Cd-Free Flexible Cu(In,Ga)Se2Thin Film Solar Cells with ZnS(O,OH) Buffer Layers on Ti Foils

Cd-free flexible Cu(In,Ga)Se 2 (CIGS) solar cells with ZnS(O,OH) buffer layers were fabricated on Mo-coated Ti foils. The CIGS thin films were deposited by the three-stage process at substrate temperatures of 460–550 °C. The influence of the substrate temperature during CIGS deposition on film properties and cell performance has been mainly investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), and secondary ion mass spectroscopy (SIMS) analyses. A double grading of Ga/(In+Ga) atomic ratio required to achieve high efficiency devices was formed at a substrate temperature of 500 °C. The best ZnS(O,OH)/CIGS solar cell yielded a total area efficiency of 17.9% by optimizing cell fabrication conditions. The importance of surface smoothness of Ti foil substrates is also discussed.