Computational Design of Mixed-Valence Tin Sulfides as Solar Absorbers

Binary tin sulfides, such as SnS and SnS₂, are appealing because of their simple stoichiometry and semiconducting properties and are, therefore, being pursued as potentially cost-effective materials for optoelectronic applications. The multivalency of Sn, that is, Sn(+2) and Sn(+4) allows yet more intermediate compositions, SnS, whose structures and properties are of interest. Sn₂S₃ is already under consideration as a mixed-valence semiconductor. Other intermediate compositions, for example, Sn₃S₄ and Sn₄S₅ have remained elusive, although their existences have been alluded to in literature. Here we report a comprehensive study of phase stability of the SnS series compounds, utilizing swarm-intelligence crystal structure search method combined with first-principles energetic calculations. We find that the stability of mixed-valence SnS compounds with respect to decomposition into pure-valence SnS and SnS₂ is in general weaker than the SnO counterparts, likely due to differences in chemical bonding. Besides identifying the experimentally discovered stable phases of Sn₂S₃, our calculations indicate that the Sn₃S₄ phase is another mixed-valence composition which shows marginal stability with respect to decomposition into SnS and SnS₂. Other studied compositions may be metastable under ambient conditions, with slightly positive formation enthalpies. We find two structures of Sn₃S₄ having comparably low energies, both of which feature one-dimensional chain-like fragments obtained by breaking up the edge-connected octahedral layers of SnS₂. Both structures indicate lattice phonon stability and one shows quasi-direct band gap with a calculated value of 1.43 eV, ideal for solar absorbers. A further analysis of the composition-structure-property relationship supports the notion that low-dimensional Sn-S motifs and van der Waals interaction may lead to diverse structure types and chemical compositions, having functional properties that are yet to be identified in the SnS series with mixed valency.