The dependence of the crystalline volume fraction on the crystallite size for hydrogenated nanocrystalline silicon based
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The dependence of the crystalline volume fraction on the crystallite size for hydrogenated nanocrystalline silicon based solar cells K. J. Schmidt1, Y. Lin2, M. Beaudoin3, G. Xia2, S. K. O'Leary1, G. Yue4, and B. Yan4 1
School of Engineering, The University of British Columbia, Kelowna, BC, Canada. Department of Materials Engineering, The University of British Columbia, Vancouver, BC, Canada. 3 Advanced Materials and Process Engineering Laboratory, The University of British Columbia, Vancouver, BC, Canada. 4 United Solar Ovonic LLC, Troy, MI, United States. 2
ABSTRACT We have performed an analysis on three hydrogenated nanocrystalline silicon (nc-Si:H) based solar cells. In order to determine the impact that impurities play in shaping the material properties, the XRD and Raman spectra corresponding to all three samples were measured. The XRD results, which displayed a number of crystalline silicon-based peaks, were used in order to approximate the mean crystallite sizes through Scherrer's equation. Through a peak decomposition process, the Raman results were used to estimate the corresponding crystalline volume fraction. It was noted that small crystallite sizes appear to favor larger crystalline volume fractions. This dependence seems to be related to the oxygen impurity concentration level within the intrinsic nc-Si:H layers. INTRODUCTION Hydrogenated nanocrystalline silicon (nc-Si:H) based solar cells, as the low bandgap component cell in multi-junction thin film silicon solar cells, have attracted a considerable amount of attention in recent years owing to their improved long wavelength response and lower light induced degradation when contrasted with the case of hydrogenated amorphous silicon (aSi:H) based solar cells [1]. Record solar cell and module efficiencies have been attained using aSi:H/nc-Si:H/nc-Si:H triple-junction solar cells. Owing to its complexity as a material system, the material structure of nc-Si:H, and the role that impurities and the substrate play in influencing these material properties, are the subject of current intensive investigation. It has been observed that nc-Si:H based solar cells are more sensitive to impurities than a-Si:H based solar cells. A recent study, by Yue et al. [2], examined the impurity profiles found within a series of n-i-p nc-Si:H based solar cells. The corresponding device performance was also probed, i.e., the fill-factor, the open-circuit voltage, the quantum efficiency, and the efficiency of the solar cells themselves were determined. In this analysis, we study the material properties of the nc-Si:H found within these solar cells using X-ray diffraction (XRD) and Raman spectroscopy. Correlations between the impurity concentration profiles and the material properties are sought. EXPERIMENT We considered three n-i-p nc-Si:H based solar cells for this analysis; in particular, sample numbers 21821, 21886, and 21916 were considered, these samples also being considered by Yue
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et al. [2]. These cells were deposited on stainless steel substrates with textured
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