Metal halide semiconductors have emerged as attractive materials for solar cells with power-conversion efficiencies now exceeding 26%, however, these record efficiencies have all relied on incorporation of lead as the metal. The search for less toxic ingredients has led to the emergence of a plethora of new bismuth-based semiconductors, including bismuth halides and chalcogenides. Power conversion efficiencies around 6% have been realised for such materials, triggering new research efforts to explore and eliminate current limitations to performance.

Here, we show that an ultrafast charge-carrier self-trapping process limits long-range charge-carrier transport in most bismuth-based semiconductors.[1-7] We have examined the evolution of photoexcited charge carriers in the double perovskite Cs₂AgBiBr₆ using a combination of temperature-dependent photoluminescence, absorption and optical pump−terahertz probe spectroscopy.[1] We observe rapid decays in terahertz photoconductivity transients that reveal an ultrafast, barrier-free localization of free carriers on the time scale of 1.0 ps to an intrinsic self-trapped small polaronic state. Alloying Cs₂AgBiBr₆ with Cs₂AgSbBr₆ on the trivalent metal site interestingly leads to significantly stronger self-localisation,[2] which we attribute to self-localised charge carriers probing the energetic landscape more locally thus turning an alloy’s low-energy sites (here, Sb sites) into traps, which dramatically deteriorates transport properties. We further demonstrate the novel lead-free semiconductor Cu₂AgBiI₆ which exhibits a low exciton binding energy of ~29 meV and a lower and direct band gap near 2.1 eV,[3,4] making it a significantly more attractive lead-free material for photovoltaic applications. However, charge carriers in Cu₂AgBiI₆ are found to exhibit similarly strong charge-lattice interactions[4,5]. Further work examining five compositions along the AgBiI₄–CuI solid solution line (stoichiometry Cu_4x(AgBi)_1−xI₄) shows that increased Cu+ content enhances the band curvature around the valence band maximum, resulting in lower charge-carrier effective masses, reduced exciton binding energies, and higher mobilities, as well as partly mitigating the extent of such ultrafast self-localisation.[5] Interestingly, we show that thin films of BiOI lack of such self-trapping, with good charge-carrier mobility maintained over longer time scales, reaching ∼3 cm² V^–1 s^–1 at 295 K and increasing gradually to ∼13 cm² V^–1 s^–1 at 5 K, indicative of prevailing bandlike transport.[6] Finally, we examine thin films of AgBiS₂ nanocrystals as a function of Ag and Bi cation-ordering,[7] which is modified via thermal-annealing. We show that homogeneous cation disorder reduces charge-carrier localization, most likely because cation-disorder engineering flattens the disordered electronic landscape, removing tail states that would otherwise exacerbate Anderson localization of small polaronic states.[7]

Overall, self-trapping of charge carriers therefore emerges as a clear challenge for this class of materials. Our findings explore the parameter space governing such self-localization, highlighting the effects of local energetic disorder as an exacerbating factor that may pose new challenges to alloying strategies. In addition, or findings show that cation-disorder engineering may partly mitigate such effects through flattening of the local energy landscape.