Two-dimensional lead halide perovskites have emerged as promising materials for photovoltaic and light-emitting applications owing to their excellent environmental stability, tunable quantum confinement, and chemical compatibility with three-dimensional perovskites [1,2]. However, their reduced structural dimensionality gives rise to enhanced excitonic effects and highly anisotropic charge transport, making it essential to understand excitation and charge-carrier diffusion, particularly in the out-of-plane direction. In this presentation, we discuss two complementary studies that establish how thin-film structure governs transport in Ruddlesden–Popper-type lead-iodide 2D perovskites.

First, we demonstrate an effective method for monitoring inter-layer diffusion of photoexcitations in (PEA)₂PbI₄ thin films by tracking time-dependent photoluminescence spectral changes induced by photon reabsorption effects [3]. By selectively exciting the films from either the substrate or air side, we reveal depth-dependent diffusion dynamics across the film profile. Time-dependent diffusion coefficients are extracted using a one-dimensional diffusion model coupled with an interference correction that accounts for refractive-index variations near the strong excitonic resonance. This analysis reveals a low out-of-plane excitation diffusion coefficient of (0.26 ± 0.03) × 10⁻⁴ cm² s⁻¹, corresponding to a diffusion anisotropy of approximately four orders of magnitude [3].

Second, we extend this dynamic photon reabsorption approach to a systematic series of 2D perovskite thin films incorporating non-conjugated alkylammonium spacer cations with chain lengths from three to eight carbon atoms. Pronounced odd–even effects are observed in absorption coefficients, photoluminescence energies and lifetimes, out-of-plane excitation diffusion, and in-plane charge-carrier mobility measured by optical pump–terahertz probe spectroscopy. Grazing-incidence wide-angle X-ray scattering reveals that these transport trends arise from cation-controlled nanostructural orientation: even-numbered alkyl spacers promote highly ordered lead-iodide planes lying within the film plane, whereas odd-numbered spacers induce more disordered stacking. Furthermore, the observed 1/d² dependence of inter-layer diffusion on the inter-plane distance d indicates that Förster resonance energy transfer underpins excitation transport between lead-iodide layers. Together, these findings establish a direct structure–transport relationship in 2D perovskite thin films and provide design guidelines for anisotropic optoelectronic devices [4].