Restraining the Band Fluctuation of CBD‐Zn(O,S) Layer: Modifying the Hetero‐Junction Interface for High Performance Cu2ZnSnSe4 Solar Cells With Cd‐Free Buffer Layer

Zn(O,S) film is a promising low‐cost and environment‐friendly Cd‐free buffer layer for chalcopyrite and kesterite thin film solar cells. However, the devices with Zn(O,S) buffer layer usually suffer from poor interface performance, resulting in a much lower efficiency, especially for kesterite solar cells. Here, the band fluctuation caused by ZnO secondary phase in Zn(O,S) layer is identified as the main reason deteriorating the device performance. By a concentrated ammonium etching and subsequent soft annealing treatment, the detrimental ZnO and Zn(OH)2 secondary phases are eliminated from the Zn(O,S) layer and the hetero‐junction performance is improved significantly. Consequently, the power conversion efficiency of the Zn(O,S)/CZTSe solar cells was improved from 1.17% to a favorable value of 7.2%. Temperature dependent J–V properties reveal a defect level assisted charge carrier transport mechanism across the Zn(O,S)/CZTSe interface. These encouraging results imply that Zn(O,S) buffer layer is a promising substitution for toxic CdS in future manufacturing of high performance thin film solar cells. The hetero‐junction interface performance of Cd‐free CuZnSnSe4/Zn(O,S) solar cell is improved significantly by restraining the band fluctuation of chemical bath deposited Zn(O,S) layer, using an etching treatment in concentrated ammonium and a soft annealing process. This interface treatment consequently gives rise to the significantly improved device performance with 7.2% efficiency.