Effect of Al addition on the characteristics of Cu(In,Al)Se2 solar cells

Cu(In,Al)Se2 (CIAS) thin films are expected as the absorber of high-efficiency solar cells similar to Cu(In,Ga)Se2 (CIGS). To realize high-efficiency CIAS solar cells, we attempted to deposit CIAS thin films using three-stage evaporation process, which could realize the large crystal grain size and the V-shape band profiling. CIAS thin films with Al/(In+Al) composition ratios of 0.07–0.34 were prepared and the band gap energy was controlled from 1.10 to 1.38 eV. At low Al contents of Al/(In+Al)⩽0.21, a relatively large crystal grain size of 2 μm was obtained, while that became small down to 0.5–1 μm for Al/(In+Al)⩾0.27. As the band gap increases the voltage dependence of photocurrent increases, indicating a poor minority carrier collection. This behavior was similar to that of wide band gap CIGS solar cells.