Metal-halide perovskites (MHPs) and “perovskite-inspired materials” (PIMs) have recently emerged as versatile materials for solar cells and photocatalytic applications. Ultrafast optical probes of photoconductivity dynamics are particularly useful here, uncovering the generation, localisation and ultimate recombination of charge carriers following photon absorption.

Probing charge-carrier motion in highly anisotropic semiconductors poses particular challenges. We examine such charge transport in layered, two-dimensional metal halide perovskites (2DPs),[1,2,3,4] whose electronic landscape is moderated through quantum confinement. We examine the effects of the high anisotropy of transport in thin films comprising layers that are highly oriented either parallel or perpendicular to the substrate plane [1]. We further utilise a powerful non-contact technique to assess the degree of transport anisotropy in 2DP films [2], based on time-dependent photon reabsorption effects and THz conductivity probes [2-4]. We show that (PEA)₂PbI₄ exhibits strongly suppressed out-of-plane diffusion coefficients of 0.26x10⁻⁴ cm² s⁻¹, consistent with a diffusion anisotropy of about 4 orders of magnitude [2]. We further investigate the effect of spacer cation length on the electronic and optical properties of (Cx)₂PbI₄ 2DPs, using Cx alkylammonium cations of varying carbon chain lengths. We reveal that such films exhibit pronounced odd–even dependence on the carbon number in both optical and transport properties, including absorption coefficients, photoluminescence energies and lifetimes, and excitation diffusion dynamics [3,4]. Out-of-plane diffusion of photoexcitations displays an opposite odd–even trend to in-plane charge-carrier mobility, causing a pronounced odd–even modulation of the thin-film mobility anisotropy [4].

We further report on charge-carrier conduction in new metal-halide based PIMs, which have been shown to exhibit dynamic transitions from large to small polaronic states [5,6,7]. We show that in Mixed-metal chalcohalides (A₂BCh₂X₃) charge-carrier localisation can be overcome through judicious chemical substitution substitution of the M(II) cation on the A-site [7]. We show that a shift in lattice symmetry from the lower-symmetry monoclinic P21/c phase in Pb₂SbS₂I₃ to thehigher-symmetry orthorhombic Cmcm phase in Sn₂SbS₂I₃ has a crucial effect on such charge self-localisation, which is observed only for Pb₂SbS₂I₃, whereas Sn₂SbS₂I₃ maintains a longer-lived nanosecond photoconductivity. We attribute this observation to the higher electronic dimensionality of the Cmcm Sn₂SbS₂I₃ structure, influenced by its more symmetric lattice. These findings establish a direct link between structural and optoelectronic properties in metal chalcohalides, demonstrating how facile chemical tuning can be harnessed to overcome charge-carrier localisation in PIM absorbers for solar energy harvesting.