Enhanced stability of triple-halide perovskites CsPbI3-x-yBrxCly (x and y = 0-0.024): understanding the role of Cl doping from ab initio calculations.

Abstract

Doping of chloride in mixed iodide-bromide perovskites has been shown experimentally to suppress the photo-induced halide-ion segregation and enhance the stability of triple-halide perovskites (THP). However, a fundamental understanding of the effects of Cl doping is yet to be achieved especially when the doping concentration is low. Here we report the results of a state-of-the-art ab initio study of the atomic structure of THP by considering small doping concentrations of Br and Cl in CsPbI3. We find a reduction in the Pb-I bond lengths and tilting of PbI6 octahedra with Cl doping which lead to exothermic heat of mixing and therefore higher stability of THP. Moreover, using quasi-chemical approximation, our results show that there is a very small contribution of configurational entropy to Gibbs free energy at such low doping concentrations and at the operational temperature of 50 °C. This suggests that the favorable heat of mixing value is more important for the stability at low doping concentrations of Cl while a higher concentration of Cl increases the risk of halide segregation. Further calculations on Frenkel defect formation energy of I or Br-interstitial shows that the doping of Cl in I/Br mixed binary-compounds hinders the formation of Frenkel defects. These results support experiments and help to understand the role of chloride in suppressing the halide ion mobility with only a slight increase in the band gap. Accordingly, the THPs manifest a promising pathway for developing single-phase perovskites for solar cells and light-emitting diodes with improved performance and enhanced stability.