Photoinduced Frenkel Pair Formation Drive Phase-Segregation in Halide Perovskite Alloys*

Alloys of ABX₃ halide perovskite (HP) display unique phase behavior distinct from traditional III-V and II-VI semiconductor alloys. While the latter alloys phase-segregate based on size mismatch, alloys of HP with different halogens exhibit good miscibility in the dark but segregate under light. Our DFT alloy calculations¹ reveal that the key factors are (i) light-induced defect formation and (ii) atomic migration of dissociated members. Specifically, Halogen vacancies (V_X) and interstitials (X_i) form Frenkel-pair defects, which drive phase segregation in HP alloys. At a threshold bromine composition, photogenerated holes localize, creating a doubly-charged iodine Frenkel-pair (V_I+I_i)²⁺. Faster iodine migration over bromine interstitials leads to iodine-rich and iodine-depleted regions, which results in phase segregation. Dark thermal annealing removes mobile defects, reversing the segregation. This understanding enables control of phase segregation by selecting elements (e.g., replacing Pb with Sn) that cannot form stable Frenkel defects on the B site, offering potential in solar technology.

[1] F.P. Sabino et al. Adv. Energy Mater. 2023, 2301539.