Enhanced Thermoelectric Performance through Tuning Bonding Energy in Cu₂Se_1-xS_x Liquid-like Materials

Thermoelectric materials require an optimal carrier concentration to maximize electrical transport and thus thermoelectric performance. Element doping and composition off-stoichiometry are the two general and effective approaches for optimizing carrier concentrations, which have been successfully applied in almost all semiconductors. In this study, we propose a new strategy called bonding energy variation to tune the carrier concentrations in Cu₂Se-based liquid-like thermoelectric compounds. By utilizing the different bond features in Cu₂Se and Cu₂S, alloying S at the Se sites successfully increases the bonding energy to fix Cu atoms in the crystal lattice to suppress the formation of Cu vacancies, leading to greatly reduced carrier concentrations toward the optimal value. Via a combination of the lowered electrical and lattice thermal conductivities and the relatively good carrier mobility caused by the weak alloy scattering potential, ultrahigh zT values are achieved in slightly S-doped Cu₂Se with a maximal value of 2.0 at 1000 K, 30% higher than that in nominally stoichiometric Cu₂Se.