Nanostructured Silicon Oxide Dual-Function Layer in Amorphous Silicon Based Solar Cells
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Nanostructured Silicon Oxide Dual-Function Layer in Amorphous Silicon Based Solar Cells Tining Su, Baojie Yan, Laura Sivec, Guozhen Yue, Jessica Owens-Mawson, Jeffrey Yang, and Subhendu Guha United Solar Ovonic LLC, 1100 W Maple Rd, Troy, MI 48084, U.S.A. ABSTRACT We report the results of using n-type hydrogenated nanocrystalline silicon oxide alloy (nc-SiOx:H) in hydrogenated nanocrystalline silicon (nc-Si:H) and amorphous silicon germanium alloy (a-SiGe:H) single-junction solar cells. We used VHF glow discharge to deposit nc-SiOx:H layers on various substrates for material characterizations. We also used VHF glow discharge to deposit the intrinsic layer in nc-Si:H solar cells. RF glow discharge was used for the deposition of the doped layers and the intrinsic layer in a-SiGe:H solar cells. Various substrates such as stainless steel (SS), Ag coated SS, and ZnO/Ag coated SS were used for different cell structures. We found that by using nc-SiOx:H to replace the ZnO and the a-Si:H n-layer in nc-Si:H solar cells, the cell structure is greatly simplified, while the cell performances remain nearly identical to those made using the conventional n-i-p structure on standard ZnO/Ag BR’s. Solar cells with nc-SiOx:H as the n layer directly deposited on textured Ag show similar quantum efficiency (QE) as the n-i-p cells on ZnO/Ag BRs. In both cases, QE is higher than that in the n-i-p cells made directly on Ag coated SS. This effect is probably caused by the shift of surface plasmonpolariton resonance frequency due to the difference in index of refraction of ZnO, nc-SiOx:H, and Si. INTRODUCTION Light trapping is one of the most critical techniques for obtaining high efficiency in thin film silicon solar cells. ZnO coated silver (ZnO/Ag) back reflectors (BR) are commonly used in n-i-p structured hydrogenated amorphous silicon (a-Si:H), nanocrystalline silicon (nc-Si:H), and amorphous silicon germanium (a-SiGe:H) solar cells. Extensive experimental data show that cells made on ZnO/Ag BRs produce higher short-circuit current density (Jsc) than those deposited directly on Ag. The exact mechanism of the enhancement of Jsc by ZnO is still being debated. However, from a manufacturing point of view, it is highly desirable to replace the ZnO with materials that can be deposited in-line with the solar cell, and therefore reduce cost. Recently, silicon-rich hydrogenated nanocrystalline silicon oxide thin films (nc-SiOx:H), capable of deposition by conventional PECVD, have been extensively studied for solar cell applications. It has been reported that nc-SiOx:H can be used as an intermediate reflective layer in multi-junction solar cells [1-3] for effective light management, and it can also replace ZnO in a-Si:H cells with a p-i-n structure on glass superstrates [4]. To apply nc-SiOx:H films in manufacturing, it is crucial to show that cells deposited on nc-SiOx:H based BR can produce efficiencies as high as, or even better than, cells deposited on standard ZnO/Ag BR. In this report, we show that by replacing ZnO with nc-SiOx:H, we can fa
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