優化薄晶片混合式有機矽太陽能電池

Abstract

根據國際太陽能技術路線組(International Technology Roadmap for Photovoltaic, ITRPV)的統計，矽基太陽能電池的晶片成本佔整體製造成本的三成，為了能夠大幅地降低製造成本並帶來可彎性(Bendability)，使用薄型晶片是目前太陽能產業的趨勢，但矽是屬於非直接能隙材料，基板的減薄會導致長波段的吸收下降。本論文中我們研究不同厚度的混合型有機矽太陽能電池的光電特性，首先以溼式蝕刻法來進行表面結構的製備，並進一步地研究其吸收表現。我們利用氫氧化鈉來減薄並拋光矽晶片，通過此方法能使晶圓從200μm減薄至50μm，並使其表面粗糙度從167.78 nm下降至13.66 nm，且能於直徑15 mm的圓柱彎曲貼合。接著我們以金屬輔助化學蝕刻法於厚度180μm的微米級金字塔結構上製作矽奈米線形成多重結構(Hierarchical)，此多重結構能讓全波段的反射率進一步降低，且積分光電流能提升至40.01 mA/cm2。此積分光電流的表現可媲美於厚度為625μm的矽奈米線結構基板(Jph = 40.84 mA/cm2)。而元件的最佳光電轉換效率分別可達到12.77 %及13.51 %，兩者差距僅只有0.74 % 但整體製程成本卻下降了30 %。 此外基板減薄時，除了元件的吸收會下降，表面複合問題也會變得更加嚴重，因此我們以簡易且快速的刮刀製程，將小分子有機材料OXD-7刮塗於混合型太陽能電池的背面。利用背表面場對不同厚度的元件做開路電壓、填充因子與轉換效率的分析。當基板從180μm減薄至100μm甚至50μm時，整體的淨開路電壓差從0.8 mV上升至9.6 mV，淨填充因子差則從0.5% 提升至1.65%，而整體的轉換效率增益幅度分別為0.96 %、3.01 %和6.92 %。通過此結果，我們可以證實OXD-7可以部份改善表面複合的問題，提升混合型薄矽太陽能電池的表現。

According to the International Technology Roadmap for Photovoltaics (ITRPV), wafer cost accounts for one-third of overall manufacturing costs of silicon based solar cells. In order to significantly reduce the fabrication costs and bring bendability, thin substrate is the current trend for the PV industry. However silicon is an indirect bandgap material, reducing substrate thickness will decrease the optical absorption, particularly for long wavelength components. In this study, we investigate the photovoltaic characteristics of hybrid organic-silicon solar cell with different substrate thickness. First, isotropic and anisotropic wet etch techniques are employed for fabricating different surface textures on silicon to improve optical absorption. We took sodium hydroxide (NaOH) solutions to thin and polish silicon wafers from 200μm to 50μm such that the surface roughness also declines from 167.78 nm to 13.66 nm. Moreover, by fabricating hierarchical micro-pyramids and nanowires structures, we achieve broadband antireflection, corresponding to an equivalent photocurrent (Jph) of 40.01 mA/cm2 for a 180μm-thick substrate. The optical absorption is comparable to that obtained from a 625 μm-thick substrate with only nanowire structures (Jph= 40.84 mA/cm2). The best power conversion efficiencies (PCE) of the two device structures reach 12.77% and 13.51 %, respectively. There is only 0.74 % PCE difference, but the whole fabrication cost has been decreased by 30 %. Furthermore, the surface recombination of photogenerated charge carriers also become a serious problem, as the substrate thickness decreases. Therefore we demonstrate a simple and fast blade coating process to introduce a small-molecule organic material, OXD-7 as the back surface field (BSF) to the rear side of hybrid solar cells. We investigate the Voc, F.F. and PCE characteristics of hybrid solar cells with OXD-7 for different wafer thicknesses. When substrates are thinned to 180μm, 100μm, and 50μm, the net Voc difference increases from 0.8 mV to 9.6 mV; the net F.F. difference increases from 0.5 % to 1.65 %. Overall, the enhancement factors of the PCE reach 0.96 %, 3.01 % and 6.92 % respectively, for devices with OXD-7. Based on these results, we conclude that the OXD-7 BSF can alleviate the surface recombination issue for thin-substrate hybrid solar cells.