Role of Mixed Halide on Photophysical Properties of Organic-Inorganic Hybrid Perovskite Compounds

Increasing energy demands and global warming stimulate research in alternative energy for subsequent generations. Utilizing solar energy can be ultimate goal to satisfy energy demands in the future. Organic-inorganic hybrid perovskite is currently one strong candidate for next generation solar cells. To apply the perovskite compounds to solar cell devices, it is undebatable that one must study the fundamental photophysical properties and basic chemistry of these perovskite.

Varying the halide ratio (e,g., Br^-:I^-) is a convenient approach to tune the bandgap of organic lead halide perovskites. The complexation between Pb²⁺ and halide ions is the primary step in dictating the overall composition, and optical properties of the annealed perovskite structure. Preferable complexation between Pb²⁺ and Br^- dominates the overall absorption/emission properties of the mixed halide perovskite.

In the presence of both bromide and iodide anions in the perovskite film, a reversible photoinduced segregation process occurred generating bromide-rich and iodide-rich domains. This intriguing aspect of halide ion migration can be monitored by both emission and transient absorption spectroscopic characterizations. Both the emission and transient absorption spectroscopy elucidates photophysical evidence of the two segregated domains and ultrafast charge transfer from the bromide-rich to iodide-rich domains. Also, tracking formation and recovery processes through absorption/transient absorption studies, we found that phase separation occurs with a rate constant of 0.1-0.3 s^-1. The recovery occurs over a time period of several minutes-hour. Furthermore, at higher halide concentrations (more than stoichiometric composition), the segregation effects become less prominent as evident from the faster recovery kinetics in the preferred CH₃NH₃PbBr₃ film formation. This finding suggests that the exclusive binding of Br^- to Pb²⁺ plays a role in minimizing the photoinduced segregation process. Also, the slower formation and recovery rates observed with halide deficient films indicate the involvement of defect sites in influencing the segregation process. The findings from this study further reflects the importance of halide treatment (e.g., CH₃NH₃PbI₃) of perovskite films to improve the performance of solar cells.