Modification of Bulk Heterojunction and Cl Doping for High-Performance Thermoelectric SnSe₂/SnSe Nanocomposites

SnSe₂ is a wide band gap semiconductor ( E_g = 1.05 eV) with a typical two-dimensional hexagonal crystal structure of the prototype CdI₂ phase, resulting in an intrinsically low thermal conductivity, which is favorable for thermoelectrics. Herein, we reported the remarkable role of Cl doping in SnSe₂/SnSe nanocomposites. Doping with Cl in the system not only increases the carrier concentration by an order of magnitude, but it also modifies the heterojunction from that of the Schottky junction type (p-n junction) in undoped samples to junctions having an ohmic contact (n-n junction) when the samples are doped with Cl, increasing their carrier mobility in the process. On account of the simultaneously boosted carrier concentration and carrier mobility upon Cl doping, the electrical conductivity and the power factor are greatly increased. Moreover, the enhanced point defect phonon scattering induced by Cl doping, coupled with the interface phonon scattering, results in a suppression of the thermal conductivity. As a consequence, the maximum ZT value of 0.56 at 773 K is achieved in the 6% Cl-doped SnSe₂/SnSe nanocomposite measured in the direction parallel to the pressing direction. This is an almost 6 times larger value than that measured on the undoped composite. In addition, unlike the conventional layered compounds (Bi₂Te₃ and SnSe), the ZT value measured parallel to the pressing direction is much higher than the one measured perpendicular to the pressing direction. This study provides a new way for optimizing the thermoelectric properties of materials through interface regulation.