Multiscale Defect-Assisted Enhancement of Thermoelectric Transport in Sn-Doped Black Phosphorus Polycrystals

Black phosphorus (BP) has a low electrical conductivity and a high thermal conductivity, despite its narrow band gap and high Seebeck coefficient. Here, it is experimentally demonstrated that the Sn atoms are substitutionally doped into BP polycrystals and that multiscale microstructural defects, such as point defects, dislocations, and amorphous phases, as phonon scattering sites, are independently incorporated into them. The Sn doping into the BP polycrystal increases the carrier concentration up to 3.7 × 10¹⁸ cm^-3 at 300 K without a significant degradation of the Seebeck coefficient and effectively decreases the thermal conductivity due to the phonon-impurity scattering. The multiscale defects (point defects, dislocations, grain boundaries, amorphization) synergistically suppress the lattice thermal conductivity (from 13 to 6.3 W/m K) with a decoupling of the electronic transport. As a result, the thermoelectric figure-of-merit, ZT, is significantly enhanced more than 1 order of magnitude, by controlling the carrier concentration and the multiscale microstructural defects, independently.