應用奈米結構於同質與異質接面 矽太陽能電池的製作與分析

Abstract

矽基太陽能電池，包含晶片型和薄膜型佔據了當今的太陽能電池市場。其中，晶片型太陽能電池由於其純熟的製程發展技術，將繼續領導未來十年的太陽能電池工業。然而，其售價仍須進一步的下降以加速其普及化。因此，開發新的技術來降低矽材料的使用以及減低製程成本是目前的當務之急。 在本論文裡，我們首先介紹奈米科技以及其應用於矽太陽能電池。接著，我們實際使用奈米小球微影技術搭配反應式離子蝕刻製作出大面積的矽奈米洞陣列結構並應用於太陽能電池。此種結構有優異的寬頻譜與廣角度抗反射特性。另外，我們更進一步建立奈米洞陣列的光學模型，來研究奈米洞陣列結構應用於不同厚度矽基板的光學吸收特性。結果顯示奈米洞陣列結構在薄矽時可達到抗反射與光侷限的效果，進而增加吸收。與使用平面單層抗反射膜的200微米矽晶片比，奈米洞陣列可有效降低其材料使用達95%，只需要5%的厚度即可達到相同的吸收。 為了達到有效節省矽太陽能電池製程成本的目的，我們開發出使用水溶液的製程製作太陽能電池。其使用單晶矽作為基板，利用低成本的濕蝕刻製作大面積的微米金字塔結構，此結構能均勻且快速的製作，並且能大幅降低半導體表面反射至10%。並接著在表面旋塗一層導電高分子材料PEDOT:PSS，經由介面處理以及改變旋塗條件來提升異質接面的覆蓋性與品質。另外，我們也研發出使用銀奈米線作為透明導電電極材料，可達到大面積與高電流密度下傳導的功效。我們成功利用這種快速簡單的方法，製作出轉換效率10%的混和型電池。我們進一步建立理論模型來研究此種具異質接面太陽能電池的物理機制以及潛力。從中發現其異質接面品質對於這種有機結合無機的太陽能電池轉換效率有極大的影響。除此之外，有機層的導電度提升也對其太陽能電池電性提升有很大幫助。藉由表面處理以及有機層的改質，預測此種太陽能電池具有超過20%轉換效率的潛力，對於矽基太陽能電池新製程的開發提供了一個確切可行的發展方向。

Wafer-based silicon photovoltaics are currently dominating the solar cells industry, and they are likely to continue dominate the market share due to the mature technology. However, the price of silicon photovoltaics is necessary to further decrease to accelerate the wide-spread use. Therefore, new techniques to save the material usage and to reduce the fabrication costs are essential. In this thesis, we firstly introduced the nano-technology and its applications on silicon photovoltaics. Then, we presented solar cells based on silicon nanohole arrays which employ polystyrene nanosphere lithography and reactive-ion etching (RIE) techniques for large-area processes. Moreover, optical modeling has been established to perform the wafer thickness dependence of active layer absorption. The SiNH arrays reveal great potential for efficient light harvesting in thin silicon photovoltaics with a 95% of material saving compared to a typical cell thickness of 200 μm. To address the goal of simplifying the fabrication procedures in silicon photovoltaics, we developed hybrid heterojunction solar cells based on a conjugate polymer directly spun-cast on micro-textured n-type crystalline silicon wafers. Moreover, we presented solution-processed silver-nanowire meshes which uniformly cover the micro-textured surface of hybrid heterojunction solar cells to enable efficient carrier collection for large device area. A remarkable power conversion efficiency of 10.1% is achieved with a device area of 1×1 cm2. A one-dimensional drift-diffusion model is then developed based on fitting the device characteristics with experimentally determined PEDOT:PSS parameters and projects an ultimate efficiency above 20% for organic/inorganic hybrid photovoltaics. The simulation results reveal the impacts of defect densities, back surface recombination, doping concentration, and band alignment.