利用奈米小球技術製作表面粗化以結合抗反射與光捕捉之薄膜太陽能電池

Abstract

由於近期能源危機及環境污染議題逐漸升溫，綠色能源成為目前最主要的研究議題，尤其以太陽能電池為最。目前各國爭先發展高效率太陽能電池，太陽能產業相關的專利及市場布局競爭激烈。傳統以矽晶圓太陽能電池為主要產品，但由於矽晶圓太陽能電池材料成本昂貴，且製作已趨近成熟，研究及產業已把焦點轉放在薄膜太陽能電池。但由於薄膜太陽能電池較薄，通常為數百奈米的厚度，使一般薄膜型太陽能電池不足以完全吸收在能隙內的光子，因此薄膜太陽能電池的光學吸收已演變成一重要之研究課題。太陽能電池的光學特性研究中，抗反射層(antireflection coatings)與光捕捉(light trapping)為主要課題，過去的研究是把抗反射層放在入光面而把光捕捉結構置放在電池的背面。於本論文中，我們創造出一全新的概念:結合抗反射及光捕捉機制在同一結構上，並將此結構放在入光面上。我們利用膠體奈米球微影術(colloidal lithography)去製作不同的奈米結構，而此奈米結構同時具有抗反射及光捕捉機制。並且，將這樣的奈米結構置放在入光面可以有效增加大角度入射光的入光量，進而提升太陽能電池效率。此研究中，我們利用格耦合波分析(rigorous coupled wave analysis)模擬軟體先去系統化驗證我們的概念。在實驗部分我們製作不同的奈米結構去增加有機太陽能電池及a-Si 太陽能電池效率。過程中，我們架設一台變角度反射率光譜儀來量測這樣的奈米結構的變角度抗反射的效果。我們相信，此論文的研究結果對於太陽能電池研究領域有極大的幫助及影響。

In the past few years, thin film solar cells have emerged as the possible candidate to be adopted in the major solar cell market owing to the dramatic reduce of cost from material usage. However, the cell efficiency is too low to meet the market demand of below 1$/W. The thin film solar cells are usually a few hundred nano-meters thick and therefore the optical loss is one of the major losses in the thin films. Antireflection and light trapping are both needed to enhance the cell efficiency by increasing the photon absorption. Recent researches have demonstrated separate engineering processes to provide both mechanisms. Conventionally, antireflection layers are set at the front surface and the light trapping structures are set in the back of the cell. In this thesis, we innovate to combine antireflection and light trapping together by putting antireflective nanostructures at the front surface of thin film solar cells through a simple process. The antireflective nanostructures that provide both mechanisms attributed from the taper shape of each groove and horizontal arrangement which scatters the incident light into the cell. These nanostructures also sufficiently couple the angular incident wave to the cell and hence enhance the angular absorption of the cell. We utilized colloidal lithography for large area nanostructure fabrication on various substrates (ITO, silicon nitride) and implemented the nanostructure in various types of thin film solar cells (organics, amorphous silicon). We systematically studied the antireflective and scattering mechanisms of these nanostructures through rigorous couple wave analysis (RCWA) simulation and experimental measurement. An angle-resolved reflective spectroscopy system was setup to confirm the omnidirectional antireflective properties of the nanostructures. We believe these results can directly impact on the attainment of scalable renewable energy from thin-film solar cells.