Influence of Te vacancies on the thermoelectric properties of n-type Cu_0.008Bi₂Te_2.7-xSe_0.3

In this study, we report the influence of Te vacancy formation on the thermoelectric properties of n-type Cu_0.008Bi₂Te_2.7Se_0.3 alloys, including their electronic and thermal transport properties. Te-deficient Cu_0.008Bi₂Te_2.7-xSe_0.3 (x = 0, 0.005, 0.01 and 0.02) samples were systematically synthesized and characterized. Regarding electronic transport properties, carrier concentration was increased with Te vacancies, while carrier mobility was maintained. As a result, the electrical conductivity significantly increased while the Seebeck coefficient reduced moderately, thus, the power factor was enhanced from 3.04 mW/mK² (pristine) to 3.22 mW/mK² (x = 0.02) at 300 K. Further analysis based on a single parabolic band model revealed that the weighted mobility of the conduction band increased, which is favorable for electron transport, as Te vacancies were generated. Regarding thermal transport properties, lattice thermal conductivity decreased with Te vacancies due to additional point defect phonon scattering, however, total thermal conductivity increased due to larger electronic contribution as Te vacancies increased. Analysis using the Debye-Callaway model suggests that the phonon scattering by the Te vacancies is as efficient as the substitution point defect scattering. Consequently, the thermoelectric figure of merit zT increased at all temperatures for x = 0.005 and 0.01. The maximum zT of 0.95 was achieved for Te-deficient Cu_0.008Bi₂Te_2.69Se_0.3 (x = 0.01) at 400 K.