Coupling to Octahedral Tilts in Halide-Perovskite Nanocrystals Induces Phonon-Mediated Attractive Interactions between Excitons

Here, we show that photoexcitation drives a reduction of the lead-halide-lead bond angles, a result of deformation potential coupling to low energy optical phonons. We accomplish this by performing femtosecond-resolved, optical-pump, electrondiffraction- probe measurements to quantify the lattice reorganization occurring as a result of photoexcitation in nanocrystals of FAPbBr3 and CsPbBr3.

Our results indicate a stronger coupling in FAPbBr3 compared to CsPbBr3. We attribute the enhanced coupling in FAPbBr3 to its disordered crystal structure, which persists down to cryogenic temperatures. We find the reorganizations induced by each exciton in a multi-excitonic state constructively interfere, giving rise to a coupling strength which scales quadratically with the exciton number.