Property comparison of flexible Cu(InGa)Se2 thin film solar cells on Ti and Ni foils without diffusion barrier

Flexible Cu(InGa)Se2 (CIGS) solar cells on metallic substrates are highly desirable for versatile energy applications. However, related researches reported on CIGS devices usually used a diffusion barrier, which involves an issue of thermal expansion coefficient matching and complicates the fabrication process. In this study, CIGS solar cells were fabricated on Ni and Ti foils through selenization of metal stacks deposited by e-beam evaporation, without the use of diffusion barrier. The films on Ti foils showed (112) preferred orientation while those on Ni foils exhibited (220) preferred orientation, due to the different crystal orientation of substrates. In–Ni and Ga–Ni phase were observed in films prepared on Ni foils, which led to the existence of Cu2–xSe. As a result, the CIGS solar cells fabricated on Ni foils showed inferior photoelectric properties, whereas those on 50 and 100 μm Ti foils presented conversion efficiency of 4.9% and 6.6%. The discrepancy on efficiency between two cells on Ti foils was ascribed to the different structural properties induced by the thermal stress. After 100 bending cycles, efficiency of cells on 50 and 100 μm thick Ti foils decreased by 1.0% and 2.6% respectively, indicating that CIGS solar cell on 50 μm Ti foil is more promising as flexible device applied on harsh circumstance.