Investigating Chemistry of Metal Dissolution in Amine–Thiol Mixtures and Exploiting It toward Benign Ink Formulation for Metal Chalcogenide Thin Films

Solution processing of metal chalcogenides using elemental metals dissolved in an amine–thiol solvent mixture has recently received a great deal of attention for the fabrication of thin-film optoelectronic devices. However, little is known about the dissolution pathway for metallic precursors in such mixtures. To exploit the full potential of this method, it is essential that a detailed understanding of the dissolution chemistry be developed. In this study, we use several characterization techniques to examine these solutions and then propose reaction mechanisms for In and Cu dissolutions in a hexylamine/1,2-ethanedithiol mixture. These dissolutions are rather found to be reactions resulting in metal oxidation with coevolution of H2 forming bis(1,2-ethanedithiolate)indium(III) in the case of indium dissolution and high nuclearity Cu(I) thiolate compounds in case of copper dissolution. This understanding allowed us to address the issue of toxicity and corrosivity associated with amine–thiol solvent by utilizing it as a reactant rather than a solvent for ink formulation. Here, we demonstrate a new approach whereby metal complexes formed by dissolving a range of metals including Cu, In, Zn, Sn, Se, and Ga with Se in amine–thiol solution can first be isolated by evaporation of the precursor solution and then dissolved in a variety of weakly coordinating organic solvents to provide a benign and stable solution free of unreacted amine and thiol for thin-film fabrication of various chalcogenide semiconductors. We utilize this new approach to demonstrate the fabrication of CuIn(S,Se)2 solar cells using dimethyl sulfoxide as a fabrication solvent.