  Combined with first-principles calculations and semiclassical Boltzmann
transport theory, Janus {\alpha}-STe2 and {\alpha}-SeTe2 monolayers are
investigated systematically. Janus {\alpha}-STe2 and {\alpha}-SeTe2 monolayers
are indirect semiconductors with band gaps of 1.20 and 0.96 eV. It is found
they possess ultrahigh figure of merit (ZT) values of 3.9 and 4.4 at 500 K,
much higher than that of the pristine {\alpha}-Te monolayer (2.8). The higher
ZT originates from Janus structures reduce lattice thermal conductivities
remarkably compared with pristine {\alpha}-Te monolayer. The much higher phonon
anharmonicity in Janus monolayers leads to significant lower lattice thermal
conductivity. It is also found electronic thermal conductivity can play an
important role in thermoelectric efficiency for the materials with quite low
lattice thermal conductivity. This work suggests the potential applications of
Janus {\alpha}-STe2 and {\alpha}-SeTe2 monolayers as thermoelectric materials
and highlights Janus structure as an effective way to enhance thermoelectric
performance.
