SCANNING PROBE MICROSCOPY STUDY ON NANOSTRUCTURED MATERIALS IN ORGANIC AND PEROVSKITE OPTOELECTRONIC DEVICES

다국어 초록 (Multilingual Abstract)

Since discovery of electrical conductivity in organic substances, organic optoelectronics have been developed and diversified for several decades. Studies on organic optoelectronics and organic-inorganic hybrid optoelectronics, including organic light emitting diodes (OLEDs), organic solar cells (OSCs) and perovskite solar cells (PSCs) have enlarged its application to practical industries. In order to understand charge carrier behaviors in multi-stacked thin film devices of optoelectronics, nano-scale characterization is important to investigate practical excitonic reaction. Moreover, by adapting nanomaterials and nano-scaled layers onto devices, their performance is able to be improved on efficiency and stability.
In this thesis, we adapt nanomaterials and nano-scaled layers on organic and perovskite optoelectronic devices. Flexible and efficient OLEDs are fabricated using dual-scale AgNWs electrode. ZnO NRs array as electron transporting layer and Liq interlayer for work function matching is applied on excitonic solar cells such as organic solar cells and perovskite solar cells.
First, we fabricate flexible electrode using dual-scale AgNWs which is made by mixing short/thin AgNWs and long/thick AgNWs. Dual-scale AgNWs have excellent properties as flexible electrode for organic optoelectronics which includes conductivity, transmittance, roughness and coverage. Rouhness and coverage is analyzed through AFM and EFM. The green emission phosphorescent organic light emitting diodes is fabricated onto the electrode and compared to mono-scale AgNWs electrode. The OLEDs based on dual-scale AgNWs are superior to the others and there is no spectral difference in luminescence.
Organic solar cells are fabricated on vertically grown ZnO NRs array as electron transporting layer. ZnO NRs are fabricated using facile method of hydrothermal synthesis with a linearly controllable length profile. There is orientation increment of ZnO NRs proportional to length after germination. ZnO NRs with different lengths are utilized as efficient electron transport layers in organic solar cells. The length of ZnO NRs is optimized under trade-off relationship in charge collection and UV absorption.
And finally, nano-scaled layer of perovskite and Liq is characterized to improve photovoltaic performance and stability. The physical process of perovskite degradation is investigated through the FFM and conductive AFM. The perovskite fabricated under humid air and argon atmosphere is compared and it is found that degradation of perovskite by moisture is initiated from local islands. Liq has effective work function tunability to electrode for optoelectronic devices. Its coverage onto PCBM electron transporting layer is increased by deposition thickness but the uniformity is deteriorated after full coverage. The current enhancement through the Liq interlayer is measured through conductive AFM and its potential change is measured through SKPM.
In conclusion, this thesis proposes the practical utilization of nanomaterials and nano-scaled layers to improve flexibility, performance and stability of excitonic optoelectronic devices. These approaches and versatile measurement techniques in film-level and device-level characterization of individual cell are expected to be applicable to other researches which need nanoscale investigation and fabrication.

목차 (Table of Contents)

• Abstract i
• Contents v
• List of Figures xi
• List of Tables xvii
• Chapter 1 Introduction 1
• 1.1 Emergence of organic optoelectronics 3
• 1.2 Nanostructured Materials for Device Application 8
• 1.3 Instances for Utilization of Nanostructure in Optoelectronic Devices 10
• 1.3.1 Silver Nanowire Mesh Electrode as Transparent Conductive Electrode of Optoelectronic Devices 11
• 1.3.2 Oriented and Crystalline ZnO Nanorods for Photovoltaic Devices 16
• 1.4 Outline of Thesis 21
• Chapter 2 Experimental Methods 23
• 2.1 Materials 23
• 2.1.1 Preparation of Silver Nanowires 23
• 2.1.2 Preparation of Organic Materials 24
• 2.1.3 Chemical structures of Organic Materials 25
• 2.2 Scanning Probe Microscopy 27
• 2.2.1 Principles of Scanning Probe Microscopy 27
• 2.2.2 Contact AFM and Noncontact AFM 28
• 2.2.3 Electrostatic Force Microscopy (EFM) 31
• 2.2.4 Conductive Atomic Force Microscopy (Conductive AFM) 32
• 2.2.5 Lateral Force Microscopy (LFM) 33
• Chapter 3 Dual-scale metal nanowire network for flexible OLEDs 37
• 3.1 Fabrication and Characterization of Dual-scale Silver Nanowire Electrode 40
• 3.1.1 Fabrication of dual-scale silver nanowire electrode 40
• 3.1.2 Mechanical and Chemical Stability of dual-scale silver nanowire electrode 42
• 3.2 Comparison of the Silver Nanowire Electrodes with Different Scale 45
• 3.2.1 Roughness comparison of silver nanowire electrodes 45
• 3.2.2 Coverage comparison of silver nanowire electrodes 47
• 3.3 Flexible OLED Fabrication onto Dual-Scale Silver Nanowire Electrodes 54
• 3.4 Summary 60
• Chapter 4 Germinant ZnO Nanorods as a Charge-Selective Layer in Organic Solar Cells 61
• 4.1 Length Controllability of Vertically Aligned ZnO Nanorods 64
• 4.1.1 Fabrication of ZnO Nanorods 64
• 4.1.2 Linearity in Hydrothermal Growth of ZnO Nanorods 65
• 4.1.3 Orientation and Vertical Alignment of ZnO nanorods Analysis by X-ray Diffraction 67
• 4.1.4 UV-vis Absorption Measurement for Bilayer of ZnO Nanorods and Active Material 68
• 4.2 Vertically Aligned ZnO Nanorods Layer for Electron Extraction Layer of Organic Solar Cells 70
• 4.2.1 Fabrication of Organic Solar 70
• 4.2.2 Photovoltaic Performance of Organic Solar Cells with ZnO Nanorods as Electron Extraction Layer 72
• 4.3 Summary 77
• Chapter 5 AFM Characterization of Work Function Tuning in Liq and Degradation in Perovskite for Solar Cell Devices 79
• 5.1 Local Degradation of Perovskite Thin Films by Humid Atmosphere 82
• 5.1.1 AFM Characterization of Degraded Perovskite Films 82
• 5.1.2 Friction Force Microscopy of Degraded Perovskite Films 84
• 5.1.3 Photovoltaic Device Application with Perovskite Thin Films in Humid Environment 87
• 5.2 Interfacial Engineering for Efficient Electron Extraction of Perovskite Solar Cells 92
• 5.2.1 Liq Interlayer as Electron Injection Layer for Perovskite Solar Cells 92
• 5.2.2 Coverage and Potential Tuning Measurement using AFM 93
• 5.2.3 Effect of Liq Thickness on Application for Perovskite Solar Cell 97
• 5.3 Summary 99
• Chapter 6 Conclusion 101
• Bibliography 103
• Publication 119
• 한글 초록 123