Structural diversity and electronic properties of Cu₂SnX ₃ (X=S, Se): A first-principles investigation

The ternary semiconductors Cu₂SnX₃ (X=S, Se) are found frequently as secondary phases in synthesized Cu₂ZnSnS ₄ and Cu₂ZnSnSe₄ samples, but previous reports on their crystal structures and electronic band gaps are conflicting. Here we report their structural and electronic properties as calculated using a first-principles approach. We find that (i) the diverse range of crystal structures such as the monoclinic, cubic, and tetragonal phases can all be derived from the zinc-blende structure with tetrahedral coordination. (ii) The energy stability of different structures is determined primarily by the local cation coordination around anions, which can be explained by a generalized valence octet rule. Structures with only Cu₃Sn and Cu ₂Sn₂ clusters around the anions have low and nearly degenerate energies, which makes Cu and Sn partially disordered in the cation sublattice. (iii) The direct band gaps of the low-energy compounds Cu ₂SnS₃ and Cu₂SnSe₃ should be in the range of 0.8-0.9 and 0.4 eV, respectively, and are weakly dependent on the long-range structural order. A direct analogy is drawn with the ordered vacancy compounds found in the Cu(In,Ga)Se₂ solar-cell absorbers.