Organic-inorganic metal halide perovskites have emerged as attractive materials for solar cells with power-conversion efficiencies now exceeding 25%. However, challenges and opportunities remain relating to material microstructure, ionic migration and toxicity. While tin halide perovskites offer lead-free alternatives to the currently best-performing lead halide perovskites, their prevalence towards tin vacancy formation and oxidation makes these materials particularly unstable [1].

We have recently investigated ultrafast charge-carrier dynamics in lead-free silver-bismuth semiconductors[2-4] which promise lower toxicity and potentially higher barriers against ion migration than their more prominent lead-halide counterparts. We 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.[2] 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 small polaronic state. While the initially photogenerated delocalized charge carriers show bandlike transport, the self-trapped, small polaronic state exhibits temperature-activated mobilities, allowing the mobilities of both to still exceed 1 cm²V⁻¹s⁻¹ at room temperature. Self-trapped charge carriers subsequently diffuse to color centers, causing broad emission that is strongly red-shifted from a direct band edge. Overall, our observations suggest that strong electron−phonon coupling in this material induces rapid charge-carrier localization which may inhibit the use of this material as an efficient light harvester in photovoltaic devices.

We further demonstrate the novel lead-free semiconductor Cu₂AgBiI₆ which exhibits several advantages over Cs₂AgBiBr₆, namely a low exciton binding energy of ~29 meV and a lower and direct band gap near 2.1 eV,[3,4,5] 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 coupling strength[3] to that in Cs₂AgBiBr₆, suggesting a link with the presence of AgBi. Tuning such charge-lattice interactions therefore emerges as a serious challenge for this class of materials.