A plethora of new semiconductors have recently emerged as versatile materials for solar cells and photocatalytic applications. Combinatorial analytical probes have played a pivotal role in uncovering the mechanisms underpinning light-harvesting performance even before device optimisation has been attempted. 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 show how such charge transport can be probed successfully 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 technique to assess the degree of transport anisotropy in thin films of 2DPs,^[2] based on time-dependent photon reabsorption effects^[2] and THz conductivity probes ^[3]. We show that in (PEA)₂PbI₄ films, time-dependent diffusion coefficients arise from minute misalignment of 2DPs planes occurring 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. We extract a low out-of-plane excitation diffusion coefficient of 0.26x10⁻⁴ cm² s⁻¹, consistent with a diffusion anisotropy of about 4 orders of magnitude. We further demonstrate the effect of spacer cation length on the electronic and optical properties of lead-iodide-based 2DPs, using alkylammonium cations of varying chain lengths, revealing pronounced odd-even effects on transport anisotropies.^[3]

We further report on charge-carrier conduction in new bismuth-halide based semiconductors.^[5,6,7] We show that such materials exhibit dynamic transitions from large to small polaronic states that dominate the dynamics of charge carriers, and discuss how such transitions are affected by lattice softness in (AgI)_x(BiI₃)_y Rudorffites.^[5] In addition, we examine the quality of the electronic interfaces formed between Cu₂AgBiI₆ (CABI) and commonly used charge transport layers.^[6] We reveal that while organic transport layers, such as PTAA and PCBM, form a relatively benign interface, inorganic transport layers, such as CuI and SnO₂, induce the formation of unintended impurity phases within the CuI−AgI−BiI₃ solid solution space, significantly influencing structural and optoelectronic properties.^[6]