Double-layered Ag-Al back reflector on stainless steel substrate for a Si:H thin film solar cells

An effective light trapping method for substrate-type hydrogenated amorphous silicon (a-Si:H) thin film solar cells is the use of a back reflector (BR) of high roughness, e.g., ‘hot silver’, which is deposited at temperatures higher than 450 °C. In this work, textured silver-aluminum (Ag–Al) BR films were fabricated by depositing Ag on Al film at Ag-deposition temperatures (TAg) ranging from 25 to 350 °C. The surface morphology and roughness of Ag–Al films were strongly affected by TAg. The Al and Ag films were formed entirely of Ag2Al alloy at TAg of 330 °C or higher, while the Ag–Al films maintained a double-layered structure at 290 °C or below. Although the films did not undergo alloying at TAg of 290 °C, the Ag–Al films have a well-developed surface structure with high diffuse-reflectance, compared to Ag films deposited at the same temperature. The conversion efficiency of an a-Si:H thin film solar cell on a flexible stainless steel substrate increased from 7.63% to 8.44% as TAg was increased from 25 to 290 °C, as a result of more effective light scattering by Ag–Al BRs, producing increased short-circuit current. However, at higher TAg, Ag2Al alloy films with sharp crystallite edges were formed, and were not appropriate as BRs. The present work clearly shows that double-layered Ag–Al films fabricated at temperatures as low as 290 °C could be useful back reflectors for substrate-type thin film solar cells.