Inter-Layer Diffusion of Excitations in 2D Perovskites Revealed by Photoluminescence Reabsorption

Abstract

2D lead halide perovskites (2DPs) offer chemical compatibility with 3D perovskites and enhanced stability, which are attractive for applications in photovoltaic and light-emitting devices. However, such lowered structural dimensionality causes increased excitonic effects and highly anisotropic charge-carrier transport. Determining the diffusivity of excitations, in particular for out-of-plane or inter-layer transport, is therefore crucial, yet challenging to achieve. Here, an effective method is demonstrated for monitoring inter-layer diffusion of photoexcitations in (PEA)₂PbI₄ thin films by tracking time-dependent changes in photoluminescence spectra induced by photon reabsorption effects. Selective photoexcitation from either substrate- or air-side of the films reveals differences in diffusion dynamics encountered through the film profile. Time-dependent diffusion coefficients are extracted from spectral dynamics through a 1D diffusion model coupled with an interference correction for refractive index variations arising from the strong excitonic resonance of 2DPs. Such analysis, together with structural probes, shows that minute misalignment of 2DPs planes occurs at distances far from the substrate, where efficient in-plane transport consequently overshadows the less efficient out-of-plane transport in the direction perpendicular to the substrate. Through detailed analysis, a low out-of-plane excitation diffusion coefficient of (0.26 ± 0.03) ×10⁻⁴ cm² s⁻¹ is determined, consistent with a diffusion anisotropy of ≈4 orders of magnitude.