Thermoelectric performance in disordered Cu2ZnSnSe4 nanostructures driven by ultra-low thermal conductivity

High-energy reactive mechanical alloying (ball milling) was used to synthesize tetragonal and cubic polymorphs of Cu2ZnSnSe4. The ordered tetragonal (I-4) polymorph undergoes a phase transition above 400 K into a Cu-Zn disordered tetragonal (I-42 m) polymorph, while the cubic (F-43 m) polymorph with full cation disorder is stabilized at room temperature. Both polymorphs show ultra-low thermal conductivities, 0.42 W m−1 K−1 at 722 K and 0.21 W m−1 K−1 at 523 K for the disordered tetragonal and cubic phases respectively. The cubic polymorph has a higher zT in the low-temperature range, peaking at 0.26 (523 K), while the disordered tetragonal has a maximum zT of 0.46 at 712 K. The latter is the highest reported zT for stoichiometric Cu2ZnSnSe4, comparable to the best-performing doped materials in the literature. A combination of experimental results and ab-initio calculations point to a coupling between structural disorder and microstructure as the mechanism behind the reported performance.