Symposium ES4 : Thermoelectric Polymers and Composites—Nontraditional Routes to High Efficiency

Organic semiconductors are an emerging class of thermoelectric materials because of their promising electrical properties and their low lattice thermal conductivities. The limits of the thermoelectric properties of organic materials are currently not well understood. We will present our results on how processing methods during electric doping affect the thermopower in both p- and n- type semiconducting polymers. Using solution and vapor processing methods, we can increase both the electrical conductivity and thermopower in high performance p-type polymers. Electrical conductivities above 200 S/cm can be readily achieved in PBTTT doped by F4TCNQ and can be attributed to the alignment of polymer chains, observed using polarized resonant soft X-ray scattering. We have also studied n-type doping in low band gap polymers and find that steric effects have a significant influence on the ultimate electrical conductivity. These results suggest important structure-property relationships between the dopant and the host for that should be considered when designing new polymers for thermoelectrics. Using these results, we will present an outlook for the future performance of polymers as thermoelectric materials.

The recent interest in doped polymeric semiconductors for thermoelectric applications has led to a large body of experimental work that show promising results. As most work so far has been empirical, the field somewhat lacks a formal and predictive understanding of thermoelectric properties, making it, amongst others, difficult to assess the application potential of various material systems. Here, we will focus on a novel analytical model to calculate the conductivity σ and Seebeck coefficient S, and concomitantly the power factor, of doped organic semiconductors. The model describes, with a single set of parameters, experiments covering a wide range of doping concentrations for various polymers in which doping gives rise to integer charge transfer. Moreover, it is benchmarked to kinetic Monte Carlo simulations; a previous, and currently popular model is shown to be inconsistent with kMC.

The key ingredient of our model is the fact that the electrostatic potential of ionized dopants broadens the original density of states (DOS) of the organic semiconductor, giving rise to a new, deep tail of trap sites. As a consequence of the competition between DOS broadening and state filling, the effective mobility becomes a non-monotonous function of dopant concentration. For increasing doping concentration, the conductivity and power factor increase while the Seebeck coefficient decreases. Interestingly, for initially low-disorder materials, the continuous broadening of the DOS with increasing doping concentration gives rise to an apparent power law relation between conductivity and Seebeck coefficient with slope around -1/4, i.e. S ∝ σ ^-1/4, in good agreement with recent experiments by Glaudell et al.

We used our model to assess attainable performances for doped organic semiconductors. Using realistic-optimistic parameters we find that, within the model framework, the thermoelectric figure of merit ZT will not easily become 0.1 or more.

In addition, we will address the problem of probing the Seebeck coefficient of modestly conducting materials in thin film geometry. We show that geometries that seem desirable from a signal-to-noise perspective may induce systematic errors in the measured value of S by a factor 3 or more. The enhancement of the apparent Seebeck coefficient by the device geometry is related to competing conduction paths outside the region between the electrodes. We derive a universal scaling curve that allows correcting for this and show that structuring the semiconductor is not needed for the optimal electrode configuration, being a set of narrow, parallel strips.

Carrier transport depends strongly on morphology in electrically doped semicrystalline polymers. [1] There has been significantly less study of doping of amorphous polymers particularly at high carrier concentrations. PTAA (polytriarylamine) is an amorphous hole-transport polymer where transport is by hopping, that is, phonon-assisted tunneling, of charge carriers between localized states. [2] Electrochemical doping of PTAAs have been reported demonstrating electrochromic switching induced by external electric field. [3] However, the molecular doping of PTAAs and their electrical/thermal properties has not been fully investigated.
We have studied the influence of electrical doping on the electrical conductivity and thermoelectric (TE) effects of PTAA to determine if this model system is well described by hopping theories of conduction.
We electrically doped PTAAs with metal salts with varying anions. From UV-Vis-NIR spectroscopy, we observed new absorption peaks at about 500 nm and the NIR region. We attribute these spectral changes to the formation of polaron states onto the main chain of PTAAs. This doped material was stable in air for several weeks. Differential scanning calorimetry thermograms revealed that a new melting peak in doped PTAAs compared to pristine ones demonstrating a significant structural change and the potential for simple processing of these conductive materials. The observed electrical conductivity of doped PTAA films was relatively low compared to semicrystalline materials ∼ 0.1 S/cm and the relationship between the observed conductivity, carrier concentration and thermopower will be presented. [4] This doping technique will be a promising candidate for organic TE materials and initial studies of its properties in composites with carbon nanotubes will be presented. [5-6]
References
[1] S. N. Patel et al., ACS Macro Lett. 5, 268 (2016). [2] J. Veres et al., Adv. Funct. Mater. 13, 199 (2003). [3] S.-H. Hsiao et al., RSC Adv. 5, 90941 (2015). [4] A. M. Glaudell et al., Adv. Energy Mater. 5, 1401072 (2015). [5] M. Nakano et al., Jpn. J. Appl. Phys. 54, 04DN03 (2014) [6] M. Nakano et al., RSC Adv. 6, 2489 (2016).

Organic thermoelectric materials are emerging as a class of solid-state energy converters enabling new paths to a more sustainable energy landscape without the need of expensive, or even toxic metal-based compounds. Building efficient thermoelectric devices requires high-performance complementary p-type (hole-transporting) and n-type (electron-transporting) materials. However, all-organic thermoelectric devices have been difficult to manufacture so far primarily due to the limitations encountered by the electron-transporting conducting polymers, which show conductivities of typically less than 10⁻² S cm⁻¹. Theoretical and experimental studies on n-doped donor-acceptor polymers reveal highly localized charge carriers (polarons), suggesting an intrinsic upper limit to the conductivity of this class of materials. The low conductivity directly translates into relatively low thermoelectric properties.
Here we show that ladder-type conducting polymers can achieve conductivities values that are three orders of magnitude higher than those of the best performing n-type donor-acceptor co-polymers. Extensive calculations reveal a larger polaron delocalization length, therefore suggesting also a higher polaron mobility. In this frame, the high electron conductivity of ladder-type polymers can be already justified at the single chain level.

Solution processible, scalable polymers have the potential to have high electrical conductivity (σ), high Seebeck coefficient (also called thermopower, S), and low thermal conductivity (κ), all necessary attributes for good thermoelectric performance. Organic materials also offer new opportunities to design materials with complex transport properties. In comparison to inorganic materials, ionic conductivities can be relatively high in organic materials near room temperature, e.g. as high as 1 S/cm. These high ionic conductivities can be obtained alongside electronic conduction leading to organic mixed ion-electron conductors. We suggest that this ability to simultaneously carry significant electronic and ionic charge can lead to unique thermoelectric properties, but the way that these transport properties influence each other within a single material is poorly understood. Conductive polymers such as PEDOT:PSS hold great promise as flexible thermoelectric devices, where solution processing techniques can lead to flexible devices in novel geometries. 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. In this talk, I will discuss how electrochemistry can be utilized to stabilize the Seebeck enhancement leading to stable improvements to power factor in mixed conductor thermoelectrics. I will also discuss new mixed conductor systems and device geometries that allow us to gain insight to mixed conductor effects in thermoelectrics.

Thermoelectric materials enable conversion of heat to electrical energy. The performance of traditional electronic thermoelectric materials, like semi-metals and inorganic semiconductors, are typically evaluated using a figure-of-merit ZT = σα²T/λ, where σ is the conductivity, α is the so-called Seebeck coefficient and λ is the thermal conductivity. However, it is not suitable to evaluate the performance of a new emerging class of ionic thermoelectric materials, like ionic solids and electrolytes. These systems cannot be directly used in a traditional thermoelectric generator, because they are based on ions that will be blocked at the interface between the electrolyte and the external metal electrode, which prevents current from passing through the circuit. Instead, energy can be harvested from the ionic thermoelectric effect by charging of the electric double layer capacitor (EDLC) of a super-capacitor. In this report, we investigate the ionic thermoelectric properties at varied relative humidity for the polyelectrolyte polystyrene sulfonate sodium (PSS:Na) and correlate these properties with the charging efficiency when used in an ionic thermoelectric super-capacitor (ITESC). In analogy with electronic thermoelectric generators, the results show that the charging efficiency of the ITESC increases with the ionic conductivity σ_i and the ionic Seebeck coefficient α_i, while it decreases with λ. In particular, we show that the efficiency can be quantitatively related to the figure-of-merit ZT_i = σ_iα_i²T/λ. This means that the performance of ionic thermoelectric materials can also be compared and predicted based on the ZT, which will be highly valuable in the design of high-performance ITESCs.

The thermoelectric (TE) phenomenon of direct energy conversion from a temperature gradient to electricity has captured attention as an economical, pollution-free form of energy conversion and plays a key role in developing alternative energy technologies that would reduce the dependence on fossil fuel and decrease greenhouse gases. Among various materials, mixed ion-electron conductors (MIEC) are recently being explored as potential thermoelectrics, primarily due to their low thermal conductivity. The combination of electronic and ionic charge carriers in those inorganic or organic materials leads to complex evolution of the thermovoltage (V_oc) with time, temperature and/or humidity. One of the most promising organic thermoelectric materials, poly(3,4-ethyelenedioxythiophene)-polystyrene sulfonate (PEDOT-PSS), is a MIEC. In addition to its competitive thermoelectric properties at temperatures below 150^oC, in comparison to inorganic counterparts, it is also easily processable, composed of naturally abundant elements and compatible with well-established solution-based manufacturing processing. Previous study showed that at high humidity, PEDOT-PSS undergoes an ionic Seebeck effect due to mobile protons. However, this phenomenon is not well understood. Here we report the time dependence of the V_oc and explain its behavior from the contribution of both charge carriers namely, holes and protons. From the electrical and spectroscopic evidence, we identify the presence of a complex reorganization of the charge carriers induced by the ionic thermoelectric effect, promoting an internal electrochemical reaction within the polymer film. We also demonstrate that the time dependence behavior of V_oc is a way to distinguish between three classes of polymeric materials: electronic conductor, ionic conductor and mixed ionic- electronic conductor.

Hybrid thermoelectric materials are promising for enhanced zT values (the TE figure of merit) for environment-friendly energy conversion because of their tunable structures and components, variable electrical conductivity and Seebeck coefficients. The current efforts on these materials are focuing on the precisely optimization of the orangic and inorganic components as well as the lattice structures. High-dimensional conductive coordination polymers with well-defined structures exhibit extened polymeric structures, adjustable organic buliding blocks and inorganic metal centers, as well as great thermalstability. They are becoming a unique type of molecular materials especially for the correlation construction between structures and performances. In the past few years, a series of hybrid molecular materials including functional coordination polymers have been achieved in our research. In this presentation, we shall update with the current progress on coordination thermoelectric materials: design and assembly through green chemical reactions; nontraditional thermoelectric films fabrication and composition; and their performance evaluations.
Acknowledgement: We appreciate the Institute of Materials Research and Engineering, A*STAR of Singapore for financial support.
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The abundance of heat at temperatures < 150 °C indicates the potential for low grade waste heat recovery. Thermoelectric generators (TEG), consisting of p- and n-type semiconductors, enable direct conversion of this waste heat into electricity. These devices operate without moving parts and are projected to have long lifetimes, which make them attractive for energy harvesting applications. Conducting polymers are lightweight, flexible and can be processed from solution thereby enabling new device architectures. In this study, we investigate and fabricate two new polymer-based architectures for TEGs. Based on characteristic length scales for polymers, we demonstrate our first design, which is a radial architecture that consists of disks of alternating p-type and n-type polymers stacked coaxially to create an appreciable voltage. Unlike conventional flat-plate TEGs, the radial design enables heat spreading which eliminates the need for active cooling. The radial architecture also reduces electrical contact resistance which enables higher power densities compared to flat-plate TEGs. In our second design, we employ inkjet printing for rapid and low-cost manufacturing of close-packed and thin-film architectures in which the thermoelectric junctions are printed in accordance with fractal space-filling curves. This architecture not only allows for minimizing the contact resistance, but also allows for easy tuning of the TEG to make it thermally and electrically impedance matched to a variety of applications. These novel architectures could potentially enable rapid scalability onto flexible substrates at low $/W costs for self-powered sensors and wearable electronics.

Of crucial importance for the thermoelectric properties of composites used in electronic and thermoelectric applications is the total thermal conductance of metal-nonmetal interfaces and their temperature dependence.

The mechanism of heat transfer and the contribution of electron-phonon coupling to thermal conductance of a metal-semiconductor interface is analysed taking into account the pathway: coupling between electrons and phonons within the metal, and then subsequently the coupling between phonons of the metal and phonons of the nonmetal.

The conductance and its temperature dependence for electron-phonon coupling within the metal, interfacial electron-phonon coupling and then the total thermal conductance of metal-nonmetal interfaces are estimated.

For energy transfer to occur between electrons and phonons, there must be some degree of nonequilibrium between them. To study such a non-equilibrium process, the Boltzmann transport equations for both electrons and phonons that are coupled through an electron-phonon scattering are numerically solved. The results are verified by comparing them with published experimental and analytical results for some compositions.

We report an enhanced thermoelectric performance of PEDOT:PSS via simple H2SO4 chemical treatment. The spin coated PEDOT:PSS film was treated by immersing in H2SO4 .The PEDOT:PSS film electrical conductivity significantly improved from 0.3 S cm-1 to 3478.26 Scm-1 after simple chemical treatment with H2SO4 solution that signifies an improve of four orders of magnitude related to the analogous factors of the untreated thermoelectric materials and the Seebeck coefficient reduced from 27.5 μW/ m K2 for untreated to 16.2 μW/ mK2 for H2SO4 treated PEDOT:PSS. The power factors of PEDOT:PSS after three times H2SO4 treatment enhanced to 91.28 μW/ mK2. And the thermal conductivity before and after H2SO4 treatment were 0.22 and 0.18 W/mK respectively. The highest figure of merit ZT (0.15) was observed at 300 K. Various characterization approaches including AFM,XPS, Raman spectra and UV-vis-NIR absorbance spectra were employed to examine the motive behind the significantly improved TE performance.
Key Words: PEDOT:PSS, H2SO4 chemical treatment , Electrical conductivity, Seebeck coefficient, Thermal conductivity, Figure of merit

Recently, hybrid organic/inorganic composites have been investigated in the thermoelectric fields in an effort to interdependently manipulate the charge carriers which are interconnected with electrical and thermal transport. The decoupling of a charge carrier’s role in the hybrid structures can demonstrate the enhanced thermoelectric performance by accommodating the only merit from each component, for example, the relatively high Seebeck coefficient originated from the inorganic part as well as the highly maintained electrical conductivity from the organic part.
In this work, we demonstrated the enhanced thermoelectric performance of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) by employing the nanoimprinted Ge₂Sb₂Te₅ nanowire arrays to form a vast of PEDOT:PSS/Ge₂Sb₂Te₅ nanowire composites, which show approximately ten times higher Seebeck coefficient than the PEDOT:PSS itself without damaging the electrical conductivity. In addition, the ethylene glycol (EG) was used for selective de-doping of PSS to enhance the electrical conductivity of the composites. The variation of EG concentration to optimize the de-doping effect revealed that the best thermoelectric power factor of ~1.6 x 10³ μW/mK² was achieved with the EG level of 6 vol. % in our configuration. Meanwhile, the electrical and thermoelectric measurement was conducted in both parallel and perpendicular directions to observe dependence of transport property on charge carrier pathway under the existence of the interface so as to achieve a high enough thermoelectric power factor.

Nowadays, inorganic thermoelectric (TE) nanomaterials with ZT values above unity are well established, nevertheless the commercialisation and wide application of thermoelectric generators are mainly limited to Bi-Sb-Te-Se and Pb-Te-Se materials system. Due to economic and environmental issues of these materials, the organic thermoelectric materials are attracting more attention over the last decade. Compared to traditional inorganic TE materials, organic materials have shown various advantages, e.g., light weight, flexibility, low thermal conductivity and expected low-cost of fabrication. However, TE polymers have shown some drawbacks as well, such as low electrical conductivity and Seebeck coefficient. Mentioned disadvantages might be overcome by constructing so called polymer composite with the addition of inorganic materials such as Te nanorods [1] and Bi2Te3 powders [2].
In order to gain more physical insight into the structural and electronic properties of the polymer composites based on PEDOT and P3HT conjugated polymers and Te chalcogenides, we performed ab initio calculations. Finally, the transport properties of these material systems are calculated within the framework of Boltzmann theory and constant relaxation time approximation.

[1] K. C. See, J. P. Feser, C. E. Chen, A.Majumdar, J. J. Urban, and R. A. Segalman, Nano Letters 10, 4664 (2010).
[2] B. Zhang, J. Sun,H. E. Katz, F. Fang, and R. L.Opila, ACS Applied Materials and Interfaces 2, 3170 (2010)

Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT/PSS) is a widely used intrinsically conductive polymer with high electrical conductivity. This type of conductive polymers has been commercialized and they have been applied for a variety of industry applications including electrodes in touch screens and sensors, printed electronics and anti-statics. Its conductivity is, in general, below 1 S/cm in an aqueous solution, but it could be significantly increased up to a few 100 to 1,000 upon addition of a small amount of organic high boiling-point polar solvents such as dimethyl sulfoxide and ethylene glycol. Recently, it was reported that its conductivity was able to approach to 5,000 S/cm through proper acid treatment. In this presentation, we would like to update our recent systematical investigation of PEDOT/PSS by treatment with a series of organic polar solvents including organic acids, amides and alcohols. The results show that the treatment with different organic acids, alkanols, etc., will considerably remove the excess PSS present in the PEDOT/PSS as evidenced by UV-Vis spectroscopy. Its electrical conductivity is remarkably increased from around 0.5 to approximately close to 4000 S/cm. The Seebeck coefficients however almost remain intact, leading to a relative high figure of merit (ZT) of 0.1-0.2.

Using the frequency-domain perfectly match layer (FD-PML) method recently developed by our group to simulate mode-resolved phonon scattering events, we investigate the transport of thermal wavelength phonons across interdiffused interfaces. In particular, on a mode-by-mode basis we probe the validity of the popular Diffuse Mismatch Model (DMM) for phonon transmission for increasing levels of interdiffusion. The DMM assumes that incident phonons scatter diffusely at sufficiently rough interfaces, losing memory of their incident wavevector, and scattering randomly into any available wavevector with equal energy/frequency. For simulations of the interface between two fcc lattices in 3D, we show that as the level of interfacial interdiffusion increases, some aspects of the DMM are reproducible. In particular, modes of transmission that are forbidden at a perfect interface, become available, and thus disorder can increase the thermal interface conductance. We show that DMM-like transmission coefficients are recovered for submonolayer levels of interdiffusion, but the validity is confined to only certain incident modes in a cone on the Brillouin zone directed toward the interface. Based on a mode resolved analysis of the transmission coefficient over the entire Brillouin zone we hypothesize that there are still important restrictions on the wavevectors available to the scattered waves. Furthermore, we find that with increasing levels of interdiffusion, we do not converge toward limiting transmission coefficients; rather, the transmission coefficients of many modes continue to decrease as the size of the interdiffused region is increased, indicating that phonon localization may play an important role determining interface conductance.

Organic semiconductors, owing to their flexibility and low-fabrication-cost merit, have demonstrated the promising application potentials in the field of wearable thermoelectric devices. The thermal conductivity of organic semiconductors is the vital factor to adjust the figure of merit (ZT) of these organic materials, and thus the heat transporting mechanism is an important area worth in-depth investigation. Here we applied non-equilibrium molecular dynamics (NEMD) to simulate thermal transport in a high mobility and air-stable small molecule organic semiconductor, dinaphtho-[2,3-b:20,30-f]-thieno-[3,2-b]-thiophene (DNTT). In the simulation, the general Amber force field (GAFF) is used to describe the atomic interaction. We noticed that the thermal transport of DNTT shows a strong anisotropic property. The bulk thermal conductivity of DNTT in a, b, and c crystal directions at room temperature are 2.92, 0.91 and 2.60 W/m-K, respectively. By use of kinetic theory, the corresponding phonon mean free paths (MFP) in each direction are 34, 12, and 18 nm. Our 3-w experimental result shows the thermal conductivity of 50nm DNTT in c-direction is around 0.45 W/m-K which is only 22% of the MD simulated result (2.01 W/m-K). As the selected area electron diffraction (SAED) pattern of DNTT thin film is in a ring pattern, it is believed that the DNTT thin film is under a polycrystalline structure, hence the thermal boundary resistance (TBR) would play an important role in the experimental results which is not considered in the MD simulation. To address the TBR effect, we further investigate the interfacial thermal transport of the a-b interface, a-c interface and b-c interface, respectively. The simulated TBR values are 2.23, 1.99 and 1.23×10^-9 m²-K/W, which are comparable with the literature results of other organic-organic interface TBRs. By comparing the TBR values with the experimental measured thermal conductivity, one can estimate the crystallinity of the organic thin film by quantifying the number of boundary interface in the organic crystal. To match the MD results to experimental result, we find there are around 5-8 crystal interfaces in the DNTT thin film. Furthermore, we also investigate the void vacancy effect in the thermal conductivity of the DNTT thin film. When the vacancy concentration varies from 0 to 6 %, the MD predicted thermal conductivities of DNTT can drop 72, 50 and 62% in a, b, and c crystal directions respectively, which demonstrates the vacancy in the DNTT crystal also has an unneglectable effect on the thermal transport of organic thin films. Our findings provide valuable information on how to modulate the thermal conductivity of organic semiconductors to achieve the desired thermoelectric properties.

Polymer based thermoelectric (TE) devices begin to attract attention as a potential energy source for low-power consuming devices. Due to the high flexibility in TE devices and modules design and fabrication, they can be used for various applications that the traditional inorganic thermoelectric devices were hardly applicable to, such as flexible and stretchable devices. By the fundamental understandings and multilateral efforts on the development of novel materials and devices, the TE device performance has been noticeably improved recently. However, the fabrication method of polymer TE modules is not thoroughly investigated. A novel fabrication method is required to realize practical polymer TE generators (TEGs) because the polymer TE devices have an extremely thinner active layer compared to traditional inorganic TEGs. In this study, we introduce a novel and simple fabrication method for flexible polymer TEGs by connecting four TE devices in series. The TEGs based on poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS, as purchased) exhibited an output voltage of ~1 mV under the temperature difference of 30 K at room temperature, while each TE device in a module showed the Seebeck coefficient of 8–10 μV/K. It is also notable that the surface of the polymer layer was passivated during fabrication, which enabled us to observe stable operation for several days.

Due to their low thermal and high electrical conductivities, organic conducting polymers have attracted extensive investigations for thermoelectric (TE) applications. Highly conductive p-toluenesulfonate (Fe-Tos) doped poly(3,4-ethylenedioxythiophene) (PEDOT) films were first prepared by solution casting and thermal polymerization of EDOT monomers with the oxidant cum dopant (Fe-Tos). UV-vis-NIR absorption and X-ray photoelectron spectroscopic studies of the conducting PEDOT films clearly indicated that the monomers were polymerized. Post-treatment of the polymer films was then carried out with the environment-friendly L-ascorbic acid (vitamin C) as a reducing agent. Upon the treatment, increases in absorption in the visible spectral range and decreases in the NIR region were observed as compared with the untreated ones, indicating that reduction of the polymer films was achieved. Furthermore, enhancement in TE responses and drop in electrical conductivities were also measured. Experimental details on the treatment and characterization of the conducting PEDOT films at different oxidation levels will be presented.

The Transient Hot Bridge method, which is used to measure the thermal transport properties of materials, is an enhancement of the Hot Wire or the Transient Hot Strip method (DIN EN 993-14, DIN EN 993-15). The measuring method is a transient, time depended measuring method. The advantage of this method compared to stationary methods is a much shorter measuring time, and the thermal diffusivity is measured in parallel to the thermal conductivity and Specific heat. Transient Hot Bridge has proved to be a useful approach for materials having a thermal conductivity range of 0.01-100 W/mK over temperature range of -50C to 700C.

It is well known that a good thermoelectric material should find its optimal doping concentration. However, much less attention has been paid to the optimization of the dopant’s energy level. Thermoelectric materials doped with shallow level impurities may experience a dramatic zT reduction at high temperature region due to the excitation of minority carriers, which reduces Seebeck coefficient and increases bipolar heat conduction. Doping with deep level impurities can delay the excitation of minority carriers as it requires a higher temperature to ionize all dopants. We find through modeling that, depending on the material type and temperature range of operation, different impurity level (shallow or deep) will be desired to increase the thermoelectric performance by increasing the engineering zT. We further answer the question on what is the most preferable position of the impurity level inside the band gap for different materials. Our research provides insights in choosing the most appropriated dopants for a thermoelectric material in order to maximize the device efficiency. This work is supported by DOE EFRC (Grant No. DE-SC0001299).

Conducting polymers are promising candidates for wearable thermoelectric energy devices thanks to their low-temperature and solution processability and flexibility. Enhancing the electrical conductivity(σ) of conducting polymers using traditional routes such as chemical doping, which shift the Fermi energy (E_F) level closer to the conduction band, has been studied for application in practical devices. However, the increase of σ via the intensive doping of carriers accompanies reduction of the Seebeck coefficeint resulting in decrease of efficiency of organic thermoelectric such as power factor. In this study, we propose a new strategy for enhancing power factor of organic thermoelectric materials by fabricating multilayer structure using two different conducting polymers- Poly (3,4-ethylendioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and polyaniline doped with camphorsulfonic acid (PANI-CSA). Charge transfer arising from different band structure between PEDOT:PSS and PANI-CSA can result in only density of states around E_F, thus maintaining the Seebeck coefficient.

At first, PANI-CSA film on the glass substrate was annealed at 50°C for 1 hr in N₂ ambience. PEDOT:PSS film was then deposited on top of the PANI-CSA film and then baked at 120°C for 15 min in N₂ atmosphere. The above process was repeated 1 to 5 times for fabricating multilayer structure. The minimum thickness of both PANI-CSA and PEDOT:PSS component layer is around 20 nm. The electrical conductivity and power factor of five PEDOT:PSS/PANI-CSA multilayer films are 1.3 and 2 times higher than those of a single PEDOT:PSS layer. Transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) showed distinct interfaces. The enhancement of electrical conductivity occurs via stretching of PEDOT and PANI chains and a hole diffusion from PANI-CSA layer to PEDOT:PSS layer.

Thermoelectric (TE) materials, which can directly convert heat into electricity and vice versa, have attracted tremendous interest because of their potential applications in waste heat recovery and solar thermal utilization. Traditional commercial TE materials such as Bi₂Te₃ and PbTe, however, have inevitable drawbacks such as high cost, scarcity, toxicity, and brittleness, and hence lack scalability of production. On the contrary, organic TE materials offer unique advantages of low cost, light weight and solution processability, thus holding great potential of high-throughput production of flexible TE modules. Nevertheless, their intrinsically low electrical conductivity limits the improvements of power factor (PF) and zT. Recently, the development of organic-inorganic TE nanocomposites can resolve the above issues. On one hand, they possess high electrical conductivity of inorganic part. On the other hand, the organic component imparts the hybrids with low thermal conductivity, rational molecular design, and mechanical flexibility. However, most of these TE nanocomposites employed carbon-based nanomaterials and inorganic semiconductors. Major obstacles limiting applications of TE nanocomposites are low PF and lack of n-type TE materials. As native n-type conductors, metals with not only high electrical conductivity but also low cost are barely reported as inorganic component. Here we report the solution fabrication of flexible n-type TE nanocomposites comprising metallic Ni nanowires (NWs) embedded in an insulating polyvinylidene fluoride (PVDF) matrix. It is found that the electrical conductivity and Seebeck coefficient are decoupled and both remarkably increase with Ni content. A maximum PF up to 220 μW m⁻¹ K⁻² is obtained at 380 K with 80 wt% Ni NWs, which is among the best for n-type TE nanocomposites. The nanocomposites exhibit typical temperature dependences of magnetic metals such as Ni, that is, positive in electrical conductivity while negative in absolute Seebeck coefficient. The resulting PF is progressively enhanced over temperature. This work demonstrates an effective avenue towards high-performance n-type TE nanocomposites while unraveling the unique transport mechanism.

Thermoelectric generators are typically operating under high external thermal resistances. Realistic heat exchange processes may induce significant temperature differences between the thermoelectric generator and its heat source and/or sink. Since available thermoelectric modules with limited geometric variability typically have comparably low thermal resistances, the actual temperature difference across thermoelectric elements tends to become much smaller than the temperature difference between the heat source and sink, which in turn reduces the performance of power generation. There are two ways of resolving the problem. One is to reduce the external thermal resistances, which is practically difficult. The other method is to increase the thermal resistance of thermoelectric modules in order to better match the external resistances. In this study, we take the second approach, and propose to introduce porosity into thermoelectric elements. Several recent studies revealed that micro-sized or nano-sized pores may improve the performance of materials, but the impact of increase of thermal resistances by the introduction of macro-sized pores is seldom emphasized. In this study, we present the result of our analytic study on the effect of macro-szied pores in thermoelectric elements with special emphasis on device performances. Potential economic benefit of the introduction of pores is also investigated, using a model for economic analysis of thermoelectric generation.

P- and n-type doping through charge transfer interaction has attracted more attention as an important technique to functionalize carbon materials such as for thermoelectric power generation [1-2]. Nonoguchi et al. reported that absorption of molecular dopants alters the type of majority carriers of pristine single walled carbon nanotubes (SWNTs) from positive to negative, and they displayed a relatively large Seebeck coefficient (S = –72 μV K^–1 at 310 K) and a thermoelectric figure of merit (ZT = 0.073 at 310 K) of the film fabricated from phosphine-doped SWNTs [1]. Matsumoto et al. reported that thermoelectric properties of commercially available graphite sheets could be manipulated by intercalating chemical species into the graphite interlayers, and they showed a Seebeck coefficient (S = −58 μV Κ⁻¹ at room temperature) and a large power factor (σS² > 10⁻² W m⁻¹ K⁻² at room temperature) of the sheet with potassiun-graphite intercalation compounds (K-GICs) [2]. In spite of their successes, there has been essentially no report about the functionalization of carbon blacks with such doping even though it should be easy to use them for mass production because of the lower price and the higher availability compared with other carbon materials.
In the present study, therefore, we tried to fabricate an n-type thermoelectric material using a carbon black (acetylene carbon black (ACB) and Ketjenblack), as a main raw material. N-type doping of those carbon blacks, generally p-type material (S ~ 12 μV K^–1 at room temperature), was conducted with a molecular dopant, triphenylphosphine (tpp) (S = –15 μV K^–1 at room temperature in the case of ACB). The key was to modify the surface of carbon blacks with silver (Ag) nanoparticles to attach tpp molecules through interaction between the π electrons of aromatic rings and the surface of Ag. A systematic study of the structural and thermoelectric properties of the tpp-doped Ag-modified carbon mateirals will be discussed in our presentation at the meeting.
1. Y. Nonoguchi, K. Ohashi, R. Kanazawa, K. Ashiba, K. Hata, T. Nakagawa, C. Adachi, T. Tanase, and T. Kawai, Sci. Rep. 3, 3344 (2013).
2. R. Matsumoto, Y. Okabe, and N. Akuzawa, J. Electron. Mater. 44, 399 (2015).

Nanocomposites of conjugated polymers and carbon nanotubes are promising materials for large-area thermoelectric generators that operate at room temperature. They combine the high electrical conductivity of nanotubes with the low thermal conductivity and facile solution processability of polymers. While encouraging power factors have been achieved for composites with positive Seebeck coefficients (S>0, p-type) [1], more work is needed on air-stable organic n-type materials.

Here, we present work done on composites of regio-regular poly(3-hexylthiophene-2,5-diyl) (rr-P3HT) and nitrogen-doped multi-walled carbon nanotubes (nCNTs), which can exhibit either S>0 or S<0. N-type behaviour can be obtained by increasing the nCNT content of a p-type composite, and simpler still by UV-irradiating a solution of p-type composite during deposition [2]. Using the developed method, we demonstrate a thermoelectric module, where both types of legs are deposited from the same solution, thereby significantly simplifying processing.


[1] C. Bounioux, P. Díaz-Chao, M. Campoy-Quiles, M. S. Martín-González, A. R. Goñi, R. Yerushalmi-Rozen, C. Müller, “Thermoelectric composites of poly(3-hexylthiophene) and carbon nanotubes with a large power factor”, Energy & Environmental Science, Vol. 6, No. 3, (2013), 918-925.
[2] B. Dörling, J. D. Ryan, J. D. Craddock, A. Sorrentino, A. El Basaty, A. Gomez, M. Garriga, E. Pereiro, J. E. Anthony, M. C. Weisenberger, A. R. Goñi, C. Müller, M. Campoy-Quiles, "Photoinduced P- to N-Type Switching in Thermoelectric Polymer-Carbon Nanotube Composites", Advanced Materials, (2016), 28, 2782–2789.

Nanostructuring and doping are common and promising methods to improve thermoelectric materials. We performed density functional theory-based calculations, validated by experimental measurements, to investigate the structural, electrical and thermal properties of new candidate – transition-metal dichalcogenides, and conventional compounds - skutterudites. Two-dimensional transition metal dichalcogenides (2D-TMDs) are of broadening research interest due to their novel physical, electrical, and thermoelectric properties. Having the chemical formula MX₂, where M is a transition metal and X is a chalcogen, there are many possible combinations to consider for materials-by-design exploration. In order to develop these materials into wide-scale use, it is crucial to comprehensively understand the compositional affects. We investigated the structure, electronic, and phonon properties of eighteen different TMD MX₂ materials compositions as a benchmark to explore the impact of various elements. Our results identified key factors to optimize MX₂ compositions for desired performance. The key factors include atomic weight, radius, oxidation state and interfacial lattice mismatching. To further enhance the thermoelectric properties, we studied substitutional doping and heterostructure effects on the TMD materials. In addition, skutterudites inherently have a high electrical conductivity, but difficulty of obtaining figure of merit, ZT greater than unity. Substitutional doping in the skutterudites can increase electrical conductivity and decrease thermal conductivity by means of scattering phonons. However, the pnicogen rings (i.e. B-side rings) in the skutterudites are structurally distorted as substitution concentration increases, exhibiting a deviation from the cubic symmetry of the skutterudite compounds. In the presentation, we will also discuss such structural distortion effect on the thermoelectric properties of the skutterudite compounds.

Polymer semiconductors are a promising class of thermoelectric materials that currently receive considerable attention because they can be used to cost-effectively convert (waste) heat into electricity and vice versa. Molecular dopants are widely explored as a means to modulate the charge carrier density and hence achieve optimal thermoelectric properties. A complete picture of underlying structure-property relationships is lacking because polymers and dopants are typically co-processed, which disrupts the nanostructure of the semiconductor. We are exploring doping of the model semiconductor poly(3-hexylthiophene) with F4TCNQ vapour, which permits to disentangle the solution-processing and doping step. As a result, we are able to independently control formation of the semiconductor nanostructure through e.g. the choice of processing solvent and polymer regio regularity. Subsequent vapour doping equips us with a tool to establish structure-property relationships such as the impact of crystalline order on the doping efficiency and hence electrical conductivity and Seebeck coefficient.

Since the discovery of conducting polyacetylene in 1977, π-conjugated systems, especially low band-gap conjugated oligomers and polymers, have attracted considerable interest for their peculiar properties such as third-order nonlinear optical properties, electrical conductivity, organic light-emitting diodes (OLEDs), field-effect transistors (FETs), electrochromic or smart windows, chemical sensing and photovoltaic properties (OPVs) and potential applications in optoelectronic devices. Those conducting materials are indispensable units for many technologies. The market for conducting polymers and corresponding has been huge and is still expanding. Among those conducting polymers, PEDOT/PSS is quite excellent, which shows high electric conductivity over 3000 S/cm and highest ZT of 0.42. Such PEDOT/PSS indicates the great application in p-type thermoelectric materials. However, currently, high-performance thermoelectric materials, especially n-type thermoelectric materials, are quite limited. Therefore, in our research, we develop a series of diketopyrrolopyrrole (DPP) containing polymers, which show great n-type thermoelectric performance. Such polymers can be promising candidate for real application.

The thermoelectric properties of PEDOT-based materials have attracted attention because of their remarkable electrical conductivity. In the first part of this presentation, we report the use of a solid-state photoinduced charge-transfer reaction to precisely control the carrier density in PEDOT/PSS. We find a maximum power factor of ~42 μW/mK² at a carrier density of ~5 × 10²⁰ cm⁻³. Measurement of the Seebeck coefficeint of PEDOT/PSS in water and ethylene glycol shows an enhanced value from 17 μV/K to ca.35 μV/K, and the calculated power factor could as high as 100 μW/mK². We believe this is because PEDOT significantly doped by proton and the doping state could be affected by water and ethylene glycol through hydrogen bonding. This assumption is supported by the film absorption in different solvents. We will also describe our recent works on fabricating of high density thermoelectric devices using thermal lamination aiming on a power density of 50 μW/cm².
In the second part, we report the methods to study through-plane electrical conductivity and in-plane thermal conductivity. We have shown that PEDOT/PSS films have anisotropic carrier transport properties and thermal conductivity. There is pronounced change in the through-plane electrical conductivity activation energy at about 250 K, but almost no change of the in-plane electrical conductivity activation energy, suggesting different transport mechanisms at through-plane direction and in-plane direction.

References
[1] Q.S. Wei, et al., Adv. Mater., 2013, 25, 2831.
[2] Q.S. Wei, et al., Appl. Phys. Express, 2014, 7, 031601.
[3] Q.S. Wei, et al., ACS Macro Lett., 2014, 3, 948.
[4] Q.S. Wei, et al., RSC Adv., 2014, 4, 28802.
[5] Q.S. Wei et al., ACS Appl. Mater. Interfaces, 2016, 8, 2054.

Freestanding poly(3,4-ethylenedioxythiophene) (PEDOT) nanowires were synthesized by template-confined in-situ polymerization, and then integrated into PEDOT:PSS host. The hybrid morphologies were characterized by atomic force microscopy, indicating homogenous dispersion of PEDOT nanowires. Thermoelectric properties of the resultant hybrids were measured, and the power factor was found to be enhanced by 9-fold in comparison to PEDOT:PSS mixed with 5 vol% dimethyl sulfoxide while the low thermal conductivity was still maintained. Such a significant improvement could be attributed to the synergistic effects of interfacial energy filtering, component contributions, and changes of carrier concentration in the host materials. Upon addition of 0.2 wt% PEDOT nanowires, the resultant composites demonstrated a power factor as high as 102.7 μW m^-1 K^-2 and thermoelectric figure of merit could reach 0.1 at room temperature. These findings open up a new route towards high-performance polymer thermoelectric materials and devices.

Thermoelectric polymers display the highest sustainable power factors when conductivity is entirely electronic and when mobility is maintained even at high doping levels. We present results on two approaches towards these goals, based on poly(bisdodecylquaterthiophene) (PQT12)-type structures. In the first, we use the static charge stored in a nonvolatile fluoropolymer gate dielectric to modulate the Seebeck coefficients and hole conductivities, with the expected opposite dependencies on doping levels. In the second, we copolymerize dodecylthiophene and ethylenedioxythiophene (EDOT) to form a new regioregular copolymer. Doping this latter polymer with tetrafluorotetracyanoquinodimethane (F4TCNQ) increases both conductivity and mobility, and results in a power factor among the highest reported for hole-only, nonionic single-polymer materials. This work shows that the EDOT monomer can be incorporated into thiophene polymer designs without relying on the usual electrochemical syntheses and sulfonate dopants. Results from both systems fit well with models that relate Seebeck coefficient and electrical conductivity.

Over the past several decades, thermoelectric (TE) materials have been developed because they offer new avenues to energy generation. Today, the enhancement of TE conversion efficiency is achieved through new developments and improvements in nanostructuring of materials, for example by using nanoparticles as building blocks for constructing the semiconducting materials. The performance of TE materials is defined by the dimensionless figure of merit, ZT=σS²T/κ, where σ, κ, S, and T represent electrical conductivity, thermal conductivity, Seebeck coefficient and average temperature between the hot and cold sides, respectively. In nanostructured TE materials, ZT value is enhanced because κ should be reduced through phonon scattering at grain boundaries, while σ should be essentially maintained because of the smaller mean free path of charge carriers. The approach has led to the discovery of many TE materials with elevated ZT value, but the full potential of nanostructuring has not been achieved yet because of a lack in understanding for how the nanoparticle size, shape, composition and structure affect the overall thermoelectric characteristics. Furthermore, rare metals such as tellurium, selenium or antimony are still used extensively in the best inorganic TE materials, which limits their widespread use on a practical scale. In order to achieve more efficient TE technology, we must investigate new nanoparticle materials composed of non-traditional and sustainable compositions with a focus on control of the nanoparticle characteristics such as particle size, shape, composition and structure. With these criteria in mind, we have developed a series of sustainable TE materials based on nanoparticles composed of transition metal sulfide compounds. By using a bottom-up wet-chemical technique, we maintain control over the nanoparticle characteristics, which we then process into large scale materials while also maintaining the particle attributes. The results have been promising with typical ZT values of about 1 for these earth abundant materials. The presentation will focus on the synthesis, processing and characterization of our most recent materials including Cu₂XS₃ and Cu₂MXS₄ (X = Sn or Ge, M = transition metal) nanoparticles. We will also talk about effects on the fundamental physical characteristics such as quantum confinement effect, as well as techniques to control the nanoparticle characteristics.

Organic semiconductors with conjugated electron system are currently intensively investigated for many novel electronic and optoelectronic applications. The key advantages are flexibility, low cost and low resources usage since the materials mostly consist of carbon.
For use as thermoelectrics, control of the Fermi level is critical since it largely determines the Seebeck coefficient which is crucial for thermoelectric applications. In this talk, I will summarize results on controlled doping of semiconductors which is known to improve devices since considerable time /1/. However, the detailed understanding of the doping effects and Fermi level control has turned out to be difficult. Based on recent measurements of the Seebeck coefficient /2/ and comparison with photoemission spectroscopy data /3/, a comprehensive understanding has been achieved.

/1/ K. Walzer et al., Chem. Rev. 107, 1233 (2007)
/2/ T. Menke et al. Phys. Status Solidi B 252, 1877 (2015); T. Menke et al, Organic Electronics 15, 365 (2014)
/3/ M. Tietze et al., Adv. Funct. Mat. 25, 2701 (2015)

Organic semiconductors are candidates for use in next generation light emitting devices, thin film transistors (TFTs), photovoltaics, and thermoelectrics. Many of these devices benefit from electrical doping that can boost the effective carrier mobilities by filling trap states, enhancing electrical conductivity by increasing the density of free charge carriers, and lowering barriers to charge-carrier injection or collection at electrodes. High performance semiconducting polymers have been reported with comparable hole and electron charge-carrier mobilities,^1,2 however generally the electrical conductivities of n-doped organic semiconductors have been substantially lower than that of p-doped organic semiconductors. Additionally, there are to-date no reports of a single polymer doped to be p-type and n-type with comparable conductivities.

Here, we report the n-doping of the non-planar ambipolar polymer Poly((E)-3-(5-([8,8'-biindeno[2,1 b] thiophenylidene]-2-yl)thiophen-2-yl)-2,5-bis(2-octyldodecyl)-6(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione) (P(BTP-DPP)) with the organometallic dimer pentamethylcyclopentadienyl mesitylene (RuCp*mes)₂ processed via sequential spin-casting. Maximum n-type conductivities of 0.45 S/cm are observed, which are amongst the highest reported for n-type semiconducting polymers. Using a combination of photoemission, spin resonance and optical spectroscopy we have studied the impact of processing conditions on the resulting electronic structure of the doped polymer. Significantly, we observe evidence of the coexistence of polarons and bipolarons in optimally doped films. Furthermore, upon doping these systems were found to be highly optically transparent in the visible region of the electromagnetic spectrum, making them excellent candidates for next generation n-type transparent conducting electrodes.

The origin of the increased electrical conductivity observed in this system was probed using structural methods such as grazing incidence X-ray scattering and atomic force microscopy. Our results suggest that sequential processing allows for formation of efficient percolation pathways for charge transport. This work provides us with a framework for developing future high conductivity systems using sequential casting to process semiconducting polymers with non-planar backbones that enable the efficient packing of dopants.

1) Li, J.; Zhao, Y.; Tan, H. S.; Guo, Y.; Di, C.-A.; Yu, G.; Liu, Y.; Lin, M.; Lim, S. H.; Zhou, Y.; Su, H.; Ong, B. S. A Stable Solution-Processed Polymer Semiconductor with Record High-Mobility for Printed Transistors. Sci. Rep. 2012, 2.
2) Lee, J.; Han, A.-R.; Yu, H.; Shin, T. J.; Yang, C.; Oh, J. H. Boosting the Ambipolar Performance of Solution-Processable Polymer Semiconductors via Hybrid Side-Chain Engineering. J. Am. Chem. Soc. 2013, 135 (25), 9540–9547.

Organic semiconductors have recently gained considerable attention as thermoelectric materials due to their mechanical flexibility and compatibility with low-temperature manufacturing techniques. However, the power factors of organic thermoelectrics are lower compared to champion inorganic materials (such as Bi₂Te₃) at the same electrical conductivity. Because the carrier concentration cannot be readily determined in many organic materials, its relation to thermopower is poorly understood. This makes it challenging to find the upper bound of power factor in organic materials.

In this work, we have investigated the thermoelectric performance of a p-type semiconducting polymer (PBTTT) in a field-effect transistor (FET) configuration to directly modulate the carrier concentration via gating. The transistor was operated in the high carrier sheet density (10¹² – 10¹³ /cm²) regime by employing a high-capacitance (~ 1µF/cm²) polymerized ionic liquid (PIL) dielectric. PILs contain one ion covalently bonded to the polymer backbone and one mobile ion, which limits electrolyte diffusion into the semiconductor. The Seebeck coefficient measured in situ during the operation of the transistor was found to systematically decrease from ~300 µV/K to 20 µV/K as a function of increasing gate bias, while the electrical conductivity increased from ~0.1 to 200 S/cm. A comparison of our results with bulk doping will be presented. Our studies demonstrate that gating with PILs provides a route to understand the connection between carrier concentration and thermopower.

Among the various categories of thermoelectric materials, significant attention has been devoted to polymer thermoelectric materials that particularly enjoy low thermal and high electrical conductivities while being solution processable and flexible. Case of study, poly(3, 4-ethylenedioxythiophene) –PEDOT as a conducting polymer has been shown to possess figure of merit (ZT) values as high as 0.42 and promises to rival their inorganic thermoelectric counterparts. Although PEDOT enjoys low thermal conductivities, the electrical conductivity spans a wide range from 10^-8 to 10³ Scm^-1_. Moreover, as observed in inorganic thermoelectrics, the electrical conductivity, thermal conductivity, and thermopower properties of PEDOT are all correlated thus making the optimization of the figure of merit tremendously challenging. As such, the ability to increase the electrical conductivity without sacrificing the thermopower is imperative for promoting the use of PEDOT as a thermoelectric material. PEDOT –in particular has been shown to possess high thermopowers (≈100-500 μV K^-1) due to electron-phonon scattering in the crystalline grains and electron-phonon coupling in the counter ion introduced to control their crystallinity and processability. On the other hand, the charge transport in PEDOT is typically dominated by phonon-assisted hopping between the metallic domains within the polymer chains which turns out to be a less effective charge transport mechanism than that observed in inorganic materials. Consequently, the electrical conductivity and thermopower are strongly affected by the morphology, composition, temperature, dopant levels, and orientation of this conducting polymer. Herein, we will discuss the effects that dopants have on the thermoelectric properties of PEDOT relative to the counter ion present in the polymer backbone. We will also present experimental details on an electrically detected magnetic resonance (EDMR) study of the spin dynamics in assembled devices comprised of PEDOT thin films having different counter ions and dopant levels. EDMR studies at low temperatures (5K) for our PEDOT thin film diode shows a pronounced resonance line that is strongly dependent on dopant levels and counter ions. This characteristic resonance feature is due to weakly spin-spin coupled recombining charge carrier pairs (polarons). Moreover, we observe that this resonance feature displays a decreasing linewidth with increasing doping concentration regardless of the counter ion. We conclude that dopants added to PEDOT thin films result in less localized electronic charge carrier states that ultimately impact their thermoelectric performance.

Recent years have seen the emergence of organic semiconductors (OSCs) with values of ZT approaching those of inorganic counterparts. Among high-performance organic thermoelectric materials are disordered polymers such as PEDOT:PSS and PEDOT:Tos, with power factors as high as 469 µW/mK². Crystalline small-molecular OSCs such as rubrene and pentacene, on the other hand, have exhibited lower thermoelectric power factors, as a result of less efficient carrier transport. Carrier localization lengths, for example, are typically smaller in such materials (1-2 nm) than in high-performance disordered polymers (~10 nm), suggesting that transport is less band-like. In spite of this, the less disordered electronic landscape of crystalline small-molecular OSCs offers more straightforward means of tuning material properties that can improve ZT, such as the carrier density of states (DOS) and the carrier localization length, through the design of novel supramolecular architectures.

The thermoelectric power factors of crystalline small-molecular OSCs can be enhanced by engineering distortions in the DOS (similar to methods previously applied to crystalline inorganic thermoelectric materials) or by increasing the carrier localization length. In this study, we present a computational model that demonstrates how microscopic properties such as the intermolecular vibration frequency and the electron-phonon coupling strength can be systematically tuned in order to realize high thermoelectric power factor. The model accurately accounts for a mixture of coherent and incoherent charge transport and is thus able to successfully model both high-temperature and low-temperature (e.g., metal-insulator transition) behavior. By recursively solving the Green’s function for a one-dimensional tight binding Hamiltonian with off-diagonal disorder, we unambiguously show for the first time the extent to which the on-site potential (diagonal disorder) and the intermolecular interaction (off-diagonal disorder) can be realistically tuned in order to engineer DOS distortions that enhance the thermoelectric power factor.

Organic semiconductors have attracted increasing attention as low-temperature thermoelectric materials, offering the possibility of fabricating low-cost, large-scale and mechanically flexible thermoelectric modules. The thermal conductivity of organic semiconductors is intrinsically low, leaving the power factor as the most important parameter for optimization. Careful control of the doping level is required to improve the power factor. However, solution processed n-type organic semiconductors typically suffer from poor miscibility with molecular dopants. This results in poor host/dopant morphology, low doping efficiency, and limited conductivity.

In this contribution, we show how careful tailoring of the polarity of organic semiconductors can drastically improve host/dopant miscibility. We use a fullerene derivative (PTEG1) with a hydrophilic triethylene oxide side chain doped with (4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)dimethylamine (n-DMBI). AFM and SEM studies show that the morphology of this system is much better than that of the less polar PCBM-based equivalent, implying improved host/dopant miscibility. The resulting conductivities are up to 2.05 S/cm, caused by enhanced efficiency of charge transfer doping. Additionally, the thermal stability of the PTEG-1 based system was improved by increasing the amount of dopant. Finally, a power factor of 17 μW/K² is achieved with a Seebeck coefficient of -285 μV/K at 40% dopant concentration. Our work introduces a new strategy for improved conductivity of n-type organic thermoelectrics.

Significant development in the optimization of thermoelectric properties of conducting polymers has taken place in the recent decade. Inorganic-organic hybrid materials provide a new playground to enhance the properties further. While crystalline conjugated polymers can have bandlike properties, achieving bandlike transport in aligned polymers has proven challenging. We study the effect of morphology and molecular weight of conducting polymers on the transport of charge carriers towards achieving bandlike transport demonstrated by measuring temperature dependent Seebeck and electrical conductivity. In particular, we employ high resolution E-Beam Lithography to create 1D structures of confined conjugated polymers. Further, we explore different solution treatments of prototypical PEDOT:PSS in order to tune it's electronic and thermoelectric properties. Finally, I will present some work on 2D materials and their derivatives and demonstrate their feasibility for thermoelectrics.

Heat transport by atomic and molecular vibrations is a source of entropy production in thermoelectric materials that reduces the efficiency of thermal energy conversion. Here, we report studies of the vibrational thermal conductivity of a total of 26 electrically insulating amorphous polymers, polymer salts, and cage-like macromolecules to advance fundamental understanding of the lower and upper limits to heat conduction in macromolecular materials. We prepare films with a thickness on the order of 100 nm on Si substrates and measure their thermal conductivities and heat capacities using time-domain thermoreflectance (TDTR). Varying the modulation frequency allows us to span from thermally thick to thermally thin and modulate the relative sensitivities of the TDTR measurement to thermal conductivity and heat capacity. The thermal conductivities vary by an order of magnitude, from 0.06 W m^-1 K^-1 for functionalized fullerenes to 0.7 W m^-1 K^-1 for poly(vinylphosphonic calcium salt). 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 and frequencies on the order of a few GHz. Overall, the thermal conductivities are well correlated with the scaling of the model of the minimum thermal conductivity with heat capacity and sound velocities.

The process of generating electric power by conventional means dissipates two thirds of the energy by heat. Thermoelectric devices represent a clean way to convert derived-heat into electricity. The efficiency of thermoelectric generators depends critically on the material performance, estimated by the figure of merit ZT. The latter, depends on the Seebeck coefficient, electrical conductivity (σ), temperature and thermal conductivity (κ).
Polymers potentially offer some peculiar features compared to inorganic materials for thermoelectrics (low κ, production cost and solution-processability). Notably, poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonic acid) (PEDOT:PSS) exhibit amongst the highest σ reported for conducting polymers so far. Understanding its thermal transport is crucial for further uses. Few methods are currently established for that purpose: 3ω or Frequency-dependent Time-Domain Thermoreflectance. However, these methods generally require expensive dedicated instrumentation and mathematical modelling.
Here, are presented through a pump-probe technique the thermal properties of different formulations of conductive PEDOT:PSS and carbon nanotubes-polymer composites. Main advantage of this approach is that it opens up opportunities to study thermal properties with standard pump-probe setups with no further mathematical simulations needed. The material’s surface is heated through a pump-beam (700ps-FWHM), and a time-delayed probe-beam (150fs-FWHM) monitors the change of the reflectivity, interpreted as the resulting temperature change on the material. Our approach is to fit with the same parameters (polymer κ and the interfacial thermal resistances) the data obtained from different material thicknesses considering one-dimensional diffusion equation of the heat flow within the polymer. To validate our approach we used PMMA and the results are in agreement with the reported ones. We further tested three different commercial PEDOT:PSS formulations. The formulation of PEDOT:PSS, Orgacon™ICP-1050, in-plane σ_//=11.32 S/cm, has the lowest value of thermal conductivity (0.339 W m^-1 k^-1). The other two formulations show a little higher κ of 0.446 and 0.461 W m^-1 k^-1 for the Clevios™PJet700 (σ_//=267.12 S/cm) and Orgacon™IJ-1005 respectively (σ_//=462.86 S/cm).

There has been growing interest in hybrid organic−inorganic crystals and superlattices, which can combine the superior properties of both components, such as the good electronic properties from inorganic materials and the superb flexibility from organic materials. The properties of organic−inorganic hybrid crystals and superlattices could be easily tuned through using different organic components and changing the dimensionality of the lattice. Furthermore, the thermal conductivity of hybrid crystals can be reduced due to the heterogeneous bonding and interfaces. As such, we witness significant efforts in developing flexible thermoelectrics using some of these hybrid crystals and superlattices.
As compared to organic−inorganic nanocomposites with a characteristic structure size in the order of 1−100 nm, the blending of organic and inorganic components in hybrid crystals and superlattices happens at an atomic scale by forming strong chemical bonds. Because of the unique structural features, the electronic and optical properties of organic−inorganic hybrid crystals have been investigated extensively in the past two decades for their potential applications in electronics and photonics. However, the thermal conductivity of organic−inorganic hybrid crystals and superlattices is rarely studied. Even though first-principles-based calculations are increasingly used to predict thermal conductivity of inorganic crystals. It is challenging to use standard first-principles calculations for the thermal conductivity of hybrid materials due to the complexity of crystal structures and computational costs. In this talk, we discuss our efforts in developing the first-principles-based atomistic simulation methodology to predict thermal conductivity of hybrid crystals and superlattices. The phonon transport mechanisms in some novel hybrid crystals, such as hybrid β-ZnTe(en)_0.5 and hybrid perovskite CH₃NH₃PbI₃, are discussed. We also present the challenges in modeling the thermal conductivity of organic-intercalated TiS2superlattices.
References:
[1]. Xin Qian, Xiaokun Gu, and Ronggui Yang, Lattice Thermal Conductivity of Organic-Inorganic Hybrid Perovskite CH₃NH₃PbI₃, Applied Physics Letters, Vol. 108, Art No. 063902, 2016.
[2]. Xin Qian, Xiaokun Gu, and Ronggui Yang, Anisotropic thermal transport in organic-inorganic hybrid crystal β-ZnTe(en)_0.5, Journal of Physical Chemistry C, Vol. 119, pp. 28300-28308, 2015.
[3]. Chunlei Wan, Xiaokun Gu, Feng Dang, Tomohiro Itoh, Yifeng Wang, Hitoshi Sasaki, Mami. Kondou, Kenji Koga, Kazuhisa Yabuki, G. Jeffrey Snyder, Ronggui Yang, and Kunihito Koumoto_, " Flexible N-type thermoelectric materials by organic intercalation of layered transition metal dichalcogenide TiS₂”_, Nature Materials, Vol. 14, pp. 622-627, 2015.

Heat transport has shown non-diffusive behaviour in many nanostructures, including nanowires, thin films and porous materials. The strong thermal transport reduction in such materials has sparked much attention especially for thermoelectric applications. However, very few material geometries have been exploited mainly because of the lack of computational tools having both accuracy and flexibility. We propose a multiscale deterministic model that combines the predictive power of ab-initio calculations (i. e. Density Functional Theory) with the flexibility of the Boltzmann transport equation (BTE). By going beyond the commonly used frequency-dependent approach, our model captures the full wave-vector dependence of the phonon-boundary scattering. The development of our method is facilitated by the introduction of the vectorial mean free paths (MFPs) distribution. Unlike the well-known MFP distribution, this new quantity retains the information of the direction of the group velocity, which turns out to be critical in material with complex geometries. We employ our parallel program to compute thermal reduction in nanoporous silicon with aligned pores. In accordance with previous results [1], thermal transport is suppressed by two orders of magnitude with respect to the bulk. We have also performed calculations for different angles between the main crystalline direction with respect to the pores lattice, obtaining an interesting modulation in the thermal conductivity. We provide preliminary conclusions by connecting these results with the information provided by the vectorial mean MFPs distribution. By considering the actual crystalline structure with respect to the material geometry, our results add an additional degree of freedom in tuning lattice thermal conductivity, paving the way to the development of next-generation thermoelectric devices.
[1] G. Romano, K. Esfarjani, D. A. Strubbe, D. Broido, A. M. Kolpak "Temperature-dependent thermal conductivity in silicon nanostructured materials studied by the Boltzmann transport equation." Physical Review B 93.3 (2016): 035408.

Nanocomposite materials can be employed to control thermal and electrical properties and to increase the efficiency in thermoelectric energy conversion. Understanding surface scattering in nanocomposite materials is critically important to manipulate their thermal transport properties, which in turn affect the thermoelectric figure of merit ZT. A rigorous understanding and manipulation of phonon interface scattering mechanisms and thermal transport in nanocomposites is still challenging. A current fundamental question is how surface conditions affect the amount of specular reflection and transmission across interfaces and the resultant thermal conductivities, which involves consideration of the different physical properties and dispersion relations of both media forming the nanocomposite. To account for thermal phonon interface scattering, we use an extension of electromagnetic wave scattering theory developed for rough surfaces by P. Beckmann and A. Spizzichino along with the Fuchs-Sondheimer theory to understand and manipulate thermal transport in layered composites. A rigorous analysis involving complete dispersion relations, refraction conditions, and shadowing effects is presented. The resultant thermal conductivities are calculated and compared with experiments. A detailed analysis of the heat spectrum is also presented which allows to predict the amount of heat carried by phonons of different frequencies and mean free paths. The presented accurate description of phonon surface scattering and prediction of heat spectra would allow for the design of nano-engineered composite materials and devices with improved thermoelectric properties.

Engineering of nanostructured materials with very low thermal conductivity is a necessary step towards the realization of efficient thermoelectric devices. Superlattices, either in the form of thin films or nanowires, are promising candidates with a considerably high ZT coefficient, as their thermal conductivity can be dramatically reduced. However, the thermal conductivity of superlattices cannot be reduced further as periodic length drops below a certain value (usually about a few nanometers), due to the presence of coherent phonons facilitating the heat conduction of some specific phonons. We present here a new superlattice structure to effectively block the transport of coherent phonons by performing non-equilibrium molecular dynamics (NEMD) simulation. We explore the behind mechanism by comparing the cases between traditional superlattices and the new structures. Our study provides a new route for manipulating phonons in superlattices, which could be beneficial for enhancing energy conversion performance of thermoelectric materials.

Flexible, lightweight, and wearable thermoelectric textiles are needed for diverse applications, from harvesting the waste heat from car engines, electronic devices, and industrial waste streams to powering wearable devices. We report the fabrication of flexible woven textiles based on thermoelectric yarns that are made by a novel process, which involves electrospinning polyacrylonitrile nanofibers into highly oriented nanofiber sheets, sputtering Sb₂Te₃ and/or Bi₂Te₃ sheaths onto these nanofibers, and then twisting the nanofiber sheets into yarns. Using these yarns, we fabricate and characterize highly flexible plain weave, zigzag stitch, and garter stitch textiles that harvest thermal energy from temperature gradients in the through-thickness direction. A high power output of up to 8.6 W m^-2 was achieved for a temperature gradient of 200^oC. Further improvements in thermoelectric performance and the thermal stability of the core fiber could result in efficient energy-harvesting textiles that can be wrapped around a car’s exhaust pipe or the hot waste-steam pipes that exit chemical or power plants.

Improvements in the thermoelectric materials’ performance over the past two decades have largely been based on decreasing the phonon thermal conductivity. Enhancing the power factor had less success in comparison. In this work, a peak power factor of ~106 μW cm^-1 K^-2 is achieved by increasing the hot pressing temperature up to 1373 K in p-type half-Heusler Nb_0.95Ti_0.05FeSb. The power factor is the highest among the semiconductor based thermoelectric materials above room temperature. A record output power density of ~22 W cm^-2 is also demonstrated based upon a single-leg device operating between 293 and 868 K. Such a high output power density can be beneficial for large-scale power generation applications.Improvements in the thermoelectric materials’ performance over the past two decades have largely been based on decreasing the phonon thermal conductivity. Enhancing the power factor had less success in comparison. In this work, a peak power factor of ~106 μW cm^-1 K^-2 is achieved by increasing the hot pressing temperature up to 1373 K in p-type half-Heusler Nb_0.95Ti_0.05FeSb. The power factor is the highest among the semiconductor based thermoelectric materials above room temperature. A record output power density of ~22 W cm^-2 is also demonstrated based upon a single-leg device operating between 293 and 868 K. Such a high output power density can be beneficial for large-scale power generation applications.

Composite materials play a distinct role in emerging technology due to their exceptional properties. For this reason, it is of interest that thermoelectric composite materials are going to be one of the areas that will attract the researchers’ attention. It is well known that oxides may take place of conventional thermoelectric materials (TE) because they show promising TE properties and they are thermodynamically stable at elevated temperatures, nontoxic, well-supplied, cheap and easy to fabricate. Among them; well known Ca₃Co₄O₉ (C-349) metal oxide and recently evidenced 1111 oxychalcogenide with the formula BiSeCuO are two of the best oxide based high-temperature TE materials. Both show ZT around 1 and low lattice thermal conductivity at high-temperature. Thus, it is necessary to combine these two valuable materials in a composite structure BiSeCuO/Ca₃Co₄O₉ (BC) to improve their efficiencies for energy conversion from waste heat in an air. In addition, hetero-structured materials; such as micro and nano mixtures, might be effective way to modify the path of carriers and phonons. Based on this idea, phonon mean free path could be decreased to enhance phonon-scattering at grain boundaries and as a result, reducing thermal conductivity without much degrading electrical conductivity or suppressing thermo-power (S²σ) to increase the total ZT. Hence, in our study, C-349 metal oxide TE materials with average grain sizes 18 nm and micro-structured BiSeCuO with average grain size around 5 mm are mixed in a solid solution in the amount of 1, 3 and 5 %, which corresponds to BC1, BC3 and BC5 and one C-349, BC0. 50 nm C-349 material in powder form achieved using citrate sol-gel processing with the help of PEG-400 and BiSeCuO material has been synthesized using the solid-state method. The formation process of BCs and the characterization of powders were conducted using BET, XRD and FE-SEM. Thermoelectric properties such as thermo-power, electrical resistivity, and thermal conductivity of BC thermoelectric materials have been studied to investigate the thermoelectric efficiency of these materials. Measurements were taken by fully computer controlled measurement system from 300K to 1100K, which is established in our laboratory. The surface areas of nano powder were determined as 80 m²g^-1 using BET analysis. Resistivity values of BC0, BC1, BC3, and BC5 samples were measured by four-point probe technique from 0.26 to 0.19 mΩ.m and Seebeck coefficient as 121 to 165 μVK^-1 at room temperature. We have seen great decrease in resistivity and increase in Seebeck coefficient. To conclude, micro/nano composite structure shows extraordinary TE properties as they were compared with their counterparts. It is highly beneficial to optimize these compositions in a wide range to obtain high-efficient level for practical applications.

Nanostructured materials are currently emerging as promising materials for energy generation and storage devices, nano-electronics and thermoelectric devices^1-2 via increasing contribution of complex nano-interfaces to thermal transport through them. However lack of 3D order and connectivity restricts the flow of electrons and phonons through the structure. But, theoretical predictions of thermal properties through periodically arranged nanoporous silicon suggests significant control over thermal conductivity³. In the present work, a 3D ordered and precisely doped Si nanostructure, namely an inverse opal metalattice^4-5, with periodicities on the order of 1-60 nm is synthesized using unique infiltration of Si into the voids of closely packed silica spheres nanotemplates through high pressure chemical vapor deposition.
Subsequently, 3ω and Time domain thermoreflectance methods are utilized to measure the thermal transport through such a metalattice sample and observe and appropriately control the effect of interfacial thermal resistance on the thermal phenomenon and energy flow through it. Initial measurement of thermal conductivity on a 30nm silica sphere size template infiltrated with Si shows a promising result of 2.27 W/m-K in contrast to a value of 33.48 W/m-K obtained using rule of mixtures on 25% silicon dispersed through 75% silica matrix indicating the dominant contribution of interfacial resistance on thermal conductivity. Hence, the composition and periodicity of such metalattices would be be varied to observe the corresponding effect on phonon transport and aim towards achieving ZT much higher than current state of art.
References
¹ Arun Majumdar, “Thermoelectricity in semiconductor nanostructures” Science(2004), Vol. 303, Issue
5659, pp.777-778
² Sanghamitra Neogi, J. Sebastian Reparaz, Luiz Felipe C. Pereira, Bartlomiej Graczykowski, Markus
R. Wagner, Marianna Sledzinska, Andrey Shchepetov, Mika Prunnila, Jouni Ahopelto, Clivia M.
Sotomayor-Torres,and Davide Donadio “Tuning Thermal Transport in Ultrathin Silicon Membranes
by Surface Nanoscale Engineering” ACS Nano, 2015, 9(4), pp 3820-3828
³ J.H. Lee, G.A. Galli, & J.C. Grossman, "Nanoporous Si as an Efficient Thermoelectric Material".
Nano Lett 8 (11), 3750-3754 (2008) http://dx.doi.org/10.1021/nl802045f
⁴ Han, J.E. & Crespi, V.H., Tuning fermi-surface properties through quantum confinement in metallic
metalattices: New metals from old atoms. Physical Review Letters 86 (4), 696-699 (2001).
⁵ Han, J.E. & Crespi, V.H., Abrupt topological transitions in the hysteresis curves of ferromagnetic
metalattices. Physical Review Letters 89 (19) (2002)

The development of alternative approaches to thermal energy conversion beyond conventional inorganic semiconductors is critical for achieving future breakthroughs in thermoelectric device performance. Two broad categories of promising alternatives are magnetic materials and composites, both of which introduce additional adjustable parameters (spin and interfaces, respectively) that can be tuned to achieve enhanced thermoelectric performance.
In this talk, we first discuss progress we have recently made in our understanding of how magnetization dynamics in elemental 3d ferromagnets (Fe, Co) influence the materials’ transverse (Nernst) and longitudinal (Seebeck) thermopower via magnon-electron drag (MED) [1]. Building on that work, we report here a study of MED in Fe, Co and Ni with different levels of defects. We show that MED remains dominant even in porous samples, while phonon-electron drag (PED) effects disappear, establishing defect studies as a useful experimental tool for separating the two phenomena. These results also suggest MED should dominate even in magnetic alloys, thereby allowing other material properties to be tuned without significantly affecting the Seebeck coefficient. We then show how MED is actually a “high temperature” effect, in contrast to PED, making it relevant for applications at room temperature and above. Its robustness to both temperature and defects establishes MED as an appealing approach to achieving large power factors and possibly high zT in magnetic conductors.
The second part of the talk will discuss our successful demonstration of the spin Seebeck effect in bulk ferromagnetic nanocomposites [2], which manifests as an enhancement in the transverse thermopower, thereby offering a completely new approach to designing and optimizing transverse thermoelectric devices such as Nernst-Ettingshausen coolers.

References:
[1] S. J. Watzman et al., arXiv:1603.03736 (2016)
[2] S.R. Boona et al., arXiv:1604.05626 (2016)

Funding:
ARO MURI W911NF-14-1-0016
NSF CEM DMR-1420451

We report the first paper thermoelectrics fabricated via directly impregnating cellulose paper, the most abundant biopolymer on Earth, with p- or n-type colloidal semiconductor quantum dots (CSQD). CSQD are a versatile class of nanomaterials that offer low-temperature, large-area integration with the ease of material processing. Celluloses, on the other hand, are traditionally used for thermal insulation materials and are abundant and flexible. These advantageous properties taken together motivate us to fabricate and study cellulose-CSQD nanocomposites for thermoelectric applications.

To date, CSQD solutions were only studied as a material to deposit uniform thin-films on flat substrates. Adopting a new point of view, CSQDs dissolved in an organic solvent can be used as a “penetrating solution” that directly impregnates and converts a common paper into a semiconductor through simple dip-casting. Limitations arising from paper’s porosity and surface roughness in forming a uniform film do not apply to this approach while being amenable to high-throughput processing. Furthermore, paper is by far the cheapest flexible substrate used in daily life. The yearly production of paper is about 100 million tons and its roll-to-roll processing technology can exceed 100 km / h.

In this presentation, a complete suite of thermoelectric transport measurements including temperature-dependent Seebeck coefficient, electrical conductivity and in-plane thermal conductivity are reported and a strategy toward maximizing the thermoelectric figure of merit is discussed. Furthermore, we demonstrate equipment-free fabrication of flexible thermoelectric modules using p- and n-type paper strips. Unlike bulk thermoelectric materials that are rigid and brittle, these prototype paper power generators have the potential to efficiently utilize heat available in natural and man-made environments by maximizing the thermal contact to heat sources of arbitrary geometry.

We report here the results of the first studies of optimization of the magnon-drag thermopower in iron alloys. Binary alloys Fe_1-xM_x (M=Co, Ni, Ga) are ferromagnets in which magnons are involved in the transport of electrons and induce an additional thermopower by drag effects. Unlike traditional inorganic semiconductor thermoelectric materials, that are brittle, require elaborate defect control during processing and operation, and necessitate complex contact technologies, metallic alloys are strong, easy to produce and form into net shapes, and can be welded. We concentrate on alloys of elements that are earth-abundant, with Fe as major constituent. Metals are not traditionally considered good thermoelectrics, because their high electron density limits their Seebeck coefficients to a few μV / K, but the use of drag effects enhances that strongly. A recently developed theory (1) derives the magnon drag thermopower of elemental 3-d metals, and shows that it is an order of magnitude larger than the classical diffusion thermopower, a rare instance in which a spin-based effect is larger than a charge-based effect. The theory predicts that the magnon-drag thermopower is inversely proportional to the number of s and p electrons while the sign of the thermopower is determined by the sign of the effective mass of majority carriers. Combining this theory with our orbitally-resolved band structure calculations of the DOS of the alloys, we predict a change of sign of the thermopower of the Fe-Co alloys with x, and the magnitude of their thermopower. We synthesize the alloys and measure the temperature dependence of their resistivity, thermopower and Nernst coefficients from 77 to 1000 K. We find that magnon-drag contributes greatly to the thermopower and show that the predictions about sign and magnitude are verified experimentally. We further investigate the coupling of acoustic phonons to the magnons, which is governed by the magnetostriction coeffient (2), by comparing Fe-Ga alloys with very high magnetostriction to Fe-Ni alloys with very low magnetostriction.
(1) S. J. Watzman et al., arXiv1603.03736 (2016)
(2)C. Kittel Phys. Rev. 110, 836 (1958); V.V. Gudkov and J.D. Gavenda, Magnetoacoustic Polarization Phenomena in Solids, Springer, New York (2000)
Funding: ARO MURI grant number W911NF-14-1-0016

To reduce the greenhouse effect, thermoelectric materials are believed to be a promising technology with respect to waste heat energy conversion. A good thermoelectric material needs to demonstrate a low electrical resistivity and thermal conductivity and have a high thermoelectric power. Ca₃Co₄O₉ is one of the promising materials among all other oxides with its high thermo-power, low thermal conductivity and resistivity values at high temperatures and environmentally friendly. Nano grained structure is one of the key approaches to enhance the thermoelectric efficiency for high temperature applications. On the other hand, porosity could reduce the thermal conductivity due to the presence of air in the pores that work as an insulator layer among the grains in the bulk sample; that is, porosity would additionally limit the mean free path of some of the phonon modes and thus, contribute to a further decrease in thermal conductivity. Based on this idea, samples with different nanometer grain sizes ranging from 18 nm to 50 nm have been synthesized via citrate sol-gel method. Self-assembled pores have been observed after the samples prepared as pellet for electrical and thermal characterizations. It is of interest that as the size of the grains decreases, the degree of porosity increases drastically. This self-occurred heterostructure gives us an idea of considering this structure as matrix: nano and porous mixture. It has also appeared in all the samples. It is observed that decrease in the thermal conductivity is around 30% . However, the increase in electrical resistivity does not much affect the total improvement of thermoelectric properties of Ca₃Co₄O₉. This significant reduction presents 12% enhancement in ZT. Nano structuring significantly reduces the total thermal conductivity down to 0.32 W/mK at 300K of Ca₃Co₄O₉ material via grain boundary scattering without much degrading the thermopower. The ratio of k/kph is calculated as % 99 indicating that the main contribution to the total thermal conductivity is lattice thermal conductivity. Indeed, the porosity affect for the reduction in total thermal conductivity was demonstrated. The highest thermoelectric efficiency (ZT) is calculated as 0.63 at 1000K. The dimensionless figure of merit of the nano/pore composite structure provided enhanced properties for practical applications.

Thermoelectric power generation is in the focus of considerable attention because of the potential for environmentally benign and cost-effective conversion of waste heat to electricity. A single crystalline form of p-type SnSe has recently shown an exceptionally high ZT of ~2.6 at 923 K along the b-axis due to a highly anharmonic bonding. Since then polycrystalline SnSe based materials have been quite actively investigated for practical applications. However, it is quite difficult to make a high ZT polycrystalline SnSe material because of both low dopability and solubility limit with other elements. Thus, we investigated the effects of atomic substitution and doping on the thermoelectric properties of polycrystalline SnSe materials. In this study, we present detailed investigations of electrical and thermal transport measurement as well as structural data on polycrystalline SnSe based materials. Their thermoelectric properties are highly enhanced and its origin will be discussed.

Nanocarbons including carbon nanotubes have been widely studied for the construction of human-friendly and wearable electronics and power modules. However, the absence of air- and thermally-stable n-type nanocarbon materials has hindered the development of practical PN junction devices such as photovoltaic cells and thermoelectric modules. Here we solve this challenge by using ordinary salt with crown ethers. We report on stable and efficient n-type nanocarbon doping based on simple salts: ordinary halide and hydroxide anions with tetraalkylammonium cations and cationic crown ether complexes with alkali ions. Thermoelectric properties measurements revealed that various ordinary salts convert p-type carbon nanotubes to air-stable n-type forms efficiently. In particular, n-type carbon nanotubes were effectively stabilized by the coordination of alkali metal-crown ether host-guest complexes. This material showed excellent n-type thermoelectric properties with a power factor exceeding 200 μW/K², and unprecedented thermal stability in air for more than one month at 373K. This air-stable n-type conductors were implemented in the thermoelectric series circuit, enabling the powering of a commercially-available LED upon hair-dryer heating.

A lot of efforts such as doping, band engineering, and nanostructuring have been tried to enhance figure of merit (ZT) values of bulk thermoelectric materials. However, since most methods to develop the materials with high ZT values require many efforts and high costs for materials fabrication, their practical applications have stayed elusive. For example, the most widely used method includes melting of elements with appropriate dopants, annealing, grinding ingots to a fine powder, and sintering. Here, we propose a surprisingly simple and cost-effective method to produce materials having high ZT values. The element tellurium (Te) was subjected to spark plasma sintering at the relatively low temperature of 573K under the pressure of 50 MPa. The resulting thermoelectric samples were chemically transformed into silver telluride (Ag₂Te) at room temperature by simple dipping pure Te samples into a silver (Ag) ions-containing solution. Using this simple approach a power factor increases more than 3 times compared to pure Te at the temperature of 623.

Thermoelectric materials enable direct recovery of waste heat into useful electrical energy. Ceramic oxides are now being considered as a new class of such materials because of their high stability at elevated temperatures. The present investigation uses lead plumbate (BaPbO₃) as the base material, the thermoelectric properties of which have been altered by judicious cation substitutions. The perovskite structure of BaPbO₃ is known for its large interstitial spaces which can accommodate a variety of “impurity” ions. Such substitutions are being made to trigger a metal-insular transition where major property changes are expected. As BaPbO₃ has high electrical conductivity, σ=2.25x10⁵ Ω^-1m^-1 at room temperature, its thermopower, S, is relatively low: 23μV/K, as expected. With a thermal conductivity, k, of 4.83W/m.K, the figure of merit (ZT=S²σTk^-1) of BaPbO₃ is only 0.01 at T = 300 K. The objective of this investigation is to study the variation of thermoelectric properties of BaPbO₃ as Ba and Pb ions are systematically substituted by alkali and alkaline earth ions.

Today, the enhancement of thermoelectric (TE) conversion efficiency is achieved through new developments and improvements in nanostructuring of materials, for example by including nanoscale crystalline domains in an otherwise bulk material (nano-bulk). In nano-bulk materials, the thermoelectric figure of merit, ZT=σS²T/κ (where σ, κ, S, and T are electrical conductivity, thermal conductivity, Seebeck coefficient and average temperature between the hot and cold sides, respectively), is enhanced because κ should be reduced through phonon scattering at grain boundaries, while σ should be effectively maintained. The approach has led to the discovery of many materials with elevated ZT value, but the full potential of nanostructuring has not been achieved yet. Furthermore, rare and toxic metals such as Te, Se or Sb are still used extensively in the best inorganic TE materials, which limits their widespread use on a practical scale. In order to proliferate the use of TE materials to a wide degree, it is necessary to make them low cost, earth abundant, environmentally benign, and much more efficient in terms of energy conversion. With these criteria in mind, we have developed a series of sustainable nano-bulk TE materials composed of transition metal sulfide compounds. By using a bottom-up wet-chemical method, we synthesize transition metal sulfide nanoparticles maintaining control over the uniformly of nanoparticle size, shape, composition and structure, which we then process into large scale nano-bulk materials. The results have been promising with ZT~1 for these earth abundant materials. The presentation will focus on the characterization of nano-bulk TE materials including our most recent materials, for example, Cu₂SnS₃ and Cu₂GeS₃ compounds. These nanoparticle based materials retain their independent grain size even after processing into a pellet which is an important aspect of the work, which we achieve by using a spark plasma sintering technique. We will discuss the TE characteristics such as σ, κ, and S as well as overall ZT value of the nano-bulk materials with various compositions and structures.

Enhanced thermoelectric performance can be achieved in composite thermoelectric materials due to the reduction in lattice thermal conductivity. In the present work, we have explored the effect of SnSe incorporation in In₄Se₃ matrix on electrical and thermal transport properties of In₄Se₃-SnSe. Five different compositions of In₄Se₃-SnSe composite with 1% - 5% SnSe were synthesized by melt grown technique followed by spark plasma sintering at 693 K for 5 min. Structural and microscopic characterization were performed to examine the phase purity, crystal structure and grain size of samples. Electrical measurements were performed in helium atmosphere in the temperature range of 300 - 700 K using a bar shaped sample with dimension 2.5 x 2.4 x 8 mm. Electrical resistivity measurement exhibit the non-degenerate semiconducting behaviour. Negative Seebeck coefficient confirmed the n-type conduction with enhanced values in In₄Se₃-SnSe composite materials than the pristine In₄Se₃. It is due to the high energy charge carrier filtering by the potential barrier formed at the interfaces. Thermal conductivity was found to decrease with the incorporation of SnSe due to lattice thermal conductivity reduction by phonon scattering at the interfaces. Single parabolic band model with acoustic phonon scattering approximation was used to explain the transport mechanism in the sample. Maximum power factor was observed in the sample with 2% SnSe which results in the highest ZT ~ 0.9 at 700 K.

Heat conduction in superlattices is usually governed by incoherent interface scattering that randomizes phonon phase information and results in a decrease in thermal conductivity with increasing interface number density. In this presentation, we show that in short-period epitaxial, lattice-matched TiN/(Al,Sc)N metal/semiconductor superlattices, thermal transport is dominated by phonon wave effects as the wavelengths of phonons that carry significant amounts of heat become comparable to the superlattice period thicknesses.

Epitaxial and coherent (Ti,W)N/(Al,Sc)N superlattices with period thickness (a) ranging from 0.8-240 nm were deposited on (001) MgO substrates by reactive dc-magnetron sputtering. High-resolution XRD analysis along with reciprocal space mapping indicates that the superlattices are pseudomorphic. The x-ray reflectivity (XRR) analysis suggests that the superlattice interfaces are extremely sharp with interface roughness of the order of one to two atomic layers. High-resolution transmission electron microscopy (HRTEM) along with HAADF-STEM shows cube-on-cube epitaxial growth with a very low density of extended defects.

The cross-plane thermal conductivity in the short-period TiN/(Al,Sc)N superlattices increases with decreasing period thicknesses resulting in a distinct minimum of thermal conductivity at a period thickness of about 4 nm at room temperature. Thermal conductivity of the superlattices also decreases with an increase in the temperature, which supports the wave nature of phonon transport. Thermal conductivity of the (Ti,W)N/(Al,Sc)N superlattices however saturates at short period thicknesses due to increasing effects of alloy scattering. Even though electronic contribution to thermal conductivity is significant in thick metallic layers; thermal conductivity of short period superlattices seems to be dominated by phonon transport. Extensive first-principles density functional theory based analyses have also been performed to explain various aspects of the thermal conductivity results.

Our results show that lattice matched, epitaxial TiN/(Al,Sc)N superlattices behave as an effective medium with respect to phonon transport at short periods and the wave nature of phonons dominates the heat conduction.

Ref. 1. B. Saha, Y. R. Koh, J. Comparan, S. Sadasivam, J. L. Schroeder, J. Birch, T. S. Fisher, A. Shakouri, T. D. Sands, “Cross-plane thermal transport in (Ti,W)N/(Al,Sc)Nmetal/semiconductor superlattice.” Phys. Rev. B, 93, 045311 (2016).
2. B. Saha, Y. R. Koh, J. P. Feser, S. Sadasivam, A. Shakouri, T. S. Fisher, and T. D. Sands, “ Phonon Wave-effects in the thermal transport of epitaxial TiN/(Al,Sc)N metal/dielectricsuperlattices.” (in-review).

The direct conversion between thermal and electrical energies by thermoelectric devices has become an important alternative for the clean energy generation. Low-dimensional materials seem to be promising candidates, whose efficiency is determined by the dimensionless figure-of-merit defined as ZT=σS²T/(κ_el+κ_ph), where the Seebeck coefficient (S), electrical conductivity (σ), electronic (κ_el) and phononic (κ_ph) thermal conductivities can be calculated by using the Boltzmann formalism [1]. The inherent correlation between these thermoelectric quantities makes difficult to improve the value of ZT. In this work, we study thermoelectric properties of periodic and quasiperiodically segmented core-shell nanowires by means of a real-space renormalization plus convolution method [2] developed for the Kubo-Greenwood formula, in which tight-binding and Born models are respectively used for the study of electric and lattice thermal conductivities [3]. Analytical results are found for periodic nanowires showing a maximum ZT in the temperature space, as occurred in the carrier concentration one. This maximum ZT can be improved by introducing periodically arranged segments and a core-shell cross section into nanowires. Finally, the quasiperiodicity seems to be another important ZT enhancing factor, since it significantly diminishes the thermal conduction of long wavelength acoustic phonons, which are responsible of the phononic conductivity at low temperature and not easy to block their transmission since they do not feel local defects neither impurities.
This work has been partially supported by UNAM-IN113714. Computations were performed at Miztli of DGTIC, UNAM.
[1] T. M. Tritt (Ed.), Thermal Conductivity - Theory, Properties and Applications, Kluwer Academic-Plunum Pub., New York, 2004, p. 3.
[2] V. Sanchez and C. Wang, Phys. Rev. B 70, 144207 (2004).
[3] C. Wang, F. Salazar, and V. Sánchez, Nano Lett. 8, 4205 (2008).

Bi₂Te₃-based alloys, (Bi,Sb)₂Te₃ and Bi₂(Te,Se)₃, are the best thermoelectric materials near room temperature, whose dimensionless figures of merit (ZT) are around unity. Although these classical materials have been studied for more than 50 years, the thermoelectric properties are still improving by employing various nanostructuring strategy, which is effective in increasing phonon scattering while preserving the high electrical conductivity, the so-called phonon glass electron crystal (PGEC) approach. Among the many ideas for nanostructuring, nanocomposite synthesis adding extrinsic nanoscale inclusions into the matrix is quite simple and effective, and this process is more controllable than inducing the second phase formation in the matrix by precipitation or spinodal decomposition. Up to now, various nanoparticles were tested for Bi₂Te₃-based nanocomposite synthesis, and contrary to general expectations, nonmetallic nanoinclusions have shown promising results, for example SiC^[1], boron carbide^[2],Al₂O₃^[3], SiO₂^[4], and carbon nanotubes^[5]. In this study, MgO nanoparticles (>99 % metals basis purity, average particle size of about 100 nm) were used to synthesize n-type Bi₂Te_2.7Se_0.3 nanocomposites, and the thermoelectric properties were analyzed. Bi₂Te_2.7Se_0.3 nanocomposites containing x vol% (x≤1.5) MgO nanoparticles were produced by high-energy ball milling and plasma activated sintering at 693 K. The resistivity of Bi₂Te_2.7Se_0.3–MgO nanocomposites increased with x from 10.5 μΩm (x = 0) to 15.1 μΩm (x = 1.5) at 298 K, and the maximum Seebeck coefficient also increased from -171 μV/K (x = 0) to -194 μV/K (x = 1.5). The thermal conductivity decreased with x, showing the minimum value of 0.909 W/Km (x = 1.5) at 373 K, and upon analysis it was observed that the lattice thermal conductivity decreased with x at temperatures below 400 K, whereas the tendency was reversed above 500 K. As a whole, addition of MgO nanoparticles enhanced the maximum dimensionless figure of merit ZT of the polycrystalline Bi₂Te_2.7Se_0.3 about 8.3 %, from 0.806 (x = 0) to 0.875 (x = 1.5). SEM observation confirmed that MgO nanoparticles were properly dispersed within the matrix without any special process or treatment, making it very advantageous for immediate utilization in the industry.

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[3] K.T. Kim, H.Y. Koo, G.-G. Lee, G.H. Ha, Mater. Lett. 82, 141 (2012).
[4] Y.C. Dou, X.Y. Qin, D. Li, L.L. Li, T.H. Zou, Q.Q. Wang, J. Appl. Phys. 114, 044906 (2013).
[5] F. Ren, H. Wang, P.A. Menchhofer, J.O. Kiggans, Appl. Phys. Lett. 103, 221907 (2013).

Thermoelectric energy extraction from industrial waste heat is a promising technology for the production of direct electric power. Generation of electric power takes palace due to thermal gradient rising due to Seebeck effect. Traditionally Bi₂Te₃, skutterudite, clathrate, Zintyl, and half-Heusler based alloy materials have been used. However, most of these materials possess figure of merit (ZT) values in 1.0 to 1.7 range. Enhancement in the transport properties has been accomplished through microstructural tailoring and structural engineering, and examined the variations of homovalent, trivalent substituted dopants to improve the Seebeck coefficient (S) and ZT values.
This work demonstrates simultaneous doping of lanthanum (La) and nickel (Ni) in Ca₃Co₄O₉ with different doping concentrations to form alloys of Ca_3-xLa_xCo_4-y Ni_yO₉, with x = y = 0.0, 0.05, 0.1, 0.15, 0.2, and 0.25, and the resulting nanocomposites are nomenclatured as S0, S1, S2, S3, S4, and S5, respectively. The choice of simultaneous doping is based on the expectation that Ni will be substituted for cobalt Co⁴⁺ site and lanthanum La²⁺ substituted for Ca²⁺ and the combination will reduce the electrical resistivity and increase in the power factor (S²/ρ), where, S is the Seebeck coefficients and ρ is the electrical resistivity. The samples have been prepared by solid states method and the resulting mixture was reground and treated in vacuum (10⁻³ bar) in the hot-pressed sintering apparatus (Insamrat Systems, Hyderabad, India). For hot-pressing, 8 g of Ca₃Co₄O₉ powders is loaded in a graphite die with an inner diameter of 20 mm, applied a pulsed electric current through the assembly to heat for 40min by maintaining 850°C by applying a uniaxial pressure of 60 MPa, and cooled to room temperature at a rate of 100 °C/min. The SEM micrographs exhibit highly compact microstructure having bulk density (99% theoretical density i.e 4.67 g/cm³). Reitveld analysis for the XRD data of the sintered pellets Ca₃Co₄O₉ using Topas software shows two monoclinic layered structure with lattice parameters a = 4.841 Å, b1 =4.560 Å, c =10.881 Å, b2 = 2.80 Å, and β= 98.92 with b1/b2 = 1.62 of the pure samples and almost similar behavior of all doped samples. Resistivity of the matrix reduces after the co doping with La and Ni as dopants to a minimum value of 0.79mΩcm at 750 K for S1 sample. The pure Ca₃Co₄O₉ hot-pressed pellets indicate an electrical resistivity of 3.922 mΩcm and S of 149 µV/K, leading to an estimation for the power factor of 0.56mW/K²m at 300K, which increases upto 2.01 mW/K²m at 750K. On co-doping with nickel and lanthanum, its power factor increased by S1 samples by 3.86 mW/K²m at 750K. This work has produced useful parameters for device applications and value obtained for this pellets is best at 750K. Further work is in progress to obtain still higher power factor and to understand the mechanism underlying this improvement.

Bi₂Te₃ –based compounds are representative of thermoelectric materials that operate near room temperature. Here, we investigate thermoelectric properties of (CuI)_x (x=0, 0.003, 0.006, 0.009) doped Bi₂Te_2.7Se_0.3 which are prepared both single and poly crystalline. In single crystalline, we found that the weighted mobility and carrier density are increased with doping CuI. For this reason, the highest power factor (x=0.003, 0.009) is recorded about 4.2~4.3 mW/mK². Also, obtained single crystalline exhibit a high ZT of 0.97 at 373K for x=0.003. Additionally, polycrystalline obtained by controlling textures have still high power factor of 4.07mW/mK² at 300K and reduced lattice thermal conductivity as compare with single crystalline of the same composition. As a result, we obtained ZT value of 1.07 at 473K which higher than single crystalline.

The ability to deterministically control the thermal conductivity for semiconductors is of fundamental importance in fields of phononics and thermoelectrics. Enhancing scattering during phonon transport provides a means to achieve this control. This can be achieved by engineering the high-densities of interfaces in nanostructured materials such as superlattices, nanowires, quantum dots, and nanocomposites. When the size of the nanostructure is smaller than the phonon mean free path, phonons can be strongly scattered, giving rise to a decrease in the thermal conductivity.

In ferroelectrics, it was found that the ferroelastic domain walls acted as interfaces that can scatter phonons resulting in a net decrease in the effective thermal conductivity. For thin films, the size of ferroelectric domains ranges from several nanometers to hundreds of nanometers depending on the chemical composition, material size, and mechanical and electric boundary conditions of the film, which is generally compatible with those phonon mean free paths that carry thermal energy at room temperature. Recently, altering the effective thermal conductivity via domain structures engineering either by controlling the film-growing conditions or by applying an electric field has been demonstrated. In this work, taking BiFeO₃ and PbZr_1-xTi_xO₃ thin films as examples, we demonstrate that the effective thermal conductivity can be tuned by engineering the ferroelectric domain structure. In order to achieve this, we employ the phase-field model of ferroelectric thin films to evolve the domain structures of PbZr_1-xTi_xO₃. Then, the effective thermal conductivity as a function of domain structure is obtained by solving the heat conduction equation using a spectral iterative perturbation algorithm in materials with inhomogeneous thermal conductivity distribution. Using this proposed algorithm, the experimentally measured effective thermal conductivities of domain-engineered {001}_p-BiFeO₃ thin films are quantitatively reproduced. By combining this algorithm with the phase-field model of ferroelectric thin films, the effective thermal conductivity for PbZr_1-xTi_xO₃ films under different composition, thickness, strain, and working conditions is predicted. It is shown that the chemical composition, misfit strain, film thickness, film orientation, and a Piezoresponse Force Microscopy tip can be used to engineer the domain structures and tune the effective thermal conductivity [1]. Therefore, we expect our findings will stimulate future theoretical, experimental and engineering efforts on developing devices based on the tunable effective thermal conductivity in ferroelectric nanostructures.

[1] J. J. Wang, Y. Wang, J. F. Ihlefeld, P. E. Hopkins, L. Q. Chen, Tunable Thermal Conductivity via Domain Structure Engineering in Ferroelectric Thin Films: A Phase-Field Simulation, Acta Materialia 111, 220(2016)

AgSbTe₂ is known as a good thermoelectric material by forming a pseudo-binary alloy with GeTe. There is a discontinuous decreasing of the thermal conductivity without changing of the electric conductivity in (GeTe)_x(AgSbTe₂)_1−x alloys. However, the mechanism of the drastic change of the thermal conductivity has yet to be understood, and even the crystal structures of the (GeTe)_x(AgSbTe₂)_1−x alloys are still under discussion. In this presentation, we discuss the problem and propose a new general design rule for high-efficient thermoelectric materials based on AgSbTe₂ as typical case, by the first principles calculations.
First of all, as the crystal structure of AgSbTe₂ has not been well investigated [1], I predict the several candidates of the stable structure theoretically by means of the cluster expansion approach. The structural stability is attributed to the formation of the Ag-Te-Sb chain structure which strongly reduces the formation enthalpy. The generalized perturbation method based on the Korringa-Kohn-Rostoker (KKR) method also showed the strong attractive interaction between the Ag and Sb atoms in the Ag-Te-Sb chain structure. It is found, by the analysis of the crystal orbital Hamilton population (COHP) and maximally localized Wannier functions (MLWFs), that the Te-5p anti-bonding states are dominant at the valence band maximum by the Sb-5s and Te-5p hybridization. This feature is quite different from the case of general semiconductors such as Si, GaAs, ZnSe, etc., where the main components in the valence band are the bonding states orbitals. Due to the inherent instability by the anti-bonding coupling, AgSbTe₂ produces various mutation phases by introducing the defect complex 2V_Ag+Sb_Ag in the Ag-poor and Sb-rich crystal growth conditions. These mutation phases and grain boundary might work as the phonon scattering center and decrease the lattice thermal conductivity in AgSbTe₂. Additionally, the calculations of the mixing energy clearly show that the defect complexes 2V_Ag+Sb_Ag in AgSbTe₂ spontaneously gather together, so that the vacancy-rich region also works as the scattering center. Thus, one can synthesize the AgSbTe₂ with high thermoelectric efficiency by constructing nano-structures of the mutation phase, changing the Ag and Sb vapor pressures. Above proposed design rule can be applied to other thermoelectric materials which have anti-bonding contributions at the valence band maximum. [2]

Reference:
[1] H. Shinya, A. Masago, T. Fukushima, H. Funashima and H. Katayama-Yoshida, Japanese Journal of Applied Physics 53, 111201 (2014).
[2] H. Shinya, A. Masago, T. Fukushima and H. Katayama-Yoshida, accepted to Japanese Journal of Applied Physics 55, 041801 (2016).

Germanium doped SnSe polycrystalline specimens were prepared by melting and spark plasma sintering. The influence of germanium doping on the thermoelectric properties of solid solution SnSe was investigated and compared with the pristine sample. The substitution of germanium resulted in a reduction in thermal conductivity and enhancement of Seebeck coefficient in the Sn_1-xGe_xSe series. Extremely low thermal conductivity was achieved in all Ge-doped compounds due to phonon scattering from disordered dopant atoms and high anharmonicity bonding nature of SnSe. Because of the extremely low thermal conductivity and moderate power factor of Sn_0.99Ge_0.01Se, led to a maximum zT value of 0.8 was obtained at 800 K. Large Seebeck coefficient, moderate power factor, and low thermal conductivity can be achieved in Sn_1-xGe_xSe compound, which make them promising candidates for high efficient thermoelectric materials.

Thermoelectric (TE) materials, which can generate electricity from heat or convert electricity to temperature gradient, have great potential for waste heat recovery and heating/cooling applications. The performance of thermoelectric devices is strongly related to the figure of merit ZT, which is determined by Seebeck coefficient, electrical conductivity, and thermal conductivity of TE materials [1]. Most TE devices are fabricated in a traditional manufacturing process consisting of cutting ingots, setting n-type/p-type legs in array and connecting with conductive metals, which is labor-intensive and costly. In order to employ cost-effective manufacturing technologies, such as printing techniques, flexible TE materials with high ZT are required [2]. In particular, the performance of n-type flexible TE has not advanced compared to the p-type materials. Herein, we discuss experimental studies about thermoelectric properties on n-type flexible thermoelectric material that is Bi₂Te₃ and carbon nanotubes (CNTs) composite, and the annealing effect on this material.
Bi₂Te₃ nanowires and CNTs composites were successfully synthesized through a facile wet chemical process. Bi₂Te₃ nanowires were synthesized using a tellurium template in a polyvinylpyrrolidone (PVP) phase synthetic process. PVP plays a key role in the Bi₂Te₃ nanowire synthesis as a surfactant, enabling one-dimensional growth and controlling a growth rate. The CNT and Bi₂Te₃ nanowires were mixed uniformly using ultra-sonication and the composite films were formed by vacuum-filtration. The thermoelectric properties were evaluated on samples with different compositions. It showed the introduction of CNTs into Bi₂Te₃ nanowires not only enabled flexible, but also dramatically enhanced the thermoelectric power factor of n-type flexible composite films. Furthermore, a post-annealing effect on n-type Bi₂Te₃ nanowire/carbon nanotubes composite films was studied. After the post heat treatment, a thermoelectric power factor of Bi₂Te₃ nanowires and CNTs composites was significantly improved. The annealing effects on Bi₂Te₃ nanowires and CNTs materials and thermoelectric properties were analyzed using material characterization tools, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray Photoelectron Spectroscopy (XPS). Understanding these phenomena is necessary to improve thermoelectric efficiency of flexible TE devices.

[1] L. E. Bell, Cooling, heating, generating power, and recovering waste heat with thermoelectric systems. Science 321, 1457-1461 (2008).
[2] S. J. Kim, J. H. We, B. J. Cho, A wearable thermoelectric generator fabricated on a glass fabric. Energ Environ Sci 7, 1959-1965 (2014).

PbTe-based thermoelectric materials are promising candidates for energy conversion applications, e.g., waste heat recovery. Nanoscale precipitates, such as SrTe, PbS, or SeTe, in bulk Na-doped PbTe decreases the lattice thermal conductivity and improves the thermoelectric figure of merit of the material. Therefore, characterization of the morphology, dimensions, composition, and number density of these nanostructures is necessary to understand their effects on the figure of merit. Moreover, understanding the nanostructures’ nucleation and growth could eventually enable nanometer-scale engineering of the material to achieve higher figures of merit. However, conventional electron microscopies are unable to provide accurate composition measurements of nanometer size nanostructures in three-dimensions with subnanoscale spatial resolution. In this study, different concentrations of Sr, S, and Se for different heat treatment temperatures and times exhibit nanoscale precipitates with different morpholgies and dimensions. We characterized the nanoscale morphology of precipitates and the segregation of the Na-dopant at the interfacial regions between the precipitates and the PbTe matrix, utilizing ultraviolet (355 nm wavelength) laser-assisted local-electrode atom-probe (LEAP) tomography employing picosecond laser pulsing on a superior spatial resolution (0,1 nm) of atom-by-atom and atomic {hkl} plane-by-plane basis. Significant amount of Na dopant atoms segregate at the interface between PbS precipitate and PbTe matrix. Calculated Gibbsian interfacial excess is 4.09 +0.24 atoms nm^-2.

As the supply of fossil fuels, the main energy source in human’s life, is going to be exhausted eventually, people are seeking not only the renewable and sustainable energy sources, but also the technology for the recovery and reuse of waste energy. This technology is known as Waste Heat-to-Useful Energy conversion technology. Thermoelectric materials have been developed rapidly and drawn the significant attention of the public as of yet due to their ability to directly convert heat to electricity. Ca₃Co₄O₉, as one of the most pronounced bulk thermoelectric materials, was reported to possess an extraordinary figure-of-merit (ZT) of ~1.0 at 1000K in the pure single crystal form. Later on polycrystalline Ca₃Co₄O₉ was researched heavily by introducing several different kinds of dopants in the system with an aim to enhance its thermoelectric response i.e., lower its thermal conductivity and improve the Seebeck coefficient and electrical conductivity. Improvement in the thermoelectric properties can also be realized by introducing porosity in the material. In this presentation, we will present some very exciting work going on in this area, in our lab at the university of Utah. We will discuss a novel bio-friendly synthesis approach that we developed for making porous Ca₃Co₄O₉. Samples with varying degrees of porosity were prepared and thoroughly characterized using several state-of-the-art techniques. The porosity and topography of all the samples were determined using Scanning Electron Microscopy (SEM). Crystal structure and lattice parameters were determined using Transmission Electron Microscopy (TEM) and X-ray Diffraction (XRD). Laser Flash Technique and Differential Scanning Calorimetry (DSC) were utilized to determine the thermal conductivity (κ) of the samples. Seebeck coefficient (S) and resistivity (ρ) measurements were performed separately by using an indigenous Seebeck coefficient and four-point probe resistivity measurement system. By using experimentally determined S, ρ and κ values, figure of merit for various samples was determined. Our results showed that as the porosity in the material increases, its thermal conductivity declines giving rise to a significantly enhanced thermoelectric response of the material.

Low grade thermal energy from body heat is a promising energy source to harvest and/or store electrical energy for various wearable and flexible electronic devices. Here fully organic composite materials made of conducting polymers and carbon nanotubes for thermal-to-electrical energy conversion is presented. In particular, rarely studied n-type polymer composites made of carbon nanotubes and conducting polymers are included. To maximize the performance, nanotubes were barely percolated in a polymer matrix, and the pristine composites were reduced to have n-type doping. In addition, novel bi-functional simultaneous conversion and storage of electrical energy from thermal energy are discussed. Here thermally-driven ion diffusion was employed to greatly increase the output voltage with polystyrene sulfonic acid film. Polyaniline coated electrodes containing graphene and carbon nanotube sandwiched the polystyrene sulfonic acid film where thermally induced voltage enabled electrochemical reactions, resulting in a charging behavior without an external power supply. Details of these novel approaches including material synthesis and characterization as well as energy carrier transport behaviors are obtained.

Recently organic hybrid thermoelectric (TE) materials composed of poly(3,4-ethylenedioxy-thiophene) poly(styrenesulfonate) (PEDOT-PSS) and carbon nanotubes (CNTs) have received much attention due to their high TE power factors.^1-2) In addition their TE performance was reported to depend on the kind of the CNT. Single-walled and double-walled CNTs, or the CNTs with high Seebeck coefficient were recommended.^3-4) However, both the commercially available PEDOT-PSS and CNTs are very expensive. Instead, here we used mass-produced CNT and polymers, i.e., super-growth CNT (SG-CNT)⁵⁾ and poly(vinyl chloride) (PVC) or polyimide (PI). Nippon ZEON Corporation, Ltd. has started to produce SG-CNTs. Thus, the SG-CNTs are less expensive than the common single-walled CNTs. However, they have some disadvantages, such as more defective and less electroconductive than common ones. In this presentation we have succeeded in constructing hybrid TE materials by using SG-CNTs, PVC or PI, and nanoparticles like nano-dispersed poly(nickel ethylenetetrathiolate) (n-PETT),⁶⁾ where n-PETTs might play a role of carrier transport promoter by covering the defects of SG-CNTs to improve the electrical conductivity. Although we reported high TE figure-of-merit for the hybrid of the CNTs produced by arc-discharge process (Arc-CNT), PVC and n-PETTs,⁷⁾ here we could construct the hybrid TE materials with the similar TE performance by using less expensive and defective SG-CNTs instead of Arc-CNTs.
Acknowledgements: This research was supported by NEDO, Japan and Nippon ZEON corporation, Ltd.
References:
1. C. Yu, et al., ACS Nano, 2011, 5, 7885.
2. H. Song, et al., RSC Adv., 2013, 3, 22065.
3. G. P. Moriarty, et al., Energy Technol., 2013, 1, 265.
4. Y. Naka, et al., Appl. Phys. Express, 2014, 7, 025103.
5. K. Hata, et al., Science, 2004, 306, 1362.
6. K. Oshima, et al., Chem. Lett., 2015, 44, 1185.
7. N. Toshima, et al., Adv. Mater., 2015, 27, 2246.

Harvesting waste heat by thermoelectric generators (TEG) acts as a prospective alternative to conventional petroleum based fuels. Currently, TEGs fabricated with thermoelectric polymer materials (PTEG) have gained the attention of the scientific community, due to their low cost, low toxicity and high processability [1,2]. Nevertheless, in comparison to their inorganic counterparts, the PTEGs have poorer efficiencies, due to the inherent lower Seebeck coefficient of the polymer systems [3,4]. Additionally, the n-type polymers needed to fabricate a PTEG are usually much less efficient than their p-type equivalents and are relatively unstable in ambient conditions [5]. In our study we are overcoming these drawbacks by combining inorganic and polymer materials, taking advantage of the effects undergoing in the interface of the material [6,7]. By controlling these effects, we manage to tune the thermoelectric power output of the composite and fabricate a 5-leg hybrid thermoelectric generator with record high power of 1.3 μW for ΔΤ = 10^oC at room. Considering that conventional TEGs are fabricated with more than 100 legs, our findings make reality the dream of using polymers for thermoelectric applications.
References :
[1] Bubnova, O. et al. Optimization of the thermoelectric figure of merit in the conducting polymer poly(3,4-ethylenedioxythiophene). Nat. Mater. 10, 429–433 (2011).
[2] Kim, G.-H., Shao, L., Zhang, K. & Pipe, K. P. Engineered doping of organic semiconductors for enhanced thermoelectric efficiency. Nat. Mater. 12, 719–723 (2013).
[3] Zhao, L.-D. et al. Ultrahigh power factor and thermoelectric performance in hole-doped single-crystal SnSe. Science 351, 141–144 (2016).
[4] Bubnova, O. & Crispin, X. Towards polymer-based organic thermoelectric generators. Energy Environ. Sci. 5, 9345 (2012).
[5] Russ, B. et al, Power factor enhancement in solution-processed organic n-type thermoelectrics through molecular design. Adv. Mater, 26, 3473–3477 (2014)
[6] Malen, J.A. et al, Fundamentals of energy transport, energy conversion, and thermal properties in organic–inorganic heterojunctions. Chemical Physics Letters, 491, 109-122 (2010)
[7] Coates, N. et al, Effect of interfacial properties on polymer–nanocrystal thermoelectric transport. Adv. Mater. , 25, 1629–1633 (2013)

ACKNOWLEDGEMENTS
The authors acknowledge financial support from Arkema and the Région Aquitaine as well as
from the Industrial Chair (Arkema/ANR) within the grant agreement no. AC-2013-365. I.P. is
grateful to the Région Aquitaine for financial support (Ph.D. grant #20111101004).
This work was performed within the framework of the Labex
AMADEUS ANR-10-LABEX-0042-AMADEUS with the help of the French state Initiative
d’Excellence IdEx ANR-10-IDEX-003-02.

Low-cost, mechanically flexible, and efficient thermoelectric (TE) materials provide the potential for widespread and novel uses of TE devices, such as converting wasted thermal energy from automobiles into useful electrical energy, utilizing body heat to power wearable electronics, and enabling low cost and mechanically robust refrigeration. Organic – inorganic nanocomposites present an emerging class of potentially low-cost, mechanically flexible, and efficient TE materials; however, the critical parameter (ZT) in determining the TE performance of these materials is currently an order of magnitude lower than that of high-performing pure inorganic materials. The performance of a TE material is determined by the Seebeck coefficient (α), electrical conductivity (σ), and thermal conductivity (κ), ZT= α²σT/κ. The Seebeck coefficient is a measure of the electrical potential difference generated across a material upon application of a temperature differential, and it will depend on the energy dependence of charge transport in the material. Organic – inorganic nanocomposites consisting of π-conjugated host polymers and inorganic nanowires are particularly exciting in that the charge transport properties, and thus the Seebeck coefficient, can be manipulated by altering interfacial energy landscapes and interfacial charge-transfer rates. In this work we show that interfacial energetics can be readily manipulated by the choice of the surface ligand on the nanowire, with ultraviolet photoelectron spectroscopy measurements showing that the work functions of silver nanowires (AgNWs) and lead sulfide nanowires (PbSNWs) can be adjusted by over 0.8 eV. This work function variation results in significantly altered energy level alignments with the transport states of the organic host polymer. We show that in blends of these NWs with PEDOT:PSS and P3HT, the Seebeck coefficient depends on both the energy level alignments between the polymers and NWs as well as the NW junction resistance. At low NW junction resistance, which results from a fusing of NW junctions during thermal annealing, the Seebeck coefficient and power factor (α²σ) are nearly equal to that of the pure nanowires. However, with higher NW junction resistance the Seebeck coefficient and electrical conductivity depend on the work function of the NWs relative to the ionization energy of the conjugated polymer host. Variation in the energy level alignments between these two components thus presents a promising potential strategy to significantly increase TE performance in these materials.

The desire for low cost, lightweight, and flexible devices has drawn attention to the prospects of using organic thermoelectrics for energy harvesting. While significant progress has been made with high performing p-type thermoelectrics, n-type polymers have lagged behind due to their instability in air. Metallo-organic complexes with nickel as the metal center have been shown to exhibit high electrical conductivities but their thermoelectric potential warrants investigation as they tend to be n-type. Here, we report the synthesis, characterization and thermoelectric properties of materials containing poly(nickel-ethenetetrathiolate), Ni(dmit)₂ (dmit=2-thioxo-1,3-dithiole-4,5-dithiolato), and other similar metal coordination polymers. We discuss the impact of oxidation on the electrical conductivity (σ) of these materials. X-ray photoelectron spectroscopy (XPS) results indicate clear differences in binding environments of sulfur as a function of the extent of oxidation, indicating that the ligand may play an important role in determining the electrical conductivity of these polymers. Interestingly, the Seebeck coefficient (S) remains largely unchanged over the course of oxidation indicating that this may be a viable technique to decouple S and σ in such materials. Elemental analysis is employed to confirm the composition of these materials as well as to ensure the synthesis is reproducible. In order to enhance the Seebeck coefficient, we blend these metallo-organic polymers with n-channel polymers to form a hybrid thermoelectric material. As expected, the hybrid films have a larger Seebeck coefficient, which enables optimization of the power factor. We report the thermoelectric properties of pristine pellets as well as composite films; the temperature dependent properties for these polymers show semi-conducting behavior that is consistent with hopping transport.

In an effort to create a paintable/printable thermoelectric material, comprised exclusively of organic components, polyaniline (PANi), graphene, and double-walled carbon nanotubes (DWNT) were alternately deposited from aqueous solutions using the layer-by-layer assembly technique. Graphene and DWNT are stabilized with an intrinsically conductive polymer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). A 1 µm thick film, composed of 80 PANi/graphene-PEDOT:PSS/PANi/DWNT-PEDOT:PSS quadlayers (QL) exhibits electrical conductivity (σ) of 1.88 X 10⁵ S/m and a Seebeck coefficient (S) of 120 µV/K, producing a thermoelectric power factor (S²σ) of 2710 µW/(mK²). This is the highest value ever reported for a completely organic material measured at room temperature. Furthermore, this performance matches or exceeds that of commercial bismuth telluride. These outstanding properties are attributed to the highly ordered structure in the multilayer assembly. The thermoelectric power output increased with the number of cycles deposited, yielding 8.5 nW at 80 QL for ΔT = 5.6 K. A simple thermoelectric generator was prepared with selectively-patterned, fabric-based system. The electric voltage generated by each TE device increased in a linear relationship with both ΔT and the number of TE legs, producing ~ 5 mV with just five legs and a ΔT of 9.5 K. This unique TE coating system is water-based and uses only organic components. For the first time, there is a real opportunity to harness waste heat from unconventional sources, such as body heat to power devices in an environmentally-benign way.

The adverse correlation of electrical conductivity and thermopower have been the main roadblocks for large improvement in thermoelectric properties of organic thermoelectric materials. Here, the electrical properties of our composites based on poly(3,4-ethylenedioxythiophene) : poly(styrenesulfonate) (PEDOT:PSS) and carbon nanotube (CNT) treated by polar solvent (dimethyl sulfoxide, DMSO) and acid (formic acid, FA) deviate from the typical behaviors, making possible a remarkable thermoelectric power factor up to 464 μW/m-K² at room temperature. The promising power factor can be mainly attributed to the unchanged thermopower despite large increase in electrical conductivity. The origins of the unusual transport properties - decoupled thermopower and electrical conductivity, maximized thermopower at a CNT loading, and the simultaneous enhancement of electrical conductivity and thermopower - were studied. From XPS, AFM, Hall measurement, and tunneling AFM current mapping results, the removal of insulating PSS and film morphology changes, such as smaller grain size and more percolated PEDOT network after solvent/acid treatments, were found to be the factors for the great conductivity improvement. The thermopower was minimally affected due to carrier mobility enhancement by PSS removal. Prominent energy filtering effect with an optimum number of PEDOT:PSS-CNT junctions peaks thermopower at CNT concentration of 6.7 wt%. These factors synergetically lead to the optimized power factor. This study provides better understanding of polymer/CNT composites for TE applications and offers possibility for further performance improvement.

While the low lattice thermal conductivity of conducting organic materials has motivated efforts to increase their thermoelectric power factor for use as low-cost, flexible thermoelectric materials, organic molecules are also employed to tune the in-plane and cross-plane thermoelectric properties of layered inorganic materials. Here, several experiments are discussed for better understanding thermoelectric transport in these organic and hybrid thermoelectric materials. These experiments include a study of electronic thermal conductivity of poly(3,4-ethylenedioxythiophene) (PEDOT) thin films, which are charcterized with the use of a four probe thermoelectric measurement capability of a suspended micro-device. A similar experimental method is employed to investigate the effects of tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) coating on the thermoelectric properties of bismuth antimonite telluride nanoplates, where the organic surface functionalization serves to tune the surface band bending. In another experiment, inelastic light scattering is used to examine phonon transport in layered inorganic materials intercalated with organic ligands.

Semiconductor nanocrystals (NCs) are prized for their size- and shape-dependent electronic and optical properties and as building blocks in the assembly of NC solids. However, the long, insulating ligands commonly employed in the synthesis of colloidal NCs inhibit strong interparticle coupling and charge transport once NCs are assembled into the solid state as NC arrays. In this talk, I will describe methods to introduce atoms, ions, and more compact molecules at the NC surface that allows us to increase interparticle coupling and dope NC solids. NC coupling and doping provide control over the density of states, the carrier statistics and the Fermi energy. I will also describe the importance of engineering device interfaces to study the fundamental physics of NC solid transport and to design device architectures for applications. Examples of strong coupling and doping in II-VI and IV-VI semiconductor NC solids will be given that yield high-mobility, high-conductivity NC solids. Temperature--dependent transport measurements of these materials are consistent with a transition from localized to extended-state charge transport.

Over recent years several new classes of conjugated polymer and molecular semiconductors have shown relatively high carrier mobilities. We have been studying the Seebeck coefficient of a range of these materials using gated field-effect transistor configurations, both to better understand the transport physics of these materials but also to understand better their suitability for thermoelectric applications as well as relevant the structure-property relationships. We have also developed novel strategies for bulk doping of self-organised conjugated polymers, for which the incorporation of dopant molecules does not degrade the structural order of the polymer. Polymers doped in this way exhibit excellent charge transport and thermoelectric properties.

Thermoelectric composites can be used to yield large figure of merit, even though the effective medium theory limits zT of a composite made from two non-interacting materials, A and B, to the larger of the two [1]. We describe a mechanism that can lift this limitation by treating charge and heat flux separately. Insulating beads coated with a conducting shell are distributed throughout a thermoelectric host material. Data shows that thermal conductivity decreases as beads are added, but the electrical conductivity can be made to decrease less than the thermal conductivity. Here we study a composite of lead-coated silica beads in a p-type (Bi,Sb)₂Te₃ host, which gave zT = 1.3. To maximize zT, the host material and beads must be optimized simultaneously. To enhance the zT, we reduce the hole concentration in the host material to have a higher Seebeck coefficient and reduce the high temperature thermal conductivity. Then we dope the material around the beads to a higher hole concentration to maintain the electrical conductivity. This requires finding an optimal heat treatment and an optimal bead concentration in the composite.

References:

[1] David J. Bergman and Ohad Levy, J. Appl. Phys. 70 6821 (1991); David J. Bergman and Leonid G. Fel, J. Appl. Phys. 85 8205 (1999)

The inability to independently manipulate the thermal and electronic properties of conventional thermoelectric materials has impeded their progress and delayed their broad deployment into terrestrial applications. Hybrid organic/inorganic thermoelectric materials based on conducting polymers and inorganic nanostructures have been demonstrated to combine both the inherently low thermal conductivity of the polymer and the superior charge transport properties (high power factors) of the inorganic component. Unfortunately, only a handful of such successful examples currently exist in literature, primarily due to the lack of any general design rules for generating hybrid thermoelectric materials. In this talk, I will demonstrate a modular approach that combines molecular engineering at the organic/inorganic interface and simple processing techniques enabling de novo design of complex hybrid thermoelectric systems. We chemically modify the surfaces of inorganic nanostructures and graft conductive polymers to yield robust solution processable p-type and n-type inorganic/organic hybrid nanostructures. The thermoelectric properties of these hybrid materials are tailored by varying the composition of the organic and inorganic components, which yields novel non-monotonic increases in the electronic properties due to strong chemical interactions between the components. As a result, we are able to obtain enhanced thermoelectric performance in a wide variety of hybrid systems at intermediate concentrations. This strategy establishes a unique platform with broad handles to custom tailoring of thermal and electrical properties through hybrid material tunability and enables independent control over inorganic material chemistry, nanostructure geometry, and organic material properties, thus providing a robust pathway to major performance enhancements.

Engineering the electronic band structures of certain materials can increase the thermoelectric efficiency of a material, due to an increase in band degeneracy at the band edges. As the band structure of a material is deeply linked to the underlying chemical bonds,^[1,2] changing bonding interactions has shown to be most effective in engineering the band degeneracy of thermoelectric materials. Successful strategies include changing the orbital overlap in PbTe via doping,^[1] or through chemical pressure exerted on the ligand field splitting in quaternary chalcopyrites and Zintl phases.^[3,4]

Skutterudite CoSb₃, a well known thermoelectric material, has recently been shown to exhibit band convergence at elevated temperatures when heavily doped.^[5] In this study, we aim to explain the observance of the experimentally observed changes in the thermoelectric transport from a chemical and structural point of view.^[6] We present temperature dependent synchrotron diffraction data, showing differences in the thermal expansion coefficient of local bonds in the structure. Using the experimentally obtained temperature dependent structural information, density functional theory is employed to assess the changes in the band structure with temperature. These experimental and computational results reveal connections between bonding interactions and electronic structure, providing explanation for observed changes in thermoelectric transport. In addition, we will show how n-type doping in this system alters the bonding situation and the structure due to population of anti-bonding bands. This study shows the direct effect of chemical bonding and orbital population on the structure and transport of thermoelectric materials.

[1] W. G. Zeier, A. Zevalkink, Z. M. Gibbs, G. Hautier, M. G. Kanatzidis, G. J. Snyder, Angew. Chem. Int. Ed. 2016, doi:10.1002/anie.201508381.
[2] W. G. Zeier, J. Schmitt, G. Hautier, U. Aydemir, Z. M. Gibbs, C. Felser, G. J. Snyder, Nat. Mater. Rev. 2016, 16032.
[3] W. G. Zeier, H. Zhu, Z. M. Gibbs, G. Ceder, W. Tremel, G. J. Snyder, J. Mater. Chem. C 2014, 2, 10189–10194.
[4] J. Zhang, L. Song, G. K. H. Madsen, K. F. F. Fischer, W. Zhang, X. Shi, B. B. Iversen, Nat. Commun. 2016, 7, 10892.
[5] Y. Tang, Z. M. Gibbs, L. Agapito, G. Li, H.-S. Kim, M. B. Nardelli, S. Curtarolo, G. J. Snyder, Nat. Mater. 2015, 14, 1223–1228.
[6] R. Hanus, X. Guo,Y. Tang, G. Li, G.J. Snyder, W.G. Zeier submitted