The Effect of Moderate Hydrogen Dilution on Stability and Structure of Amorphous Silicon Deposited by Hot-Wire CVD
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The Effect of Moderate Hydrogen Dilution on Stability and Structure of Amorphous Silicon Deposited by Hot-Wire CVD Urban Weber and Bernd Schroeder Department of Physics and Research Center of Materials Science, University of Kaiserslautern, Kaiserslautern, Germany. ABSTRACT The effect of moderate hydrogen dilution of the process gas, F(H2)/F(SiH4) = 0 to 3, on the properties of amorphous silicon is discussed for material and solar cells deposited by Hot-Wire CVD. Dielectric properties were obtained from spectroscopic ellipsometry and are related to stability and hydrogen bonding configuration of films deposited with varying hydrogen dilution at different substrate temperatures. The stability was determined by comparing defect densities obtained from photoconductivity spectroscopy in the constant photocurrent mode (CPM) before and after pulsed-light soaking. At low substrate temperatures, which are relevant for the preparation of pin-type solar cells (160-200°C), moderate hydrogen dilution (~0.3) improves material quality regarding density and network disorder (oscillator bandwidth) as obtained from spectroscopic ellipsometry, resulting in a higher stability. At higher substrate temperatures (300°C), stability and hydrogen bonding configurations are generally better, but moderate hydrogen dilution already deteriorates these properties compared to material prepared without dilution. The incorporation of Hot-Wire-a-Si:H into pin-type solar cells is also discussed and a good correlation of ellipsometric results with bulk-related properties of solar cell performance is observed. The optimum hydrogen dilution is found to be 0 to 0.3 for i-layer deposition yielding initial efficiencies of up to 8.9% for solar cells entirely fabricated by Hot-Wire CVD. INTRODUCTION Hot-Wire (HW) or thermo-catalytic CVD of amorphous silicon, a-Si:H, was following two routes in the past few years: low substrate temperature (Ts< 250°C) deposition, which is mandatory for the application in pin (superstrate) solar cells [1-3], and high substrate temperature (Ts> 350°C) deposition for nip (substrate) solar cells [4,5] and TFT’s [6]. Only in the high Ts case, full use of the high deposition rate and stability potential of this method can be made. The stability potential of a-Si:H by HWCVD at low Ts is associated with the influence of hydrogen dilution of the process gas on film and solar cell properties, as in PECVD [7-9]. Bauer et al. addressed this issue in HWCVD and found moderate hydrogen dilution to be appropriate to enhance the stability of low-Ts a-Si:H films [10] and solar cells [1]. Here, we examine the effect of moderate hydrogen dilution, R= F(H2)/F(SiH4)= 0 to 3, on the properties of a-Si:H films at different Ts (160-300°C) in order to gain insight into the underlying mechanisms of hydrogen dilution in HWCVD. The incorporation of these films into pin solar cells is also discussed. EXPERIMENTAL DETAILS Amorphous silicon films were prepared by HWCVD using a “W”-shaped tantalum filament at a distance of 5 cm from the substrate. In-situ sp
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