電子槍蒸鍍法成長之氧化銦錫奈米結構及其光伏元件的應用

Abstract

氧化銦錫材料過去已被大量應用於透明導電薄膜上，利用其透明導電的特性，可應用在太陽能電池或發光二極體上，已經成功提升了這些元件的光電轉換效率。利用可變角度的電子槍蒸鍍系統，我們發展了一種氧化銦錫奈米結構的製作方法，成功製作出奈米柱狀結構，奈米梳狀結構，並分別應用於砷化鎵、矽、有機太陽能電池，提升其元件的轉換效率。 我們探討了氧化銦錫奈米結構的成長機制，其主要由tin-induced self-catalytic vapor-liquid-solid (VLS)及vapor-solid (VS)共存的成長機制所主導的。其成長過程主要包含了凝核形成、柱狀成長、側壁分支的成長。藉由初期的凝核結構的形成，我們可以發現這種材料成長的初期確實有液態的凝核過程。利用利用元素分析法，我們則發現了具有較高濃度的錫分布在奈米柱狀結構的外圍，這層外層能夠幫助奈米結構的軸向成長。因此，在成長的過程中，這外層將可提供吸附氣體分子的作用，也是促使奈米結構成長的關鍵。 而後，我們開始探討氧化銦錫奈米結構的應用。我們斜向蒸鍍具有方向性的氧化銦錫奈米柱於砷化鎵太陽能電池上，此奈米柱提供了寬頻譜的抗反射特性。因此，相較於沒有製作抗反射層的元件，氧化銦錫奈米柱砷化鎵太陽能電池的轉換效率增益達28%。另外，我們也可沉積氧化銦錫奈米梳狀結構在表面粗糙化的矽基太陽能電池上，發展出一種結合奈米、微米尺度的抗反射層。可增加矽基太陽能電池在紅外波段的光學吸收，提升轉換效率。這種複合式抗反層矽基太陽能電池的轉換效率可達17.2%，而作為對照組的傳統矽基太陽能電池則為16.1%。最後，我們結合氧化銦錫奈米柱及薄膜結構，發展出一種三維的奈米結構電極，並應用於有機太陽能電池的製作。此種奈米結構電極可提升有機太陽能電池內的電洞收集效率。因此，這種奈米電極的有機太陽能電池相較於傳統的平板電極，其光電轉換效率及元件壽命分別增加了10%及一倍。我們總結氧化銦錫奈米結構的成長機制及應用，並提出一些可繼續研究的方向。

Indium-tin-oxide (ITO) has been a useful material as transparent conductive electrodes for the last two decades. Both solar cells and light-emitting-diodes benefit from the property of ITO to improve the conversion efficiency or light extraction, respectively. In this work, we developed a growth method to deposit ITO nanostructures, including the nano-columns, nanowhiskers, and nanorods. These nanostructures were applied for the GaAs-based, Si-based, and polymer-based solar cells, to reduce the surface reflectance or increase carrier collection. We further investigated the growth mechanism of ITO nanostructures which was dominated by the tin-induced self-catalytic vapor-liquid-solid (VLS) and the vapor-solid (VS) growth mechanism. The growth process could be divided into three steps: (1) nucleation, (2) column growth, and (3) side branch growth. We show evidence of the initial droplets formation to confirm the existence of the liquid phase. The core-shell structure had been observed in the TEM image of ITO nanorods, and hence the EDX analysis demonstrated higher concentration of tin in the shell than that in the core. The shell layer could absorb ITO vapor during the growth of ITO nanowhiskers. After investigation of the growth mechanism of ITO nanostructures, the applications had been discussed. First, we deposited the oriented ITO nano-columns on GaAs-based solar cell to provide broadband antireflection. Therefore, the conversion efficiency of the ITO nano-columns GaAs-based solar cell increased by 28% compared to a cell without any AR treatment. Next, we deposited the ITO nanowhiskers on the micro groove textured Si-based solar cell to combine the nano-and micro-textured antireflective coating. The compound antireflective structures increased light harvesting in the near-infrared. The conversion efficiency of the combined antireflective coated Si-based solar cell achieved 17.2%, compared to 16.1% of the conventional Si based solar. Finally, the ITO nanorods were prepared on ITO glass which functioned as three-dimensional (3D) nanoelectrode. The nano-electrode increased the hole collection efficiency for the organic solar cell. Compared to the organic solar cell with a flat electrode, the conversion efficiency and lifetime of the ITO nano-electrode organic solar cell increased by 10%, and 100%, respectively. We then conclude the growth and photovoltaic applications of the ITO nanostructures and provide future outlooks.