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  • 學位論文

有機光電半導體:載子傳輸特性及光電元件應用之研究

Charge transport properties of organic semiconductors and their relevance in organic optoelectronic devices

指導教授 : 李君浩

摘要


有機薄膜已被廣泛的應用在有機發光二極體或有機太陽能電池等光電子元件的主動層材料中。本篇論文中我們將探討單一小分子有機薄膜或混層薄膜結構,以及高分子混層薄膜的載子傳輸特性,研究結構對於有機光電子元件效率表現的影響。第一部份將討論兩種以不同分子構形所組成之有機半導體混層結構,分別為平面形分子搭配球形分子(NPB:Alq3);及平面形分子搭配四面體形分子(NPB:Bebq2)之組成。本部分中以時間暫態飛行量測法(Time-of-Flight; TOF),配合Poole-Frenkel模型分析其電子與電洞之載子遷移率以及傳輸特性,目的在深入探討有機混層結構中的載子傳輸機制。第二部分中,首先我們以TOF分析四面體形分子Bebq2在改變蒸鍍速率下之載子遷移率以及傳輸特性之變化,並使用Bebq2作為有機發光二極體之元件發光層以及電子傳輸層,結合此鍍率控制的方式以增加有機發光二極體之電流效率以及元件壽命。近年來,以P3HT及PCBM混層結構為主的有機太陽能電池在太陽能領域中已被廣泛的研究與發展。第三部分中我們將探討P3HT與PCBM的比例分配對此混層結構的載子遷移率以及電荷傳輸特性的影響。在提升PCBM比例的過程中我們發現PCBM的叢集有助於提升P3HT的聚集與結晶,進而提升此混層結構中電子與電洞的載子遷移率以及提升元件之光電流。另外由實驗中我們可以發現加入過量的PBCM將會使電子與電洞的傳輸特性被PCBM叢集所主導,破壞了其異質結構特性,而形成較平坦的表面形貌以及較差的元件特性。

並列摘要


Organic thin-films have been used as active materials in optoelectronic devices, such as organic light-emitting diodes (OLEDs) and organic solar cells. In this thesis, we study the charge transporting properties or carrier mobilities of pure or mixed organic thin films, polymer blended films, and their influences on the device performances. The first part of this thesis presents the time-of-flight (TOF) photocurrent study of drift mobilities and transport properties of holes and electrons in the mixed organic semiconductors. Two kinds of mixed layer systems, i.e. planar vs. spherical (NPB:Alq3) and planar vs. tetrahedron (NPB:Bebq2) molecular packings, are studied by a Poole-Frenkel Model and the analysis of disorder parameters. This study is to obtain insightful information about the carrier transport mechanism in the mixed organic solids. In the second part of this thesis, we study the thin film growth of a tetrahedron Bebq2 molecular material. Charge transporting properties and carrier mobilities of Bebq2 thin film are extracted and deciphered by TOF technique. Due to the tetrahedron molecular structure, we have demonstrated a simple fabrication control, the deposition rate of Bebq2, to increase the current efficiency and the operation lifetime of OLEDs, of which both light-emitting and electron-transport layers are Bebq2 thin films. Organic solar cell based on the blends of P3HT and PCBM bulk heterojunction is the other chosen optoelectronic device in this study. The charge transporting properties and carrier mobilities of P3HT/PCBM blended films are studied in the third part of this thesis. From our experimental results, the initial increase of the blending concentration of PCBM induces P3HT aggregation in the formation of percolation paths, which leads to an enhancement of both hole and electron mobilities and the output photocurrent of organic solar cells. However, when the blending concentration of PCBM above certain level, the resulting PCBM clusters may dominate the charge transporting in hole and electron percolation paths, impair the efficiency of exciton dissociation (charge transfer) in such donor-acceptor bulk heterojunction. In addition to the charge carrier mobilities of the thin films determined by TOF technique, this is indirectly evidenced by the smooth surface morphology of the thin film (by AFM surface images) and the low photocurrent output from the solar cell devices.

參考文獻


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Chapter 1
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