Given the prominent success of the Ga gradient in CuIn 1−x Ga x Se 2 (CIGSe) solar cells, Ge gradient implementation is a promising way to boost Cu 2 ZnSn(S,Se) 4 (CZTSSe) solar cells. However, Ge-graded CZTSSe solar cells only possess a low efficiency of 9.2%, far from that of Geincorporated CZTSSe without a gradient (12.3%). Herein, we demonstrated a shallow Ge gradient CZTSe solar cell with an improved efficiency over 10%. The Ge gradient was achieved through a GeSe 2 −Se coselenization process, where GeSe 2 acts as a low-temperature fluxing agent to assist crystallization and induce Ge transport toward the back interface. The relieved band tails and improved junction quality, leading to a better carrier separation, were found to take a primary responsibility for device improvement. These results highlight a remarkable breakthrough for Ge-graded CZTSe solar cells and offer a promising way to develop Ge-involved solar cells.

Sn-free Cu2ZnGeSe4 (CZGSe) is emerging as a promising non-toxic and earth-abundant photovoltaic absorber material due to its attractive electrical and optical properties as well as its high theoretical conversion efficiency....