開發摻雜型氫化微晶矽氧於非晶矽/非晶矽鍺串疊型薄膜太陽能電池作為穿隧復合接面及中間反射層的研究

Abstract

非晶矽/非晶矽鍺雙接面電池對於得到高效率的三接面太陽能電池來說是很重要的建立基點。為了改善非晶矽/非晶矽鍺雙接面電池在太陽光曝曬後的穩定性，非晶矽上部電池的吸收層厚度勢必要有所減薄，而此目的可以藉由引用一層中間反射層在非晶矽上部電池及非晶矽鍺底部電池的中間來達成。中間反射層的折射率相較於非晶矽及非晶矽鍺較低，也因此折射率的差異會使部分未被吸收的光再次返回非晶矽上部電池做吸收，進而使非晶矽吸收層可在減薄的厚度下依然保有較好的光電流響應。氫化微晶矽氧材料可藉由氧得參入得到可調控的折射率，也因此很適合做為中間反射層的材料，然而，氧的參入對於電性及光性有競爭的效果。本篇論文的材料皆由27.12 MHz的電漿輔助化學氣相沉積系統所製成，在實驗結果上，我們發現氧對於氫化非晶矽極其重要，當氧含量從0 at.%提升到23 at.%時，光吸收能隙從1.89電子伏特增加到2.20電子伏特，折射率從3.80降低至2.85，且電性從1.41x101 降低至 6.59x10-1 S/cm。另外我們也發現到，加入23 at.%氧含量的氫化非晶矽氧中間反射層可以有效的減少非晶矽吸收層厚度從160到140 nm，並適時的降低光輻射性能衰退效應。最後，優化之中間反射層應用在非晶矽/非晶矽鍺之雙接面太陽能電池的開路電壓、短路電流密度、填充系數、轉換效率被可提升至1.6 V、8.32 mA/cm2、67.1%及8.82%。

The a-Si:H/a-Si1-xGex:H tandem cell is an important building block for high-efficiency triple-junction solar cells. To improve the stability of a-Si:H/a-Si1-xGex:H tandem cells against light exposure, the thickness of the a-Si:H top cell needed to be reduced because high-energy photons were mostly absorbed by a-Si:H absorber. The reduction of thickness can be realized by employing intermediate reflecting layer (IRL) between a-Si:H top and a-Si1-xGex:H bottom cell. The IRL typically exhibited lower refractive index than a-Si:H and a-Si1-xGex:H that established a difference in refractive index, reflecting unabsorbed photons back into the a-Si:H top cell. This increased the photocurrent of a-Si:H top cell and thus entailed the possibility to reduce the thickness of a-Si:H. The microcrystalline silicon oxide (μc-SiOx:H) having tunable and reduced refractive index with the incorporation of oxygen is suitable for the application as IRL. However, there is a trade-off between its optical and electrical properties with the incorporation of oxygen. In this thesis, we employed a 27.12 MHz radio-frequency plasma-enhanced chemical vapor deposition (PECVD) system for the preparation of thin-film silicon materials. We found that oxygen incorporation was a key factor to influence the characteristics of μc-SiOx:H(n) IRL. As the oxygen content was increased from 0 at.% to 23 at.%, the E04 increased from 1.89 to 2.20 eV, refractive index decreased from 3.80 to 2.85, and the dark conductivity decreased from 1.41x101 to 6.59x10-1 S/cm. We also found that the employed 23 at.% oxygen content of μc-SiOx:H(n) IRL could reduce the thickness of a-Si:H absorber from 160 to 140 nm leading to decreased Staebler-Wronski effect. Furthermore, the VOC, JSC, FF, and efficiency of a-Si:H/a-Si1-xGex:H tandem cell with the optimized μc-SiOx:H(n) IRL were improved to 1.60 V, 8.32 mA/cm2, 67.1% and 8.82%, respectively.