ZnS buffer layer for Cu₂ZnSn(SSe)₄ monograin layer solar cell

Copper zinc tin sulfo-selenide Cu₂ZnSn(SSe)₄ (CZTSSe) is a low-cost alternative semiconductor material that can be used as an absorber in solar cells. CdS deposited by chemical bath deposition (CBD) is the most efficient buffer layer for Cu₂ZnSn(SSe)₄ and Cu(InGa)(SSe)₂ (CIGS) solar cells. However, there is a strong demand for the development of a Cd-free buffer layer due to the toxicity of Cd and its associated concerns with respect to the disposal and long term safety of Cd containing solar modules. In this work, we report for the first time, the successful use of a ZnS buffer layer for CZTSSe monograin solar cell that shows similar functionality level as a CdS buffer layer. ZnS buffer layer was deposited onto CZTSSe absorber layer by employing a scalable non-vacuum CBD method. The effect of morphology, thickness, as well as chemical composition of the ZnS buffer layer on the efficiency of the CZTSSe solar cell was investigated, and the best CZTSSe monograin solar cell had an efficiency of 4.50 (±0.16)%. External quantum efficiency (EQE) showed higher transmission in the blue light region for the ZnS buffer compared to CdS. Increased number of ZnS layers decreased the EQE signal in 400-800nm regions, resulting in decreased J-V parameters, suggesting a single layer of 10-25nm ZnS as a most efficient alternative buffer for CZTSSe.