Microstructure and Thermoelectric Properties of Bi-doped β-Cu2Se Film

The Cu2Se material has attracted more attentions in the field of thermoelectric materials due to its high figure of merit, "electronic crystal phonon liquid" structure, rich constituent elements in the crust, low price and other advantages. Compared with bulk material, the film with a nearly two-dimensional structure can effectively improve the Seebeck coefficient and reduce the thermal conductivity, showing excellent thermoelectric properties. At present, the main methods to improve the thermoelectric properties include element doping, composites containing nano-sized second phase, low dimensionalization, nano structure, etc. Element doping can modulate the carrier concentration and change the energy band structure, further modulating the Seebeck coefficient and electrical conductivity. Therefore, element doping has been proved to be one of the most effective methods to improve the thermoelectric properties of materials. Bi has a larger atomic radius compared with Cu. This means that doping Bi may cause lattice distortion and more point defects in Cu2Se lattice. At the same time, phonons are scattered in transmission due to mass fluctuation and periodic stress field destruction due to Bi doping, thus reducing the thermal conductivity. Thus, Bi doping helps to improve the thermoelectric performance of materials. In this work, The β-Cu2‒xBixSe thermoelectric films with different Bi contents were prepared by magnetron sputtering on single crystal Si substrate containing SiO2 layer with high vacuum powder sintered Cu-Bi-Se alloy target. The phase composition of deposited films was determined by XRD patterns and the surface and cross-sectional morphology of deposited films was observed by SEM. The content and distribution of the constituent elements were measured and analyzed by EDS. The Seebeck coefficient and electrical conductivity of deposited films were measured by LSR-3 resistivity and Seebeck system. The carrier concentration and mobility of deposited films at room temperature were measured by Hall experiments. The results showed that deposited films were mainly composed of single β-Cu2Se phase at room temperature. The films with the maximum Bi doping amount of 1.07at.% also contained very small amount of α-Cu2Se phase and β-Cu2Se phase. Cu atom in β-Cu2Se lattice was substituted by Bi atom and (Cu,Bi)2Se solid solution formed in the deposited films. The deposited β-Cu2‒xBixSe films with ([Bi]+[Cu])/[Se]>2.0 possessed p-typed conductive characteristics. In the range of measured temperature from 25 to 395 ℃, the electrical conductivity decreased and the Seebeck coefficient increased with increasing measured temperature, showing the conductive properties of degenerate or semi-degenerate semiconductors. The carrier concentration and electrical conductivity decreased, but the mobility and Seebeck coefficient increased with increasing Bi content in deposited films at room temperature. The power factor of the Bi-doped films was higher than that of the films without Bi. The power factor of the deposited film increased with Bi content increasing to 225 ℃. Above 225 ℃, the film with doping amount of 0.29at.% Bi had the highest power factor and the power factor of deposited film decreased gradually with further increasing Bi content. The power factor of β-Cu2Se film can be significantly enhanced by doping a proper amount of Bi in films. © 2023 Chongqing Wujiu Periodicals Press. All rights reserved.