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 I-2-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 (I (4) over bar 2m) and orthorhombic (Cmc2(1)) 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-xS 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.

Automated summary

This study reports the ability to dissolve Cu2S and SnO precursors in thiol-amine solvent mixtures to generate molecular inks. Polymorphic control over Cu2SnS3 films was achieved by simply switching the thiol identity. The control of polymorphism is determined by the differences in the resulting molecular metal-thiolate complexes and their decomposition profiles.