Transient Spectroscopic Indications of Optical Excitation Induced Phase Separation in MAPbBr_XI_3-X

Hybrid organic inorganic perovskites exhibit multifold advantages in their charge-transport performance and applications in optoelectronic devices. Mixtures of them during the synthesis process enable largely enhanced tunability in both the optical spectroscopic response of the materials and structural design of the light-emitting or photodetection devices. However, phase separation is usually observed in the mixture materials CH₃NH₃PbBr_XI_3-X (MAPbBr_XI_3-X) under optical excitation, which is based on the induced aggregation of iodine ions (I^-) to the positively charged MA⁺, leading to a much modulated photoluminescence (PL) spectrum by a new feature at longer wavelengths. The available reports still lack insights into the dynamics of the responsible mechanisms. In this work, we employ CH₃NH₃PbBr_1.5I_1.5 as a typical example and 80 ps laser pulses at about 405 nm with varied repetition rate as the excitation. In addition to the intrinsic emission of MAPbBr_1.5I_1.5 at about 658 nm, a new spectral feature centered at about 734 nm is identified as the emission from the new phase with I^--enrichment. Focusing on the emission spectrum around 734 nm, we were able to investigate how the phase-separation effect depends on the repetition rate, pulse energy, average power, and interaction time duration. Thus, these transient spectroscopic indications facilitate comprehensive understanding of the phase-separation mechanisms and determine the typical spectroscopic features accordingly in MAPbBr_XI_3-X.