The temperature dependence of the electrical properties is typically to doping level. At the larger n required to obtain reasonable PFs, the S 2 σ decrease with temperature due to the metallic-like decrease in σ with T typical of heavily-doped ("degenerate") semiconductors (Figs S13/S14 † ). As in Pnma SnS/SnSe, this is partially offset by an increase in the |S|, but for n-type π-SnSe there is a ∼30% reduction in the PFs from ∼2 to 1.4 mW m -1 K -2 from 300-700 K at our reference n = 4 × 10 19 cm -3 .