High thermoelectric performance in p-type ZnSb upon increasing Zn vacancies: an experimental and theoretical study

The high thermoelectric performance of dopant-free, low-cost and eco-friendly p-type Zn_1−xSb (x = 0, 0.01, 0.03, and 0.06) is demonstrated by synergistically optimizing its electrical and thermal properties via Zn-vacancy engineering. Upon increasing Zn-vacancies in ZnSb, the bandgap is observed to reduce due to the formation of the impurity states above the valence band, which is theoretically validated using density functional theory (DFT). Remarkably, Zn vacancy-driven point defects significantly influence the hole concentration within the Zn_1−xSb samples. At 300 K, the hole concentration (n_H) is boosted from 3.6 × 10¹⁸ cm⁻³ (in ZnSb) to 3.4 × 10¹⁹ cm⁻³ for the Zn_0.94Sb sample, culminating in a marked enhancement in electrical conductivity (σ) from 1.80 × 10⁴ S m⁻¹ to 7.57 × 10⁴ S m⁻¹ for Zn_0.94Sb. Equally noteworthy is the substantial decrease in thermal conductivity (κ) observed in the Zn_0.94Sb sample at 673 K, plunging from 2.29 W m⁻¹ K⁻¹ (in ZnSb) to 1.41 W m⁻¹ K⁻¹. This decline in thermal conductivity is attributed to the effective phonon scattering arising from Zn-vacancy-assisted point defects, combined with the efficient coupling of optical and acoustic phonons and the characteristic low group velocity, evidenced by the theoretically calculated phonon dispersion curve. Overall, the high thermoelectric figure of merit, zT of ∼0.8 at 673 K, is achieved for the sample with a 6 mol% Zn deficiency. Furthermore, a maximum theoretical conversion efficiency of ∼7% is predicted at a temperature gradient of 625 K, showing high potential for use in practical devices for mid-temperature applications, and the present work features the effect of a reliable dopant-free approach in improving the overall zT of eco-friendly and low-cost ZnSb.