砷化鎵基混合式異質接面太陽電池

Abstract

原油價格節節攀升吹響能源危機的號角，而太陽電池已成為各項綠色能源的要角之一。為了降低原料與製程成本，有機導電高分子材料搭配無機半導體之混合式太陽電池成為近年新興研究主題。砷化鎵由於其直接能隙(direct bandgap)所帶來的高光吸收特性以及材料本身具有的高電子遷移率(electron mobility)之電學特性，因此本論文採用砷化鎵取代矽作為基板來製作有機/無機混合式太陽電池。根據一維電性模擬軟體分析結果，在同樣的材料表面與內部缺陷濃度情況下，因為能帶匹配(band alignment)較合適，砷化鎵基元件之開路電壓較高，又因能隙較大所致吸收截止波長較短，故短路電流較低，效率則預期比矽基效率高。依據太陽電池設計原理，基板的摻雜濃度應要較低使空乏區寬度較大以利載子收集。但砷化鎵晶圓的摻雜濃度普遍較難低於1017cm-3，因此我們以MOCVD成長緩衝層和低摻雜(1016cm-3)的吸收層，元件各項電學特性有顯著提升，效率平均達到6.6%，是原先以晶圓製作元件的203%。正電極圖案為條狀與柵狀相連接，故遮蔽率是主導入光量和載子傳輸的重要參數，平面元件經過優化後得出當遮蔽率接近14%時(柵狀部分每根寬度60μm)可以達到最高效率7.66%。此外，針對平面元件內部電性也進行了模擬分析。為進一步提升效率，我們蝕刻表面奈米結構，期望藉由降低正面反射率以增加光吸收。由於非等向性蝕刻與準確控制表面形貌之需求而選擇乾蝕刻，利用聚苯乙烯奈米小球自組裝微影技術，使小球單層排列於基板表面，接著進行兩階段反應式離子蝕刻(Reactive Ion Etching, RIE)。第一階段為縮球，預留奈米柱間距與控制頂部形狀；第二階段為砷化鎵蝕刻，主要決定奈米柱長度。雖有進行損傷移除蝕刻(Damage Removal Etching, DRE)，但殘存的缺陷與增大的表面積和傳輸路徑仍使元件開路電壓略微下降，不過短路電流平均提升33%，效率最高能可達到7.74%。

The price of crude oil climbs progressively, which winds the horn of energy crisis. Photovoltaics have been one of the critical roles in various green energies. In order to reduce raw material and fabrication cost, hybrid solar cells combining organic conductive polymers and inorganic semiconductors have become rising topic in recent years. Due to the properties of direct energy gap and high electron mobility, GaAs has high absorption coefficient in visible wavelengths and good carrier transport characteristics. In this work, we substitute GaAs for Si to fabricate organic/inorganic hybrid devices. According to the analysis of one-dimensional simulation, at the same surface and bulk defect level, the open-circuit voltage of GaAs-based cell is larger, but the short-circuit current density is lower because of proper band alignment and larger bandgap resulting in shorter cut-off wavelength of absorption spectrum respectively. It is expected that the cell performance would exceed Si-based one. In conventional solar cell design, base doping should be lower in order that the width of depletion region on base side would lengthen, which is beneficial for carrier collection. Generally, doping concentration of GaAs wafer is hard to be manufactured below 1017cm-3, therefore we grow buffer layer and low-doped(1016cm-3) absorber by MOCVD. All electrical properties are enhanced, and average efficiency achieves 6.6%, which is 203% of the one without these epilayers. The pattern of top contact is composed of one bar and several grids connected together, so the shading ratio is a significant parameter that dominates the amount of incident light and the condition of carrier transport. Optimized planar devices with shading ratio around 14% (60μm width of grid) can reach a high efficiency of 7.66%. In addition, we perform simulations for planar devices to analyze the internal electrical properties. For further cell performance improvement, we etch nanostructure on front surface to reduce reflectance and increase light absorption. Preferring anisotropic etching and precise morphology control, we choose dry etching method. By use of self-assembly polystyrene nanosphere lithography technique, monolayer of nanospheres is deposited on substrate, followed by two stages of reactive ion etching. First step is to shrink spheres, reserving the spacing among nanorods and controlling the upper part morphology of nanorods. Second step is GaAs etching, dominating the length of nanorods. Although damage removal etching is performed, residual defects and enlarged surface area slightly lower the Voc. However, compared to planar devices, the average Jsc enhances 33%, leading to the highest efficiency of 7.74%.