Factors Affecting Success of Serial Crowdfunding: From Heuristic and Systematic Perspectives

Much of the current crowdfunding literature focuses on revealing determinants of one-time crowdfunding performance. However, the impacts of existing platform cues on serial crowdfunding performance remain largely unexplored. Drawing heuristic-systematic model, this study examines how performance-based heuristics cues and opinion-based systematic cues exert differential impacts on subsequent crowdfunding performance. This paper will fill the research gap in the crowdfunding literature by examining how backers are processing performance-related and opinion-related information when making decisions in serial crowdfunding context

유기발광다이오드에서 발광체의 극성이 재결합 기작과 소자 성능에 미치는 영향

학위논문 (박사)-- 서울대학교 대학원 : 공과대학 재료공학부, 2018. 2. 김장주.재결합 현상은 유기 광전자 소자의 구동에 직접적으로 연관된 핵심적인 현상이다. 유기발광다이오드 (OLED)의 경우, 재결합은 광자를 생성하는 엑시톤을 형성하여 발광에 기여하며, 유기 광전지 (OPV)의 경우 재결합은 광자에 의해 생성된 에너지가 손실되기 때문에 최소화 해야할 현상이다. 유기 반도체에서 고려되는 두 가지의 주요한 재결합 메커니즘이 있다. 하나는 정공과 전자 사이에서 발생하는 랑제빈 재결합 현상이며, 다른 하나는 트래핑 된 전하와 반대 부호의 전하 사이에 발생하는 트랩 보조 재결합 현상이다. 각 메커니즘이 미치는 영향은 소자 종류에 따라 다르지만, 이러한 현상을 원인을 파악하고 이해하는 것은 소자의 성능을 향상시키는데 공통적으로 중요하다. 이 논문에서 연구한 염료 도핑된 OLED 시스템에서는, 재결합 메커니즘은 구동 전압 및 효율을 결정짓는 중요한 요소이다. 이러한 중요성에도 불구하고 재결합 메커니즘을 결정하는 요인에 대한 연구는 많이 이루어지지 않았다. 트랩 심도의 효과가 그 중 하나 이지만, 깊은 트랩 준위를 가지는 시스템에서도 랑제빈 재결합 현상이 지배적인 시스템을 구축하는 것이 가능하다고 보고 되었으며, 이러한 현상을 설명할 수 있는 이론에 대한 연구가 필요하다. 이 논문에서는 첫번째로, 엑시플렉스 형성 공동 호스트 시스템 구조의 가상 소자를 연구 하였다. 층간 장벽 효과, 발광층의 전하 이동도 및 트랩 심도에 따라 표동-확산 모델링을 이용하여 소자에서의 전하 밀도, 재결합 속도 및 랑제빈 재결합 비율을 계산하였다. 그 결과 발광층의 전하이동도가 낮아질수록, 층간 에너지 장벽의 크기가 커질수록 발광층에서 정공 또는 전자 더 축적된다는 결과를 얻었으며, 이로 인해 소자에서의 랑제빈 재결합의 비율이 증가된다는 사실을 밝혔다. 또한 트랩 심도의 경우, 트랩을 빠져나가는 현상에 영향을 주어 트랩된 정공의 밀도와 관련이 있으며, 트랩 심도가 0.3 eV 이상으로 커질 경우, 이 파라미터의 크기에 상관없이 일정해 진다는 사실을 알았다. 이 결과를 엑시플렉스 공동 호스트에 대응시켜 볼 때, 이러한 구조가 랑제빈 재결합이 우세한 시스템을 만들기에 적합한 호스트 라는 사실을 알 수 있었다. 두 번째로, 도펀트의 쌍극자 모멘트가 염료로 도핑된 OLED의 재결합 메커니즘에 영향을 미치는 가장 중요한 요인 중 하나임을 밝혔다. 우선, 전류-전압 및 시간 전계 발광 특성을 통하여, 5 Debye 이상의 큰 쌍극자 모멘트를 가진 동종 리간드 구조의 이리듐 발광체는 트랩 보조 재결합에 의한 발광 현상을 가진다는 것을, 그러나 2 Debye 미만의 작은 쌍극자 모멘트를 갖는 이종 리간드의 이리듐 발광체는 랑제빈 재결합에 의한 발광 특성을 가진다는 것을 밝혔다. 또한 우리는 표동-확산 모델에서 쌍극자 모멘트의 트랩 효과를 추가적으로 고려하여, 트랩 깊이가 0.25 eV 보다 큰 경우 트랩 깊이에 의한 효과는 무시 가능하며, 쌍극자 모멘트가 소자의 재결합 메커니즘을 결정짓는 중요한 요인이 된다는 것을 밝혀냈다. 이 발견은 형광 또는 열활성 지연 형광 OLED를 포함하여 다양한 종류의 OLED 연구에 유용할 것으로 예측된다.1. Introduction 1 1.1 Organic Light Emitting Diodes 1 1.2 Langevin and trap assisted recombination 4 1.3 Device with Exciplex Forming Co-host system 11 1.4 Outline of the thesis 13 2. Electrical Modeling 15 2.1 Introduction 15 2.2 Governing Equations in the model 18 2.3 Implementation of the model 24 3. Factors Affecting the Recombination Mechanisms in Dye-doped Organic Light Emitting diodes 31 3.1 Introduction 31 3.2 Experimental 34 3.3 Result and discussion 37 3.4 Conclusion 46 4. Unveiling the role of Dopant Polarity on the Recombination and Performance of Organic Light Emitting Diodes 47 4.1 Introduction 47 4.2 Experimental 50 4.3 Device Characteristics (J-V-L, transient EL) 51 4.4 Simulation parameters 65 4.5 Langevin recombination against trap-assisted recombination 70 4.6 Effect of disorder due to dipoles 68 4.7 Discussion 75 4.8 Conclusion 79 Appendix MATLAB code of drift-diffusion model 92 Bibliography 124 Abstract 134 CURRICULUM VITAE 137 List of publication 139Docto

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