Development of Advanced Deposition Technology for Microcrystalline Si Based Solar Cells and Modules: Final Technical Report, 1 May 2002-31 July 2004

The key objective of this subcontract was to take the first steps to extend the radio-frequency plasma-enhanced chemical vapor deposition (RF-PECVD) manufacturing technology of Energy Photovoltaics, Inc. (EPV), to the promising field of a-Si/nc-Si solar cell fabrication by demonstrating 'proof-of-concept' devices of good efficiencies that previously were believed to be unobtainable insingle-chamber reactors owing to contamination problems. A complementary goal was to find a new high-rate deposition method that can conceivably be deployed in large PECVD-type reactors. We emphasize that our goal was not to produce 'champion' devices of near-record efficiencies, but rather, to achieve modestly high efficiencies using a far simpler (cheaper) system, via practical processingmethods and materials. To directly attack issues in solar-cell fabrication at EPV, the nc-Si thin films were studied almost exclusively in the p-i-n device configuration (as absorbers or i-layers), not as stand-alone films. Highly efficient, p-i-n type, nc-Si-based solar cells are generally grown on expensive, laboratory superstrates, such as custom ZnO/glass of high texture (granular surface)and low absorption. Also standard was the use of a highly effective back-reflector ZnO/Ag, where the ZnO can be surface-textured for efficient diffuse reflection. The high-efficiency 'champion' devices made by the PECVD methods were invariably prepared in sophisticated (i.e., expensive), multi-chamber, or at least load-locked deposition systems. The electrode utilization efficiency, defined asthe surface-area ratio of the powered electrode to that of the substrates, was typically low at about one (1:1). To evaluate the true potential of nc-Si absorbers for cost-competitive, commercially viable manufacturing of large-area PV modules, we took a more down-to-earth approach, based on our proven production of a-Si PV modules by a massively parallel batch process in single-chamber RF-PECVDsystems, to the study of nc-Si solar cells, with the aim of producing high-efficiency a-Si/nc-Si solar cells and sub-modules.