Direct Sputtering Deposition of a Zn1−xGexOx+1 Buffer Layer With a Controllable Conduction Band for Cd-Free Cu(In,Ga)S2 Solar Cells

Abstract

Cu(In,Ga)S₂ (CIGS) is a promising candidate material for the absorber layer in tandem solar cells, but the unfavorable conduction band offset with the conventional CdS buffer layer means that an alternative n-type buffer layer material is required. Here, n-type Zn_1−xGe_xO_x+1 with x = 0–1 (ZnGeO) thin films are fabricated to serve as the buffer layer for CIGS solar cells. The [Ge]/([Zn]  +  [Ge]) ratio can be adjusted by a cosputtering deposition technique to control the optical bandgap of the ZnGeO films from 3.3 to 4.7 eV and the electron affinity from 4.1 to 3.2 eV. In experiments, ZnGeO buffer layers with varying [Ge]/([Zn]  +  [Ge]) ratios (0–0.44) are directly deposited by sputtering on CIGS absorber layers. Increasing the [Ge]/([Zn]  +  [Ge]) ratio increases the open-circuit voltage from 0.282 to 0.701 V and enhances the built-in potential from 1.03 to 1.54 eV, which indicates a strengthened electric field at the heterojunction. The device with the ZnGeO buffer layer demonstrates a conversion efficiency comparable to that of a device fabricated with a conventional CdS buffer layer, which highlights the potential of the sputtering deposition of ZnGeO as a promising and eco-friendly alternative n-type buffer layer for CIGS solar cells.