where e is the elementary charge and µ is the carrier mobility determined by the lifetime τ and conductivity effective mass m * σ , all of which are obtained from a weighted average over the bands that contribute to the transport. The Seebeck coefficient is related to the Seebeck effective mass m * where q = ±e for hole and electron carriers, respectively, h is the Planck constant and k B is the Boltzmann constant. The m * σ and m * S for the Pnma and π phases of SnS and SnSe were determined according to the procedure in Ref. at a nominal n = 4 × 10 19 cm -1 and T = 700 K and are presented in Table . † We calculate m * σ between 1.1-2.5 m e for the π phases, which are ∼ 5 -10× larger than the 0.15-0.27 m e for the Pnma phases. On the other hand, the m * S of the π phases are 3-4× larger than those of the Pnma phases. These differences partially account for the lower conductivity and higher Seebeck coefficients of the π phases. Furthermore, the m * σ for electrons are around 30 and 45-50% lower than holes in the Pnma and π phases, respectively, whereas the m * S differ by < 10%, which explains why these materials are predicted to show better n-type conductivity and power factors.