Ge‐Doped CZTSe Solar Cell Efficiency Beyond 14% by Suppressing Recombination

Abstract Recombination is one of the main factors that cause the large open‐circuit voltage deficit (V_{oc, def}) in Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells. To mitigate this issue, an effective strategy is developed to suppress the recombinations in CZTSe devices by doping Ge in the absorber. A certified efficiency of 14.67%(total area: 0.3143 cm², without an anti‐reflective layer) was achieved in optimally Ge‐doped CZTSe solar cells. In addition to promoting the grain growth, Ge doping induces more pronounced downward band‐bending, effectively passivating GBs and suppressing recombination in the quasi neutral region (QNR). While optimal Ge doping reduces the number of Cu_Zn defects, excessive Ge doping leads to additional Sn loss, resulting in the formation of deep‐level defects such as V_Sn. Furthermore, the incorporation of Ge resulted in the formation of residual SnSe_x secondary phases near the rear interface, highlighting the need for precise cation ratio adjustments in the precursor films. This work reveals the mechanism underlying Ge doping, demonstrating it potential to mitigate the high recombination velocity at GBs, providing a pathway for achieving high‐efficiency CZTSe solar cells.