摘要
本論文主要針對能夠轉換太陽光能量的再生能源元件作相關研究，利用三五族氮化鎵/氮化鎵銦(GaN/InGaN)材料製作光伏(photovoltaic, PV)元件與光電化學(photo-electro-chemical, PEC)水解出氫氣之元件。首先在PV元件的研究上，我們改變n型高摻雜層(n+-layer)的有效深度(teffn)與金屬電極分佈來縮短受光激發的載子在元件中的傳輸距離，藉此降低材料缺陷對元件特性的影響。轉換效率上，teffn為40 nm比起teffn 740 nm之元件減少了35%的光電流損耗；指叉狀電極分佈的光電流比起網狀電極高出30%。在SiO2光學薄膜的應用上，適當選擇的光學薄膜除了減少光反射(Fresnel reflection loss)外，元件表面形成的鈍化層亦減少半導體材料的表面復合速率，當元件本身表面缺陷較少的情況下對元件的開路電壓(VOC)有明顯提升，並且在轉換效率上有高達22%的提升。
PEC水解氫氣元件的研究上，我們設計不同金屬電極密度，改變電場在半導體內的延伸，降低受光激發載子在傳輸過程被復合的比例，且為了避免金屬電極參與光電化學反應而剝落，以SiO2薄膜做為金屬的保護層，使用的光源為300 W的氙燈、電解液為一個莫耳濃度之氯化鈉(NaCl : 1 mol/L)。比較光電化學反應的產氣速率，網狀金屬電極分佈比起直接藉由n+-GaN傳輸載子的元件高出72%，而且在外加1V參考偏壓時氣體反應速率更有161%的提升效果。

Abstract
InGaN-based photovoltaic (PV) devices with GaN/InGaN superlattice absorption layers epitaxially grown on sapphire substrates by metal-organic vapor phase epitaxy techniques are performed to reach band edge absorption at around 445 nm. The stacked superlattice structures with barrier and well layers form a miniband with lower bandgap energy instead of thick InGaN junctions with high indium compositions. PV devices designed with different structures and post-treatment are performed to alleviate the loss in photovoltaic effects of the InGaN-based absorption layers. In this study, shorter effective thickness of n+-layer (teffn ~ 40 nm) for PV-A reduces 35% in photo-generated current loss as compared with that of PV-B (teffn ~ 740 nm). On the other hand, the conversion efficiency of the devices with electrodes in digitated arrangement is improved by around 30% as compared to that of PV devices with electrodes in grid arrangement. In addition, SiO2 antireflection layers applied to the PV devices could effectively reduce the reflection of incident light and reduce surface recombination of surface states. As a result, the overall conversion efficiency is enhanced by approximately 22%.
Hydrogen generation by direct photoelectrolysis of aqueous solutions was also studied by using n+-GaN films to serve as working electrodes. To enhance the photocurrent, i.e., gas generation rate, meshed Cr/Au contacts on the n+-type GaN films with SiO2 protection layers were immersed in the NaCl electrolyte. The meshed Cr/Au contacts could effectively reduce the transit time of photogenerated electrons before they reach the Cr/Au Ohmic contacts while the SiO2 protection layers could isolate the Ohmic contacts from electrolyte. We designed two different n+-type GaN photoelectrodes. One is the n+-type GaN photoelectrode with Ohmic contacts in grid arrangement and SiO2 protection layers deposited on the Ohmic contacts (labeled as PEC2). For comparison, the carrier collection was directly extracted to the external circuit by working n+-GaN electrodes without the grid Ohmic contacts (labeled as PEC1). Under zero bias, the gas generation rate of PEC2 is enhanced by approximately 72% as compared to PEC1. The high generation rate could be attributed to the fact that the electrons generated by the light irradiation on the n-GaN working electrodes might recombine with holes or other material defects before they reach to the electrodes (i.e., external circuits) if the diffusion length of the electrons is shorter than the distance from the the n+-type GaN photoelectrode to the Ohmic contact. When a reference bias of 1V (compared with Pt electrodes) was applied to the n+-GaN working electrodes (PEC1 and PEC2), the generation rate was increased to 161% .

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