Intrinsic defect compensation in the space charge region enables cadmium-free kesterite solar cells to achieve 13.9% certified efficiency

Abstract

Cadmium-free Zn_xSn_1−xO (ZTO) buffer layers are an attractive alternative to conventional CdS in Cu₂ZnSn(S,Se)₄ (CZTSSe) thin-film solar cells, especially with advantages of Cd-related toxicity elimination and parasitic absorption mitigation at short wavelengths. However, the performance of ZTO-based CZTSSe devices has lagged behind that of CdS-involved counterparts, largely due to high-density detrimental defects and non-ideal band alignment at the CZTSSe/ZTO heterojunction. Here, we develop a junction heat-treatment process that, upon thermal activation, selectively drives Zn cations to diffuse from the ZTO buffer into the CZTSSe absorber along the chemical potential gradient. This diffusion-driven Zn incorporation compensates for intrinsic bulk defects such as Cu_Zn and V_Cu, particularly in the space charge region. The cation diffusion also contributes to a more favorable conduction-band offset, prolonged minority-carrier lifetime, and extended carrier-diffusion length. Collectively, this results in optimization of carrier dynamics with simultaneously enhanced carrier separation, extraction, and transport efficiencies. As a result, we achieve an efficiency of 14.39% (certified at 13.90%) and a large-area (1.03 cm²) efficiency of 12.24%, representing the highest-to-date efficiency for Cd-free CZTSSe solar cells.