Increasing the Thermoelectric Figure of Merit of Tetrahedrites by Co-Doping with Nickel and Zinc

We report a more than 30% increase in the thermoelectric figure of merit of Ni-doped tetrahedrite (Cu12Sb4S13) by the addition of Zn. We show that this enhancement is due to the combination of two effects: (1) a tuning of the Fermi energy to optimize the Seebeck coefficient and thermoelectric power factor and (2) a reduction in thermal conductivity due to decreases in both the electronic and lattice thermal conductivities. Unlike tetrahedrites doped solely with Zn, which can become electrically insulating at high Zn concentrations, Ni doped samples remain conducting due to the existence of an additional valence band from spin-split Ni states. By adding Zn to these Ni-doped materials, electrons can fill the holes in the valence band, allowing for tuning of the thermoelectric properties. Tetrahedrites optimally doped with both Ni and Zn have thermal conductivity approaching theoretical minimum values, which helps boost the thermoelectric figure of merit zT in these compounds above unity.