Enhanced Thermoelectric Properties of Cu2SnSe3-Based Materials with Ag2Se Addition

In this work, (Ag, In)-co-doped Cu₂SnSe₃-based compounds are prepared using a self-propagating high-temperature synthesis process. Ag₂Se and as-synthesized (Ag, In)-co-doped Cu₂SnSe₃-based powders are mixed in a proportion according to the formula of Cu_1.85Ag_0.15Sn_0.91In_0.09Se₃/x Ag₂Se (x = 0, 3, 4, and 5%), which is followed by a subsequent plasma-activated sintering (PAS) to obtain consolidated composite bulks. A sandwich experiment is designed to reveal the evolution of the microstructure and phase composition of the composite samples during the PAS process. We investigate the reaction mechanism between Ag₂Se and Cu₂SnSe₃-based matrix as well as the influence of Ag₂Se on the phase composition, microstructure, and thermoelectric transport properties of the composites. Ag₂Se addition is proven to be effective to improve Ag solubility in the Cu_1.85Ag_0.15Sn_0.91In_0.09Se₃ matrix and introduce a CuSe secondary phase and an Ag-rich phase at grain boundaries. The electrical conductivity of Cu_1.85Ag_0.15Sn_0.91In_0.09Se₃/x Ag₂Se (x = 0, 3, 4, and 5%) composites decreases while the Seebeck coefficient increases with increasing Ag₂Se addition, resulting in an optimized power factor. Moreover, benefiting from the collective phonon scattering at various defects induced by Ag₂Se addition, the composite samples exhibit significantly suppressed lattice thermal conductivity, which reaches as low as 0.11 W m^-1 K^-1 at 700 K for the x = 5% sample. A peak figure-of-merit (ZT) of 1.26 at 750 K and an average ZT of 0.75 at 300-800 K are obtained for Cu_1.85Ag_0.15Sn_0.91In_0.09Se₃/5% Ag₂Se. This work provides an efficient way to improve average ZT values of Cu₂SnSe₃-based compounds for promising power generation at intermediate temperatures.

Synergistically Enhanced Thermoelectric Performance of Cu2SnSe3-Based Composites via Ag Doping Balance

As an ecofriendly and low-cost thermoelectric material, Cu₂SnSe₃ has recently drawn much attention. In this work, the thermoelectric properties of Cu₂SnSe₃-based materials have been synergistically optimized. Ag doping at the Cu site enables strong phonon scattering via large strain field fluctuations and increases the effective mass of carriers through band engineering, which results in a reduced lattice thermal conductivity and enhanced Seebeck coefficient. Thus, a peak ZT value of 1.04 at 800 K is obtained for Cu_1.85Ag_0.15SnSe₃. Then, In is further doped at the Sn site to further increase the carrier concentration and power factor; the ZT values of Cu_1.85Ag_0.15Sn_1-yIn_ySe₃ samples are highly improved in the temperature range of 300-800 K, and a peak ZT value of 1.12 is obtained at 800 K for the Cu_1.85Ag_0.15Sn_0.91In_0.09Se₃ sample. Considering that Ag₂S decomposes to Ag and S completely under vacuum and high current field, the sulfur vapor would be pumped away by a vacuum pump, and the generated Ag not only enriches at the grain boundary of the Cu_1.85Ag_0.15Sn_0.91In_0.09Se₃ bulk material but also enters the matrix and occupies the Cu site, leading to the extruding Cu and Se forming the second phase of CuSe nanoparticles. Thus, the power factor greatly improves to 13.8 μW cm^-1 K^-2 at 700 K, and the lattice thermal conductivity is as low as 0.12 W m^-1 K^-1 at 800 K for the Cu_1.85Ag_0.15Sn_0.91In_0.09Se₃/4% Ag₂S composite. Finally, a high ZT value of 1.58 is obtained at 800 K for the Cu_1.85Ag_0.15Sn_0.91In_0.09Se₃/4% Ag₂S composite, which is nearly an increase of 204% compared to that of Cu₂SnSe₃. This work provides an effective solution to optimize the conflicting material properties for Cu₂SnSe₃-based thermoelectric materials.

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.

Improved Figure of Merit of Cu2SnSe3 via Band Structure Modification and Energy-Dependent Carrier Scattering

As an ecofriendly thermoelectric material with intrinsic low thermal conductivity, ternary diamond-like Cu₂SnSe₃ (CSS) has attracted much attention. Nevertheless, its figure of merit, ZT, is limited by its small thermopower (S) and power factor (PF). Here, we show that an increase in thermopower by 63% and a carrier-mobility rise of 81% at 300 K can be simultaneously achieved through 5% substitution of Fe for Sn due to both enhancement of electronic density of states and degeneracy of multiple valence band maxima, which lead to high PF = 10.3 μW·cm^-1·K^-2 at 823 K for Fe-doped CSS (CSFS). Besides, an ultrahigh PF of 14.8 μW·cm^-1·K^-2 (at 773 K) and 45% reduction of lattice thermal conductivity (at 823 K) are realized for CSFS-based composites with 0.125 wt % of MgO nanoinclusions, owing to further enhancement of S via energy-dependent scattering and strong phonon scattering by the embedded nanoparticles. Consequently, a maximum ZT = 1 at 823 K is reached for the CSFS/f MgO composite samples with f = 0.125 wt %, which is around 2.5 times larger than that of the CSS compound.

Thermoelectric Properties of Cu2SnSe3-SnS Composite

Heavily doped degenerate semiconductors such as Cu₂SnSe₃ (CTSe) attracted attention in thermoelectric (TE) and optoelectronic fields, due to their high electrical conductivity and small band gap. The small Seebeck coefficient of undoped CTSe, however, is the major issue in achieving high TE performance (figure of merit, ZT). Here, we report that the Seebeck coefficient of CTSe can be controlled by adding SnS within a CTSe matrix. CTSe-SnS composite has not only high Seebeck coefficient in the range of 300–500 µVolt/K but thermal conductivity which is lower than that of pristine CTSe due to the scattering at the interface between the matrix and the SnS particles. A reasonable ZT of 0.18 is achieved at 570 K by adding a small amount (3 wt.%) of SnS to the CTSe matrix.

Enhanced Thermoelectric Performance of Cu₂SnSe₃-Based Composites Incorporated with Nano-Fullerene

In this study, nano-sized fullerene C₆₀ powder was sufficiently mixed with Cu₂SnSe₃ powder by ball milling method, and the C₆₀/Cu₂SnSe₃ composites were prepared by spark plasma sintering technology. The fullerene C₆₀ distributed uniformly in the form of clusters, and the average cluster size was less than 1 μm. With increasing C₆₀ content, the electrical conductivity of C₆₀/Cu₂SnSe₃ composites decreased, while the Seebeck coefficient was enhanced. The thermal conductivity of composites decreased significantly, which resulted from the phonon scattering by the C₆₀ clusters located on the grain boundaries of the Cu₂SnSe₃ matrix. The highest figure of merit ZT of 0.38 was achieved at 700 K for 0.8% C₆₀/Cu₂SnSe₃ composite.