Controlling Structural Evolution by VHF Power Profiling Technique for High-efficiency Microcrystalline Silicon Solar Cel
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1153-A07-21
Controlling Structural Evolution by VHF Power Profiling Technique for High-efficiency Microcrystalline Silicon Solar Cells at High Deposition Rate Guofu Hou 1 , Xiaoyan Han, Changchun Wei, Xiaodan Zhang, Guijun Li, Zhihua Dai, Xinliang Chen, Jianjun Zhang, Ying Zhao, and Xinhua Geng Institute of Photoelectronics, Nankai University, Tianjin 300071, P.R. China ABSTRACT High rate deposition of hydrogenated microcrystalline silicon (μc-Si:H) films and solar cells were prepared by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) process in a high power and high pressure regime. The experiment results demonstrate that in high-rate deposited μc-Si:H films, the structural evolution is much more dramatic than that in low-rate deposited μc-Si:H films. A novel VHF power profiling technique, which was designed by dynamically decreasing the VHF power step by step during the deposition of μc-Si:H intrinsic layers, has been developed to control the structural evolution along the growth direction. Another advantage of this VHF power profiling technique is the reduced ion bombardments on growth surface because of decreasing the VHF power. Using this method, a significant improvement in the solar cell performance has been achieved. A high conversion efficiency of 9.36% (Voc=542mV, Jsc=25.4mA/cm2, FF=68%) was obtained for a single junction μc-Si:H p-i-n solar cell with i-layer deposited at deposition rate over 10 Å/s. INTRODUCTION Because of its improved stability and superior long wavelength response, μc-Si:H has been widely used as a stable and narrow bandgap absorber layers in both single-junction and multi-junction solar cells.[1~3] Since μc-Si:H is an indirect band-gap material, a thicker layer (~3μm) is necessary to make full use of solar spectrum, especially the infrared light(>800nm). Thus, high-rate deposition of μc-Si:H is a critical issue for low-cost production.[1] As well known that the high efficiency μc-Si:H solar cell are usually prepared with an intrinsic layer deposited near the a-Si:H/μc-Si:H transition region.[1~3] Such transition film is strongly dependent on the substrate and deposition condition, and its microstructure varies dramatically along the growth direction: an amorphous incubation layer regularly forms in the initial growth stage of layers, until fully microcrystalline growth sets in.[2] Then, the crystallinity increases gradually along the increase of thickness. This will cause serious deterioration for the device performance.[4, 5] In order to control the microstructure evolution of μc-Si:H, B. Yan et al. developed a hydrogen dilution profiling technique.[6] Since then, this method has been adopted by many groups.[3, 7~8] Besides improving microstructure evolution by hydrogen dilution profiling technique, J. Gu reported that the μc-Si:H films by HWCVD also demonstrate higher compactness and thus high stability against the oxygen diffusion[7], while C. Niikura found improved carrier transport properties along the growth direction.[8] Optimized hydrogen dilution pro
