Continuous-wave optically pumped lasing from quasi-two-dimensional (2D) perovskite has been demonstrated recently. Understanding the key factors limiting the lasing performance is highly relevant for pursuing electrically driven lasing based on such solution-processed quasi-2D perovskite materials. In this work, we compare the photoluminescence (PL) and amplified spontaneous emission (ASE) of quasi-2D perovskite (CsPbBr3 with 80% butylammonium bromide (BABr) spacer) and its 3D counterpart formed by thermal removing the organic spacers. Surprisingly, the superior PL (at low excitation fluence) of quasi-2D perovskite does not translate into a lower ASE threshold (~600 μJ cm^-2). Annealing the quasi-2D perovskite into the 3D perovskite results in a lower ASE threshold (~130 μJ cm^-2) despite the much poorer PL efficiency at low excitation fluence of the 3D perovskite.

Power-dependent time-resolved PL revealed that the excited-state population of the quasi-2D perovskite based on the 80% BA spacer is dominated by excitons. This accounts for the superior PL at low fluence, as the excitonic emission is efficient at low excited-state densities, whereas the free-carrier-recombination-based PL of the 3D materials is a second-order process and needs higher excited-state densities to become efficient. However, at the rather low excited-state density of 10¹⁶ cm^-3, exciton–exciton annihilation sets in for the quasi-2D sample and results in a decreasing radiative efficiency at high excitation power. In contrast, the free-carrier radiative recombination leads to a high radiative efficiency steadily increasing to the transparency carrier density (10¹⁸ cm^-3), which explains its lower ASE threshold.

We further systematically investigate the ASE of a series of quasi-2D perovskites through varying the spacer type and concentration. The experimental results show that the optimum ASE threshold of quasi-2D perovskite films can be achieved by minimizing the surface roughness and thin QWs volume fraction through decreasing the spacer contents or utilizing 1-naphthylmethylamine bromide spacer. Time-resolved photophysical studies manifested that the increase of thick QWs volume fraction correlates with an increased contribution of free carrier radiative recombination to the total emission process of the quasi-2D perovskites. Our work suggests a detailed understanding of the excited-state population(s) in quasi-2D perovskites, and their dynamics is essential for engineering their gain performance. The concrete guidance for material development that our results suggest is that quasi-2D perovskite gain materials should target fast free carrier recombination by engineering the thickness and size of QW, but not maximum PL quantum yields under low power excitation.