Controlling the S/(S + Se) ratio in Co-doped CZTSSe films via optimizing the annealing condition for highly efficient solar cells

• A new method to improve V oc and PCE of CZTSSe solar cell by Co doping is put forward. • This is the first report on the effect of post-selenization on Co-doped CZTSSe. • Adjusting S/(S + Se) ratio can improve surface morphology and passivate defects. • The effect of annealing conditions on PCE is ascribed to change of V OC and FF. Selenium-rich Cu 2 ZnSn(S,Se) 4 (CZTSSe) absorber with large-grained structure is a significant contribution to the preparation of efficient CZTSSe solar cells. Therefore, it is important to find an applicable method to adjust the S/(S + Se) ratio in CZTSSe films. Here, the influence of annealing temperature and time on the regulation of S/(S + Se) ratio in the absorption layer was studied based on the preparation of Co-doped CZTSSe films by sol–gel method. The results show that the ratio of S/(S + Se) in Co-doped CZTSSe films decreases with increasing annealing temperature and time. Adjusting the S/(S + Se) ratio can significantly improve the surface morphology of the Co-doped CZTSSe absorber and passivate the grain boundary defects. Co-doped CZTSSe films have the best surface morphology under the optimal annealing conditions of 560 °C for 15 min, which significantly inhibits Cu Zn antisite defects and [2Cu Zn + Sn Zn ] defect clusters. At last, the Co-doped CZTSSe solar cells with the highest efficiency of 7.46 % were obtained under the optimum annealing condition. The optimal filling factor reaches 62.10 %, which is at the high level of the reported kesterite solar cells.