1 Molecular and Supramolecular Parameters Dictating the2 Thermoelectric Performance of Conducting Polymers: A Case Study3 Using Poly(3-alkylthiophenes)4 Balazs Endrodi,†,‡ Janos Mellar,§ Zoltan Gingl,§ Csaba Visy,† and Csaba Janaky*,†,‡

5†Department of Physical Chemistry and Materials Science, University of Szeged, Rerrich Square 1, Szeged, H-6720, Hungary

6‡MTA-SZTE “Lendulet” Photoelectrochemistry Research Group, Rerrich Square 1, Szeged, H-6720, Hungary

7§Department of Technical Informatics, University of Szeged, Arpad Square 2, Szeged, H-6720, Hungary

8 *S Supporting Information

9 ABSTRACT: In this study, we investigated the impact of a molecular and10 supramolecular structure of conducting polymers (CPs) on their thermo-11 electric properties. As a model system, poly(3-alkylthiophene)s (P3ATs)12 with different side-chain lengths were prepared through oxidative chemical13 polymerization and were recrystallized to a well-ordered lamellar structure,14 resulting in one-dimensional self-assembled nanofibers (evidenced by15 transmission electron microscopy, X-ray diffraction, and UV−vis spectro-16 scopic measurements). Thermoelectric characterization was performed at17 different doping levels (precisely tuned by doping in the redox reaction with18 Ag+ and Fe3+ cations), and the highly doped samples exhibited the best19 performance for all studied polymers. By varying the length of the alkyl side20 chain, the supramolecular structure and consequently the electronic21 properties were varied. The highest electrical conductivity was measured22 for poly(3-butylthiophene), rooted in its densely packed structure. The established structure−property relationships, concerning23 the monotonous decrease of the electrical conductivity with the alkyl side chain length, highlight the importance of the24 supramolecular structure (interchain distance in this case). These findings may contribute to the rational design of organic25 thermoelectric materials.

26 ■ INTRODUCTION27 Among other alternative energy harvesting methods, thermo-28 electric power generation attracts significant and continuously29 growing interest. Direct conversion of heat to electricity aims to30 reduce heat losses in industrial processes, and to recover energy31 from high temperature exhaust fumes. Long lifetime, offered by32 the lack of any moving-parts in such devices makes this33 approach even more attractive.1

34 The efficiency of thermoelectric materials is represented by35 their figure of merit (Z, most often used in its dimensionless36 form as ZT), or by the power factor (P):

σκ

σ= =ZTS T

P Sand2

2

37 where T is the absolute temperature, S is the Seebeck38 coefficient, σ is the electrical conductivity, and κ is the thermal39 conductivity.40 Rivaling their inorganic counterparts, conducting polymers41 (CPs) are promising alternatives for low temperature thermo-42 electric applications.2−5 Their notably high Seebeck coefficient43 coupled with small thermal conductivity as well as their low44 cost, induced significant scientific efforts in this area.3,6

45 Thermoelectric characterization of many different CPs have46 been performed so far;7 however, values of their thermoelectric

47figure of merit (ZT) remained inferior compared to that of their48best inorganic counterpart. On the other hand, recently49reported increased ZT values, (approaching 0.5 in the case of50PEDOT-Tosylate and PEDOT-PSS systems) project a51promising future for organic thermoelectrics.8,9

52A major drawback of CPs in thermoelectrics is their usually53low electrical conductivity in their neutral form. This issue can54be circumvented by either doping (increasing charge carrier55concentration) or by enhancing the charge carrier mobility. The56effects of increased charge carrier density (i.e., higher doping57level), however, are complex because it affects thermal58conductivity and the Seebeck coefficient as well (in an59undesired manner). This fact emphasizes the importance of60the charge carrier mobility, which is strongly correlated to both61the regioregularity of the CP, and the increased probability of62interchain electron hopping.63Studies on traditional CPs, most importantly on P3HT, led64to the recognition that the monomer’s structure can have a65decisive impact on the charge carrier mobility in the polymer.66Unlike for other, first (and some second) generation CPs,

Received: January 6, 2015Revised: February 9, 2015

Article

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67 where the hole mobility was mostly below 10−2 cm2 V−1 s−1,68 this value could be even one magnitude larger in the case of69 P3HT.10,11 This large improvement could be rooted in at least70 two factors: (i) self-ordering effect of the alkyl side-chains; (ii)71 high regioregularity of properly synthesized P3HT. Elaborating72 on these results, a new era, the rational design of new73 monomers has been emerged within the organic electronics74 (more precisely the OFET) community.12 By including fused75 aromatic rings (e.g., pyrrolopyrrole) into the polymer back-76 bone, extended rigidity of the polymer (compared to P3HT)77 leads to further enhanced charge carrier mobility. Thus, with78 the use of bulky monomers, such as diketopyrrolopyrrole or79 cyclopentadithiophene, several orders of magnitude higher hole80 mobility value was reached.13,14

81 Beyond the above-discussed effect of the monomer molecule,82 precise control of the supramolecular and morphological83 features of the polymer is also of importance in optimizing84 the thermoelectric performance. Different strategies have been85 exploited to form CP structures with enhanced electrical86 conductivity recently.15−17 These methods aim to form ordered87 CP structures, in which the high electrical conductivity is88 ensured by 1D electron pathways.89 Electrospinning is a general route to prepare polymer90 (micro)fibers. Note, however, that application of such high91 voltage requires complex instrumentation, making the synthesis92 expensive, thus diminishing the benefits of CPs versus inorganic93 semiconductors. Another possibility is the template synthesis of94 CP nanofibers. Both hard and soft templates can be employed95 in this manner. Infiltration of CP solutions into solid, porous96 templates (e.g., nanoporous Al2O3) leads to parallel standing97 CP fibers.18 The prominence of such structures in analytical98 applications is obvious due to the multiplied surface area. The99 lack of interaction between the individual fibers, however, limits100 the electrical conductivity of such architectures.101 The application of soft templates offers an easy, one-step102 synthesis of highly conducting CPs. The presence of polyanions103 (e.g., polystyrenesulfonate (PSS)) during the synthesis leads to104 highly conductive products, because of the orientation of105 positively charged polymer chains along the (PSS) macro-106 molecules.16,17 Detailed investigation on the structure of107 PEDOT/PSS however, revealed the presence of PSS aggregates108 in the formed polymer. The insulating effect of these109 nonconducting parts can be partially overcome by treating110 PEDOT/PSS with polar solvents, or acid solutions.19−23

111 Although most promising thermoelectric results were gathered112 on PEDOT/PSS systems, all the above facts emphasize the113 importance of template free, solution phase methods.114 Despite that the π-stacking interaction between the115 independent polymer chains has a slight ordering effect, most116 CPs have a globular, disordered structure after synthesis. In the117 case of alkyl-substituted regioregular polythiophenes, however,118 formation of highly ordered, crystalline nanofibers (NFs) was119 reported earlier.24−28 The self-assembly was attributed to the120 interaction between the alkyl-side chains, leading to an121 interdigitated, “zipper-like” structure. The decreased distance122 between the polymer chains facilitates interchain electron123 hopping, resulting in enhanced charge carrier mobility.25,29

124 It is important to notice that the above-described approaches125 are far from being independent from each other, contrarily, they126 are rather convoluted. For example, introducing an alkyl-chain127 to the thiophene ring not only affects the molecular structure,128 but enables the self-assembly of the polymer to form129 nanofibers. Therefore, when designing thermoelectric materials,

130it is crucial to understand the individual role and contribution131of the different parameters to the thermoelectric properties.132Elaborating on our recent communication on poly(3-133hexylthiophene) nanofibers,29 we present the systematic134investigation on the effect of both molecular and supra-135molecular features as well as that of doping on the136thermoelectric properties of poly(3-alkylthiophenes) (P3ATs)137with different side-chain length. Our attempt to deconvolute138the effect of various parameters affecting the thermoelectric139performance may contribute to the rational design of new CP-140materials for thermoelectric applications.

141■ EXPERIMENTAL SECTION

142Oxidative chemical polymerization was employed to obtain143polythiophene and P3ATs with different side-chain length (A =144(M) methyl, (B) butyl, (H) hexyl, (O) octyl, (D) dodecyl).30 3-145Alkylthiophene and FeCl3 solutions (in chloroform) were146mixed, having final reagent concentrations of 0.1 and 0.25 M,147respectively. The reaction mixtures were kept in closed vessels148for 6 h on ice bath, under continuous stirring. The precipitated149polymers were filtered on 12−15 μm pore sized filter paper,150and washed repeatedly with ethanol to remove iron-traces. The151final products were dried in air, under infrared lamp.152The whisker method was used to form nanofibers from the153polymer powders.25 First, a larger molecular weight fraction of154the respective P3AT was dissolved by tetrahydrofuran (THF).155Owing to the insolubility of polythiophene and P3MT, the156recrystallization, and hence further studies were carried out157only on the other four P3ATs. After evaporating THF, the158polymers were redissolved in a mixed (9:1 ratio) anisole/159chloroform solvent (final concentrations were 2.0 g dm−3 in the160case of P3BT, while 2.5 g dm−3 in the case of other P3ATs).161Finally, the solutions were heated up to 70 °C and then cooled162down to room temperature on ice bath.163P3AT nanonets were drop-casted from P3AT solutions, on164plastic substrates (2 cm × 2.5 cm), previously patterned with165four gold electrodes for four-point probe electrical measure-166ments.167Neutral polymer films were oxidized by dipping them into168AgClO4 or FeCl3 solutions (in nitrobenzene). The concen-169tration of the oxidant was varied between 0.1 mmol dm−3 and17025 mmol dm−3 (saturated). At a selected oxidant concentration,171the doping level of P3ATs was controlled by the dipping time.172Since the best thermoelectric results were gathered on the most173highly doped polymers,29 all the characterization data (X-ray174diffraction (XRD), Fourier transform infrared (FT-IR) spec-175troscopy, scanning electron microscopy (SEM)) are presented176for these samples in this paper.177UV−vis−NIR spectra were recorded using an Agilent 8453178UV−visible diode array spectrophotometer in the range of179200−1100 nm. FT-IR were recorded using a Bio-Rad Digilab180Division FTS-65A/896 instrument, equipped with a Harrick’s181Meridian SplitPea single reflection diamond attenuated total182reflectance (ATR) accessory. All spectra were recorded183between 400 and 4000 cm−1, using 4 cm−1 optical resolution,184averaging 512 interferograms.185Raman spectroscopic studies were carried out on a DXR186Raman microscope with a red laser (λ = 780 nm), operating at1871 mW laser power.188To record transmission electron microscopic (TEM) images,189P3AT solutions were drop-casted on copper mesh TEM grids190covered by carbon film. Ag-doping occurred in situ on the

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191 P3AT coated grids. A FEI Tecnai G2 20 X-Twin instrument192 was used, operating at an acceleration voltage of 200 kV.193 Scanning electron microscopic (SEM) images were recorded194 using a Hitachi S-4700 field emission scanning electron195 microscope (coupled with a Rontec EDX detector), operating196 at 10 kV acceleration voltage.197 XRD patterns were recorded between 2Θ = 3−80° at 1°198 minute−1 scan rate on a Rigaku Miniflex II instrument,199 operating with a Cu Kα,1 radiation source (λ = 0.1541 nm).200 A custom designed setup (Supporting Information, Figure201 S1) was used to measure the Seebeck coefficient. Electrical202 conductivity was determined by the four-point method, using a203 Keithley 2400 type general purpose source meter.

204 ■ RESULTS AND DISCUSSION205 Recrystallization of P3ATs to Nanofibers. The recrystal-206 lization procedure is accompanied by the color change of the207 polymer solutions from orange to ruby-purple. On the UV−

f1 208 visible spectra (Figure 1.) significant changes could be detected

209 when the solvent was changed from pure THF to 9/1 ratio210 anisole/chloroform mixture. The first important difference is211 the red shift of the absorption maximum in all cases. Such a212 decrease in the transition energy can be related to the extension213 of the conjugation length. Furthermore, appearance of the fine214 vibronic structure (π−π* absorption bands at higher wave-

215lengths of about 510, 550, and 600 nm) is a clear indication of216the nanofiber formation.26−28

217Supramolecular structure of the bulk and nanofibrillar P3AT218samples was investigated by XRD, after drop-casting them on219 f2glass substrates. As seen in Figure 2A, bulk P3ATs show both220sharp reflections and a notable, broad, hill-type reflection too.221While the first reflection (100) can be attributed to the lamellar222ordering of the polymer chains, the latter is typically observed223for amorphous, regiorandom P3ATs, obtained by simple224oxidative chemical polymerization.25

225Contrarily, the presence of the crystalline phase is almost226exclusive after the recrystallization: beyond the nearly complete227disappearance of the hill-type reflection; this is further228supported by the appearance of the (200) and (300) reflections229(Figure 2B). This alteration can be attributed to a highly230ordered structure, in which the individual P3AT chains are231stacked together due to the interaction of the overlapping232aromatic rings (π-stacking) and the “zipper-like” connection of233the alkyl side chains.25,31,32

234Direct evidence on the nanofiber formation was furnished by235 f3TEM measurements. As seen in Figure 3A, the recrystallization236process results in a randomly oriented network of CP237nanofibers. Interestingly, the average diameter of the individual238nanofibers is almost identical for all P3ATs (around 18 nm). At239the same time, the length of the nanofibers is highly dependent240on the alkyl side-chain: while it is about 1 μm for P3BT and241P3HT, for polymers with longer side side-chains it decreases242drastically (in the case of P3DT larger aggregates of very short243nanofibers were detected) (Figure 3B).244Effect of Silver Perchlorate Doping. To tune the245oxidation level of the P3AT samples, the nanonets were246gradually doped in their reaction with silver cations:

+ = +° + −P3AT AgClO P3AT ClO Ag4 40

247 f4Raman spectra (Figure 4A for P3BT and Supporting248Information, Figure S2 for all other polymers) of the neutral249and the doped P3ATs nanonets are consistent with that of250other P3ATs studied earlier.33 For example, as shown for251P3BT, changes in the relative intensities of the Cα−S−Cα′252deformation (721 cm−1) and Cα−Cα′ stretching vibration253(1212 cm−1, characteristic to the head-to-tail structure), and the254shift of the band at 1446 cm−1 (related to the CαCβ

255stretching) to a significantly lower wavenumber (1407 cm−1)256confirm the formation of a heavily doped form of the polymer.257This fact was further supported by FTIR measurements. As the258doping reaction proceeds, the characteristic tenor of the

Figure 1. UV−visible spectra of P3AT solutions. Spectra recorded inTHF and anisole/chloroform solutions (after cooling) are overlaid foreach P3AT.

Figure 2. XRD pattern of (A) bulk and (B) nanofibrillar P3ATs.

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259 spectrum ceases (Figure 4B for P3BT and Supporting260 Information, Figure S3 for all other polymers), indicating the261 formation of the oxidized, conducting form of P3ATs.34

262 Modifications in the crystal structure were monitored by263 XRD. Upon oxidation, the XRD pattern of the P3ATs changed264 significantly. The sharp reflection at lower 2Θ values−265 corresponding to the distance between the interdigitated266 chains−gradually shifted to lower values (larger distances)

f5 267 (Figure 5 and Supporting Information, Table S1). This268 phenomenon is a side-effect of the oxidation: as the polymer269 chains get positively charged, Coulomb repulsion leads to larger270 interchain distances. Furthermore, appearance of the new271 reflections at 2Θ = 27.7, 32.0, 38.1, 44.4, and 46.1° confirms the272 formation of metallic silver particles on the P3AT nanonets.

273Although XRD measurements are not quantitative, the high274relative intensity of the latter reflections indicates the formation275of a large amount of silver, and indirectly the high doping level276of the polymers.277SEM images together with the XRD data proved the278formation of silver nanoparticles in the doping reaction. Using279concentrated oxidant solution, the doping level of the polymers280was about 0.4 in all cases (as derived from elemental ratio of S281to Ag, measured by EDX measurements). This value indicates282the presence of a highly doped form of polymers and the283formation of a large amount of silver. According to the SEM284images, the Ag nanoparticles form large aggregates, with a size285of about 420 ± 44 nm in the case of P3BT and about 320 ± 50286 f6nm in the case of the other P3ATs (Figure 6 and Supporting287Information, Figure S4). Careful analysis of TEM images (e.g.,288Figure S5) also confirmed the above average particle sizes.289Clearly, the deposited Ag particles do not form a percolation290pathway through the bulk nanonet structure.35 This observa-291tion indicates an intimate electronic contact among the292individual P3AT nanofibers in the self-assembled network.293Thermoelectric Properties of Silver-Doped P3AT294Nanonets. Changes in the thermoelectric properties of295P3ATs upon the doping reaction were monitored at different296oxidant concentrations. Most importantly, the oxidant concen-297tration had a large impact on both the reaction rate and on the298maximum doping level, reached in the reaction. Higher299electrical conductivities, registered with higher concentrations,300indicate higher doping level (see data in Supporting301Information, Figure S6 for P3BT). Similarly to P3HT302nanofibers29 the best thermoelectric performance was achieved

Figure 3. (A) TEM image of P3BT nanofiber network; (B) average diameter and length of the formed nanofibers for the different P3ATs.

Figure 4. (A) Raman- and (B) absorption FTIR-spectra of the neutral and the doped P3BT nanofiber network.

Figure 5. XRD pattern of the doped P3ATs.

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303 at the highest oxidant concentration for all polymers in this304 study.

f7 305 As presented in Figure 7A−D, results obtained for all the306 polymers follow a similar pattern: Electrical conductivity307 increases with the doping level, while the Seebeck coefficient308 ceases to a minimum value. This trend is entirely consistent to309 earlier reports on P3HT.29,36 The maximum power factor,310 registered for the studied P3ATs, however, was highly311 dependent on the length of the alkyl side-chain. While the312 best power factor was 10 μW m−1 K−2 in the case of P3BT, it313 was about 40 times lower for P3DT.314 Differences in the thermoelectric performance can be further315 highlighted by comparing and contrasting the electrical316 conductivity and the Seebeck coefficient values for the best317 performing P3AT samples. Similarly to earlier findings on318 different CPs, the value of the Seebeck coefficient is totally

f8 319 independent from the length of the alkyl side chain (Figuref8 320 8A).37−39 This means, that the large difference in the power

321 factors is mostly dictated by the electrical conductivity values:322 as the length of the alkyl side chain increased, the reasonable323 electrical conductivity measured for P3BT (35 S cm−1)324 decreased to 0.77 S cm−1 (about a 45 times lower value) in325 the case of P3DT.326 To link the above observations with structural attributes, and327 thus obtain structure−property relationships, electrical con-328 ductivity values were plotted as a reciprocal function of the329 lamellar interchain distance of the highly doped polymers

330(derived from XRD data, see Supporting Information, Table331S1) in the self-assembled architecture (Figure 8B). The more or332less linear trend indicates that besides the interchain electron333hopping facilitated by π-stack interactions,40 the closely packed334structure (observed for P3ATs with shorter side-chains) is also335a major contributor to the electrical conductivity.336Thermoelectric Properties of FeCl-Doped P3ATs. To337evaluate the role of the dopants’ nature41,42 and the possible338contribution of the in situ formed silver particles to the339enhanced electrical conductivity, doping was also performed340using FeCl3, an oxidant, forming no metal precipitate on the341polymer upon the redox reaction (Fe3+ → Fe2+).342As coined from Raman and FTIR measurements, there is no343significant difference between the effects of the two344substantially different dopants. Similarly to the above-discussed345case (using AgClO4 as oxidant), both vibrational spectroscopic346techniques confirmed the presence of heavily doped P3ATs347 f9(Figure 9A,B).348As it was shown above, the XRD pattern of P3BT changes349strongly upon oxidation (Figure 2B and Figure 5). Beyond the350obvious disappearance of the Ag-related sharp reflections, the351shift of the reflection at lower 2Θ values upon oxidation is352almost identical in the Fe3+ case (Figure 9C). This fact reveals353that the larger distance between the polymer chains in the354oxidized state originates only from the charge repulsion, and355not from the presence of Ag particles.

Figure 6. SEM images the Ag-doped P3ATs: (A) P3BT, (B) P3HT, (C) P3OT, (D) P3DT.

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356 As shown in Figure 9D, changes in both the electrical357 conductivity and Seebeck are very similar to those observed in358 the case of AgClO4 doping (Figure 7). Although the kinetics of359 the reaction differs using a different oxidant, both the Seebeck360 coefficient and the electrical conductivity (and hence the power361 factor) values are almost identical for the heavily doped362 polymers. Since no nanoparticle formation occurred during363 Fe3+ doping, the identical value of the power factor in the two364 cases suggests that the in situ formed Ag particles do not365 contribute to the thermoelectric performance.

366 ■ CONCLUSIONS367 As a result of our systematic study on the influence of both368 molecular/supramolecular structure and doping on the thermo-369 electric properties of P3ATs we may conclude that the

370observed effects of the above factors are complex and371convoluted.372Both shorter alkyl side chains (molecular structure) and the373ordered nanofibrillar structure (supramolecular structure)374manifested in an enhanced electrical conductivity. At the375same time, the value of the Seebeck coefficient was376independent from these parameters. As a result, the best377thermoelectric power factor was registered for P3BT, the378polymer with the shortest alkyl side chain among the studied379P3ATs. Selecting the most appropriate parameters from our380studies, the highest achieved thermoelectric power factor (10381μW K−2 m−1 for heavily doped P3BT nanofibers) showed a382notable improvement compared to the values reported earlier383for this class of polymers.

Figure 7. Doping level-dependent thermoelectric properties of different P3ATs: (A) P3BT, (B) P3HT, (C) P3OT, (D) P3DT) with saturatedAgClO4 (c = 25 mM).

Figure 8. (A) Electrical conductivity and Seebeck coefficient values for highly doped P3ATs; (B) maximum electrical conductivity values in functionof the reciprocal chain distance of the highly doped polymers (derived from XRD measurements).

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384 The effect of the dopant’s nature was also investigated, by385 comparing the thermoelectric power factor of Fe3+- and Ag+-386 doped P3BT nanonets. Even though in the latter case the387 formation of silver particles was evidenced, the best power388 factor reached with the two very different oxidants is almost389 identical, indicating that the enhanced electrical conductivity is390 rooted in molecular changes upon doping.391 Finally we note that by evaluating the individual (yet392 convoluted) contribution of the above parameters on the393 thermoelectric performance, new avenues may be paved for the394 rational design of CP thermoelectrics. Supramolecular and395 morphological engineering of CPs with intrinsically high charge396 carrier mobility (e.g., CPs containing diketopyrrolopyrrole397 moieties) may contribute to unprecedently high ZT values, and398 such studies are in progress in our laboratory.

399 ■ ASSOCIATED CONTENT

400 *S Supporting Information

401 Scheme of the thermoelectric setup, Raman, FT-IR, TEM, and402 thermoelectric results. This material is available free of charge403 via the Internet at http://pubs.acs.org.

404 ■ AUTHOR INFORMATION

405 Corresponding Author406 *E-mail: janaky@chem.u-szeged.hu. Fax: +36 62 546-482. Tel:407 +36 62 546-393.

408 Notes409 The authors declare no competing financial interest.

410■ ACKNOWLEDGMENTS411Funding support of the Hungarian Academy of Sciences412through its Momentum Excellence Grant is gratefully acknowl-413edged.

414■ ABBREVIATIONS415P3HT, poly(3-hexylthiophene); SEM, scanning electron416microscopy; CP, conducting polymer; P3AT, poly(3-alkylth-417iophene); XRD, X-ray diffraction; TEM, transmission electron418microscopy; FT-IR, Fourier-transform infrared spectroscopy;419PT, polythiophene; P3MT, poly(3-methylthiophene; P3BT,420poly(3-butylthiophene; P3HT, poly(3-hexylthiophene; P3OT,421poly(3-octylthiophene; P3DT, poly(3-dodecylthiophene

422■ REFERENCES(1) 423DiSalvo, F. J. Thermoelectric Cooling and Power Generation.

424Science 1999, 285, 703−706.(2) 425Zhang, Q.; Sun, Y.; Xu, W.; Zhu, D. Organic Thermoelectric

426Materials: Emerging Green Energy Materials Converting Heat to427Electricity Directly and Efficiently. Adv. Mater. 2014, 26, 6829−6851.

(3) 428He, M.; Qiu, F.; Lin, Z. Towards High-Performance Polymer-429Based Thermoelectric Materials. Energy Environ. Sci. 2013, 6, 1352−4301361.

(4) 431Chen, Y.; Zhao, Y.; Liang, Z. Solution Processed Organic432Thermoelectrics: Towards Flexible Thermoelectric Modules. Energy433Environ. Sci. 2015, DOI: 10.1039/c4ee03297g.

(5) 434McGrail, B. T.; Sehirlioglu, A.; Pentzer, E. Polymer Composites435for Thermoelectric Applications. Angew. Chem., Int. Ed. 2014,436DOI: 10.1002/anie.201408431.

(6) 437Dubey, N.; Leclerc, M. Conducting Polymers: Efficient438Thermoelectric Materials. J. Polym. Sci., Polym. Phys. 2011, 49, 467−439475.

Figure 9. (A) Raman and (B) FTIR spectra; (C) XRD pattern of the neutral and the FeCl3 doped P3BT nanofiber network; (D) doping level-dependent thermoelectric properties of P3BT using 50 mM FeCl3 as oxidant.

The Journal of Physical Chemistry C Article

DOI: 10.1021/acs.jpcc.5b00135J. Phys. Chem. C XXXX, XXX, XXX−XXX

G

(7)440 Poehler, T. O.; Katz, H. E. Prospects for Polymer-Based441 Thermoelectrics: State of the Art and Theoretical Analysis. Energy442 Environ. Sci. 2012, 5, 8110−8115.

(8)443 Bubnova, O.; Khan, Z. U.; Malti, A.; Braun, S.; Fahlman, M.;444 Berggren, M.; Crispin, X. Optimization of the Thermoelectric Figure445 of Merit in the Conducting Polymer Poly(3,4-Ethylenedioxythio-446 phene). Nat. Mater. 2011, 10, 429−433.

(9)447 Kim, G.; Shao, L.; Zhang, K.; Pipe, K. P. Engineered Doping of448 Organic Semiconductors for Enhanced Thermoelectric Efficiency. Nat.449 Mater. 2013, 12, 719−723.

(10)450 Heeger, A. J. Semiconducting Polymers: The Third Generation.451 Chem. Soc. Rev. 2010, 39, 2354−2371.

(11)452 Horowitz, G. Organic Field-Effect Transistors. Adv. Mater.453 1998, 10, 365−377.

(12)454 Sirringhaus, H. 25th Anniversary Article: Organic Field-Effect455 Transistors: The Path Beyond Amorphous Silicon. Adv. Mater. 2014,456 26, 1319−1335.

(13)457 Tseng, H.; Phan, H.; Luo, C.; Wang, M.; Perez, L. A.; Patel, S.458 N.; Ying, L.; Kramer, E. J.; Nguyen, T.; Bazan, G. C.; et al. High-459 Mobility Field-Effect Transistors Fabricated with Macroscopic Aligned460 Semiconducting Polymers. Adv. Mater. 2014, 26, 2993−2998.

(14)461 Yun, H.; Lee, G. B.; Chung, D. S.; Kim, Y.; Kwon, S. Novel462 Diketopyrroloppyrrole Random Copolymers: High Charge-Carrier463 Mobility from Environmentally Benign Processing. Adv. Mater. 2014,464 26, 6612−6616.

(15)465 Long, Y.; Li, M.; Gu, C.; Wan, M.; Duvail, J.; Liu, Z.; Fan, Z.466 Recent Advances in Synthesis, Physical Properties and Applications of467 Conducting Polymer Nanotubes and Nanofibers. Prog. Polym. Sci.468 2011, 36, 1415−1442.

(16)469 Lang, U.; Muller, E.; Naujoks, N.; Dual, J. Microscopical470 Investigations of PEDOT:PSS Thin Films. Adv. Funct. Mater. 2009, 19,471 1215−1220.

(17)472 Nardes, A. M.; Kemerink, M.; Janssen, R. A. J.; Bastiaansen, J. A.473 M.; Kiggen, N. M. M.; Langeveld, B. M. W.; van Breemen, A. J. J. M.;474 de Kok, M. M. Microscopic Understanding of the Anisotropic475 Conductivity of PEDOT:PSS Thin Films. Adv. Mater. 2007, 19,476 1196−1200.

(18)477 Martin, C. R. Template Synthesis of Electronically Conductive478 Polymer Nanostructures. Acc. Chem. Res. 1995, 28, 61−68.

(19)479 Alemu, D.; Wei, H.; Ho, K.; Chu, C. Highly Conductive480 PEDOT:PSS Electrode by Simple Film Treatment with Methanol for481 ITO-Free Polymer Solar Cells. Energy Environ. Sci. 2012, 5, 9662−482 9671.

(20)483 Kim, J. Y.; Jung, J. H.; Lee, D. E.; Joo, J. Enhancement of484 Electrical Conductivity of Poly(3,4-Ethylenedioxythiophene)/Poly(4-485 Styrenesulfonate) by a Change of Solvents. Synth. Met. 2002, 126,486 311−316.

(21)487 Kim, Y. H.; Sachse, C.; Machala, M. L.; May, C.; Muller-488 Meskamp, L.; Leo, K. Highly Conductive PEDOT:PSS Electrode with489 Optimized Solvent and Thermal Post-Treatment for ITO-Free490 Organic Solar Cells. Adv. Funct. Mater. 2011, 21, 1076−1081.

(22)491 Mengistie, D. A.; Ibrahem, M. A.; Wang, P.; Chu, C. Highly492 Conductive PEDOT:PSS Treated with Formic Acid for ITO-Free493 Polymer Solar Cells. ACS Appl. Mater. Interfaces 2014, 6, 2292−2299.

(23)494 Ouyang, J. Solution-Processed PEDOT:PSS Films with495 Conductivities as Indium Tin Oxide through a Treatment with Mild496 and Weak Organic Acids. ACS Appl. Mater. Interfaces 2013, 5, 13082−497 13088.

(24)498 Ihn, K. J.; Moulton, J.; Smith, P. Whiskers of Poly(3-499 alkylthiophene)s. J. Polym. Sci., Polym. Phys. 1993, 31, 735−742.

(25)500 Samitsu, S.; Shimomura, T.; Heike, S.; Hashizume, T.; Ito, K.501 Effective Production of Poly(3-alkylthiophene) Nanofibers by Means502 of Whisker Method Using Anisole Solvent: Structural, Optical, and503 Electrical Properties. Macromolecules 2008, 41, 8000−8010.

(26)504 Oosterbaan, W. D.; Vrindts, V.; Berson, S.; Guillerez, S.;505 Douheret, O.; Ruttens, B.; D’Haen, J.; Adriaensens, P.; Manca, J.;506 Lutsen, L.; et al. Efficient Formation, Isolation and Characterization of507 Poly(3-Alkylthiophene) Nanofibres: Probing Order as a Function of508 Side-Chain Length. J. Mater. Chem. 2009, 19, 5424−5435.

(27) 509Kiriy, N.; Jahne, E.; Adler, H.; Schneider, M.; Kiriy, A.;510Gorodyska, G.; Minko, S.; Jehnichen, D.; Simon, P.; Fokin, A. A.; et al.511One-Dimensional Aggregation of Regioregular Polyalkylthiophenes.512Nano Lett. 2003, 3, 707−712.

(28) 513Samitsu, S.; Shimomura, T.; Ito, K. Nanofiber Preparation by514Whisker Method Using Solvent-Soluble Conducting Polymers. Thin515Solid Films 2008, 516, 2478−2486.

(29) 516Endrodi, B.; Mellar, J.; Gingl, Z.; Visy, C.; Janaky, C. Reasons517Behind the Improved Thermoelectric Properties of Poly(3-hexylth-518iophene) Nanofiber Networks. RSC Adv. 2014, 4, 55328−55333.

(30) 519Jarvinen, H.; Lahtinen, L.; Nasman, J.; Hormi, O.; Tammi, A.-L.520A New Method to Prepare 3-Octylthiophene and Poly-(3-octylth-521iophene). Synth. Met. 1995, 69, 299−300.

(31) 522Coakley, K. M.; Srinivasan, B. S.; Ziebarth, J. M.; Goh, C.; Liu,523Y.; McGehee, M. D. Enhanced Hole Mobility in Regioregular524Polythiophene Infiltrated in Straight Nanopores. Adv. Funct. Mater.5252005, 15, 1927−1932.

(32) 526Lu, G.; Bu, L.; Li, S.; Yang, X. Bulk Interpenetration Network of527Thermoelectric Polymer in Insulating Supporting Matrix. Adv. Mater.5282014, 26, 2359−2364.

(33) 529Baibarac, M.; Lapkowski, M.; Pron, A.; Lefrant, S.; Baltog, I.530SERS Spectra of Poly(3-hexylthiophene) in Oxidized and Unoxidized531States. J. Raman Spectrosc. 1998, 29, 825−832.

(34) 532Pinter, E.; Fekete, Z. A.; Berkesi, O.; Makra, P.; Patzko, A; Visy,533C. Characterization of Poly(3-octylthiophene)/Silver Nanocomposites534Prepared by Solution Doping. J. Phys. Chem. C 2007, 111, 11872−53511878.

(35) 536Roussel, F.; Chen Yu King, R.; Kuriakose, M.; Depriester, M.;537Hadj-Sahraoui, A.; Gors, C.; Addad, A.; Brun, J. Electrical and Thermal538Transport Properties of Polyaniline/Silver Composites and Their Use539as Thermoelectric Materials. Synth. Met. 2015, 199, 196−204.

(36) 540Xuan, Y.; Liu, X.; Desbief, S.; Leclere, P.; Fahlman, M.;541Lazzaroni, R.; Berggren, M.; Cornil, J.; Emin, D.; Crispin, X.542Thermoelectric Properties of Conducting Polymers: The Case of543Poly(3-hexylthiophene). Phys. Rev. B 2010, 82, 115454.

(37) 544Russ, B.; Robb, M. J.; Brunetti, F. G.; Miller, P. L.; Perry, E. E.;545Patel, S. N.; Ho, V.; Chang, W. B.; Urban, J. J.; Chabinyc, M. L.; et al.546Power Factor Enhancement in Solution-Processed Organic N-Type547Thermoelectrics through Molecular Design. Adv. Mater. 2014, 26,5483473−3477.

(38) 549Mai, C. K.; Schlitz, R. A.; Su, G. M.; Spitzer, D.; Wang, X.;550Fronk, S. L.; Cahill, D. G.; Chabinyc, M. L.; Bazan, G. C. Side-Chain551Effects on the Conductivity, Morphology, and Thermoelectric552Properties of Self-Doped Narrow-Band-Gap Conjugated Polyelec-553trolytes. J. Am. Chem. Soc. 2014, 136, 13478−13481.

(39) 554Mai, C.; Zhou, H.; Zhang, Y.; Henson, Z. B.; Nguyen, T.;555Heeger, A. J.; Bazan, G. C. Facile Doping of Anionic Narrow-Band-556Gap Conjugated Polyelectrolytes during Dialysis. Angew. Chem., Int.557Ed. 2013, 52, 12874−12878.

(40) 558Nielsen, C. B.; McCulloch, I. Recent Advances in Transistor559Performance of Polythiophenes. Prog. Polym. Sci. 2013, 38, 2053−5602069.

(41) 561Glaudell, A. M.; Cochran, J. E.; Patel, S. N.; Chabinyc, M. L.562Impact of the Doping Method on Conductivity and Thermopower in563Semiconducting Polythiophenes. Adv. Energy Mater. 2014,564DOI: 10.1002/aenm.201401072.

(42) 565Zhang, Q.; Sun, Y.; Xu, W.; Zhu, D. Thermoelectric Energy566from Flexible P3HT Films Doped with a Ferric Salt of Triflimide567Anions. Energy Environ. Sci. 2012, 5, 9639−9644.

The Journal of Physical Chemistry C Article

DOI: 10.1021/acs.jpcc.5b00135J. Phys. Chem. C XXXX, XXX, XXX−XXX

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