Effect of composition gradient in Cu(In,Al)Se2 solar cells

Cu(In,Al)Se2 (CIAS) thin films were prepared by a three-stage evaporation process. In this experiment, the composition ratio of Cu/(In+Al) at the end of the second stage (Cu/III2nd) was changed from 1.1 to 1.7. The CIAS films showed an Al distribution with a V-shape profile. The valley depth of the V-shape from the surface increased with increasing the Cu/III2nd ratio. The valleys of the V-shape for the films with the Cu/III2nd ratio of 1.1–1.7 were located at approximately 0.3–1.0 ?m from the film surface, respectively. The rms surface roughness increased from 40 nm for Cu/III2nd=1.1 to 90 nm at Cu/III2nd=1.3 and then saturated for greater Cu/III2nd ratios. Solar cells with the Al/ITO/ZnO/CdS/CIAS/Mo/soda-lime glass structure were fabricated. The fill factor was seen to decrease while the product of short-circuit current and open-circuit voltage remained constant. The reverse saturation current increased when the Cu/III2nd ratio is greater than 1.3 which is a behavior of the surface roughness. Cu/III2nd ratios greater than 1.3 lead to the distant position of V-shape from the surface and the increase in surface roughness.