Correlation between film and cell properties for DC plasma deposited amorphous silicon
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Correlation between film and cell properties for DC plasma deposited amorphous silicon Jennifer Heath1, Suman B. Iyer1, Yoram Lubianiker1, J. David Cohen1 and Gautam Ganguly2 1 Department of Physics, University of Oregon, Eugene, OR 97403 U.S.A. 2 BP Solar Corporation, Toano, VA 23168, U.S.A. ABSTRACT We have carried out measurements to try to correlate amorphous silicon film properties with companion solar cell device performance. The dc plasma deposited intrinsic films were prepared with various hydrogen dilution levels, and increasing power levels to increase growth rate. The electronic properties were determined using admittance spectroscopy and drive-level capacitance profiling (DLCP) techniques as well as transient photocapacitance and photocurrent spectroscopy. Cell and film performance were explored in both as-grown and light-soaked states. We observed that, although cell performance decreased systematically with increasing growth rate, it depended on factors other than the deep defect density in the matched films. On the other hand, we did observe that increases in defect density caused by the lightinduced degradation led to fairly predictable decreases in the cell fill factors. INTRODUCTION Reducing the manufacturing costs of amorphous silicon based photovoltaic modules is currently an important goal in the development of this technology. One active area of research is finding methods to substantially increase the deposition rate of the intrinsic a-Si:H absorber layer from the roughly 1Å/s level known to produce the most stable, highest efficient solar cell devices. Unfortunately most attempts to increase this deposition rate much beyond 3Å/s have led to significant deterioration in device performance. At present the reasons for this are not well understood. In particular, it is not at all clear which material properties of the i-layer absorber might be good predictors of the actual efficiency once a given material is fabricated into a solar cell device. In the study reported below we have examined a range of a-Si:H matched films and devices in an attempt to correlate the film electronic properties with the solar cell performance in both their as-grown and light-degraded states. The materials properties examined included the deep defect densities, electrical conductivities, sub-band-gap spectra and, in some cases, the minority carrier µτ products. The series of i-layer a-Si:H samples for this study was deposited by the dc glow discharge method covering a wide range of hydrogen dilution and power densities. This resulted in the growth rates being varied by roughly a factor of eight, from 0.65Å/s to 6Å/s. SAMPLE PREPARATION The hydrogen diluted undoped a-Si:H films were deposited at BP Solar in a double loadlocked research reactor using dc plasma decomposition of silane as detailed elsewhere [1]. For materials characterization, the i-layers were deposited on p+ crystalline silicon and/or SnO2 coated specular glass substrates. Series of samples were deposited varying two of the external deposition parameters;
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