Fate of Optical Excitons in FAPbI<sub>3</sub> Nanocube\nSuperlattices

Understanding the\nnature of the photoexcitation and ultrafast charge\ndynamics pathways in organic halide perovskite nanocubes and their\naggregation into superlattices is key for potential applications as\ntunable light emitters, photon-harvesting materials, and light-amplification\nsystems. In this work, we apply two-dimensional coherent electronic\nspectroscopy (2DES) to track in real time the formation of near-infrared\noptical excitons and their ultrafast relaxation in CH(NH<sub>2</sub>)<sub>2</sub>PbI<sub>3</sub> nanocube superlattices. Our results\nunveil that the coherent ultrafast dynamics is limited by the combination\nof the inherent short exciton decay time (≃40 fs) and the dephasing\ndue to the coupling with selective optical phonon modes at higher\ntemperatures. On the picosecond time scale, we observe the progressive\nformation of long-lived localized trap states. The analysis of the\ntemperature dependence of the excitonic intrinsic line width, as extracted\nby the antidiagonal components of the 2D spectra, unveils a dramatic\nchange of the excitonic coherence time across the cubic to tetragonal\nstructural transition. Our results offer a new way to control and\nenhance the ultrafast coherent dynamics of photocarrier generation\nin hybrid halide perovskite synthetic solids.