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 Zn1-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 Zn1-xSb samples. At 300 K, the hole concentration (nH) is boosted from 3.6 x 1018 cm-3 (in ZnSb) to 3.4 x 1019 cm-3 for the Zn0.94Sb sample, culminating in a marked enhancement in electrical conductivity (sigma) from 1.80 x 104 S m-1 to 7.57 x 104 S m-1 for Zn0.94Sb. Equally noteworthy is the substantial decrease in thermal conductivity (kappa) observed in the Zn0.94Sb 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 similar to 0.8 at 673 K, is achieved for the sample with a 6 mol% Zn deficiency. Furthermore, a maximum theoretical conversion efficiency of similar to 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. Zn-vacancy driven high power factor and strong coupling of acoustic and optical modes enhance the zT in p-type ZnSb.