Interplay of Structure, Charge‐Carrier Localization and Dynamics in Copper‐Silver‐Bismuth‐Halide Semiconductors

Silver‐bismuth based semiconductors represent a promising new class of materials for optoelectronic applications because of their high stability, all‐inorganic composition, and advantageous optoelectronic properties. In this study, charge‐carrier dynamics and transport properties are investigated across five compositions along the AgBiI4–CuI solid solution line (stoichiometry Cu4x(AgBi)1−xI4). The presence of a close‐packed iodide sublattice is found to provide a good backbone for general semiconducting properties across all of these materials, whose optoelectronic properties are found to improve markedly with increasing copper content, which enhances photoluminescence intensity and charge‐carrier transport. Photoluminescence and photoexcitation‐energy‐dependent terahertz photoconductivity measurements reveal that this enhanced charge‐carrier transport derives from reduced cation disorder and improved electronic connectivity owing to the presence of Cu+. Further, 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. Finally, ultrafast charge‐carrier localization is observed upon pulsed photoexcitation across all compositions investigated, lowering the charge‐carrier mobility and leading to Langevin‐like bimolecular recombination. This process is concluded to be intrinsically linked to the presence of silver and bismuth, and strategies to tailor or mitigate the effect are proposed and discussed. Five semiconductors with compositions Cu4x(AgBi)1−xI4 are investigated via X‐ray diffraction, optical, and terahertz spectroscopy. Silver and bismuth are found to be intrinsically linked to the formation of small polarons, while the addition of copper leads to reduced charge‐carrier effective masses, enhanced photoluminescence, and improved charge‐carrier transport. Further, small polaron formation is shown to cause Langevin‐like charge‐carrier recombination in silver‐bismuth semiconductors.