Versatile Thiol-Amine Chemistry for The Synthesis of Complex Chalcogenides and Chalcohalides: New Opportunities to Develop Earth-Abundant Materials for Energy Applications

Earth-abundant chalcogenides and chalcohalides are garnering substantial attention as innovative semiconductors compatible with solar energy harvesting, thermoelectric, pyroelectric applications and other optoelectronic technologies.¹ Among them, van der Waals materials possess an anisotropic crystal structure resulting from covalently-bonded ribbons along one crystallographic direction, which leads to unique electrical properties (e.g. enhanced carrier transport) when the material is correctly oriented. Among recent successes Sb₂(S,Se)₃solar cells have achieved efficiencies above 10%², and SbSX (X=I,Br) micro-scale devices have been synthesized for the first time showing excellent optoelectronic properties³. However, although the synthesis techniques used so far have enabled significant progress, it is necessary to explore manufacturing routes that are more cost-effective, scalable and versatile in terms of composition and chemical doping.<br/><br/>Molecular precursor ink deposition is an excellent approach for thin film synthesis, being a low-energy, low-cost and versatile method, allowing various compositional analysis by modifying the stoichiometry of the precursor solutions, its integration into distinctive scalable deposition methods (including roll-to-roll coating), and deposition on flexible and non-planar substrates. In particular, thiol-amine solvent systems have recently demonstrated great potential to synthesize chalcogenide compounds (typically poorly soluble in polar solvents), producing promising results for Cu₂ZnSnS₄ and BaZrS₃, but there is still little information regarding van der Waals materials with low-dimensionality.^4,5<br/><br/>In this work, Sb₂Se₃, Sb₂S₃ and Sb₂(S,Se)₃ thin films have been synthesized by dissolving either the chalcogenide itself or the constituent elements into different mixtures of ethandithiol and amines/diamines, followed by spin-coating and a short hot-plate annealing at 300 C. Then, different post-deposition treatments have been explored using a tubular furnace under chalcogen atmosphere to coalesce crystallites and to selectively orient texture in the (00l) direction, improving the quality of devices. Additionally, different doping strategies have been investigated to tune the conductive type and carrier concentration. For n-type doping, Sb₂Se₃ samples have been prepared incorporating NH₄Cl and SbCl₃ to the precursor solution (Cl-doping).⁶On the other hand, p-type doping strategies have been investigated through Sn-doping and synthesis of Se-rich samples, resulting in an increase of the V_oc of photovoltaic devices. Structural, compositional and electronic characterization have been performed by X-ray diffraction, microscopy, X-ray and UV photoelectron spectroscopy (determination of valence band and Fermi level), Seebeck coefficient and JV measurements.<br/><br/>Additionally, the universality of the thiol-amine solvent for chalcogenides will be discussed, presenting its successful implementation for synthesis on other systems, including emerging chalcohalides (e.g. SbSI, SbSeI) and Ag-based anti-perovskites (Ag₃SI). The latter has a cubic structure analogous to standard perovskite (switching anions by cations and vice-versa), bandgap around 1 eV, and shows superionic behavior above room temperature. Overall, this work demonstrates the viability of thiol-amine solvents to synthesize a broad range of cutting-edge complex chalcogenide materials, allowing composition and doping control, and opening the door to explore new properties for energy harvesting applications.<br/><br/><br/>1 <i>Chem. Rev. </i><b>123</b>, 1, 327-278 (2023)<br/>2 <i>Nat Energy</i> <b>5</b>, 587–595 (2020)<br/>3 <i>J. Mater. Chem. A</i>, <b>11</b>, 17616-17627 (2023)<br/>4 <i>Chem. Mater.</i> <b>27</b>, 6, 2114-2120 (2015)<br/>5 <i>Chem. Mater.</i> <b>31</b>, 15, 5674-5682 (2019)<br/>6 <i>Chem. Mater.</i> <b>32</b>, 6, 2621-2630 (2020)