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 ( V OC ) deficit in Cu 2 ZnSn(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 GeO 2 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 cm 2 ). Further investigation reveals that Ge bidirectional diffusion simultaneously occurs toward the CZTSSe absorber and MoSe 2 layer at the back interface while being selenized. That is, some Ge element from the GeO 2 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 MoSe 2 at the back interface, thus enhancing the work function of MoSe 2 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.