Phase Transition Engineering of Cu2S to Widen the Temperature Window of Improved Thermoelectric Performance

Cu 2 S compounds are promising thermoelectric (TE) candidate materials with environmentally friendly and earth abundant chemical constituents. A series of phase transitions occur with temperature whereas only the high temperature stabilized cubic structure (α-Cu 2 S) exhibits desirable TE properties. In this work, by alloying Cu sites with Mn, Zn, Ga, and Ge, profound influence on β- to α-Cu 2 S phase transition and thermoelectric transport properties is observed. Both phase transition temperature ( T c ) and the enthalpy of phase change (Δ H ) decreases with doping; remarkably, for Cu 1.95 Mn 0.03 S, T c reduces by ≈156 K. The Seebeck anomaly near the critical point of phase transition also vanishes. The electrical conductivity is remarkably improved for doped samples due to the largely elevated hole concentration. In comparison with pristine Cu 2 S, not only is the peak TE power factor substantially enhanced (by ≈272%), but also the average ZT for 500–823 K is highly improved (by ≈145%) due to the successful stabilization of α-Cu 2 S at lower temperatures. The present work offers a clue to enlarge the temperature regime of high TE properties, which is practically useful for a variety of polymorphous thermoelectric compounds.