開發具成本效益之透明導電氧化物與能帶調變工程應用於 光伏元件

Abstract

本研究開發具量產潛能與成本效益的製程技術，期望提升矽基薄膜太陽能電池轉換效率與成本競爭力。首先，本研究針對單接面微晶矽鍺合金薄膜太陽能電池提出能帶調變的結構。在p型矽與本徵矽鍺吸收層之間設計漸變微晶矽鍺緩衝層，降低因異質接面結構所產生價電帶位能差並增加電洞在元件內的萃出率。此外，本研究提出一新穎結構以提升吸收層內建電場。傳統方式藉增加p型矽與n型矽參雜濃度來提升內建電場，但此方法大量增加入射光子的損失，進而造成電池轉換效率不佳。本研究於n型矽與本徵矽鍺吸收層間加入微晶矽電場增益層，並探討電場增益層厚度對電池轉換效率之影響。經實驗發現電場增益層在適當的厚度下，其電池轉換效率比傳統結構電池之轉換效率相對提升28%，填充因子也相對提升19%。針對此新穎結構進行模擬，其計算結果驗證吸收層內建電場有大幅度的提升。此新穎結構運用在非晶矽/微晶矽鍺雙接面太陽能電池，其電池轉換效率可達11.46%。本研究在非晶矽/微晶矽雙接面太陽能電池也得到非常好的結果。在p型矽與本徵微晶矽吸收層間加入低結晶率微晶矽層，進一步提升底層電池其短波長波段光吸收效率。同時，利用低結晶率微晶矽層做為電場增益層並插入n型矽與本徵微晶矽吸收層之間，本研究探討此微晶矽電場增益層其厚度與結晶率對於電池特性的影響。在製程條件優化情況下，非晶矽/微晶矽雙接面太陽能電池轉換效率可達到11.87%。 為使矽基薄膜太陽能電池更具有價格競爭能力,本研究同時開發具成本與量產效益之氧化鋅鎵薄膜材料做為其所必備的基板。有別於傳統製程，本研究結合大氣電漿與濺鍍系統成功開發自我絨化透明導電基板。大氣電漿所沉積的有機矽薄膜掌握透明導電基板其光電特性。因此，大氣電漿之掃描速度與前驅物氣體載體流率為本研究探討的重點。此外，我們也探討氧化鋅鎵厚度與結晶情況如何影響其表面形貌與光學霧度。本研究將非晶矽單接面太陽能電池製作在不同絨化透明導電基板上並探討其電性表現，並利用三維空間時域有限差分法模擬探討電池內部光侷限效應。採用自製氧化鋅鎵基板，目前非晶矽單接面太陽能電池轉換效率最高可達8.78%。

In this study, we develop novel and cost-effective materials and techniques to enhance the performance of Si-based thin film solar cells. We invent a new structure of band-gap engineering for hydrogenated microcrystalline silicon-germanium (μc-SiGe:H) single junction solar cells. μc-Si:H buffer layers are deposited between p-Si:H and μc-SiGe:H to reduce potential barriers due to the valence-band discontinuity. Moreover, we implement μc-Si:H field-enhancement layers (FELs) between n-Si:H and μc-SiGe:H to increase the built-in electric field in absorbers, which is demonstrated by the Atlas device simulation. Instead of increasing doping concentration in p-Si:H and n-Si:H layers, this technique can avoid the loss of incoming light. Here, we investigate the relation between the crystallinity and the thickness of FELs and the performance of solar cells. Compared with the traditional cells, the cells with optimized FELs show 19% and 28% enhancements in fill factor (FF) and conversion efficiency (η), respectively. Furthermore, a-Si:H/μc-SiGe:H tandem cells with optimized μc-Si:H FELs can reveal η of 11.48%. We also implement band-gap engineering in a-Si:H/μc-Si:H tandem cells. In order to further enhance light absorption efficiency in a short wavelength range in bottom cells, μc-Si:H layers with low crystallinity are deposited between p-Si:H layers and μc-Si:H absorbers. μc-Si:H FELs with different crystallinity and thickness deposited between n-Si:H layers and μc-Si:H absorbers are investigated. a-Si:H/μc-Si:H tandem cells can reveal η of 11.87% in a optimized condition. Meanwhile, we successfully develop self-texturing ZnO:Ga (GZO) substrates for solar cells by an atmospheric-pressure plasma jet (APPJ) and sputtering. The electro-optical properties of the textured GZOs are mainly controlled by the haze of organosilicon underlayers deposited by the APPJ. In this study, we investigate the characteristics of organosilicon layers and texturing GZOs deposited in different process conditions. Moreover, light-trapping effects resulting from these textured substrates in Si-based thin-film solar cells are examined by the 3D finite-difference time domain (FDTD) simulation. In a optimized condition, a-Si:H single junction solar cells fabricated on homemade GZO substrates can exhibit η of 8.78%.