Multiple conduction bands with strong valley anisotropy yielding ultrahigh thermoelectric power factors in n-type elemental Ge

Materials with high power factors are of great importance for thermoelectric power generation. However, currently most state-of-the-art thermoelectric compounds with excellent ZT values mainly benefit from thermal conductivity reduction. Herein, we report an ultrahigh power factor of 64.8 μWcm−1K−2 at 673 K in phosphorus-doped n-type elemental Ge. This is achieved because of the presence of multiple conduction bands with strong valley anisotropy that ensures both large band degeneracy and high electron mobility. By contrast, for the gallium-doped p-type Ge whose electrical properties are governed by one valence band, the optimized power factor is only 26.2 μWcm−1K−2, far less than that of Ge1-xPx. Consequently, a decent ZT of ∼0.5 at 873 K is achieved for n-type elemental Ge. Our work highlights the importance of multi-valley electronic structure and band anisotropy in optimizing thermoelectric performance.