(209e) Rethinking the Synthesis of Chalcogenide Perovskites for Moderate Temperature Processing

Chalcogenide perovskites are an emerging class of semiconducting materials that differentiate themselves from other established semiconductors through an interesting combination of properties.¹ These materials have an ABX₃ composition where A is Ca, Sr, or Ba, B is Zr or Hf, and X is S. As the most researched chalcogenide perovskite, BaZrS₃ provides a useful example for study. Like the halide and oxide perovskites, the chalcogenide perovskites take a distorted perovskite crystal structure of corner sharing BX₆ octahedra. This crystal structure may play an important role in imbuing a direct bandgap and high absorption coefficient, similar to those seen in the halide perovskites.² But while the halide perovskites have intrinsic instability issues and concerns over the use of toxic lead, the chalcogenide perovskites are composed of earth abundant and non-toxic elements and have excellent stability, more in line with the high-bandgap oxide perovskites. The chalcogenide perovskites also differentiate themselves from other established semiconductors due to the ionic nature of their bonding, unlike Si, CuInSe₂, and CdTe which have relatively covalent bonding nature.¹

To date, the biggest challenge for chalcogenide perovskites has been difficulty in synthesis and reliance on extreme conditions to obtain high purity material. For example, common routes have included the sulfurization of oxide perovskites and solid state reactions of the binary sulfides, with both routes frequently using temperatures around 1000 °C.^3,4

In this work, we will show that an understanding of the thermodynamic landscape around the synthesis of chalcogenide perovskites explains the historical reliance on excessively high temperatures. Furthermore, with this knowledge, the thermodynamic landscape can be manipulated to allow for the chalcogenide perovskites to be synthesized at moderate temperatures below 600 °C. We first show that by careful selection of reactive precursors that are both soluble and contain metal sulfur bonding can enable solution deposition of BaZrS₃ and BaHfS₃ following a moderate temperature sulfurization step.⁵ Second, we show that the oxide perovskite BaZrO₃ can be converted to BaZrS₃ in a moderate temperature sulfurization step if a thermodynamic driving force is created through the inclusion of a powerful oxygen sink.

In summary, this work seeks to develop an understanding of the thermodynamics of chalcogenide perovskite synthesis and to exploit this knowledge to develop moderate-temperature synthesis methods, opening doors for further research into this interesting class of semiconductors.

(1) Sopiha, K. V.; Comparotto, C.; Márquez, J. A.; Scragg, J. J. S. Chalcogenide Perovskites: Tantalizing Prospects, Challenging Materials. Adv. Opt. Mater. 2022, 10 (3), 2101704. https://doi.org/10.1002/ADOM.202101704.

(2) Sun, Y.-Y.; Agiorgousis, M. L.; Zhang, P.; Zhang, S. Chalcogenide Perovskites for Photovoltaics. Nano Lett. 2015, 15, 581–585. https://doi.org/10.1021/nl504046x.

(3) Márquez, J. A.; Rusu, M.; Hempel, H.; Ahmet, I. Y.; Kölbach, M.; Simsek, I.; Choubrac, L.; Gurieva, G.; Gunder, R.; Schorr, S.; et al. BaZrS3 Chalcogenide Perovskite Thin Films by H2S Sulfurization of Oxide Precursors. J. Phys. Chem. Lett. 2021, 12, 2148–2153. https://doi.org/10.1021/acs.jpclett.1c00177.

(4) Meng, W.; Saparov, B.; Hong, F.; Wang, J.; Mitzi, D. B.; Yan, Y. Alloying and Defect Control within Chalcogenide Perovskites for Optimized Photovoltaic Application. Chem. Mater 2016, 28, 821–829. https://doi.org/10.1021/acs.chemmater.5b04213.

(5) Turnley, J. W.; Catherine Vincent, K.; Pradhan, A. A.; Panicker, I.; Swope, R.; Uible, M. C.; Bart, S. C.; Agrawal, R. Solution Deposition for Chalcogenide Perovskites: A Low-Temperature Route to BaMS3 Materials (M = Ti, Zr, Hf). J. Am. Chem. Soc. 2022, 144 (40), 18234–18239. https://doi.org/10.1021/jacs.2c06985.