Thermoelectric Properties of Strained β-Cu₂Se

In recent years, the high-temperature phase of Cu₂Se, namely, β-Cu₂Se, has attracted increasing attention due to its outstanding thermoelectric efficiency. The Cu ions in β-Cu₂Se show a distinct ion-liquid behavior between the framework of a Se ion lattice. Many experimental operations may lead to a discrepancy in the lattice structure. Experimentally synthesized β-Cu₂Se was p-type, and a thermoelectric device needs both p-type and n-type materials with a close figure of merit. Studying the effect of strain and the possibility of n-type β-Cu₂Se is essential. Utilizing first-principles calculations and molecular dynamics simulations, we investigate the thermoelectric performance of strained β-Cu₂Se. The results show that the n-type β-Cu₂Se can exhibit superior zT values like the p-type one. Applying compressive strain is an effective way to promote the power factor. The tensile strain will lead to a low lattice thermal conductivity and thus boost the p-type zT values. The predicted maximum zT values for n-type and p-type β-Cu₂Se can reach 1.65 and 1.71 at 800 K, respectively. Considering the fact that applying strain is challenging in experiments, we propose a feasible strategy to manipulate the lattice structure and carrier type: doping halogen elements. Our results provide a guide for Cu₂Se-based thermoelectric devices.