Light‐Induced Frenkel Defect Pair Formation Can Lead to Phase‐Segregation of Otherwise Miscible Halide Perovskite Alloys

Alloys of ABX 3 halide perovskites (HP) exhibit unique phase behavior compared to traditional III‐V and II‐VI semiconductor alloys used in solar cells. While the latter typically have good mutual miscibility when their mixed components are size matched, and phase‐segregate when size mismatched, HP alloys show good miscibility in the dark but can phase‐segregate under light. Quantum mechanical calculations described herein reveal light‐induced defect formation and migration hold the key. Specifically, the interaction between a halogen vacancy V X with halogen interstitial X i forming together a Frenkel‐pair defect emerges as the enabler for phase‐segregation in HP alloys. At a threshold bromine composition in the Br‐I alloys, the photogenerated holes in the valence band localize, creating thereby a doubly‐charged iodine Frenkel‐pair (V I + I i ) 2+ . Faster migration of iodine over bromine interstitial into the vacant iodine V I site leads to the formation of iodine‐rich and iodine‐depleted regions, establishing phase‐segregation. Removal of the mobile defects–the agent of segregation–by dark thermal annealing, supplies the opposing force, leading to reversal of phase‐segregation. This atomistic understanding can enable some control of the phase‐segregation by selecting substituting elements on the B site–such as replacing some Pb by Sn–that are unable to form stable Frenkel defects.