Size matters - Amplified spontaneous emission limited by accumulation of dark excitons in thin quasi-2D perovskites

Two-dimensional lead halide perovskites have received much attention in the field of light emission and amplification due to their bright emission and synthetic tunability. Recently, mixed films comprised of perovskite grains with different layer thickness have been shown to act as carrier concentrators in light-emitting devices while outcompeting trap mediated recombination. However, it has been suggested that quasi-two-dimensional (Q2D) slabs stabilize the formation of unwanted triplet excitons, potentially resulting in parasitic nonradiative losses and the quenching of remaining bright singlet states. In our investigation on the effect of out-of-plane confinement on the ability to accumulate long-lived triplet excitons, ultrathin highly-confined domains were found to stabilize these dark states. The ensuing ultrafast annihilation of bright states was shown to outpace the energy transfer to the amplified spontaneous emission (ASE) sites, preventing the build-up of the critical carrier density for population inversion and lasing. Our results highlight the need to control the inorganic layer thickness in order to avoid the presence of highly excitonic slabs in the development of efficient Q2D perovskite films for optical gain applications.