Comparison of Structural Properties and Solar Cell Performance of a-Si:H Films Prepared at Various Deposition Rates usin
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Comparison of Structural Properties and Solar Cell Performance of a-Si:H Films Prepared at Various Deposition Rates using 13.56 and 70 MHz PECVD Methods S.J. Jones*, D.L. Williamson**, T. Liu*, X. Deng*, and M. Izu* *Energy Conversion Devices, Inc., Troy, MI 48084 ** Department of Physics, Colorado School of Mines, Golden, CO 80401 ABSTRACT The advantage of using very high frequencies for preparation of a-Si:H materials at high rates (above 5 Å/s) for intrinsic layers (i-layer) of solar cells has been well documented. In an effort to identify film properties which may be related to this superior device performance, a study of the structural, optical and electrical properties of films made at various deposition rates between 1 and 15 Å/s using rf frequencies of 13.56 and 70 MHz has been made. The films were characterized using a number of techniques including small-angle x-ray scattering, infrared absorption spectroscopy, and scanning electron microscopy. For the films made using the 70 MHz frequency, the amount of nanovoids with sizes of < 100Å increases systematically as the deposition rates increases beyond 5 Å/s. Accompanying the increase in void fraction in the films are increases in the hydrogen content and the amount of 2070 cm-1 mode in the infrared absorption spectra. In addition to an increase in the amount of nanovoids in the films as the deposition rate exceeds 5 Å/s, the films made using the 13.56 MHz and high deposition rates have large amounts of SAXS related to scattering features with sizes > 200 Å. This scattering is associated with large bulk density fluctuations and/or enhanced surface roughness. None of the films in the study displayed signs of having columnar-like microstructures. The nanovoids are not related to changes in the solar cells with increasing i-layer deposition rate for both fabrication processes, perhaps due to the relatively small volume fractions of less than 0.2% and/or good void-surface passivation. However, the larger-scale structures detected in the films made using the 13.56 MHz technique could cause poorer performance in cells prepared at high growth rates. INTRODUCTION The advantage of using very high frequencies (70-100 MHz) to prepare i-layers for a-Si:H solar cells at deposition rates between 5 and 10 Å/s has been reported by several research groups[1-4]. As the deposition rate for the a-Si:H i-layer growth is increased from 1 to 10 Å/s, virtually no change is observed in the solar cell properties, such as the fill factor (FF), the open circuit voltage (Voc) and the short circuit current (Jsc). Only a 5% drop in Jsc is noted for a-Si:H cell with the same thickness prepared with current-enhancing Ag/ZnO backreflectors. In contrast, extensive attempts to increase the i-layer deposition rates using the standard 13.56 MHz PECVD process has led to cells with poor fill factors and cell efficiencies. This poorer performance is usually attributed to enhanced powder and polyhydride formation in the plasma that leads to ilayers with defect ridden, heterogeneous microstructures. He
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