Ultrahigh power factors in P-type 1T-ZrX₂ (X = S, Se) single layers

The thermoelectric performances of 1T-ZrX₂ (X = S and Se) single layers were investigated using a combination of density functional calculations and semi-classical Boltzmann transport theory. Because of the high hole mobilities at 300 K, ultrahigh power factors (PF=S²σ) were found in the P-type compounds; these values were ∼ 11.95 and ∼13.58 mW K^-2 m^-1 for 1T-ZrS₂ and 1T-ZrSe₂ single layers, respectively. However, because of the Lorenz relation between the electrical conductivity (σ) and an electron's thermal conductivity (κ_el) given by the Wiedemann-Franz law, the electronic figures of merit (Z_elT=PF·T/κ_el) at 300 K were approximately 0.67 and 0.75 for the N- and P-type 1T-ZrSe₂, respectively. In addition, the lattice thermal conductivities (κ_ph) were calculated, giving values of ∼1.43 and ∼0.97 W K^-1 m^-1 for 1T-ZrS₂ and 1T-ZrSe₂ single layers, respectively. Therefore, because of the lower κ_ph/κ_el ratio, the P-type 1T-ZrX₂ single layers possess higher figure-of-merits (ZT=Z_elT/1+κ_phκ_el) than their counterparts. This signifies that the P-type samples demonstrate better thermoelectric performance than the N-type ones. The thermoelectric properties of metastable 2H-ZrX₂ (X = S and Se) single layers were also investigated.