Energy-Level Control at Hybrid Inorganic/Organic Semiconductor Interfaces (eBook)

Supervisor’s Foreword 7
Abstract 9
Acknowledgements 11
Contents 13
Abbreviations 16
1 Introduction 18
References 22
2 Fundamentals 24
2.1 Organic Semiconductors: --Conjugated Molecules 24
2.2 Electronic Structure Theory 27
2.2.1 Nearly Free Electrons 28
2.2.2 Model of Tight Binding/LCAO 29
2.2.3 The Density of States 33
2.2.4 Gap States 34
2.3 Models and Processes of Energy--Level Alignment 38
2.3.1 Doping and Fermi--Level 39
2.3.2 Band Bending 41
2.3.3 Vacuum Level Alignment 49
2.3.4 Fermi--Level Pinning 51
2.3.5 Push Back 54
2.3.6 Molecular Film Structure 56
References 58
3 Theory of Experimental Methods 63
3.1 Photoelectron Spectroscopy (PES) 63
3.1.1 The Three Step Model of Photoemission 66
3.1.2 Interpretation and Analysis of Photoemission Data 74
3.2 Near Edge X-ray Absorption Fine Structure (NEXAFS) 79
3.2.1 Angular Dependence of Molecular NEXAFS Resonances 81
3.3 Scanning Tunneling Microscopy (STM) 85
3.4 Low Energy Electron Diffraction (LEED) 87
References 89
4 Methodology and Experimental Setups 93
4.1 Materials and Sample Preparation 93
4.1.1 ZnO Crystals 93
4.1.2 Organic Materials 95
4.2 Experimental Equipment and Settings 98
4.2.1 PES, NEXAFS and LEED at BESSY II 98
4.2.2 PES, STM/AFM and LEED Setup at the HU Berlin 100
4.2.3 PES and LEED at Chiba University 101
4.2.4 Optical Characterization of HIOS Structures 101
4.3 Data Analysis 102
4.3.1 Processing Photoemission Data 102
4.3.2 Processing NEXAFS Data 103
4.3.3 Processing AFM/STM Data 104
4.3.4 PES Peak Fitting and Determination of Positions 105
References 105
5 Results and Discussion 107
5.1 Characterization of the ZnO Faces 108
5.1.1 Investigation of the Surface Structure 109
5.1.2 Electronic Characterization of the Clean ZnO Faces 116
5.2 Donor and Acceptor Molecules on ZnO 119
5.2.1 Strong Work Function Increases of ZnO Using F4TCNQ 120
5.2.2 F6TCNNQ: A Similar, but Not as Volatile, Acceptor as F4TCNQ 131
5.2.3 HATCN, an Application Relevant Acceptor for Tuning the Work Function of ZnO 135
5.2.4 A Weak Acceptor, Pentacenetetrone, on ZnO 143
5.2.5 Donors on ZnO 149
5.3 ZnO Gap State Density of States Limits Adsorption Induced Internal Band Bending 155
5.4 Universal Energy-Level Alignment of Para-Sexiphenyl on ZnO 162
5.5 Adjusting HIOS Energy-Level Alignment 168
5.5.1 Energy-Level Adjustment of 6P on ZnO by HATCN Monolayers 169
5.5.2 Demonstration of Continuous Energy-Level Adjustment: SP6 on ZnO(0001) 173
5.5.3 Efficient Light Emission from Inorganic and Organic Semiconductor Hybrid Structures by Energy-Level Tuning 176
References 186
6 Conclusion 197
Appendix A Mathematica Code for Band Bending in Depletion Approximation 201
Appendix B Main Excitation Lines/Satellites 204
Appendix C Program Code for Satellite Removal 205
Appendix D NEXAFS ``Dark Current''/Offset Related Artifact 208
Appendix E Mathematica Code for Band Bending Using Fermi--Dirac Statistics 211
Appendix F Additional Experimental Spectra 215
F.1 Work Functions of Differently Prepared ZnO 215
F.2 STM of Ex Situ Hot Furnace Annealed ZnO 216
F.3 XP Spectra of NMA on ZnO(000bar1) 216
F.4 Relationship Between GDOS and ZnO Energy--Level Shifts / UV Irradiation 218
F.5 Thick--Film NEXAFS of 6P on ZnO 219
Curriculum Vitae 221