Organic thin-film photovoltaics [Elektronische Ressource] / Miaoyin Liu

Organic Thin-film Photovoltaics




Dissertation
zur Erlangung des Grades
‘Doktor der Naturwissenschaften’

am Fachbereich Chemie, Pharmazie und Geowissenschaften
der Johannes Gutenberg-Universität in Mainz



Miaoyin Liu
Geboren in Shandong Province / China
Mainz, 2010



























Dedicated to my family



















The impossible exists only until we find a way to make it possible.
Mike Horn



CONTENTS


Chapter 1 Motivation and Outline 1
1.1 Photovoltaic Effect…………………………………………………………………………. 1
1.2 Motivation……………………………….3
1.2.1 Self-organizable Donors for Bulk-heterojunction Solar Cells………....................3
1.2.1.1 Discotic Liquid Crystals as Donor Materials…………………………........4
1.2.1.2 Conjugated Polymers for Polymer-Fullerene Solar Cells………………..5
1.2.2 Organic Sensitizers for Dye-sensitized Solar Cells………………………………..6
1.3 Outline.………………………………………………………………………………………..6
1.4 References……………………………...9

Chapter 2 Introduction to Organic Thin-film Photovoltaics 11
2.1 Photovoltaic Parameters………………………………………………………………….11
2.1.1 External Quantum Efficiency (EQE)………………………………………………11
2.1.2 I-V Curve……………………………………………………………………………..13
2.2 Bulk-heterojunction Solar Cells (BHJ)…………………………………………………...15
2.2.1 Concept………………………………………………………………………………15
2.2.2 Fundamental Physical Processes…………………………………………………17
2.2.2.1 Creation of Excitons………………………………………………………..18
2.2.2.2 Diffusion of Excitons…………………………….........19 3 Dissociation of Charge Carriers at the Donor/Acceptor Interface........ 20
2.2.2.3.1 Photoinduced Charge Separation Process………………….20 2.2.2.3.2 Loss Mechanisms………………………………………………24
3 Marcus Theory…………………………………......................25
2.2.2.4 Charge Transport…………………………………………………………..26
2.2.2.5 Extraction of the Charge Carriers at the Electrodes……………………27
2.2.3 Molecular and Morphology Requirements for the Photoactive Layer………….28
2.2.3.1 Discotic Liquid Crystals…………………………………………………….30
2.2.3.2 Conjugated Polymers………………………………………………………31
2.3 Dye-sensitized Solar Cells (DSSCs)……………………………………………………..37
2.3.1 Liquid-state Dye-Sensitized Solar Cells…………………………………………...38 2.3.2 Solid-state Dye-sensitized Solar Cells…………………………………………….40
2.3.2.1 TiO Films………………………………………………41 2
2.3.2.2 Sensitizers.…………………………………………………………………..42
2.3.2.2.1 Ruthenium Complexes…………………………………………..43 2.3.2.2.2 Organic Sensitizers………………………………………………47
2.3.2.3 Hole-transport Materials (HTM)……………………………………………51 2.4 Counter Electrode…………………………………………………………...53
2.4 References………………………………………………………………………………….54


Chapter 3 Supramolecular Organization and Photovoltaics of the Triangle-
shaped Discotic graphenes 61
3.1 Introduction…………………………………………………………………………………61
3.2 Synthesis of Triangle-shaped Discotic Graphenes…………………………………….63
3.3 Optical Properties………………………………………………………………………….64
3.4 Energy Levels………………………………………………………………………………65
3.5 Thermotropic Properties…………………………………………………………………..66
3.6 Photovoltaic Device………………………………………………………………………..69
3.7 Morphology View…………………………………………………………………………..70
3.8 Comparison with the Swallow-tailed HBC Analogues…………………………………71
3.9 Conclusion………………………………………………………………………………….72
3.10 References……………………………………..74


Chapter 4 A Self-organizable Polymer for Photovoltaic Applications 77
4.1 Introduction…………………………………………………………………………………77
4.2 Benzodithiophene Building Block………………………………………………………...79
4.3 Synthesis of PBTT…………………………………………………………………………81
4.4 Optical Properties……………………...…………………………………………………..82
4.5 Energy Level Matching…………………………………………………………………….83
4.6 X-ray Diffraction (XRD) Measurements on the Film Morphology……………………..84
4.7 Solar Cell Device Fabrication and Performance..………………………………………86
4.7.1 Morphology Control….………………………………………………………………86 4.7.1.1 D/A Ratio..…………………………………………………………………86
4.7.1.2 Processing Additive..……………………………………………………..88
4.7.1.3 Annealing Effect..…………………………………………………………91
4.7.1.3.1 Annealing Effect in the Aspect of AFM Topography..………94 4.7.1.3.2 Annealing Effect in the Aspect of Shunt Resistance..………95
4.7.2 PBTT/[70]PCBM Cell Performance…..………………………………………….100
4.7.3 PDI as Acceptor…………………………………………………………………...101
4.8 PBTT Compared to P3HT……………………………………………………………….102
4.8.1 Regarding Structure and Energy Level…………………………………………102
4.8.2 Regarding Primary Variables of Polymers...……………………………………103
4.8.3 Regarding Aggregation..………………………………………………………….103
4.8.4 Regarding Photovoltaic Performance…………………………………………...105
4.9 Thought of Utilizing More Ribbon Structures………………..…………………..........107
4.9.1 D-A Copolymer SCS312 with More Ribbon Structure………………………...107
4.9.2 Optical and Electrochemical Properties of SCS312……………….................109
4.9.3 Photovoltaic Performance and Further Possible Optimizations……………...110
4.10 Conclusion……………………………………………………………………………….112
4.11 References………………………………………………………………………………113


Chapter 5 Novel Organic Sensitizers for Solid-state Dye-sensitized Solar
Cells 117
5.1 A Stable Perylene Dye for Solid-state Dye-sensitized Solar Cells….………………118
5.1.1 Introduction …………………………...……………………………………………118
5.1.2 Optical Properties………………………………………………………………….121
5.1.3 Electrochemical Properties……………………………………………………….123
5.1.4 Device Fabrication…………………………………………………………………125
5.1.5 Solvent Effects-Solvatochromic Solar Cells…………………………………….125
5.1.6 Device Performance……………………………………………………………….127
5.1.7 Device Stability……….......………………………………………………………..131
5.1.8 Conclusion………………………………………………………………………….132
5.2 An Isophorone Dye for Solid-state Dye-sensitized Solar Cells……………………...133
5.2.1 New Isophorone Dye Ind-1……………………………………………………….134
5.2.2 Optical Properties……………………………………………………...................135 5.2.3 Electrochemical Properties……………………………………………………….136
5.2.4 Photovoltaic Performance………………………………………………………...137
5.2.5 Possibility of Building Molecular Cocktail DSSCs…….………………………..139
5.2.6 Conclusion.…………………………………………………………………………140
141 5.3 References………………………………………………………………………………..


Chapter 6 Experimental Part 145
6.1 Device Preparation……………………………………………………………………….145
6.1.1 Organic Bulk-heterojunction Solar Cells..….……………………………………145
6.1.2 Solid-state Dye-sensitized Solar Cells…..………………………………………148
6.2 Device Testing Setup.……………………………………………………………………152
6.3 Absorption Spectroscopy………………………………………………………………..153
6.4 Cyclic Voltammetry (CV)..….……………………………………………………………154
6.5 Microscopic Techniques…………………………………………………………………156
6.5.1 Atomic Force Microscopy………………………………………………………...156
6.5.2 Scanning Electron Microscopy (SEM)…………………………………………..158
6.6 X-ray……………………………………………………………………………………….159
6.6.1 2D-WAXS measurements………………………………………………………..159
6.6.2 X-ray Diffraction………….………………………………………………………..160
6.7 Differential Scanning Calorimetry (DSC)………………………………………………163
6.8 References………………………………………………………………………………..164


Chapter 7 Conclusion and Outlook 165
7.1 Conclusion………………………………………………………………………………...165
7.2 Outlook…………………………………………………………………………………….167
7.2.1 Hybrid Tandem Solar Cell………………………………………………………..168
7.3 References………………………………………………………………………………..170

Acknowledgement
List of Publications
Curriculum Vitae
Index of Abbreviations

AFM Atomic force microscope
ASTM American Society for Testing and Materials
BHJ Bulk-heterojunction solar cells
CB Conduction band
CV Cyclic voltammetry
D/A Donor/acceptor
DLCs Discotic liquid crystals
DSSCs Dye-sensitized solar cells
EA Electron affinity
EPFL Éco