(333g) Shaping the Future: Progress in Emerging BaZrS3 Chalcogenide Perovskite Synthesis

Several countries have set ambitious plans and have pledged to reduce their carbon footprint by investing in renewable energy. Among various options, solar power is a promising resource. It is projected that by 2050, solar energy will be the largest resource for electricity generation. This rapid rise in anticipated demand requires rapid technological innovations in new promising semiconductor materials and optimum device architectures.

Although several promising photovoltaic absorber materials have been discovered, there is still a need for stable materials that would match the potential and unprecedented growth of halide perovskites. The halide perovskites can easily be synthesized at low temperatures (<200 °C) via cost-effective solution processing and possess tunable bandgap and high light-absorption coefficients. These materials have also been shown to be defect-tolerant and have superior charge transport properties, making them suitable candidates for optoelectronic applications. However, along with all these excellent properties, these exciting materials also suffer from intrinsic instability to oxygen and moisture, limiting their use for practical applications. The high-performing halide perovskites also employ toxic lead in their crystal structure. While significant progress has been made in stabilizing their structure by various means, exploring alternatives for lead in the structure, and trying ways to encapsulate lead to avoid harm to the environment, much more still needs to be done before these materials can be widely deployed for harnessing solar power.

Certain studies have proposed that the remarkable properties observed in halide perovskites are attributed to their perovskite crystal structure, enticing the exploration of other stable alternatives crystallizing in the perovskite structure. This led to the exploration of chalcogenide perovskites for optoelectronic applications. These materials crystallize in a similar ABX₃ crystal structure as halide perovskites but consist of earth-abundant elements, A=Ba, Sr, Ca, B=Zr, Hf, and X=S. They have been shown to possess high light absorption coefficients and even surpass halide perovskites in some cases. They also have high dielectric constant and tunable bandgap and are suited for top layers in tandem solar cells. An exciting property is their stability in the presence of air, moisture, and heat. Consequently, chalcogenide perovskites hold significant promise as a class of materials for diverse optoelectronic applications, including solar cells.

Unfortunately, until recently, these materials had only been synthesized at temperatures exceeding 900 °C, with reports primarily utilizing powders to evaluate their properties, making them unsuitable for incorporation in solar cells. Similarly, the reported thin films also utilized high-temperature methods. In addition, oxophilicity has been a significant challenge, leading to the presence of oxide secondary phases in the film. Consequently, variations in composition throughout the film depth were observed due to these impurities. Additionally, due to the unoptimized methods, varying bandgaps have been reported for the synthesized material in the wide range of 1.7-1.9 eV, while the ideal bandgap lies around 1.8 eV. Notably, no reports have been available using low-temperature, cost-effective methods such as solution processing. Furthermore, the solution chemistries for late and post-transition metal chalcogenides do not work for this class. The lack of a comprehensive understanding of the requisite factors for reducing synthesis temperatures has further impeded progress.

In our research, we have systematically investigated the factors influencing the high-temperature growth of these materials and have identified the reactivity of precursors, the use of a liquid flux or transport agent, and the presence of an oxygen sink as crucial for the synthesis of impurity-free BaZrS₃. By prioritizing precursor reactivity to enhance kinetics and ensure complete conversion to the desired products, we have developed a range of solution-processing methods utilizing various metal precursors, including metal acetylacetonates, metal halides, metal sulfides, and organometallic compounds, covering a broad spectrum of commonly employed materials. We have devised a liquid flux to enable low-temperature growth by addressing mass-transfer barriers and have implemented an oxygen sink to eliminate oxide impurities from oxophilic early transition elements. We used this knowledge to successfully synthesize BaMS₃ (M=Zr, Hf) compounds at temperatures below 600 °C. Furthermore, our methodologies yielded highly uniform films of BaZrS₃ chalcogenide perovskites exhibiting an optimal bandgap of 1.82 eV. Our research offers promising prospects for advancing chalcogenide perovskite studies, overcoming synthesis challenges, and facilitating the fabrication of optoelectronic devices at feasible temperature ranges—an area of critical importance yet to be extensively explored.