Gigantic Phonon-Scattering Cross Section To Enhance Thermoelectric Performance in Bulk Crystals

In thermoelectric energy conversions, thermal conductivity reduction is essential for enhancing thermoelectric performance while maintaining a high power factor. Herein, we propose an approach based on coated-grain structures to effectively reduce the thermal conductivity to a much greater degree when compared to that done by conventional nanodot nanocomposite. By incorporating CdTe coated layers on the surface of SnTe grains, the thermal conductivity is as low as 1.16 W/m-K at 929 K, resulting in a thermoelectric figure of merit, i.e., zT, of 1.90. According to our developed theory, phonons scatter coherently due to the phase lag between phonons passing through and around the coated grain. Such scattering is induced by the acoustic impedance mismatch between the coated layer and the grain, resulting in a gigantic phonon-scattering cross section. The phonon-scattering cross section of the coated grains is several orders of magnitude larger than that of the nanodots with the same impurity concentration. The power factor was also slightly increased by the energy filtering effect at the coated surface and additional minority carrier blocking by the heterointerfaces. This scheme can be utilized for various bulk crystals, meaning a broad range of materials can be considered for thermoelectric applications.