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

Aiming at a large open‐circuit voltage (VOC) 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 VOC 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. Ge bidirectional diffusion, simultaneously toward the Cu2ZnSn(S,Se)4 (CZTSSe) absorber and MoSe2 at the back interface, is realized by an easy‐handling method. The crystal growth of the absorbers is regulated and the work function of MoSe2 is enhanced as well. Up to 13.14% efficiency is achieved for Ge‐involved CZTSSe solar cells, which present a certified efficiency of 12.8%.