Self-trapping effect on the excitonic and polaronic properties of a single-layer 2D metal-halide perovskite

Abstract

Two-dimensional metal-halide perovskites (2DMHPs) have attracted tremendous interest due to their promising applications in the light-harvesting and light-emitting diodes. Recently, intensive experiment observations suggested the lattice of 2DMHPs should be very soft, such that photoexcitations would interact to the surrounding lattice distortions through the electron-lattice (e-l) couplings and led to the formation of self-trapped exciton (STE). However, the detailed formation mechanism has never been clearly clarified from the theoretical point of view. In this theoretical study, we build an effective tight-binding model for a single-layer 2DMHP (S2DMHP) with the e-l coupling considered as the Holstein type. This model successfully describes the ground-state optoelectronic properties and generates the primary non-linear elementary excitations which have been observed in a series of experiments. Our calculation reveals that the exciton state of a S2DMHP will exhibit significant polaronic characters. Depending on the e-l coupling strength and the level-occupancy configuration of the electron-hole pairs, such polaronic states could behave as either large polarons or small polarons. Large polarons range over multiple unit cells and induce the local deformation of the surrounding crystalline structure. Contrarily, small polarons are strongly bound and being trapped within the single octahedras. The existence of large and small polarons respectively explain the relative lower charge mobilities and the peculiar broadened white-light emissions in some selected S2DMHPs. The present work highlights the essential role of e-l couplings in determination of the excitonic properties of S2DMHPs and provides a theoretical foundation to deeply understand the optoelectronic behavior of S2DMHPs-based semiconductor devices.