Use of a GAS Jet Technique to Prepare Microcrystalline Silicon Based Solar Cells at High I-Layer Deposition Rates
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ABSTRACT Using a Gas Jet thin film deposition technique, microcrystalline silicon (ltc-Si) materials were prepared at rates as high as 15-20 A/s. The technique involves the use of a gas jet flow that is subjected to a high intensity microwave source. The quality of the material has been optimized through the variation of a number of deposition conditions including the substrate temperature, the gas flows, and the applied microwave power. The best films were made using deposition rates near 16 A/s. These materials have been used as i-layers for red light absorbing, nip singlejunction solar cells. Using a 610nm cutoff filter which only allows red light to strike the device, pre-light soaked currents as high as 10 mA/cm2 and 2.2-2.3% red-light pre-light soaked peak power outputs have been obtained for cells with i-layer thicknesses near 1 micron. This compares with currents of 10-11 mA/cm 2 and 4% initial red-light peak power outputs obtained for high efficiency amorphous silicon germanium alloy (a-SiGe:H) devices. The AMI.5 white light efficiencies for these microcrystalline cells are 5.9 -6 .0%. While the efficiencies for the aSiGe:H cells degrade by 15-20% after long term light exposure, the efficiencies for the microcrystalline cells before and after prolonged light exposure are similar, within measurement error. Considering these results, the Gas Jet deposition method is a promising technique for the deposition of jtc-Si solar cells due to the ability to achieve reasonable stable efficiencies for cells at i-layer deposition rates (16 A/s) which make large-scale production economically feasible. INTRODUCTION The poor performance of red light absorbing a-SiGe:H cells as compared with the blue-green light absorbing a-Si:H cells has been widely documented[l]. Larger defect densities, weaker hydrogen bonds and heterogeneous microstructures have all been attributed to the poorer performance for the alloy material. With a-SiGe:H alloys commonly used as red light absorbing i-layers for the component cells of high stable efficiency triple-junction devices, the poorer performance limits the achievable efficiencies for the triple-junction modules presently made in production. In addition, the cost of germane gas presently used to make the alloy material is a significant expense in the manufacturing process while silane gas is a low material cost item. Thus, it would be desirable in terms of both stable cell efficiency and module cost to find an alternative low bandgap, red light absorbing material to replace a-SiGe:H as i-layers for bottom cells of the triple-junction structure. One possible alternative material is microcrystalline Si (lic-Si), which has a bandgap near 1.1 eV. Shah et. al.[2] have shown that high quality ltc-Si pin devices with 8.5% efficiencies that absorb a significant amount of the red light and do not degrade with long term light exposure can be made using the Very High Frequency PECVD technique (VHF). However in using this or any standard PECVD technique, the deposition rates of the pc-Si materials are
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