以分子動力學研究結晶型太陽電池表面粗糙化結構對效率之影響

Abstract

本研究藉由分子動力學的模擬，探討太陽電池表面粗糙化對太陽電池效率的影響。太陽電池的發電機制包含光子與電子、聲子的能量轉換，非常複雜。本研究發展了一個簡易的傳統分子動力學模型，以取代複雜的電子-光子-聲子的能量交換，藉以與拋光的太陽電池表面的理論效率做比較，求得各種粗糙化方式對結晶型太陽電池效率提升之量化指標。 本研究發現，無論是各種粗糙的形狀，當高低落差愈大，其效率提升愈高，若高低落差達到14埃，效率幾可提升3倍。若粗糙結構距離愈近，其效率提升愈高，距離縮小至1埃時，效率幾可比未做表面粗糙處理之太陽電池提升2倍。若將光的入射角度變化，則入射與垂直的夾角愈小，其效率愈高。 此外，若探討多晶態矽經鹼性溶液蝕刻後產生之金字塔型結構對太陽電池效率的影響，則因為金字塔角度的關係，會在某些角度範圍提昇轉換效率，但是其他角度則否；入射方位角改變亦會影響轉換效率；而金字塔角度愈大，則轉換效率愈提高。 在砷化鎵太陽電池方面，本研究採用Tersoff 勢能模型與Lennard-Jones勢能模型之平均值來模擬原子與光子之間之交互作用，我們發現其趨勢類似於單晶矽太陽電池所得到之結果，當表面愈粗糙，其效率之提升愈大。 另一方面，我們也對原來的分子動力學模型做一些改進，根據前人的論文，聲子的傳遞對於光電效應的效率有重要的影響，因此我們加入了聲子放射的概念。新的模型顯示，在平板太陽電池的光電轉換效率降低到20％以下，表示一部份的能量被以熱的形式散失掉，但因此也更加接近太陽電池操作的實際狀況。同時降低聲子的傳遞可以使效率提升。這個結果也提供一個改進太陽電池效率的方向。 本文的研究，對於瞭解太陽電池表面粗糙化對效率的影響有很大的幫助。

The purpose of this paper is to develop a simple new model, based on the classic molecular dynamics simulation (MD), alternative to complex electron-photon-phonon interactions to analyze the surface texturization of solar cells. This methodology can easily propose the absorptance differences between texturing and non-texturing solar cells. To verify model feasibility, this study simulates square, pyramidal, and semicircular texturization surfaces. Simulations show that surface texturization effectively increases the absorptance of incident light for solar cells, and this paper presents optimal texturization shapes. The MD model can also be potentially used to predict the efficiency promotion in any optical reflection-absorption cases. In this study, we found higher drop of texturization increases the solar cells’ efficiency. The closer distance of texturizations also increases the efficiency. The smaller incident angle also enhances the absorptance of solar cell to light. Besides, in case of multi-crystalline silicon etching by alkaline solutions, the tilted angle of pyramids promotes the efficiency. Incident angle of light also affects the efficiency. We also get a similar result in the case of GaAs solar cells. A new potential energy, the average of Tersoff potential model and Lennard-Jones potential model, was adapted to govern the interaction between GaAs atoms and photon. In this case, we also found the similar result as that from mono-crystalline silicon solar cell. The efficiency increased with the increase of GaAs solar cell surface’s roughness. We also appropriately modified our model with the concept of phonon emission. The absorptance that was higher than 60% decreased to almost 50%. Thus the results from the simulations with phonon emission were close to reality. Besides, the results with phonon emission also lead to a new thought that the decreasing of heat loss can increase the efficiency of photovoltaic effect. In the mechanism of PV effect, decreasing phonon’s transportation is one of feasible direction. Our studies were helpful to realize the effect of surface texturization on the efficiency of crystalline solar cells.