Progress in Solution-Processed Chalcogenide Perovskite Synthesis and Application

Solar power offers a promising solution for reducing carbon emissions, but stable photovoltaic materials are needed to match the potential of halide perovskites. These materials are cost-effective, easily synthesized at low temperatures, and provide tunable bandgaps, high absorption, defect tolerance, and excellent charge transport. However, they are unstable in oxygen and moisture and contain toxic lead, raising environmental concerns. While progress has been made in improving stability and reducing toxicity, further advancements are required for their widespread use in solar applications.

Studies suggest that the remarkable properties of halide perovskites are due to their unique crystal structure, prompting interest in exploring other stable materials with similar characteristics. Chalcogenide perovskites, which also crystallize in the ABX₃ structure using earth-abundant elements (A = Ba, Sr, Ca; B = Zr, Hf; X = S), have shown high light absorption, often surpassing halide perovskites. They also have a high dielectric constant, tunable bandgaps, and excellent stability against air, moisture, and heat, making them promising for optoelectronic applications like solar cells, especially in tandem structures.

Until recently, these materials have only been synthesized at temperatures exceeding 900 °C, with most studies using powders to evaluate their properties, rendering them unsuitable for integration into solar cells. Similarly, thin film reports have primarily relied on high-temperature methods. The strong oxophilicity of these materials presents a significant challenge, leading to the formation of oxide secondary phases, which introduce compositional inhomogeneities throughout the film. Moreover, due to unoptimized synthesis methods, a wide range of bandgaps (1.7–1.9 eV) has been reported, despite the ideal bandgap being around 1.8 eV. Notably, there are no studies employing low-temperature, cost-effective techniques such as solution processing, as the solution chemistries used for late and post-transition metal chalcogenides are incompatible with this class of materials. The lack of a comprehensive understanding of the key factors needed to lower synthesis temperatures has further hindered advancements in this area.

In our work, we have systematically investigated the factors influencing the high-temperature growth of these materials and have identified precursor reactivity, the use of a liquid flux or transport agent, and the presence of an oxygen sink as critical for synthesizing impurity-free BaZrS₃. By enhancing precursor reactivity to accelerate kinetics and ensure complete conversion, we have developed a suite of solution-processing methods using various metal precursors, including metal acetylacetonates, halides, sulfides, and organometallic compounds. We have engineered a liquid flux to facilitate low-temperature growth by overcoming mass-transfer barriers and have employed an oxygen sink to eliminate oxide impurities from oxophilic early transition metals. This approach has enabled us to successfully synthesize BaMS₃ (M=Zr, Hf) compounds at temperatures below 600 °C, producing highly uniform BaZrS₃ films with an optimal bandgap of 1.82 eV. Additionally, we have established a framework to synthesize nanoparticles of BaZrS₃, BaHfS₃, BaTiS₃, BaNbS₃, α-SrZrS₃, and α-SrHfS₃, broadening their potential for optoelectronic applications. Furthermore, we have demonstrated the superior performance of solution-processed BaZrS₃ by fabricating a photodetector with the fastest rise and decay times for transient photoconductivity reported in the chalcogenide perovskite literature. Lastly, we have demonstrated the alloying of various elements in BaZrS₃ and BaHfS₃ to engineer their bandgaps to span the single junction and tandem solar cell range. Our findings have presented new pathways for advancing chalcogenide perovskite research, overcoming synthesis challenges, and enabling optoelectronic device fabrication at lower temperatures.