Post-transition-metal chalcogenides, such as SnSe, SnSe₂, In₄Se₃, and In₂Se₃, have attracted renewed attention as promising thermoelectric materials mainly due to their inherent low lattice thermal conductivities, originating from the atomically layered structure. Herein, we demonstrate the enhanced thermoelectric transport properties of n-type InSe in an effort to search for new thermoelectric materials within post-transition-metal chalcogenide systems. By Si doping at the In site, significantly enhanced electrical conductivity is obtained, mainly due to the simultaneous increase in both carrier concentration and mobility. Meanwhile, the large Seebeck coefficient is maintained despite the increase of carrrier concentration with Si doping. Based on theoretical considerations for band structure change by Si doping, this unconventional trade-off between electrical conductivity and Seebeck coefficient is due to the generation of heavy flat energy levels near the conduction band minimum in the presence of Si at the In site. The doped Si also acts as an effective point defect phonon scattering center, resulting in reduced lattice thermal conductivity. Due to this synergetic effect, a 210% improved thermoelectric figure of merit (zT) of 0.14 at 795 K compared with pristine InSe was obtained by 7% Si doping.

The emergence of metallic conduction in layered dichalcogenide semiconductor materials by chemical doping is one of key issues for two-dimensional (2D) materials engineering. At present, doping methods for layered dichalcogenide materials have been limited to an ion intercalation between layer units or electrostatic carrier doping by electrical bias owing to the absence of appropriate substitutional dopant for increasing the carrier concentration. Here, we report the occurrence of metallic conduction in the layered dichalcogenide of SnSe₂ by the direct Se-site doping with Cl as a shallow electron donor. The total carrier concentration up to ~10²⁰ cm⁻³ is achieved by Cl substitutional doping, resulting in the improved conductivity value of ~170 S·cm⁻¹ from ~1.7 S·cm⁻¹ for non-doped SnSe₂. When the carrier concentration exceeds ~10¹⁹ cm⁻³, the conduction mechanism is changed from hopping to degenerate conduction, exhibiting metal-insulator transition behavior. Detailed band structure calculation reveals that the hybridized s-p orbital from Sn 5s and Se 4p states is responsible for the degenerate metallic conduction in electron-doped SnSe₂.