Charge Generation of n-doped Organic Semiconductors and Polaron Transport in Organic Light-Emitting Diodes for Operational Stability
n-도핑된 유기반도체의 전하생성효율과 유기발광소자의 구동 안정성을 위한 폴라론 거동 분석

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dc.description학위논문 (박사)-- 서울대학교 대학원 : 공과대학 재료공학부(하이브리드 재료), 2018. 8. 김장주.-
dc.description.abstractDiscovery of organic semiconductors enables a variety of organic electronic devices involving organic light-emitting diodes (OLEDs), organic photovoltaics, organic thin-film transistors. Especially, OLEDs has been commercialized and became practical display based on the establishment of chemical and physical understanding of organic semiconductors. In this research, charge in electrically doped organic semiconductors and OLEDs is explored. First, charge generation mechanism of n-type-doped organic semiconductors is investigated. Second, operational stability of OLEDs is correlated with polaron behaviors in view of polaron and exciton distribution. In chapter 2, charge generation process in electrically doped organic semiconductors is quantitatively analyzed. Electrical doping is an important technology to enhance the conductivity of organic layers, which is related to driving voltage and corresponding efficiency of organic electronic devices. In organic semiconductors, the charge generation efficiency, the ratio between generated charge carriers and dopants, shows a few percents, which is a different behavior to conventional inorganic semiconductors. Many reports revealed that doping effect is realized by the formation of charge transfer complex and dissociation into free charge carrier. In this research, for n-type doped organic semiconductors, the charge transfer complex formation efficiency and dissociation efficiency depending on dopant concentration are quantitatively analyzed. As a result, it is investigated that dissociation into free charge carrier is relatively inefficient compared to formation of charge transfer complex.

In chapter 3 and 4, electrical properties of materials and device are correlated with the operational stability of OLEDs. In chapter 3, from a comparison of the co-host based device and single host based device, it is verified that polaron and exciton distribution under the operation is dependent on the balance between electron and hole mobilities. This finding means that quantitative characterization of mobility balance in the EML is crucial to enhance device stability of OLEDs in addition to the use of co-host or bipolar host. In chapter 4, the role of charge transport layer in device stability of OLEDs is revealed. With the modification of HTLs only in the device structure, device lifetime is seven-times enhanced in comparison of two kinds of OLEDs. From various static and dynamic electrical characterization of materials and devices, it is unraveled that charge transport layers control the number of charge carriers in the EML and induces charge transport path in the EML. These two effects are important in carrier balance in the EML in terms of carrier density balance and carrier mobility balance, which affects exciton and polaron density distribution. Therefore, it implies that electrical consideration of charge transport layers should be performed in comprehensive views in order to enhance operational stability of OLEDs.
dc.description.tableofcontentsChapter 1. Introduction 1

1.1 Electrical doping in organic semiconductors 1

1.2 Operational stability of organic light-emitting diodes 8

1.3 Theoretical description of electrical characteristics of OLEDs (Drift-diffusion numerical modeling) 15

1.4 Impedance spectroscopy 21

Chapter 2. N-Type Molecular Doping in Organic Semiconductors: Formation and Dissociation Efficiencies of a Charge Transfer Complex 36

2.1 Introduction 36

2.2 Experimental methods 39

2.3 Charge transfer complex formation efficiency 40

2.4 Decomposition of Rb2CO3 based on in situ XPS measurements. 46

2.5 Charge generation efficiency and dissociation efficiency. 49

2.6 Conclusion. 55

Chapter 3. Mobility Balance in the Light-emitting Layer Governs the Polaron Accumulation and Operational Stability of Organic Light-Emitting Diodes 57

3.1 Introduction 57

3.2 Experimental methods 59

3.3 Structure design and performance of OLEDs 59

3.4 Understanding of charge behavior by impedance spectroscopy and drift-diffusion numerical modeling 63

3.5 Recombination zone depending on the mobility ratio 71

3.6 Conclusion 73

Chapter 4. Charge Transport Layers Manage Mobility and Carrier Density Balance in Light-emitting Layers Influencing the Operational Stability of Organic Light-emitting Diodes 75

4.1 Introduction 75

4.2 Experimental methods 77

4.3 Device structure and performance of OLEDs 79

4.4 Charge transport in single carrier devices 83

4.5 Impedance analysis of the OLEDs 88

4.6 Correlation between the charge transport and operational stability of the OLEDs 92

4.7 Conclusion 95

Chapter 5. Summary and Conclusion 97

Bibliography 100

초록 113


List of Publications 117

List of Presentations 118

List of Patents 121
dc.publisher서울대학교 대학원-
dc.titleCharge Generation of n-doped Organic Semiconductors and Polaron Transport in Organic Light-Emitting Diodes for Operational Stability-
dc.title.alternativen-도핑된 유기반도체의 전하생성효율과 유기발광소자의 구동 안정성을 위한 폴라론 거동 분석-
dc.contributor.AlternativeAuthorKim Jae-Min-
dc.contributor.affiliation공과대학 재료공학부(하이브리드 재료)-
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College of Engineering/Engineering Practice School (공과대학/대학원)Dept. of Material Science and Engineering (재료공학부) Theses (Ph.D. / Sc.D._재료공학부)
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