Suppressing Deep-Level Trap and Energy Band Alignment Enable Air-Solution-Processed Cu2ZnSnS4 Thin Film Solar Cell with Certified Record Efficiency

The sulfide kesterite Cu2ZnSnS4 (CZTS) is widely recognized as a highly competitive photovoltaic material, especially for use in multi-junction solar cells, due to its excellent photoelectric properties. However, the power conversion efficiency (PCE) of CZTS solar cells has remained stagnant for years. The presence of deep level defects, such as VS (sulfur vacancies), has been demonstrated to provide a kind of effective non-radiative recombination pathway, leading to serious open-circuit voltage (VOC) deficit and reduced efficiency in carrier migration and separation. Herein, we propose a novel directional passivation strategy for VS through a an easily operable heterojunction heat treatment. By introducing O2 during this process, sulfur vacancies can be readily occupied by oxygen atoms in an oxygen-rich environment, thus contributing to eliminating the already-existing VS defects. In addition, the thermal movement of ions during this process induces Cd diffusion to the absorption layer to occupy the Cu or Zn lattice sites, these collective effects lead to a reduced charge recombination and more favorable band alignment. As a result, CZTS thin film solar cell with a PCE of 11.89% and a certified record efficiency of 11.51% was achieved. The present study offers valuable insights into defect passivation mechanism of kesterite thin-film solar cells.