Non-equilibrium transport in polymer mixed ionic–electronic conductors at ultrahigh charge densities A tandem transistor allowing for both electrochemical gating and field-effect gating is designed to achieve unprecedented doping and correlated physics in polymer semiconductors.
Non-equilibrium transport in polymer mixed ionic–electronic conductors at ultrahigh charge densities Mixed ionic–electronic transport in conducting polymers remains poorly understood. Here the authors observe non-equilibrium electronic transport when counterions are unable to equilibrate in response to gate-injected electronic carriers.
Mobile ion confinement for better thermoelectrics Restricting the directional segregation of mobile ions via strategic local ion confinement allows remarkable thermoelectric performance with better stability.
Highly stabilized and efficient thermoelectric copper selenide Cu2Se is of interest for thermoelectrics as it is environmentally sustainable and has a high figure of merit ZT; however, copper ion migration impacts device stability. Here a co-doping strategy that combines steric and electrostatic effects is shown to improve device stability as well as improving ZT to 3.
Thermoelectric measurements Joseph Heremans and Joshua Martin discuss the reproducibility of thermoelectric measurements and conclude that the uncertainty on the figure of merit zT is of the order of 15–20%.
Dynamic crystallography reveals spontaneous anisotropy in cubic GeTe Cubic materials such as GeTe have low lattice thermal conductivity, thought to arise from a non-cubic local structural transition. Here, using a variable-shutter pair distribution function method, the average structure is shown to be crystalline but with anisotropic dynamics at higher temperatures.
Anisotropy boosts transverse thermoelectrics Despite its tiny magnetization, the non-collinear antiferromagnet YbMnBi2 is shown to possess exceptional transverse thermoelectric performance owing to its anisotropic transport properties.
Giant anomalous Nernst signal in the antiferromagnet YbMnBi2 The anomalous Nernst effect (ANE) in topological materials with large Berry curvature shows great potential for transverse thermoelectrics, but antiferromagnets typically show small ANEs. The antiferromagnet YbMnBi2 has an ANE thermopower of 3 μV K −1 , similar to ferromagnets, and a larger ANE conductivity.
High-performance thermoelectrics and challenges for practical devices Thermoelectric materials can generate energy from a heat differential. This Review provides an overview of mid- to high-temperature thermoelectrics, their application in modules, and the issues that need to be addressed to enable commercial implementation of state-of-the-art materials.
Breaking thermoelectric performance limits Through meticulous care for detail, researchers have now shattered the ceiling on thermoelectric performance, achieving a figure of merit above 3 for bulk SnSe polycrystalline powder.
Polycrystalline SnSe with a thermoelectric figure of merit greater than the single crystal SnSe has a very high thermoelectric figure of merit ZT, but uncommonly polycrystalline samples have higher lattice thermal conductivity than single crystals. Here, by controlling Sn reagent purity and removing SnOx impurities, a lower thermal conductivity is achieved, enabling ZT of 3.1 at 783 K.
Quantifying charge carrier localization in chemically doped semiconducting polymers A model describing the behaviour of charge carriers in semiconducting polymers both in the hopping-like and metal-like regimes is developed, and used to quantify charge carrier localization and other transport parameters in organic semiconductors.
Twisting the thermoelectric potential A multifunctional device produces a much-improved thermoelectric-driven transverse voltage by exploiting a thermoelectric current to drive an anomalous Hall effect in a ferromagnet.
Seebeck-driven transverse thermoelectric generation Transverse thermeoelectrics can simplify devices as the electric field and heat gradient are perpendicular, but the power output is much less than in standard devices. Here, by forming a closed circuit of thermoelectric and magnetic materials, a much larger transverse thermopower is generated.
Thermoelectric cooling materials Thermoelectric materials can generate electricity from waste heat but can also use electricity for cooling. This Perspective discusses coefficients of performance for these systems and the state-of-the-art for materials, and suggests strategies for the discovery of improved thermoelectric materials.
Thermoelectrics with a twist Inducing a topological phase transition by applying pressure is shown to be a successful strategy for improving the performance of thermoelectric materials.
Enhancement of thermoelectric performance across the topological phase transition in dense lead selenide By applying a pressure of 2.8 GPa using a diamond anvil cell, a topological phase transition is found to occur in Cr-doped PbSe. This enables a thermoelectric figure of merit ZT of 1.7 at room temperature.
The Fermi surface geometrical origin of axis-dependent conduction polarity in layered materials A single-band metal whose carriers behave as electrons or holes depending on the direction of travel is observed. The effect arises from a particular type of Fermi surface geometry.
Liquid-like thermal conduction in intercalated layered crystalline solids Investigation of the thermal transport properties of AgCrSe2 reveals complete suppression of the transverse acoustic phonons by ultrafast dynamic disorder with only the longitudinal acoustic mode surviving, resembling the thermal conduction of liquids.
A new member of the Hall family The spin Nernst effect — a spin accumulation in a ferromagnet in the direction normal to an applied thermal gradient and external magnetic field — has been experimentally demonstrated.
Turn your phonon Heat travelling down a thermal gradient has been found to undergo a significant deflection by a magnetic field in a multiferroic insulator.
Thermoelectric materials step up Xun Shi and Lidong Chen summarize recent progress in the field of thermoelectric materials in China, and discuss steps towards the realization of commercially viable devices.
Probing disorder in isometric pyrochlore and related complex oxides Disordering in complex oxides is important for their radiation resistance. It is now shown that pyrochlores disorder by the formation of a weberite-like phase, with similar behaviour observed in spinels, adding complexity to their disordering.
Feeling the squeeze Optimizing the electronic functionality of a thermoelectric molecular junction depends on both the chemistry of the molecule and external environmental conditions.
Molecular design and control of fullerene-based bi-thermoelectric materials The thermoelectric response of a fullerene–gold electrode single-molecule junction has been studied in a scanning tunnelling microscope. The junction exhibits positive and negative thermopower, dependent on molecule orientation and applied pressure.
Better thermoelectrics through glass-like crystals Twenty years ago, the 'phonon-glass, electron-crystal' concept changed thinking in thermoelectric materials research, resulting in new high-performance materials and an increased focus on controlling structure and chemical bonding to minimize irreversible heat transport in crystals.
Convergence of multi-valley bands as the electronic origin of high thermoelectric performance in CoSb3 skutterudites It is shown that the large thermoelectric capability of CoSb3 skutterudite can be associated with a secondary conduction band with high valley degeneracy, which can converge with the light conduction band at high temperatures.
Flexible n-type thermoelectric materials by organic intercalation of layered transition metal dichalcogenide TiS2 A flexible n-type material has been developed with a thermoelectric figure of merit of 0.28 at 373 K via the intercalation of organic cations between titanium disulphide monolayers.
Behind organics' thermopower Conjugated polymers with high electrical conductivity and high thermopower are now demonstrated. The electronic structure of these materials is that of a semi-metal, a previously unreported state for organic conductors.
Crossover from incoherent to coherent phonon scattering in epitaxial oxide superlattices Understanding the thermal transport properties of superlattice structures is relevant to a number of possible practical applications. Now, the scattering of phonons in oxide superlattices is shown to undergo a crossover from an incoherent to a coherent regime, which in turn strongly alters their thermal behaviour.
Thermopower enhancement by encapsulating cerium in clathrate cages Clathrate materials have been the subject of intense investigation because of their beneficial properties, in particular their low thermal conductivities. Now, improved thermopower at high temperatures arising from strong electron correlation effects has been achieved in a type-I clathrate containing cerium guest atoms.
Suppression of thermal conductivity by rattling modes in thermoelectric sodium cobaltate Sodium cobaltate has latterly received attention due to its appealing thermoelectric properties. By combining inelastic X-ray and neutron scattering results with detailed first-principles calculations, it is now shown that low-energy rattling modes of sodium ions within multi-vacancy clusters play a central role in determining the low thermal conductivity of this material.
Thermoelectric imaging of structural disorder in epitaxial graphene Heat is a form of energy that is transported from a hot to a cold region, but it is not a notion that is associated with the microscopic measurement of electronic properties. It is now shown that local thermoelectric measurements can be used for imaging structural disorder in graphene, with high sensitivity, on the atomic and nanometre scales, uncovering soliton-like domain-wall line-patterns separating different graphene regions.
Engineered doping of organic semiconductors for enhanced thermoelectric efficiency The conversion efficiency of heat to electricity in thermoelectric materials depends on both their thermopower and electrical conductivity. It is now reported that, unlike their inorganic counterparts, organic thermoelectric materials show an improvement in both these parameters when the volume of dopant elements is minimized; furthermore, a high conversion efficiency is achieved in PEDOT:PSS blends.
Extremely high electron mobility in a phonon-glass semimetal The silver chalcogenide semimetals are known for their appealing magnetoresistive properties. It is now shown that when copper silver selenide is doped with nickel, these properties are maintained, resulting in high electron mobilities and, in turn, a significant thermoelectric effect.
Surface chemistry mediates thermal transport in three-dimensional nanocrystal arrays The optical and electronic performance of inorganic nanocrystal assemblies stabilized by organic ligands has been extensively investigated, whereas less attention has been paid to their thermal transport properties. It is now shown that the thermal conductivity of these composite systems is determined by the vibrational states of both inorganic and ligand regions, as well as by their relative volumes.
Spin-current-driven thermoelectric coating A thin layer of yttrium iron garnet coating on different materials can transform wasted heat into voltage. The process is based on the spin Seebeck effect and could lead to new types of application that make use of omnipresent wasted heat.
Spin caloritronics Spin caloritronics focuses on the interaction of electron spins with heat currents. This Review describes newly discovered physical effects that have re-invigorated the field by stimulating further research into understanding the fundamentals of spin–phonon interactions, and providing new avenues to explore to improve current thermoelectric technology.
Copper ion liquid-like thermoelectrics A common route to obtain efficient thermoelectrics is to optimize the ratio between electrical and thermal conductivity. Typically, materials with a complex, glass-like phonon structure and therefore a very low thermal conductivity are studied. Now, a route showing that solid ions in a liquid-like state can have a low enough thermal conductivity to compete with the best existing thermoelectrics is proposed.
A new class of doped nanobulk high-figure-of-merit thermoelectrics by scalable bottom-up assembly In the quest for more efficient thermoelectrics, a common strategy has been to introduce nanostructures in bulk crystals, thus reducing the thermal conductivity without affecting the electrical transport properties. A route is now presented in which the aggregation of nanoplatelets creates nanostructured materials that have higher thermoelectric efficiencies than their bulk counterparts.
Thermal properties of graphene and nanostructured carbon materials The thermal properties of nanostructures have become a fundamental topic owing to the necessity of heat removal in increasingly smaller electronic devices. Carbon allotropes present a range of intriguing thermal features, with the thermal conductivity spanning five orders of magnitude at room temperature. The topic is reviewed here with particular emphasis on graphene, which exhibits the highest thermal conductivity observed.
The thermoelectric alternative Results show that achievable improvements may make solar thermoelectric generators competitive with other solar power conversion methods.
Giant anharmonic phonon scattering in PbTe Neutron scattering and first-principles calculations show that the small thermal conductivity of PbTe is due to anharmonic coupling between the acoustic phonon modes and the optical ferroelectric ones. The results provide a microscopic picture of why many good thermoelectrics are found near a ferroelectric lattice instability.
Green energy from a blue polymer Efficient energy harvesting from temperature gradients requires thermoelectric materials with low thermal and high electrical conductivities. A conducting polymer can fulfil these conditions if its doping level is controlled precisely.
High-performance flat-panel solar thermoelectric generators with high thermal concentration The conversion of sunlight into electricity has been dominated by photovoltaic and solar thermal power generation. A highly efficient solar to electric energy conversion device based on nanostructured thermoelectric materials and high solar concentration is now demonstrated. The results show potential for cost effective solar thermoelectric generation.
Optimization of the thermoelectric figure of merit in the conducting polymer poly(3,4-ethylenedioxythiophene) Organic materials are rarely considered for thermoelectric applications, because their low electrical conductivity limits the thermoelectric figure of merit (ZT). It is now shown that by optimizing the oxidation level in a polymer, ZT can reach 0.25, which approaches the values desirable for devices.
Dimensional crossover of thermal transport in few-layer graphene The ability to propagate heat in a film should improve with increasing thickness. However, graphene has a higher thermal conductivity than graphite, despite having a smaller thickness. The crossover from two-dimensional to bulk graphite is now studied experimentally and explained theoretically. The results may pave the way to thermal management applications in nanoelectronics.
Precise control of thermal conductivity at the nanoscale through individual phonon-scattering barriers Tailoring the thermal conductivity of nanostructured materials is a fundamental challenge for nano- and microelectronics heat management. It is now demonstrated how to modify the thermal conductivity of SiGe by engineering nanodot inclusions in regions as short as 15 nm. A similar approach could used on other materials, extending the range of thermal conductivities available.
An explosive thrust for nanotubes Carbon nanotubes direct chemically produced thermal waves, providing propulsion and thermopower waves that create electrical energy.
