Flexible high-efficiency CZTSSe solar cells on stainless-steel substrates

Stainless steel (SS) foil is made of abundant materials and is a durable and flexible substrate, but the efficiency of a solar cell on SS foil deteriorates via the diffusion of impurities from the SS substrate into a Cu2ZnSn(S,Se)4 (CZTSSe) absorber layer. In this work, the properties of the diffusion barrier for CZTSSe solar cells is investigated by X-ray diffraction (XRD), secondary ion mass spectrometry (SIMS), and scanning electron microscopy (SEM). The industrially relevant oxide materials ZnO and SiO2 are used as diffusion barriers against impurities. The formation of a ZnSe reaction with Se degrades the barrier properties of the ZnO barrier layer. As a result, ZnO fails to act as a diffusion barrier, and Fe is observed in the absorber layer. On the other hand, the intrinsic diffusion barrier properties of SiO2 are superior to those of ZnO, and SiO2 is a stable diffusion barrier even after selenization. Therefore, SiO2 was applied to flexible solar cells, and a power conversion efficiency of 10.30%, the highest efficiency for CZTSSe on SS foil, was obtained.