Ge Bidirectional Diffusion to Simultaneously Engineer Back Interface and Bulk Defects in the Absorber for Efficient CZTSSe Solar Cells

Abstract Aiming at a large open‐circuit voltage (V_OC) deficit in Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells, a new and effective strategy to simultaneously regulate the back interface and restrain bulk defects of CZTSSe absorbers is developed by directly introducing a thin GeO₂ layer on Mo substrates. Power conversion efficiency (power‐to‐efficiency) as high as 13.14% with a V_OC of 547 mV is achieved for the champion device, which presents a certified efficiency of 12.8% (aperture area: 0.25667 cm²). Further investigation reveals that Ge bidirectional diffusion simultaneously occurs toward the CZTSSe absorber and MoSe₂ layer at the back interface while being selenized. That is, some Ge element from the GeO₂ diffuses into the CZTSSe absorber layer to afford Ge‐doped absorbers, which can significantly reduce the defect density and band tailing, and facilitate quasi‐Fermi level split by relatively higher hole concentration. Meanwhile, a small amount of Ge element also participates in the formation of MoSe₂ at the back interface, thus enhancing the work function of MoSe₂ and effectively separating photoinduced carriers. This work highlights the synergistic effect of Ge element toward the bulk absorber and the back interface and also provides an easy‐handling way to achieve high‐performance CZTSSe solar cells.