Use of Very-High-Frequency Plasmas to Prepare a-Si:H-Based Triple-Junction Solar Cells at High Deposition Rates: Annual Technical Status Report, 11 March 1998 - 11 March 1999

This report describes work performed by Energy Conversion Devices, Inc. (ECD) during this phase of this subcontract. ECD researchers have made significant progress in advancing the very high frequency (VHF), high-rate technology. They demonstrated that 8.0% stable efficiencies can be achieved for a-Si:H cells whose i-layers are prepared at rates near 10 ?/s using the VHF technique. Presently,there is not a great difference in the performance of a-Si:H cells made using the VHF technique and i-layer deposition rates near 10 ?/s and that for cells made using the standard 13.56 MHz technique and rates near 1 ?/s in the same deposition system. In terms of the a-SiGe:H cells, researchers have completed a number of studies of devices with properties appropriate for middle-junctioncells-that is, cells without Ag/ZnO back-reflectors having Voc values near 0.75V and Jsc values near 8.0 mA/cm2 when measured using AM1.5 light filtered using a 530-nm, low-band-pass filter. The stabilized properties for these cells prepared at i-layer rates near 10 ?/s are again similar to a-SiGe:H cells made using the same deposition hardware and the low-rate 13.56 MHz method. Establishing aninitial 10.5% for a triple-junction cell whose i-layers are prepared at the high rates sets the baseline for ECD's future studies. The triple-junction cell degradation (10%-13%) with prolonged light soaking is similar to that regularly obtained for cells prepared at low i-layer deposition rates (1 ?/s). This is important because the use of high-rate methods to prepare i-layers typically leads toless-stable materials and cells. Increasing the buffer-layer deposition rate to 6 ?/s leads to nearly a 15-min decrease in the total deposition time, whereas the increase in the n-layer and p-layer deposition rates both decrease the total time by 5 and 5.8 min, respectively. Thus, besides the i-layer growth rates, increasing the buffer layer growth rate has the strongest effect on the depositiontime of the triple-junction semiconductor structures.