Critical Roles of Ultrafast Energy Funnelling and Ultrafast Singlet-Triplet Annihilation in Quasi-2D Perovskite Optical Gain Mechanisms

Quasi-2D (Q2D) perovskite possess considerable potential for light emission and amplification technologies. Recently, mixed films containing Q2D perovskite grains with varying layer thicknesses have shown great promise as carrier concentrators, effectively mitigating trap-mediated recombination. In this strategy, photo-excitations are rapidly funnelled down an energy gradient to the thickest grains, leading to amplified spontaneous emission (ASE). However, the quantum-confined Q2D slabs also stabilize the formation of unwanted triplet excitons, resulting in parasitic quenching of emissive singlet states. Here, a novel ultrafast photoluminescence spectroscopy is used to study photoexcitation dynamics in mixed-layer Q2D perovskites. By analysing spectra with high temporal and energy resolution, this is found that sub-picosecond energy transfer to ASE sites is accompanied by excitation losses due to triplet formation on grains with small and intermediate thicknesses. Further accumulation of triplets creates a bottleneck in the energy cascade, effectively quenching incoming singlet excitons. This ultrafast annihilation within 200 femtosecond outpaces energy transfer to ASE sites, preventing the build-up of population inversion. This study highlights the significance of investigating photoexcitation dynamics on ultrafast timescales, encompassing lasing dynamics, energy transfer, and singlet-triplet annihilation, to gain crucial insights into the photophysics of the optical gain process in Q2D perovskites.