Free Carriers versus Self-Trapped Excitons at Different Facets of Ruddlesden–Popper Two-Dimensional Lead Halide Perovskite Single Crystals

The physical origin of sub-band gap photoluminescence in Ruddlesden–Poppers two-dimensional (2D) lead halide perovskites (LHPs) is still under debate. In this paper, we studied the photoluminescence features from two different facets of 2D LHP single crystals: the in-plane facet (IF) containing the 2D inorganic layers and the facet perpendicular to the 2D layers (PF). At the IF, the free carriers (FCs) dominate due to the weak electron–phonon coupling in a symmetric lattice. At the PF, the strain accumulation along the 2D layers enhances the electron–phonon coupling and facilitates self-trapped exciton (STE) formation. The time-resolved PL studies indicate that free carriers (FCs) at the IF can move freely and display the trapping by the intrinsic defects. The STEs at the PF are not likely trapped by the defects due to the reduced mobility. However, with increasing STE density, the STE transport is promoted, enabling the trapping of STE by the intrinsic defects.