Thermoelectric Properties of Cu-added Polycrystalline Bi₂O₂Se Oxyselenide

Bi₂O₂Se oxyselenide has been actively studied as a potential n-type thermoelectric material because of its intrinsically low thermal conductivity and high Seebeck coefficient (S). However, Bi₂O₂Se has very low electrical conductivity (σ), resulting in relatively poor thermoelectric performance. Herein, we investigate the effect of Cu addition on the electrical and thermal transport of n-type polycrystalline Bi₂O₂Se. A series of Cu_xBi₂O₂Se (x = 0, 0.0025, 0.005, and 0.0075) polycrystalline samples were synthesized by a conventional solid-state reaction. Tetragonal Bi₂O₂Se was successfully synthesized, and its lattice parameters gradually decreased with the addition of Cu. Further, σ decreased and the magnitude of S increased with increasing Cu content, according to the trade-off relationship between these parameters. Consequently, a maximum power factor of 0.106 mW m^-1 K^-2 was achieved for the sample with x = 0.0025 at 300 K, owing to the increase in the magnitude of S. The Hall carrier concentration decreased exponentially with the addition of Cu, which is mainly attributed to the possible enlargement of the band gap of the Cu-added samples. The lattice thermal conductivity decreased with increasing x, which was attributed to point-defect phonon scattering via Cu addition. Therefore, a maximum zT of 0.222 was obtained at 790 K for the Cu_0.0025Bi₂O₂Se (x = 0.0025) sample, which was approximately 8% higher than that of the pristine Bi₂O₂Se sample.