具奈米結構之氧化鋅光電元件特性分析

Abstract

摘要 自從1970 年代發生能源危機以及現今環保意識的抬頭，可回收再利用的能源越來越受重視，其中以太陽能電池的應用最為廣泛。在太陽能電池方面目前有幾個問題有待被解決其中包含:界面反射率、寬廣化的吸收頻譜、入射光的接收角以及元件內載子復合等等所造成的問題。過去幾十年內，為了提高矽基(Si)為主太陽能電池的效率，發展出一套利用化學濕蝕刻的方式，在太陽能電池的表面製造出具有週期性的粗糙化結構、倒金字塔結構以及蜂窩狀結構，使得太陽能電池表面材料的折射係數形成漸進式的變化以及造成入射光有效的被元件捕捉，進而降低表面的反射率。然而由於化學濕蝕刻容易受到濕度和溫度的影響，所以很難利用化學濕蝕刻成功的製作出具奈米結構的光電元件。 在太陽能電池轉換效率方面，目前以利用多層接面結構的砷化鎵(GaAs)為主的太陽能電池效率最高，但由於其製作步驟繁複且材料成本昂貴，所以在應用上還是以矽基(Si)為主要的太陽能電池材料。而在太陽能電池的光接收角改善方面，目前技術上是利用太陽光追蹤系統來控制入射光保持在接近垂直入射以達到吸收能圓的最大化，但是太陽光追蹤系統卻需要消耗額外的能源進而導致能源的浪費。至於在元件內的載子復合問題，主要是藉由材料品質的改善以及材料與金屬接面的材料重參雜來增加載子的生命周期。然而，傳輸路徑的過長也會使得載子在傳遞到金屬的過程中被缺陷復合的機率提高。基於以上的問題，我們提出利用結合寬能隙材料氧化鋅(ZnO)以及矽基(Si)， 製作出具有寬廣且平坦吸收頻譜的光電元件，接著再利用最佳化的奈米小球鋪排技術，使光電元件的光響應和接收角都有大幅的提升。 同時，我們還利用鋪排奈米小球的技術以及蝕刻製程，製作出在材料接面上具備奈米結構的n-GZO/a-Si(i)/p+-Si 異質結構光偵測器，由於奈米結構可以有效的降低表面反射率以及表面型態較接近圓柱狀，所以此元件具有較高的光響應以及廣接收角的特性。除此之外，我們還發現在奈米結構的元件中其載子傳輸時間較短，而較短的載子傳輸時間可以降低載子在元件內被復合的機率，進而有機會造成較高的光響應。最後，具備奈米結構的光電元件擁有較高的光響應、寬廣的接 收角以及較快的載子傳輸時間，其在太陽能電池的應用上具有相當大的潛力。

Abstract Renewable energy was attracted more attentions due to the energy crisis in 1970 and environmental issue in the world. There are several critical problems on high conversion efficiency solar cell manufacturing, low reflectivity, multiple band absorption, wide acceptance angle and low carrier recombination, et al. For the past decades, photovoltaic scientists developed the high conversion efficiency crystalline Si based solar cells by manufacturing the periodic rough structures, pyramid, inverted pyramid and honeycomb, et al., using wet etching on the solar cell surface. And, the reflectance could be drastic decreased due to the gradual refractive index in the surface textured devices. However, the wet etching process is hard to control due to the humidity and temperature in the clean room. Another, the size of textured structure is hard to control at the nanoscale. Furthermore, multiple junction solar cells could produce the higher conversion efficiency compared to the single junction solar cells due to the broad band absorption spectrum. The highest solar cell conversion efficiency was observed in the GaAs based semiconductor. The cost and difficult process result in the smaller market sharing than Si based solar cells. The acceptance angle of incident light for solar cells is another important issue for absorbing maximum light intensity by daytime. Although solar tracking system could provide the solution for this issue, it will cause the extra energy consumption. Another key issue for high conversion efficiency is carrier recombination. More carriers which were recombined in the devices will reduce the conversion efficiency. However, shortening the carrier transit paths could possibly decrease the probability of recombination when carrier transported to the contact electrodes. From the above mentions, we proposed another approach to manufacture the high conversion efficiency solar cells by combining the material of GZO and Si. By the overlapping of band gaps, the characteristic of broad band absorption ranged from 400nm to 800nm is realized in the n-GZO/p-Si heterojunction photodiodes. By the simple and novel technique of silica nanosphere spraying, the enhanced responsivity and wide acceptance angle could be achieved in the nanoparticle coated devices. The wide acceptance angle in n-GZO/p-Si photodiodes is due to the Bragg diffraction effect, Litterow configuration. This work could have the potential application to solar cells. Furthermore, we investigate the nanostructure n-GZO/a-Si(i)/p+-Si heterojunction photodiodes by using the self-masked nanosphere lithography. The characteristics of high responsivity and wide acceptance angle are achieved in the nanostructure photodiodes which is due to reducing surface reflectivity and nanostructure morphology. Moreover, nanostructure photodiodes have shorter transit time compared to planar photodiodes which is due to the shorter and more transit paths in the nanostructure devices. Nanostructure photodiodes possess the higher photoresponsivity compared to planar one which is possible due to the lower reflectance and shorter transit time. The n-GZO/a-Si(i)/nanopatterned p+-Si heterojunction photodiodes has the shortest transit time compared to planar n-GZO/a-Si(i)/p+-Si and n-GZO/nanopatterned a-Si(i)/p+-Si heterojunction photodiodes. Finally, self-masked nanosphere lithography and nanopatterned photodiodes possessed the enhanced photoresponsivity, wider acceptance angle and shorter transit time compared to the planar photodiodes. They have the potential applications to solar cells.