Excitonic cuprophilic interactions in one-dimensional hybrid organic-inorganic crystals

The everlasting pursuit of hybrid organic-inorganic lead-free semiconductors has directed the focus towards eco-friendly copper-based systems, perhaps because of the diversity in chemistry, controlling the structure-property relationship. In this work, we report single crystals of a Cu(i) halide-based perovskite-like organic-inorganic hybrid, (TMA)Cu₂Br₃, (TMA = tetramethylammonium), consisting of unusual one-dimensional inorganic anionic chains of -(Cu₂Br₃)-, electrostatically stabilized by organic cations, and the Cu(i)-Cu(i) distance of 2.775 Å indicates the possibility of cuprophilic interactions. X-ray photoelectron spectroscopy measurements further confirmed the presence of exclusive Cu(i) in (TMA)Cu₂Br₃ and electronic structure calculations based on density functional theory suggested a direct bandgap value of 2.50 eV. The crystal device demonstrated an impressive bulk photovoltaic effect due to the emergence of excitonic Cu(i)-Cu(i) interactions, as was clearly visualized in the charge-density plot as well as in the Raman spectroscopic analysis. The single crystals of a silver analogue, (TMA)Ag₂Br₃, have also been synthesized revealing a Ag(i)-Ag(i) distance of 3.048 Å (signature of an argentophilic interaction). Unlike (TMA)Cu₂Br₃, where more density of states from Cu compared to Br near the Fermi level was observed, (TMA)Ag₂Br₃ exhibited the opposite trend, possibly due to variation in the ionic potential influencing the overall bonding scenario.