  For high efficiency thermoelectric power conversion not only improvement of
materials properties but also prediction and synthesis of new thermoelectric
materials is needed. Here we have carried out a systematic investigation on
thermoelectric performance of newly predicted two dimensional (2D)
semiconducting SiS2 and SiSe2 monolayers of group IVA-VIA family using density
functional theory (DFT) and Boltzmann transport equation (BTE). Our computed
values of lattice thermal conductivity (kph) are ultralow which result very
high thermoelectric figure of merit (ZT) value of 0.78 (0.80) at 500K in SiS2
(SiSe2) monolayer. The ultralow values of kph are attributed to phonon-phonon
coupling of acoustic and low frequency optical branches which leads to larger
scattering, low group velocity, smaller mean free path and shorter lifetime of
phonons. It is also found from our investigation that p-type doping is more
effective than n-type doping to get optimal power factor (PF) and ZT. Our
theoretical investigation suggests that newly predicted semiconducting SiS2 and
SiSe2 monolayers can be very promising thermoelectric materials for fabrication
of high efficiency thermoelectric power generator to convert wastage heat into
electricity.
