Flexible high-efficiency CZTSSe solar cells on stainless steel substrates

Abstract

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.