  Two-dimensional (2D) materials have garnered considerable attention as an
emerging thermoelectric (TE) material owing to their unique density of state
(DOS) near the Fermi level. We investigate the TE performance of Janus
$\beta$-PdXY (X/Y=S, Se, Te) monolayer materials as a function of carrier
concentration and mid-temperature range (300 to 800 K) by combining density
functional theory (DFT) and semi-classical Boltzmann transport theory. The
phonon dispersion spectra and AIMD simulations confirm their thermal and
dynamical stability. The transport calculation results reveal the highly
anisotropic TE performance for both n and p-type Janus $\beta$-PdXY monolayers.
Meanwhile, the coexistence of low phonon group velocity and converged
scattering rate leads to lower lattice thermal conductivity (K_l) of 0.80 W/m
K, 0.94 W/m K, and 0.77 W/m K along y-direction for these Janus materials.
While the high TE power factor is attributed to the high Seebeck coefficient
(S) and electrical conductivity, which is due to the degenerate top valance
bands of these Janus monolayers. The combination of lower K_l and high-power
factor at 300K (800 K) leads to an optimal figure of merit (ZT) as 0.68 (2.21),
0.86 (4.09) and 0.68 (3.63) for p-type Janus PdSSe, PdSeTe and PdSTe
monolayers. To capture rational electron transport properties, the effects of
acoustic phonon scattering ($\tau$_ac), impurity scattering ($\tau$_imp), and
polarized phonon scattering ($\tau$_polar) are included in the
temperature-dependent electron relaxation time. These findings indicated that
the Janus $\beta$-PdXY monolayers are promising candidates for TE conversion
devices.
