The role of chemical composition in determining the charge-carrier dynamics in (AgI)x(BiI3)yrudorffites

Abstract

Silver-bismuth-based perovskite-inspired materials (PIMs) are increasingly being explored as non-toxic materials in photovoltaic applications. However, many of these materials exhibit an ultrafast localization of photogenerated charge carriers that is detrimental for charge-carrier extraction. In this work, such localization processes are explored for thermally evaporated thin films of compositions lying along the (AgI)_x(BiI₃)_y series, namely BiI₃, AgBi₂I₇, AgBiI₄, Ag₂BiI₅, Ag₃BiI₆, and AgI, to investigate the impact of changing Ag⁺/Bi³⁺ content. A persistent presence of ultrafast charge-carrier localization in all mixed compositions and BiI₃, together with unusually broad photoluminescence spectra, reveal that eliminating silver will not suppress the emergence of a localized state. A weak change in electronic bandgap and charge-carrier mobility reveals the resilience of the electronic band structure upon modifications in the Ag⁺/Bi³⁺ composition of the mixed-metal rudorffites. Instead, chemical composition impacts the charge-carrier dynamics indirectly via structural alterations: Ag-deficient compositions demonstrate stronger charge-carrier localization most likely because a higher density of vacant sites in the cationic sublattice imparts enhanced lattice softness. Unraveling such delicate interplay between chemical composition, crystal structure, and charge-carrier dynamics in (AgI)_x(BiI₃)_y rudorffites provides crucial insights for developing a material-by-design approach in the quest for highly efficient Bi-based PIMs.

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