NOVELTY - An ink comprises: i) a copper source selected from copper complexes of nitrogen-, oxygen-, carbon-, sulfur-, and selenium-based organic ligands, copper sulfides, and/or copper selenides; ii) a tin source selected from tin complexes of nitrogen-, oxygen-, carbon-, sulfur-, and selenium-based organic ligands, tin hydrides, tin sulfides, and/or tin selenides; iii) optionally, a zinc source selected from zinc complexes of nitrogen-, oxygen-, carbon-, sulfur-, and selenium-based organic ligands, zinc sulfides, and/or zinc selenides; iv) optionally, a chalcogen compound (I).    USE - As ink (claimed) for preparing films of copper zinc tin sulfide and its selenium analogues on a substrate; in preparing photovoltaic devices.    ADVANTAGE - The ink formulation provides simple, low-cost, scalable materials and processes with a low number of operations that provide high-quality, crystalline copper tin sulfide and copper zinc tin sulfide films with tunable composition and morphology; provides low-temperature routes to materials using solvents and reagents with relatively low toxicity.    DETAILED DESCRIPTION - An ink comprises: i) a copper source selected from copper complexes of nitrogen-, oxygen-, carbon-, sulfur-, and selenium-based organic ligands, copper sulfides, and/or copper selenides; ii) a tin source selected from tin complexes of nitrogen-, oxygen-, carbon-, sulfur-, and selenium-based organic ligands, tin hydrides, tin sulfides, and/or tin selenides; iii) optionally, a zinc source selected from zinc complexes of nitrogen-, oxygen-, carbon-, sulfur-, and selenium- based organic ligands, zinc sulfides, and/or zinc selenides; iv) optionally, a chalcogen compound (I) selected from: elemental S, elemental Se, CS2, CSe2, CSSe, R1S-Z', R1Se-Z', R1S-SR1, R1Se-SeR1, R2C(S)S-Z', R2C(Se)Se-Z', R2C(Se)S-Z', R1C(O)S-Z', and/or R1C(O)Se-Z'; ; v) optionally, a solvent. Provided that: if there is no solvent, then at least one of the chalcogen compound and the tin source is a liquid at room temperature; and if the copper source is selected from copper sulfide and copper selenide, and the tin source is selected from tin sulfide and tin selenide, then the solvent is not hydrazine.    Z'=H, (NR4)4, or (SiR5)3;    R1 and R5=hydrocarbyl, O-, N-, S-, halogen- or tri(hydrocarbyl)silyl-substituted hydrocarbyl;    R2=hydrocarbyl, O-, N-, S-, Se-, halo-, tri(hydrocarbyl)silyl-substituted hydrocarbyl, or O-, N-, S-, and Se-based functional groups;    R4=H, O-, N-, S-, Se-, halogen-, tri(hydrocarbyl)silyl-substituted hydrocarbyl, or O-, N-, S-, Se- based functional groups.