As an eco-friendly thermoelectric material, Cu 2 SnSe 3 has recently drawn much attention. However, its high electrical resistivity ρ and low thermopower S prohibit its thermoelectric performance. Herein, we show that a widened band gap and the increased density of states are achieved via S alloying, resulting in 1.6 times enhancement of S (from 170 to 277 μV/K). Moreover, doping In at the Sn site can cause a 19fold decrease of ρ and a 2.2 times enhancement of S (at room temperature) due to both multivalence bands' participation in electrical transport and the further enhancement of the density of states effective mass, which allows a sharp increase in the power factor. As a result, PF = 9.3 μW cm −1 K −2 was achieved at ∼800 K for the Cu 2 Sn 0.82 In 0.18 Se 2.7 S 0.3 sample. Besides, as large as 44% reduction of lattice thermal conductivity is obtained via intensified phonon scattering by In-doping-induced formation of multidimensional defects, such as Sn vacancies, dislocations, twin boundaries, and CuInSe 2 nanoprecipitates. Consequently, a record high figure of merit of ZT = 1.51 at 858 K is acquired for Cu 2 Sn 0.82 In 0.18 Se 2.7 S 0.3 , which is 4.7-fold larger than that of pristine Cu 2 SnSe 3 .
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