High dimensionless figure of merit of the ZrI2 monolayer identified based on intrinsic carrier concentration and bipolar effect

A 2D ZrI2 monolayer with a high dimensionless figure of merit (ZT) is analyzed in terms of intrinsic carrier concentrations, transport coefficients, and a bipolar effect. The ZrI2 monolayer with a space group of P21/m is fully optimized. The dynamic and thermal stabilities are verified by computing the phonon dispersion in addition to performing ab initio molecular dynamics simulation. The thermal conductivity of the lattice is evaluated by employing the phonon Boltzmann transport theory and the first-principle second and third force constants. The Seebeck coefficients, electronic thermal conductivities, and electric conductivities of the monolayer are determined by solving the relaxation time approximation semiclassical Boltzmann transport equation. To further explore the chance for promoting ZT, we investigate the manipulating effect of the carrier concentrations. The largest ZT with the bipolar effect can reach 7.86 at 700 K, implying that the ZrI2 monolayer has excellent thermoelectric performance.