Hot-Phonon Bottleneck Enables Biphasic Carrier Cooling with Density-Independent Rates in CsPb(Cl/Br)(3) Perovskite Quantum Dots.

Hot-carrier solar cells can achieve higher efficiency by utilizing excess carrier energy before thermalization. The hot-phonon bottleneck delays phonon decay and allows their reabsorption by carriers, delaying carrier cooling and retaining excess energy. This study investigates the hot-phonon bottleneck effect and biphasic cooling dynamics in CsPb(Cl/Br) perovskite quantum dots. We modeled time-resolved emission spectra to analyze carrier cooling dynamics. At high carrier densities, the cooling curves revealed two distinct phases. Phase I, driven by Fröhlich interactions, exhibits cooling times independent of carrier density, indicating a constant average number of longitudinal optical (LO) phonon emission per carrier. Similarly, phase II, influenced by the hot-phonon bottleneck, also displayed cooling times unaffected by carrier density. However, the overall average cooling time increased with increasing carrier density due to a fluence-dependent amplitude for phase II. Increased LO phonon lifetimes at high carrier densities facilitate their reabsorption by carriers, delaying cooling. These findings elucidate the dual-phase cooling dynamics and, more importantly, their carrier-density-independent rates, offering critical insights into slow carrier cooling necessary for hot-carrier solar cell advancement.