Publication date: 15th December 2025
Two-dimensional lead halide perovskites (2DPs) offer chemical compatibility with three-dimensional perovskites and enhanced stability, which are attractive for applications in photovoltaic and light-emitting devices[1-2]. However, such lowered structural dimensionality causes increased excitonic effects and highly anisotropic charge-carrier transport[3-4]. Determining the diffusivity of excitations, in particular for out-of-plane or inter-layer transport, is therefore crucial, yet challenging to achieve. Here, we demonstrate an effective method 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.[5] Through selective photoexcitation from either substrate- or air-side of the films we reveal differences in diffusion dynamics encountered through the film profile. We extract time-dependent diffusion coefficients from spectral dynamics through a one-dimensional diffusion model coupled with an interference correction for refractive index variations arising from the strong excitonic resonance of 2DPs.[5] 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, we determine a low out-of-plane excitation diffusion coefficient of (0.26 ± 0.03) × 10-4 cm2s-1, consistent with a diffusion anisotropy of ~4 orders of magnitude[5].
The authors gratefully acknowledge funding from the Engineering andPhysics Sciences Research Council (EPSRC) UK. K.A.E. acknowledges thesupport of the Rank Prize through a Return to Research grant. L.M.H. ac-knowledges support through a Hans Fischer Senior Fellowship from theTechnical University of Munich's Institute for Advanced Study, funded bythe German Excellence Initiative.
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