Boosting the thermoelectric performance of zinc blende-like Cu₂SnSe₃ through phase structure and band structure regulations

As a variant of the zinc blende structure, p-type semiconducting Cu₂SnSe₃ exhibits a high potential in the field of thermoelectric energy conversion, due to the low lattice thermal conductivity and large abundance of the constituent elements. Till now, the bottleneck of achieving a comparable thermoelectric performance in Cu₂SnSe₃ with state-of-the-art thermoelectric material systems is its unsatisfactory electrical power factor. In this work, we realized a simultaneous increment of charge carrier concentration and mobility through In/Sb co-doping at the Sn site; detailed X-ray diffraction (XRD), Rietveld refinement, and density functional theory (DFT) band structure calculations revealed a gradual phase structure (and an associated band structure) transition from a low-symmetry monoclinic phase to a high-symmetry cubic one, which was further verified by Cs-corrected scanning transmission electron microscopy (Cs-corrected STEM) characterization. Eventually, we achieved a peak figure of merit, ZT_max ∼0.90 at 773 K and an average ZT_avg ∼0.36 (323–773 K) for the composition of Cu₂(Sn_0.85In_0.05Sb_0.05Ti_0.05)Se₃, representing the state of the art for all Cu₂SnSe₃-based thermoelectric materials reported thus far.