Thermodynamics, Electronic Structure, and Vibrational Properties of Sn n (S 1– x Se x) m Solid Solutions for Energy Applications

The tin sulphides 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 sulphide and selenide phases could provide scope for optimising physical properties against sustainability requirements, but this has not been comprehensively explored. This work presents a detailed modelling study of the Pnma and rocksalt Sn(S1-xSex), Sn(S1-xSex)2 and Sn2(S1-xSex)3 solid solutions. All four show energetically favourable and homogenous mixing at all compositions, but rocksalt Sn(S1- xSex) and Sn2(S1-xSex)3 are predicted to be metastable and accessible only under certain synthesis conditions. Alloying leads to predictable variation of the bandgap, density of states and optical properties with composition, allowing SnS2 to be “tuned down” to the ideal Schockly-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(S1-xSex) 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 optimised alongside a variety of practical considerations.