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Combining inorganic thermoelectric (TE) materials with conductive polymers is one promising strategy to develop flexible thermoelectric (FTE) films and devices. As most inorganic materials tried up until now in FTE composites are composed of scarce or toxic elements, and n -type FTE materials are particularly desired, we combined the abundant, inexpensive, nontoxic Zn-doped chalcopyrite (Cu 1- x Zn x FeS 2 , x = 0.01, 0.02, 0.03) with a flexible electrical network constituted by poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) and graphene for n -type FTE films. Hybrid films from the custom design of binary Cu 1- x Zn x FeS 2 /PEDOT:PSS to the optimum design of ternary Cu 0.98 Zn 0.02 FeS 2 /PEDOT:PSS/graphene are characterized. Compared with the binary film, a 4-fold enhancement in electrical conductivity was observed in the ternary film, leading to a maximum power factor of ∼ 23.7 μW m -1 K -2 . The optimum ternary film could preserve >80% of the electrical conductivity after 2000 bending cycles, exhibiting an exceptional flexibility due to the network constructed by PEDOT:PSS and graphene. A five-leg thermoelectric prototype made of optimum films generated a voltage of 4.8 mV with a Δ T of 13 °C. Such an evolution of an inexpensive chalcopyrite-based hybrid film with outstanding flexibility exhibits the potential for cost-sensitive FTE applications.

Metal oxides are considered suitable candidates for thermoelectric materials owing to their high chemical stabilities. The formation of ordered nanopores within these materials, which decreases thermal conductivity (κ), has attracted significant interest. However, the electrical conductivity (σ) of reported nanoporous metal oxides is low, owing to electron scattering at the thin pore walls and many grain boundaries formed by small crystallites. Therefore, a novel synthesis method that can control pore walls while forming relatively large crystallites to reduce κ and retain σ is required. In this study, we used indium tin oxide (ITO), which is a typical example among metal oxides with high σ. Nanoporous ITOs with large crystallite sizes of several hundred nanometers and larger were successfully prepared using indium chloride as a source of indium. The pore sizes were varied using colloidal silica nanoparticles with different particle sizes as templates. The crystal phase and nanoporous structure of ITO were preserved after spark plasma sintering at 723 K and 80 MPa. The κ was significantly lower than that reported for bulk ITO due to the phonon scattering caused by the nanoporous structure and thin pore walls. There was a limited decrease in σ even with high porosity. These findings show that κ and σ are independently controllable through the precise control of the structure. The control of the thickness of the pore walls at tens of nanometers was effective for the selective scattering of phonons, while almost retaining electron mobility. The remarkable preservation of σ was attributed to the large crystallites that maintained paths for electron conduction and decreased electron scattering at the grain boundaries.

Polycrystalline higher manganese silicides Mn 1- x Cr x Si 1.74 ( x = 0, 0.10, 0.20) with Cr single doping and Mn 1-2 y Cr y Fe y Si 1.74 ( y = 0.10, 0.20) with Cr-Fe double doping have been prepared by arc melting and spark plasma sintering. Hall effect results and thermoelectric transport properties measurements demonstrate that Cr doping effectively increases the carrier concentration, thereby giving rise to enhanced electrical conductivity and power factor. Coupled with an enlarged effective mass and a reduction in the lattice thermal conductivity, a maximum zT is realized in Mn 0.90 Cr 0.10 Si 1.74 . It is also proved that the carrier concentration and carrier scattering mechanism could be altered through further doping on the Mn site by Fe, which leads to a lower electrical conductivity and higher Seebeck coefficient. Factors related to the suppression of the lattice thermal conductivity, like mass and strain field fluctuation scattering and electron-phonon scattering, are also analyzed. This work reveals the effects of Cr single doping and Cr-Fe dual-element doping on the carrier concentration, carrier scattering mechanism, and lattice thermal conductivity of higher manganese silicides.

We propose a simple, low-cost, and large-area method to increase the thermoelectric figure of merit (ZT) in silicon membranes by the deposition of an ultrathin aluminum layer. Transmission electron microscopy showed that short deposition of aluminum on a silicon substrate covers the surface with an ultrathin amorphous film, which, according to recent theoretical works, efficiently destroys phonon wave packets. As a result, we measured 30-40% lower thermal conductivity in silicon membranes covered with aluminum films while the electrical conductivity was not affected. Thus, we have achieved 40-45% higher ZT values in membranes covered with aluminum films. To demonstrate a practical application, we applied this method to enhance the performance of a silicon membrane-based thermoelectric device and measured 42% higher power generation.