聚三己烷噻吩與苯基碳61丁酸甲酯混成有機太陽能電池之介面物理、化學與電性分析

聚合物/富勒烯組成的塊材異質接面太陽能電池為當今前端之研究題材，其中，又以聚三己烷塞吩(P3HT)混和苯基碳61丁酸甲酯(PCBM)構成的主動層吸光材料最為熱門。本論文使用介面分析的方法，如紫外光電子頻譜(UPS)及X光電子頻譜(XPS)等儀器，分析太陽能電池介面間的電子結構及化學反應。同時，配合上各種不同結構的元件製作，直接和介面分析的結果做對照，探討各種不同條件的製程處理、緩衝層的使用對元件效能影響之背後機制。首先，論文從電極金屬與主動層有機材料間介面開始做探討。在一般P3HT:PCBM為吸光層的太陽能電池當中，大多使用鋁或者鈣來當作陰極，而使用鈣當陰極的元件擁有明顯較大的開路電壓，元件整體轉換效率也較高，導致此增益的機制藉由XPS被成功觀察到。XPS發現在鈣與P3HT的介面有明顯的化學反應發生，所產生的電子轉移現象反應在UPS觀測的能階變化，此能階變化拉大了施體材料及受體材料間的能階差異，因而提升元件開路電壓及整體效能。接著在第二主題中，本論文研究了元件製作中最常被用來提升效能的熱退火機制。經由介面分析發現，使用純鋁陰極的元件退火後，明顯的P3HT能階變化促使元件的開路電壓大增；反之在使用鈣陰極的元件中，熱退火反而造成P3HT能階反向移動，縮小元件開路電壓。另一方面，不論使用鈣或鋁當陰極，被這兩種金屬覆蓋的主動層表面在經過熱退火之後，都明顯觀察到PCBM向上擴散之現象，此擴散提供整體異質接面元件更接近理想的垂直結構分布，因而增大元件短路電流值。除了熱退火之外，第三和第四主題針對廣泛被使用的緩衝層來做探討。由於在聚合物太陽能電池當中，緩衝層往往扮演極重要的腳色，本論文特別針對提升元件效能非常多的bathocuproine (BCP)以及PCBM兩種緩衝層做深入研究。在使用BCP當陰極緩衝層的部分，介面分析發現只要2奈米厚的BCP，就可有效抑制高溫鋁金屬蒸鍍時對主動層造成的破壞；同時，原子力顯微鏡的影像指出，BCP層能緩和地填入原本粗糙度相當大的P3HT:PCBM層表面，截斷載子漏電之路徑，並在主動層和鋁陰極之間產生奈米紋理，提供更高的有效接觸面積，增加光電流值。而在PCBM的部分，由於其本身即為主動層受體材料，因此以熱蒸鍍方式額外沉積的PCBM緩衝層，在經過我們適當的熱退火處理之後，能產生類似主動層漸層延伸的效果，此效應使得吸光材料在整個元件中的縱向分布更趨近理想的異質接面狀態，元件之功率轉換效率也因載子分離效率大幅提升，因而產生約20%幅度的增加。

關鍵字

聚合物有機太陽能電池；光電子頻譜；介面分析；能階分佈

並列摘要

In this dissertation, the polymer/fullerene bulk heterojunction (BHJ) solar cells based on poly(3-hexylthiophene) (P3HT) mixed with [6,6]-phenyl C61-butyric acid methyl ester (PCBM) are studied through surface analysis and device characterization. Via ultraviolet and x-ray photoemission spectroscopy (UPS and XPS), the electronic structures and interfacial chemical reactions are investigated. The results are directly compared with the performance of photovoltaics, discovering the relationship between surface modifications and device characteristics. The first topic of this dissertation investigates the mechanisms leading to the power conversion efficiency improvement in P3HT:PCBM based organic solar cells using calcium (Ca) in cathode structures. Both XPS and UPS results show that chemical reactions occur at P3HT/Ca interface, pulling down the energy levels of P3HT. The presence of Ca thus enlarges the energy difference between the highest occupied molecular orbital (HOMO) of P3HT and the lowest unoccupied molecular orbital (LUMO) of PCBM at the cathode interfaces, resulting in the increase of open circuit voltage and the enhancement of device efficiency. In addition to the contacts between active layers and electrodes, the second topic focuses on the phase segregations and the evolutions of energy levels in overall solar cells during annealing. Upon aluminum (Al) deposition onto top of the P3HT:PCBM layer, the HOMO level of P3HT exhibits a large downward shift after annealing, resulting in the larger Voc in devices. Furthermore, both UPS and XPS results illustrate the annealing treatment will induce the out-diffusion of PCBM toward cathode, which can provide more ideal hetero-structures for electron extractions. The third and fourth topics investigate the effects of cathode buffer layers on the efficiency of solar cells. In the case of the device using bathocuproine (BCP) as buffer layer, photoemission results shows that no reaction occurs at the active layer/BCP interface and only 2 nm of BCP could effectively suppress the chemical reactions between Al and active layers. Besides, Atomic force microscope images indicate that BCP layers can provide smoother contact surfaces with Al cathodes and suppress the generation of leakage current. On the other hand, for using PCBM as the buffer layer, the device performance shows a 20% improvement in power conversion efficiency as compared to the conventional devices without extra evaporated-PCBM layers. Results of surface analysis indicate that thermal annealing treatments lead to better mixture structures of PCBM layers with P3HT at the blended surfaces, providing more ideal hetero-structures and vertically graded PCBM distribution to optimize the carrier extraction efficiency.

並列關鍵字

Polymer solar cell ； P3HT ； PCBM ； photoemission spectroscopy ； surface analysis ； energy level

參考文獻

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[2] John, "The Silicon Solar Cell Turns 50", National Renewable Energy Laboratory (Perlin, 2004).

[6] C. W. Tang, “Two-layer organic photovoltaic cell,” Appl. Phys. Lett. 48, 183 (2008).

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聚三己烷噻吩與苯基碳61丁酸甲酯混成有機太陽能電池之介面物理、化學與電性分析

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