Microstructure Evolution with Thickness and Hydrogen Dilution Profile in Microcrystalline Silicon Solar Cells
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Microstructure Evolution with Thickness and Hydrogen Dilution Profile in Microcrystalline Silicon Solar Cells Baojie Yana, Guozhen Yuea, Jeffrey Yanga, Subhendu Guhaa, D. L. Williamsonb, Daxing Hanc, and Chun-Sheng Jiangd a United Solar Ovonic Corporation, 1100 West Maple Road, Troy, MI 48084 b Department of Physics, Colorado School of Mines, Golden, CO 80401 c Department of Physics & Astronomy, University of North Carolina, Chapel Hill NC 27599-3255 d National Renewable Energy Laboratory, Golden, CO 80401 ABSTRACT Hydrogenated microcrystalline silicon (µc-Si:H) solar cells with different thicknesses were deposited on specular stainless steel substrates and on textured Ag/ZnO back reflectors using RF and modified very high frequency glow discharge at various deposition rates. Raman spectra and X-ray diffraction patterns exhibit a significant increase of microcrystalline volume fraction and in grain size with film thickness. Atomic force microscopy reveals an increase in the size of microstructural features and the surface roughness with increasing thickness. Based on these results, we believe that the increase of the microcrystalline phase with thickness is the main reason for the deterioration of cell performance with the thickness of the intrinsic layer. To overcome this problem, we have developed a procedure of varying the hydrogen dilution ratio during deposition. Using this method, we have been successful in controlling the microstructure evolution and achieved an initial active-area efficiency of 8.4% for a µc-Si:H single-junction solar cell, and 13.6% for an a-Si:H/a-SiGe:H/µc-Si:H triple-junction solar cell. INTRODUCTION Hydrogenated microcrystalline silicon (µc-Si:H) as the intrinsic absorbing layer in the bottom cell of hydrogenated amorphous silicon (a-Si:H) based multi-junction structures has been intensively studied since it offers a better stability against prolonged illumination than hydrogenated amorphous silicon germanium alloys (a-SiGe:H) [1-5]. The band gap of µc-Si:H with high microcrystalline volume fraction (fc) is normally close to 1.1 eV, which is the same value as crystalline silicon. Therefore, µc-Si:H solar cells generally have higher short-circuit current density (Jsc), but lower open-circuit voltage (Voc) than a-Si:H and a-SiGe:H cells. However, a thick intrinsic µc-Si:H layer is required for obtaining high Jsc due to the indirect optical transition in this material. We find that Jsc reaches a maximum around 22-24 mA/cm2 with a thickness in the range between 1 and 2 µm under our current deposition conditions. Increasing the thickness further leads to a decrease in Jsc, caused by a reduction of spectral response in the long wavelength region. Two mechanisms could be responsible for the low Jsc for thick µc-Si:H solar cells. One is microcrystallite collision due to the textured substrate [6]. It has been reported that grains in µc-Si:H deposited on a textured back reflector (BR) grow perpendicular to the local substrate surface and collide with each other when the film exceeds a ce
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