非晶與微晶矽鍺合金作為薄膜太陽能電池吸收層之特性分析與最佳化

Abstract

在本研究中，藉著改變氫流率、電漿功率以及製程氣體來研究非晶矽鍺與微晶矽鍺之薄膜特性以及非晶矽鍺單接面太陽電池之轉換效率。製程氣體我們使用矽甲烷(SiH4)、鍺甲烷(GeH4)以及氫氣，利用射頻電漿輔助化學氣相沉積系統(PECVD)來沉積非晶矽鍺薄膜。由本篇實驗發現通入適當氫氣可提升光電導，而鍺原子濃度亦影響光學能隙以及電性。電漿功率除了會影響薄膜沉積速率外同時亦可提升電性。在元件的表現上可分為以下幾個部分：非晶矽鍺吸收層的厚度大約在150 nm 至 300 nm 之間為佳，過厚或過薄都會對效率有不良影響；較高的沉積功率增強了對表面的離子轟擊效應因此讓元件填充因子下降；在p/i 與 i/n 介面間加入一緩衝層可有效阻擋磷原子擴散至吸收層，使得界面品質改善；最後，當我們將吸收層中的鍺含量做漸變的變化，元件有較大的開路電壓且效率可達8.5 %。 在微晶矽鍺合金方面，當我們逐漸提升沉積時的放電功率可發現一個由非晶矽鍺合金轉變到微晶矽鍺合金的區域，且越接近此轉變的區間，膜中的結晶範圍越大。調整氫氣稀釋比例也可改變膜中的結晶率，但由光暗電導可發現此時的膜大多為高度結晶。改變膜中的鍺含量會大幅影響微晶矽鍺薄膜的特性，氣態的鍺濃度大約在 9 % 至 16 % 為最佳的區間，光暗電導的差距約為一至兩個數量級。

In this study, the effects of systematic variation in hydrogen dilution, RF power, gas phase ratio of germane (RGeH4) on the hydrogenated amorphous silicon germanium (a-SiGe:H) and hydrogenated microcrystalline silicon germanium (μc-SiGe:H) alloys properties and a-SiGe:H single junction solar cell performance have been studied. The a-SiGe:H and μc-SiGe:H films were deposited from the mixture of SiH4, GeH4 , and H2 dilution by the plasma enhanced chemical vapor deposition (PECVD) system at 27.12 MHz. It has been found that hydrogen dilution plays an important role in the properties of films. Adequate hydrogen dilution ratio can improve the photo photosensitivity, but both higher and lower hydrogen dilution ratio deteriorates the film properties. Power density influences on the deposition rate and electronic properties. Photo conductivity is higher for films deposited at higher power density for all hydrogen dilution ratios. Dark conductivity is almost the same for films deposited at two different power densities for all hydrogen dilution ratios. The device performance is better for the cell deposited at 20 W then 30 W, which means that ion bombardment plays a more important role than i-layer film quality.