矽摻雜氮化鎵之光伏特性：中間能帶太陽能電池的潛力評估; Photovoltaic characteristics of Si-doped GaN: the potential for intermediate band solar cells

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题名:	矽摻雜氮化鎵之光伏特性：中間能帶太陽能電池的潛力評估;Photovoltaic characteristics of Si-doped GaN: the potential for intermediate band solar cells
作者:	廖仁熊;Liao,Jen-hsiung
贡献者:	光電科學與工程學系
关键词:	中間能帶太陽能電池;氮化鎵;intermediate band solar cell;GaN
日期:	2013-07-23
上传时间:	2013-08-22 11:43:59 (UTC+8)
出版者:	國立中央大學
摘要:	中間能帶太陽能電池利用導電帶與價電帶之間的額外能帶，得到寬頻譜的吸光能力，理論上可以打破Shockley-和Queisser 在1963年提出的轉換效率極限(The SQ limit)。中間能帶的存在為太陽能電池帶來兩個額外的次能階躍遷，分別是從價電帶到中間能帶以及中間能帶到導電帶，因此能讓同質的半導體接面吸收多種波長的光。在本論文中，我們利用矽摻雜之n型氮化鎵製成中間能帶太陽能電池。氮化鎵的能隙為3.4eV，在過去二十年，n型氮化鎵的黃色螢光特性已被科學家所熟知，並被廣泛地研究，黃色螢光的成因為電子從導電帶至中間能帶的能量釋放，而此中間能帶為鎵原子空缺(Ga vacancy)所誘發而成的能帶，該能帶離價電帶頂端約1.2eV。因此n型氮化鎵能吸收的太陽光能量為:3.4 eV、2.2 eV以及1.2 eV。本研究用簡單的p-n-n氮化鎵同質接面，在吸光層中藉由三種不同的矽摻雜濃度，來調變中間能帶太陽能電池的吸光能力。三種吸光層的電子濃度分別為: 1×1017 cm-3(PUN元件)、5×1017 cm-3(Pn-N元件)以及1×1018 cm-3(PnN元件)。在AM1.5G太陽光模擬器的照射下，PnN元件具有最大的短路電流，比PUN元件高出80%。雖然PnN元件的開路電壓有些微的下降，但該元件的轉換效率約為PUN元件的1.4倍。此研究結果說明:利用矽摻雜所產生的氮化鎵中間能帶電池，能有效增加元件的吸光能力，並提升整體的光伏效能。 Intermediate band solar cells (IBSCs) are expected to break the SQ limit with a single junction structure, which is due to the additional energy level induced in the forbidden gap between the conduction band (CB) and the valence band (VB). This stepping-stone-like energy level brings two sub-bandgap transitions within the bandgap (i.e. VB-IB and IB-CB), and therefore allows multi-color absorption with a single junction. In this work, we demonstrate the IBSC with Si-doped n-type GaN. The yellow luminescence (YL) of Si-doped GaN has been extensively discussed in the last two decades, and the origin was attributed to the energy transition from CB to the impurity level induced by gallium vacancies and the associate complexes. The yellow-emitting impurity level is employed in the active region to absorb the solar energy of 3.4eV, 2.2eV and 1.2eV. The entire device structure is similar to a simple p-i-n homojunction, except for the slightly Si-doped i-layer. Three types of active regions are fabricated with different doping concentrations: the undoped (1×1017 cm-3, hereafter called PUN), the lightly doped (5×1017 cm-3, called Pn-N), and the moderately doped (1×1018 cm-3, called PnN). The J-V measurement under AM1.5G illumination shows greatly enhanced JSC with the PnN sample, which is 80% higher than that of the PUN sample. Although the VOC is slightly decreased, the PnN sample exhibits the efficiency improvement of 40%, indicating the significant contribution of photocurrents from the IB absorption.
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