Thermodynamic insights into the Ba–S system for the formation of BaZrS3 perovskites and other Ba sulfides

Abstract

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.