Unraveling the Radiative Pathways of Hot Carriers upon Intense Photoexcitation of Lead Halide Perovskite Nanocrystals

The slowdown of carrier cooling in lead halide perovskites (LHP) may allow the realization of efficient hot carrier solar cells. Much of the current effort focuses on the understanding of the mechanisms that retard the carrier relaxation, while proof-of-principle demonstrations of hot carrier harvesting have started to emerge. Less attention has been placed on the impact that the energy and momentum relaxation slowdown imparts on the spontaneous and stimulated light-emission process. LHP nanocrystals (NCs) provide an ideal testing ground for such studies as they exhibit bright emission and high optical gain, while the carrier cooling bottleneck is further pronounced compared to their bulk analogues due to confinement. Herein, the luminescent properties of CsPbBr3, FAPbBr(3), and FAPbI(3) NCs in the strong photoexcitation regime are investigated. In the former two NC systems, amplified spontaneous emission is found to dominate over the radiative recombination at average carrier occupancy per nanocrystal larger than 5-10. On the other hand, under the same photoexcitation conditions in the FAPbI(3) NCs, a longer lived population of hot carriers results in a competition between hot luminescence, stimulated emission, and defect recombination. The dynamic interplay between the aforementioned three emissive channels appears to be influenced by various experimental and material parameters that include temperature, material purity, film morphology, and excitation pulse width and wavelength.