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Development of Organic Semiconducting Materials for High Performance Organic-based Solar Cells

  • 박성민 (대학원 화학과 유기화학전공)

초록/요약

Organic conjugated molecules have attracted significant attention in the organic electronics area due to their advantageous properties such as low cost, light weight, and solution processability. In particular, simple solution process can easily provide a large or flexible electronic device such as organic based solar cell, organic light emitting diode (OLED), and organic field-effect transistor. Moreover, the properties of molecules such as energy bandgap, energy levels, and conductivity are tunable through the designing of molecular structures facile synthesizing by cross-coupling methods. To be applied as a light harvesting material in organic photovoltaic (OPV), the molecules should have an optical bandgap in visible light range which can be achieved by the design strategies such as quinoid structure or electron donor-acceptor alternation. To be applied as an interlayer material in OLED or organic based perovskite solar cells (PSC), the molecules should be designed by mainly focusing on the high conductivity which can be achieved through the oxidative doping. In this thesis, the conjugated molecules are designed and synthesized by considering suitable properties for application as light harvesting materials of OPV and hole transporting materials (HTM) of PSC. Firstly, when organic conjugated molecules in OPV absorb the light in active layer, the hole and electron are bound to each other, known as exciton, via strong Coulombic interaction. To overcome the binding energy of exciton, the active layer of OPV device commonly consists of donor and acceptor materials as bulk heterojunction to give a driving force for exciton separation. In this regard, the blend morphology control is a critical issue to efficient exciton separation and charge carrier transport. Here, the donor polymers was developed incorporating o-dichlorobenzyl group compatible with fullerene derivative as acceptor material to optimize the blend morphology and enhance the performance of the OPV device. The effects of the effective distance and volume occupied by the flexible motions of alkyl side chains were investigated on the blend morphology and photovoltaic properties. Secondly, a series of conjugated small molecules was designed and synthesized with different fluorine substitution patterns (0F-4F). The properties of designed molecules are very different depending on the symmetricity and the numbers of incorporated fluorine atoms. 2F and 4F, which featured symmetric and even-numbered fluorine substitution patterns, show improved molecular packing structures and higher crystalline properties in a film compared with 1F and 3F. 2F achieves the higher performance compared with less ordered 0F molecule in the both as a donor material in OPV device and as a dopant free HTM in PSC. The effects of the local dipole moment and the distribution of conformer are demonstrated on intermolecular interaction and charge transport properties in solid state. Finally, a conjugated small molecule, tetra-TPA, based on [2,2]paracyclophane (PCP) with triphenylamine (TPA) units was designed and synthesized to be applied for a HTM in PSCs. Tetra-TPA shows higher hole mobility and photovoltaic performance of PSC compared with well-known HTM, spiro-OMeTAD, due to effective intermolecular aggregation in solid state. A series of HTM based on PCP was designed and synthesized with the various numbers of TPA substitution to investigate the effect of the chemical structure of the HTM on photovoltaic performance of PSC. The multi-armed structure, tetra-TPA, shown higher conductivity and photovoltaic performance of PSC compared with that of di-TPA and tri-TPA HTMs.

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목차

CHAPTER 1: General Introduction
1.1. Conjugation System 2
1.2. Low Bandgap Conjugated Molecules 6
1.2.1. Quinoid Structure 6
1.2.2. Donor−Acceptor-type Structure 9
1.2.3. D−A-type Conjugated Polymers 13
1.2.4. D−A-type Conjugated Small Molecules 17
1.3. Organic Conductive Molecules 19
1.4. History of Photovoltaics 27
1.5. Organic Photovoltaics 29
1.5.1. Working Mechanism of OPVs 31
1.5.2. Characterization of OPV Device Performance 33
1.5.3. Bulk-heterojunction Device 36
1.6. Perovskite Solar Cell 39
1.7. Reference 46

CHAPTER 2: Development of Conjugated Polymer for Optimized Blend Morphology in Organic Photovoltaics
Part 1: Control of Nanoscale Morphology in Polymer Solar Cells using Conjugated Polymers Incorporating Fullerene Compatible Side Chains
2.1.1. INTRODUCTION 53
2.1.2. RESULTS AND DISCUSSION 56
2.1.2.1. Synthesis 56
2.1.2.2. Properties of Polymers 58
2.1.2.3. Photovoltaics Properties 61
2.1.2.4. Blend Morphology 64
2.1.2.5. Grazing Incidence X-ray Diffraction 68
2.1.2.6. Contact Angles 71
2.1.2.7. Carrier Mobility 75
2.1.2.8. Charge Generation Efficiencies 77
2.1.2.9. Bimolecular Recombination 77
2.1.2.10. Photoluminescence Quenching 80
2.1.2.11. Density Functional Theory 82
2.1.3. CONCLUSIONS 85
2.1.4. REFERENCE 86

Part 2: Effect of Dynamic 3D-Volume of Side chain in Conjugated Polymers on Nanoscale Morphology and Solar Cell properties
2.2.1. INTRODUCTION 92
2.2.2. RESULTS AND DISCUSSION 94
2.2.2.1. Polymer Syntheses 94
2.2.2.2. Optical and Electrochemical Properties of the Polymers 96
2.2.2.3. Grazing Incidence X-ray Diffraction 100
2.2.2.4. Photovoltaic properties of the polymers 104
2.2.3. CONCLUSIONS 115
2.2.4. REFERENCE 116

CHAPTER 3: Development of Conjugated Small Molecules by Controlling Fluorine Substitution for Organic-based Solar Cells
3.1. INTRODUCTION 120
3.2. RESULTS AND DISCUSSION 123
3.2.1. Synthesis and Physical Properties 123
3.2.2. Photophysical and Electrochemical Properties 125
3.2.3. Theoretical Calculations 129
3.2.4. Grazing Incidence X-ray Diffraction 131
3.2.5. Solar Cells 134
3.3. CONCLUSIONS 152
3.4. REFERENCE 153

CHAPTER 4: Development of Hole Transporting Materials for High Performance Perovskite Solar Cells
Part 1: A [2,2] paracyclophane triarylamine-based hole-transporting material for high performance perovskite solar cells
4.1.1. INTRODUCTION 159
4.1.2. RESULTS AND DISCUSSION 162
4.1.2.1. Synthesis 162
4.1.2.2. Properties of PCP-TPA 164
4.1.2.3. Density Functional Theory Calculation 170
4.1.2.4. Hole Mobility 172
4.1.2.5. Perovskite Solar Cells 174
4.1.3. CONCLUSIONS 180
4.1.4. REFERENCE 181

Part 2: Effect of multi-armed triphenylamine-based hole transporting materials for high performance perovskite solar cells
4.2.1. INTRODUCTION 185
4.2.2. RESULTS AND DISCUSSION 188
4.2.2.1. Synthesis and Characterization 188
4.2.2.2. Time-Resolved Photoluminescence 194
4.2.2.3. Conducting Properties 196
4.2.2.4. Perovskite Solar Cells 199
4.2.3. CONCLUSIONS 206
4.2.4. REFERENCE 207

APENDIX
CHAPTER 2: Part 1
2.1.5. EXPERIMENTAL SECTION 209
2.1.5.1. General Procedure 209
2.1.5.2. Synthesis of Monomers 210
2.1.5.3. Synthesis of Polymers 217
2.1.5.4. Electrochemical Properties 227
CHAPTER 2: Part 2
2.2.5. EXPERIMENTAL SECTION 228
2.2.5.1. General Procedure 228
2.2.5.2. Synthesis of Monomers and Polymers 229
2.2.5.3. Electrochemical Properties 239
CHAPTER 3:
3.5. EXPERIMENTAL SECTION 240
3.5.1. General Procedure 240
3.5.2. Characterization Methods 240
3.5.3. Synthesis 242
3.5.4. Theoretical Calculation Method 256
CHAPTER 4: Part 1
4.1.5. EXPERIMENTAL SECTION 257
4.1.5.1. General Procedure 257
4.1.5.2. Optical Properties Measurements 258
4.1.5.3. Thermal Properties Measurements 258
4.1.5.4. Electrochemical Properties 258
4.1.5.5. Synthesis of PCP-TPA 259
CHAPTER 4: Part 2
4.2.5. EXPERIMENTAL SECTION 264
4.2.5.1. General Procedures 264
4.2.5.2. Synthesis of HTMs 265
4.2.5.3. Photoluminescence Lifetime Measurements 280
4.2.5.4. Conductive Atomic Force Microscopy (c-AFM) Measurements 280
REFERENCE OF EXPERIMENTAL SECTIONS 281
PERMISSION OF COPYRIGHT 282
ACKNOWLEDGEMENT 287

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