Development of High Stable- Efficiency, Triple-Junction a-Si Alloy Solar Cells: Annual Subcontract Report, 18 July 1994 - 17 July 1995

This report describes work performed by Energy Conversion Devices, Inc. (ECD) under a 3-year, cost-shared amorphous silicon (a-Si) research program to develop advanced technologies and to demonstrate stable 14%-efficient, triple-junction a-Si alloy solar cells. The technologies developed under the program will then be incorporated into ECD's continuous roll-to-roll deposition process to furtherenhance ECD's photovoltaic manufacturing technology. In ECD's solar cell design, triple-junction a-Si alloy solar cells are deposited onto stainless-steel substrates coated with Ag/ZnO back-reflector layers. This type of cell design enabled ECD to use a continuous roll-to-roll deposition process to manufacture a-Si PV materials in high volume at low cost. Using this cell design, ECD previouslyachieved 13.7% initial solar cell efficiency using the following features: (1) a triple-junction, two-band-gap, spectrum-splitting solar cell design; (2) a microcrystalline silicon p-layer; (3) a band-gap-profiled a-SiGe alloy as the bottom cell i-layer; (4) a high-performance Ag/ZnO back- reflector; and (5) a high-performance tunnel junction between component cells. ECD also applied thetechnology into its 2-MW/yr a-Si production line and achieved the manufacturing of 4-ft2 PV modules with 8% stable efficiency. During this program, ECD is also further advancing its existing PV technology toward the goal of 14% stable solar cells by performing the following four tasks: (1) improving the stability of the intrinsic a-Si alloy materials; (2) improving the quality of low-band-gapa-SiGe alloy; (3) improving p+ window layers; and (4) developing high stable-efficiency triple-junction a-Si alloy solar cells.;