where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, and κ e and κ l are the electronic and lattice (phonon) components of the thermal conductivity. The best performing materials thus far have achieved ZT > 2 in a laboratory setting, including PbTe (2.5 at 973 K) and SnSe (2.6 at 973 K). Ideally, to compete with traditional energy conversion methods, materials with ZT > 3 are needed to achieve a 20-30% thermal-to-electrical energy conversion efficiency.