10% ternary blend of 30% T-8-T/10%
ehDPP, all ternary films showed higher intensity peaks than the DPP:PC61BM binary reference.
The GIXRD patterns for 30% T-8-T/10% ehDPP ternary blends are shown in Figure 2.2. The
peak intensity at 10% CBS in the blend is slightly lower than the fully conjugated reference sample.
For this polymer with good structural compatibility with the host (10% ehDPP in both polymers)
and the highest amount of CBS (30%), the intensity of the lamellar peak increases by addition of
CBS polymers in the system, where 15 and 25% incorporation of the CBS polymer shows a
significant enhancement in crystallinity relative to 10% incorporation and the reference binary.
Although, the lamellar packing distance is increased compared to the binary cell, increasing the
content of CBS polymer in the system (from 10% to 25%) does not change the d100. The increasing
crystallinity is correlated with the observed JSC for these ternary solar cells.

97

Figure 2.2 Grazing-incidence X-ray diffraction patterns of P3HTT-ehDPP-10%:30% T-8-T/10%
ehDPP:PC 61BM thin films spin-cast from o-dichlorobenzene (o-DCB) and dried under N 2 for 30
min. P3HTT-ehDPP-10%: PC 61BM data provided for fully conjugated reference
Although introducing a third component has proven to be an effective way to enhance the
photovoltaic performance compared with binary devices in many cases, rationally designing a
miscible component for the host binary system to achieve a well ‐developed morphology is quite
challenging.
73–75
It has been shown that structural similarity between the host and the guest helps
to maintain the film morphology of the binary host.
76,77
Peng et al. reported that synergistic effects
of two structurally compatible components with good miscibility enhanced the charge transport in
a ternary system.
78
Our results demonstrate that constructing ternary devices by incorporating
structurally similar polymer donors is an effective strategy to maintain (for example in 10% T-8-
T/10% ehDPP and 20% T-8-T/10% ehDPP systems) and further improve (in 20% T-8-T/10%
ehDPP system with 25% of CBS) the photovoltaic performances of the devices.
Space-charge limited current (SCLC) mobility measurements were performed on hole-only
devices for all the ternary blend samples and the binary reference (Figure 2.3 and Table A-5).
Although all CBS polymers showed significantly lower hole mobility compared to the fully
conjugated parent polymer, it could be expected that the ternary active layers would show similar
mobility to the fully conjugated binary reference blend. Mei et al. has shown that in complementary
98

semiconductor blends of CBS polymers, only a few percent of a fully conjugated polymer (as little
as 1 wt%) is needed to induce a nearly 2 order of magnitude improvement in the charge carrier
mobility of the blend.
79–81
Our findings provide a quantitative verification that increased ratio of
CBS polymers in the blend does not undermine hole transport. However, not all blends resulted in
the same mobility as the binary reference. A similar trend in hole mobilities of the ternary blends
was observed in most cases. Specifically, the hole mobility for the CBS ternary blends was
generally observed to increase when the composition increased from 10% to 15% of the donor
fraction and then decrease at the higher loading of 25%.
With the Class I CBS polymers, the ternary devices with 10% T-8-T/10% ehDPP, in which
CBS polymers had the lowest fraction of spacers, showed lower hole mobility compared to the
reference binary for all ratios. With 20% T-8-T/20% ehDPP, while the same trend was observed,
for this ternary system at 15% content, the mobility is comparable to the binary reference.
Although both 10% T-8-T/10% ehDPP and 20% T-8-T/20% ehDPP ternary blends show similar
crystallinity, higher mobility in the latter system could be attributed to the higher content of DPP
in 20% T-8-T/20% ehDPP polymer.
With the Class II CBS polymers, the same general trend, in which mobility was increased up
to 15% content and then decreased, was observed. Moreover, it is clear that as the content of the
CBS in the polymers is increased (from 20 to 30 to 40%), the mobility of the ternary blends
decreased. It was found that the 15% blends of the 20% T-8-T/20% dtdDPP polymer could enhance
the mobility relative to the binary reference. This result is in correlation with photovoltaic
performances, which compared to the P3HTT-ehDPP10%: PC61BM reference, the ternary blend
delivered slightly higher current and similar PCE at the 15% ratio. Note that the GIXRD pattern
for P3HTT-ehDPP10%: 20% T-8-T/20% dtdDPP: PC61BM at 15% is almost the same, in terms of
99

intensity, as the binary reference. Meaning that the optimized binary morphology was maintained
and this polymer at this ratio did not perturb the host system. The mobility in ternary devices with
30% T-8-T/30% dtdDPP does not follow the general trend, however, it is still comparable with the
binary reference up to 15%. The calculated crystallite size at 15% with this polymer is exactly the
same as the binary reference (9.98 nm, Table A-4). Although the crystalline correlation length
(CCL) was slightly lower than the binary, which could be due to the morphology disturbance,
similar crystallite size could have resulted in similar mobilities compared to the binary (1.01 x 10
-
3
cm
2
V
-1
s
-1
and 1.22 x 10
-3
cm
2
V
-1
s
-1
, respectively). The lowest charge mobility for the devices
were observed for 40% T-8-T/40% dtdDPP which has the highest fraction of CBS.
39

For the Class III CBS polymers the trend in 20% T-8-T/10% ehDPP is quite different than the
general trend. Here, the mobility for ternary devices reached the highest value at 25% (1.72 x 10
-
3
cm
2
V
-1
s
-1
). In 30% T-8-T/10% ehDPP ternaries, the difference between charge mobilities at
15% and 25% is very small (0.2 cm
2
V
-1
s
-1
) and the highest current achieved for all ternary solar
cells (11.04 mA cm
−2
) belonged to P3HTT-ehDPP10%:30% T-8-T/10% ehDPP:PC61BM at 25%.
The mobility values for this device and the reference are the most similar among all other blends
(Table A-5, 1.18 x 10
-3
and 1.22 x 10
-3
cm
2
V
-1
s
-1
, respectively).
Although the mobility for these ternary blends showed a general trend with composition (with
the exception of 30% T-8-T/30% dtdDPP and 20% T-8-T/10% ehDPP), no overall correlation was
found between the hole mobility, photovoltaic performances and GIXRD. However, in terms of
hole mobility, in each class, it was shown that ternary devices with CBS polymers containing 20%
of break spacers, have overall better mobilities. Moreover, 20% T-8-T/10% ehDPP and 20% T-8-
T/20% ehDPP systems showed higher GIXRD peak intensity for all ratios compared to the
reference, suggesting an optimum CBS content in designing ternary systems. However, the
100

highest observed PCE’s in the ternary blends were with 10% of 40%-T-8-T/40%-dtdDPP and 25%
of 30%-T-8-T/10% ehDPP. Hence, more investigation is necessary in order to correlate the
efficient charge transport and the degree of disorder in these systems with OPV performance.
Nonetheless, the trend in hole mobilities in ternary devices clearly suggests that there is an optimal
content of CBS polymer in an optimized ternary device, which is generally about 15%. Indeed,
we observed that the PCE values reached the maximum with 10-15% loading in nearly all cases
(with the exception of 30% T-8-T/10% ehDPP).

Figure 2.3 Hole mobility trends in P3HTT-ehDPP-10%:CBS:PC 61BM ternary blends at different
donor ratios.

2.3 Conclusions
Here, seven CBS polymers were successfully incorporated into ternary blend solar cells based
on a fully conjugated polymer:PC61BM binary reference system. The purpose of this study is to
demonstrate that highly elastic CBS polymers can be incorporated into solar cells without
inhibiting device function. Although no mechanical properties were considered in this study (and
would likely not be relevant with a fullerene acceptor), it can be concluded that the binary system
could tolerate up to 25% of the elastic CBS polymer without compromising device performance.
101

In this study it was also found that 25% donor content of 30% T-8-T/10%DPP showed better
performance than the fully conjugated reference binary blend. This polymer also has the highest
fracture strain (432%) among the CBS polymers investigated here. Although CBS polymers suffer
from poor mobility in neat films, they can occupy a significant fraction of the active layer of a
solar cell without compromising electronic properties.
Our results indicate that employing CBS polymers in a ternary blend is a potentially promising
route to advance mechanical properties in polymer-based solar cells. Although PCE in the present
examples is low, this strategy opens an avenue for further development of ternary blends with both
electronic and mechanical function. Such a strategy could enable a method to rationally transform
a binary system into an enhanced ternary blend with improved mechanical properties.

102

2.4 References
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(3) Noriega, R.; Rivnay, J.; Vandewal, K.; Koch, F. P. V.; Stingelin, N.; Smith, P.; Toney,
M. F.; Salleo, A. A General Relationship between Disorder, Aggregation and Charge Transport in
Conjugated Polymers. Nat. Mater. 2013, 12 (11), 1038–1044.
(4) Fratini, S.; Nikolka, M.; Salleo, A.; Schweicher, G.; Sirringhaus, H. Charge Transport
in High-Mobility Conjugated Polymers and Molecular Semiconductors. Nat. Mater. 2020, 19 (5),
491–502.
(5) Root, S. E.; Savagatrup, S.; Printz, A. D.; Rodriquez, D.; Lipomi, D. J. Mechanical
Properties of Organic Semiconductors for Stretchable, Highly Flexible, and Mechanically Robust
Electronics. Chem. Rev. 2017, 117 (9), 6467–6499.
(6) Bruner, C.; Dauskardt, R. Role of Molecular Weight on the Mechanical Device
Properties of Organic Polymer Solar Cells. Macromolecules 2014, 47 (3), 1117–1121.
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109

Chapter 3: A Universal Strategy to Improve the Mechanical Properties of Polymer
blends by Incorporating Ductile Conjugated Polymers with Conjugation Break Spacers
3.1 Introduction
Organic photovoltaic technologies have been used in variety of applications with many
economic and environmental advantages, such as solar refrigerators, watches, power banks, phone
chargers, and outdoor lighting devices.
1
Recently, stretchable electronics have attracted attention
due to their extraordinary potential in the field of wearable electronics such as skin-like and bio-
implanted electronics.
2
One of the key advantages of organic photovoltaic materials and in
particular conjugated polymers, is their potential to be flexible and stretchable on different
substrates. Flexible organic solar cells have been extensively investigated in different aspects, such
as materials design, interfacial layers and device fabrication processes.
3,4
However, their efficiency
and more importantly, their mechanical properties are still lagging behind the requirements for
applications in wearable devices (often crack onset strain (COS) of at least 20–30% required).
5

Lee et al. showed that only ~50% of the performance of stretchable PM6:Y7 devices under 15%
strain is maintained and they failed performing at higher strain (~20%).
6
Therefore, for commercial
applications, enhancing long-term performance and mechanical robustness is indispensable.
The presence of rigid backbones in conjugated polymers has some drawbacks, such as
decreasing their solubility and inducing strong aggregation of polymers in the solid state. As a
result, the active layer exhibits unoptimized morphology and unsatisfactory mechanical
properties.
7
Different approaches have been investigated to overcome this drawback including a
ternary strategy inserting an insulating polymer (PS and SBS)
8
into the active layer, and
introducing a flexible spacer in conjugated backbone of the polymer.
9
Although these strategies
have shown positive impact on COS, there are still some challenges. For example, the addition of
110

insulating polymers into the active layer will dilute the conjugated compound, and the presence of
amorphous regions will lower charge carrier mobilities and PCEs in devices.
10
Moreover,
polymers containing discrete non-conjugated segments are found to display inferior electrical
properties relative to fully conjugated polymers. These challenges motivated us to combine these
two strategies. In our previous study, a large library of random conjugation break spacer (CBS)
containing polymers
11,12
were incorporated into a binary host blend of a fully conjugated polymer
and a fullerene acceptor (P3HTT-ehDPP-10%:PC61BM, Chapter 2). It was found that blends of a
CBS polymer and PC61BM resulted in low PCE (<1.0%), likely due to the lower hole mobility of
the CBS polymers. However, in ternary blends with up to 25% CBS polymer in the donor fraction,
PCE values similar to or exceeding the binary reference were observed in several cases.
13
This
work demonstrated that CBS polymers can effectively be used as a synergistic component in a
ternary blend without detriment to PCE and could thus offer a path to enhance the mechanical
properties of OPVs through ternary blends. Due to the brittle nature of PC61BM in the studied
model systems compelling mechanical properties were not expected. Recently, high fracture
resistance of all-polymer solar cells is of great importance to ensure long-term mechanical
stability. Introducing CBS polymers with superior mechanical properties as the third component
may be a facile and effective strategy to enhance stretchability and ductility of polymer blends.
However, the relation between the morphology and stretchability in ternary blends and the
regulation mechanism of stretchability need to be more explored.
In this work, our motivation was to investigate the effect of CBS polymers on mechanical
properties of the host polymers, with or without structural similarity. Two ductile polymers (P4:
n=0.4, p=0.3, m=0.3 and P5: n=0.2, p=0.4, m=0.4, respectively) are mixed with P3HTT-ehDPP-
10%
13
and one of the best-performance polymers (PTQ10)
14
to modulate the stretchability of the
111

polymers blends (Scheme 3.1). In the former, P4 and P5 have analogue structures with the host
polymer (future work) and in the latter, there is no structural similarity between the ductile
polymers and the conjugated host. We observe that adding 25% content ratio of P4 into PTQ10
can decrease its elastic modulus to almost half (from 450.8 MPa to 239.9 MPa) and increase the
COS to more than double (from 10.98% to 23.2%). The effect of P5 on the elastic modulus is little
lower (decreased by 33% to 300.0 MPa) when it is mixed with PTQ10 at 25%; however, the COS
remains almost the same (~10%). Thus, this work demonstrates a universal strategy to improve
the mechanical properties of organic solar cells. This strategy proposes that any binary blend is
capable of being promoted electronically (chapter 2) and mechanically with CBS polymers, which
sheds light on the future development of stable OPV for flexible, stretchable, and infrastructure
integrable applications.
112

Scheme 3.1 The structure of P4, P5 (ductile polymers) and PTQ10, P3HTT-ehDPP-10% (fully
conjugated polymers)
3.2 Results and Discussion
We synthesized two ductile polymers (P4 and P5 according to our previous study
12
), and a
fully conjugated polymer (PTQ10)
14
(Table 3.1). The aim was to leverage the effect of these
ductile polymers on PTQ10, where fully conjugated:ductile polymer weight ratio is 75:25. Adding
the ductile CBS polymers can reduce the content of brittle PTQ10, which can be effective in
improving the stretchability of the blend.

113

Table 3.1 SEC and mechanical properties for pristine P4, P5, PTQ10 and their blends.
Polymer M n
a
(kDa) Đ
a
COS (%) E Modulusa (MPa)
P4 43.4 1.42 17.5 145.3
P5 27.4 1.51 12.39 158.8
PTQ10 33.1 2.3 10.98 450.8
PTQ10:P4 - - 23.2 239.9
PTQ10:P5 - - 9.85 300.0
a
Obtained through size-exclusion chromatography (SEC).
b
Obtained from strain at failure.
c
Derived from linear regime of stress-strain curves.
The film-on-water (FOW) methodology (Figure 3.1) was adopted to measure the mechanical
properties and obtain stress-strain graphs (Figure 3.2). The blend films were prepared using o-
DCB as the solvent with the total concentration of 12 mg/mL. P4 and P5 demonstrated higher
ductility with much lower modulus (E). Several studies, including our own, have shown that CBS
units can improve mechanical properties relative to fully conjugated polymers.
11,15,16
These results
indicate that P4 and P5 are quite soft and can be applied as a ductile agent to the PTQ10 polymer.
114

Figure 3.1 Overview of the film-on-water methodology used to measure the mechanical
response, which includes a floating film of polymer, linear actuator, clamp attached to a load cell,
and a trough filled with water.
The stress-strain curves of the blend films acquired from FOW are shown in Figure 3.2. The
results show that the stiffness (E modulus) of the blend films reduced significantly with 25% of
P4 or P5 in PTQ10. Compared to pristine PTQ10, adding 25 wt% of P4 can increase the COS of
the blend film by 111%. However, adding P5 into the conjugated PTQ10 does not show any
improvement. This difference can be attributed to the fact that P5 has more brittle DPP unit content
(40%) compared to P4 (30%). Although adding P5 dilutes the fully conjugated polymer in the
blend, P5 itself contains large amount of conjugated DPP unit which may result in increasing
crystallinity in the blend. Furthermore, the molecular weight of polymers have profound impacts
on mechanical properties where increasing the molecular weight can improve the stretchability.
17

The molecular weight of P4 is higher than P5 which indicates that P4 can make more efficient
entanglement of the polymer chain.
18
The more entanglement of the polymer chains in the blend
115

can reduce the crystallinity, thus more external stress is loaded on the deformable amorphous
region.
17
From the above results, we confirm that introducing ductile CBS polymer as an additive
up to 25 wt% into a fully conjugated polymer is a simple and effective strategy to enhance the
mechanical performance of host polymer films.

Figure 3.2 Representative stress-strain graphs were obtained through film-on-water
methodology with pristine P4, P5, PTQ10 and their blends.
116

3.3 Conclusion
The mechanical stability is correlated to the microstructural robustness, which is the ability of
structures against mechanical loads. Introducing a ductile component into a polymer can enhance
the stretchability and structural robustness of the blend by increasing entanglement and tie chains
to dissipate strain. This work provides an instructive methodology for designing stretchable
electronics by using ductile CBS polymers. It was shown that incorporating 25 wt% of CBS
polymers can reduce the stiffness of a fully conjugated brittle polymer. Although the effect of a
ductile polymer on mechanical properties and morphology needs more investigation, this study
describes a universal method for the preparation of stretchable and mechanically stable
photovoltaics.
3.4 References
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J. B.-H.; Lee, G. H.; Jin, L.; Bao, Z. Tough-Interface-Enabled Stretchable Electronics Using Non-
Stretchable Polymer Semiconductors and Conductors. Nat. Nanotechnol. 2022, 1–7.
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Efficient and Mechanically Robust Ultraflexible Organic Solar Cells Based on Mixed Acceptors.
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Jiang, Y.; Hou, J.; Fukuda, K.; Someya, T.; Zhou, Y. Robust Metal Ion-Chelated Polymer
Interfacial Layer for Ultraflexible Non-Fullerene Organic Solar Cells. Nat. Commun. 2020, 11 (1),
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T.-S.; Kim, B. J.; Wang, E. Polymer Acceptors with Flexible Spacers Afford Efficient and
Mechanically Robust All-Polymer Solar Cells. Adv. Mater. 2022, 34 (6), 2107361.
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Kim, B. J.; Lee, J.-Y. Intrinsically Stretchable Organic Solar Cells with Efficiencies of over 11%.
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Properties of Organic Semiconductors for Stretchable, Highly Flexible, and Mechanically Robust
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(10) Ding, Z.; Liu, D.; Zhao, K.; Han, Y. Optimizing Morphology to Trade Off Charge
Transport and Mechanical Properties of Stretchable Conjugated Polymer Films. Macromolecules
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(11) Melenbrink, E. L.; Hilby, K. M.; Alkhadra, M. A.; Samal, S.; Lipomi, D. J.;
Thompson, B. C. Influence of Systematic Incorporation of Conjugation-Break Spacers into Semi-
Random Polymers on Mechanical and Electronic Properties. ACS Appl. Mater. Interfaces 2018,
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Break Spacer Content on the Mechanical and Electronic Properties of Semi-Random Polymers.
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Solar Cells Incorporating Ductile Conjugated Polymers with Conjugation Break Spacers. ACS
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(14) Sun, C.; Pan, F.; Bin, H.; Zhang, J.; Xue, L.; Qiu, B.; Wei, Z.; Zhang, Z.-G.; Li, Y. A
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118

(17) Balar, N.; Rech, J. J.; Henry, R.; Ye, L.; Ade, H.; You, W.; O’Connor, B. T. The
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119

Chapter 4: Improving the Performance of All-polymer Solar Cells by Blending with
Syndiotactic and Atactic Poly((N-carbazolylethylthio)propyl methacrylate)
4.1 Introduction
With the expanding demand for clean energy, bulk heterojunction (BHJ) organic photovoltaics
(OPVs) have progressed toward realizing their potential as inexpensive, large-area, and
lightweight solar energy conversion technologies.
1
The extensive exploration and great
breakthroughs in the development of conjugated materials, with tunable properties that differ from
traditional inorganic solar cells; make them promising candidates for portable power supplies and
building-integrated photovoltaics of various shapes with excellent mechanical properties.
2,3

At present, the power conversion efficiencies of traditional binary organic solar cells (OSCs)
have already surpassed 19% due to the materials development and advanced device engineering.
4

In the near future, OSCs are gradually developing toward commercialization requirements, which
is strongly dependent on the improvement of their photovoltaic conversion efficiency (PCE) and
stability.
Indeed, the emergence of high-performance narrow band gap small-molecule acceptors
(SMAs) accelerated the development of OSCs.
5
This type of SMA is known for better long-
wavelength absorption and tunable electronic structures with significantly lower energy loss that
leads to high-performance OSCs even under a very small driving force.
6,7
However, improving the
short circuit current density (JSC) and PCE in traditional binary PSCs which is due to the intrinsic
properties of conjugated polymers and small-molecule acceptors is still challenging. Moreover,
morphological stability to thermal and mechanical stress and better compatibility with various
donor materials for practical application as flexible devices is another limitation of SMAs.
8
To
overcome these challenges, Zhang et al. synthesized a high-performance polymer acceptor (PZ1)
120

by incorporating an acceptor-donor-acceptor (A-D-A) unit (IDIC as the key building block) and a
thiophene as a linking units in 2017.
9
It was shown that the polymer acceptor could preserve the
advantages of IDIC such as strong absorption and narrow band gap while also showing the
advantages of polymers such as good film formation and mechanical flexibility. As a result, all
polymer solar cells (all-PSCs) with the wide-band-gap polymer donor (PBDB-T) and PZ1 as the
acceptor, showed a record-high PCE of 9.19%. Recently, the strategy of polymerizing small
molecule acceptors (PSMAs) has been reported as an efficient approach towards boosting the
performance of all-PSCs to over 18%.
10
Despite the outstanding performance of fully conjugated
polymers in optoelectronic applications, obtaining an effective synthetic route, narrow dispersity
and high molecular wight polymers is still challenging.
11,12
To overcome this challenge, pendant
polymers are an underexplored class of semiconducting polymers in OPVs that offer a number of
contrasts and potential advantages relative to conjugated polymers.
13
Specifically, pendant
polymers offer the potential for significantly higher molecular weights, which is critical for
achieving robust mechanical properties.
10,14

Ternary organic solar cells offer another strategy to further enhance photovoltaic
performances, by improving the light absorption of the ternary film through incorporation of an
additional donor or acceptor into a traditional binary photoactive layer.
2,15
Moreover, the ternary
strategy has been demonstrated to be beneficial in overcoming the shortcomings of electronic
structures and morphologies of binary blends.
16,17
As a result, ternary OSCs have surpassed the
PCE of 19% in single junction solar cells.
4,6,18
It also has been shown that blends of conjugated
polymers and commodity polymers (e.g., HDPE, PS, PMMA, and PEO) have given rise to
improved mechanical properties.
19,20
Further, blends based on P3HT/PCBM/HDPE have shown
good OPV performance and flexible, robust free-standing films.
21
The same effects have been
121

observed with P3HT-PE block copoylmers.
22
An advantage of such polymers is the high molecular
weight, improving dramatically the ductility and toughness of conjugated polymer thin films.
Notably, the addition of poly(N-vinyl carbazole) (PVK) (up to 50 weight%) to a PBDB-T/N2200
blend resulted in significant improvement in environmental stability, device scalability, and
reduction in nonradiative recombination, although the polymer did not participate directly in the
charge generation and transport processes.
23

While optical and charge transport properties have been studied in pendant polymers, there are
few examples of application to OPV.
24
Recently, our group has identified that the atactic nature of
pendant polymers is a primary limiting factor in achieving high charge carrier mobility.
25,26

In this work, we synthesized three novel polymers (so called PAMA (Scheme 4.1a)) with
different tacticity (syndiotactic and atactic) and molecular weight, and introduced them as a third
component in all-polymer OPV cells (Scheme 4.1b). It is worth noting that although PAMA with
isotactic structure was synthesized successfully, we have not applied this polymer to our binary
system because of solubility and processability issues. The ternary blend photovoltaic performance
was explored, and the results show that devices with up to 10% of pendant polymers (of any
tacticity and molecular weight) outperform the binary references. However, our results show that
with syndiotactic polymers at lower molecular weight (49 kg/mol) the PCE shows 19%
improvement compared to the binary reference. It was shown that although these polymers have
lower absorption than conjugated polymers, gains in device performance outweigh this deficiency.
Furthermore, all devices were fabricated in air with up to 35% humidity with no thermal or solvent
treatment. To the best of our knowledge, this is the first ternary solar cell study elucidating the
effect of tacticity of pendant electroactive guest polymers on the device performance.
122

Scheme 4.1 (a) The synthetic route of Poly(acrylate) with syndiotactic or atactic backbone. (b)
The structure of PM7 as the host polymer donor and PZ1 as the polymer host acceptor.

123

4.2 Results and Discussion
PM7 and PZ1 are among the highest performing and most widely studied polymers for all-
PSCs.
27,28
In this report, we compared the device performance of PM7:PZ1 all-PSCs with a range
of blend ratios of PAMA (from 0% to 30%) as the third component. The PAMA polymers were
synthesized with different tacticity (syndiotactic and atactic) and molecular weight (syndiotactic
49 and 122 kg/mol).
13,25,26
To evaluate the potential of these PAMAs on the photovoltaic
performance of all-PSCs with different contents of PAMA. Ternary bulk heterojunction solar cells
were fabricated with an ITO/ZnO/PM7:PAMA:PZ1/MoO3/Ag inverted device architecture
(Figure 4.1a). As a first step toward constructing a ternary organic photovoltaic device, PAMA
with concentration of 7 mg/mL was added to the binary blend with the donor:acceptor ratio of 2:1
in CF with an overall concentration of 7 mg/mL and the addition of 2% CN as an additive. This
ratio yields the best PCE for the PM7:PZ1 binary cell. Although in this study, defining the role of
the pendant electroactive polymers is not the focus, for convenience and according to the nature
of the pendant structure, we assign them as the second donor in the system. Therefore, the
donor:acceptor ratio found to continuously increase as the PAMA content was increasing in the
blend (to 3.3:1 for 30% blend). All devices were fabricated and characterized under the same
condition, in air and humidity around 30%. Figure 4.1b shows the energy levels of the materials,
and the HOMO/LUMO energy levels were estimated to be −5.52:–3.57, −5.67:-2.27, and −5.70:–
4.15 eV, respectively for PM7
29
, PAMA
13
, and PZ1.
9
Appropriate HOMO and LUMO energy
levels can provide a sufficient driving force to separate excitons to free charge carriers and obtain
a lower VOC loss in PSCs.
30
PAMAs and PZ1 have a very similar HOMO level, which suggests
that the energy loss in PSCs would be effectively reduced.
31
The HOMO levels of these three
124

materials provide a cascade alignment for hole transfer, thereby facilitating charge transfer from
PZ1 to PM7.

Figure 4.1 (a) Polymer structures and device architecture. (b) Energy levels of related materials
and HUMO and LUMO of PAMAs.
13

The J-V characteristics of the devices were measured at an active area of 5.18 mm
2
. For ternary
devices, PAMA polymers were added at 5%, 10%, 15%, 25% and 30% of the total polymer
concentration. Higher ratios of PAMA polymers and a PAMA:PZ1 binary device were also tested;
however, it was observed that device performance deteriorated and the reference PAMA binary
was not functioning. Therefore, here we focus on the photovoltaic properties of ternary devices
with up to 30% PAMAs. Figure 4.2 provides the J–V curves of the binary and ternary devices, and
the corresponding device parameters are summarized in Table 4.1. The JSC values were well
matched (within 5% error) with the integrated JSC values obtained from the EQE spectra (see Table
B-2 for the mismatch factor and JEQE).
125

Figure 4.2 Photocurrent–voltage curves of (a) PM7:PAMA1:PZ1, (b) PM7:PAMA2:PZ1 and (c)
PM7:PAMA3:PZ1.
Under the optimized conditions, the binary PM7:PZ1 device showed a high VOC of 0.97 V, a
JSC of 11.09 mA cm
–2
, and an optimized FF of 0.60, delivering a PCE of 6.46%. In the first set of
ternary devices with the blend ratio of PAMA1, the largest PCE of 7.65% with a VOC of 0.98 V, a
high JSC of 12.59 mA cm
–2
, and a remarkable FF of 0.62 was achieved with 10% PAMA1 in blend.
Similarly in the second group of ternary devices, the highest PCE of 8.00% with a VOC of 0.98 V,
JSC of 13.63 mA cm
–2
, and a high FF of 0.60 was obtained by addition of 10% PAMA2. In the
third group of ternary devices, 5% PAMA3 in the host blend resulted in the highest PCE of 7.78%,
VOC of 0.96 V, JSC of 13.97 mA cm
–2
, and FF of 0.58. The results showed that PM7 as the host
126

donor with broad light absorption could retain the advantages of high current, and possibly PAMA
as the third component with suitable HOMO energy level could contribute to proper charge
transfer; thus, the ternary device has a qualitative improvement in PCE relative to the binary
devices.
Table 4.1 Photovoltaic characteristics of the best-performing photovoltaic devices based on
various PM7:Pendant:PZ1 blend films, under the illumination of AM 1.5G 100 mW cm
−2
.
Ternary System Ratio J (mA Cm
-2
) V (V) FF PCE (%)
PM7:PAMA1:PZ1 0% 11.09 0.97 0.60 6.46
PM7:PAMA1:PZ1 5% 13.14 0.98 0.58 7.47
PM7:PAMA1:PZ1 10% 12.59 0.98 0.62 7.65
PM7:PAMA1:PZ1 15% 12.67 0.98 0.61 7.57
PM7:PAMA1:PZ1 20% 10.39 0.98 0.59 5.99
PM7:PAMA1:PZ1 25% 10.13 0.97 0.60 5.92
PM7:PAMA1:PZ1 30% 10.38 0.98 0.55 5.60
PM7:PAMA2:PZ1 0% 13.21 0.97 0.52 6.68
PM7:PAMA2:PZ1 5% 14.29 0.98 0.56 7.78
PM7:PAMA2:PZ1 10% 13.63 0.98 0.60 8.00
PM7:PAMA2:PZ1 15% 12.63 0.98 0.59 7.36
PM7:PAMA2:PZ1 20% 12.68 0.98 0.58 7.25
PM7:PAMA2:PZ1 25% 11.58 0.98 0.58 6.55
PM7:PAMA2:PZ1 30% 12.06 0.98 0.52 6.18
PM7:PAMA3:PZ1 0% 13.42 0.90 0.57 6.91
PM7:PAMA3:PZ1 5% 13.97 0.96 0.58 7.78
PM7:PAMA3:PZ1 10% 12.59 0.97 0.58 7.14
PM7:PAMA3:PZ1 15% 12.06 0.96 0.56 6.52
PM7:PAMA3:PZ1 20% 10.14 0.97 0.54 5.28
PM7:PAMA3:PZ1 25% 12.18 0.98 0.44 5.28
PM7:PAMA3:PZ1 30% 10.46 0.96 0.49 4.92

Furthermore, these ternary devices exhibited composition tolerance. For example, the PCEs of
the ternary devices with the blend ratio of PAMA1, PAMA3 and PAMA2 ranging from 5-15%, 5-
127

15% and 5-20%, respectively, are all higher than the corresponding binary reference. The low
composition sensitivity of these ternary devices is beneficial to commercialization of PSCs in
future. Predictably, the losses in efficiency at high contents of PAMAs are observed since the
optoelectronic properties of these polymers are inferior to PM7, which leads to decreases in the
JSC and FF.
13
The external quantum efficiency (EQE) spectra were characterized to evaluate the
spectral responses of the binary and ternary PSCs. For example, as shown in Figure 4.3, the
PM7:PZ1 binary device showed a strong signal in the range of 400–720 nm. The EQE spectra in
the range of 450–700 nm of ternary devices gradually improved with increasing PAMA1 up to
15% content, and the ternary device with 10% PAMA1 exhibited the optimal balance.

Figure 4.3 Corresponding external quantum efficiency (EQE) spectra of PM7:PAMA1:PZ1
devices.
Figure 4.4 records the variation of device parameters (VOC, JSC, FF and PCE) with the change
of blend ratios. A similar trend is obvious for these parameter values. For instance, JSC went up
first up to 5% PAMAS, then decreased. FF and PCE reaching a maximum when adding 10%
PAMAs; and the VOC had a constant linear trend.
128

Figure 4.4 Variation of device parameters as a function of PAMAs content.
The absorption coefficient spectra of the PM7, PZ1 and PAMAs are displayed in Figure 4.5a.
The light absorption of PM7 covers a broad range from ~500–700 nm, and it has an overlap with
PZ1 from ~600-700 nm. Compared to the fully conjugated polymer donor and acceptor, PAMAs
show lower absorption coefficient in the range of ~300-400 nm.
129

Figure 4.5 (a) Absorption coefficient spectra. (b) Photoluminescence spectra of the photoactive
material films (excited at 530 nm).
130

The surface energy of components plays an important role in controlling nanoscale mixed
morphology.
32
The measurements in Table 4.2 show that PM7 and PZ1 have the closest surface
energies which promote an interpenetrating network in both horizontal and vertical directions. This
morphology is critical for both exciton dissociation and charge transport.
23
PAMAs with higher
surface energies tend to aggregate at the bottom of the blend film.
33
It was shown that the excellent
electronic properties (charge carrier mobility) of conjugated polymers can still be maintained if
they are mixed with a high-Tg polymer up to 60% concentration.
34
The higher current and FF in
ternary blends at 10% PAMAs could be attributed to the maintained charge transport channels at
the bottom substrate which is crucial for efficient charge collection at the bottom electrode.
Moreover, photoluminescence (PL) measurements were performed on the neat PM7 and blend
films to verify the potential charge/energy transfer process (Figure 4.5b). The polymer donor
showed wide emission ranges with an absorption peak at 725. The PM7:PZ1 blend film showed
an emission peak of ∼840 nm, which should be ascribed to the emission peak of PZ1; and the PM7
emission peak was quenched. The intensity of this peak is even lower in a ternary blend (PM7:10%
PAMA1:PZ1).

131

Table 4.2 Measured contact angles 𝞱 of water and Glycerol for polymers. Surface energies were
calculated according to Wu model.
Surface 𝞱 Water 𝞱 Glycerol Surface Energy (mN m
-1
)
PM7 105.18 92.74 22.12
PZ1 103.26 89.68 24.22
PAMA1 95.98 78.98 31.69
PAMA2 95.50 78.00 32.59
PAMA3 91.90 72.42 37.48

To interpret the effect of the PAMAs on charge transport dynamics, the space-charge-limited
current (SCLC) measurement was used to evaluate the charge mobility, which is realized by the
two different device structures.
1
The hole-only and electron-only device were fabricated with the
structures of ITO/PEDOT:PSS/active layer/Al and ITO/ZnO/active layer/Al, respectively.
35,36
The
μh/µe of the binary reference as well as the ternary devices are summarized in Table 4.3. In general,
the electron mobility is lower compared to hole mobility for all devices. For NFA systems, it has
been reported that a polymer:NFA blend may undergo some phase separation which results in rich
acceptor domains.
37
These domains might broaden the density of states and reduce electron
mobility under weak electric field.
38
One hypothesis for this observation is that some small-scale
isolated PZ1 domains below the percolation threshold are formed.
39
The hole mobility of pristine
PM7 (1.64E-05 cm
2
V
–1
S
–1
) however remained almost constant after addition of PMA1 up to 15%
(see SI). For all three PAMA polymers, μh showed a decreasing trend in the optimal ternary blend
film relative to the binary blend films. However, for 5% PAMA2 ternary blend, the hole mobility
increased from 2.09E-05 to 6.57E-05 cm
2
V
–1
S
–1
. On the other hand, the electron mobility for all
PAMAs ternary blends showed a peak in the range of 5-10% and then decreased gradually.
Therefore, the μh/µe reached its lowest values which represents the better-balanced electron and
hole extraction ability, and it can account for simultaneously improvement in JSC and FF. As shown
132

in Table 4.3 and Figure 4.6, the μh/µe was found to be 5.82 for the binary blend film. The μh/µe
was improved with the increase of the PAMA1 percentage, and the most balanced μh/µe was
obtained for the optimized 10% PAMA1 ternary blend films (Figure 4.6a and 4.6d). Upon further
increasing the PAMA1 loading in the ternary blends, the μe slightly decreased, however the μh,
significantly dropped. This less balanced μh/µe then resulted in a reduced FF and efficiency, likely
due to the exacerbated charge recombination. Similar trend was observed for both PAMA2 and
PAMA3. PAMA2 showed the lowest μh/µe at 10% loading among all devices (Figure 4.6b and
4.6e). An appropriate PAMAs loading plays a vital role in optimizing charge carrier transport and
collection in all ternary blend films which can be due to the fact that an optimized charge transport
channel was formed.
31

133

Table 4.3 Charge mobility determined from dark J-V curves for hole and electron dominated
carrier devices for binary and ternary blend films.
Ternary System Ratio
µ e
(cm
2
V
–1
S
–1
)
µ h
(cm
2
V
–1
S
–1
)
µ h/µ e
PM7:PAMA1:PZ1 0% 2.09E-05 1.21E-04 5.82
PM7:PAMA1:PZ1 5% 2.30E-05 1.10E-04 4.78
PM7:PAMA1:PZ1 10% 2.15E-05 7.38E-05 3.43
PM7:PAMA1:PZ1 15% 1.15E-05 4.88E-05 4.24
PM7:PAMA1:PZ1 20% 4.72E-06 3.70E-05 7.85
PM7:PAMA1:PZ1 25% 3.36E-06 3.24E-05 9.64
PM7:PAMA1:PZ1 30% 2.03E-06 3.71E-05 18.28
PM7:PAMA2:PZ1 0% 2.09E-05 1.21E-04 5.82
PM7:PAMA2:PZ1 5% 6.57E-05 1.36E-04 2.07
PM7:PAMA2:PZ1 10% 6.40E-05 8.76E-05 1.37
PM7:PAMA2:PZ1 15% 1.81E-05 5.35E-05 2.95
PM7:PAMA2:PZ1 20% 1.04E-05 3.12E-05 2.99
PM7:PAMA2:PZ1 25% 4.36E-06 2.63E-05 6.02
PM7:PAMA2:PZ1 30% 3.18E-06 2.05E-05 6.44
PM7:PAMA3:PZ1 0% 2.09E-05 1.21E-04 5.82
PM7:PAMA3:PZ1 5% 3.16E-05 8.12E-05 2.57
PM7:PAMA3:PZ1 10% 2.49E-05 6.64E-05 2.67
PM7:PAMA3:PZ1 15% 9.72E-06 6.44E-05 6.62
PM7:PAMA3:PZ1 20% 7.81E-06 7.38E-05 9.46
PM7:PAMA3:PZ1 25% 8.04E-06 6.25E-05 7.78
PM7:PAMA3:PZ1 30% 4.52E-06 5.62E-05 12.42

134

135

136

Figure 4.6 The hole and electron mobilities of the binary ternary blends with different PAMAs
contents.
4.3 Conclusion
The PAMA polymers are designed by introducing conjugated electroactive units to the
nonconjugated polymer backbone. Although the resulting polymers do not exhibit photovoltaic
performances in binary systems (blend with an acceptor), introducing them into OPV cells as the
guest polymer, the PM7:PZ1-based cells achieve a much higher PCE than that of the binary
137

reference device. Meanwhile, the more balanced charge carrier mobility with 10% of PAMAs in
all-polymer OPV cells enable higher JSC and fill factor (FF). Furthermore, our results show that
although PAMAs with any tacticity enhances the photovoltaic performance, lower molecular
weight (49 kg/mol) syndiotactic PAMA2 could improve the PCE up to 19% from 6.68 To 8% .
To the best of our knowledge, this is the first ternary system investigating the overlooked role
of the tacticity of the polymers on device performance. Our results suggest that tacticity could have
a distinctive and important role related to charge transport; however, its impact on morphology
might be minor and needs to be investigated in more detail. To sum up, our work indicates that
designing new polymer components with controlled tacticity plays a vital role in enhancing the
photovoltaic performance of all-polymer OPV cells.
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142

Biographical Sketch
Negar Kazerouni was born in Tehran, Iran. She received her Bachelor of Engineering degree
in Materials Science and Engineering from Amirkabir University (Tehran Polytechnique) in 2012.
She then started her master’s at University of Erlangen-Nuremberg and accomplished her thesis in
Prof. Christoph J. Brabec's group focusing on fabrication and characterizations of ternary solar
cells. Currently, she is performing her PhD studies under the guidance of Prof. Barry C. Thompson
at the University of Southern California. Her research is focused on the synthesis and
characterization of application-oriented small molecule organic compounds, their polymerization,
solar cell design, engineering, measurements, and characterizations for polymer-based ternary
solar cells.

143

Appendix A
Chapter 2: Ternary Blend Organic Solar Cells Incorporating Elastic Conjugated
Polymers with Conjugation Break Spacers
A.1 Materials and Methods
All reagents and solvents from commercial sources were used without further purification,
unless otherwise noted. Polymers were synthesized according to our previous studies (Scheme
1).
1,2
For thin-film measurements, ternary blend solutions in o-dichlorobenzene at a total
concentration of 23 mg/mL were spin-coated onto precleaned glass slides. For all solutions the
ratio of the donor to acceptor was 1:1.3. The thickness of the thin films and grazing- incidence X-
ray diffraction (GIXRD) measurements were obtained using Rigaku Diffractometer Ultima IV
using Cu Kα radiation source (λ = 1.54 Å ) in the reflectivity and grazing-incidence X-ray
diffraction mode, respectively.
144

Scheme A.1 Semi-random polymers with conjugation-break spacers synthesized for this study.

145

A.2 Device Fabrication
All devices were fabricated and characterized in air. ITO-coated glass substrates (10 Ω/sq,
Thin Film Devices Inc.) were cleaned in a sonication bath sequentially in detergent, deionized
water, tetrachloroethylene, acetone, and isopropyl alcohol, then dried overnight in a nitrogen
stream. A thin layer of PEDOT:PSS (Clevios PH500, filtered with a 0.45 μm poly(vinylidene
fluoride) syringe filter-Pall Life Sciences) was first spin-coated on the precleaned ITO-coated glass
substrates and baked at 120 °C for 50 min under vacuum. Separate binary solutions of P3HTT-
DPP-10%:PC61BM (1:1.3) and CBS:PC61BM (1:1.3) in o-dichlorobenzene were prepared with the
polymer concentration of 10 mg/mL. The solutions were stirred for 6 hours to completely dissolve
in the solvent at 60 ˚C. Then ternary solutions were made at the desired ratios and stirred for 24
hours and 60 ˚C to form a homogeneous mixture. Subsequently, the solutions were spin-coated at
900 rpm (after filtration with a 0.45 μm polytetrafluoroethylene syringe filter-Pall Life Sciences)
on top of the PEDOT:PSS layer. After casting the active layers, the films were dried under N2 for
30 min and then placed in the vacuum chamber for aluminum deposition. At the final stage, the
substrates were exposed to high vacuum (<9 × 10−7 Torr), and aluminum (100 nm) was thermally
evaporated at 3−4 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 4.4 mm
2
. The current − voltage
(I−V) characteristics of the devices were measured under ambient conditions using a Keithley
2400 source-measurement unit.

146

A.3 SCLC Device Fabrication and Characterization
All steps of device fabrication and testing were performed in air. ITO-coated glass substrates
(10 Ω/m, Thin Film Devices Inc.) were sequentially cleaned by sonication in detergent, de-ionized
water, tetrachloroethylene, acetone, and isopropyl alcohol, and dried in a nitrogen stream. A thin
layer of PEDOT:PSS (Baytron® P VP AI 4083, filtered with a 0.45 μm PVDF syringe filter – Pall
Life Sciences) was first spin-coated on the pre-cleaned ITO-coated glass substrates and baked at
120 ˚C for 60 minutes under vacuum. Solutions of polymers:PC61BM were prepared in o-
dichlorobenzene solvent at a polymer concentration of 10 mg/ml under the same conditions as
solar cells and stirred for overnight. Subsequently, the active layer was spin coated (with a 0.45
μm PTFE syringe filter – Pall Life Sciences) on top of the PEDOT:PSS layer. Upon spin coating
of solutions, films were first placed under N2 for 30 min and then placed in the vacuum chamber
for aluminum deposition. At the final stage, the substrates were pumped down to high vacuum (<
2.5×10-6 Torr) and aluminum (100 nm) was thermally evaporated at 3 – 5 Å/sec using a Denton
Benchtop Turbo IV Coating System onto the active layer through shadow masks to define the
active area of the devices as 5.18 mm
2
. The current−voltage (I−V) characteristics of the devices
were measured under ambient conditions using a Keithley 2400 source-measurement unit.

147

A.4 Thin film measurements
Solutions were spin-cast onto pre-cleaned 2.5 cm
2
glass slides (sonicated for 10 min in water,
5 min in acetone, and 5 min in isopropyl alcohol then dried under high N2 flow) from 23 mg mL
-
1
o-dichlorobenzene solutions. The thickness of the thin films and grazing- incidence X-ray
diffraction (GIXRD) measurements were obtained using a Rigaku Diffractometer.

Figure A-1 J-V curve of P3HTT-ehDPP-10%:P1:PC 61BM ternary solar cells.
148

Figure A-2 J-V curve of P3HTT-ehDPP-10%:P2:PC 61BM ternary solar cells.
149

Figure A-3 J-V curve of P3HTT-ehDPP-10%:P3:PC 61BM ternary solar cells.
150

Figure A-4 J-V curve of P3HTT-ehDPP-10%:P4:PC 61BM ternary solar cells.
151

Figure A-5 J-V curve of P3HTT-ehDPP-10%:P5:PC 61BM ternary solar cells.
152

Figure A-6 J-V curve of P3HTT-ehDPP-10%:P6:PC 61BM ternary solar cells.
153

Figure A-7 J-V curve of P3HTT-ehDPP-10%:P7:PC 61BM ternary solar cells.

154

Figure A-8 EQE spectra of P3HTT-ehDPP-10%:P1:PC 61BM ternary solar cells.
155

Figure A-9 EQE spectra of P3HTT-ehDPP-10%:P2:PC 61BM ternary solar cells.
156

Figure A-10 EQE spectra of P3HTT-ehDPP-10%:P3:PC 61BM ternary solar cells.
157

Figure A-11 EQE spectra of P3HTT-ehDPP-10%:P4:PC 61BM ternary solar cells.
158

Figure A-12 EQE spectra of P3HTT-ehDPP-10%:P5:PC 61BM ternary solar cells.
159

Figure A-13 EQE spectra of P3HTT-ehDPP-10%:P6:PC 61BM ternary solar cells.
160

Figure A-14 EQE spectra of P3HTT-ehDPP-10%:P7:PC 61BM ternary solar cells.

161

A.5 GIXRD
The thickness of films and GIXRD measurements were obtained using Rigaku Diffractometer
Ultima IV using Cu Kα radiation source (λ = 1.54 Å ) in the reflectivity and grazing incidence X-
ray diffraction mode, respectively. Crystallite size was estimated using Scherrer’s equation:
τ = Kλ/(β cosθ) (1)
where τ is the mean size of the ordered domains, K is the dimensionless shape factor (K = 0.9),
λ is the x-ray wavelength, β is the line broadening at half the maximum intensity (FWHM) in
radians, and θ is the Bragg angle.

162

Figure A-15 Grazing-incidence X-ray diffraction patterns of P3HTT-ehDPP-10%:P1:PC 61BM
thin films spin-cast from o-dichlorobenzene (o-DCB) and placed in a N 2 cabinet for 30 min. P3HTT-
ehDPP-10%:PC 61BM absorption spectrum provided for fully conjugated reference.
163

Figure A-16 Grazing-incidence X-ray diffraction patterns of P3HTT-ehDPP-10%:P2:PC 61BM
thin films spin-cast from o-dichlorobenzene (o-DCB) and placed in a N 2 cabinet for 30 min. P3HTT-
ehDPP-10%:PC 61BM absorption spectrum provided for fully conjugated reference.
164

Figure A-17 Grazing-incidence X-ray diffraction patterns of P3HTT-ehDPP-10%:P3:PC 61BM
thin films spin-cast from o-dichlorobenzene (o-DCB) and placed in a N 2 cabinet for 30 min. P3HTT-
ehDPP-10%:PC 61BM absorption spectrum provided for fully conjugated reference.
165

Figure A-18 Grazing-incidence X-ray diffraction patterns of P3HTT-ehDPP-10%:P4:PC 61BM
thin films spin-cast from o-dichlorobenzene (o-DCB) and placed in a N 2 cabinet for 30 min. P3HTT-
ehDPP-10%:PC 61BM absorption spectrum provided for fully conjugated reference.
166

Figure A-19 Grazing-incidence X-ray diffraction patterns of P3HTT-ehDPP-10%:P5:PC 61BM
thin films spin-cast from o-dichlorobenzene (o-DCB) and placed in a N 2 cabinet for 30 min. P3HTT-
ehDPP-10%:PC 61BM absorption spectrum provided for fully conjugated reference.
167

Figure A-20 Grazing-incidence X-ray diffraction patterns of P3HTT-ehDPP-10%:P6:PC 61BM
thin films spin-cast from o-dichlorobenzene (o-DCB) and placed in a N 2 cabinet for 30 min. P3HTT-
ehDPP-10%:PC 61BM absorption spectrum provided for fully conjugated reference.
168

Figure A-21 Grazing-incidence X-ray diffraction patterns of P3HTT-ehDPP-10%:P7:PC 61BM
thin films spin-cast from o-dichlorobenzene (o-DCB) and placed in a N 2 cabinet for 30 min. P3HTT-
ehDPP-10%:PC 61BM absorption spectrum provided for fully conjugated reference.

169

Table A-1 SEC and HOMO/E g data for the P3HTT-ehDPP-10% and CBS semi-random polymer
family
Polymer Mn
a
(kDa)

Ð
a
HOMO (eV)
b
Eg
c

P3HTT-ehDPP-10% 9.9 4.7 5.52 1.5
P1 19.7 6.23 5.41 1.55
P2 14.2 5.15 5.49 1.61
P3 35.5 2.59 5.56 1.61
P4 47.6 2.37 5.52 1.66
P5 44.1 2.54 5.54 1.69
P6 8.5 8.18 5.48 1.61
P7 12.8 3.75 5.50 1.66
a) Obtained through size-exclusion chromatography (SEC)
b) Calculated from oxidation onset versus ferrocene
c) Calculated from the absorption band edge in thin films, where Eg = 1240/λedge.

170

Table A-2 Raw Photovoltaic data for ternary solar cells and binary controls without MM
correction.
Device composition D:A ratio CBS content J (mA/cm
2
) V (V) FF PCE (%)
P3HTT-ehDPP-10%:PC61BM 1:1.3 0% 10.8 0.60 55 3.6
P3HTT-ehDPP-10%:P1:PC61BM 0.95:0.05:1.3 5% 11.3 0.61 55 3.8
P3HTT-ehDPP-10%:P1:PC61BM 0.9:0.1:1.3 10% 11 0.61 54 3.6
P3HTT-ehDPP-10%:P1:PC61BM 0.85:0.15:1.3 15% 10.7 0.61 56 3.7
P3HTT-ehDPP-10%:P1:PC61BM 0.75:0.25:1.3 25% 10.1 0.61 52 3.2
P3HTT-ehDPP-10%:P1:PC61BM 0.5:0.5:1.5 50% 6.3 0.63 47 1.8
P1:PC61BM 1:1.7 100% 3.2 0.65 34 0.7
P2:PC61BM 1:1.3 100% 3.6 0.72 27 0.7
P4:PC61BM 1:1.3 100% 1.5 0.74 37 0.41
P5:PC61BM 1:1.3 100% 1.1 0.74 40 0.32
P7:PC61BM 1:1.3 100% 1.5 0.58 29 0.24
171

Table A-3 Photovoltaic properties of binary reference and ternary solar cells.

J (mA/cm
2
) V (V) FF PCE (%) MM
Jcor.
(mA/cm
2
)
JEQE
(mA/cm
2
)
Error (%)
PCEcor
(%)
Number
of Pixels
P3HTT-ehDPP-10% 10.80±0.17 0.60±0.00 55.08±0.39 3.57±0.05 0.82±0.00 13.77±0.38 13.23±0.08 3.47 4.37±0.03
24
P1

0.10 9.56±0.04 0.61±0.00 52.44±0.26 3.03±0.02 0.78±0.00 12.84±0.11 12.00±0.36 1.15 3.91±0.04
16
0.15 9.56±0.17 0.59±0.00 48.94±0.37 2.77±0.06 0.78±0.00 11.94±0.47 11.31±0.30 3.21 3.56±0.16
16
0.25 8.25±0.17 0.60±0.00 48.38±0.43 2.41±0.05 0.78±0.01 10.42±0.20 10.30±0.06 4.18 3.09±0.06
13
P2

0.10 9.47±0.23 0.59±0.00 50.80±0.37 2.86±0.09 0.78±0.00 12.13±0.26 12.32±0.08 4.03 3.66±0.09
15
0.15 9.55±0.10 0.57±0.00 46.40±0.36 2.52±0.04 0.80±0.01 11.88±0.16 11.70±0.18 2.69 3.17±0.08
15
0.25 8.27±0.22 0.58±0.01 44.25±0.45 2.13±0.09 0.79±0.01 11.92±0.43 10.30±0.08 3.69 2.68
16
P3

0.10 10.00±0.16 0.59±0.00 52.44±0.51 3.07±0.03 0.78±0.01 12.84±0.20 12.44±0.30 1.17 3.94±0.05
16
0.15 10.98±0.16 0.59±0.00 52.73±0.25 3.40±0.07 0.78±0.00 14.15±0.19 13.19±0.10 0.21 4.39±0.11
15
0.25 8.44±0.13 0.59±0.00 48.13±0.74 2.41±0.08 0.77±0.01 10.83±0.21 11.55±0.10 0.26 3.11±0.14
15
P4

0.10 10.60±0.07 0.59±0.00 54.25±0.37 3.38±0.04 0.78±0.00 13.66±0.24 13.74±0.25 1.7 4.34±0.13
16
0.15 10.30±0.14 0.59±0.00 51.23±1.03 3.11±0.05 0.78±0.00 13.17±0.33 13.64±0.11 2.95 4.03±0.04
13
0.25 10.30±0.28 0.60±0.00 52.73±0.76 3.25±0.12 0.77±0.00 13.43±0.44 12.47±0.29 2.24 4.19±0.19
11
P5

0.10 10.70±0.26 0.60±0.00 58.07±0.57 3.70±0.06 0.77±0.01 14.89±0.20 13.59±0.23 3.50 4.81±0.10
15
0.15 9.30±0.18 0.59±0.00 5513±0.90 3.05±0.10 0.75±0.00 12.19±0.20 11.70±0.33 4.47 4.04±0.13
14
0.25 9.83±0.12 0.59±0.00 49.69±0.83 2.82±0.08 0.75±0.00 13.29±0.23 13.51±0.24 2.42 3.84±0.19
16
P6

0.10 9.27±0.14 0.60±0.00 53.19±0.49 2.94±0.03 0.80±0.01 11.98±0.13 10.67±0.35 0.43 3.70±0.05
16
0.15 9.54±0.10 0.60±0.00 53.69 3.09±0.04 0.79±0.01 12.17±0.17 11.25±0.32 2.69 3.92±0.07
16
0.25 8.90±0.23 0.60±0.00 49.21±0.33 2.61±0. ±0.08 0.79±0.00 11.17 10.47 0.42 3.30±0.15
14
P7

0.10 7.39±0.20 0.58±0.00 45.86±0.34 1.95±0.07 0.78±0.01 10.04±0.08 10.11±0.02 1.02 2.51±0.08
8
0.15 8.87±0.08 0.61±0.00 51.75±0.31 2.78±0.04 0.79±0.01 11.51±0.13 12.08±0.05 4.71 3.54±0.11
8
0.25 11.04±0.07 0.60±0.00 54.88±0.30 3.63±0.03 0.79±0.00 13.75±0.10 13.39±0.06 2.74 4.60±0.06
8

172

Table A-4 2θ, interchain distances (100), GIXRD intensities, full-width at half maximum
(FWHM) values, and crystallite size (nm) calculated from Scherrer’s equation for binary and ternary
as-cast films.
Polymer Composition 2θ (deg.) d 100 (Å)
Intensity
(counts)
FWHM
(deg.)
Crystallite
Size (nm)
Crystalline Correlation
Length (CCL) (nm)
Thickness
(nm)

P3HTT-
ehDPP-
10%
5.9 14.97 25.7 0.78 9.98 8.06 61.9

P1
10% 5.85 15.10 53.5 0.71 12.29 8.85 62.2

15% 5.85 15.10 40.2 0.77 10.65 8.16 60.7

25% 5.8 15.23 28.5 0.77 10.65 8.16 60.7

P2
10% 5.85 15.10 39.4 0.71 12.29 8.85 61.1

15% 5.85 15.10 39.3 0.70 12.29 8.98 60.6

25% 5.85 15.10 33.6 0.71 11.41 8.85 60.4

P3
10% 5.85 15.10 43.9 0.79 9.98 7.95 62.0

15% 5.8 15.23 26.1 0.75 10.65 8.38 61.5

25% 5.7 15.49 24.7 0.84 9.98 7.48 59.5

P4
10% 5.85 15.10 27.9 0.79 10.65 7.95 60.2

15% 5.8 15.23 15.7 0.79 9.98 7.95 60.8

25% 5.8 15.23 12.7 0.75 10.65 8.38 62.2

P5
10% 5.9 14.97 20 0.82 10.65 7.66 60.9

15% 5.8 15.23 22.9 0.7 11.41 8.98 60

25% 5.75 15.36 11.3 0.75 12.28 8.38 60.6

P6
10% 5.9 14.97 45.6 0.81 9.98 7.76 61.0

15% 5.8 15.23 31 0.8 9.98 7.85 61.9

25% 5.75 15.36 27.7 0.77 10.65 8.16 62.1

P7
10% 5.8 15.23 22.6 0.79 10.65 7.95 61.4

15% 5.8 15.23 42.5 0.78 9.98 8.06 60.4

25% 5.8 15.23 38.7 0.81 10.65 7.76 60.5

173

Mobility was measured using a hole-only device configuration of
ITO/PEDOT:PSS/Polymer:CBS:PC61BM /Al in the space charge limited current regime (SCLC).
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 equation 2:
𝐽 𝑆 𝐶 𝐿 𝐶 =
9
8
𝜀 𝑅 𝜀 0
𝜇 𝑉 2
𝐿 3
(2)
where JSCLC is the current density, ε0 is the permittivity of space, εR 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 = Vapplied – Vbi – Vr), and L is the polymer layer thickness.
The series and contact resistance of the hole-only device (18 – 23 Ω) was measured using a blank
(ITO/PEDOT/Al) configuration and the voltage drop due to this resistance (Vr) was subtracted
from the applied voltage. The built-in voltage (Vbi), 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. Film thicknesses were measured using GIXRD in the reflectivity mode.

174

Table A-5 Hole mobility data for ternary P3HTT-ehDPP-10%:CBS:PC 61BM devices.
Polymer
ratio
P3HTT-DPP-10%::CBS:PC61BM
a

µh
a

(cm
2
V
-1
s
-1
)
Thickness (nm)
P3HTT-ehDPP-10% 1:0:1.3 (0%) 1.22E-03±1.64E-04 61.9

P1
0.9:0.1:1.3 (10%) 4.20E-04±3.77E-05 62.2
0.85:0.15:1.3 (15%) 8.74E-04±1.20E-04 60.7
0.75:0.25:1.3 (25%) 5.06E-04±3.45E-05 60.7

P2
0.9:0.1:1.3 (10%) 4.68E-04±4.80E-05 61.1
0.85:0.15:1.3 (15%) 1.38E-03±2.20E-04 60.6
0.75:0.25:1.3 (25%) 3.15E-04±3.27E-05 60.4

P3
0.9:0.1:1.3 (10%) 1.63E-03±4.19E-04 62.0
0.85:0.15:1.3 (15%) 2.84E-03±5.99E-04 61.5
0.75:0.25:1.3 (25%) 5.09E-04±7.15E-05 59.5

P4
0.9:0.1:1.3 (10%) 2.46E-03±9.48E-04 60.2
0.85:0.15:1.3 (15%) 1.01E-03±1.57E-04 60.8
0.75:0.25:1.3 (25%) 2.83E-04±2.60E-05 62.2

P5
0.9:0.1:1.3 (10%) 1.90E-04±1.10E-05 60.9
0.85:0.15:1.3 (15%) 2.84E-04±8.85E-07 60
0.75:0.25:1.3 (25%) 9.84E-05±1.64E-04 60.6

P6
0.9:0.1:1.3 (10%) 6.45E-04±8.49E-05 61.0
0.85:0.15:1.3 (15%) 1.62E-03±4.07E-04 61.9
0.75:0.25:1.3 (25%) 1.72E-03±4.25E-04 62.1

P7
0.9:0.1:1.3 (10%) 2.29E-04±1.39E-05 61.4
0.85:0.15:1.3 (15%) 1.38E-03±3.52E-04 60.4
0.75:0.25:1.3 (25%) 1.18E-03±1.84E-04 60.5
a) Calculated from charge carrier mobility measurements in the space charge limited current
(SCLC) regime in hole-only devices

175

A.6 References
(1) Melenbrink, E. L.; Hilby, K. M.; Alkhadra, M. A.; Samal, S.; Lipomi, D. J.;
Thompson, B. C. Influence of Systematic Incorporation of Conjugation-Break Spacers into Semi-
Random Polymers on Mechanical and Electronic Properties. ACS Appl. Mater. Interfaces 2018,
10 (38), 32426–32434. https://doi.org/10.1021/acsami.8b10608.
(2) Melenbrink, E. L.; Hilby, K. M.; Choudhary, K.; Samal, S.; Kazerouni, N.; McConn,
J. L.; Lipomi, D. J.; Thompson, B. C. Influence of Acceptor Side-Chain Length and Conjugation-
Break Spacer Content on the Mechanical and Electronic Properties of Semi-Random Polymers.
ACS Appl. Polym. Mater. 2019, 1 (5), 1107–1117. https://doi.org/10.1021/acsapm.9b00115.

176

Appendix B
Chapter 3: Improving the Performance of All-polymer Solar Cells by Blending
Syndiotactic and Atactic Poly((N-carbazolylethylthio)propyl methacrylate)
B.1 Materials and Methods
All reagents and solvents from commercial sources were used without further purification,
unless otherwise noted. Polymers were synthesized according to literature and our previous studies
(Scheme 1).
1–3
Number average molecular weights (Mn) and dispersity (Ð) were determined by
size exclusion chromatography (SEC) on four 300 x 7.5 mm PL1110 Mixed high grade organic
columns (Agilent) at 140 °C using an Agilent PL-GPC separation module and an Agilent 1260
Infinity II RI detector. All samples were dissolved in HPLC grade trichlorobenzene at a
concentration of 1.0 mg/mL, briefly heated and then allowed to cool to room temperature prior to
filtering through a 0.2 µm PTFE filter. The instrument was calibrated vs. polystyrene standards
(1050−3,800,000 g/mol), and data were analysed using Agilent GPC/SEC software.
For thin-film measurements, ternary blend solutions in chloroform at a total concentration of
7 mg/mL were spin-coated onto precleaned glass or silicon slides. For the binary PM7:PZ1
solution the ratio of the donor to acceptor was 2:1. The pristine PAMA solution was added at
different ratios (% v/v) to binary mixture to make ternary blends and the solutions were stirred at
room temperature for at least one hour before coating. UV-Vis absorption spectra were obtained
on a PerkinElmer Lamda 950 spectrophotmeter. Photoluminescence (PL) measurements were
performed on a Photon Technology International QuantaMaster C-60 Spectrometer using the
Qm/Ex300 Em400 light source with an excitation wavelength of λ = 310 nm and 510 nm, a 5 mm
slit-width, a step-size of 2 nm and an integration of 0.5 seconds and analyzed using the Felix GX
software. The thicknesses of the thin films were obtained using a Film-Sense FS-1 Ellipsometer
177

and the Film-Sense FS-1 analysis software version 1.59 in Single Measurement mode using the
Cauchy on Si with k3 model as the average of five measurements across the slides.
Table B-1 SEC and HOMO/Eg data for the P3HTT-ehDPP-10% and CBS semi-random polymer
family
Polymer Mn

(kDa)

Ð
PM7 26.47 2.4
PZ1 24.8 1.5
PAMA1 122.1 1.2
PAMA2 49.4 1.5
PAMA3 26.1 1.6

B.2 SCLC Device Fabrication and Characterization
Mobility was measured using a hole-only device and electron-only device configuration of
ITO/PEDOT:PSS/Polymer/Al and ITO/ZnO/Polymer/Al, respectively, in the space charge limited
current regime (SCLC). All steps of the device fabrication and testing were performed in air. ITO-
coated glass substrates (10 Ω/square, Thin Film Devices Inc.) were subsequently cleaned by
sonication in water, acetone and isopropyl alcohol and dried in a N2 stream. For hole-only devices,
a thin layer of PEDOT:PSS (Baytron® P VP Al 4083, filtered with a 0.45 μm PVDF syringe filter
– Pall Life Science) was first spin-coated on the pre-cleaned ITO-coated glass substrate and
annealed at 120 °C for 50 minutes under vacuum. For electron-only devices, the ZnO precursor
was prepared by dissolving zinc acetate dihydrate (ZnAc, Zn(CH3COO)2·2H2O, Aldrich, 99.9 %,
0.24 g) and ethanolamine (MEA, NH2CH2CH2OH, Aldrich, 99.5 %, 0.83 µl) in 2-
methoxyethanol (CH3OCH2CH2OH, Aldrich, 99.8 %, 3 ml). The solution was vigorous stirred
for 18 h in air for the hydrolysis reaction. The ZnO solution then was spin coated on the top of
glass/ITO patterned substrates. The films were annealed at 200 °C for 60 minutes and the thickness
of the ZnO film was ~40 nm. The polymer solution was spin-coated on top of the PEDOT:PSS or
ZnO layer. The substrates were pumped down to a high vacuum and aluminum (100 nm) was
178

thermally evaporated at 2-3 Å/s using an Angstrom 01353 Coating System onto the active layer
through shadow masks to define the active area of the devices. 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 is described by the
following equation where JSCLC is the current density, ε0 is the permittivity of space, εR 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 = Vapplied – Vbi – Vr), and L is the
polymer layer thickness: The series and contact resistance of the hole-only and electron-only
devices were measured using a blank ITO/PEDOT/Al and ITO/ZnO/Al configuration and the
voltage drop due to this resistance (Vr) was subtracted from the applied voltage. The built-in
voltage (Vbi), 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 a Film-Sense FS-1 Ellipsometer and the Film-Sense FS-1
analysis software version 1.59 in Single Measurement mode using the Cauchy on Si with k3 model
as the average of five measurements across the slides.
𝐽 𝑆 𝐶 𝐿 𝐶 =
9
8
𝜀 𝑅 𝜀 0
𝜇 𝑉 2
𝐿 3

179

B.3 Device Fabrication and Characterization
All devices were fabricated and characterized in air. ITO-coated glass substrates (10 Ω/sq,
Thin Film Devices Inc.) were cleaned in a sonication bath sequentially in water, acetone, and
isopropyl alcohol, then dried overnight in a nitrogen stream. A thin layer of ZnO solution was first
spin-coated on the precleaned ITO-coated glass substrates and baked at 200 °C for 50 min in air.
A separate binary solution of PM7:PZ1 (2:1) and pristine solutions of PAMA in chloroform were
prepared with the total concentration of 7 mg/mL. The solutions were stirred overnight to
completely dissolve in the solvent at room temperature. Then ternary solutions were made at the
desired ratios and stirred for at least one hour to form a homogeneous mixture. Subsequently, the
solutions were spin-coated on top of the ZnO layer. The films were then placed in the vacuum
chamber for silver deposition. At the final stage, the substrates were exposed to high vacuum (<9
× 10−7 Torr), and silver (100 nm) was thermally evaporated at 2-3 Å /s 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 devices were measured under ambient conditions using a Keithley
2400 source-measurement unit.

180

Table B-2 Photovoltaic properties of binary reference and ternary solar cells.

J (mA/cm
2
) V (V) FF PCE (%) MM Jcor. (mA/cm
2
)
PCEcor
(%)
JEQE Error (%)
PM7:PZ1 9.85 0.97 0.60 5.75 0.888 11.09 6.46 10.63 4.39
PAMA1 11.70 0.98 0.58 6.61 0.890 13.14 7.47 13.06 0.68
5% 11.24 0.98 0.62 6.85 0.893 12.59 7.65 12.24 2.89
10% 11.17 0.98 0.61 6.66 0.882 12.67 7.57 12.30 2.97
15% 9.16 0.98 0.59 5.26 0.882 10.39 5.99 10.77 3.57
20% 8.93 0.97 0.60 5.20 0.881 10.13 5.92 10.45 3.05
25% 9.11 0.98 0.55 4.92 0.877 10.38 5.60 10.03 3.55
30% 11.70 0.98 0.58 6.61 0.890 13.14 7.47 13.06 0.68

Figure B-1 J-V curve of PM7:PAMA1:PZ1 ternary solar cells.

181

Figure B-2 J-V curve of PM7:PAMA2:PZ1 ternary solar cells.
182

Figure B-3 J-V curve of PM7:PAMA3:PZ1 ternary solar cells.

Figure B-4 Contact angle images of glycerol and water droplets on the neat films PM7, PZ1 and
PAMAs.
183

B.3 References
(1) Zhang, Z.-G.; Yang, Y.; Yao, J.; Xue, L.; Chen, S.; Li, X.; Morrison, W.; Yang, C.;
Li, Y. Constructing a Strongly Absorbing Low-Bandgap Polymer Acceptor for High-Performance
All-Polymer Solar Cells. Angew. Chem. Int. Ed. 2017, 56 (43), 13503–13507.
https://doi.org/10.1002/anie.201707678.
(2) Samal, S.; Schmitt, A.; Thompson, B. C. Contrasting the Charge Carrier Mobility of
Isotactic, Syndiotactic, and Atactic Poly((N-Carbazolylethylthio)Propyl Methacrylate). ACS
Macro Lett. 2021, 10 (12), 1493–1500. https://doi.org/10.1021/acsmacrolett.1c00622.
(3) Fan, Q.; Zhu, Q.; Xu, Z.; Su, W.; Chen, J.; Wu, J.; Guo, X.; Ma, W.; Zhang, M.; Li,
Y. Chlorine Substituted 2D-Conjugated Polymer for High-Performance Polymer Solar Cells with
13.1% Efficiency via Toluene Processing. Nano Energy 2018, 48, 413–420.
https://doi.org/10.1016/j.nanoen.2018.04.002.

Abstract (if available)

Abstract The past three years have witnessed rapid growth in the field of organic solar cells with intensive efforts being devoted to material development, device engineering, and understanding of device physics. The power conversion efficiency of single-junction organic solar cells has now reached high values of over 19%. In addition to efficiency, stability, which is still the main barrier to the commercial application of organic solar cells, needs to be investigated.
Developing a strategy which includes non-halogenated green solvent processability, uniform large-area module fabrication, annealing-free production, high efficiency, and excellent stability, are among the critical requirements for industrial development of organic solar cells.
The ternary strategy is a promising technology to overcome these limits towards high power conversion efficiency for practical application. Although the efficiency for ternary organic solar cells has surpassed 19%, the stability, degradation mechanisms and mechanical properties in the ternary strategy are still not well investigated. Therefore, the scientific community is changing the focus to the stability constraints of organic solar cells, largely driven by the recent critical need to understand and optimize the stability and reliability of organic photovoltaic based products. In some cases, the third component in ternary organic solar cells is chosen such that although it exhibits complementary absorption in the solar spectrum with the host blend, it does not show great photovoltaic performance in binary blends. However, at low ratios in ternary blends, it has a synergistic benefit on performance and stability, which demonstrates a significant advantage for developing highly efficient and stable organic solar cells.
In this dissertation, ternary strategies are presented with the aim to provide useful insights of the role of the third component not only on the photovoltaic performances but also on improvement of the device stability.
Chapter 1 emphasizes the crucial role of the third component in improving the efficiency and specifically the stability. Introducing the third element in the photoactive layer of organic photovoltaic (OPV) devices is a promising strategy towards improving the efficiency and stability of this technology while maintaining relatively low costs. The fundamental understanding of this efficient and sustainable strategy motivates us to design and apply novel polymers with specific features as the third component to enable more effective and stable solar cells, described in details in Chapters 2-4.
In Chapter 2, a broad family of ductile semi-random donor-acceptor (D-A) copolymers with 8-carbon alkyl conjugation break spacer (CBS) units were incorporated into ternary blend organic solar cells in order to determine their impact on the electronic metrics of solar cell performance. The goal of this study was to shine light on rational co-optimization strategies for photovoltaic and mechanical properties in flexible and stretchable organic solar cells. The ternary blended active layers were based on two polymer donors and the acceptor [6,6]-Phenyl-C61-butyric acid methyl ester (PC61BM). In all cases, the majority polymer donor component was the previously reported fully conjugated semi-random polymer P3HTT-ehDPP-10%, comprised of 80% 3-hexylthiophene, 10% diketopyrrolopyrrole (DPP) with 2-ethylhexyl (eh) side chains, and 10% thiophene. As the second donor, three different classes of CBS polymers were used, where the spacer length was kept constant at 8 methylene units. In the first class, the CBS unit was incorporated at 10% and 20% with eh side chains on DPP at matching contents of 10% and 20%. In the second class, the CBS unit and DPP unit were incorporated at matching contents of 20%, 30% and 40%, where the side chains of DPP was replaced with 2-decyltetradecyl (dtd). In the third class, with CBS incorporated at 20% and 30%, the DPP monomer content was fixed at 10% and eh side chains were again used. The mechanical properties of these polymers are quite notable with elastic moduli as low as 8.54 MPa and fracture strains as high as 432%. However, it was found that as elasticity increased, hole mobility decreased. In this study, we observed that the hole mobilities of the ternary active layers generally increased upon increasing the content of the CBS polymer up to 15% of the overall donor fraction. The higher carrier mobilities likely contribute to the higher JSC observed in many of the ternary devices. The as-cast ternary solar cells made in ambient environment without any pre/post treatment gave strong performance up to 25% of CBS polymer loading. This work demonstrates that introducing highly stretchable CBS polymers with poor charge mobility does not adversely affect solar cell performance, offering insights into the development of ternary strategies for flexible/stretchable organic solar cells.
In Chapter 3, two semi-random polymers were synthesized via Stille polycondensation. Conjugation-break spacers (CBS) of 8-carbon alkyl spacers were incorporated into the polymer backbone at the content of 30% and 40% (P4 and P5, respectively). Compared to a fully conjugated DPP polymer, incorporation of CBS units has been shown to enhance the fracture strain from 68% to 325% and 398%, for P4 and P5 respectively. Although these polymers displayed extraordinary mechanical properties, with elastic moduli as low as 14.8 MPa (for P4), they have low hole mobilities. However, our previous study (chapter 2) shows that in ternary blends with up to 25% CBS polymer in the donor fraction, PCE values similar to or exceeding the binary reference (DPP:PCBM) were observed in several cases. In this study, we are motivated to investigate the effect of CBS polymers specifically on mechanical properties in a blend with a well-known polymer donor, PTQ10. The goal of this study is to explore the impact of CBS polymers on the ductility of blends with polymers that are not structural analogs. This study proposes that semi-random polymers with conjugation-break spacers are promising candidates for further study in flexible electronics.
In Chapter 4, the focus is on all-polymer solar cells. On one hand, efficient all-polymer solar cells (all-PSCs) can be fabricated from polymerized small-molecule acceptors (PSMAs) with high optical absorption and electron mobilities. On the other hand, the ternary strategy can be applied as an effective method for improving the blend film morphology, absorption ability, and device performance. However, the ternary strategy has had very limited success in all-polymer solar cells (all-PSCs) because of the scarcity of new polymers and the challenges faced during third component optimization. Here, we designed nonconjugated pendant electroactive polymers containing electroactive units in the side chain with different tacticity (syndiotactic and atactic) and molecular weight. Incorporation of the pendant polymers into binary blends of PM7:PZ1 results in over 19% improvement in the photovoltaic performance of all-polymer ternary devices. As a result, all-PSCs featuring pendant polymers achieve a high power conversion efficiency (PCE = 8%), outperforming binary devices without the pendant polymer (i.e., PM7:PZ1, PCE = 6.91%). Importantly, these all-PSCs are fabricated in air, without any thermal or solvent treatment.

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Creator Kazerouni, Negar (author)

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

School College of Letters, Arts and Sciences

Degree Doctor of Philosophy

Degree Program Chemistry

Degree Conferral Date 2023-05

Publication Date 07/25/2024

Defense Date 12/09/2022

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

Tag all-polymer solar cells,ambient environment,atactic,conjugation break spacer (CBS),donor-acceptor copolymers,flexible,hole mobility,mechanical properties,nonconjugated pendant electroactive polymers,OAI-PMH Harvest,organic photovoltaic,organic solar cells,semi-random polymers,stability,Stille polycondensation,stretchable,syndiotactic,tacticity,ternary

Format theses (aat)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Thompson, Barry C. (committee chair), Narayan, Sri (committee member), Wu, Wei (committee member)

Creator Email negarkazerouni@gmail.com,nkazerou@usc.edu

Permanent Link (DOI) https://doi.org/10.25549/usctheses-oUC112718950

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Legacy Identifier etd-KazerouniN-11438

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