Thermoelectric properties and figure of merit of Cu2ZnSnS4

Thermoelectric (TE) power is a leading technology for addressing energy waste, and the quaternary sulphide Cu2ZnSnS4 (CZTS) is of considerable interest for lightweight and flexible thin-film devices. However, the large variation in the figure of merit, ZT , obtained in experimental studies highlights the need for a more comprehensive characterisation of the material properties. In this work, we apply a fully ab initio modelling approach to obtain high-quality reference predictions of the thermoelectric properties and ZT of CZTS. We find that CZTS has large bulk κlatt, due to long phonon lifetimes, which can be minimised by nanostructuring. We predict significantly higher electrical conductivity, Seebeck coefficients and power factors with p-type (hole) doping compared to n-type (electron) doping. The electrical properties are less sensitive to crystallite size than the κlatt, and we predict an industry-standard high-temperature ZT > 1 should be achievable for nanostructured p-type CZTS with crystallite sizes below 10 nm. Comparing our predictions to measurements also suggests that modelling heavily-doped CZTS or CZTS-based solid solutions may yield valuable insight into how the ZT can be optimised further.