Title Page

ABSTRACT

Contents

List of Abbreviations 19

Chapter 1. Introduction to Organic Photovoltaics (OPVs) 22

1.1. A brief overview of solar cells 22

1.1.1. Generations of PV production technologies 23

1.2. Organic photovoltaic cells (OPVs) 24

1.3. Working principle of OPV cells 25

1.3.1. Absorption of photon and exciton formation 25

1.3.2. Exciton dissociation 26

1.3.3. Charge separation 26

1.3.4. Charge extraction 27

1.4. Performance parameters for OPV cells 27

1.4.1. Power conversion efficiency (PCE) 27

1.4.2. Open circuit voltage (VOC)[이미지참조] 28

1.4.3. Short circuit current (JSC)[이미지참조] 28

1.4.4. Fill factor (FF) 28

1.5. Device architecture of OPV cells 28

1.5.1. Conventional architecture 29

1.5.2. Inverted architecture 29

1.6. Junction types in OPV cells 30

1.6.1. Single-layered OPV cells 30

1.6.2. Bi-layer OPV cells 30

1.6.3. Bulk heterojunction OPV cells 31

1.6.4. Tandem OPV cells 31

1.7. Organic semiconducting materials for OPV cells 31

1.7.1. Development of NFA materials 31

1.7.2. Development of polymer donor materials 34

1.7.3. Development of random copolymers for FAs and NFAs 36

1.7.4. Future outlook for polymers 38

1.8. Applications of OPV cells 39

1.8.1. Flexible, semitransparent, and indoor applications 39

1.8.2. Large area applications 40

1.9. Challenges and limitations from lab to fab 41

1.10. Evaluation and assessment 43

1.10.1. SWOT analysis for OPV cells 43

1.11. References 45

Chapter 2. Large area applicable terpolymers for fullerene acceptors (FAs) via manipulation of difluorobenzo-thiadiazole with its structural analogues 53

2.1. Introduction and aim of this work 53

2.2. Results and discussion 58

2.2.1. Synthesis and characterization 58

2.2.2. Optical and electrochemical properties 62

2.2.3. Photovoltaic performance 78

2.2.4. Large area implications of polymer 2FBT-OBT50 95

2.3. Conclusion 97

2.4. Experimental section 99

2.4.1. Instrumentation 99

2.4.2. Synthesis of monomers and polymers 100

2.4.3. Device fabrication and characterization 119

2.5. References 121

Chapter 3. Impact of trifluoromethyl-substitution on conjugated random copolymers for large area applications 123

3.1. Introduction and aim of this work 123

3.2. Results and discussion 126

3.2.1. Synthesis and characterization 126

3.2.2. Optical and electrochemical properties 130

3.2.3. Photovoltaic performance 142

3.2.4. Morphological analysis 153

3.2.5. Charge carrier mobilities 155

3.2.6. Charge dynamics studies 156

3.2.7. Large area implications of polymer PBTF5 157

3.3. Conclusion 160

3.4. Experimental section 161

3.4.1. Instrumentation 161

3.4.2. Synthesis of monomers and polymers 162

3.4.3. Device fabrication and characterization 171

3.5. References 173

Chapter 4. Conclusion 175

List of Tables

Table 2.1. Device performances of the of polymer solar cells based on 2FBT-OBTX: PC₇₁BM. 79

Table 2.2. Chloroform polymer batches of 2FBT-OBT50 varying in MW. 80

Table 2.3. Device performances of the of polymer solar cells based on 2FBT-OBT50 batches (varying in MW under microwave conditions): PC₇₁BM. 81

Table 2.4. Polymer batches of 2FBT-OBT50 varying in MW under thermal conditions. 82

Table 2.5. Device performances of the of polymer solar cells based on 2FBT-OBT50 batches (varying in MW under thermal conditions): PC₇₁BM. 83

Table 2.6. Device performances of the of polymer solar cells based on 2FBT-OBT50:IT-4F. 84

Table 2.7. Device performances of the polymer solar cells based on 2FBT-OBT50-H/F/Cl: PC₇₁BM. 85

Table 2.8. Device performances of the polymer solar cells based on 2ClBT-OBTX: PC₇₁BM. 86

Table 2.9. Device performances of the polymer solar cells based on 2FBT-OBT50:IT-4F and 2FBT-OBT50:PC₇₁BM. 87

Table 2.10. Device performances of the polymer solar cells based on 2FBTE-OBT50: PC₇₁BM and 2FBT-FOBT50:PC₇₁BM. 88

Table 2.11. Device performances of the polymer solar cells based on 2FBT-OBT50-HF: PC₇₁BM and 2FBT-OBTP50-HF:PC₇₁BM. 89

Table 2.12. Device performances of the polymer solar cells based on 2FQx-FBT50-F: PC₇₁BM and 2FQx-FBTP50-H: PC₇₁BM. 90

Table 2.13. Device performances of the polymer solar cells based on 2FQx-C14P50, 2FQx-BOP50, 2FQx-HDP50, and 2FQx-ODP50: PC₇₁BM. 92

Table 2.14. Device performances of the polymer solar cells based on 2FQx-C14BT50, 2FQx-BOBT50, 2FQx-HDBT50, and 2FQx-ODBT50: PC₇₁BM. 93

Table 2.15. A co-relation table for device parameters of all the terpolymer donors for PC₇₁BM. 94

Table 2.16. Device performances of the polymer solar cells based on 2FBT-OBT50:PC₇₁BM and 2FBT-OBT50:IT-4F on large area (54 cm²). 96

Table 3.1. Device performances of the of polymer solar cells based on polymer donors PBTFX: Y6-BO. 143

Table 3.2. Device performances of the polymer solar cells based on PB5, PTF5, PMO5, PMEH5, PFO5, PFEH5 and Y6-BO. 145

Table 3.3. Device performances of the polymer solar cells based on PM6-TFOX and PM7-TFOX and Y6-BO. 146

Table 3.4. Device performances of the polymer solar cells based on PM6-TFX and PM7-TFX and Y6-BO. 148

Table 3.5. Device performances of the polymer solar cells based on DTFOX and Y6-BO 149

Table 3.6. Device performances of the polymer solar cells based on DBTFX and Y6-BO 151

Table 3.7. A co-relation table for device parameters of all the terpolymer donors of PM6 and D18. 152

Table 3.8. Summary of crystallographic parameters for donor polymer:Y6-BO blend films. 155

Table 3.9. Electron and hole mobilities of the polymers PBTFX. 156

Table 3.10. Device performances of the of polymer solar cells based on PM6 and PBTF5: Y6-BO photoactive layer with aperture size of 54.45 cm². 159

List of Figures

Figure 1.1. Three generations of PV cells. 22

Figure 1.2. The latest chart on record cell efficiencies by National Renewable Energy Laboratory (NREL). 24

Figure 1.3. Working principle/mechanism of the OPV. 25

Figure 1.4. Relationship between VOC, JSC, Vₘₐₓ, and Jₘₐₓ.[이미지참조] 27

Figure 1.5. OPV architectures: (a) conventional (b) inverted. 29

Figure 1.6. Junction types in OPV cells. 30

Figure 1.7. The National Renewable Energy Laboratory (NREL) has documented the highest certified efficiencies for OSCs between 2001 and 2021. 32

Figure 1.8. Structure of A-D-A type ITIC and related molecular engineering techniques. 33

Figure 1.9. Structure of A-DA′D-A type Y6 and corresponding molecular engineering strategies. 34

Figure 1.10. The PffBT4T-2OD polymer donor's structure and associated molecular engineering approaches for alternating copolymers. 35

Figure 1.11. The polymer donor PM6's structure and associated molecular engineering techniques for alternating copolymers. 36

Figure 1.12. Some representative terpolymers for FAs and NFAs. 38

Figure 1.13. Promising applications of OSCs. 40

Figure 1.14. According to device type and area, the highest certified power conversion efficiencies (PCEs) of OPV devices. 40

Figure 1.15. (a) High-performance small-area (＜1 cm²), large-area (≥1 cm²) and OSC modules (≥1 cm²), (b) Efficiency progress of large-area (≥1 cm²) OSCs... 42

Figure 2.1. Design strategy of terpolymers of PffBT4T-2OD. 54

Figure 2.2. Rationale material (terpolymer) design with regulated miscibility and crystallinity. 55

Figure 2.3. UV-vis absorption spectra of polymer donor's 2FBT-OBTX in a) solution and b) thin film states processed from chlorobenzene. 63

Figure 2.4. (a) Cyclic voltammograms for the polymers 2FBT-OBTX and (b) Energy level diagrams of polymer donors. 64

Figure 2.5. (a-c) UV-vis absorption spectra of polymer donor's 2FBT-OBT50-H, 2FBT-OBT50-F, 2FBT-OBT50-Cl in solution (dash lines) and thin film (solid lines)... 65

Figure 2.6. (a) Cyclic voltammograms for the polymers 2FBT-OBT50-T, 2FBT-OBT50-F, 2FBT-OBT50-Cl and (b) Energy level diagrams of polymer donors. 66

Figure 2.7. UV-vis absorption spectra of polymer donor's 2ClBT-OBTX in a) solution and b) Thin film states processed from chloroform. 66

Figure 2.8. (a) Cyclic voltammograms for the polymers 2ClBT-OBTX, and (b) Energy level diagrams of polymer donors. 68

Figure 2.9. (a) UV-vis absorption spectra of polymer donor's 2ClBT-OBT50 and 2FBT-OBT50 in solution (dash lines) and thin film (solid lines) states processed... 68

Figure 2.10. (a) UV-vis absorption spectra of polymer donor's 2FBT-OBT50 and 2FBTE-OBT50 in solution (dash lines) and thin film (solid lines) states processed... 69

Figure 2.11. (a) UV-vis absorption spectra of polymer donor's 2FBT-OBT50 and 2FBT-FOBT50 in solution (dash lines) and thin film (solid lines) states processed... 70

Figure 2.12. (a) Cyclic voltammograms for the polymers 2FBT-OBT50, 2FBT-FOBT50 and (b) Energy level diagrams of polymer donors. 71

Figure 2.13. (a) UV-vis absorption spectra of polymer donor's 2FBT-OBT50-HF and 2FBT-OBTP50-HF in thin film states processed from chloroform and (b, c)... 72

Figure 2.14. (a) UV-vis absorption spectra of polymer donor's 2FQx-FBT50-H and 2FQx-FBT50-F in thin film states processed from chlorobenzene and (b, c)... 73

Figure 2.15. (a, b) UV-vis absorption spectra of polymer donor's 2FQx-C14P50, 2FQx-BOP50, 2FQx-HDP50 and 2FQx-ODP50 c, d) 2FQx-C14BT50, 2FQx-... 75

Figure 2.16. Cyclic voltammograms and energy level diagrams for the polymers (a, b) 2FQx-C14P50, 2FQx-BOP50, 2FQx-HDP50 and 2FQx-ODP50 (c, d) 2FQx-... 76

Figure 2.17. J-V curves of the photoactive materials 2FBT-OBTX processed from o-xylene. 79

Figure 2.18. J-V curves of the different batches of the polymer 2FBT-OBT50 processed from o-xylene. 81

Figure 2.19. J-V curves of the thermally synthesized batches of the polymer 2FBT-OBT50 processed from o-xylene. 82

Figure 2.20. J-V curves of the polymer solar cells based on polymer 2FBT-OBT50:IT-4F. 84

Figure 2.21. J-V curves of the photoactive materials 2FBT-OBT-H/F/Cl processed from o-xylene. 85

Figure 2.22. J-V curves of the photoactive materials 2ClBT-OBTX processed from o-xylene. 86

Figure 2.23. J-V curves of the polymer solar cells based on 2FBT-OBT50:IT-4F and 2FBT-OBT50:PC₇₁BM. 87

Figure 2.24. J-V curves of the photoactive materials 2FBTE-OBT50 and 2FBT-FOBT50. 88

Figure 2.25. J-V curves of the photoactive materials 2FBT-OBT50-HF and 2FBT-OBTP50-HF. 89

Figure 2.26. J-V curves of the photoactive materials 2FQx-FBTP50-H. 90

Figure 2.27. J-V curves of the photoactive materials 2FQx-C14P50, 2FQx-BOP50, 2FQx-HDP50, and 2FQx-ODP50. 91

Figure 2.28. J-V curves of the photoactive materials 2FQx-C14BT50, 2FQx-BOBT50, 2FQx-HDBT50, and 2FQx-ODBT50 93

Figure 2.29. (a, b) Schematic device illustration and fabrication of the large-area OSC. 96

Figure 3.1. Design strategy of terpolymers of PM6 and D18. 124

Figure 3.2. UV-vis absorption spectra of polymer donors PBTFX in a) solution and b) thin film states processed from o-xylene. 131

Figure 3.3. (a) Cyclic voltammograms for the polymers PBTFX and (b) Energy level diagrams of polymer donors and Y6-BO acceptor. 131

Figure 3.4. UV-vis absorption spectra of polymer donors PTF5, PB5, PMO5, PMEH5, PFO5, PFEH5 in a) solution and b) thin film states processed from o-xylene. 132

Figure 3.5. (a) Cyclic voltammograms for the polymers donors PTF5, PB5, PMO5, PMEH5, PFO5, PFEH5 and (b) Energy level diagrams of polymer donors and Y6-... 133

Figure 3.6. UV-vis absorption spectra of polymer donors PM6-TFOX, PM7-TFOX in a, b) solution and c, d) thin film states processed from o-xylene. 135

Figure 3.7. (a) Cyclic voltammograms for the polymers PM6-TFOX, PM7-TFOX and (b) Energy level diagrams of polymer donors and Y6-BO acceptor. 136

Figure 3.8. UV-vis absorption spectra of polymer donors PM6-TFX, PM6-TFX in a, b) solution and c, d) thin film states processed from o-xylene. 137

Figure 3.9. (a) Cyclic voltammograms for the polymers PM6-TFX, PM7-TFX and (b) Energy level diagrams of polymer donors and Y6-BO acceptor. 137

Figure 3.10. UV-vis absorption spectra of polymer donors DTFOX and DBTFX in a, c) solution and b, d) thin film states processed from o-xylene. 138

Figure 3.11. (a, c) Cyclic voltammograms for the polymers donors DTFOX and DBTFX and (b, d) Energy level diagrams of polymer donors. 140

Figure 3.12. Photoactive materials PBTFX processed from o-xylene. a) Best J-V curves, b) EQE spectra with integrated JSC.[이미지참조] 142

Figure 3.13. J-V curves of the photoactive materials PTF5, PB5, PMO5, PFO5, and PFEH5 processed from o-xylene. 144

Figure 3.14. J-V curves of the photoactive materials PM6-TFOX and PM7-TFOX processed from o-xylene. 146

Figure 3.15. J-V curves of the photoactive materials PM6-TFX and PM7-TFX processed from o-xylene. 148

Figure 3.16. J-V curves of the photoactive materials DTFOX processed from o-xylene. 149

Figure 3.17. J-V curves of the photoactive materials DBTFX processed from o-xylene. 150

Figure 3.18. a) AFM height images and for PBTFX: Y6-BO blend films. b) Contact angles of PM6, terpolymers, and Y6-BO to water and diiodomethane. 154

Figure 3.19. a) 2D GIWAXS scattering pattern images of PBTFX: Y6-BO blend films. Line-cut profiles for b) in-plane (IP) direction and c) out-of-plane (OOP)... 155

Figure 3.20. SCLC characteristic curves of (a) hole-only and (b) electron-only devices based on PBTFX: Y6-BO photoactive layers. 156

Figure 3.21. a) Jph-Veff characteristics of OSCs based on PBTFX: Y6-BO. Dependence of b) VOC and c) JSC on the incident light intensity for PBTFX: Y6-...[이미지참조] 157

Figure 3.22. a) Schematic representation of the large-area OSC. b) Large-area OSCs with optimal J-V characteristics based on PBTF5:Y6-BO and PM6:Y6-BO.... 159

List of Schemes

Scheme 2.1. Synthetic routes of polymers 2FBT-OBTX, 2FBT-OBT50-H/F/Cl and 2ClBT-OBTX. 59

Scheme 2.2. Synthetic routes of polymers 2FBT-FOBT50, 2FBTE-OBT50, 2FBT-OBT50-HF and 2FBT-OBTP50. 60

Scheme 2.3. Synthetic routes of polymers 2FQx-FBT-H/F, 2FQx-OBT50 and 2FQx-OBTP50. 60

Scheme 2.4. Synthetic routes of monomers M1, M2 and M3. 100

Scheme 2.5. Synthetic routes of monomers M4, M5 and M6. 100

Scheme 2.6. Synthetic routes of intermediate thiophene (X3, R-Th-SnMe₃). 104

Scheme 2.7. Synthetic routes of monomers M8, M9 and M10. 113

Scheme 3.1. Synthetic routes of polymers PTFX, PBTFX, and PBX. 127

Scheme 3.2. Synthetic routes of polymers PFEHX, PFOX, PMEHX, and PMOX. 128

Scheme 3.3. Synthetic routes of polymers PM6/PM7-BTFX and PM6/PM7-TFOX for their impact of fluorination vs chlorination. 128

Scheme 3.4. Synthetic routes of polymers DTFOX and DBTFX. 129

Scheme 3.5. Synthetic routes of reference polymers PM6 and D18. 129

Scheme 3.6. Synthetic routes of monomers M1, M2, and M3. 162

Scheme 3.7. Synthetic routes of monomers M5. 167

 In order to create commercially feasible polymer solar cells (PSCs), a great deal of research has been done in the last several years, either by developing novel donor materials or by improving device fabrication techniques. Major improvements are required to capitalize on the low cost, flexibility, light weight, and large-scale roll to roll (R2R) fabrication of PSCs into commercialization. These improvements include the use of high-throughput device fabrication conditions, easily scalable high-efficiency low cost donor materials, high stability under real weather conditions, and fabrications using environmentally friendly solvents.

From this perspective, we concentrate on the synthesis of effective polymer donors which may find use in large-scale bulk heterojunction solar cells using non-halogenated solvents. Random copolymerization can result in modest molecular structural modifications. These modifications can be done by adding the donor or acceptor unit as third component in to the polymer backbone. These modifications can have a major impact on the polymer band-gap, solubility, miscibility, processability, blend morphology, charge carrier mobility, device performance and large area applicability. Consequently, the creation of high-performing photovoltaic materials requires a thorough knowledge of how these alterations affect optoelectronic and photovoltaic capabilities. We synthesized the various polymer structures involving donor and acceptor (D-A) polymer design in this contribution. We then investigated the relationship between these modifications and the photovoltaic performances of the resulting copolymers.

The first chapter serves as an introduction to organic solar cells. A brief history of the creation of organic solar cells, their working mechanism, performance parameters, device design, junction types, applications (semi-transparent OSCs, Indoor OSCs, and flexible OSCs), large area implications and their SWOT analysis are included. In-depth explanations of the developments in material designs and synthesis (polymer and small molecules) were provided. The role of random copolymers was highlighted for the large area applications of the organic solar cells, moreover, challenges and limitations from lab to fab are discussed.

The second chapter discuss the synthesis of various terpolymers' for their room temperature processability with fullerene acceptors and large area applicability by adding the third component structurally very similar to the main backbone of the polymer. This way modified polymer's optoelectronic properties remain conserved along with non-halogenated solvent processability. Herein, we developed a series of terpolymers by adding structurally different third units like symmetrically asymmetrically substituted alkoxy-benzothiadiazole (BT), ester functionalized-BT, and quinoxaline analogues of benzothiadiazoles. Furthermore, alkyl chain modification and thiophene spacer presence/absence was inspected on quinoxaline for rational terpolymer design. Evermore, quaternary approach was also touched superficially for polymers development. Optical, electrochemical and photovoltaics properties were thoroughly examined. In the end, we find out the rational material exhibited PCE of 9.73% on small area (0.12cm²) and 6.32% on large area (54cm²) in non halogented solvent (o-xylene).

The third chapter encloses the role of trifluoromethyl (CF3-) group for the modifications of the state of the art polymers for non-fullerene acceptors (PM6 & D18) towards large area adaptability. Many different kinds of CF₃ contained benzene units with varied electron withdrawing ability were introduced into the main backbone of these discussed ultra-modern polymers via random copolymerization to ease their over aggregation. Synthesis of the polymers was controlled and optimized in the long run. The influence of these third components on the optoelectronic and photovoltaic characteristics of the resulting random copolymers was systematically investigated and compared. In the huge gallery of developed polymers, PCE of 18.2% was shown by one of the polymer on small area (0.12cm²) and 11.5% on large area (54 cm²) in environmentally friendly solvent (o-xylene). We further examined that polymer series and explore the function of the CF₃ towards these plausible results by means of crystallinity, morphology, charge recombination, and charge carrier mobility measurements.