Polymorphic Control of Solution-Processed Cu₂SnS₃ Films with Thiol–Amine Ink Formulation

There is increasing demand for tailored molecular inks that produce phase-pure solution-processed semiconductor films. Within the Cu-Sn-S phase space, Cu₂SnS₃ belongs to the I₂-IV-VI₃ class of semiconductors that crystallizes in several different polymorphs. We report the ability of thiol-amine solvent mixtures to dissolve inexpensive bulk Cu₂S and SnO precursors to generate free-flowing molecular inks. Upon mild annealing, polymorphic control over phase-pure tetragonal (I4̅2m) and orthorhombic (Cmc2₁) Cu₂SnS₃ films was realized simply by switching the identity of the thiol (i.e., 1,2-ethanedithiol vs 2-mercaptoethanol, respectively). Polymorph control is dictated by differences in the resulting molecular metal-thiolate complexes and their subsequent decomposition profiles, which likely seed distinct Cu_2-x S phases that template the ternary sulfide sublattice. The p-type tetragonal and orthorhombic Cu₂SnS₃ films possess similar experimental direct optical band gaps of 0.94 and 0.88 eV, respectively, and strong photoelectrochemical current responses. Understanding how ink formulation dictates polymorph choice should inform the development of other thiol-amine inks for solution-processed films.