Conductive polymers for thermoelectric power generation

In spite of the fact that conducting polymers, during the past decade, have made inroads into various flexible devices including electronics, supercapacitors, sensors, transistors and memories etc. their exploration in the field of thermoelectric power-generation has not yet been significant. This review provides a comprehensive study regarding thermoelectric performance of various conducting polymers depending upon their specific structural and physico-chemical properties. Recent trends in organic thermoelectrics are discussed as: (i) factors affecting thermoelectric performance; (ii) strategies required for improvement of the power factor (due to inherent low thermal conductivity); and (iii) challenges that still lie ahead. A detailed analysis of electrical and thermal transport mechanisms suggests that various processes such as stretching, controlled doping and addition of inorganic materials/carbon nanostructures, may be applied for enhancement of the thermoelectric figure-of-merit. The attempts are made for highlighting as to how these conducting polymers can be realized into efficient thermoelectric generators by summarizing various reported architectural-designs. These devices have a tremendous potential for tapping low-temperature heat (e.g. body/appliances' heat, geo-thermal/oceanic heat etc.) to power wearable medical sensors and smart electronic devices. Finally, the efforts are put together to familiarize the reader with the big breakthrough that can be created by light-weight, flexible, non-toxic conducting polymers in thermoelectric domain

Nanostructured polypyrrole: enhancement in thermoelectric figure of merit through suppression of thermal conductivity

Semi-crystalline polypyrrole (PPy) nanotubes were synthesized through a chemical polymerization route using methyl orange-ferric chloride (MO-FeCl3) as a template for growth. The thermoelectric properties of these PPy nanotubes have been studied in the temperature range 300-380 K after treatment with various dopants such as hydrochloric acid (HCl), p-toluene-sulphonic acid monohydrate (ToS), and tetrabutyl ammonium hexaflurophosphate (PF6). It has been observed that these dopants affect the electrical and thermal transport properties of PPy nanotubes in different ways. The temperature dependence of electrical resistivity suggests that pure PPy and ToS-doped PPy nanotubes exhibit a critical regime of metal-to-insulator transition, and doping with HCl drives them into the metallic regime. In contrast, PF6 doping of PPy nanotubes carries them into the insulating regime and these samples exhibited the highest figure of merit of similar to 3.4 x 10(-3) at 380 K, which was 240% higher than the value obtained in the case of pristine PPy nanotubes. Strongly repressed thermal conductivity along with moderately high Seebeck coefficient and electrical conductivity have been found to be responsible for the high figure of merit observed in PF6-doped PPy nanotubes. The suppression of thermal conductivity in PF6-doped PPy nanotubes is attributed to the scattering of the spectrum of phonons via hierarchical length-scale defect structures present in the sample

Electron beam induced modifications in electrical properties of Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) films

Conducting polymer PEDOT:PSS [Poly(3,4-ethylenedioxythiophene):poly(styrenesulphonate)], owing to its high electrical conductivity, enviornmental stability and low cost, is presently getting most attention for various device applications including thermoelectric, organic light emitting diodes and photovoltaics. We have investigated the irradiation effect of high energy electron beam on the electrical transport properties of PEDOT:PSS films to manifest the scope of this polymer in high radiation field and its suitability for radiation dosimeter applications. PEDOT:PSS films were deposited on flexible polyimide (Kapton) sheets using drop-cast method and irradiated up to 75 kGy dose with 1 MeV electron beam. The electrical conductivity of as deposited polymer film was similar to 3.2 S/cm which consistently falls to similar to 0.76 S/cm on irradiation dose of 75 kGy. Detailed characterization of the samples using x-ray photoelectron spectroscopy, contact angle measurement and solubility test conclusively suggested that the lowering of electrical conductivity in irradiated sample is attributed to the crosslinking of PEDOT chains and dissociation of PSS

Flexo-green Polypyrrole - Silver nanocomposite films for thermoelectric power generation

Conducting polymers offer various advantages over inorganic thermoelectric materials such as ecofriendliness, a reduced manufacturing cost, flexibility, low thermal conductivity and amenability to tuning of electrical properties through doping; have recently drawn much attention for conversion of low temperature waste heat (<= 150 degrees C) into electricity. In this study, we investigated the thermoelectric properties of hybrid films of polypyrrole and silver (PPy-Ag). These films were prepared on biaxially oriented polyethylene terephthalate (BOPET) flexible substrates by eco-friendly one pot photo-polymerization method using aqueous solution of silver nitrate (AgNO3) as photo initiator. Detailed characterization of the samples revealed that morphology of composite films reorganized with the change in AgNO3 concentration during synthesis. Increasing AgNO3 concentrations resulted in PPy films containing Ag nanopartides, nanoclusters as well as macroclusters. With alteration in concentration and size of Ag particles in PPy matrix, it has been observed that the electrical conductivity of the films increased (1.5-17.3 S cm(-1)), thermal conductivity decreased (0.16-0.002 Wm(-1) K-1), while Seebeck coefficient moderately reduced from 10.9 mu V/K to 5.8 mu V/K. Nearly same doping (N+/N similar to 0.35) content, improved conjugation length and incorporation of Ag between the PPy chains resulted in improved charge carrier mobility/electrical conductivity in the PPy-Ag films. It is proposed that the interface of Ag and PPy served as scattering sites for phonons, thus leading to reduction of thermal conductivity. This synergetic combination of high electrical conductivity, extremely low thermal conductivity along with moderate Seebeck coefficient in the PPy-Ag films resulted in the highest figure-of-merit of similar to 7.4 x 10(-3) at 335 K among reported PPy based materials. A prototype thermoelectric power generator was fabricated by integrating six numbers of PPy-Ag films. The fabricated device exhibited maximum voltage and power respectively as 6 mV and similar to 30 pW. The present work opens new avenues for the thermoelectric applications of rarely explored flexible PPy-Ag films prepared by a simple nature-friendly photo-chemical process at room temperature

Elucidating the mechanisms behind thermoelectric power factor enhancement of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) flexible films

Conducting polymers that have shown their potential in flexible electronics and sensorics for the last one decade can be looked upon as promising materials for room temperature thermoelectric applications. Among all the existing popular conducting polymers, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is being widely studied because of its extraordinary high electrical conductivity and environmental stability. This article explains the possible mechanisms behind power factor enhancement of solvent-mixed flexible PEDOT:PSS films. The films drop-tasted on polyimide sheets by using both pristine and organic solvent (dimethyl sulphoxide i.e. DMSO) pre-mixed solutions were optimized for both annealing temperature and solvent concentrations. The detailed characterization of these films suggested that PSS was detached from PEDOT:PSS after DMSO addition. Selective eviction of PSS from typical core-shell structure of PEDOT:PSS not only caused conformational change in PEDOT chains from benzoid (coiled structure) to quinoid (linear structure) but also re-arranged PSS in more stretched form. Such a modification of the chemical structure caused improvement in power factor mainly due to enhanced charge carried mobility rather than increased doping/carrier concentration. A flexible thermoelectric generator consisting of an array of thirty elements was also fabricated by drop-casting DMSO-mixed PEDOT:PSS solution through a patterned mask. This array resulted in an output voltage of similar to 17.6 mV under a temperature gradient of 80 degrees C

Free-standing flexible multiwalled carbon nanotubes paper for wearable thermoelectric power generator

Most of the thermoelectric research, these days, is being focussed towards development of flexible thermoelectric power generators (TEG) for wearable applications. Organic thermoelectric materials being flexible and solution processable, though, seem promising cannot render themselves for designing of conventional thermoelectric devices which require both p- and n-type of legs; due to lack of air-stable and flexible n-type materials. This work shows the possibility of conversion of a p-type flexible free-standing multiwalled carbon nanotubes (MWCNTs) paper into n-type on treatment with polyethylenimine (PEI). X-ray photoelectron spectroscopy results suggest that 'as-grown' MWCNTs attained n-type nature due to electron donation by imine group of PEI. Power factors of similar to 0.2 and similar to 0.06 mu W/mK(2) observed respectively for p- and n-type MWCNTs papers, owing to extremely low thermal conductivity (similar to 0.05W/mK), resulted in figure-of-merit (ZT) of 1.27 x 10(-3) and 3.02 x 10(-4) at 68 degrees C. A prototype TEG designed using 'as-grown' and 'PEI-modified' MWCNTs as p- and n-type thermoelements respectively exhibited output of 227 mu V/7.6 mu A for a temperature difference of 40 degrees C. In short, facile scalability of MWCNTs when collaborated with such a low cost, environment friendly method that can easily modify its conduction to n-type can certainly open opportunities for scalable production of flexible roll-to-roll type wearable thermoelectric modules

Boosting thermoelectric power factor of free-standing Poly (3,4ethylenedioxythiophene):polystyrenesulphonate films by incorporation of bismuth antimony telluride nanostructures

We demonstrate that introduction of p-type Bi0.5Sb1.5Te3 nanostructures into the polymer matrix not only causes highly adherent drop-casted films of PEDOT:PSS (on Kapton sheets) to attain a free-standing nature but also brings a significant improvement in their thermoelectric properties. Hall and ESR measurements of these hybrid films clearly show that both the carrier concentration and mobility can be varied with Bi0.5Sb1.5Te3 content. Whereas, results of X-ray diffraction, Raman and X-ray photoelectron spectroscopy confirm the enhancement in chain alignment and better connectivity among PEDOT:PSS and Bi0.5Sb1.5Te3 nanosheets; leading to remarkable enhancement of electrical conductivity. These hybrid films, due to energy filtering of charge carriers at the organic/inorganic interface, exhibit improvement in the Seebeck coefficient also. In fact, such a synergetic combination of improved electrical conductivity and Seebeck coefficient expertly tailors the power factor (from order of similar to 10(-4) to 8.3 mu W/mK(2)) over a vast range. The optimized films are tested for their power conversion ability and a single thermoelement based device exhibits an open circuit voltage similar to 536 mu V and current similar to 134 mu A for a temperature difference of 53 degrees C. Such an evolution of organic-inorganic hybrid films in a flexible, free-standing motif with enhanced thermoelectric properties exhibit good potential for recovering heat from the curved hot surfaces