High roughness Ag back reflector on a metal underlayer for thin film solar cell applications

•High roughness Ag film was obtained by introducing metal (Al and Cu) underlayers.•Increase of Ag film roughness on Al underlayer is attributed to Ag–Al alloying.•Rough Ag films enhance the light trapping by decreasing reflectance of incoming light.•Ag–Al alloy phase was controlled by varying thickness of Al underlayer and Ag film.•Ag–Al alloy film with 75–118nm roughness was obtained at a low temperature, 350°C.