BaZrS₃ stands out as one of the most extensively researched chalcogenide perovskites. Unlike its halide counterpart, this Pb-free alternative boasts superior intrinsic chemical stability. Notably, all three chemical elements are among the 20 most abundant elements in Earth's crust. With a band gap of approximately 1.8 eV, it is theoretically well suited to augment Si in tandem cells. Additionally, BaZrS₃ exhibits one of the highest band-edge absorption levels among all known solar cell materials and maintains stability in air up to 400 °C. However, the synthesis of BaZrS₃ thin films—essential for typical optoelectronic devices—remains a challenge. The primary obstacle lies in the elevated process temperatures required for achieving a high degree of crystallinity, potentially hindering integration into tandem photovoltaic devices. Nonetheless, the formation of high-order Ba polysulfides as intermediate phases can notably decrease the growth temperature of BaZrS₃. The purpose of the present work is to produce a pressure-temperature phase diagram for the Ba–S system, defining the domains of stability of binary Ba sulfides. By independently varying the temperatures of the sample and the S vapor source, an experimental phase diagram is initially constructed. Then a first-principles thermodynamic model for the sulfurisation of Ba is built and the theoretical results are compared with the experimental results. Good agreement is found for the BaS₂–BaS₃ transition, while the discrepancy observed for the BaS₂–BaS transition is attributed to equipment limitations. In the process, the easily overlooked roles of thermal gradients and thermal transport in the flow reactor are also highlighted. The insights gleaned are relevant to general thin-film sulfurisation systems, where achieving and maintaining a controlled high partial vapor pressure of S present greater challenges compared to solid-state chemistry. This study offers valuable thermodynamic guidance for the synthesis of a wide range of Ba sulfides and is of particular relevance for the formation of BaZrS₃ perovskites at moderate temperature.

Chalcogenide perovskites are being considered for various energy conversion applications, not least photovoltaics. BaZrS3 stands out for its highly stable, earth-abundant, and nontoxic nature. It exhibits a very strong light-matter interaction and an ideal band gap for a top subcell in a two-junction photovoltaic device. So far, thin-film synthesis-necessary for proper optoelectronic characterization as well as device integration-remains underdeveloped. Sputtering has been considered, among others, but the need for an annealing step of at least 900 degrees C has been a cause for concern: such a high temperature could lead to damaging the bottom layers of prospective tandem devices. Still, a solid-state fabrication route has already demonstrated that BaZrS3 can form at much lower temperatures if excess S is present. In this work, sputtered Ba-Zr precursors capped by SnS are sulfurized at under 600 degrees C for 20 min. Although some Sn is still present at the surface after sulfurization, the resulting crystalline quality is comparable to samples synthesized at much higher temperatures. The results are rationalized, and the effect of key process variables is examined. This study represents the first successful synthesis of BaZrS3 perovskite that is compatible with conductive substrates-an important step forward for device integration.

Tandem solar cells based on hybrid organic-inorganic metal halide perovskites have reached efficiencies up to 28%, but major concerns for long-term stability and the presence of Pb have raised interest in searching for fully earth-abundant, intrinsic chemically stable, and nontoxic alternatives. With a direct band gap around 1.8 eV and stability in air up to at least 500 degrees C, BaZrS3 is a promising candidate. This work presents the first approach of synthesizing a thin film of such compound by sputtering at ambient temperature with a subsequent rapid thermal process. Despite the short fabrication time, the width of the XRD diffraction peaks and the energy and distribution of the photoluminescence response show comparable crystalline quality to that from bulk synthesis methods. Good crystallization required around 900 degrees C. Such a high temperature could be incompatible with fabrication of tandem solar cells.