Unveiling the roles of halogen ions in the surface passivation of CsPbI3 perovskite solar cells

Halide ion passivation is an effective way to improve the stability and the power conversion efficiency (PCE) of perovskite solar cells. In this work, the passivation mechanism of the surface iodine vacancies of inorganic perovskite CsPbI3 films by halogen ions (F-, Cl-, and Br-) has been studied using the first-principles method. Due to its high electronegativity, the F ion withdraws electron density out of its neighboring atoms, readily forms ionic bonds with Pb atoms and has a coupling effect with the nearest neighbor Cs atoms, which can alleviate the generation of cation vacancy and ion migration to locally stabilize the structure of the perovskite. The fluorinated CsPbI3 (001) surface has a lower surface energy, which improves the grain growth of perovskite films. Different from F-, the passivation via Cl- or Br- ions can effectively prevent the charge accumulation on the film surface, reduce the exciton binding energy of CsPbI3, and eliminate the loss of optical absorption intensity in the visible light range caused by iodine vacancies. These results provide a deep understanding about surface passivation by halogen ions for perovskite solar cells.