功能性硒化鉍氧化物之電子結構及其於電子元件之相關應用

Electronic Structure of Oxidized Bismuth Selenide and its Application in Electronic Devices

葉雲傑(Yun-Chieh Yeh)

指導教授 ： 陳俊維

國立臺灣大學/工學院/材料科學與工程學研究所/博士(2014年)

https://doi.org/10.6342/NTU.2014.01422

摘要

本研究乃包含兩大部分，第一部分之研究乃以氧電漿(O2 plasma)轟擊單晶硒化鉍(Bi2Se3)表面，通過控制轟擊時間以調控之單晶表面之氧化程度。X光光電子能譜縱深分析顯示氧化深度可達約30奈米，縱向成分分析亦顯示氧化物分布呈梯度變化，表面氧化層部分以氧化鉍及氧化硒為主，而由於表面氧化層之屏蔽作用，晶體內部因氧分子穿透較少量故晶體內部主要以較低氧化態之元素態硒及硒化鉍為主，表面氧化層至內部未被氧化且呈現單晶狀態之硒化鉍間，其氧化態呈現遞減之趨勢，根據電子顯微鏡之切面觀察，於氧化層與非氧化層間，其晶相亦呈現散亂之現象，顯示氧電漿處理乃屬於一破壞性之氧化過程。進一步將氧化處理所成長之氧化層與下方為氧化之硒化鉍晶體整合可製得一具不對稱蕭基障礙(schottky barrier)之金屬-半導體-金屬接面電晶體，根據變溫電性量測，表面氧化層具半導體特性並即有機會可進一步應用為電子元件之功能性傳輸層。
第二部分之研究主要著以水溶液製程(solution-processable)且具大範圍電子結構調控區間(electronic structure controlling window)之功能性材料並將其分別應用於電子(electron)及電洞(hole)傳輸層(transporting layer)中並根據材料之差異對於元件之傳輸行為逕行系統性之載子動態分析(carrier dynamic) 。電子傳輸曾方面以溶液凝膠法(sol-gel method)合成不同化學劑量比之氧化鈦（TiOx中，x = 1.56-1.93 ）藉由化學劑量比之調變TiOx之能帶結構亦有所變動直接影響最高佔據軌道(higest occupied molecular orbital, HOMO)及最低為佔據軌道(lowest unoccupied molecular orbital)位置致使有機太陽能電池之內建電場產生系統性之改變，藉此抑制減電荷複合率(recombination rate)以提升整體之元件效率。
電動傳層方面，乃利用硒化鉍電子結構可調控之性質，以氧電漿轟擊對於塗佈於氧化銦錫(indium tin oxide, ITO)基板之硒化鉍進行表面氧化並配合聚3-己基噻吩 (poly(3-hexylthiophene), P3HT)緩衝層(buffer layer)以達陽極功函數修飾之目的，電洞之傳輸性質可根據整體功函數之改變加以控制。以上之介面修飾層均具大範圍 電子結構可調及可以水溶液製程之性質，對於不同能帶對準(band alignment)之主動層(active layer)可藉由氧電漿處理調控功函數以達到能帶匹配之目的，可廣泛應用有機太陽能電池或發光二極管(light emitting diode)等元件中。

關鍵字

硒化鉍 ； 拓樸絕緣體 ； 縱深分析 ； 有機太陽能電池 ； 載子傳輸層

並列摘要

In this study, we mainly focus on two parts. The first part demonstrates the controllable oxidation toward single crystal Bi2Se3 via O2 plasma bombardment. The oxidation depth could be reached from ~3 to ~30 nm in accordance with oxidation duration. Longitudinal composition and component evolution reveals gradient oxidation states evolution with respect to probing depth. Transition from Bi2Se3 to oxidized species, including BiOx, SeOx and elemental Se was observed in surface oxide layer. Further integrate the oxide layer with Bi2Se3 beneath, a device with metal-semiconductor-mental (M-S-M) is successfully fabricated. Electrical properties revealed from temperature dependent I-V analysis indicates semiconductor nature toward oxygen doped Bi2Se3, which gets great potential to be further implemented as a functional transport layer in electronic devices.
The second part in this thesis is focus on developing of solution-processible functional materials with wide-range electronic structure controllable window for cathode and anode modificaiton, respectively. In anode modifier, poly(3-hexylthiophene) (P3HT) grifted oxidized bismuth selenide (Bi2Se3) through lithium intercalation approach is utilized. We found that work function of the solution-processed Bi2Se3 cluster on indium tin oxide (ITO) can be controlled via O2 plasma treatment. Degree of oxidation is found to highly correlated with device performance according to the established hole transporting barrier by individual modified anode. The optimized device exhibits a promising power conversion efficiency, compatible with the device using poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), which is conventionally used for hole transport layers.
In cathode modifier part, we systematically investigated the stoichiometric dependence of titanium oxide (TiOx, x=1.56-1.93) as a cathode modifier on polymer solar cells. Electronic structures of the synthesized TiOx modifier layers were controlled by tuning the compositions of various O/Ti ratios, meanwhile, band edges of individual TiOx is alerted. With TiOx incorporation the cathode workfunctions and the corresponding device performances of polymer solar cells are systematically changed with respected to the established built-in potential. Regarding the advantage of controllable electronic structure and being solution processable, the functional materials developed in this research could further be implanted as new interfacial modifier for band edge match with respect to different active material to facilitate hole and electron transport.

並列關鍵字

Bi2Se3 ； topological insulator ； depth profile ； organic solar cells ； carrier transporting layer

參考文獻

Chapter 1

1. C. L. Kane, E. J. Mele, Phys. Rev. Lett. 2005, 95, 146802.

3. Y. L. Chen, J. G. Analytis, J. -H. Chu, Z. K. Liu, S. -K. Mo, X. L. Qi, H. J. Zhang, D. H. Lu, X. Dai, Z. Fang, S. C. Zhang, I. R. Fisher, Z. Hussain, Z.-X. Shen, Science 2009, 325, 5937.

5. L. Fu, Phys. Rev. Lett. 2009, 103, 266801.

8. Y. Zhang, K. He, C. –Z. Chang, C. -Li Song, L. –L Wang, X. Chen, J. –F. Jia, Z. Fang, X. Dai,W. –Y. Shan, S. –Q. Shen, Q. Niu, X. –L. Qi, S. –C. Zhang, X. –C. Ma, Q. -K Xue, Nature Phys.2010, 6, 584.