Novel synthesis of semiconductor chalcohalide anti-perovskites by low-temperature molecular precursor ink deposition methodologies

Abstract

In recent years, a growing interest in the development of new energy harvesting technologies based on earth-abundant, environmentally-friendly semiconductors, has led to the re-discovery of hitherto overlooked materials. Among them, Ag-based chalcohalides stand out for their abundancy and low-toxicity, as well as the crystal structure analogous to perovskite, albeit with cations in place of anions and vice-versa (i.e. anti-perovskite). Until now, inorganic anti-perovskites have generally been studied as solid-state electrolytes. Indeed, Ag₃SI was identified in the 1960s as a superionic conductor. On the other hand, theorical calculations have demonstrated bandgaps in the visible range, suggesting that they could be suitable for PV applications. However, there is little published information on their potential as energy harvesting materials and so far, thin films have been prepared by solid-state reactions or physical vapor deposition techniques at high temperature and/or vacuum conditions, which limits their commercial viability owing to costly, non-scalable processes. In this work, we present a new procedure to synthesize Ag-based chalcohalides by a low-temperature solution-based methodology, using an thiol-amine reactive solvent system to dissolve Ag₂S and AgX (X = Br, I) precursors, followed by spin coating deposition to obtain polycrystalline films. Through this process, it has been possible to synthesize Ag₃S(I_xBr_1−x) (x = 0–1) films for the first time, which have been characterized, demonstrating the formation of the anti-perovskite phase and a linear correlation between structural parameters and composition. Optical characterization shows bandgap ranging from 0.9 eV (Ag₃SI) to 1.0 eV (Ag₃SBr), with a bowing effect for the intermediate solid solutions. First solar cells prototypes demonstrate photo-response and promising electrical characteristics.