With these electrical and thermal transport properties, we can calculate the theoretical dimensionless figure of merit ZT for Sb2Si2Te6 and Sc2Si2Te6 as functions of temperature and carrier concentration (Figures and). It is found that for Sb2Si2Te6, the p-type system achieves a superior ZT compared to the n-type system, while the results are opposite in Sc2Si2Te6. For p-type doping, Sb2Si2Te6 consistently exhibits higher ZT than Sc2Si2Te6 regardless of the transport direction, which can be attributed to the lower lattice thermal conductivity (Figure ) and higher electrical conductivity (Figure ). However, for n-type doping, we find an extremely large calculated ZT of 3.51 at 1000 K for the in-plane direction in Sc2Si2Te6, greatly surpassing the optimal ZT observed (1.69 -2.06) for n-type Sb2Si2Te6. The exceptional ZT observed for n-type Sc2Si2Te6 originates from the dramatically enhanced Seebeck coefficient and PF (Table ) caused by ultra-high band degeneracy, as discussed above, in combination with the low lattice thermal conductivity (Figure ). This value surpasses that of typical layered thermoelectric materials, such as p-type SnSe (measured ZT of ~ 2.6 at 923 K) and BiCuOSe ( predicted ZT of 1.32 at 1000 K). Similar predictions of high ZT through high Seebeck coefficient and PF have also been observed in other materials, such as Na2TISb (ZT ~ 4.81) and InBrSe (ZT ~ 5.12). We note that while the in-plane ZT is extremely high for n-type Sc2Si2Te6, the cross-plane ZT is very low (0.06 at 1000 K, the ZT without colour scale of n-type Sc2Si2Te6 along the cross-plane direction as shown in Figure ), which can be attributed to the low cross-plane n-type conductivity (Figure ) -itself a consequence of the relatively localised Sc-d orbitals in the Sc2Si2Te6 CBM, giving weak interlayer electronic interactions. Overall, these results