Co dopant enhance the thermoelectric properties of Cu2Se films via magnetism and smooth surface

Abstract

Cu₂Se has a liquid-like structure and is a promising thermoelectric material. However, the defects in Cu₂Se thin films are difficult to control, which limits the improvement of the thermoelectric performance of the films. Co doping inhibits the thermally-induced phase transition of Cu₂Se thin films and enhances the thermoelectric performance by leveraging the interaction between the magnetism of Co and the film carriers. Changes in the Co content will influence the surface morphology by regulating the phase composition. Moreover, a relatively dense-surfaced Cu₂Se thin film can be obtained through Co doping. When the doping amount is 0.98 %, Co atoms exist in the form of a small number of interstitial defects, Co_i. When the doping amount increases to 2.38 %, a large number of Co_i are filled into the sub-lattice of the β-phase. This inhibits the formation of the α-phase and leads to the formation of a stable sub-lattice structure. As a result, it increases the hopping activation energy of carriers, enhances lattice coupling, and promotes acoustic phonon scattering. The magnetism of Co-doped Cu₂Se thin films is related to the quantity of Co_i. The small amount of Co_i can provide impurity energy levels, reduce the hopping activation energy of carriers, and generate ionized impurity scattering. Meanwhile, a small amount of Co_i gives rise to a built-in magnetic field in micro-regions, which restricts the migration of low-energy carriers in local regions. The relatively large effective mass makes the 0.98 %-Co film exhibit a relatively high Seebeck coefficient, effectively increasing the power factor of the film.