Synthesis of Ag₃SX (X = Br, I) Chalcohalide Anti-Perovskites Thin Films

Metal chalcohalides constitute an extended family of semiconductor materials encompassing different compositions, structural and optoelectronic properties. Indeed, since the 1960s, several chalcohalide materials have been reported to possess photoconductive, electro-optical and ferroelectric effects, and their wide variety of compositions and structures could lead to a broad bandgap range, potentially suitable for photovoltaics (PV). However, these semiconductors have been largely overlooked among the PV field, likely shadowed by the prevalence of Si in the solar cell market, and the success of other thin film technologies such as Cu(In,Ga)Se₂ and halide perovskites. Nevertheless, recent studies have reported excellent performance of several chalcohalide materials such as SbSI and SbSeI, reaching efficiencies up to 5% in a very short period of time, benefited by their quasi-1D structure, which favors a high mobility and excellent transport properties. But the chalcohalide family includes other members which have properties suitable for PV, as well as being constituted by low-toxicity and earth-abundant elements.<br/><br/>Among these, silver chalcohalides stand out for their crystalline structure analogous to the successful perovskite, suggesting that they could possess great tolerance to defects. Indeed, their structure corresponds to that of a perovskite, switching cation sites by anions, and anion sites by cations; hence denominated an anti-perovskite. Also, theoretical calculations have reported that silver chalcohalides have bandgaps in the 0.9-2.0 eV range, making them ideal for single-junction or tandem PV devices. Despite these promising properties, there is no published information on their implementation in solar cells, and so far, they have only been synthesized by solid-state reactions (powder) and laser ablation at high temperatures (ultra-thin films).<br/><br/>In this work, we present a procedure to prepare Ag₃SBr and Ag₃SI by low-temperature solution-based methodologies, using the amine-thiol solvent to dissolve Ag₂S and AgX (Br,I) precursors, followed by solution deposition by blade or spin-coating to obtain polycrystalline thin films on Mo and Al₂O₃-coated glass substrates. The structural properties of these films have been characterized by X-ray diffraction, confirming the formation of the anti-perovskite phase. Also, a series of samples have been subjected to different annealing treatments to determine the optimal synthesis conditions, whereby it has been observed that the anti-perovskite phase starts to form at 200^oC coexisting with Ag₂S (which decreases upon increasing temperature). Likewise, formation of AgBr has been detected at temperatures above 250^oC, bounding the range of optimum growth. Ag₃SBr and Ag₃SI films have also been characterized by scanning electron microscope imaging, Raman and transmission spectroscopies measurements.<br/><br/>Thus, this work demonstrates for the first time the viability of solution-processing methods to prepare Ag chalcohalide anti-perovskite thin films, using the amine-thiol (“alkahest”) solvent. Importantly, this methodology has reduced the synthesis temperature to 200^oC, opening the door to its implementation with different substrates and the manufacture of solar cells. In addition, this versatile chemical system tolerates different cation and anion substitutions, offering a viable approach for bandgap tuning. Strategies to incorporate Cu₂S to the precursor solutions to prepare (Cu,Ag)₃SX films and replace S by Se will also be presented and discussed.