Correlation of Hydrogen Dilution Profiling to Material Structure and Device Performance of Hydrogenated Nanocrystalline
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1066-A03-03
Correlation of Hydrogen Dilution Profiling to Material Structure and Device Performance of Hydrogenated Nanocrystalline Silicon Solar Cells Baojie Yan1, Guozhen Yue1, Yanfa Yan2, Chun-Sheng Jiang2, Charles W. Teplin2, Jeffrey Yang1, and Subhendu Guha1 1 United Solar Ovonic LLC, 1100 West Maple Road, Troy, MI, 48084 2 National Renewable Energy Laboratory, 1617 Cole Blvd, Golden, CO, 80401
ABSTRACT We present a systematic study on the correlation of hydrogen dilution profiles to structural properties materials and solar cell performance in nc-Si:H solar cells. We deposited nc-Si:H single-junction solar cells using a modified very high frequency (VHF) glow discharge technique on stainless steel substrates with various profiles of hydrogen dilution in the gas mixture during deposition. The material properties were characterized using Raman spectroscopy, X-TEM, AFM, and C-AFM. The solar cell performance correlates well with the material structures. Three major conclusions are made based on the characterization results. First, the optimized nc-Si:H material does not show an incubation layer, indicating that the seeding layer is well optimized and works as per design. Second, the nanocrystalline evolution is well controlled by hydrogen dilution profiling in which the hydrogen dilution ratio is dynamically reduced during the intrinsic layer deposition. Third, the best nc-Si:H singlejunction solar cell was made using a proper hydrogen dilution profile, which caused a nanocrystalline distribution close to uniform throughout the thickness, but with a slightly inverse nanocrystalline evolution. We have used the optimized hydrogen dilution profiling and improved the nc-Si:H solar cell performance significantly. As a result, we have achieved an initial active-area cell efficiency of 9.2% with a nc-Si:H single-junction structure, and 15.4% with an a-Si:H/a-SiGe:H/nc-Si:H triple-junction solar cell structure. INTRODUCTION Hydrogen dilution is a key parameter for controlling the transition from a-Si:H to ncSi:H. The structure analyses showed that nc-Si:H films deposited with constant hydrogen dilution normally exhibit an initial amorphous incubation layer, followed by nucleation and growth of cone-like nanocrystalline clusters. During the growth of the nanocrystalline conestructures, the crystalline volume fraction and average grain-size increase with the film thickness [1-4], a phenomenon called nanocrystalline evolution. It was also found that nc-Si:H solar cells made with a hydrogen dilution close to the amorphous/nanocrystalline transition showed a better performance than those with a higher crystalline volume fraction [5]. The nc-Si:H materials with a high crystalline volume fraction normally have a high density of micro-voids and cracks, which results in ambient degradation caused by impurity diffusion into the material [6]. Obviously, with a constant hydrogen dilution, it is difficult to maintain the material structure close to the transition regime throughout the sample thickness. In order to solve the nanocrys
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