Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
A Planar Cyclopentadithiophene−Benzothiadiazole-Based Copolymer with sp2‑Hybridized Bis(alkylsulfanyl)methylene Substituents for Organic Thermoelectric Devices Jiae Lee,† Jaeyun Kim,§ Thanh Luan Nguyen,† Miso Kim,† Juhyung Park,§ Yeran Lee,† Sungu Hwang,∥ Young-Wan Kwon,‡ Jeonghun Kwak,*,§ and Han Young Woo*,† †
Department of Chemistry, College of Science, and ‡KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul 02841, Republic of Korea § School of Electrical and Computer Engineering, University of Seoul, Seoul 02504, Republic of Korea ∥ Department of Nanomechatronics Engineering, Pusan National University, Miryang 627-706, Republic of Korea S Supporting Information *
ABSTRACT: A semicrystalline p-type thermoelectric conjugated polymer based on a polymer backbone of cyclopentadithiophene and benzothiadiazole, poly[(4,4′-(bis(hexyldecylsulfanyl)methylene)cyclopenta[2,1-b:3,4-b′]dithiophene)-alt-(benzo[c][1,2,5]thiadiazole)] (PCPDTSBT), is designed and synthesized by replacing normal alkyl sidechains with bis(alkylsulfanyl)methylene substituents. The sp2hybridized olefinic bis(alkylsulfanyl)methylene side-chains and the sulfur−sulfur (S−S) chalcogen interactions extend a chain planarity with strong interchain packing, which is confirmed by density functional calculations and morphological studies, i.e., grazing incidence X-ray scattering measurement. The doping, electrical, morphological, and thermoelectric characteristics of PCPDTSBT are investigated by comparison with those of poly[(4,4′-bis(2-ethylhexyl)cyclopenta[2,1-b:3,4-b′]dithiophene)-alt-(benzo[c][1,2,5]thiadiazole)] (PCPDTBT) with ethylhexyl side-chains. Upon doping with a Lewis acid, B(C6F5)3, the maximum electrical conductivity (7.47 S cm−1) of PCPDTSBT is ∼1 order higher than that (0.65 S cm−1) of PCPDTBT, and the best power factor is measured to be 7.73 μW m−1 K−2 for PCPDTSBT with doping 9 mol % of B(C6F5)3. The Seebeck coefficient−electrical conductivity relation is analyzed by using a charge transport model for polymers, suggesting that the doped PCPDTSBT film has superb charge transport property based on a high crystallinity with olefinic side-chains. This study emphasizes the importance of side-chain engineering by using the sp2hybridized olefinic substituents to modulate interchain packing, crystalline morphology, and the resulting electrical properties.
1. INTRODUCTION Recently, thermoelectric (TE) materials, which can directly convert thermal energy into electricity, have attracted great attention. In particular, organic TE materials have high potential for energy harvesting and temperature sensing at low temperatures with unique advantages of facile chemical tunability and solution processability.1−4 In addition, great mechanical flexibility of organic materials enables the realization of deformable TE devices and modules applicable in wearable devices. The efficiency with which a material converts a thermal gradient to electrical energy can be described in terms of the dimensionless figure of merit, ZT = (S2σT)/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity, and T is the absolute temperature.3,5 As the carrier concentration (n) increases, typically, σ and κ increase and S decreases. Thus, to obtain high ZT and a power factor (S2σ), it is necessary to fine-tune the parameters correlated to the carrier concentration of the material. Great efforts have been devoted to enhancing the power factor of organic TE devices. The most extensively studied organic © XXXX American Chemical Society
material is poly(ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) because of its high electrical conductivity and great solution processability demanded for the facile fabrication of TE devices.6 Seebeck coefficients up to hundreds of μV K−1 for commercially available PEDOT:PSS have also been reported using post-treatments with chemical agents such as organic polar solvents [e.g., dimethyl sulfoxide (DMSO) or ethylene glycol (EG)],7−11 acids,12−14 and small molecules.15 However, a few issues, such as the hygroscopic property and counterion effects of PEDOT:PSS,16 increase the necessity of finding alternative organic TE molecules. Recently, several research groups make efforts to develop high-performance TE devices with either controlling the morphology of the film or synthesizing new p- and n-type TE polymers. For instance, in 2009, Leclerc et al. reported a TE device showing a high power factor of 19 μW m−1 K−2 by using FeCl3-doped poly[(N-9′Received: February 26, 2018 Revised: April 14, 2018
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DOI: 10.1021/acs.macromol.8b00419 Macromolecules XXXX, XXX, XXX−XXX
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C double bond. PCPDTSBT exhibits not only the extended polymer chain planarity by the olefinic bis(alkylsulfanyl)methylene group at the 4-position of CPDT but also strong interchain packing in film because of chain planarity and the sulfur−sulfur (S---S) chalcogen interaction. We investigated the electrical, morphological, and thermoelectric properties of PCPDTSBT by comparing with those of poly[(4,4′-bis(2ethylhexyl)cyclopenta[2,1-b:3,4-b′]dithiophene)-alt-(benzo[c][1,2,5]thiadiazole)] (PCPDTBT) with ethylhexyl side-chains. The higher HOMO energy level and red-shifted absorption of PCPDTSBT with the stronger shoulder peak compared to PCPDTBT suggest a stronger electron-donating ability of bis(alkylsulfanyl)methylene substituent and tighter interchain packing in PCPDTSBT. The strong (100) lamella and π−π stacking (010) scatterings in the grazing-incidence wide-angle X-ray scattering (GIWAXS) confirm the crystalline morphology with strong face-on packing interactions in the PCPDTSBT film. Upon doping with tris(pentafluorophenyl)borane (B(C6F5)3), the electrical conductivity increases by up to 4−5 orders of magnitude compared to the undoped polymer for both PCPDTSBT and PCPDTBT structures. PCPDTSBT shows electrical conductivity as high as 7.47 S cm−1 without ionic contribution and optimal power factor up to 7.73 μW m−1 K−2. In contrast, the doped PCPDTBT shows maximal electrical conductivity of 0.65 S cm−1 and power factor up to 4.41 μW m−1 K−2. Under the optimal doping conditions, the conductivity of PCPDTSBT is 10 times higher than PCPDTBT, probably owing to higher carrier mobility due to chain planarity and strong interchain packing. We also analyzed the S−σ relation of the doped PCPDTSBT films based on the recent charge-transport model.23
heptadecanyl-2,7-carbazole)-alt-5,5′-(4′,7′-di-2-thienyl-2′,1′,3′benzothiadiazole)] (PCDTBT).17 In 2014, Zhu’s group investigated the TE performance of donor−acceptor (D−A) copolymers and showed the performance was comparable to well-known donor polymers, poly(3-hexylthiophene) (P3HT) and poly(2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT-C12).18 Chabinyc’s group recently reported a remarkably high power factor over 100 μW m−1 K−2 using doped poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2b]thiophene) (PBTTT-C14) thin films. Through exposure of the polymer film to (tridecafluoro-1,1,2,2-tetrahydrooctyl)trichlorosilane (FTS) vapor or immersion in 4-ethylbenzenesulfonic acid solution, they achieved a σ value of ∼103 S cm−1, a S value of ∼33 μV K−1, and a resulting power factor of ∼110 μW m−1 K−2.19 Furthermore, by doping the PBTTT-C14 films with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) in a vapor phase, the performance improved with σ = 670 S cm−1, S = 42 μV K−1, and the resulting power factor of 120 μW m−1 K−2, respectively.20 In 2017, Katz et al. introduced several TE polymers possessing different highest occupied molecular orbital (HOMO) energy levels by modifying poly(bis(dodecyl)quaterthiophene) (PQT12) to improve the F4TCNQ doping efficiency.21 With this strategy, power factors up to 10 μW m−1 K−2 were obtained without ionic contribution. Most of the researches described above studied how dopants affected the polymer microstructure and TE properties, emphasizing the importance of fine-tuning for the optimal combination of an electrical conductivity and a Seebeck coefficient to maximize the power factor. The electrical conductivity is associated with the number of charge carriers and their mobility. The carrier concentration can be controlled by doping (e.g., chemical and electrochemical doping). However, it is hard to measure the carrier concentration accurately because of the localized charge carriers in organic molecules and polymers. Thus, the detailed investigation of TE properties in organic thin films enabled us to understand the charge-transport characteristics based on the S−σ relation.22 Meanwhile, the mobility is strongly dependent on the crystalline film morphology.23 To optimize the molecular structure of semiconducting conjugated materials for improved crystalline morphology, not only the conjugated main-chain but also solubilizing side-chains should be designed carefully. Thus, numerous efforts have been devoted to studying various flexible side-chains such as alkyl, oligoether, and fluoroalkyl chains24−28 with consideration of chain length, steric bulkiness, and branch point effects.29−31 For instance, Müllen’s group introduced the vinylene CC double bond containing side-chains onto cyclopentadithiophene (CPDT) to tune the packing structure of the resulting polymers.32 McCulloch et al. also reported an alkylidene-substituted polymer, poly[9-(1′-alkylidene)fluorene-2,7′-diyl], with enhanced field-effect mobilities of thin-film organic field-effect transistors (OFETs) compared to the alkyl-substituted counterpart, owing to its higher planarization of the conjugated polymer backbone.33 Thus, side-chain engineering plays a critical role to modulate not only the intermolecular ordering (packing) and film morphology but also charge carrier transport and device performance.24−28,34,35 In this work, we report a new TE conjugated polymer based on CPDT, poly[(4,4′-(bis(hexyldecylsulfanyl)methylene)cyclopenta[2,1-b:3,4-b′]dithiophene)-alt-(benzo[c][1,2,5]thiadiazole)] (PCPDTSBT), where the bis(alkylsulfanyl)methylene side-chains are attached via the sp2-hybridized C
2. EXPERIMENTAL SECTION 2.1. General. 1H and 13C NMR spectra were recorded using a Bruker Advance III HD system operating at 500 and 125 MHz, respectively. UV−vis spectra were obtained using a JASCO V-630 spectrophotometer. CV experiments were performed with a Versa STAT 3 analyzer. All CV measurements were carried out in 0.1 M Bu4NBF4 acetonitrile solution with a conventional three-electrode configuration, employing a platinum wire as the counter electrode, a platinum electrode coated with a thin polymer film as the working electrode, and Ag/Ag+ electrode as the reference electrode (scan rate: 50 mV s−1). Thermogravimetric analysis (TGA) was carried out in Ar atmosphere (99.999%, flow rate = 50 mL min−1) in the temperature range of 25−500 °C (heating rate = 20 °C min−1) using a Scinco TGA-N 1000. The thermal properties of two polymers were measured on a Mettler 821e differential scanning calorimeter (DSC) (Mettler, Greifensee, Switzerland). DSC measurements were performed at a heating and cooling rate of 10 °C min−1 in nitrogen (99.999% purity). Surface morphology of film was measured by AFM using Park Systems XE 100 model in a noncontact mode. GIWAXS measurements were performed at a 9A (U-SAXS) beamline at Pohang Accelerator Laboratory, Korea. Electrical conductivity was measured using the two- and four-point probe techniques with a semiconductor parameter analyzer (Keithley 2634B). Electron paramagnetic resonance (EPR) spectra were collected using a JES-FA200 X-band spectrometer (JEOL, Japan). The magnetic field was swept from 3110 to 3610 G at 0.9980 mW microwave power, and modulation amplitude was 0.5 G at 100 kHz modulation frequency. The total sweep time was 1.5 min at a constant time of 0.03 s, and the receiver gain was 1 × 1. Doping concentrations were calculated relative to a standard reference sample (4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl, TEMPOL).36 2.2. Thermoelectric Device Fabrication and Characterization. PCPDTSBT (10 mg mL−1), PCPDTBT (10 mg mL−1), and B(C6F5)3 (5 mg mL−1) were dissolved in chloroform to prepare the stock solutions. The polymer and B(C6F5)3 stock solutions were B
DOI: 10.1021/acs.macromol.8b00419 Macromolecules XXXX, XXX, XXX−XXX
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a
Reagents and reaction conditions: (i) t-BuONa, DMSO, CS2, C16H33Br, room temperature, 4 h; (ii) n-butyllithium, 2,2,6,6-tetramethylpiperidine, trimethylstannyl chloride, THF, −78 °C; (iii) Pd2(dba)3, tri(o-tolyl)phosphine, toluene, microwave. mixture was stirred at −78 °C for an additional 2 h. The reaction was quenched by adding water, and the organic layer was extracted with ether, washed with water (and brine), and dried over anhydrous MgSO4. After the solvent was removed under reduced pressure, the residue was washed several times with ethanol to give a reddish sticky liquid (yield: 85%). 1H NMR (500 MHz, CDCl3): δ (ppm) 7.93 (s, 2H), 2.99−2.98 (d, J = 5 Hz, 4H), 1.64 (m, 2H), 1.45−1.23 (m, 48H), 0.87 (m, 12H), 0.42 (s, 19H). 13C NMR (125 MHz, CDCl3): δ (ppm) 146.4, 143.8, 139.3, 135.7, 134.8, 132.9, 39.4, 38.5, 33.2, 31.9, 31.4, 30.0, 29.7, 29.6, 29.3, 26.7, 26.6, 22.7, 22.6, 14.1, 14.0. Poly[(4,4′-(bis(hexyldecylsulfanyl)methylene)cyclopenta[2,1b:3,4-b′]dithiophene)-alt-benzo[c][1,2,5]thiadiazole] (PCPDTSBT). Compound 2 (257.2 mg, 0.25 mmol), 4,7-dibromobenzo[c][1,2,5]thiadiazole (3) (73.5 mg, 0.25 mmol), tris(dibenzylideneacetone)dipalladium (0) (4 mol %), tri(o-tolyl)phosphine (8 mol %), and chlorobenzene (2 mL) were added in a 5 mL microwave vial. The reaction mixture was heated at 80 °C (5 min), 100 °C (5 min), 140 °C (5 min), and 160 °C (60 min) in a microwave reactor. The reaction mixture was cooled down to room temperature, and the polymer was end-capped by addition of 2-tributylstannylthiophene (25 mg, 0.07 mmol); the mixture was further reacted at 140 °C for 20 min. The solution was cooled down, and 2-bromothiophene (45 mg, 0.28 mmol) was added by syringe. The reaction solution was heated at 140 °C for another 20 min. After the reaction was finished, the crude polymer was precipitated into the methanol and further purified by Soxhlet extraction with acetone, hexane, and chloroform. The PCPDTSBT polymer was filtered and dried under vacuum (yield: 80%). 1H NMR (500 MHz, C6D5Cl, 100 °C): δ (ppm) 9.0 (br, 2H), 7.7 (br, 2H), 3.4 (br, 4H), 2.0−0.5 (br, 62H).
mixed with changing the dopant concentration (0−34 mol %) at 45 °C for 12 h before spin-casting onto the precleaned glass substrates at 2000 rpm for 50 s. Finally, Au electrodes (50 nm) were deposited by thermal evaporation under vacuum Et σE(E , T ) = ⎨ 0 ⎝ kBT ⎠ ⎪ ⎪ 0, E < Et ⎩
where σE0(T) is a transport coefficient, Et is a transport edge, and s is a transport parameter. Using this charge transport model for the Seebeck coefficient, the S−σ relationship can be obtained (see ref 23 for details). Figure 4 plots the experimental
Figure 4. Seebeck coefficient−electrical conductivity relation of doped PCPDTSBT films (circles with error bars) and their curve fits with the parameters given in the figure.
values of S versus σ (circles) of the B(C6F5)3-doped PCPDTSBT films, and their curve fits to the above model (dashed lines). We fixed the parameter s = 3 (i.e., appropriate for most of polymers) and found σE0 that fits the data. The optimal transport coefficient for all the experimental data points was calculated to be σE0 = 2.4 × 10−3 S cm−1. This value is comparable to or slightly higher than those of the previously reported conducting polymers;23 for instance, P3HT and polyacetylene have σE0 around 10−5−10−3 S cm−1 depending on the film conditions, while PBTTT-C12 and poly[(4,4′-bis(2ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl] (PSBTBT) have σE0 around 10−4−10−3 F
DOI: 10.1021/acs.macromol.8b00419 Macromolecules XXXX, XXX, XXX−XXX
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Figure 5. Two-dimensional GIWAXS images of (a) PCPDTSBT and (b) PCPDTBT and line-cut profiles of (c) PCPDTSBT and (d) PCPDTBT with different dopant concentrations.
S cm−1. Doped poly[(2,5-bis(2-hexyldecyl)-2,3,5,6-tetrahydro3,6-dioxopyrrolo[3,4-c]pyrrole-1,4-diyl)-alt-((2,2′:5′,2″-terthiophene)-5,5″-diyl)] (PDPP3T) showed an exceptionally high σE0 around 10−3−10−2 S cm−1. As shown in Figure 4, the transport coefficients of the doped PCPDTSBT films are ranging from 1 × 10−3 to 3 × 10−2 S cm−1, indicating that this material has superb charge transport ability due to its chain planarity and well-ordered packing structure. 3.3. Morphology Study. The charge carrier transport properties in organic semiconductors are closely related to their thin film morphology. The solid film morphology was investigated with changing [B(C6F5)3] by atomic force microscopy (AFM) and grazing-incidence wide-angle X-ray scattering (GIWAXS). As shown in Figure S6, the AFM topography images of undoped PCPDTSBT and PCPDTBT films show smooth surfaces with root-mean-square (RMS) roughness of 1.06 and 1.66, respectively. Upon addition of B(C6F5)3, the RMS roughness of PCPDTSBT increases slightly, showing 1.07 and 1.47 nm at 9 and 34 mol % B(C6F5)3, respectively. In contrast, the doped PCPDTBT shows significantly increased roughness of 1.91 and 7.31 nm at [B(C6F5)3] = 9 and 34 mol %, respectively. The surface roughness of PCPDTBT with B(C6F5)3 is significantly higher than that of the doped PCPDTSBT film. Two-dimensional GIWAXS images and the corresponding line-cut profiles with the packing parameters are shown in Figure 5 and Table S3. The pristine PCPDTSBT film shows a strong in-plane (IP) (100) lamellar peak at qxy = 0.25 Å−1 (d = 25.1 Å) and π−π stacking (010) peak at qz = 1.66 Å−1 (d = 3.78 Å) in the out-ofplane (OOP) direction, suggesting a face-on orientation of
polymer chains. The PCPDTBT film also shows a similar faceon ordering with a (010) scattering at qz = 1.58 Å−1 (d = 3.97 Å) in the OOP direction. Owing to the pronounced chain planarity of PCPDTSBT with bis(alkylsulfanyl)methylene substituents, the higher crystallinity and tighter π−π stacking were measured for PCPDTSBT compared to PCPDTBT. In the PCPDTSBT thin films with the addition of B(C6F5)3 dopant, the crystalline ordering was disrupted negligibly with [B(C6F5)3] = 9−34 mol %, showing similar GIWAXS images with the lamella spacing of 25.1−27.3 Å and π−π stacking distance of 3.67−3.69 Å. Interestingly, the OOP (010) peak of the doped PCPDTSBT films shifts to larger q values (1.66 → 1.71 Å−1), indicating the tighter π−π stacking in the doped state, which also contributes to improving the charge carrier transport and electrical conductivity (by ∼105 times) of the doped PCPDTSBT film compared to the pristine film. In the doped state, the Coulombic interaction between the positively charged conjugated polymer and negatively charged dopant may tighten the interchain packing structures of doped polymers. The scattering intensity becomes weak at [B(C6F5)3] = 34 mol % owing to the film quality with partial precipitation. The crystal coherence length (CCL) was also calculated for the pristine and doped films for both polymer films.45,46 The PCPDTSBT pristine film shows CCL = 26.0 Å based on the OOP (010) peak, and doped PCPDTSBT shows increased CCL of 27.2 and 30.1 Å at [B(C6F5)3] = 9 and 23 mol %, respectively. The enhancement of interchain interactions with the shortened π−π stacking distances in the doped films improves the crystallinity, as revealed by the decrease in the full width at half-maximum of the (010) peak. In contrary, in the G
DOI: 10.1021/acs.macromol.8b00419 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules case of the doped PCPDTBT films, the interchain packing was significantly disturbed by addition of B(C6F5)3 where the π−π stacking becomes isotropic with random orientation of polymer chains, showing the OOP (010) peak in undoped film turns to a hump-shape peak with [B(C6F5)3] = 34 mol %. The pristine PCPDTBT film shows CCL = 17.3 Å, which is much smaller than that (26.0 Å) of pristine PCPDTSBT film. With [B(C6F5)3] = 9 and 23 mol %, the doped PCPDTBT films showed the decreased CCL of 14.6−16.8 Å compared to the pristine film. The GIWAXS morphological data show a good agreement with the measured electrical properties. In the excess doping (34 mol %) of PCPDTBT, the CCL decreased further (13.6 Å) with the poor film quality. The GIWAXS data confirm an important role of the bis(alkylsulfanyl)methylene side-chains in the determination of interchain packing and the resulting film morphology. It is expected that the tighter interchain packing in PCPDTSBT may hinder the penetration of B(C6F5)3 dopants into the polymer matrix which induces the lower polaron doping concentration in PCPDTSBT compared to PCPDTBT. In addition, doping concentration should be optimized carefully according to the structures of semiconducting polymers and dopants by considering the film quality.47,48
Author Contributions
J.L., J.K., and T.L.N. contributed equally to this work. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Research Foundation (NRF) of Korea (2016M1A2A2940911, 2015R1D1A1A09056905, 2015M1A2A2057506, and 2017R1C1B2010776).
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(1) Poehler, T. O.; Katz, H. E. Prospects for polymer-based thermoelectrics: state of the art and theoretical analysis. Energy Environ. Sci. 2012, 5, 8110−8115. (2) McGrail, B. T.; Sehirlioglu, A.; Pentzer, E. Polymer composites for thermoelectric applications. Angew. Chem., Int. Ed. 2015, 54, 1710− 1723. (3) Bubnova, O.; Crispin, X. Towards polymer-based organic thermoelectric generators. Energy Environ. Sci. 2012, 5, 9345−9362. (4) Russ, B.; Glaudell, A.; Urban, J. J.; Chabinyc, M. L.; Segalman, R. A. Organic thermoelectric materials for energy harvesting and temperature control. Nat. Rev. Mater. 2016, 1, 16050. (5) Wan, C.; Gu, X.; Dang, F.; Itoh, T.; Wang, Y.; Sasaki, H.; Kondo, M.; Koga, K.; Yabuki, K.; Snyder, G. J.; Yang, R.; Koumoto, K. Flexible n-type thermoelectric materials by organic intercalation of layered transition metal dichalcogenide TiS2. Nat. Mater. 2015, 14, 622. (6) Elschner, A.; Kirchmeyer, S.; Lovenich, W.; Merker, U.; Reuter, K. PEDOT: Principles and Applications of an Intrinsically Conductive Polymer; CRC Press: 2010. (7) Alemu, D.; Wei, H.-Y.; Ho, K.-C.; Chu, C.-W. Highly conductive PEDOT: PSS electrode by simple film treatment with methanol for ITO-free polymer solar cells. Energy Environ. Sci. 2012, 5, 9662−9671. (8) Luo, J.; Billep, D.; Waechtler, T.; Otto, T.; Toader, M.; Gordan, O.; Sheremet, E.; Martin, J.; Hietschold, M.; Zahn, D. R.; Gessner, T. Enhancement of the thermoelectric properties of PEDOT: PSS thin films by post-treatment. J. Mater. Chem. A 2013, 1, 7576−7583. (9) Van Reenen, S.; Scheepers, M.; van de Ruit, K.; Bollen, D.; Kemerink, M. Explaining the effects of processing on the electrical properties of PEDOT: PSS. Org. Electron. 2014, 15, 3710−3714. (10) Xiong, J.; Jiang, F.; Zhou, W.; Liu, C.; Xu, J. Highly electrical and thermoelectric properties of a PEDOT: PSS thin-film via direct dilution−filtration. RSC Adv. 2015, 5, 60708−60712. (11) Zhu, Z.; Liu, C.; Jiang, Q.; Shi, H.; Xu, J.; Jiang, F.; Xiong, J.; Liu, E. Green DES mixture as a surface treatment recipe for improving the thermoelectric properties of PEDOT: PSS films. Synth. Met. 2015, 209, 313−318. (12) Bae, E. J.; Kang, Y. H.; Jang, K.-S.; Cho, S. Y. Enhancement of thermoelectric properties of PEDOT: PSS and tellurium-PEDOT: PSS hybrid composites by simple chemical treatment. Sci. Rep. 2016, 6, 18805. (13) Kim, J.; Jang, J. G.; Hong, J.-I.; Kim, S. H.; Kwak, J. Sulfuric acid vapor treatment for enhancing the thermoelectric properties of PEDOT: PSS thin-films. J. Mater. Sci.: Mater. Electron. 2016, 27, 6122−6127. (14) Wang, J.; Cai, K.; Shen, S. A facile chemical reduction approach for effectively tuning thermoelectric properties of PEDOT films. Org. Electron. 2015, 17, 151−158. (15) Tomlinson, E. P.; Willmore, M. J.; Zhu, X.; Hilsmier, S. W.; Boudouris, B. W. Tuning the thermoelectric properties of a conducting polymer through blending with open-shell molecular dopants. ACS Appl. Mater. Interfaces 2015, 7, 18195−18200. (16) Wang, H.; Ail, U.; Gabrielsson, R.; Berggren, M.; Crispin, X. Ionic Seebeck effect in conducting polymers. Adv. Energy Mater. 2015, 5, 1500044.
4. CONCLUSION In conclusion, we synthesized a new p-type TE polymer, PCPDTSBT, and characterized its thermoelectric characteristics. Based on CPDT and BT moieties with sp2-hybridized bis(alkylsulfanyl)methylene side-chains, the PCPDTSBT films showed the higher crystallinity owing to the extended chain planarity with the stronger interchain packing compared to PCPDTBT with normal ethylhexyl substituents. Also, the PCPDTSBT films doped with the Lewis acid B(C6F5)3 exhibited similar or even higher interchain ordering. As a result, the electrical conductivity and thermoelectric properties of the doped PCPDTSBT films were drastically improved by doping without ionic contribution. We also analyzed the Seebeck coefficient−electrical conductivity relation and found that PCPDTSBT has superb charge transport properties comparable to or higher than other reported polymers. This study shows that the optimization of the electronic structure and crystalline morphology of conjugated polymers are important to achieve high electrical and thermoelectric characteristics. On the basis of the results, we believe that the sp2-hybridized bis(alkylsulfanyl)methylene side-chains can be a good strategy for the molecular design of p- and n-type thermoelectric materials.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00419. DFT calculation, additional GIWAXS, and device characterization data (PDF)
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REFERENCES
AUTHOR INFORMATION
Corresponding Authors
*(H.Y.W.) E-mail
[email protected]. *(J.K.) E-mail
[email protected]. ORCID
Han Young Woo: 0000-0001-5650-7482 H
DOI: 10.1021/acs.macromol.8b00419 Macromolecules XXXX, XXX, XXX−XXX
Article
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DOI: 10.1021/acs.macromol.8b00419 Macromolecules XXXX, XXX, XXX−XXX
