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Kirigami-Inspired Conducting Polymer Thermoelectrics from Electrostatic Recognition Driven Assembly Ying-Shi Guan, Haoqi Li, Fei Ren, and Shenqiang Ren ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b02489 • Publication Date (Web): 09 Jul 2018 Downloaded from http://pubs.acs.org on July 10, 2018

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Kirigami-Inspired Conducting Polymer Thermoelectrics from Electrostatic Recognition Driven Assembly Ying-Shi Guan1,2, Haoqi Li3, Fei Ren3, Shenqiang Ren1,2* 1

Department of Mechanical and Aerospace Engineering, University at Buffalo, The State

University of New York, Buffalo, NY, 14260, USA 2

Research and Education in eNergy, Environment and Water (RENEW) Institute, University at

Buffalo, The State University of New York, Buffalo, NY, 14260, USA 3

Department of Mechanical Engineering, Temple University, Philadelphia, PA 19122, USA

* E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT: Two-dimensional (2D) conducting polymers are expected to offer emergent topological, structural, and physical properties, which has become the "holy grail" for the development of plastic electronics. Here we report the assembly of free-floating metallic polymer layer, consisting of poly(3,4-ethylenedioxythiophene) complexed with poly(styrene sulfonate) anions, directed by electrostatic recognition, amphiphilicity, and aromatic interactions. The obtained large-area crystalline nanosheets exhibit excellent environmental stability and mechanical robustness, meanwhile showing an electrical conductivity of 1216 S•cm-1, the highest among the nanometer-thick conducting polymers. The kirigami-inspired freestanding polymer thermoelectrics, repeatedly stretching up to 200% strain, is demonstrated with high Seebeck coefficient with an optimized power factor of 95 µWm-1K-2. The large-scale assembly and aqueous compatibility of 2D conducting polymers provide an exciting platform for integrating thermoelectricity into free-floating polymer nanostructures.

KEYWORDS: freestanding, polymer nanosheet, self-assembly, thermoelectrics, kirigami.


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Solution-processable and air-stable conducting polymers are a critical class of materials capable of semiconducting,1-3 and even metallic transport properties.4-6 However, the ‘metallic state’ of the conducting polymer is still not typical due to the combination of structural inhomogeneity and molecular disorder. The electrical conductivity of conducting polymers varies significantly subject to its oxidation levels, molecular ordering and alignment, interchain interactions, and morphology, etc.7,8 With molecular-scale disorder, the scattering can cause the electronic states to become localized near the Fermi level and lead to a metal-to-insulator transition, as defined by the Anderson localization.9 And therefore, most conducting polymers show semiconducting behaviors at room temperature and the transport properties are governed by thermally activated hopping.10 High-aspect-ratio conducting polymer is a significantly important geometry for the confinement-enabled exotic structural and physical properties, due to their extended lateral dimensions and nanoscale thickness.11,12 However, the conjugated polymers frequently have imperfect ordering due to their entangled chains, leading to an electronic structure that relies strongly on their morphology. The spontaneous assembly of two-dimensional (2D)-like nanostructures can rationally control the folding of conjugated polymers that order the entangled chains into hierarchically functional structures.13 The predominant assembly route to prepare these 2D polymer materials relies on liquid/liquid interfacial assembly, driven by surface tension induced spontaneous spreading. The advantage of interfacial assembly is that it can provide a defect-free surface as the substrate, and control over the morphology and thickness of the twodimensional film by tuning the area of spreading surface and the concentration of polymer solution. In addition, it is a highly reproducible approach for the assembly of the polymer materials. 14,15


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Despite ongoing advances, the requirement of solution spreading and miscibility is still a significant challenge for the assembly of water-soluble polymers, such as poly(3,4ethylenedioxythiophene) complexed with poly(styrene sulfonate) anions (PEDOT:PSS). The water-soluble PEDOT:PSS conducting polymer, selected for this study, is well known for its high conductivity, high transparency, and excellent stability in air, being regarded as the promising candidate for next-generation polymer thermoelectrics.16-21 Here we present an electrostatic recognition assisted assembly of water-soluble conducting polymer for the preparation of free-floating crystalline layers. The obtained polymer nanosheets show molecular ordering and uniform morphology, demonstrating its practicality for the device fabrication. The hydrochloric acid modulated electrostatic interactions enable a large-area free-floating PEDOT:PSS nanosheet formation (~ 1 cm2), which shows excellent environmental stability and robust mechanical property. Meanwhile, it exhibits a high electrical conductivity of 1,216 S·cm-1 after treatment with the sulfuric acid, among the highest conductivity reported in the nanometerthick freestanding conducting polymer films, which can serve as the flexible and robust polymer conductors in organic electronics, as well as polymer thermoelectrics that interconvert heat and electricity. A high Seebeck coefficient of PEDOT:PSS nanosheets is therefore shown, achieving an optimized power factor of 95 µWm-1K-2. The excellent stretchability of PEDOT:PSS thermoelectrics is demonstrated using the kirigami-inspired structure by repeatedly stretching up to 200% strain.22,23 RESULTS AND DISCUSSION The free-floating PEDOT:PSS nanosheets are prepared by adding 4M hydrochloric acid (HCl) dispersion into the aqueous PEDOT:PSS solution at room temperature (Figure 1a). The formation of freestanding PEDOT:PSS nanosheets can be explained as follows. The


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PEDOT:PSS consists of hydrophobic PEDOT and hydrophilic PSS components which enables the solubility in the aqueous solution. The PSS acts as both a dispersant of PEDOT and a counter anion for the PEDOT backbone, resulting in a molecular-level PEDOT:PSS structure. These two components bond together through the electrostatic interactions that lead PEDOT:PSS to form a dispersion in water. After adding HCl into the PEDOT:PSS dispersion, the displacement reaction takes place according to the following equation: PEDOT:PSS + HCl → PEDOT:Cl + PSS:H That’s to say, the PSS chains can be protonated in the hydrochloric acid solution, which would lead to the weaker electrostatic interaction between PEDOT and PSS domains. Due to the solubility of PSS in water, part of the periphery PSS can be removed from the PEDOT:PSS complex and dissolve into the aqueous solution, resulting in the exposure of hydrophobic PEDOT group to water. To minimize the exposure of hydrophobic groups to water, the PEDOT:PSS polymer chains tend to align and assemble in the layered nanostructures. The strong π-π stacking nature of PEDOT induces PEDOT:PSS for a particular lamella stacking between two distinct alternate orderings of PEDOT and PSS chains.19 The enhanced hydrophobic and strong π-π stacking interactions between PEDOT molecules induce the ordered assembly, leading to the formation of the 2D nanosheets. The key interaction to grow free-floating PEDOT:PSS nanosheets is the acid-modulated electrostatic recognitions between the PEDOT and PSS chains. The obtained freestanding PEDOT:PSS nanosheets can be transferred onto any artificial substrates for further studies. Figure S1 and Figure 1b show the optical and scanning electron microscopy (SEM) images of a large-area freestanding PEDOT:PSS nanosheet, where the partially folding sheets and wrinkles reveal its high flexibility. The X-ray diffraction (XRD) is


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carried out to gain insight into its molecular packing structures (Figure S2). The characteristic diffraction peak at 2ɵ=6.6o can be attributed to the stacking distance of the alternate orderings of the PEDOT and PSS chains,19,24 which confirm highly ordered lamella structures dominant in the freestanding PEDOT:PSS nanosheets. The transmission electron microscopy and high resolution transmission electron microscopy (HR-TEM) images (Figure 1c) further confirm its high crystalline nature within the freestanding PEDOT:PSS nanosheets. To examine the relationship between molecular ordering, crystallinity and nanomechanical response, nanoindentation is employed to study freestanding PEDOT:PSS nanosheets and the spun-cast control films (Figure 1d-1e and S3). The load-displacement curves and ultra-fast extreme property mapping (XPM) demonstrate the average Young’s modulus 3.19 GPa of freestanding PEDOT:PSS nanosheets, higher than that of the spun-cast films (1.98 GPa), confirming its highly ordered and crystalline nature through electrostatic recognition assisted assembly. Freestanding PEDOT:PSS nanosheets exhibit adjustable thickness within a large range based on the solution concentration during the aqueous assembly, where its typical thicknesses can vary from 100 nm to 1 µm. Figure 2a-2b show the thickness-dependent electrical conductivity of freestanding PEDOT:PSS nanosheets, ranging from 39 S·cm-1 to 58 S·cm-1, which is higher than that of the spun-cast control films (～1 S·cm-1). The optimum conductivity is achieved at the average layer thickness of 153 nm, which can be rationalized by Coulomb scattering26-28 from the charged impurities and a resistor network mechanism. As the thickness increases, the increased carrier concentration decreases the effect of charged impurities, leading to the rising conductivity, while the contact resistance reduces the conductivity for the further increase of the nanosheet thickness. The temperature dependent conductivity of freestanding PEDOT:PSS nanosheets is studied in the range between 100 K and 300 K (Figure 2c-2d). The conductivity is


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observed to gradually increase from 31 S·cm-1 to 58 S·cm-1 with increasing the temperature, suggesting that the charge transport is dominant by thermally activated interchain hopping probability within the freestanding PEDOT:PSS nanosheets.10 It should be noted that the acidic environment could induce the conformational change of PEDOT chains, leading to the enhancement of the interchain interactions for the physical crosslinks.19 We further select the sulfuric acid (H2SO4) to tune the crystallinity and electrical conductivity of the PEDOT:PSS nanosheets. The representative current-voltage (I-V) curves of H2SO4-treated PEDOT:PSS nanosheets are measured using two-point-probe and four-pointprobe schemes, respectively (Figure 3a). It is essential to utilize a four-terminal configuration because of the substantial contact resistance in two-terminal configuration shown in Figure 3b.25 Figure 3c shows that the four-point-probe measured conductivity of the freestanding PEDOT:PSS, which significantly increases after H2SO4 treatment. The optimum electrical conductivity of 1,216 S·cm-1 is achieved in PEDOT:PSS nanosheets under the treatment of 8M H2SO4. The freestanding PEDOT:PSS nanosheets combine high electrical conductivity and excellent mechanical durability, opening the possibility as the polymer metal conductor. Under the bending and twisting cycles, a negligible change in conductivity is observed in the circuit of light-emitting diodes. To evaluate the reversibility and reproducibility of freestanding PEDOT:PSS conductor, we further conduct the fatigue test by bending more than 500 cycles, while the current-voltage (I-V) curves has a slight change after bending and twisting cycles (Figure 3d). To understand the mechanism of the increased conductivity for H2SO4-treated PEDOT:PSS nanosheets, the resistance before and after treatment is measured from room temperature to 80 K (Figure 4a), which gradually increases as decreasing the temperature. The Arrhenius plot is


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performed in Figure 4b, where freestanding PEDOT:PSS nanosheets with and without H2SO4 treatment exhibit a temperature-activated behavior following the Arrhenius law, ln[1/R] = ln[1/R0] - Ea/kBT, Ea is the activation energy, and kB is the Boltzmann constant.6 From the Arrhenius equation, we can extract the activation energy of freestanding PEDOT:PSS nanosheets is 3.3 meV, while the H2SO4-treated freestanding PEDOT:PSS nanosheets is 2.1 meV. The reduced activation energy indicates that H2SO4 treatment enables the energy barrier for interchain and inter-domain charge hopping, improving the charge transport within PEDOT:PSS nanosheets. To further understand the acid treatment effect on electrical properties of PEDOT:PSS nanosheets, the XRD is carried out to gain insight of molecular packing structures and crystallinity (Figure 4c). After H2SO4 treatment, the XRD pattern of PEDOT:PSS nanosheets exhibits a stronger diffraction peak at 2ɵ = 6.6o and a second-order diffraction peak at 2ɵ = 12.6o, indicating its highly ordered crystalline structure after H2SO4 treatment. These observations demonstrate that H2SO4 treatment enables the structural rearrangement, in which PEDOT chains align densely to form PEDOT-rich domains. These results account for the significantly improved conductivity of H2SO4-treated freestanding PEDOT:PSS nanosheets. In addition, HRTEM (Figure 4d) and atomic force microscope (AFM) images (Figure 4e) of PEDOT:PSS nanosheets further show the visible large crystalline domains appeared after 8 M H2SO4 treatment. As depicted in Figure 4f, the weak electrostatic interaction between PEDOT and PSS lead to PSS partially be removed, which allows PEDOT molecular backbone to aggregate to form a highly conductive network channel, leading to high conductivity of H2SO4-treated freestanding PEDOT:PSS nanosheets. Polymer PEDOT:PSS material is regarded as potential thermoelectrics, due to its lightweight, flexibility, high electric and low thermal conductivities.8 The freestanding PEDOT:PSS


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nanosheets exhibit tunable electrical conductivity based on the acidic treatment. As shown in Figure 5a, the Seebeck coefficient of the freestanding PEDOT:PSS nanosheets is increased from 17µV K-1 to 29 µV K-1 when the concentration of H2SO4 solution increased from zero to 2 M and then remained unchanged when the concentration is further increased from 2 M to 8 M. As the acid treatment can change the crystallographic and morphological structures of the freestanding PEDOT:PSS nanosheets, this facilitates the migration of charge carriers relative to the temperature gradient. While further increasing the acid concentration from 2 M to 8 M, it can increase the conductivity and carrier concentration, which leads to the decrease and increase of the Seebeck coefficient, respectively. These two competing effects between the conductivity and carrier concentration result in the acid concentration-dependent Seebeck coefficient, and eventually reaching a plateau as shown in Figure 5a. Figure 5b shows that the thermoelectric power factor of freestanding PEDOT:PSS nanosheets treated with different concentrations of H2SO4. The power factor is observed to gradually increase from 1.8 µWm-1K-2 to 61 µWm-1K-2 with increasing concentrations of H2SO4. Despite the maximum Seebeck coefficient achieved with a H2SO4 concentration of 2 M, the power factor appears an upward trend as increasing the concentration of H2SO4, due to the monotonic increase trend in the conductivity. The maximum power factor measured in this study is 61 µWm-1K-2, achieved with a H2SO4 concentration of 6 M. The enhanced Seebeck coefficient and power factor obtained by the acid treatment could be potentially utilized to fabricate nanoscale temperature sensors or thermal energy harvesters for flexible and stretchable electronics. The Kirigami-inspired structure is employed as the scaffold to improve the stretchability of PEDOT:PSS thermoelectrics. The freestanding PEDOT:PSS nanosheets treated with 6 M H2SO4 are transferred onto the tracing paper and then laser cut into a Kirigami structure, where the repeatable 200% strain could be applied onto the device (Figure


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5c). The electrical conductivity and Seebeck coefficient of Kirigami PEDOT:PSS thermoelectrics show a subtle change over the entire stretching process, while the Seebeck coefficient is in the range between 32.4 µV K-1 to 35.5 µV K-1 (Figure 5d). Figure 5e illustrates the stretching effect on the power factor of the Kirigami PEDOT:PSS device, which shows a negligible change with a power factor of ~ 95 µWm-1K-2 during the entire stretching process from 0% to 200%. These results indicate that the Kirigami-based devices provide the potential in the application of flexible and stretchable thermoelectric devices. CONCLUSIONS In conclusion, highly conductive and freestanding PEDOT:PSS nanosheets are achieved through the electrostatic recognition assisted self-assembly. The 2D-like flexible nanosheets exhibit a high electrical conductivity of 1216 S cm-1, the highest value reported in 2D polymer films. Furthermore, a high Seebeck coefficient is achieved for the freestanding PEDOT:PSS nanosheets after H2SO4 treatment, showing very promising thermoelectric properties. More importantly, Kirigami-inspired PEDOT:PSS thermoelectrics enables the stretchability of 200% strain with a power factor of 95 µWm-1K-2, which can provide a platform to develop stretchable and wearable thermoelectric devices.


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METHODS Materials and instruments. PEDOT:PSS purchased from Sigma-Aldrich and used as received. The SEM images were taken from FEI Quanta450FEG. UV-Vis spectrum were recorded on an Agilent Model HP8453. I–Vcharacteristics of the freestanding PEDOT:PSS films were recorded with a Keithley 2400 semiconductor analyzer. A Bruker Apex II Duo single-crystal X-ray diffractometer was used to obtain the XRD pattern at a step of 0.1° per minute from 3° to 30°. Mechanical properties were investigated on TI 980 Triboindenter System. The fabrication of PEDOT:PSS freestanding films. The typical PEDOT:PSS freestanding film can be obtained by adding 1 mL 4 M HCl aqueous solution into 5 mL commercially available PEDOT:PSS dispersion drop by drop and shaking 3-5 mins at room temperature. Then, the freestanding PEDOT:PSS films can be observed in the aqueous solution. The obtained freefloating PEDOT:PSS film can be transferred onto many kinds of substrates such as glass, silicon, tracing paper and so on for further studies. The transfer process is illustrated in the scheme 1.in supporting information. Acid Treatment of PEDOT:PSS freestanding films. PEDOT:PSS freestanding films obtained above were transferred onto glass, silicon substrates, and then were immersed into H2SO4 solutions with different concentrations for 12 h at room temperature. Then, the films were washed in a deionized water bath and dried at 120oC for 30 mins to remove residual water completely. The silver electrodes were evaporated onto the dried film deposited on glass substrates using a shadow mask for I-V test. The dried films on silicon substrates were used for SEM, XRD, AFM measurements.


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The freestanding PEDOT:PSS films were transferred into H2SO4 solutions directly using pipet with large diameter aperture from the as-formed freestanding film solution, then transferred into deionized water bath for washing. The films were then transferred onto the TEM grid for TEM characterization using the transfer process illustrated in scheme 1. The films on TEM grid were dried at 120oC for 30 mins to completely remove the residual water before the TEM study. The fabrication of polymer conductor based on PEDOT:PSS. The freestanding PEDOT:PSS film treated with 8 M H2SO4 solution was deposited on PDMS (Polydimethylsiloxane), then dried naturally overnight in the ambient conditions. After completely dried, another layer PDMS was laminated on the top of the PEDOT:PSS film for the protection. The Schematic representation of an commercial LED device bridged by PEDOT:PSS film conductor to the power source is shown in scheme 2 in supporting information. The fabrication of kirigami devices. The PEDOT:PSS films treated with H2SO4 were transferred onto the tracing paper. Then, the tracing paper covered PEDOT:PSS film were dried 72 h naturally in air. All kirigami samples were fabricated using laser cut (EPILOG LASER 40 W) to generate the patterned cuts. Electrical characterization of the PEDOT:PSS freestanding Films. Electrical conductivity of the PEDOT:PSS freestanding films were measured by the standard four-point probe method using Keithley 2400 semiconductor analyzer. Seebeck coefficient measurement. Seebeck coefficient (thermopower) was measured using an in-house apparatus. The PEDOT:PSS freestanding films were deposited on the glass slides, which were placed on top of two Peltier stages. The temperature was detected by thermocouples and thermovoltage across the samples was tested.


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ASSOCIATED CONTENT The authors declare no competing financial interest. Supporting Information. The supporting information is available free of charge. AUTHOR INFORMATION Corresponding Author E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEGMENTS The U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering supports S.R. under Award DE-SC0018631.

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Transparent Electrode of Polymer Solar Cells. ACS Appl. Mater. Interfaces 2016, 8, 1162911638. 18. Xia, Y.; Ouyang, J. Significant Conductivity Enhancement of Conductive Poly(3,4ethylenedioxythiophene):Poly(styrenesulfonate) Films through a Treatment with Organic Carboxylic Acids and Inorganic Acids. ACS Appl. Mater. Interfaces 2010, 2, 474-483. 19. Kim, N.; Kee, S.; Lee, S. H.; Lee, B. H.; Kahng, Y. H.; Jo, Y. R.; Kim, B. J.; Lee, K. Highly Conductive PEDOT:PSS Nanofibrils Induced by Solution-Processed Crystallization. Adv. Mater. 2014, 26, 2268-2272. 20. Yeon, C.; Yun, S. J.; Kim, J.; Lim, J. W. PEDOT:PSS Films with Greatly Enhanced Conductivity via Nitric Acid Treatment at Room Temperature and Their Application as Pt/TCOFree Counter Electrodes in Dye Sensitized Solar Cells. Adv. Electron. Mater. 2015, 1, 1500121. 21. Worfolk, B. J.; Andrews, S. C.; Park, S.; Reinspach, J.; Liu, N.; Toney, M. F.; Mannsfeld, S. C. B.; Bao, Z. Ultrahigh Electrical Conductivity in Solution-Sheared Polymeric Transparent Films. Proc. Nat. Acad. Sci. USA. 2015, 112, 14138-14143. 22. Tang, Y.; Lin, G.; Yang, S.; Yi, Y. K.; Kamien, R. D.; Yin, J. Programmable Kiri-Kirigami Metamaterials. Adv. Mater. 2017, 29, 1604262. 23. Shyu, T. C.; Damasceno, P. F.; Dodd, P. M.; Lamoureux, A.; Xu, L.; Shlian, M.; Shtein, M.; Glotzer, S. C.; Kotov, N. A. A Kirigami Approach to Engineering Elasticity in Nanocomposites through Patterned Defects. Nat. Mater. 2015, 14, 785-789. 24. Kim, N.; Lee, B. H.; Choi, D.; Kim, G.; Kim, H.; Kim, J. R.; Lee, J.; Kahng, Y. H.; Lee, K. Role of Interchain Coupling in the Metallic State of Conducting Polymers. Phys. Rev. Lett. 2012, 109, 106405.


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25. Alves, H.; Molinari, A. S.; Xie, H.; Morpurgo, A. F. Metallic Conduction at Organic ChargeTransfer Interfaces. Nat. Mater. 2008, 7, 574-580. 26. Li, S.-L.; Wakabayashi, K.; Xu, Y.; Nakaharai, S.; Komatsu, K.; Li, W.-W.; Lin, Y.-F.; Aparecido-Ferreira, A.; Tsukagoshi, K. Thickness-Dependent Interfacial Coulomb Scattering in Atomically Thin Field-Effect Transistors. Nano Lett. 2013, 13, 3546-3552. 27. Joo, M.-K.; Moon, B. H.; Ji, H.; Han, G. H.; Kim, H.; Lee, G.; Lim, S. C.; Suh, D.; Lee, Y. H. Understanding Coulomb Scattering Mechanism in Monolayer MoS2 Channel in the Presence of h-BN Buffer Layer. ACS Appl. Mater. Interfaces 2017, 9, 5006-5013. 28. Gámiz, F.; López‐Villanueva, J. A.; Jiménez‐Tejada, J. A.; Melchor, I.; Palma, A. A Comprehensive Model for Coulomb Scattering in Inversion Layers. J. Appl. Phys. 1994, 75, 924.


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Figure 1. The growth and characterization of the freestanding PEDOT:PSS nanosheet. (a) The schematic diagram for the formation of the freestanding PEDOT:PSS nanosheets. (b) The SEM image of PEDOT:PSS nanosheets. (c) The TEM image of PEDOT:PSS nanosheets. Inset: high resolution TEM image of PEDOT:PSS nanosheets. (d) The load-displacement curves of freestanding PEDOT:PSS nanosheets and the spun-cast control film. (e) The ultra-fast extreme property mapping (XPM) images of freestanding PEDOT:PSS nanosheets.


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Figure 2. The electric characteristics of PEDOT:PSS nanosheet. (a) The current-voltage (I-V) curve of freestanding PEDOT:PSS nanosheet with the thickness of 121 nm. Inset: the SEM image of the device. (b) The thickness-dependent conductivity of freestanding PEDOT:PSS nanosheets. (c) The I-V curves of freestanding PEDOT:PSS nanosheets with the thickness of 153 nm at different temperature. (d) The temperature-dependent conductivity of PEDOT:PSS nanosheets with the thickness of 153 nm.


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Figure 3. The acid treatment effect on electric characteristics of PEDOT:PSS nanosheet. (a) The I-V curves of freestanding PEDOT:PSS nanosheets treated with 8M H2SO4 measured using 2-points and 4-points method. (b) The R-V curves of freestanding PEDOT:PSS nanosheets treated with 8M H2SO4 measured using 2-points and 4-points method. (c) The conductivities of freestanding PEDOT:PSS nanosheets treated with different concentration of H2SO4. (d) Cycle tests of freestanding PEDOT:PSS nanosheets treated with 8M H2SO4. The inset shows the demonstration of freestanding PEDOT:PSS nanosheets as the metal conductor for LED devices (The bending curvature of PEDOT:PSS film is about 0.67 cm-1and the twisting angle is about 60˚).


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Figure 4. The mechanistic understanding of the acid-treated PEDOT:PSS nanosheet. (a) The temperature-dependent resistance of the freestanding PEDOT:PSS nanosheets before and after treatment of H2SO4 (8 M). (b) Arrhenius fitting for freestanding PEDOT:PSS nanosheets before and after treatment of H2SO4 (8 M). (c) The XRD pattern of PEDOT:PSS nanosheets treated with 8 M H2SO4. (d-e) High resolution TEM and AFM images of freestanding PEDOT:PSS nanosheets treated with 8 M H2SO4, respectively. (f) The schematic illustration of H2SO4 treatment effect on PEDOT:PSS nanosheets.


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Figure 5. The thermoelectric performance of PEDOT:PSS nanosheet. (a-b) The Seebeck coefficient and power factor of freestanding PEDOT:PSS nanosheets treated with different concentrations of H2SO4. (c) Photographs of the deformed kirigami patterned freestanding PEDOT:PSS nanosheets at different strain, and the schematic thermoelectric device using kirigami freestanding PEDOT:PSS nanosheet. (d) The Seebeck coefficient and conductivity of the kirigami patterned freestanding PEDOT:PSS nanosheets at different strain. (e) The power factor of the kirigami patterned freestanding PEDOT:PSS nanosheets at different strain.


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