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

Abstract

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 Cu2–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.

Enhanced Thermoelectric Performance of Nanostructured Cu2SnS3 (CTS) via Ag Doping

“…The external cation doping at the Sn site transforms the monoclinic polymorph into cubic ( F 4̅3 m ). It is worth mentioning that partially (SG: I 4̅2 m ) or fully (SG: F 4̅3 m ) disordered CTS polymorphs can also be obtained by synthesis methods, e.g., colloidal method and high-energy reactive ball milling. , Recently, Koskela et al reported an orthorhombic ( Cmc 2 1 ) CTS polymorph. Polymorphism in these materials offers multiple possibilities of structural manipulation and band engineering to boost the TE performance.…”