Mixtures of poly(3,4-ethylenedioxythiophene) and polystyrene sulfonate (PEDOT:PSS) can have reasonably large power factors when cast from aqueous suspensions in combination with co-solvents such as dimethyl sulfoxide (DMSO). As is often the case in thermoelectrics materials research on thin layers that are potentially anisotropic, the thermal conductivity is difficult to measure in the same direction as the power factor. We find, using time-domain thermoreflectance measurements of thermal conduction along multiple directions of relatively thick layers cast on PDMS substrates and sectioned using diamond blade microtomy, that the thermal conductivity can be highly anisotropic when the electrical conductivity is large. For highly conductive compositions, the in-plane thermal conductivity is a factor of ~3 larger than the through-thickness thermal conductivity. We relate the increase in thermal conductivity to the estimated electronic component of the thermal conductivity using the Wiedemann-Franz law. The data do not support an anomalously small or large value of the Lorenz number in PEDOT. To further examine the anisotropy of the samples and gain insight about how the sound velocities change with preparation conditions, we measure the longitudinal modulus by picosecond acoustics and the shear modulus using an elastomeric phase-shift mask that enables us to use pump-probe methods to determine surface acoustic wave velocities for acoustic wavelengths of 700 nm.

Due to their simple, low-cost processing, mechanical flexibility and toughness, and steadily improving electrical transport properties, polymers are increasingly used in electronic applications. Recently there has been renewed interest in the use of conducting polymers for thermoelectric energy conversion, for which their low thermal conductivity offers increased conversion efficiency.
The engineering of thermal conductivity in polymers has numerous complexities related to chemical bonding and nanoscale-to-microscale morphology. It is commonly held that heat transfer occurs rapidly along the polymer backbone, with a thermal bottleneck occurring at junctions between polymer chains. In this study we use molecular dynamics simulations to investigate heat transfer between two chains of PMMA in which we control the number of repeat units that interact, finding a linear relationship between the number of interacting units and the interchain thermal conductance. From the slope of this relationship we derive the basic junction thermal conductance between two PMMA units. We then extend this work to other polymers to study the junction thermal conductance for other pairs of repeat units as well as pairs of polymer side groups, examining the physical properties that link this conductance to the chemical structure and phonon density of states of these basic polymer units.

Electrically conducting polymers are interesting for applications ranging from thermoelectrics to chemical and biological sensing. Recently, thermal transport in polymers has received significant attention as very high and very low thermal conductivity polymers have been reported. The microstructure, including alignment and crystallinity, of the polymer chains significantly impacts the thermal and electrical transport. This work examines how controlling the microstructure through varying deposition conditions can yield tunable material properties for specific organic electronic applications. In this work, we measure both the thermal conductivity of microdots of electrodeposited polypyrrole using the 3omega; technique and the electrical conductivity of the polymer. The effects of deposition parameters are examined to determine the optimal deposition conditions.

Thermal transport is critical to the performance and reliability of polymer-based energy devices, ranging from solar cells to thermoelectrics. This work shows that the thermal conductivity of low band gap conjugated polymers, poly(4,8-bis-alkyloxybenzo[1,2-b:4,5-bprime;]dithiophene-2,6-diyl-alt-(alkylthieno[3,4-b]thiophene-2-carboxylate)-2,6-diyl) (PBDTTT), for photovoltaic applications can be actively tuned through side chain engineering. Compared to the original polymer modified with short branched side chains, the engineered polymer using all linear and long side chains shows a 160% increase in thermal conductivity. The polymer thermal conductivity exhibits a good correlation with the side chain lengths as well as the polymer crystallinity characterized using small-angle X-ray scattering (SAXS) and Gazing incident X-ray scattering (GIXS) experiments. Molecular dynamics simulations and atomic force microscopy are used to further probe the molecular-level local order of different polymers. It is found that the linear side chain modified polymer can facilitate the formation of more ordered structures, as compared to the branched side chain modified ones. In the PBDTTT:PCBM (([6,6]-Phenyl-C61 butyric acid methyl ester) bulk heterojunctions, film thermal conductivity change first follows a linear relationship with PCBM content when PCBM concentration is lower than 60%. It then saturates and converges to the low thermal conductivity of PCBM when the PCBM concentration further increases. The effective medium theory can describe such thermal conductivity changes only in the case where host phase switching effect and morphology factors as a result of varied binary phase ratio are considered. These results offer important guidance for actively tuning thermal conductivity of conjugated polymers through molecular level design.

Colloidal crystals are a well-established material class, which are typically fabricated by self-assembly of monodisperse colloidal particles such as polystyrene latex or silica spheres. A key feature of colloidal crystals is their periodicity, which is determined by the diameter of the constituting spheres. This results in constructive and destructive interference phenomena, which lead to photonic and phononic bandgaps.[1]
Further, the periodic and well-defined nanostructure of such colloidal crystals represent an ideal platform to study thermal transport phenomena. It allows relating structural changes to heat transfer, and thereby to develop a fundamental understanding of nano/mesostructured thermal conductivity.
In this contribution, we investigate the thermal conductivity of highly ordered colloidal crystals based on monodisperse polystyrene particles, which span a broad range of sizes. The presence of a multitude of interfaces results in highly insulating polymer films, with thermal conductivities as low as polystyrene foams, yet at much higher film densities. We also present the temperature dependent behaviour of such granular films, which feature a marked hysteresis and offer a potential route towards thermal switches.
[1] Still, T.; Cheng, W.; Retsch, M.; Sainidou, R.; Wang, J.; Jonas, U.; Stefanou, N.; Fytas, G. Phys. Rev. Lett.2008, 100, 194301

This presentation will first discuss our recent work developing a model to describe thermoelectric transport in organic semiconductors (OSCs). This model allows carrier transport parameters (e.g., carrier localization length) that are important for many OSC device applications to be independently derived from measured thermoelectric property data. The model can also be used to predict the conditions under which an OSC&’s thermoelectric power factor is maximized; we find that this maximum occurs at a relatively high carrier concentration.
Because OSCs typically have a low dopant ionization fraction, care must be taken to achieve this high carrier concentration without introducing so many non-ionized dopants that carrier transport is greatly impeded. We illustrate this through measurements of PEDOT:PSS which show that carrier mobility as well as all components of the thermoelectric figure-of-merit (ZT) improve as primarily non-ionized PSS dopants are removed from highly-doped (as formulated by H.C. Starck) PEDOT:PSS by means of a chemical treatment. We achieve a maximum ZT of 0.42 at room temperature in PEDOT:PSS through this dedoping process.
Finally, the presentation will discuss our further ongoing work aimed at better understanding and improving ZT in OSCs.

The potential performance of thermoelectric material is in general evaluated by the non-dimensional figure of merit, ZT=α²σT/κ, where α, σ, T, and κ are Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively. Considering wide class of materials where their thermoelectric properties are not necessarily limited by the Mott relation in semiconductors, there are two antithetical approaches to achieve high performance thermoelectric generators (TEGs), use of a very high σ and moderate α material, or a very high α and moderate (or even relatively low) σ one. Assuming that the low σ material also exhibits very low κ, the optimum structure of the TEG could be thinner when the low σ - low κ material is used because the influence of the contact resistances becomes minor. For the application to thin flexible TEGs, it would be beneficial to choose the latter case. Since promising thermoelectric materials for this purpose have not been established yet, quests with no preconceptions for new classes of thermoelectric materials are necessary. We have been, therefore, investigating a wide range of organic-based materials using an originally developed apparatus that can measure the Seebeck coefficients of even extremely high resistance or small volume samples [1]. In this presentation, we discuss the α-σ map that summarizes not only our survey works but also some reported values by other groups [2].
The area of carbon nanotube (CNT) composites overlaps with that of the metal-like organic conductors. Among them, properties of polymer conductors, such as PEDOT:PSS, are relatively close to those of typical inorganic thermoelectric materials. CNT/cage-shaped protein composites have a unique nature where α, σ and κ are separately controlled by the core-shell-type biomolecular junctions [3].
The thermoelectric properties of most of the typical organic semiconductors are plotted within the predicted area by the conventional theory for nondegenerated semiconductors. There are, however, several exceptional materials that exceed the limit. An organic Mott insulator exhibits a very high Seebeck coefficient at around the phase transition temperature. Seebeck coefficients of pure C₆₀ films are extremely large, around 50-120 mV/K [4]. To the best of our knowledge, these values are the highest even among inorganic and nanocarbon materials. Recently, the similar giant Seebeck effect has been observed with several other organic compounds, such as benzoporphyrins and thienothiophenes [5].
[1] M. Nakamura et al.: MRS Proceedings1197, 1197-D09-07 (2009); DOI:10.1557/PROC-1197-D09-07; [2] M. Nakamura, Oyobuturi82, 954 (2013) (in Japanese); [3] M. Ito et al.: Appl. Phys. Express7, 065102 (2014); also presented in this symposium; [4] H. Kojima et al.: presented in this symposium; [5] R. Abe et al.: presented in this symposium.

Understanding charge transport is critical to developing new organic and hybrid materials for applications ranging from solar cells to transistors and light emitting diodes. Polymeric semiconductors are also showing promise for thermoelectric applications. Thermopower, the entropy per charge carrier, offers an additional tool to probe transport and the electronic density of states. Here we investigate and compare the effect of fundamentally different doping mechanisms on thermoelectric properties, with a consistent set of thiophene-based polymers. The effort is two fold: to assess the viability of high mobility semiconducting polymers for thermoelectric applications, and to gain a deeper understanding of the underlying transport mechanism that governs the electronic, thermoelectric, and optical properties. We find that a general relationship between thermopower α and electrical conductivity σ,α prop; σ^-1/4, emerges over a very large range of conductivities, polymers, and doping schemes. The empirical relationship allows an approximate metric for comparing the performance of new materials to existing organic thermoelectric materials. We demonstrate how this empirical law can be used to understand improvements in the thermoelectric performance of polymers gained by processing conditions.

Conjugate polymers can exhibit semiconducting or metallic properties, largely depending on the level of doping and disorder. Such materials are especially attractive for thermoelectric applications due to their inherently low thermal conductivity, non-toxicity and low production cost. Despite the amount of research available, full understanding and description of the mechanisms responsible for the transport in polymer composites is still missing. We explore how thermal fluctuation-induced tunneling in disordered materials can govern the thermoelectric properties at low temperatures. The essence of this mechanism centers around the local conducting regions formed along the polymer chains upon doping. Tunneling transport between these regions is modified by local thermal fluctuations. We develop a linear response theory for charge and heat currents with applied external applied voltage and temperature gradient. The calculated properties are compared with experimental data. This work aims to further the fundamental understanding of polymers for potential thermoelectric applications.

A device with integrated on-chip architecture comprising micro-fabricated source-drain electrodes, temperature sensors, and a heater has been built to measure the Seebeck coefficients of field-effect transistors (FETs). This on-chip architecture has been used to measure the Seebeck coefficients of FETs of several solution-processed organic polymers whose thermally activated field-effect mobilities are relatively high, > 0.01 cm²/V-s at room temperature. The solution-processed organic polymers include two p-type polymers, Poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) and indacenodithiophene-co-benzothiadiazole (IDTBT), as well as an ambipolar polymer, the selenium derivative of diketopyrrolopyrrole-benzothiadiazole (PSeDPPBT). As gate voltages are increased the changing dependences of the measured Seebeck coefficients on gate voltage is a direct probe of energetic disorder in these materials. In addition to probing energetic disorder, the voltage-modulated Seebeck coefficient is also useful in mapping the transition from bulk-limited conduction to accumulation layer dominated conduction within the FET. Capacitance measurements indicate that the surface carrier densities are between 10¹¹ and 10¹³ cm^-2. The corresponding Seebeck coefficients are large, between 1000 and 200 mu;V/K, and show no discernible temperature dependences over the measured temperature range, 240 - 340 K. The implications of the voltage-modulated Seebeck measurements are examined using various accepted models that are used to explain charge transport in organic semiconductors.
References:
1. Deepak Venkateshvaran et al., Field-effect modulated Seebeck coefficient measurements in an organic polymer using a microfabricated on-chip architecture, APL Mat. 2, 032102 (2014)
2. Deepak Venkateshvaran et al., Approaching disorder-free transport in high mobility conjugated polymers, under review at Nature.

Building efficient thermoelectric architectures requires complementary p-type (hole transporting) and n-type (electron transporting) components. While new candidates for high performing p-type materials are actively being developed, the design of n-type organic based materials has proven challenging, and thermoelectric studies of organic n-type systems are scarce. Finding effective and stable n-type doping mechanisms has been a significant barrier to new material development. We recently reported that self-doping perylene diimide (PDI) derivatives are an attractive class of solution-processable n-type organic thermoelectrics displaying best-in-class performance. [1] In thin films of these self-doping PDIs, charged functional groups, tethered to the core by alkyl spacers, enable the generation of extremely high carrier concentrations (~10^21 carriers/cm^3) after mild thermal treatment. The specific pathway by which the charged groups induce the doping, however, was initially unclear. Monitoring the physical, chemical, and thermoelectric property changes of PDI variants during the charge generation process can help elucidate the mechanism involved. In this presentation, we propose a pathway through which the charged end groups lead to carrier generation; the evolution of physical, chemical, and thermoelectric properties during doping support the suggested mechanistic route. Our findings help focus the criteria for future design of n-type organic thermoelectric materials and bring the development of fully organic thermoelectric modules and their integration into applications currently inaccessible by traditional, rigid, inorganics closer to reality.
[1] Russ et al. Adv. Materials (2014), DOI: 10.1002/adma.201306116

Natural heat and waste heat are substantial, equivalent to more than half the solar/fossil/nuclear energy sources upon conversion into electricity. Thermoelectric generators could transform few percents of this heat loss into electricity if thermoelectric materials based on elements of high abundance could be designed. Conducting polymers based on carbon, oxygen and sulfur, such as poly(3,4-ethyelenedioxythiophene) PEDOT, display promising thermoelectric efficiency.
In the first part, we discuss the thermoelectric properties for the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT). In this conducting polymer, the charge carriers are electronic in nature. PEDOT displays a metallic conductivity (1500S/cm) and a large Seebeck coefficient (55mu;V/K) in its pristine form. The power factor can be enhanced by controlling its state of oxidation[1]. This optimization together with the intrinsic low thermal conductivity of the polymer (0.37 Wm^-1K^-1) yields ZT=0.25 at room temperature [2]. We rationalize why PEDOT has good thermoelectric properties compared to polyaniline. We identify the role of the doping “defects” and the link of the density of states for a (bi)polaron network with the Seebeck coefficient. Further, we show that enhancing the molecular order in PEDOT lead to an increase of both Seebeck coefficient and electrical conductivity. We believe that this observation reflect a transition from a Fermi glass to a semi-metal [3].
In the second part, we further studied the thermoelectric properties for wet PEDOT films. Wet conducting polymers have been widely investigated in electrochemical devices, where their unique properties can be ascribed to both from electronic and ionic charge carriers. Here we found that both electrical conductivity and Seebeck coefficient for ionic conductive PEDOT films increase with humidity. A large thermo-induced voltage up to several hundreds of mu;V/K is observed and rationalized as the sum of an electronic thermopower and ionic thermopower. The new findings disclose a new possible approach to improve the thermoelectric properties of conducting polymers by combining various types of charge carriers of the same sign, so as to enhance the thermoelectric efficiency of the conducting polymers.
[1] JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 134, 16456-16459.
[2] NATURE MATERIALS, 2011, 10(6), 429-433.
[3] NATURE MATERIALS, doi:10.1038/nmat3824

The entropy difference has been greatly used as the driving force to diffuse charge carriers between high and low temperature surfaces towards the development of Seebeck effects in thermoelectrics. Specifically, the high and low temperature surfaces exhibit different internal energies of charge carriers in a medium, generating an entropy difference in internal energy of charge carriers. However, this driving force from entropy difference can cause an inverse relationship between Seebeck coefficient and electrical conductivity in the thermoelectric developments. Therefore, it is necessary to develop an additional driving force to eliminate the negative effects caused by increased carrier concentration toward developing high Seebeck effects. In our work, by using the vertical metal/polymer/metal thin-film design, we have explored temperature-dependent surface polarization as an additional driving force for developing Seebeck effects. The temperature-dependent surface-electrical polarization can come from the following two effects: (i) the coupling between local polarization ad thermal vibration on the polymer film surfaces through charge-phonon coupling mechanism and (ii) the interfacial dipoles modulated by the thermally activated Fermi electrons on the metal surfaces. These two effects combine to generate a difference in electrical polarization, namely the temperature-dependent built-in electrical field, across the polymer film between high and low-temperature metal surfaces. This temperature-dependent built-in electrical field established by the surface polarization can drive the diffusion of charge carriers. To enhance the temperature-dependent surface polarization, a high dielectric layer, MoO₃, has been inserted at the metal/polymer interface, increasing the temperature-dependent built-in electrical field between high and low-temperature surfaces in the vertical metal/dielectric/polymer/metal thin-film devices. The inserted dielectric interface can also allow the electrical conduction through charge transport but further hinder the thermal conduction through interface phonon scattering. We have obtained enhanced Seebeck effects by using the high-dielectric MoO₃ layer through increasing the temperature-dependent polarization within the Al/MoO₃/P3HT:PCBM/Al thin-film device. Simultaneously, the electrical conductivity has also been increased with the high-dielectric MoO₃ layer in the thin-film device. Therefore, the temperature-dependent surface polarization with the dielectric interface provides a possibility to cooperatively enhance the Seebeck coefficient and the electrical conduction in organic thermoelectrics.

Currently, the most efficient thermoelectric materials at room temperature are bismuth telluride-based alloys. Beside the fact that they are incompatible with the industrial scale, their scarcity and toxicity limit the range of applications. Recently, important research efforts have been focused on the development of alternative materials. Among them, conducting polymers are the most promising.
We synthesized a new type of poly(3,4 ethylene dioxythiophene) (PEDOT) based conducting polymer. This polymer combines a high conductivity (from 1000 to 2200 S/cm), a low thermal conductivity and moderate thermoelectric power (in the range 15-20 µV/K), while remaining highly versatile in terms of deposition methods, flexible and semi-transparent at low thicknesses.
We developed new routes to process PEDOT thick layers into thermoelectric devices, opening brand new possibilities. Our deposition technique consists in transferring pre-deposited polymer films through a solvation step. It is compatible with a wide range of geometries that are unreachable by usual techniques.
The thermoelectric power of the polymer was enhanced by modifying the oxidation level of PEDOT:PSS up to 160 µV/K. The oxidation level controls the properties of ionic charge carriers generated on the chains. To this end, several reducing agents were used and their action was investigated. Thermoelectric and chemical characterizations were performed on the polymer layers. Several doping species were also tested on the PEDOT and their effects on the thermoelectric properties were discussed.
Based on these results, we fabricated a 100 % organic flexible thermoelectric device.¹
1- N. Massonnet, A. Carella, O. Jaudouin, P. Rannou, G. Laval, C. Celle, J.-P. Simonato, J. Mater. Chem. C2014, 2, 1278

In the last decade, nano- and mesostructuring of thermoelectric (TE) materials has become a new paradigm to enhance the efficiency of TE modules. Switching from materials with large crystal grains to nano- and mesostructured compounds brings up fundamentally important problems related to morphological stability, grain growth and inter-grain transport. The chemistry and physics of interfaces in TE devices remains largely underexplored. This is in stark contrast to, e.g., thin-film photovoltaics where grain boundaries have been studied in depth for many years.
We will first discuss the effect of interfaces on charge and heat transport in arrays of semiconductor nanocrystals (NCs) that represent a convenient model of nanostructured TE materails. Next, we will cover new chemical approaches for rational design of the interfaces in TE materials by using all-inorganic nanocrystals (NCs) that serve as a “solder” for nano- and mesoscopic grains. During mild heat treatment, NCs fill up the voids between particles and act as a “glue” joining grains in hot-pressed pellets or solution-processed films. The chemical design of NC glue allowed for the selective enhancement or reduction of the majority carrier concentration near the grain boundaries, and thus resulted in doped or de-doped interfaces in granular TE material. We show that chemically engineered interfaces can be used as to optimize power factor and thermal conductivity.

Conjugated polyelectrolytes (CPEs) are conjugated polymers with pendant ionic functionalities. This talk focuses on the thermoelectric properties of self-doped conductive CPEs and their composites with single-walled carbon natotubes (SWNTs). First, we synthesized a series of narrow band gap anionic CPEs with the same conjugated backbones, but with different counterions (Na, K, vs tetrabutylammonium) and alkyl chain lengths (C3 vs C4). These CPEs are found to be doped during dialysis as part of the purification process to furnish conducting materials. CPEs with smaller counterions and shorter alkyl chain length exhibit relatively higher electrical conductivity, but comparable thermopowers. Moreover, these CPEs can disperse SWNTs in polar media (water and methanol) for preparing conductive thin films via solution processing. Anionic systems exhibit higher electrical conductivities compared to cationic CPEs with the same conjugated backbones. These studies highlight the advantages of using self-doped conductive polymers as SWNT dispersants. Finally, we show how to tune the thermoelectric properties (electrical conductivity and thermopowers) of CPE/SWNT composites by varying the CPE structures, including the conjugated backbones and ionic functionalities. The performances of these CPE/SWNT composites will be discussed.

Recently organic thermoelectrcic materials with high thermoelectric performance are receiving much attention because they are expected to be useful for recovering electricity from waste heat below 100 degree C.^1-3 Most of the thermoelectric materials contain highly electroconducting polymers like poly(3,4-ethylenedioxythiophene) PEDOT. Here we report the development of the novel hybrid films without such conducting polymers, which showed high thermoelectric performance, thermoelectric figure-of-merit ZT of ca. 0.3 near room temperature. The thermoelectric hybrid films were prepared by drop-casting the dispersions of an insulating polymer, carbon nanotubes (CNTs), and nanoparticles of a charge transporting reagent. As an insulating polymer we chose poly(vinyl chloride) PVC, which has very low thermal conductivity, since the low thermal conductivity is effective for high thermoelectric figure-of-merit. As the CNTs we used commercial single-walled carbon nanotubes (SW-CNTs), whch had high electric conductivity. The composite films of PVC and SW-CNTs had very low electric conductivity. Addition of nanoparticles of a chage transporting reagent could increase the electric conductivity of the films so much. Thus, the three-component hybrid films, composed of PVC, CNTs, and nanoparticles of charge-transporting polymer complexes, exhibited high electric conductivity and low thermal conductivity. The nanoparticles of the polymer complexes, poly(nickel 1,1,2,2-ethenetetrathiolate) Ni-PETT, were prepared by adding surfactants in the reaction process by modifying the Sun's method.⁴ The nanoparticles, soluble Ni-PETT, were dispersed in N-methylpyrrolidone (NMP). Thus, the printing methods were applicable for preparation of the three-component thermoelectric hybrid films. The evaluation of the devices prepared by the three-component hybrid films will be also reported in the symposium.
References
1. Bubnova, O., et al., Nature Mater., 2011,10, 429.
2. Kim, G-H., Shao, L., Hang, K., Pipe, K.P., Nature Mater., 2013, 12, 719.
3. Moriarity, G.P., Briggs, K., Stevens, B., Yu, C., Grunlan, J.C., Energy Technol., 2013, 1, 265.
4. Sun, Y., et al., Adv. Mater., 2012, 24, 932.

Metal-organic-frameworks (MOFs) are hybrid organic-inorganic materials with a crystalline, microporous structure. Most applications currently under consideration capitalize on their sorbent-related properties, but their use as electronic materials has been limited by their typically low electrical conductivity. Here, we show that infiltrating thin films of the MOF HKUST-1 with redox-active molecules such as TCNQ (7,7,8,8-tetracyanoquinododimethane) increases the MOF electrical conductivity by over seven orders of magnitude. We also present the first thermoelectric measurements of this MOF. We measured the Seebeck coefficient and electrical conductivity of infiltrated HKUST-1 films and obtained Seebeck coefficient values between +300-450 µV/K, indicating p-type conduction. These films exhibit thermally-activated transport, similar to other organic thermoelectric materials of interest. These conducting molecule@MOF films show promise for optimizing the thermoelectric performance because the properties of the infiltrated molecule can be tailored as well as the loading.

There is a growing interest in solution-deposited thermoelectrics for coating on irregular surfaces. Avoidance of toxic and less common elements is desirable for such materials. Thermoelectric performance of hole-carrying (p-type) polymers has been enhanced to the point where the figure of merit ZT now significantly exceeds 0.1. There are far fewer reports of electron-carrying (n-type) thermoelectric polymers, which are equally needed for complete thermoelectric modules. We recently reported the first measurement of thermoelectric properties of an n-type polymer. A power factor of nearly 1 mu;W/mK² was later reported for commercial Polyera N2200. In this presentation, we report that n-type composites of a novel pyromellitic diimide polymer with in-situ grown common element semiconductor structures can reach power factors two orders of magnitude higher. Our polymer is related to polyimides, but has partly conjugating C-C rather than nonconjugated N-N main chain linkages. The semiconductors are largely based on tin compounds, but encompass zinc compositions as well. The most significant contribution to the high power factor is an unusually high Seebeck coefficient >1 mV/K, though the electrical conductivity is also significant and preliminary results suggest that thermal conductivity is not unduly high compared to neat polymers. Controlled growth of microstructures of the inorganic species as well as detailed electronic and thermal characterizations will be presented.

Thermoelectric systems are very effective in harvesting electricity from waste heat or heat sources with low temperature gradients relative to the environmental temperature which are inadequate for power generation using conventional systems, but are often present in the environment (like solar and geothermal energy) or generated from various power generating or consuming systems. Furthermore, it is possible to use them for refrigeration. Their simple leg-type structures, without moving parts, provide enormous advantages over conventional turbines, engines, and compressors. In addition, their high energy density (per unit weight and volume) is ideal for mobile power sources and refrigeration with robustness and silence.
This talk includes recent progress in Yu group for developing new-class of organic thermoelectric materials, which can provide light-weight (higher power density), inexpensive, and non-toxic solutions for waste heat (i.e., energy) recovery or cooling. Polymers are intrinsically poor thermal conductors, which makes them ideal for thermoelectrics, but low electrical conductivity and thermopower (or the Seebeck coefficient) have excluded them as feasible candidates in the past. Our recent results, however, demonstrated that electrical properties of organic composites can be brought into degenerate semiconductor or metallic regimes by incorporating conductive nanoparticles without significantly changing thermal conductivity. The details of our approach for material synthesis/characterization of the composites will be discussed.

Single-walled carbon nanotube (SWNT) / polyaniline (PANI) hybrid films were prepared by casting the suspension containing well-dispersed SWNTs and CSA-doped PANI. The electrical conductivity of SWNT/PANI film at first increased with the increasing SWNT content and then decreased at high SWNT content, whereas the Seebeck coefficient increased monotonically in the present SWNT content range. Moreover, the electrical conductivity values of the SWNT/PANI composites are much higher than the values calculated based on the series-connected two-component mixture model, whereas the dependence of Seebeck coefficient on the SWNT content fits well with the mixture model. Consequently, the maximum values of electrical conductivity and Seebeck coefficient of hybrid films are up to 769S/cm and 65mV/K, respectively, and the maximum thermoelectric power factor reach 176mW/mK². The optimal TE property of the SWNT/PANI hybrid film is remarkably higher than those of either individual component of the composite, and among the highest values in inorganic-organic composite materials reported so far. The abnormally enhanced thermoelectric performance is attributed to the highly ordered PANI interface layer on the SWNT surface, which formed by the synergetic effect of the chain-expanding by the chemical interactions between PANI and solvent and the chain-ordering of the p-p conjugation between PANI and CNT.

We will present our latest research on nanoparticle-coated composite flexible fabirc demonstrating a reliable power output with or without stress applied on the fabic. By tuning the growth condition and the composition of the inorganic nanoparticle coating materials, we avoid the need of high temperature sintering, thus enabling the direct fabrication of thermoelectric devices on plastic and nylon fabirics. The nanocrystal can be doped during the synthesis to exhibit either p- or n- type behavior, so only one starting material is needed to build the entire flexible thermoelectric modules. This work is sponsored by United States Air Force of Scientific Research.

Conjugated polymers have been collected strong interests for their tunability of colors, and low cost of preparation for flexible electro-optic devices.^[1-4] In particular, conjugated polymers (CPs) for thermal energy harvesting are promising because of their high electric conductivity with low thermal conductivity. Thermoelectric (TE) properties of conjugated polymers from thiophenes and selenophenes were explored for a flexible TE devices. CP films were obtained by oxidative polymerizations, which grow conductive channels as polymerization proceeds. Through an optimized polymerization condition, highly conductive CP films were obtained with lower oxidation level. Taking advantage of their high electrical conductivity, the pristine CP films could be used as an electrode itself for the precise control of the oxidation level, without using any other inorganic electrodes, and could be processed as flexible, cuttable thermoelectric films. A flexible TE device based on a CP film showed high electrical conductivity with high Seebeck coefficient when the oxidation state of CP film was optimized.^[5] As a result they exhibited a large power factor and could be used to generate electricity even by the touch of fingertips. The TE films were further explored as a transparent photo-thermo-electric film, because their absorption energy is easily controlled by the degree of oxidation or doping.^[4] Upon exposure to a near IR source, the CP films got heated, to result in temperature rise on a substrate.^{[1], [4]} The generated heat was effectively converted into electricity to confirm a photo-Seebeck effect from the CP film under a light exposure.^[6] Efficient near-IR photothermal effect and heat to electric conversion have been realized in one film that could benefit in exploiting multifunctional film displays, invisible NIR sensors, and hybrid energy harvesters.
References
[1] J. Kim, J. You, B. Kim, T. Park, E. Kim, Adv. Mater., 23, 4168 (2011).
[2] J. You, J. S. Heo, J. Kim, T. Park, B. Kim, H.-S. Kim, Y. Choi, H. O. Kim, E. Kim, ACS Nano, 7, 4119 (2013)
[3] T. Bhuvana, B. Kim, X. Yang, H. Shin, E. Kim, Angew. Chem.Inter. Ed., 52, 1180 (2013).
[4] B. Kim, J. Kim, E. Kim, Macromolecules, 44, 8791 (2011).
[5] T. Park, C. Park, B. Kim, H. Shin, E. Kim, Energ. Environ. Sci., 6, 788 (2013).
[6] B. Kim, H. Shin, T. Park, H. Lim, E. Kim, Adv. Mater., 25, 5483 (2013).

Thermal management of microelectronics is of increasing importance in today&’s world. Resins loaded with conductive filler particles are used as interface materials for enhanced thermal conductivity and thermal impedance. The thermal conductivity of the filler plays an important role in determining the effective thermal conductivity of such interface materials. The goal of this study is to investigate thermal conductivity and heat capacity of individual filler particles in a composite. Frequency Domain Thermoreflectance (FDTR) will be used to analyze individual particles of alumina, boron nitride, aluminum nitride, and magnesium oxide. A notion of particle distribution of thermal conductivity will be introduced as potential quality control tool for determining goodness of filler particles. The analysis will be conducted on cross-sectional areas of resin/filler composites. In addition we will also analyze oriented polyethylene film (Dyneematrade;) in longitudinal, normal and transverse directions.

Thermoelectric power generation device converts waste heat to power energy. Thermoelectric efficiency is determined by figure of merit, ZT and achievements of high electrical conductivity, high Seebeck coefficient and low thermal conductivity are very important for high performance thermoelectric devices. Tungsten disulfide (WS₂) is a transition metal dichalconide which has a layered structure. It has been interested as thermoelectric materials because it has high Seebeck coefficient(α) as 375-1000(#13238;/K). However, it has been reported experimentally with low ZT values due to its low electrical conductivity (10^-1-10¹S/m). On the other hand, carbon based materials usually have high electrical conductivity. Specifically, thermally reduced graphene oxide (rGO) used in this study showed high electrical conductivity of 3.11 X 10³ (S/m) and large charge carrier mobility (15,000 cm²middot;V^minus;1middot;s^minus;1). In this respect, we expect that thermally reduced graphene oxide provides an effective conductive path for transfer of charge carriers in WS₂/rGO composites and their thermoelectric property increases. Also, their layered structures induce phonon scattering and effectively decease thermal conductivity. In this study, we fabricated composites of WS₂/rGO as a function of weight percent of rGO in order to obtain optimized ZT values. As a result, electrical conductivity(σ) was increased from 7.9 to 23(S/m) even though Seebeck coefficient was decreased slightly from 780 to 670(#13238;/K). So, power factor(α²σ) of the composite increased about 2 times than that of pristine WS₂ from 5.0 to 11 (#13238;/mK²).

PEDOT-PSS (poly 3,4-ethylenedioxythiophene:polystyrenesulfonate) is an organic p-type polymer that has been widely studied due to its high electrical conductivity and excellent film forming abilities. It is easily processed from aqueous solutions, exhibits high transparency in the visible range, and is remarkably stable under enviromental, thermal, and mechanical stresses. PEDOT-PSS has been used in antistatic coatings and pyroelectric sensors, and is commonly employed as a hole injecting layer in organic electronics. In this work, thin films of PEDOT-PSS were deposited from an aqueous solution of 1.3% PEDOT-PSS and 98.7 H2O by spin coating using the following recipe: a) 500 rpm for 10 sec, b) 800 rpm for 10 sec, and c) 1500 rpm for 60 sec. We present a modelling method for measuring the thickness of PEDOT-PSS spin-coated thin films onto SiO₂ by considering both transmission and reflection ellipsometry experimental data.

Thermoelectric (TE) materials have been studied during past decades since they could generate electricity directly from waste heat source.¹ Many approaches have been used to enhance the TE performance represented by the figure of merit (ZT). For example, doping and nanostructuring can help to either increase the electric conductivity (σ) or decrease the thermal conductivity (κ). Currently, conducting polymers have attracted great attention for TE application, owing to their inherently low thermal conductivity combined with their high electrical conductivity. Conducting polymers also show low density, low cost and ease of fabrication, leading to the potential to scale up. On the other hand, antimony chalcogenides (Sb₂M₃, M = S, Se, Te) are well known as one of the promising candidates among the inorganic TE materials.² Therefore, the antimony chalcogenide - polymer nanocomposites have the potential to combine the high electric conductivity and the low thermal conductivity, resulting in novel hybrid TE materials. The electrical conductivity of these hybrid nanocomposites could be tuned depending on the solid content within the conducting polymer.
We report on the synthesis of Sb₂Te₃ via thermolysis method and the ratio between precursors was varied to observe the effect on the morphology. Hybrid nanocomposites were further fabricated from Sb₂Te₃ and a highly conducting polymer. The transport properties of the nanocomposites were characterized for the potential thermoelectric application.
1. G. H. Kim, D. H. Hwang, S. I. Woo, Phys. Chem. Chem. Phys., 2012, 14, 3530.
2. P. Christian, P. O&’Brien, J. Mater. Chem., 2005, 15, 4949.

Large-area flexible thermoelectric devices have been desired in order to generate electricity from waste heat around our lives. Generally, the performance of thermoelectric materials is evaluated by the dimensionless figure of merit, ZT = α²σT/κ (α, σ, κ, and T are Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature, respectively). Considering the influence of contact resistances against current and heat, a material with a small κ and a large α, which allows σ to be small, is beneficial to reduce the optimum thickness of the thermoelectric cell and the number of series connection of the cells to achieve a practical voltage. These can be a big advantage to fabricate thin flexible devices with high reliability. It is, however, difficult to increase α while maintaining high ZT because α and σ are interconnected by the Mott relation in the case of conventional semiconductors [1]. Therefore, a novel thermoelectric mechanism is desired to realize large-α thermoelectric materials.
In order to independently control α and σ, molecules consisting of both a moiety for electrical conduction and moieties for thermal modulation have been selected to tune the alternation of the electrical conduction mechanisms with temperature. In this work, we used BTBT, DNTT and BP skeletons as the electrical conduction moieties that are known to exhibit excellent carrier transport properties by formingπ-π stacking structures. Alkyl side chains as the thermal modulation moieties are attached to the bridging carbon atoms (namely, C8BTBT, C10DNTT and C12BP) to induce the structural alternation by thermal modulation.
According to the Seebeck measurements of these materials in vacuum after the film depositions [2], all of these materials exhibited extraordinary large α values exceeding 0.1 V/K though their ZT values were still limited by low σ. The temperature dependences of α and σ were not straightforward and different by the material. Structural change by the temperature was also investigated by differential scanning calorimetry and grazing incidence X-ray diffraction. For each material, α was found to increase in the temperature range where the alternation of heat capacity and/or activation energy of σ took place. These results suggest that a novel mechanism of thermoelectricity is expected for a certain types of organic materials, which possibly gives a breakthrough to the conventional theoretical limit of ZT.
[1] M. Cutler and N. F. Mott, Phys. Rev.181, 1336 (1969).
[2] M. Nakamura, A. Hoshi, M. Sakai, and K. Kudo, MRS Proceedings, 1197, 1197-D09-07 (2009); DOI:10.1557/PROC-1197-D09-07.

Over the past several decades with increasing of global energy demand, thermoelectric materials have gained considerable attention due to their unique ability to directly convert heat to electricity. In addition to inorganic semiconductors, polymers are potential candidates for high-performance thermoelectric applications due to their intrinsic advantages such as low thermal conductivity, solution processability, and roll-to-roll production, lightweight, and flexible thermoelectric modules.
This work aims to develop organic polymer based thermoelectric (TE) materials and to understand the fundamental behavior of these materials. To this end, we prepared cast films of commercially available poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and single walled carbon nanotubes (SWCNT). The effects of solvent and molecular weight on the thermoelectric performance of nondoped and I₂ doped samples were investigated as a function of SWCNT concentration (0-50 wt %). The composites prepared with the same molecular weight in a non-aromatic halogenated solvent (1,2-dichloroethane) showed higher power factors (0.005-0.5 µW/mmiddot;K²) with an increasing trend for higher contents of SWCNT. However, the samples made in a halogenated aromatic solvent (o-dichlorobenzene) showed a reverse trend (0.38-0.15 µW/mmiddot;K²) with higher weight loads of SWCNT. In addition, samples made from high molecular weight polymer (~150-250K Daltons) gave higher TE power factors by 3-4 fold than those of low molecular weight polymer (~70-100K Daltons) based samples that were fabricated under the same experimental conditions. Moreover, the electrical conductivity of the samples in all variable conditions gave an increasing trend with elevated concentration of SWCNT. However, Seebeck coefficients showed a decreasing trend with increased concentration of SWCNT in all conditions. Also, Seebeck coefficients of the samples made from low molecular weight polymer are slightly higher than those of high molecular weight polymer. The main reason why the thermoelectric properties of polymer are changed is differences in the morphology of the films induced by SWCNT and solvents. These morphologically induced differences are under investigation.

Thermoelectric power can be expressed in the terms of Seebeck coefficient, electrical and thermal conductivity. To achieve high conversion efficiency from thermal energy to electrical energy, we have to make a sample with high seebeck coefficient and electrical conductivity, and low thermal conductivity. Most of the metallic fillers have high electrical conductivity and thermal conductivity at the same time. However, when the metallic fillers are dispersed in a polymer matrix, high electrical conductivity and low thermal conductivity can be achieved. That's because a high degree of phonon scattering is promoted while electrons experience very little scattering to maintain the electrical conductivity. Seebeck coefficient is defined as the ratio of the resulting voltage and the temperature difference. The magnitude of seebeck coefficient tends to be reversely proportional to the carrier concentration, which is proportional to the electrical conductivity. Accordingly, trying to decouple the electrical conductivity and seebeck coefficient is important to reach the goal.
In this study, we introduced MWCNTs(Multi walled carbon nanotubes) and semiconducting SWCNTs(Single walled carbon nanotubes) as fillers. Chemical treatments change the conductivities and seebeck coefficient of each samples, functionalizing the surface of nanotubes. Especially for the single walled one, the doping effect of chemical functionalization is even able to produce both P typed and N typed SWCNTs. Non-conducting polymer and semi- or conducting polymers are used to bind the fillers, varying the weight fractions in the composite. Doping levels of them are also controlled by post-treatments like oxidation, solvent dipping, etc. Seebeck coefficients of the samples can be measured simultaneously by DTC(Differential Thermo Couple) and Voltage meter. The thermal conductivity was obtained from TC-30 (Mathis instruments Co., Canada), and four probe methode was adopted to get the electrical conductivities.

Wide-area, low-cost, and mechanically flexible thermoelectric generators are strongly desired for the energy harvesting from the waste heat around our lives. Organic-based thermoelectric materials have been gathering more and more attention for this purpose. Among many candidate materials, carbon nanotube (CNT) composites are expected to have high electrical conductivity and desirable mechanical properties for flexible devices. However, CNT itself has a very high thermal conductivity and a relatively small Seebeck coefficient. There have therefore been many attempts to decrease thermal conductivity and to increase Seebeck coefficient by selecting matrix materials for composites and forming porous structures.
We have applied a novel biological method to improve thermoelectric performance of CNT composites. Caged-shaped proteins that can include semiconductor cores have been employed. By their affinity to CNTs, CNT/protein/CNT junctions are formed in a self-assembled manner when the composite films are fabricated from their mixed dispersion. Applying a heat flow to the composite, a steep temperature gradient at the biomolecular junction is produced due to the phonon scattering by the soft protein shell but electrons or holes can selectively flow through the semiconductor core. As a result, an ideal “phonon-blocking and electron-transporting” structure is realized. We have already demonstrated that this method can dramatically improve the thermoelectric properties of CNT composites [1]. To further improve the ZT value of the CNT/protein composites, optimization of the conductivity and conduction type of CNT is required.
In this work, adsorption of cage-shaped protein molecules to various types of CNTs was studied. The increase of Seebeck coefficient with p-type semiconductor cores was observed for several types of CNTs. However, degree of the increase depended on the CNTs types. No increase was observed when the adsorption density of proteins to CNTs was small. The requirement to obtain sufficient density of adsorbed proteins will be discussed in the presentation.

Highly conductive PEDOT films were prepared using a mixture of pyridine and PEG-PPG-PEG block copolymer. The pyridine regulated the rate of polymerization working as a basic inhibitor¹ and the block copolymer enhanced the structure-directing effect.² Even though the oxidation level of the pristine polymerized EDOT (PP-PEDOT) was lower than that of conducting polymers prepared by other methods, the PP-PEDOT in the presence of Fe-Tos has high electrical conductivity for efficient thermoelectric properties. More dense structure of PP-PEDOT was confirmed and its surface morphology was rough, showing large grain size. With lower oxidation level, the pristine PP-PEDOT has a higher Seebeck coefficient. Moreover, the pristine PP-PEDOT can be used as an electrode itself for the precise control of the oxidation level due to the high electrical conductivity, without using any inorganic electrodes. A maximum power factor was obtained from the film prepared at the state that 0.1 V potential applied. This highest power factor in PP-PEDOT could originate from the precise control of the oxidation level using the electrochemical method. A flexible thermoelectric generator was achieved upon coating PP-PEDOT itself on a PET substrate. In addition, a thermoelectric module combining PP-PEDOT and n-type inorganic materials as a rigid-flexible device for a practical application will be presented.
1. B. Winther-Jensen and K. West, Macromolecules, 2004, 37, 4538-4543.
2. D. Evans, M. Fabretto, M. Mueller, K. Zuber, R. Short and P. Murphy, J. Mater. Chem., 2012, 22, 14889-14895

Hybrid perovskite based materials have raised great research attractions in photovoltaic technology. The charge transport properties of the perovskite based semiconductors result in the promising performance. However, the charge transport properties of the pervoskite materials are still elusive. To understand the charge transport properties of perovskite materials and board the application of perovskite based materials, we synthesized and characterize he thermal stability of series of pervoskite materials. The morphology of the perovskite materials were studied by Scanning Electron Microscopy (SEM). The crystal structures of the pervoskite materials are investigated from small angel X-ray diffraction (SAXRD) patterns. The electrical conductivities and Seebeck coefficients of the pervoskite materials under various temperatures were measured to understand the charge transport properties of the prepared perovskite materials. The moderate electrical conductivity and high Seebeck coefficient (over 3000uV/K at 55^oC) of the pervoskite material lead to power factor over 800uW/K, which indicates that the hybrid pervoskite materials have great potentials in the application of thermoelectric technology.

Highly efficient thermoelectric devices are expected for the application to such as mobile and wearable devices which are becoming widespread. For this purpose, some nanocarbon materials are investigated as thermoelectric candidates. Carbon nanotube is a typical material exhibiting good performance owing to excellent electrical conductivity and thermal and thermoelectric properties of graphene have been also extensively investigated. Similarly, fullerene has attracted attention in terms of not only the thermoelectric property along with other carbon materials but also its unique molecular symmetry. C₆₀ is a remarkably spherical molecule to bring about free rotation even in a solid state at ambient temperature. Although it is a promising material, the thermoelectric properties of pure C₆₀ is hard to be experimentally investigated due to its low electrical conductivity and high sensitivity to ambient gases. We have therefore modified our measurement system to make the in-situ measurement of high resistivity materials possible. Interestingly, the vacuum deposited thin film with high purity exhibits an extraordinary large Seebeck coefficient exceeding 0.1 V/K [1]. We have assumed that the giant Seebeck effect is induced as a consequence of the alternation of molecular motion modes by heat. It is known that structural and kinetic phase transitions occur at lower temperature in association with the rotational mode.
Herein we aim to elucidate the origin of the giant Seebeck effect while showing new data: temperature dependences of Seebeck coefficient and electrical conductivity. As a result, meaningful behavior of the activation energy with changing temperature is pointed out. The structural change of the solid state of C₆₀ vapor-deposited thin film is also measured by in situ grazing incidence X-ray diffraction (GIXD) technique at BL19B2 in Spring-8 with changing temperature. In the GIXD patterns, mostly isotropic diffraction peaks, which are similar to literature, appear and they shift non-monotonically with temperature. Besides, C₆₀ cluster is computationally constructed and molecular motions are simulated by molecular dynamics (MD) method. Some thermal properties are evaluated. In the presentation, we will report the latest results and discussion on the origin of the giant Seebeck effect.
[1] M. Nakamura et al., 2013 MRS Fall Meeting, BB11.06.

Transport in molecular junctions is being actively studied due to their promise for both molecular electronic and thermoelectric applications. Although recent experiments have probed thermoelectric properties of molecular junctions, control of the electronic structure of such junctions by a gate electrode, while probing their thermoelectric properties has remained out of reach. In this talk, we will describe how by careful thermal design and nanofabrication of devices we have successfully established temperature gradients exceeding 10⁹ K/m in nanoscale gaps bridged by molecules, while simultaneously controlling the electronic structure of molecules via a gate electrode. We will also describe studies performed using this platform, on prototypical Au-biphenyl-4,4&’-dithiol-Au and Au-fullerene-Au junctions, to unambiguously demonstrate that the Seebeck coefficient and electrical conductance of molecular junctions can be simultaneously increased by tuning their electronic structure. Finally, we will describe how thermoelectric properties can be dramatically enhanced when the dominant transport orbital is located close to the chemical potential.

A primary challenge of thermoelectric power generation is its heat-to-electric energy conversion efficiency that requires a material with the dimensionless figure of merit ZT exceeding 1 for wide applications. Metal-molecule-metal junctions are an emerging class of thermoelectric materials that exploit quantum confinement effects to achieve enhanced figure of merit. A key feature in such nanoscale systems is that the electron and heat transport become highly sensitive to the atomic configurations. Despite the importance, little effort has been devoted to evaluate the geometrical dependence of thermoelectricity in molecular junctions due in large part to the technical difficulty to control the atomistic configurations. Here we report strong geometrical dependence of thermoelectricity in single-molecule junctions. We measured the electrical conductance and thermopower of electrode-single molecule-electrode structures of varying configurations at room temperatures using a heater-embedded mechanically-controllable break junctions (M. Tsutsui et al., Sci. Rep. 2, 217 (2012); Sci. Rep. 3, 3326 (2013)). From the measured thermoelectric properties, we found shifting of the BDT HOMO level during mechanical stretching of junctions presumably due to a change in the molecular conformations. The electromechanical response yielded orders of magnitude difference in the single-molecule power factor, suggesting the importance to optimize junction geometries to achieve high-ZT with single-molecule junctions.

Combining thermoelectric oxides and organic molecules into hybrid inorganic-organic materials opens up the possibility of atomic-level interface engineering and nanostructuring of the oxide material. For example, the controlled preparation of inorganic-organic superlattices enables the tuning of the phonon properties and thermal conductivity of the parent oxide material. A highly controllable strategy to prepare such hybrid superlattice structures is to employ a combination of the atomic layer deposition (ALD) and molecular layer deposition (MLD) techniques; we recently demonstrated this for the ZnO:hydroquinone (HQ) thin films [1]. The ZnO:HQ thin films grown by ALD/MLD can be described as inorganic-organic superlattice structures consisting of single layers of organic molecules between thicker layers of ZnO. The ALD/MLD growth process enables precise control over the ratio of the inorganic and organic layers, providing a way to control the phonon scattering properties of the material without significantly hindering electron transport. The thermoelectric properties of the ZnO:HQ thin films can be further tuned by aluminum doping; for the (Zn,Al)O:HQ films enhanced electrical conductivity in comparison to the nondoped ZnO:HQ films was obtained [2]. With an appropriate choice of the Al doping amount and the HQ layer frequency an improved thermoelectric figure of merit can be achieved via reduced thermal conductivity through the superlattice structure [3]. To elucidate the structure-property correlations of the ZnO:HQ thin films, we have carried out a systematic structural investigation of the hybrid ZnO:HQ thin film structures using quantum chemical methods. We have derived atomic-level structural models of thin film structures with superlattice periods up to 11 nm, enabling comparisons with experimental XRD and IR data. We analyze the thermoelectric and phonon transport properties of the ZnO:HQ superlattice structures using the computational first-principles techniques that we have recently applied for thermoelectric group 14 clathrate materials [4,5]. The combined experimental-theoretical work provides useful information for the optimization of the carrier concentration and phonon scattering in the hybrid ZnO:HQ superlattice thin films.
[1] T. Tynell, M. Karppinen, Thin Solid Films2014, 551, 23-26.
[2] T. Tynell, I. Terasaki, I. ,H. Yamauchi, M. Karppinen, J. Mater. Chem. A2013, 1, 13619-13624.
[3] T. Tynell, A. Giri, J. Gaskins, P. E. Hopkins, P. Mele, K. Miyazaki, M. Karppinen, submitted.
[4] A. J. Karttunen, T. F. Fässler, ChemPhysChem2013, 14, 1807-1817.
[5] V. J. Härkönen, A. J. Karttunen, Phys. Rev. B2014, 89, 024305.

One recent trend in thermoelectrics has been the design of scalable, low-cost materials for harvesting of low-grade waste heat or on-chip cooling applications. Many of the materials studied are complex and do not obey standard physical intuition, consisting of multiple components, often combining polymers, nanocrystals, nanotubes, and even small molecule dopants. While empirical results to date have been gratifying and show potential promise for the field, it is clear that there remains much unknown territory in the basic science of how heat and charge are transported in these systems, and whether or not there are any basic rules of design. In this talk, I discuss ongoing efforts to better understand the basic physics of transport in soft and hybrid materials using model material platforms. Specifically I will discuss recent results in: 1) programmable chalcogenide exchange to controllably invert carrier type in chalcogenide nanorods and 2) high-power factor small-molecule/copper selenide nanocrystalline hybrids.

Composites of organic and inorganic semiconductors are often considered as a way to yield high thermoelectric figures of merit. The most constraining limitation in this approach is given by the effective medium theory^1,2. When one considers a composite made from two thermoelectric materials, A and B, in the absence of interactions between them the thermoelectric figure of merit of the composite cannot exceed that of the highest of the figures of merit of either A or B.¹ However, it is possible for thermoelectric power factor of the composite to exceed the highest of the power factors of either A or B.² In this talk, we will review the possible mechanisms that can lift this limitation. There typically are two strategies. One is to promote interactions between the two, whereby the properties of A or B are affected by the presence of the other. The other is to create compounds that affect the electrons and the phonons differently. Work supported by AFOSR MURI "Cryogenic Peltier Cooling", FA9550-10-1-0533.
1. David J. Bergman and Ohad Levy, J. Appl. Phys. 70 6821 (1991)
2. David J. Bergman and Leonid G. Fel, J. Appl. Phys. 85 8205 (1999)

We report terahertz frequency conductivity measurements of a high-performance thermoelectric material containing tellurium nanowires in a PEDOT:PSS matrix[1]. While the DC electrical conductivity of the hybrid material (41 S/cm) is approximately one hundred times that of pure PEDOT:PSS, the terahertz frequency (THz) conductivity of PEDOT:PSS and the hybrid material are comparable at f ~ 2THz. A frequency-dependent conductivity model indicates that the increased DC conductivity of the hybrid material results from an increase in the DC charge mobility rather than in the free charge density. We suggest that the increased DC conductivity of the hybrid material results from an increase in linkage between PEDOT domains by the tellurium nanowires. In addition, we will report time-resolved THz conductivity measurements in this material.
[1] J. N. Heyman, B. A. Alebachew, Z. S. Kaminski, M. D. Nguyen, N. E. Coates, and J. J. Urban, APL 104, 141912 (2014).

Thermoelectric devices convert thermal energy directly into electrical energy and vice-versa and hold promising contributions in waste heat recovery and refrigeration. Due to their low cost, potential for high throughput manufacturing and unique transport mechanisms; solution-processed conducting polymer/nanoparticle composite films provide an environmentally clean and efficient route to harness electricity from low-grade heat sources. However, we still lack a fundamental understanding of these hybrid systems that would provide a general and robust framework to guide materials design. In this work, we use model organic/inorganic hybrid systems to demonstrate how engineering of nanoscale interfaces can drive novel macroscale transport behavior.
Here, we study a hybrid organic/inorganic thermoelectric system comprising of Poly(3,4 ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and Tellurium nanowires (NWs) as a function of nanowire loading and chemical doping. Interesting non-monotonic electrical conductivity is observed at intermediate loadings, suggesting non-effective medium behavior. The surprising electrical conductivity behavior can be explained with a model where carrier transport is primarily through a highly conductive volume of polymer that exists at the nanoparticle-polymer interface. Additionally, by doping the individual components (NW and polymer) separately, we can boost the electrical conductivities even further and obtain higher power factors. In a complementary study, modifying the interface of hybrid Polyvinylpyrollidone (PVP) and Tellurium NWs by controlled addition of small molecules, allows us to tune the nature of carrier transport from hole-dominated to electron-dominated in thin film devices of these materials. Our approach provides a n-type thermoelectric material from essentially a p-type material simply by modifying the interface while preserving the conductivity and still rendering it solution processable. This enables use of a single material for both legs of a thermoelectric module. The approach is general and we can use it to tune the transport behavior in other promising thermoelectric materials too. Thus, a mix of traditional doping mechanisms and innovative interface engineering at the nanoscale allows us to generate high performance thermoelectric materials.