Enhanced energy dependence of carrier density of states, relaxation time, and group velocity in highly asymmetric bands yields large Seebeck coefficients.

Thermoelectric (TE) materials enable efficient conversion between thermal and electric energy, emerging as eco-friendly options for power generation. Traditionally, gapless systems have received limited attention regarding their TE performance due to very small Seebeck coefficients. However, by using first-principles calculations and Boltzmann transport theory, we demonstrated that the semimetallic InAs and InSb compounds are exceptions. They exhibit strong band asymmetry, which is different from metallic InBi and significantly enhances the energy dependence of their carrier density of states, relaxation time, and group velocity. Such characteristics lead to these systems showing large intrinsic Seebeck coefficients without doping or strain. Combined with their low lattice thermal conductivity, the room-temperatureZTvalues of InAs reach 1.1 and 1.0, respectively. Further electron-phonon coupling calculations reveal that InAs and InSb maintain a high Seebeck coefficient even with refined conductivities. The interplay between high-mobility electrons and valence-band scattering underpins this performance. Our work redefines semimetals as a promising class of TE materials, bridging the gap between conventional semiconductors and gapless systems for energy applications.