Octahedral Distortions Generate a Thermally Activated Phonon-Assisted Radiative Recombination Pathway in Cubic CsPbBr₃ Perovskite Quantum Dots

Exciton-phonon interactions elucidate structure-function relationships that aid in the control of color purity and carrier diffusion, which is necessary for the performance-driven design of solid-state optical emitters. Temperature-dependent steady-state photoluminescence (PL) and time-resolved PL (TRPL) reveal that thermally activated exciton-phonon interactions originate from structural distortions related to vibrations in cubic CsPbBr3 perovskite quantum dots (PQDs) at room temperature. Exciton-phonon interactions cause performance-degrading PL line width broadening and slower electron-hole recombination. Structural distortions in cubic PQDs at room temperature exist as the bending and stretching of the PbBr6 octahedra subunit. The PbBr6 octahedral distortions cause symmetry breaking, resulting in thermally activated longitudinal optical (LO) phonon coupling to the photoexcited electron-hole pair that manifests as inhomogeneous PL line width broadening. At cryogenic temperatures, the line width broadening is minimized due to a decrease in phonon-assisted recombination through shallow traps. A fundamental understanding of these intrinsic exciton-phonon interactions gives insight into the polymorphic nature of the cubic phase and the origins of performance degradation in PQD optical emitters.