Beneficial Effect of Na 2 CO 3 Additions on the Thermoelectric Performance of Melt‐Route Cu 2 Se

High‐performance thermoelectric materials require simultaneous reduction of thermal conductivity and electrical resistivity, among other criteria. Here it is shown that the introduction of Na 2 CO 3 into the melt‐route fabrication process for the well‐known thermoelectric Cu 2 Se has a beneficial and surprisingly strong effect. There is a significant enhancement in electrical conductivity which density functional theory calculations suggest may be due to the effect of Na and O doping in the Cu 2 Se matrix. There is also a 34% reduction in thermal conductivity which is likely due to a high density of defects causing scattering of phonons. Overall, however, there is only relatively a small change in Seebeck coefficient. A higher power factor of 12.6 µW cm −1 K −2 is achieved versus 8.8 µW cm −1 K −2 for standard Cu 2 Se. A very high value of zT of 2.3 is obtained at 804 K versus 1.1 for standard Cu 2 Se.