The p–n transformation and thermoelectric property optimization of Cu1+xFeSe2 (x = 0–0.05) alloys

Abstract

CuFeSe₂ alloys, containing earth-abundant and eco-friendly elements, are considered as environmentally friendly thermoelectric materials. In this work, the thermoelectric properties and transport were studied for nominal Cu_1+xFeSe₂ (x = 0.00, 0.01, 0.02, 0.03, 0.05) bulk alloys. The secondary phase of Cu₂Se was detected for x > 0.01, which leads to Se competitive vacancies in the major phase. The distribution of Cu₂Se was confirmed by energy-dispersive spectroscopy. Intrinsic Se vacancies were also found in all samples by X-ray fluorescence. The electrical resistivity tremendously decreased with increasing x. The Seebeck coefficients showed p–n transformation with increasing temperature for all samples. This transformation is due to the two types of Se vacancies (competitive and intrinsic), which contribute more excited electrons to the conduction band. The excess Cu significantly reduces the lattice thermal conductivity by increased phonon scatterings, including porosity, point defects, and secondary phase. The figure of merit, zT, was improved more than twice by the excess Cu. CuFeSe₂ with Se vacancies is a promising n-type thermoelectric material, and its figure of merit can be optimized by excess Cu.