Carrier-filtering and phonon-blocking AgSnSe₂-decorated grain boundaries to boost the thermoelectric performance of Cu₂Sn_0.9Co_0.1S₃

Heavily co-doped Cu₂SnS₃ 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 Cu₂Sn_0.9Co_0.1S₃ decorated with micro-nanoscale AgSnSe₂ along grain boundaries. The AgSnSe₂ 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 AgSnSe₂ 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 Cu₂Sn_0.9Co_0.1S₃-3