Efficient kesterite Ag-alloyed Cu2ZnSn(S,Se)4 thin film solar cells with a front Ag grading distribution enabled by ZnO insert layer

Abstract

Large open-circuit voltage deficit is still the main factor restricting the performance of kesterite Cu2ZnSn(S,Se)4 thin film solar cells. The high concentration of CuZn defects and [2CuZn + SnZn] defect clusters are believed to be the significant origins of severe band tailing of the kesterite absorber. A promising way to improve the open-circuit voltage and power conversion efficiency of the device is substituting Ag+ with a larger ionic radius for Cu+ to suppress the defects and construct a front-graded bandgap. However, the Ag doping in kesterite can significantly lower the melting temperature, which results in difficulties in forming an Ag grading distribution across the absorber due to the strong thermal diffusion. Herein, we innovatively introduce a ZnO insert layer in the precursor film, which is expected to slow down the Ag-Cu interdiffusion during the selenization, thereby forming a desired front-graded bandgap. The Ag grading distribution in absorbers is regulated by the thickness of ZnO insert layer. The effects of ZnO insert layer on the phase compositions, morphologies, optical properties, and photovoltaic performance of Ag-doped kesterite absorbers are investigated.