氮化銦鎵/氮化鎵多重量子井之光伏物理性質分析

Abstract

在本論文中，氮化物太陽能電池採用多重量子井式 (multiple quantum well) 的吸光層結構。在此之中，分別加入15 % 與30 % 之的銦含量，並分析整體元件的結構與物理性質。同時，透過發光頻譜、模擬太陽光源 (air mass 1.5G) 照射下所測的I-V曲線圖與外部量子效率 (external quantum efficiency) 在不同波長下的分佈，可以發現銦 (indium) 含量較高的元件，由於晶格品質較差，其所產生的光電轉換效率 (conversion efficiency) 也較低。除此之外，為了更清楚地了解吸光層中載子在不同溫度下的表現，藉由調控溫度在100 K – 400 K之間測量元件的I-V曲線，獲得轉換效率與填充因子 (filling factor) 的變化，可得知銦含量較高的元件，雖然其轉換效率較低，但因吸光層中有鎵 (gallium)、銦相分離 (phase separation) 的現象產生，故而當溫度提升時，該元件的光伏 (photovoltaic) 特性有較顯著的改變。 為了提升氮化物太陽能電池的轉換效率，利用水熱法 (hydrothermal method.) 成長低維度氧化鋅奈米柱陣列 (zinc oxide nanorod arrays) 結構作為表面的抗反射層；透過調控此一製程的相關參數與數值模擬，可獲得最佳的奈米柱陣列的幾何特徵，使氮化物太陽能電池表面的抗反射層，在可見光的波長範圍內，達到最高的穿透能力，並進一步了解結構與成長條件對電子傳輸與光特性之影響。此外，使用針狀式的氧化鋅奈米柱陣列將可大幅改善氮化物太陽能電池光伏特性。

Indium gallium nitride (InGaN) is a promising material for photovoltaic devices due to it potential to realize nearly full absorption of solar spectrum. A challenge for InGaN-based solar cells is the deteriorated crystal qualities at high indium (In) contents. In this thesis, severe In fluctuation is observed in InxGa1-xN/GaN multiple quantum well (MQW) solar cells for x = 0.3 using scanning transmission electron microscopy. The strong fluctuation and sacrificed crystal qualities lead to unsatisfactory photovoltaic performances. A strong temperature-dependent characteristic is observed for the device and is attributed to the photocurrents thermally activated from the shallow QWs. In order to enhance the conversion efficiency (η), zinc oxide (ZnO) nanorod arrays (NRAs) are applied as the antireflection (AR) coating on InGaN/GaN MQW solar cells. The NRAs are synthesized by a cost-effective hydrothermal method. The length of NRAs plays an important role in photovoltaic characteristics considering the trade-off between AR performances and bandgap absorption of ZnO. In addition, the syringe-like ZnO NRAs possessing superior advantages to the flat-top ZnO NRAs are also applied as the light-harvesting layer, resulting in the improvement of 36 % from that obtained on the solar cells with bare surface.