Carrier-filtering and phonon-blocking AgSnSe 2 -decorated grain boundaries to boost the thermoelectric performance of Cu 2 Sn 0.9 Co 0.1 S 3 .

Heavily co-doped Cu 2 SnS 3 can achieve a high power factor by relying on a high electrical conductivity ( σ ), which subsequently limits the ZT value with a large electronic thermal conductivity ( κ e ). We report here an enhanced ZT for Cu 2 Sn 0.9 Co 0.1 S 3 decorated with micro-nanoscale AgSnSe 2 along grain boundaries. The AgSnSe 2 phase served as a charge carrier filter by ionized impurity scattering, with a noticeable bottoming out of carrier mobility and a rapid increase in the Seebeck coefficient as the temperature increased from 423 to 573 K, which properly reduced the large σ and κ e while maintaining a high power factor of approximately 10 μW cm -1 K -2 at 773 K. Lattice thermal conductivity was markedly suppressed, and a low total thermal conductivity was obtained with strengthened phonon scattering by the AgSnSe 2 phase as a phonon barrier. With the synergistic effects on electrical and thermal transport, a maximum ZT of 0.93 at 773 K was achieved in Cu 2 Sn 0.9 Co 0.1 S 3 -3 wt% AgSnSe 2 .