Thermodynamics, Electronic Structure, and Vibrational Properties of Sn_n(S_{1- x}Se_x)_m Solid Solutions for Energy Applications

The tin sulfides and selenides have a range of applications spanning photovoltaics and thermoelectrics to photocatalysts and photodetectors. However, significant challenges remain to widespread use, including electrical and chemical incompatibilities between SnS and device contact materials and the environmental toxicity of selenium. Solid solutions of isostructural sulfide and selenide phases could provide scope for optimizing physical properties against sustainability requirements, but this has not been comprehensively explored. This work presents a detailed modeling study of the Pnma and rocksalt Sn(S_1-xSe_x), Sn(S_1-xSe_x)₂, and Sn₂(S_1-xSe_x)₃ solid solutions. All four show an energetically favorable and homogenous mixing at all compositions, but rocksalt Sn(S_1-xSe_x) and Sn₂(S_1-xSe_x)₃ are predicted to be metastable and accessible only under certain synthesis conditions. Alloying leads to a predictable variation of the bandgap, density of states, and optical properties with composition, allowing SnS₂ to be "tuned down" to the ideal Shockley-Queisser bandgap of 1.34 eV. The impact of forming the solid solutions on the lattice dynamics is also investigated, providing insight into the enhanced performance of Sn(S_1-xSe_x) solid solutions for thermoelectric applications. These results demonstrate that alloying affords facile and precise control over the electronic, optical, and vibrational properties, allowing material performance for optoelectronic applications to be optimized alongside a variety of practical considerations.