Ultrafast Excited-State Localization and Charge-Carrier Mobility in Cs₂AgBiBr₆ and Cu₂AgBiI₆

Understanding the nature of the charge-lattice coupling in Ag-Bi−halide semiconductors is crucial for the optimization of these materials in a variety of optoelectronic applications. In this study, I report on the evolution of photoexcited charge carriers in Cs₂AgBiBr₆ and Cu₂AgBiI₆. In both materials, I observe rapid decays in THz photoconductivity transients that reveal an ultrafast, barrier-free localization of free carriers on the time scale of a few ps to an intrinsic small polaronic state. I attribute this to the strong charge-lattice coupling in these Bi-Ag materials, while the temperature-independence of the self-trapping is indicative of low electronic dimensionality due to the electronic isolation of the distinct Ag⁺ and Bi³⁺ sites. Through a combination of temperature-dependent photoluminescence and absorption spectroscopy, I identify the electronic states occupied before and after localization, directly tracing the localization of the charge carriers with time and interpreting its influence on the charge-carrier mobility in terms of a quantitative model. Since the small polaron motion is temperature-activated, I find that their mobility still exceeds 1 cm² V⁻¹s⁻¹ in both materials, leaving open the prospect of their application in efficient photovoltaic devices.