Thermoelectric figure of merit ($ZT$)

The dimensionless material figure of merit is $ZT=S^2\sigma T/({\kappa }_e+{\kappa }_l)$. Device efficiency depends on its temperature dependence and parasitic losses, not only on one peak value.

For a constant-property pair with Seebeck difference ${\alpha }_{np}$, electrical resistance $R$ and thermal conductance $K$, $Z_{\mathrm{pair}}={\alpha }_{np}^2/(RK)$. With identical legs of individual resistivity $\rho$ and thermal conductivity $\kappa$, this is ${\alpha }_{np}^2/(4\rho \kappa )$, not ${\alpha }_{np}^2/(\rho \kappa )$.

Bringing several electronic valleys close in energy to increase useful valley degeneracy. Its transport benefit must be assessed together with mobility and scattering.

The combination $S^2\sigma$ of Seebeck coefficient and electrical conductivity. Increasing it alone need not increase ZT if thermal conductivity rises as well.