Conjugated polymers are considered for application in thermoelectric energy conversion due to their low thermal conductivity, low weight, non-toxicity, and ease of fabrication, which promises low manufacturing costs. Here, an investigation of the thermoelectric properties of poly ({4,8-bis[(2-ethylhexyl), commonly known as PTB7 conjugated polymer, is reported. Samples were prepared from solutions of PTB7 in three different solvents: chlorobenzene, 1,2-dichlorobenzene, and 1,2,4-trichlorobenzene, with and without 1,8-diiodooctane (DIO) additive. In order to characterize their thermoelectric properties, the electrical conductivity and the Seebeck coefficient were measured. We found that, by increasing the boiling point of the solvent, both the electrical conductivity and the Seebeck coefficient of the PTB7 samples were simultaneously improved. We believe that the increase in mobility is responsible for solvent-dependent thermoelectric properties of the PTB7 samples. However, the addition of DIO changes the observed trend. Only the sample prepared from 1,2,4-trichlorobenzene showed a higher electrical conductivity and Seebeck coefficient and, as a consequence, improved power factor in comparison to the samples prepared from chlorobenzene and 1,2-dichlorobenzene.
We investigated the interaction of chemical materials with flexible graphite foils which were fabricated from the expanded graphite (EG) flakes by mechanical rolling and compressing. A variety of performed experiments demonstrated that the electrical conductivity and thermoelectric power (TEP) of the graphite foils can be modified by chemical treatments. In particular, the “as prepared” p‐type graphite foil was successfully transferred into an n‐type doped material upon a treatment with amine containing compounds. Generally, the acceptor‐like chemicals increasing the concentration of the electric charge carriers enhanced the conductivity of the graphite foils, thereby, showing a decrease in the TEP reflected by the Seebeck coefficient, whereas the donor molecules significantly affected the conductivity and changed the sign and value of the TEP. Thermal and electrical insulating polymers, such as PVDF, PMMA, PVA, PS or PC, filling the inter‐lamellar spacing reduced the conductivity of the foil due to increase of the layer‐to‐layer resistance. They also blocked heat flow in the foil, and consequently increased the Seebeck coefficient. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Abstract:Chemical modification by co-solvents added to [6,6]-Phenyl-C71 butyric acid methyl ester, commonly known as an n-type semiconducting fullerene derivative PC 70 BM, is reported to change the electrical and thermoelectric properties of this system. Power factor of the casted PC 70 BM samples achieves values higher than that determined for a variety of organic compounds, including conducting polymers, such as PEDOT:PSS in the pristine form. After chemical functionalization by different solvents, namely N,N-Dimethylformamide (DMF), dimethyl sulfoxide (DMSO), N-Methyl-2-pyrrolidone (NMP), acetonitrile (AC), and 1,2-Dichloroethane (DCE), the four-probe in-plane electrical conductivity and Seebeck coefficient measurements indicate a simultaneous increase of the electrical conductivity and the Seebeck coefficient. The observed effect is more pronounced for solvents with a high boiling point, such as N,N-Dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and N-Methyl-2-pyrrolidone (NMP), than in acetonitrile (AC) and 1,2-Dichloroethane (DCE). We identified the origin of these changes using Hall mobility measurements, which demonstrate enhancement of the PC 70 BM charge carrier mobility upon addition of the corresponding solvents due to the improved packaging of the fullerene compound and chemical interaction with entrapped solvent molecules within the layers.
The thermoelectric properties of poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}), commonly known as PTB7 conducting polymer, were investigated for the first time by Rastegaralam et al. in 2017 [Crystals 2017, 7, 292]. PTB7 showed higher electrical conductivity or Seebeck coefficient (or even both) and, hence, a higher thermoelectric power factor than a variety of organic semiconductors. Therefore, it is worth working more on this semiconductor to improve its thermoelectric figure of merit. In this work, for the first time the effect of cosolvents on the thermoelectric properties of PTB7 is investigated. PTB7 conducting polymer dissolved in chlorobenzene was treated with different solvents: N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), Nmethyl-2-pyrrolidone (NMP), acetonitrile (AC), and 1,2-dichloroethane (DCE). Upon addition of DMF, DMSO, and NMP, a significant enhancement in the electrical conductivity of the samples accompanied by a reduction in the Seebeck coefficient occurred as a result of doping, while the use of AC and DCE led to simultaneous enhancement in the electrical conductivity and the Seebeck coefficient by increasing the mobility. The dopants used in this work are inexpensive, are easily available, and do not need to perform any synthesis process. The highest estimated figure of merit value obtained in this work without optimization is 0.1, which is of the highest values for organic thermoelectric materials.
Based on the electrophysical and structural data, a crystallophysical model of the ionic transport in superionic conductors Ba1-xLaxF2+x and Ca1-xYxF2+x (space group Fm¯3m), in which the charge carriers are mobile interstitial ions F− resulting from heterovalent substitutions of fluorite fragments of [M14F64] (M = Ca, Ba) by structural clusters of [M8R6F69] (R = La, Y). Single crystals of Ca1-xYxF2+x (0.02<x<0.16) and Ba1-xLaxF2+x (x = 0.31) solid solutions were obtained by directed crystallization of the melt. The mobility of ionic carriers in isostructural superionics Ba0.69La0.31F2.31, Ca0.84Y0.16F2.16, Pb0.67Cd0.33F2, and Pb0.9Sc0.1F2.1 are compared. Crystals of Ba1-xLaxF2+x and Ca1−xYxF2+x with improved conductometric and mechanical characteristics are promising active to replace the traditional CaF2 electrolyte in galvanic cells for thermodynamic research of chemicals.
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