The enhancement of conversion efficiency on solar cell with periodic grating formed by nanowires

本篇論文為針對薄膜矽太陽能電池之效率改善，經由週期性光柵型粗化之表面結構，以及奈米柱之結構兩者結合，由不同高度的奈米柱組成的週期性光柵同時具有兩者對光波長吸收增益的優點，其一，對光波具有相當卓越的抗反射效果，其二，此設計能夠增加光的繞射以及散射，以至於增益光場的吸收，並提高轉換效率。模擬使用套裝軟體COMSOL MultiphysicsR 有限元素法解Maxwell’s Equations 來計算二維的奈米柱結構中之電磁場分布。 本文中，除了對光柵形狀之內的參數如：週期、填空比、和角度進行討論外，更進一步的說明了此週期性結構的形狀如：凹陷形、三角形、及帳篷型，對於光場的影響。經由吸收比、吸收分布、電場分布、和轉換效率的相互比較驗證各個參數之間對太陽能電池的影響。以短波長而言，小的週期或填空比都可達到抗反射的效果，而長波長，因適當週期產生的繞射效應造成較大的增益。對於整體效率看來，大角度的週期性結構提供了較好的抗反射條件及較長的光路徑，可以得到結構的角度越大，轉換效率越高的趨勢。而越具有尖角形狀的週期性結構具有和大角度結構同樣的效益，同樣能夠獲得較高的轉換效率。The influence of nanostructure on the propagation of optical waves within amorphous thin-film silicon solar cells was investigated. Also, the influence of the amorphous silicon thin film (500 nm thick) with periodic gratings formed by array of nanowires with varied heights and different shaped profile on the conversion efficiency was studied via simulation for its solar energy absorption characteristics. The Finite Element Method (FEM) was used to rigorously solve the Maxwell’s equations in two dimensions. By studying the influence of the duty ratio, height and period of the nanowires of the gratings and the shapes of the grating profile including tented, triangular and concave shapes, the designs of the structures were optimized to achieve higher conversion efficiencies. Enhancement of the conversion efficiency in the blue and green wavelengths of the spectrum is achieved by using small duty ratios and periods of shaped profiles owing to its anti-reflection property, whereas the conversion efficiency in the red and infrared wavelengths of the spectrum is mainly improved by properly designed periods (P = 500 – 900 nm) and larger angles of the shaped profiles to guide light into the structures. The enhancement of conversion efficiency both occurs when no matter what polarization of the incident wave is. The conversion efficiency of solar cells covered with triangular nanowire grating profile, which is approximately optimum, can be increased by 58.21%, 42.45%, and 20.38% compared to the cases for flat solar cells without nanowires, with nanowires, and periodic triangular textures, respectively