Exciton versus free carrier emission: Implications for photoluminescence efficiency and amplified spontaneous emission thresholds in quasi-2D and 3D perovskites

Abstract: Among perovskite semiconductors, quasi-two-dimensional (2D) materials are attractive for the pursuit of electrically driven lasing given their excellent performance in light-emitting diodes (LEDs) and their recent success in continuous-wave optically pumped lasing. We investigate the spontaneous photoluminescence emission and amplified spontaneous emission (ASE) of a series of quasi-2D emitters, and their directly analogous 3D materials formed by removing the 2D organic spacer by annealing. Although the PL photoluminescence (PL) (at low optical excitation power) from quasi-2D films with high 2D spacer fractions can be much brighter than that from their 3D counterparts, the ASE thresholds of these quasi-2D materials tend to be higher. This counter-intuitive behavior is investigated through time-resolved photophysical studies, which reveal the emission in the high-spacer-content quasi-2D perovskite can be exclusively excitonic, and the exciton–exciton annihilation of quasi-2D perovskite starts to take over the exciton dynamics at a low exciton density (<1016?cm−3). To lower ASE thresholds in quasi-2D materials it is necessary to increase the volume fraction of thick quantum wells, which we achieve by decreasing the spacer content or by utilizing 1-naphthylmethylamine (NMA) linkers. The increase of the volume fraction of thick quantum wells correlates with an increased contribution of free carrier recombination to the emission process of the quasi-2D materials. These results suggest that material development of quasi-2D materials for gain applications should target fast free charge carrier recombination rates by engineering the well thickness and size and not maximum photoluminescence quantum yields under low power excitation.