The Impact of Partial Carrier Confinement on Stimulated Emission in Strongly Confined Perovskite Nanocrystals

Semiconductor lead halide perovskites are excellent candidates for realizing low threshold light amplification due to their tunable and highly efficient luminescence, ease of processing, and strong light–matter interactions. However, most studies on optical gain have addressed bulk films, nanowires, or nanocrystals that exhibit little or no size quantization. Here, we show by means of a multitude of optical spectroscopy methods that small CsPbBr₃ nanocrystals (NCs) exhibit a progressive red shift of the band-edge transition upon addition of electron–hole pairs, at least one carrier of which occupies a 2-fold degenerate, delocalized state in agreement with strong confinement. We demonstrate that this combination results in a threshold for biexciton gain, well below the limit of one electron–hole pair on average per NC. On the other hand, both the luminescent lifetime and the optical Stark effect of 4.7 nm CsPbBr₃ NCs indicate that the oscillator strength of the band-edge transition is considerably smaller than expected from the band-edge absorption. We assign this discrepancy to a mixed confinement regime, with one delocalized and one localized charge carrier, and show that the concomitant reduction of the oscillator strength for stimulated emission accounts for the surprisingly small material gain observed in small NCs. The conclusion of mixed confinement aligns with studies reporting small and large polarons for holes and electrons in lead halide perovskite nanocrystals, respectively, and creates opportunities for understanding multiexciton photophysics in confined perovskite materials.