Origin of Excitonic Absorption in Multi-Grain CsPbBr3 Perovskite Nanocrystals for Photodiode Applications

Inorganic halide perovskite nanocrystals exhibit some extraordinary properties compared to their parent materials due to the finite size effect. However, the exciton dynamics in these nanocrystals are not well understood due to the wide variation of structural properties. In this study, we have synthesized single-grain and multi-grain nanocrystals of CsPbBr₃ and analyzed the excitonic absorption and photoluminescence by correlating them with their structural properties. There exist two types of excitonic absorption; low energy localized excitons having binding energy from 64 to 156 meV and near band-edge excitons having binding energy from 18 to 23 meV. Small-size single-grain nanocrystals have a relatively higher bandgap of 2.576 eV due to increased micro-strain compared to multi-facet polyhedral with multi-grain nanocrystals. The broadening of the near band-edge excitonic absorption peak is possibly due to the presence of different sizes, shapes, and nature of the grain boundaries which also modulate the photoluminescence decay lifetime. The overall steady-state photoluminescence is higher in single-grain nanocrystals compared to multi-grain nanocrystals due to the localization of excitons. Therefore, these single-grain nanocrystals can be used for efficient light emission in perovskite photodiodes while multi-grain nanocrystals could be better suited for photodetector application.<br/><br/><b>Keywords</b>: Halide perovskites; Nanocrystals; Excitonic absorption; Grain boundaries; Photoluminescence lifetime.