Molecular and Supramolecular Parameters Dictating the

Article pubs.acs.org/JPCC

Molecular and Supramolecular Parameters Dictating the Thermoelectric Performance of Conducting Polymers: A Case Study Using Poly(3-alkylthiophene)s Balázs Endrődi,†,‡ János Mellár,§ Zoltán Gingl,§ Csaba Visy,† and Csaba Janáky*,†,‡ †

Department of Physical Chemistry and Materials Science, University of Szeged, Rerrich Square 1, Szeged, H-6720, Hungary MTA-SZTE “Lendület” Photoelectrochemistry Research Group, Rerrich Square 1, Szeged, H-6720, Hungary § Department of Technical Informatics, University of Szeged, Á rpád Square 2, Szeged, H-6720, Hungary ‡

S Supporting Information *

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

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figure of merit (ZT) remained inferior compared to that of their best inorganic counterpart. On the other hand, recently reported increased ZT values, (approaching 0.5 in the case of PEDOT-Tosylate and PEDOT-PSS systems) project a promising future for organic thermoelectrics.8,9 A major drawback of CPs in thermoelectrics is their usually low electrical conductivity in their neutral form. This issue can be circumvented by either doping (increasing charge carrier concentration) or by enhancing the charge carrier mobility. The effects of increased charge carrier density (i.e., higher doping level), however, are complex because it affects thermal conductivity and the Seebeck coefficient as well (in an undesired manner). This fact emphasizes the importance of the charge carrier mobility, which is strongly correlated to both the regioregularity of the CP, and the increased probability of interchain electron hopping. Studies on traditional CPs, most importantly on P3HT, led to the recognition that the monomer’s structure can have a decisive impact on the charge carrier mobility in the polymer. Unlike for other, first (and some second) generation CPs,

INTRODUCTION Among other alternative energy harvesting methods, thermoelectric power generation attracts significant and continuously growing interest. Direct conversion of heat to electricity aims to reduce heat losses in industrial processes, and to recover energy from high temperature exhaust fumes. Long lifetime, offered by the lack of any moving-parts in such devices makes this approach even more attractive.1 The efficiency of thermoelectric materials is represented by their figure of merit (Z, most often used in its dimensionless form as ZT), or by the power factor (P): ZT =

σS2T κ

and

P = σS 2

where T is the absolute temperature, S is the Seebeck coefficient, σ is the electrical conductivity, and κ is the thermal conductivity. Rivaling their inorganic counterparts, conducting polymers (CPs) are promising alternatives for low temperature thermoelectric applications.2−5 Their notably high Seebeck coefficient coupled with small thermal conductivity as well as their low cost, induced significant scientific efforts in this area.3,6 Thermoelectric characterization of many different CPs have been performed so far;7 however, values of their thermoelectric © 2015 American Chemical Society

Received: January 6, 2015 Revised: February 9, 2015 Published: February 10, 2015 8472

DOI: 10.1021/acs.jpcc.5b00135 J. Phys. Chem. C 2015, 119, 8472−8479

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The Journal of Physical Chemistry C where the hole mobility was mostly below 10−2 cm2 V−1 s−1, this value could be even one magnitude larger in the case of P3HT.10,11 This large improvement could be rooted in at least two factors: (i) self-ordering effect of the alkyl side-chains; (ii) high regioregularity of properly synthesized P3HT. Elaborating on these results, a new era, the rational design of new monomers has been emerged within the organic electronics (more precisely the OFET) community.12 By including fused aromatic rings (e.g., pyrrolopyrrole) into the polymer backbone, extended rigidity of the polymer (compared to P3HT) leads to further enhanced charge carrier mobility. Thus, with the use of bulky monomers, such as diketopyrrolopyrrole or cyclopentadithiophene, several orders of magnitude higher hole mobility value was reached.13,14 Beyond the above-discussed effect of the monomer molecule, precise control of the supramolecular and morphological features of the polymer is also of importance in optimizing the thermoelectric performance. Different strategies have been exploited to form CP structures with enhanced electrical conductivity recently.15−17 These methods aim to form ordered CP structures, in which the high electrical conductivity is ensured by 1D electron pathways. Electrospinning is a general route to prepare polymer (micro)fibers. Note, however, that application of such a high voltage requires complex instrumentation, making the synthesis expensive, thus diminishing the benefits of CPs versus inorganic semiconductors. Another possibility is the template synthesis of CP nanofibers. Both hard and soft templates can be employed in this manner. Infiltration of CP solutions into solid, porous templates (e.g., nanoporous Al2O3) leads to parallel standing CP fibers.18 The prominence of such structures in analytical applications is obvious due to the multiplied surface area. The lack of interaction between the individual fibers, however, limits the electrical conductivity of such architectures. The application of soft templates offers an easy, one-step synthesis of highly conducting CPs. The presence of polyanions (e.g., polystyrenesulfonate (PSS)) during the synthesis leads to highly conductive products, because of the orientation of positively charged polymer chains along the (PSS) macromolecules.16,17 Detailed investigation on the structure of PEDOT/PSS however, revealed the presence of PSS aggregates in the formed polymer. The insulating effect of these nonconducting parts can be partially overcome by treating PEDOT/PSS with polar solvents, or acid solutions.19−23 Although most promising thermoelectric results were gathered on PEDOT/PSS systems, all the above facts emphasize the importance of template free, solution phase methods. Despite that the π-stacking interaction between the independent polymer chains has a slight ordering effect, most CPs have a globular, disordered structure after synthesis. In the case of alkyl-substituted regioregular polythiophenes, however, formation of highly ordered, crystalline nanofibers (NFs) was reported earlier.24−28 The self-assembly was attributed to the interaction between the alkyl-side chains, leading to an interdigitated, “zipper-like” structure. The decreased distance between the polymer chains facilitates interchain electron hopping, resulting in enhanced charge carrier mobility.25,29 It is important to notice that the above-described approaches are far from being independent from each other, contrarily, they are rather convoluted. For example, introducing an alkyl-chain to the thiophene ring not only affects the molecular structure, but enables the self-assembly of the polymer to form nanofibers. Therefore, when designing thermoelectric materials,

it is crucial to understand the individual role and contribution of the different parameters to the thermoelectric properties. Elaborating on our recent communication on poly(3hexylthiophene) nanofibers,29 we present the systematic investigation on the effect of both molecular and supramolecular features as well as that of doping on the thermoelectric properties of poly(3-alkylthiophenes) (P3ATs) with different side-chain length. Our attempt to deconvolute the effect of various parameters affecting the thermoelectric performance may contribute to the rational design of new CPmaterials for thermoelectric applications.

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EXPERIMENTAL SECTION Oxidative chemical polymerization was employed to obtain polythiophene and P3ATs with different side-chain length (A = (M) methyl, (B) butyl, (H) hexyl, (O) octyl, (D) dodecyl).30 3alkylthiophene and FeCl3 solutions (in chloroform) were mixed, having final reagent concentrations of 0.1 and 0.25 M, respectively. The reaction mixtures were kept in closed vessels for 6 h on ice bath, under continuous stirring. The precipitated polymers were filtered on 12−15 μm pore sized filter paper, and washed repeatedly with ethanol to remove iron-traces. The final products were dried in air, under infrared lamp. The whisker method was used to form nanofibers from the polymer powders.25 First, a larger molecular weight fraction of the respective P3AT was dissolved by tetrahydrofuran (THF). Owing to the insolubility of polythiophene and P3MT, the recrystallization, and hence further studies were carried out only on the other four P3ATs. After evaporating THF, the polymers were redissolved in a mixed (9:1 ratio) anisole/ chloroform solvent (final concentrations were 2.0 g dm−3 in the case of P3BT, while 2.5 g dm−3 in the case of other P3ATs). Finally, the solutions were heated up to 70 °C and then cooled down to room temperature on ice bath. P3AT nanonets were drop-casted from P3AT solutions, on plastic substrates (2 cm × 2.5 cm), previously patterned with four gold electrodes for four-point probe electrical measurements. Neutral polymer films were oxidized by dipping them into AgClO4 or FeCl3 solutions (in nitrobenzene). The concentration of the oxidant was varied between 0.1 mmol dm−3 and 25 mmol dm−3 (saturated). At a selected oxidant concentration, the doping level of P3ATs was controlled by the dipping time. Since the best thermoelectric results were gathered on the most highly doped polymers,29 all the characterization data (X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM)) are presented for these samples in this paper. UV−vis−NIR spectra were recorded using an Agilent 8453 UV−visible diode array spectrophotometer in the range of 200−1100 nm. FT-IR were recorded using a Bio-Rad Digilab Division FTS-65A/896 instrument, equipped with a Harrick’s Meridian SplitPea single reflection diamond attenuated total reflectance (ATR) accessory. All spectra were recorded between 400 and 4000 cm−1, using 4 cm−1 optical resolution, averaging 512 interferograms. Raman spectroscopic studies were carried out on a DXR Raman microscope with a red laser (λ = 780 nm), operating at 1 mW laser power. To record transmission electron microscopic (TEM) images, P3AT solutions were drop-casted on copper mesh TEM grids covered by carbon film. Ag-doping occurred in situ on the 8473


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The Journal of Physical Chemistry C P3AT coated grids. A FEI Tecnai G2 20 X-Twin instrument was used, operating at an acceleration voltage of 200 kV. Scanning electron microscopic (SEM) images were recorded using a Hitachi S-4700 field emission scanning electron microscope (coupled with a Röntec EDX detector), operating at 10 kV acceleration voltage. XRD patterns were recorded between 2Θ = 3−80° at 1° minute−1 scan rate on a Rigaku Miniflex II instrument, operating with a CuKα,1 radiation source (λ = 0.1541 nm). A custom designed setup (Supporting Information, Figure S1) was used to measure the Seebeck coefficient. Electrical conductivity was determined by the four-point method, using a Keithley 2400 type general purpose source meter.

lengths of about 510, 550, and 600 nm) is a clear indication of the nanofiber formation.26−28 Supramolecular structure of the bulk and nanofibrillar P3AT samples was investigated by XRD, after drop-casting them on glass substrates. As seen in Figure 2A, bulk P3ATs show both sharp reflections and a notable, broad, hill-type reflection too. While the first reflection (100) can be attributed to the lamellar ordering of the polymer chains, the latter is typically observed for amorphous, regiorandom P3ATs, obtained by simple oxidative chemical polymerization.25 Contrarily, the presence of the crystalline phase is almost exclusive after the recrystallization: beyond the nearly complete disappearance of the hill-type reflection; this is further supported by the appearance of the (200) and (300) reflections (Figure 2B). This alteration can be attributed to a highly ordered structure, in which the individual P3AT chains are stacked together due to the interaction of the overlapping aromatic rings (π-stacking) and the “zipper-like” connection of the alkyl side chains.25,31,32 Direct evidence on the nanofiber formation was furnished by TEM measurements. As seen in Figure 3A, the recrystallization process results in a randomly oriented network of CP nanofibers. Interestingly, the average diameter of the individual nanofibers is almost identical for all P3ATs (around 18 nm). At the same time, the length of the nanofibers is highly dependent on the alkyl side-chain: while it is about 1 μm for P3BT and P3HT, for polymers with longer side side-chains it decreases drastically (in the case of P3DT larger aggregates of very short nanofibers were detected) (Figure 3B). Effect of Silver Perchlorate Doping. To tune the oxidation level of the P3AT samples, the nanonets were gradually doped in their reaction with silver cations:

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RESULTS AND DISCUSSION Recrystallization of P3ATs to Nanofibers. The recrystallization procedure is accompanied by the color change of the polymer solutions from orange to ruby-purple. On the UV− visible spectra (Figure 1.) significant changes could be detected

P3AT° + AgClO4 = P3AT+ClO−4 + Ag 0

Raman spectra (Figure 4A for P3BT and Supporting Information, Figure S2 for all other polymers) of the neutral and the doped P3AT nanonets are consistent with that of other P3ATs studied earlier.33 For example, as shown for P3BT, changes in the relative intensities of the Cα−S−Cα′ deformation (721 cm−1) and Cα−Cα′ stretching vibration (1212 cm−1, characteristic to the head-to-tail structure), and the shift of the band at 1446 cm−1 (related to the CαCβ stretching) to a significantly lower wavenumber (1407 cm−1) confirm the formation of a heavily doped form of the polymer. This fact was further supported by FTIR measurements. As the doping reaction proceeds, the characteristic tenor of the spectrum

Figure 1. UV−visible spectra of P3AT solutions. Spectra recorded in THF and anisole/chloroform solutions (after cooling) are overlaid for each P3AT.

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

Figure 2. XRD pattern of (A) bulk and (B) nanofibrillar P3ATs. 8474


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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.

Although XRD measurements are not quantitative, the high relative intensity of the latter reflections indicates the formation of a large amount of silver, and indirectly the high doping level of the polymers. SEM images together with the XRD data proved the formation of silver nanoparticles in the doping reaction. Using concentrated oxidant solution, the doping level of the polymers was about 0.4 in all cases (as derived from elemental ratio of S to Ag, measured by EDX measurements). This value indicates the presence of a highly doped form of the polymers and the formation of a large amount of silver. According to the SEM images, the Ag nanoparticles form large aggregates, with a size of about 420 ± 44 nm in the case of P3BT and about 320 ± 50 nm in the case of the other P3ATs (Figure 6 and Supporting Information, Figure S4). Careful analysis of TEM images (e.g., Figure S5) also confirmed the above average particle sizes. Clearly, the deposited Ag particles do not form a percolation pathway through the bulk nanonet structure.35 This observation indicates an intimate electronic contact among the individual P3AT nanofibers in the self-assembled network. Thermoelectric Properties of Silver-Doped P3AT Nanonets. Changes in the thermoelectric properties of P3ATs upon the doping reaction were monitored at different oxidant concentrations. Most importantly, the oxidant concentration had a large impact on both the reaction rate and on the maximum doping level, reached in the reaction. Higher electrical conductivities, registered with higher concentrations, indicate higher doping level (see data in Supporting Information, Figure S6 for P3BT). Similarly to P3HT nanofibers29 the best thermoelectric performance was achieved

ceases (Figure 4B for P3BT and Supporting Information, Figure S3 for all other polymers), indicating the formation of the oxidized, conducting form of P3ATs.34 Modifications in the crystal structure were monitored by XRD. Upon oxidation, the XRD pattern of the P3ATs changed significantly. The sharp reflection at lower 2Θ values− corresponding to the distance between the interdigitated chains−gradually shifted to lower values (larger distances) (Figure 5 and Supporting Information, Table S1). This phenomenon is a side-effect of the oxidation: as the polymer chains get positively charged, Coulomb repulsion leads to larger interchain distances. Furthermore, appearance of the new reflections at 2Θ = 27.7, 32.0, 38.1, 44.4, and 46.1° confirms the formation of metallic silver particles on the P3AT nanonets.

Figure 5. XRD pattern of the doped P3ATs. 8475


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

at the highest oxidant concentration for all polymers in this study. As presented in Figure 7A−D, results obtained for all the polymers follow a similar pattern: Electrical conductivity increases with the doping level, while the Seebeck coefficient ceases to a minimum value. This trend is entirely consistent to earlier reports on P3HT.29,36 The maximum power factor, registered for the studied P3ATs, however, was highly dependent on the length of the alkyl side-chain. While the best power factor was 10 μW m−1 K−2 in the case of P3BT, it was about 40 times lower for P3DT. Differences in the thermoelectric performance can be further highlighted by comparing and contrasting the electrical conductivity and the Seebeck coefficient values for the best performing P3AT samples. Similarly to earlier findings on different CPs, the value of the Seebeck coefficient is totally independent from the length of the alkyl side chain (Figure 8A).37−39 This means, that the large difference in the power factors is mostly dictated by the electrical conductivity values: as the length of the alkyl side chain increased, the reasonable electrical conductivity measured for P3BT (35 S cm−1) decreased to 0.77 S cm−1 (about a 45 times lower value) in the case of P3DT. To link the above observations with structural attributes, and thus obtain structure−property relationships, electrical conductivity values were plotted as a reciprocal function of the lamellar interchain distance of the highly doped polymers

(derived from XRD data, see Supporting Information, Table S1) in the self-assembled architecture (Figure 8B). The more or less linear trend indicates that besides the interchain electron hopping facilitated by π-stack interactions,40 the closely packed structure (observed for P3ATs with shorter side-chains) is also a major contributor to the electrical conductivity. Thermoelectric Properties of FeCl3-Doped P3ATs. To evaluate the role of the dopants’ nature41,42 and the possible contribution of the in situ formed silver particles to the enhanced electrical conductivity, doping was also performed using FeCl3, an oxidant, forming no metal precipitate on the polymer upon the redox reaction (Fe3+ → Fe2+). As coined from Raman and FTIR measurements, there is no significant difference between the effects of the two substantially different dopants. Similarly to the above-discussed case (using AgClO4 as oxidant), both vibrational spectroscopic techniques confirmed the presence of heavily doped P3ATs (Figure 9A,B). As it was shown above, the XRD pattern of P3BT changes strongly upon oxidation (Figure 2B and Figure 5). Beyond the obvious disappearance of the Ag-related sharp reflections, the shift of the reflection at lower 2Θ values upon oxidation is almost identical in the Fe3+ case (Figure 9C). This fact reveals that the larger distance between the polymer chains in the oxidized state originates only from the charge repulsion, and not from the presence of Ag particles. 8476


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Figure 7. Doping level-dependent thermoelectric properties of different P3ATs: (A) P3BT, (B) P3HT, (C) P3OT, (D) P3DT) with saturated AgClO4 (c = 25 mM).

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

observed effects of the above factors are complex and convoluted. Both shorter alkyl side chains (molecular structure) and the ordered nanofibrillar structure (supramolecular structure) manifested in an enhanced electrical conductivity. At the same time, the value of the Seebeck coefficient was independent from these parameters. As a result, the best thermoelectric power factor was registered for P3BT, the polymer with the shortest alkyl side chain among the studied P3ATs. Selecting the most appropriate parameters from our studies, the highest achieved thermoelectric power factor (10 μW K−2 m−1 for heavily doped P3BT nanofibers) showed a notable improvement compared to the values reported earlier for this class of polymers.

As shown in Figure 9D, changes in both the electrical conductivity and Seebeck coefficient are very similar to those observed in the case of AgClO4 doping (Figure 7). Although the kinetics of the reaction differs using a different oxidant, both the Seebeck coefficient and the electrical conductivity (and hence the power factor) values are almost identical for the heavily doped polymers. Since no nanoparticle formation occurred during Fe3+ doping, the identical value of the power factor in the two cases suggests that the in situ formed Ag particles do not contribute to the thermoelectric performance.

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CONCLUSIONS As a result of our systematic study on the influence of both molecular/supramolecular structure and doping on the thermoelectric properties of P3ATs we may conclude that the 8477


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

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ACKNOWLEDGMENTS Funding support of the Hungarian Academy of Sciences through its Momentum Excellence Grant is gratefully acknowledged.

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

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ABBREVIATIONS P3HT, poly(3-hexylthiophene); SEM, scanning electron microscopy; CP, conducting polymer; P3AT, poly(3-alkylthiophene); XRD, X-ray diffraction; TEM, transmission electron microscopy; FT-IR, Fourier-transform infrared spectroscopy; PT, polythiophene; P3MT, poly(3-methylthiophene; P3BT, poly(3-butylthiophene; P3HT, poly(3-hexylthiophene; P3OT, poly(3-octylthiophene; P3DT, poly(3-dodecylthiophene

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(1) DiSalvo, F. J. Thermoelectric Cooling and Power Generation. Science 1999, 285, 703−706. (2) Zhang, Q.; Sun, Y.; Xu, W.; Zhu, D. Organic Thermoelectric Materials: Emerging Green Energy Materials Converting Heat to Electricity Directly and Efficiently. Adv. Mater. 2014, 26, 6829−6851. (3) He, M.; Qiu, F.; Lin, Z. Towards High-Performance PolymerBased Thermoelectric Materials. Energy Environ. Sci. 2013, 6, 1352− 1361. (4) Chen, Y.; Zhao, Y.; Liang, Z. Solution Processed Organic Thermoelectrics: Towards Flexible Thermoelectric Modules. Energy Environ. Sci. 2015, DOI: 10.1039/c4ee03297g. (5) McGrail, B. T.; Sehirlioglu, A.; Pentzer, E. Polymer Composites for Thermoelectric Applications. Angew. Chem., Int. Ed. 2014, DOI: 10.1002/anie.201408431. (6) Dubey, N.; Leclerc, M. Conducting Polymers: Efficient Thermoelectric Materials. J. Polym. Sci., Polym. Phys. 2011, 49, 467− 475.

ASSOCIATED CONTENT

S Supporting Information *

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

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +36 62 546-482. Tel: +36 62 546-393. Notes

The authors declare no competing financial interest. 8478


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