利用擁有寬頻與全向性光擷取之奈微米結構於矽基太陽能電池之應用

Abstract

本文中，我們利用擁有寬頻與全向性光擷取之奈微米結構應用於矽基太陽能電池上以提升光伏特性，並且對其光電特性做詳細的討論。 首先，由微米金字塔以及奈米線陣列結合而成之階層結構成功製備於單晶矽晶圓上，此乃利用氫氧化鉀溶液非等向性蝕刻以及無遮罩濕式蝕刻技術製作而成。此奈微米複合結構展現出優良的光捕獲特性，在波長300奈米-1000奈米擁有極低的平均全反射(4.3%)。在1.5 AM的光照下，藉由此一階層表面結構應用於矽基太陽能電池，與拋光表面之元件相比其短路電流密度可從21.5 mA/cm2提升至28.2 mA/cm2，其轉換效率可由7.75%增加至10.47%。此第一部分研究之概念與技術將有助於下一世代的矽基太陽能電池之優化與發展。 最後，矽基異質接面太陽能電池具有達到超高光電轉換效率的潛力乃由於其較高的開路電壓，然而亦須仰賴表面結構於沉積非晶矽層之前，以增加光吸收於元件。在此，我們利用黃光微影術與氫氧化鉀溶液非等向性蝕刻製作出週期性倒金字塔結構於矽晶圓表面。相比於拋光與隨機生長的正向金字塔結構，此一倒金字塔結構在波長300奈米-1130奈米之寬頻帶中顯示出增強的光捕獲捉效應。在太陽能電池的應用上，以週期為10微米的倒金字塔結構製作在雙面的異質接面太陽能電池，其光電轉換效率可從10.38%大幅提升至14.58%，擁有604.5 mV的開路電壓，35.29 mA/cm2的短路電流密度，以及0.68的填充因子。其一顯著的提升可歸因於光在元件中吸收的增加以及沉積層鈍化的改善。此部分的研究證實了藉由表面結構的最佳化設計可有效改善矽基異質接面太陽能電池的光伏特性。

In this thesis, the broadband and omnidirectional light-harvesting scheme employing nano/microscale structures are introduced to Si solar cells for boosting the photovoltaic performances, and the photoelectric properties of the devices are discussed in detail. First, a hierarchical structure consisting of micropyramids and nanowire arrays was fabricated on the mono-crystalline Si using a KOH anisotropic etching followed by the metal-assisted chemical etching process. The hierarchical structure shows excellent light-trapping properties in the wavelength region of 300–1000 nm, with the average reflectance of 4.3 %. Upon the application of the hierarchical Si surfaces, the current density of solar cells was increased from 21.5 mA/cm2 to 28.7 mA/cm2 and the conversion efficiency could be improved from 7.75 % to 10.47 % under 1.5 AM illumination. The concept and technique presented in this study should benefit the development of next generation of Si-based solar cells. Finally, Si heterojunction solar cells have potential for high conversion efficiencies owing to very high open-circuit voltages, yet this relies on optimized surface texturing for increasing the photon absorption effectively prior to amorphous silicon (a-Si:H) deposition. In this paper, the periodic inverted pyramid (IP) textures were performed on the wafer surfaces via photolithography followed by KOH anisotropic etching process. Compared to the polished and random upright pyramid-textured Si, the IP structures show the enhanced light-trapping properties over a wide spectrum ranging from 300 to 1130 nm. For the solar cells application, the double-sided heterojunction solar cells with 10 µm periodic IP structures yield an enhancement of conversion efficiency from 10.38% to 14.58% with an open-circuit voltage of 604.5 mV, a current density of 35.29 mA/cm2, and a fill factor of 0.68, which can be attributed to the increased light absorption in the devices and the improved passivation of the deposited films. The study proves a promising way to promote the photovoltaic performances of Si heterojunction solar cells by the design optimization of surface texturing.