The efficiency of thermoelectric energy converters is limited by the material thermoelectric figure of merit ( zT ). The recent advances in zT based on nanostructures limiting the phonon heat conduction is nearing a fundamental limit: The thermal conductivity cannot be reduced below the amorphous limit. We explored enhancing the Seebeck coefficient through a distortion of the electronic density of states and report a successful implementation through the use of the thallium impurity levels in lead telluride (PbTe). Such band structure engineering results in a doubling of zT in p -type PbTe to above 1.5 at 773 kelvin. Use of this new physical principle in conjunction with nanostructuring to lower the thermal conductivity could further enhance zT and enable more widespread use of thermoelectric systems.

We studied a high-performance thermoelectric material whose chemical formula is ${\mathrm{Ag}}_9\mathrm{Tl}{\mathrm{Te}}_5$ . ${\mathrm{Ag}}_9\mathrm{Tl}{\mathrm{Te}}_5$ is simple and easy to prepare. Its highest dimensionless figure of merit $\left(ZT\right)$ value is 1.23, obtained at $700\mathrm{K}$ . The values of individual thermoelectric properties at $700\mathrm{K}$ are $2.63\times {10}^{-4}\Omega \mathrm{m}$ for electrical resistivity, $319\mu \mathrm{V}{\mathrm{K}}^{-1}$ for Seebeck coefficient, and $0.22\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ for thermal conductivity. ${\mathrm{Ag}}_9\mathrm{Tl}{\mathrm{Te}}_5$ is a unique material combining extremely low thermal conductivity and relatively low electrical resistivity.