Simultaneously engineering electronic and phonon band structures for high-performance n-type polycrystalline SnSe

n-type SnSe thermoelectrics has been seriously underdeveloped because of a lack of effective performance-enhancing strategies and doping/alloying agents. Herein, we report that conduction band electronic and phonon structures can be advantageously engineered simultaneously in both Pnma and Cmcm SnSe phases by dually incorporating Pb and Cd. They enhance the density of states near the conduction band edge in both phases by converging band minima and increasing effective mass (m₀), consequently enhancing Seebeck coefficients (S) without damaging electrical conductivity. Because exclusively divalent Pb and Cd cations reduce innate Sn vacancies, carrier mobility decreases marginally despite the increased m₀ and |S|. The tetrahedral Cd displaced from the cationic sublattice and much heavier Pb significantly soften and scatter phonon transport, depressing thermal conductivity significantly. Concurrently improved power factor and decreased thermal conductivity achieve an extraordinarily high thermoelectric (TE) figure of merit, ZT, of ∼2.23 at 873 K, a record high for all polycrystalline n-type TE materials.