Layered GaGeTe Thermoelectric Materials with Multivalley Conduction Bands

GaGeTe is interesting as a thermoelectric material since it has a layered tetradymite-like structure similar to state-of-the-art Bi2Te3. However, the electronic structure, electrical transport properties, and their rationalization in GaGeTe are rarely studied. In this study, we focus on the electronic structure and electrical transport properties of GaGeTe by combining theory and experiment. Experimentally, intrinsic p-type thermoelectric properties of pristine and Ag-doped GaGeTe polycrystalline samples are reported and found to be anisotropic due to the layered structure. Electronic structure calculation reveals GaGeTe as a semiconductor with a moderate band gap, consistent with the experimental transport properties showing no obvious bipolar effect. Based on the electronic structure, the Boltzmann transport theory is applied to calculate transport properties, leading to an excellent agreement with the experimental data of p-type GaGeTe. Strikingly, n-type electrical transport properties are predicted to be much more favorable than the p-type counterparts, which can be rationalized by the multivalley conduction bands with a primary contribution from a nontrivial 6-fold valley-degenerate conduction band. A peak zT of ∼0.7 at 800 K is estimated for n-type GaGeTe using the experimental lattice thermal conductivity of the pristine sample. We thereby expect GaGeTe to be a promising thermoelectric material if n-type doping is achievable. This work provides insights into the electronic structure and electrical transport for the further development of GaGeTe thermoelectrics.