We present a method of modeling transport coefficients from first-principles calculations. We introduce the transport distribution that contains all electronic information and from which transport coefficients can easily be calculated. We use this method to analyze ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ and calculate its transport coefficients for a comparison with experiment. The transport distribution gives an improved insight into the relationship between transport properties and electronic structure and is a valuable tool in the search for improved thermoelectric materials.

Transport coefficients from first-principles calculations

Thermoelectric (TE) materials have the capability of converting heat into electricity, which can improve fuel efficiency, as well as providing robust alternative energy supply in multiple applications by collecting wasted heat, and therefore, assisting in finding new energy solutions. In order to construct high performance TE devices, superior TE materials have to be targeted via various strategies. The development of high performance TE devices can broaden the market of TE application and eventually boost the enthusiasm of TE material research. This review focuses on major novel strategies to achieve high-performance TE materials and their applications. Manipulating the carrier concentration and band structures of materials are effective in optimizing the electrical transport properties, while nanostructure engineering and defect engineering can greatly reduce the thermal conductivity approaching the amorphous limit. Currently, TE devices are utilized to generate power in remote missions, solar-thermal systems, implantable or/wearable devices, the automotive industry, and many other fields; they are also serving as temperature sensors and controllers or even gas sensors. The future tendency is to synergistically optimize and integrate all the effective factors to further improve the TE performance, so that highly efficient TE materials and devices can be more beneficial to daily lives.

https://espace.library.uq.edu.au/view/UQ:722903/UQ722903_OA.pdf

High Performance Thermoelectric Materials: Progress and Their Applications

Template-directed synthesis represents a straightforward approach to generating one-dimensional (1D) nanostructures. In this approach, the template serves as a scaffold against which other materials are assembled with a morphology similar (or complementary) to that of the template. A variety of 1D templates have been successfully demonstrated for use with this process, and examples include channels in porous materials, 1 hexagonal assemblies of surfactants or block copolymers, 2 and 1D nanostructures synthesized using other chemical methods. 3,4 These templating processes, although very versatile, often led to the formation of polycrystalline 1D nanostructures that are limited in use for device fabrication or property measurements. Highly crystalline nanowires were only produced at temperatures around 800-1200°C when carbon nanotubes were allowed to react with proper chemicals under carefully controlled conditions. 3 A template-directed process that could generate single-crystalline nanowires in the solution phase and at room temperature is yet to be demonstrated. Here we describe a template-directed reaction, in which single-crystalline nanowires of trigonal Se ( t-Se) were quantitatively converted into single-crystalline nanowires of Ag2Se by reacting with aqueous AgNO 3 solutions at room temperature. The single-crystalline 1D morphology of the wire-like templates was retained in the final products with high fidelity. Silver selenide exhibits many interesting and useful properties. 5

Single-Crystalline Nanowires of Ag2Se Can Be Synthesized by Templating against Nanowires of Trigonal Se

Researches on flexible thermoelectric materials usually focus on conducting polymers and conducting polymer-based composites; however, it is a great challenge to obtain high thermoelectric properties comparable to inorganic counterparts. Here, we report an n-type Ag2Se film on flexible nylon membrane with an ultrahigh power factor ~987.4 ± 104.1 μWm−1K−2 at 300 K and an excellent flexibility (93% of the original electrical conductivity retention after 1000 bending cycles around a 8-mm diameter rod). The flexibility is attributed to a synergetic effect of the nylon membrane and the Ag2Se film intertwined with numerous high-aspect-ratio Ag2Se grains. A thermoelectric prototype composed of 4-leg of the Ag2Se film generates a voltage and a maximum power of 18 mV and 460 nW, respectively, at a temperature difference of 30 K. This work opens opportunities of searching for high performance thermoelectric film for flexible thermoelectric devices. Although flexible thermoelectric materials based on conducting polymers are attractive for energy harvesting, their performance is inferior to their inorganic counterparts. Here, the authors present a facile method to deliver inorganic nanowire films with high power factor and flexibility.

High performance n-type Ag 2 Se film on nylon membrane for flexible thermoelectric power generator

The development of kesterite Cu2ZnSn(S,Se)4 thin-film solar cells is currently hindered by the large deficit of open-circuit voltage (Voc), which results from the easy formation of CuZn antisite acceptor defects. Suppressing the formation of CuZn defects, especially near the absorber/buffer interface, is thus critical for the further improvement of kesterite solar cells. In this paper, it is shown that there is a large disparity between the defects in Cu- and Ag-based kesterite semiconductors, i.e., the CuZn or CuCd acceptor defects have high concentration and are the dominant defects in Cu2ZnSn(S,Se)4 or Cu2CdSnS4, but the AgZn acceptor has only a low concentration and the dominant defects are donors in Ag2ZnSnS4. Therefore, the Cu-based kesterites always show p-type conductivity, while the Ag-based kesterites show either intrinsic or weak n-type conductivity. Based on this defect disparity and calculated band alignment, it is proposed that the Voc limit of the kesterite solar cells can be overcome by alloying Cu2ZnSn(S,Se)4 with Ag2ZnSn(S,Se)4, and the composition-graded (Cu,Ag)2ZnSn(S,Se)4 alloys should be ideal light-absorber materials for achieving higher efficiency kesterite solar cells.

Engineering Solar Cell Absorbers by Exploring the Band Alignment and Defect Disparity: The Case of Cu‐ and Ag‐Based Kesterite Compounds

The Hall coefficient, electrical resistivity, and Seebeck coefficient of n-type specimens of β-Ag2Se, the low temperature polymorph of silver selenide, were measured over the temperature range from 70 to 300 K. The results showed maxima in both Hall coefficient and the electrical resistivity just below the onset of the intrinsic conduction range. This anomaly was qualitatively explained by the deviation of the Coulomb scattering from the usual assumption of independence due to the degenerate nature of the samples. The estimated energy gap for different samples of about 160 meV seems to confirm the existence of the second low temperature phase β2. This second phase is a probable reason for the relatively high thermoelectric figure of merit observed.

Thermoelectric and transport properties of β-Ag2Se compounds

Maxwell–Wagner polarization and interfacial charge at the multi-layers of thermoplastic polymers

Comparative studies between the growth characteristics of Bi2Te3 thin films deposited on SiO2, Si(100) and Si(111)

Mechanisms of spiral growth in Bi2Te3 thin films grown by the hot-wall-epitaxy technique

This paper reports on an investigation into space charge formation and decay at dielectric interfaces. In particular, the influence of temperature on the formation of the trap deep has been studied. A multi-dielectrics structure composed of two dielectric films, Low Density Polyethylene (LDPE) and Fluorinated Ethylene Propylene (FEP), was subjected to an electric stress level of +14.3 kV/mm at two temperatures, 40 °C and 60 °C, and space charge measurements were taken using the pulsed electro-acoustic technique. Space charge distributions were investigated for combinations of LDPE/FEP flat specimens. The time dependence of the space charge distribution was subsequently recorded at different temperatures under short circuit (depolarization) conditions. It was found that temperature plays a significant role in space charge dynamics at the dielectric interface, charge mobility, electrical conductivity, filling of the trap, and the formation of the shallow trap.

Marhoun Ferhat

Papers

Thermoelectric and transport properties of β-Ag2Se compounds

Maxwell–Wagner polarization and interfacial charge at the multi-layers of thermoplastic polymers

Comparative studies between the growth characteristics of Bi2Te3 thin films deposited on SiO2, Si(100) and Si(111)

Mechanisms of spiral growth in Bi2Te3 thin films grown by the hot-wall-epitaxy technique

Effect of temperature on trap depth formation in multi-layer insulation: Low density polyethylene and fluorinated ethylene propylene