氧化銦錫奈米柱狀結構應用於提升砷化鎵太陽能電池轉換效率

Abstract

我們成功展示了一種氧化銦錫奈米柱狀結構作為具導電特性的抗反射層應用在砷化鎵太陽能電池上。這種氧化銦錫柱乃是利用通入氮氣調變的電子槍斜向蒸鍍法，具有全方位的抗反射特性涵蓋了平行極化波和水平極化波，有效收光角度可達70度，頻譜範圍包括300nm~900nm，都可使反射率低於6%。應用在砷化鎵太陽能電池上，相較於沒有製作抗反射層的太陽能電池元件，提升了28%的轉換效率。根據量測數值的計算結果，在窗戶層(window layer)吸收波段以上的波段，其所產生的光電流提升了將近42%。 本篇論文的第一部分在談論到這種氧化銦錫奈米柱狀的成長機制及材料特性。我們採用電子槍斜向蒸鍍法去進行氧化銦錫的蒸鍍，依照傳統氧化銦錫導電玻璃的製作方法皆是在蒸鍍的過程通入氧氣，但本實驗在這個地方進行了改變，我們藉由通入與氧化銦錫不易起反應的氮氣，達成以氣流調變成長方向的作用，成長出具方向性的奈米柱狀結構。根據TEM分析結果顯示，內部的核心具有較低的錫分子濃度，外部包覆層的結晶性不如核心的明顯。其成長方法為一種Catalyst-free VLS的成長機制，目前最長約為1μm。氧化銦錫本身為一透明的導電材料，而由於此種柱狀結構具有顯著的方向性，我們從實驗量測及RCWA模擬計算的方法來分析這種奈米柱狀薄膜，發現其具有優異的全波段的抗反射特性，並不太受入射面及極化方向的影響，可算是一種全方位的抗反射結構，這是由於其斜向奈米柱狀結構提供了一種漸變性的折射率分佈，將可減少入射光由界面折射係數差異導致反射率的下降。在第二部分中，我們將此種奈米柱狀結構成長於砷化鎵太陽能電池的表面做為一抗反層，並自元件表面的反射率、外部量子效應、元件轉換效率，來進行元件特性的分析，發現我們元件的磊晶層的窗戶層材料形成了一個限制的條件造成效率的限制，未來可以將技術應用在其他元件上，將可有更好的提升。

We demonstrate a practical photovoltaic application of ITO nano-columns serving as a conductive AR layer for GaAs solar cells. The characteristic ITO nano-columns, prepared by glancing-angle deposition with an incident nitrogen flux, offer omnidirectional and broadband AR properties for both s- and p-polarizations, up to an incidence angle of 70° for the 300 nm-900 nm wavelength range. Calculations based on a rigorous coupled-wave analysis (RCWA) method indicate that the superior AR characteristics arise from the tapered column profiles which collectively function as a graded-refractive-index layer. The conversion efficiency of the GaAs solar cell with the nano-column AR layer increases by 28% compared to a cell without any AR treatment. Moreover, nearly 42% enhancement is achieved for photocurrents generated at wavelengths that are transparent to the window layer. In the first session of my thesis, the growth mechanism and microstructure are analized. Glancing-angle deposition was employed for preparing microscale and nanoscale porous materials based on nucleation formation and self shadowing effect. However, the characteristic ITO nanocolumn structure presented in this work is rather unique, where the formation involves either catalyst-free or self-catalyzed vapor–liquid–solid (VLS) growth assisted by the introduced nitrogen. The ITO columns become uniformly oriented at the end of deposition, following the direction of the incident vapor flux. The columns also become thicker (~100 nm), with a thinner tip ~30 nm in diameter, and resemble tilted cones with a total length of 1μm and a density of 5×109cm2. The transmission electron microscopy (TEM) image reveals the core/shell structure of an ITO nano-column. Energy-dispersive spectroscopy (EDS) analyses show that the outer shell has a higher tin content than the core region, indicating the occurrence of tin-doped indium segregation. In the second session, the GaAs solar cell with ITO nanocolumns was characterized under the AM 1.5g illumination condition and external quantum efficiency (EQE) is analyzed.