Illumination-induced phase segregation and suppressed solubility limit in Br-rich mixed-halide inorganic perovskites

Mixing halides in perovskites has emerged as an effective strategy for tuning the band gap for optoelectronic applications and tackling the stability bottleneck. However, notable photoluminescence evolution has been observed in mixed-halide perovskites under external stimuli such as light illumination, which is attributed to phase segregation with halide inhomogeneity. In this work, we investigate the light illumination effect on the optical properties of all-inorganic mixed-halide perovskite CsPb(Br₁-_xI_x)₃ in the Br-rich regime. It is found that the critical iodine concentration, defined as the solubility limit against phase segregation, is significantly suppressed by light illumination to an extremely low level (x < 0.025), although the formation energy calculation suggests a wide range of halide mixing. Furthermore, at high I concentrations (x ≥ 0.2), the phase segregation can be rectified via dark storage within 1 h, but much slower and incomplete reversibility is observed at lower I concentrations. In the all-inorganic mixed-halide perovskite films, the light-induced phase segregation above the solubility limit is also accompanied by a monotonous increase in fluorescence lifetime. Last, we propose that light-induced phase segregation enables the potential application of encrypting erasable information in perovskite films with the aid of tailored light exposure and photoluminescence mapping.