由於氮化銦鎵材料的能帶可調範圍從0.7eV 到3.4eV，此三元化合物便成為光伏元件的熱門材料，理論上，多接面太陽能電池可透過適當的銦摻雜，使得全光譜的太陽能電池得以實現，但在氮化銦鎵太陽能電池的研究上的轉換效率仍低於3 % ，其歸因於許多的材料限制，例如材料成長品質與長波長吸收的權衡，不足的載子收集等。 在此研究中，我們以漸變銦含量的氮化銦鎵作為主動層，其目的為增加載子的收集及吸收光譜的範圍。我們準備了三種樣品: 兩種漸變銦含量的氮化銦鎵太陽能電池，其厚度分別為 172 nm 及184 nm，及一固定15 % 銦含量的傳統型 p-i-n 結構。所有樣本皆由有機金屬化學氣相沉積法成長於藍寶石基板上，根據電流-電壓曲線及量子光譜圖的結果，漸變結構的太陽能電池的轉換效率高於傳統的p-i-n結構，但增加主動層的厚度卻導致較低的轉換效率。結果顯示漸變能帶結構能夠有效地增加載子的收集，除此之外，模擬結果指出極化效應亦對於載子收集有重大的影響。 ;The tunable band gaps of InGaN, spanning from 3.4eV (GaN) to 0.7eV (InN), make the ternary alloy an attractive material system for photovoltaic devices. Although the absorption of nearly full solar spectrum is theoretically possible with the multi-junction containing properly selected indium compositions, the measured conversion efficiencies of InGaN-based solar cells are typically below 3 %.The unsatisfactory performances can be attributed to many material issues such as the trade-off between long-wavelength absorption and high material qualities, insufficient carrier collection etc. In this work, a single InxGa1-xN layer with graded composition was employed as the active region for nitride-based solar cells. The goal is to increase carrier collection efficiencies and the wavelength range of optical absorption. Three types of solar cells were studied: the graded InxGa1-xN junctions with the lengths of 172 nm and 184 nm, and InxGa1-xN junction with fixed indium composition (x = 0.15). All the devices were grown by metal organic chemical vapor deposition (MOCVD). According to the results of J-V curves under solar illumination and quantum-efficiency spectra, it is found that photovoltaic performances measured with the graded junctions are superior to those obtained with fixed indium content, but lengthening the junction leads to lower efficiencies. The results indicate that carrier collection can be enhanced by the graded conduction and valence band edges. In addition, theoretical analyses based on self-consistent 1D Poisson and Schrödinger equations indicate that polarization effect also plays an important role in photo-current generation.
