Thermoelectric conversion enhanced by charged defects on a topological insulator surface

Thermoelectric (TE) conversion from thermal energy into electric energy has been attracting attention as a renewable energy harvesting. The conversion efficiency is evaluated by the dimensionless figure of merit ZT and for practical use ZT ≥ 1 (conversion efficiency is 10%) is required. Bismuth chalcogenide is not only known as a representative material with high ZT but also has received attention as a topological insulator (TI) phase in recent years. Here we theoretically study the thermoelectric effect on the surface of TIs that are ionically disordered as the consequence of hypothetical doping for systematic control of the Fermi levels. Coulomb type long range potential is introduced as the ionic disorders. Based on Boltzmann transport theory at finite temperature, we calculate the TE coefficient and ZT. As a result, we find that ZT can achieve substantially high values even for the thermal phonon. Our theory may help us understand recent TE transport measurements in TI thin films and could make an ionically disordered TI a promising material for TE conversion technology.