S-Space College of Engineering/Engineering Practice School (공과대학/대학원) Dept. of Material Science and Engineering (재료공학부) Theses (Ph.D. / Sc.D._재료공학부)
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
- 공과대학 재료공학부(하이브리드 재료)
- Issue Date
- 서울대학교 대학원
- 학위논문 (박사)-- 서울대학교 대학원 : 공과대학 재료공학부(하이브리드 재료), 2018. 8. 김장주.
- Discovery 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.
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