Understanding the Enhanced Stability of Bromide Substitution in Lead Iodide Perovskites

Lead halide perovskites have rapidly emerged as candidate materials for high-performing solar cells, but show serious issues related to long-term stability. Methylammonium (MA) lead perovskites with mixed iodide-bromide compositions, MAPb(I_1-xBr_x)₃, are reported to exhibit improved stability, but the origin of such behavior is not fully understood. Here, we report new insights into the degradation properties of MAPb(I_1-xBr_x)₃ using ab initio simulations and a range of spectroscopic techniques. Absorbance spectroscopy shows that as the Br content increases, the material stability toward oxygen and light increases. Isothermal gravimetric analysis and time-resolved single photon counting show that the amount of oxygen incorporation into perovskite films decreases significantly with increasing Br content. Ab initio simulations indicate that the degradation reaction involving superoxide species is energetically exothermic for pure MAPbI₃ but becomes less favorable with increasing Br content with an endothermic energy for pure MAPbBr₃, suggesting that the degradation of MAPbBr₃ in the presence of oxygen and light is unfavorable. The simulations indicate shorter N-H...Br hydrogen bonds between the MA⁺ cation and Br ions, which would promote greater structural stability upon bromide substitution. Thin-film passivation with iodide salts is shown to enhance the stabilities of mixed-halide perovskite films and solar cell devices. The greater fundamental understanding of mixed iodide-bromide systems gained from this study is important for the future design of stable perovskite solar cells.