A FIRST-PRINCIPLES INVESTIGATION OF THERMOELECTRIC PERFORMANCE OF 2D AND 3D MATERIALS

Abstract

In this work through first-principle calculations, we investigate and enhance the thermoelectric performance of monolayer and bulk materials from three different families of compounds. We propose two strategies to enhance thermoelectric performance: carrier optimisation and strain engineering. Enhancement through carrier optimisation we investigate the electronic, structural, elastic, and TE properties of the AgSbS2 monolayer, using density functional theory in conjunction with Boltzmann transport theory. In this study, we proposed our strategy to increase the figure of merit (ZT) by optimizing the carrier concentrations. The monolayer of AgSbS2 is found to be both mechanically and thermodynamically stable. The calculated electronic band structure shows a semiconducting-like behaviour of AgSbS2 with an indirect band gap of 1.31 eV using the Heyd-Scuderia-Ernzerhof (HSE06) exchange-correlation functional. The investigated monolayer is found to be anisotropic, hence we analysed its thermoelectric properties at various carrier concentrations along a- and b-directions. It attained a high value of Seebeck coefficient of 360 μVK-1 in the a-direction and 370 μVK-1 in the b-direction at room temperature. The maximum ZT of AgSbS2 monolayer at an optimized carrier concentration of 5×1018 cm-3 is found to be as high as 0.3 and 0.5 at 300 K in both a- and b-directions, respectively, for the n-type monolayer,. The high ZT values of AgSbS2 indicate its potential for room-temperature energy harvesting applications. Enhancement through Strain engineering Through first-principles calculations in conjunction with the semiclassical Boltzmann transport theory we study the thermoelectric performance of KCd4P3. The ternary KCd4P3 phase crystallizes in the centrosymmetric rhombohedral space group R3̅m. A good match is shown between the calculated results, such as lattice constants and bond xii lengths, with the results calculated experimentally . As the generalized gradient approximation generally underestimates the band gap, therefore, we have utilized a more accurate functional i.e., Tran Blaha modified Becke Johnson approach and obtained a direct band gap of 0.66 eV of unstrained KCd4P3. The positive value of the Seebeck coefficient which goes as high as 248 µV/K at room temperature identifies KCd4P3 as a p-type semiconducting material. We propose method to increase power factor and TE performance via strain which is useful for tuning TE parameters independently, leading to a high figure of merit of 0.78 at 300 K.