Electrochemical Effects in Thermoelectric Polymers.

Conductive polymers such as PEDOT:PSS hold great promise as flexible thermoelectric devices. The thermoelectric power factor of PEDOT:PSS is small relative to inorganic materials because the Seebeck coefficient is small. Ion conducting materials have previously been demonstrated to have very large Seebeck coefficients, and a major advantage of polymers over inorganics is the high room temperature ionic conductivity. Notably, PEDOT:PSS demonstrates a significant but short-term increase in Seebeck coefficient which is attributed to a large ionic Seebeck contribution. By controlling whether electrochemistry occurs at the PEDOT:PSS/electrode interface, the duration of the ionic Seebeck enhancement can be controlled, and a material can be designed with long-lived ionic Seebeck enhancements.

Impact of Surface-Bound Small Molecules on the Thermoelectric Property of Self-Assembled Ag2Te Nanocrystal Thin Films.

Small molecules with functional groups can show different electron affinity and binding behavior on nanocrystal surface, which in principle could be used to alternate the electrical transport in self-assembled nanocrystal thin films. These small molecules can also serve for scattering the phonons to reduce the thermal conductivity. Here, we present our research on the thermoelectric characteristic of self-assembled silver telluride (Ag2Te) nanocrystal thin films that are fabricated by a layer-by-layer (LBL) dip-coating process. We perform investigations on the electrical conductivity and Seebeck coefficient on the Ag2Te nanocrystal thin films containing hydrazine, 1,2-ethanedithiol, and ethylenediamine between 300 and 400 K. We also use photothermal (PT) technique to obtain the thermal conductivity of the films at room temperature and estimate the thermoelectric figure of merit (ZT). The experimental results suggest that the surface-bound small molecules could serve as a beneficial component to build nanocrystal-based thermoelectric devices operating at low temperature.

Thermoelectric Properties of Solution Synthesized Nanostructured Materials.

Thermoelectric nanocomposites made by solution synthesis and compression of nanostructured chalcogenides could potentially be low-cost, scalable alternatives to traditional solid-state synthesized materials. We review the progress in this field by comparing the power factor and/or the thermoelectric figure of merit, ZT, of four classes of materials: (Bi,Sb)2(Te,Se)3, PbTe, ternary and quaternary copper chalcogenides, and silver chalcogenides. We also discuss the thermal conductivity reduction associated with multiphased nanocomposites. The ZT of the best solution synthesized materials are, in several cases, shown to be equal to or greater than the corresponding bulk materials despite the generally reduced mobility associated with solution synthesized nanocomposites. For the solution synthesized materials with the highest performance, the synthesis and processing conditions are summarized to provide guidance for future work.

Large-scale solution-phase production of Bi2Te3 and PbTe nanowires using Te nanowire templates.

We report the first demonstration of large scale (>10 g) and high yield (>80%) production of ultrathin (<15 nm) PbTe and Bi2Te3 nanowires in a low-cost solution-based process using Te nanowire templates. The PbTe or Bi2Te3 nanowires can be compressed into high relative density disks with nanoscale grains.

Structure and thermoelectric properties of spark plasma sintered ultrathin PbTe nanowires.

Solution-synthesized thermoelectric nanostructured materials have the potential to have lower cost and higher performance than materials synthesized by solid-state methods. Herein we present the synthesis of ultrathin PbTe nanowires, which are compressed by spark plasma sintering at various temperatures in the range of 405-500 °C. The resulting discs possess grains with sizes of 5-30 µm as well as grains with sizes on the order of the original 12 nm diameter PbTe nanowires. This micro- and nanostructure leads to a significantly reduced thermal conductivity compared to bulk PbTe. Careful electron transport analysis shows suppressed electrical conductivity due to increased short-range and ionized defect scatterings, while the Seebeck coefficient remains comparable to the bulk value. The PbTe nanowire samples are found unintentionally p-type doped to hole concentrations of 2.16-2.59 × 10(18) cm(-3). The maximum figure of merit achieved in the unintentionally doped spark plasma sintered PbTe nanowires is 0.33 at 350 K, which is among the highest reported for unintentionally doped PbTe at low temperatures.

The effects of the size and the doping concentration on the power factor of n-type lead telluride nanocrystals for thermoelectric energy conversion.

For the first time, we demonstrate a successful synthesis of colloidal n-type lead telluride nanocrystals doped with iodine. By tuning the reaction time and iodine concentration in the precursor solution, nanocrystals with different sizes and doping concentrations are synthesized. The Seebeck coefficient and electrical conductivity of the nanocrystals are measured on nanocrystal thin films fabricated by dip-coating glass substrates in the nanocrystals solution. Investigations on the influence of size and doping concentration on the electrical properties have been performed. The results show that the size of the nanocrystals significantly influences the electrical conductivity but not the Seebeck coefficient of nanocrystal films, while higher doping concentration leads to lower Seebeck coefficient but higher electrical conductivity in the nanocrystal films. Proof-of-concept thin-film thermoelectric modules are also fabricated using both p-type and n-type PbTe nanocrystals for the conversion of thermal energy into electrical energy.

Measurement of thermal conductivity of PbTe nanocrystal coated glass fibers by the 3ω method.

Fiber-based thermoelectric materials can conform to curved surfaces to form energy harvesting devices for waste heat recovery. Here we investigate the thermal conductivity in the axial direction of glass fibers coated with lead telluride (PbTe) nanocrystals using the self-heated 3ω method particularly at low frequency. While prior 3ω measurements on wire-like structures have only been demonstrated for high thermal conductivity materials, the present work demonstrates the suitability of the 3ω method for PbTe nanocrystal coated glass fibers where the low thermal conductivity and high aspect ratio result in a significant thermal radiation effect. We simulate the experiment using a finite-difference method that corrects the thermal radiation effect and extract the thermal conductivity of glass fibers coated by PbTe nanocrystals. The simulation method for radiation correction is shown to be generally much more accurate than analytical methods. We explore the effect of nanocrystal volume fraction on thermal conductivity and obtain results in the range of 0.50-0.93 W/mK near room temperature.

Synthesis and thermoelectric properties of compositional-modulated lead telluride-bismuth telluride nanowire heterostructures.

We demonstrate the rational solution-phase synthesis of compositional modulated telluride nanowire heterostructures containing lead telluride (PbTe) and bismuth telluride (Bi2Te3). By tuning the ratio between PbTe and Bi2Te3 through adjusting the amount of critical reactants and precursors during the synthesis, the influence of composition on the thermoelectric properties of the nanowire heterostructures has been investigated in hot pressed nanocomposite pellets. Measurements of the thermoelectric properties show strongly reduced thermal conductivity that leads to an enhanced thermoelectric figure of merit (ZT) of 1.2 at 620 K.

Design principle of telluride-based nanowire heterostructures for potential thermoelectric applications.

We present a design principle to develop new categories of telluride-based thermoelectric nanowire heterostructures through rational solution-phase reactions. The catalyst-free synthesis yields Te-Bi(2)Te(3) "barbell" nanowire heterostructures with a narrow diameter and length distribution as well as a rough control over the density of the hexagonal Bi(2)Te(3) plates on the Te nanowire bodies, which can be further converted to other telluride-based compositional-modulated nanowire heterostructures such as PbTe-Bi(2)Te(3). Initial characterizations of the hot-pressed nanostructured bulk pellets of the Te-Bi(2)Te(3) heterostructure show a largely enhanced Seebeck coefficient and greatly reduced thermal conductivity, which lead to an improved thermoelectric figure of merit. This approach opens up new platforms to investigate the phonon scattering and energy filtering.

Semiconductor nanostructure-based photovoltaic solar cells.

Substantial efforts have been devoted to design, synthesize, and integrate various semiconductor nanostructures for photovoltaic (PV) solar cells. In this article, we will review the recent progress in this exciting area and cover the material chemistry and physics related to all-inorganic nanostructure solar cells, hybrid inorganic nanostructure-conductive polymer composite solar cells, and dye-sensitized solar cells.

Hierarchical assembly of micro-/nano-building blocks: bio-inspired rigid structural functional materials.

The huge diversity of hierarchical micro-/nano-rigid structures existing in biological systems is increasingly becoming a source of inspiration of materials scientists and engineers to create next-generation advanced functional materials. In the past decades, these multiscale hierarchical structures have been intensively investigated to show their contributions to high performance in mechanical properties. Recently, accompanied with the development of nanotechnology, some biologically hierarchical rigid structures have been duplicated and mimicked in artificial materials through hierarchical organization of micro-/nano-building blocks. In this critical review, we will present biological rigid structural models, functional micro-/nano-building blocks, and hierarchical assembly techniques for the manufacture of bio-inspired rigid structural functional materials (177 references).