Fine Art of Thermoelectricity

Developing High-Performance and Low-Cost Paint Thermoelectric Materials for Low-Midtemperature Applications

Most thermoelectric (TE) materials used to convert heat energy into electrical energy are expensive and, to a certain degree, toxic. Moreover, due to the chemical complexity in the synthesis process, some of the TE materials are not reproducible. Similarly, the scarcity of TE materials hampers their scalability. To address the above issues, this study presents an inexpensive, nontoxic, scalable, and highly reproducible paint-based TE module for the conversion of heat energy to electrical energy. Transport properties with structural analysis indicate that the electrical conductivity of the paint TE material is controlled by the concentration of graphite and sodium silicate, while the Seebeck coefficient is dominated by the ratio of n- and p-type Bi-Sb-Te. The results indicate that the as-developed TE module can withstand an operating temperature of up to 160 °C. At a temperature of 57 °C, the highest power factors of the as-synthesized n- and p-type TE paints are 1.34 and 1.42 μW/(cm K2), respectively. It is also found that the TE module can have a higher output voltage when the cold side of the TE module is allowed to float in the air in comparison to that when it is in contact with the human body. The performance of the paint-based TE module is measured on five parts of the body, namely, the chest, palm, leg, wrist, and neck; the wrist has the highest open-circuit voltage of 1.9 mV, indicating its suitability for wearable applications. Finally, at a temperature gradient of 30 °C, a maximum output power of 6.8 μW is attained.

Highly Flexible and Foldable Paper-Based Thermoelectric Generator Prepared with Post-Treatment-Free PEDOT:PSS Hybrid Ink

Paper-based thermoelectric (PTE) generators have recently emerged as a green technology that can help alleviate environment pollution and the energy crisis. In this work, a PTE generator was prepared by coating a post-treatment-free thermoelectric ink consisting of poly(3,4-ethylenedioxythiophene)/polystyrene sulfonate (PEDOT:PSS) doped with 1-ethyl-3-methylimidazolium:tricyanomethanide (EMIM:TCM) onto the card paper. By tuning the molar concentration of the EMIM:TCM to 0.17 M and with hot-pressing, the PTE generator showed a decent power factor (PF) value of 6.82 μW m-1 K-2, which was higher than the values of PTE in the literature. This phenomenon could be attributed to the synergistic effect of high-performance thermoelectric ink (i.e., PF = 175 μW m-1 K-2 when deposited on glass slide) and the hot-pressing. The hot-pressing enhanced the packing density of cellulose fibers and the associated PEDOT:PSS hybrid, which enabled the formation of long-range conductive paths. In addition, the PTE had good mechanical stability, indicated by no significant change of the power factor values after cyclic folding 10,000 times. Moreover, the structure of as-prepared PTE could be easily tuned into different shapes that are promising for the preparation of flexible wearable thermoelectric generators.

Copper Iodide on Spacer Fabrics as Textile Thermoelectric Device for Energy Generation

The integration of electronic functionalities into textiles for use as wearable sensors, energy harvesters, or coolers has become increasingly important in recent years. A special focus is on efficient thermoelectric materials. Copper iodide as a p-type thermoelectrically active, nontoxic material is attractive for energy harvesting and energy generation because of its transparency and possible high-power factor. The deposition of CuI on polyester spacer fabrics by wet chemical processes represents a great potential for use in textile industry for example as flexible thermoelectric energy generators in the leisure or industrial sector as well as in medical technologies. The deposited material on polyester yarn is investigated by electron microscopy, x-ray diffraction and by thermoelectric measurements. The Seebeck coefficient was observed between 112 and 153 µV/K in a temperature range between 30 °C and 90 °C. It is demonstrated that the maximum output power reached 99 nW at temperature difference of 65.5 K with respect to room temperature for a single textile element. However, several elements can be connected in series and the output power can be linear upscaled. Thus, CuI coated on 3D spacer fabrics can be attractive to fabricate thermoelectric devices especially in the lower temperature range for textile medical or leisure applications.

Sensors-on-paper: Fabrication of graphite thermal sensor arrays on cellulose paper for large area temperature mapping

This paper reports on a fabrication method to obtain multiple thermal sensors by employing an array of graphite thermocouple patterns on commonly available Xerox paper. The graphite thermocouples are patterned using two different grade graphite pencils, which show a stable and reproducible thermal sensitivity. The fabricated paper devices with multiple thermocouple arrays are capable of producing temperature mapping of the desired area. Different thermal conditions were applied to test and confirm the working of these devices. The present work shows that simple graphite trace patterns can convert a piece of paper into a thermal mapping device.

Experimental and Theoretical Investigation of the Effect of Filler Material on the Performance of Flexible and Rigid Thermoelectric Generators

Thermoelectric generators have found many applications where the heat source can be either flat or curved. For a curved heat source, flexible thermoelectric generators are generally used. A filler material with low thermal conductivity can provide additional mechanical support to the thermoelectric module and can reduce convection and radiation losses. Herein, the effect of three different filler materials on the output performance of rigid and flexible thermoelectric generators is investigated. At first, theoretical models are derived and the experimental study validated the models. The experimental study revealed that the flexible thermoelectric modules outperformed the rigid modules; this is due to the reduction of the number of thermal junctions in the flexible modules and due to the differences in the thermal conductivities of the flexible and rigid substrates. Likewise, among TE modules without filler/with air between the TE legs, with polyurethane foam filler material, and with polydimethylsiloxane filler material, air has the lowest thermal conductivity, and therefore, the thermoelectric generator without filler generates higher output power and higher power density than when the other two filler materials are used. For the fixed temperature gradient, the highest power densities for the flexible and rigid thermoelectric generators without filler are 155 and 137.7 μW/cm² for temperature gradients of 10.8 and 10.3 °C, respectively.

Physical properties of carbon nanowalls synthesized by the ICP-PECVD method vs. the growth time

Investigation of the physical properties of carbon nanowall (CNW) films is carried out in correlation with the growth time. The structural, electronic, optical and electrical properties of CNW films are investigated using electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, UV-Vis spectroscopy, Hall Effect measurement system, Four Point Probing system, and thermoelectric measurements. Shorter growth time results in thinner CNW films with a densely spaced labyrinth structure, while a longer growth time results in thicker CNW films with a petal structure. These changes in morphology further lead to changes in the structural, optical, and electrical properties of the CNW.

Optical Expediency of Back Electrode Materials for Organic Near-Infrared Photodiodes

Organic semiconductor devices, including organic photodetectors (OPDs) and organic photovoltaics (OPVs), have undergone vast improvements, thanks to the development of non-fullerene acceptors. The absorption range of such NFA-based systems is typically shifted toward the near-infrared (near-IR) region compared to early-generation fullerene-based systems, rendering organic semiconductor devices suitable for near-IR sensing applications. While most efforts are concentrated on the photoactive materials, less attention is paid to the impact of the back electrodes on the device performance. Therefore, this work focuses on the optical expediency of gold (Au), silver (Ag), aluminum (Al), and graphite as back electrode materials in organic optoelectronics. This work shows that the "one size fits all" methodology is not a valid approach for choosing the back electrode material. Instead, considering the active layer absorption, the active layer thickness, and the intended application is necessary. A traditional polymer/fullerene-based system, poly(3-hexylthiophene) with [6,6]-phenyl C₆₁ butyric acid methyl ester (P3HT:PC₆₀BM), and a state-of-the-art narrow-band gap non-fullerene-based system, poly[4,8bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b; 4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethy-lhexyl)3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-(2-6-diyl)] and 2,2'-((2Z,2'Z)-((5,5'-(4,4-bis(2-ethylhexyl)4H-cyclopenta[1,2-b:5,4-b']dithiophene-2,6-diyl)bis(4-((2ethylhexyl)oxy)thiophene-5,2-diyl))bis(methanylylidene)) bis(5,6-difluoro3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (PCE10:COTIC-4F), are investigated by combining optical transfer matrix modeling simulations with experimentally determined recombination and extraction losses. We find that the narrow-band gap system shows performance gains when employing Au as the back electrode. Furthermore, we show that these performance gains are dependent on active layer thickness, yielding the most significance for thin active layers (<100 nm). Such thin, ultra-narrow-band gap devices are the focus of near-IR sensing applications, highlighting the importance of methodically choosing the back electrode. Lastly, the impact of the back electrode on the OPV device performance is outlined.

Paper Thermoelectrics by a Solvent-Free Drawing Method of All Carbon-Based Materials

As practical interest in the flexible or wearable thermoelectric generators (TEGs) has increased, the demand for the high-performance TEGs based on ecofriendly, mechanically resilient, and economically viable TEGs as alternatives to the brittle inorganic materials is growing. Organic or hybrid thermoelectric (TE) materials have been employed in flexible TEGs; however, their fabrication is normally carried out using wet processing such as spin-coating or screen printing. These techniques require materials dissolved or dispersed in solvents; thus, they limit the substrate choice. Herein, we have rationally designed solvent-free, all carbon-based TEGs dry-drawn on a regular office paper using few-layered graphene (FLG). This technique showed very good TE parameters, yielding a power factor of 97 μW m-1 K-2 at low temperatures. The p-type only device exhibited an output power of up to ∼19.48 nW. As a proof of concept, all carbon-based p-n TEGs were created on paper with the addition of HB pencil traces. The HB pencil exhibited low Seebeck coefficients (-7 μV K-1), and the traces were highly resistive compared to FLG traces, which resulted in significantly lower output power compared to the p-type only TEG. The demonstration of all carbon-based TEGs drawn on paper highlights the potential for future low-cost, flexible, and almost instantaneously created TEGs for low-power applications.

Coupling between structural properties and charge transport in nano-crystalline and amorphous graphitic carbon films, deposited by electron-beam evaporation

Nano-crystalline and amorphous films of graphitized carbon were deposited by electron-beam evaporation of bulk graphite. Structural properties and the size of graphite nanoclusters (L ≈ 1.2-5 nm) in the films were determined from the analysis of their Raman spectra. Electrical properties of the bulk nano-crystalline graphite reference sample and the deposited graphitic carbon films were measured by means of the Hall effect technique within the temperature range from 290 to 420 K. The electrical conductivity σ and Hall mobility μ_H of all samples exhibited exponential temperature dependences, indicating on the non-metallic behavior. Electrical properties of the amorphous graphitic carbon thin films, deposited at low substrate temperatures (620 and 750 K) were analyzed in the scope of the hopping charge transport mechanism via localized states. We have shown that the charge transport in the bulk and thin film (920 K) nano-crystalline graphite samples is carried out via the tunneling and thermionic emission over potential barriers at the grain boundaries.This paper contributes towards better understanding of coupling between structural and electrical properties of graphitic carbon thin films.

MoS2-on-paper optoelectronics: drawing photodetectors with van der Waals semiconductors beyond graphite

We fabricate paper-supported semiconducting devices by rubbing a layered molybdenum disulfide (MoS₂) crystal onto a piece of paper, similar to the action of drawing/writing with a pencil on paper. We show that the abrasion between the MoS₂ crystal and the paper substrate efficiently exfoliates the crystals, breaking the weak van der Waals interlayer bonds and leading to the deposition of a film of interconnected MoS₂ platelets. Employing this simple method, which can be easily extended to other 2D materials, we fabricate MoS₂-on-paper broadband photodetectors with spectral sensitivity from the ultraviolet (UV) to the near-infrared (NIR) range. We also used these paper-based photodetectors to acquire pictures of objects by mounting the photodetectors in a homebuilt single-pixel camera setup.

Lightweight and Bulk Organic Thermoelectric Generators Employing Novel P-Type Few-Layered Graphene Nanoflakes

Graphene exhibits both high electrical conductivity and large elastic modulus, which makes it an ideal material candidate for many electronic devices. At present not much work has been conducted on using graphene to construct thermoelectric devices, particularly due to its high thermal conductivity and lack of bulk fabrication. Films of graphene-based materials, however, and their nanocomposites have been shown to be promising candidates for thermoelectric energy generation. Exploring methods to enhance the thermoelectric performance of graphene and produce bulk samples can significantly widen its application in thermoelectrics. Realization of bulk organic materials in the thermoelectric community is highly desired to develop cheap, Earth-abundant, light, and nontoxic thermoelectric generators. In this context, this work reports a new approach using pressed pellets bars of few-layered graphene (FLG) nanoflakes employed in thermoelectric generators (TEGs). First, FLG nanoflakes were produced by a novel dry physical grinding technique followed by graphene nanoflake liberation using plasma treatment. The resultant material is highly pure with very low defects, possessing 3 to 5-layer stacks as proved by Raman spectroscopy, X-ray diffraction measurement, and scanning electron microscopy. The thermal and electronic properties confirm the anisotropy of the material and hence the varied performance characteristics parallel to and perpendicular to the pressing direction of the pellets. The full thermoelectric properties were characterized both parallel and perpendicular to the pressing direction, and the proof-of-concept thermoelectric generators were fabricated with variable amounts of legs.