Near-bandgap emission in [HOC2H4NH3]2PbI4 perovskite under hydrostatic pressure: emission of a free exciton and a polaronic exciton

Abstract

Two-dimensional metal-halide perovskites, including EA₂PbI₄ (EA = HOC₂H₄NH₃⁺, ethanolammonium), are very soft materials and therefore they exhibit unique excitonic properties, significantly different from those known for conventional semiconductor quantum wells or two-dimensional van der Waals crystals. These differences should be even more pronounced for excitonic properties under hydrostatic pressure. In this work, we show that photoluminescence studies of EA₂PbI₄ under hydrostatic pressure at low temperatures very clearly reveal the nature of excitonic transitions in this crystal. The near-bandgap emission (NBE) consists of two peaks, one of which is related to the recombination of free excitons (FE), and the other observed at lower energy is assigned to a polaronic exciton, i.e., FE containing a large polaron (FEP). As hydrostatic pressure increases, the polaron formation energy increases and therefore the FEP emission is enhanced and redshifts by a factor of 1.5 relative to the FE emission. In the configuration diagram that was adapted to explain the NBE, the observed changes in emissions under hydrostatic pressure can be explained by a shift of the FEP potential minimum on both the energy axis and the configuration coordinate axis. This allows us to explain the spectral shifts of FE and FEP, as well as the appearance of additional peaks for FE and FEP emission.