Origin of Excitonic Absorption in Multigrain CsPbBr3 Perovskite Nanocrystals: Implications in Photodiodes

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 letter, we have synthesized single-grain and multigrain nanocrystals of CsPbBr3 and analyzed the excitonic absorption and photoluminescence by correlating with their structural properties. There exist two types of excitonic absorption: low-energy localized excitons having a binding energy from 65 to 115 meV and near-band-edge excitons having a binding energy from 28 to 37 meV. Small-size single-grain nanocrystals have a relative higher bandgap of 2.60 eV due to increased microstrain compared to a multifacet polyhedron with multigrain 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 multigrain nanocrystals due to localization of excitons. Therefore, these single-grain nanocrystals can be used for efficient light emission in perovskite photodiodes, while multigrain nanocrystals could be better suited for photodetector application.