Boosting Thermoelectric Performance of Cu2SnSe3 via Comprehensive Band Structure Regulation and Intensified Phonon Scattering by Multidimensional Defects

As an eco-friendly thermoelectric material, Cu₂SnSe₃ 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 19-fold 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 Cu₂Sn_0.82In_0.18Se_2.7S_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 CuInSe₂ nanoprecipitates. Consequently, a record high figure of merit of ZT = 1.51 at 858 K is acquired for Cu₂Sn_0.82In_0.18Se_2.7S_0.3, which is 4.7-fold larger than that of pristine Cu₂SnSe₃.

All-Scale Hierarchical Structure Contributing to Ultralow Thermal Conductivity of Zintl Phase CaAg0.2Zn0.4Sb

TiNiSi-type Zintl phase CaAgSb can transform into LiGaGe-type Zintl phase CaAg x Zn(1- x )/2Sb when some of the Ag atoms are substituted by Zn atoms, leading to an ultralow thermal conductivity of ≈0.4 W m-1 K-1 in the whole measured temperature range of CaAg0.2Zn0.4Sb. The microstructure is then investigated by spherical aberration-corrected electron microscopy on an atomic scale, which reveals an all-scale hierarchical structure that can scatter the phonons in a wide frequency range. There exist a large quantity of CaAgSb nanometer precipitates as well as quite a lot of edge dislocations close to these nanometer precipitates, thus releasing the stress caused by the mismatch between the precipitates and the parent phase. Many twin boundaries also exist around the CaAgSb precipitates. High-density point defects contain the randomly dispersed Ag vacancies and Zn atoms substituted for the Ag atoms. All these widely distributed multidimensional defects contribute to the decrease of lattice thermal conductivity in a wide temperature range.