High-Performance PEDOT:PSS Flexible Thermoelectric Materials and

Jun 28, 2019 - Searching an effective method to enhance the thermoelectric properties of flexible organic films can significantly widen the applicatio...
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High-Performance PEDOT:PSS Flexible Thermoelectric Materials and Their Devices by Triple Post-Treatments Shengduo Xu,† Min Hong,∥ Xiao-Lei Shi,† Yuan Wang,∥ Lei Ge,∥ Yang Bai,‡ Lianzhou Wang,‡ Matthew Dargusch,† Jin Zou,*,†,§ and Zhi-Gang Chen*,†,∥ Materials Engineering, ‡Nanomaterials Centre, School of Chemical Engineering and Australian Institute for Bioengineering and Nanotechnology, and §Centre for Microscopy and Microanalysis, The University of Queensland, Brisbane, Queensland 4072, Australia ∥ Centre for Future Materials, University of Southern Queensland, Springfield Central, Queensland 4300, Australia Downloaded via IDAHO STATE UNIV on July 18, 2019 at 06:11:55 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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ABSTRACT: Searching an effective method to enhance the thermoelectric properties of flexible organic films can significantly widen the application of flexible thermoelectric devices. Tuning the microstacking structure and oxidation level can effectively optimize the thermoelectric properties of poly(3,4-ethylenedioxithiophene):poly(styrenesulfonate) (PEDOT:PSS) organic films. Here, we adopt triple post-treatments with formamide (CH3NO), concentrated sulfuric acid (H2SO4), and sodium borohydride in sequence to engineer flexible PEDOT:PSS thermoelectric films. A high power factor of 141 μWm−1 K−2 at 25 °C has been obtained for the PEDOT:PSS film. Such a high power factor stems from the high σ (1786 Scm−1) and S (28.1 μVK−1) after posttreatment with CH3NO, H2SO4, and NaBH4 in order. The increased carrier mobility resulting from both the selective removal of excess insulating PSS within the films and the conformation transition after CH3NO and H2SO4 treatments is responsible for the enhancement of σ, while the subsequent NaBH4 treatment optimize the electrical properties (σ and S) by modulating the oxidation level. A homemade thermoelectric device has also been fabricated using the as-prepared flexible PEDOT:PSS films and had a high output power density of ∼1 μWcm−2 with human arm as a heating source. This study indicates that flexible thermoelectric devices based on cheap conducting polymers have great potential in wearable electronics.

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polymers present low intrinsic κ (0.2−0.5 Wm−1 K−1), which basically is independent from the carrier concentration9 This unique characteristic uncouples the complicated interdependency between the electrical properties (σ and S) and κ, leaving a possibility to achieve an enhanced zT. The complex of poly(3,4-ethylenedioxithiophene) (PEDOT) and poly(styrenesulfonate) (PSS) in which PSS serves as both a counterion and a soluble template for PEDOT, is regarded as one of the promising conducting polymers.25 This favorable conducting polymer (CP) is suitable for film fabrication since it is generally fabricated in the form of dispersion solution, where hydrophobic PEDOT-rich grains are encapsulated by hydrophilic PSS-rich shells.26 However, its TE performance is poor due to the low values of σ (0.1−10 Scm−1) and S (∼12 μVK−1). Such a low σ is linked to the low carrier mobility (μ) of ∼0.045 cm2 V−1 s−1 caused by the excess PSS, which increases the tunneling distance of charge carriers between the conductive host molecules, that is, the PEDOT chains with quinoid structures and thus significantly depresses the thermally activated hopping of the charge carriers.27 Accordingly, enhancing the TE performance of the

INTRODUCTION Thermoelectric (TE) technology, enabling the direct conversion between temperature gradient and electricity, can generate electricity using the waste heat from factories, automobile engines, and even human bodies.1−5 In this technology, materials that can produce an electrical voltage from temperature gradient are known as TE materials, including conventional inorganic semiconductors6−8 and the emerging conducting polymers.9−11 The performance of a TE conversion is evaluated by the dimensionless figure of merit (zT), defined as zT = S2σT/κ, where σ is the electrical conductivity, S represents the Seebeck coefficient, κ denotes the thermal conductivity, and T is the absolute temperature.12−14 Power factor (S2σ) is also used to evaluate the ability of a TE material to generate power.15−17 To date, among the states of the art inorganic TE materials,18−20 Bi2Te3-based alloys with zT > 1 are the most promising TE materials at room temperature.21,22 In addition to the conventional inorganic TE materials, conductive polymers have been regarded as new-generation TE materials because of their unique advantages, such as low cost, high mechanical flexibility, good environmental stability, and nontoxicity.23 A recent work has reported that thermoelectric devices based on conducting polymers can be fabricated by a large-area printing technique.24 In terms of their TE performance, conductive © XXXX American Chemical Society

Received: April 16, 2019 Revised: June 27, 2019 Published: June 28, 2019 A

DOI: 10.1021/acs.chemmater.9b01500 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

Figure 1. Measured S and σ at 25 °C of PEDOT:PSS films treated using (a) CH3NO within different durations; (b) CH3NO with different treating frequencies, each of which lasts for 15 min; (c) CH3NO for 15 min two times and with H2SO4 for different durations; (d) CH3NO for 15 min two times and with H2SO4 for 10 h; and with NaBH4 with various concentrations, sequentially.

PEDOT:PSS film can be achieved by two post-treatment procedures. First, using polar solvents in post-treatments, for example, formamide (CH3NO)28 and sulfuric acid (H2SO4),23 to selectively remove the excess dissociative hydrophilic PSS to increase σ. In particular, the concentrated H2SO4 functions even better, leading to a high σ value of 4380 Scm−1 due to its strong ability to significantly improve the crystallinity of the PEDOT:PSS thin film.25 Notably, the S basically remains identical during these post-treatments.23,28,29 Then, a subsequent post-treatment with reducing agents, for example, sodium hydroxide (NaOH),23 hydrazine (N2H4),30 and sodium borohydride (NaBH4),30 is implemented to improve the S by reducing the oxidation level of the sample. The second step will sacrifice the σ to some extent. Therefore, there should be an optimal σ and S for a peak power factor. Moreover, it seems that the higher the σ of the sample after the first experimental step is, the higher the peak power factor will be.23,31 An effective method for a higher σ after the first step could be a key to a better TE performance. Generally, PEDOT:PSS films are fabricated by spincoating,23 drop-casting,29 and printing.32 Although a thin film fabricated by spin-coating had led to a reported zT as high as 0.42,27 such a film with a thickness of only ∼100 nm is too fragile for large-scale fabrication. Favorably, drop-casting and printing can produce a micrometer scale (1−10 μm)-thick film that is robust for large-scale fabrication. However, the currently obtained TE performances of such thick films are much lower than that of the spin-coated thin film.27,31 Therefore, it is of great potential to optimize the TE properties of the thick film via effective structure engineering. Herein, we apply triple post-treatments with CH3NO, concentrated H2SO4, and NaBH4 to tune the TE performance of thick PEDOT:PSS film prepared via drop-casting. The treated PEDOT:PSS film shows an optimized S2σ of 141 μWm−1 K−2 with σ of 1786 Scm−1 and S of 28.1 μVK−1 at

room temperature. Moreover, a homemade device based on those treated films exhibits a peak output power density of ∼1 μWcm−2 using the human arm as a heating source.

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EXPERIMENTAL SECTION

The aqueous PEDOT:PSS solution was purchased from Heraeus. Ethanol, acetone, and isopropanol were all purchased from Aldrich. CH3NO was purchased from Life Technologies Australia. H2SO4 (98 wt %) was purchased from Merck. NaOH and NaBH4were purchased from Sigma-Aldrich. All the chemicals were used without any purification. The filter membrane with a pore size of 0.45 μm was purchased from Kuihuap. Sample Preparation. PEDOT:PSS (Clevios PH1000) was filtered using a vacuum-assisted syringe filter (0.45 μm pore size membrane) to get rid of large-size particles in the aqueous solution, and the filtrate was placed in a vacuum chamber for 10 min to remove air bubbles before use. The PEDOT:PSS (200 μL) filtrate was dropcasted onto the silicon dioxide substrates that were precleaned with detergent, deionized water, acetone, isopropanol, and plasma cleaner consecutively. After being dried out in an atmosphere oven at 60 °C overnight, the samples were treated with CH3NO in the following way. The as-prepared samples were immersed in CH3NO for 10 min at room temperature and then rinsed using deionized water to wash away the remnant CH3NO from its surface before being heated on a hot plate at 80 °C for 10 min to dry them out (called dip). After the dipping process, the CH3NO solution was dropped onto the samples, which were heated on a hot plate at 160 °C for various times (5, 10, 15, 20, 25, 30, 40, 50 min). Then, the samples were rinsed with deionized water to remove the remnant CH3NO before they were dried out at 80 °C for 10 min (called drop). A CH3NO treatment contains one dip process and one drop process. After some pristine samples had been treated twice by CH3NO treatments, the CH3NOtreated samples were immersed in concentrated H2SO4 at room temperature for various times (2, 5, 10, 20, 30, 40 h). After that, the samples were rinsed with deionized water to wash away the remnant H2SO4 four times before they were dried out at 80 °C for 10 min. Then, some CH3NO-H2SO4-treated samples were further treated with a NaBH4 solution. Considering the strong reducibility of NaBH4, a 0.5 M NaOH solution was prepared prior to the preparation of the B

DOI: 10.1021/acs.chemmater.9b01500 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials NaBH4 solution with different concentrations (5, 10, 20, 40, 60, 80, 100, 120 mM). The CH3NO-H2SO4-treated samples were thus respectively immersed into different NaBH4 solutions at room temperature for 30 min, followed by rinsing them in deionized water four times to wash away the remnant NaBH4 before being dried at 80 °C. TE Device Fabrication. Five milliliters of PEDOT:PSS filtrate was paved on the filter membrane with a diameter of 50 mm. After drying in an atmosphere oven under 80 °C overnight, the PEDOT:PSS film was immersed in acetone for 10 min. The membrane was visibly dissolved, and a free-standing PEDOT:PSS film was obtained. The PEDOT:PSS film went through the triple post-treatments as mentioned above. Then, they were cut into rectangular pieces with identical sizes. These pieces and the conducting copper wires as connections were placed in an array through a polyimide substrate, as shown in Figure 4a, and then, another two polyimide substrates were used to cover them for protection. Measurement and Characterization. Both the σ and S of the samples were measured at 25 °C using SBA458 from Netzsch. The n and μ of the samples were measured using the van der Pauw technique. The crystal structures of the samples were characterized using grazing incidence XRD (GIXRD, Rigaku SmartLab). The doping levels of the samples were characterized by XPS (Kratos Axis Ultra XPS). The oxidation levels (carrier types) of the samples were characterized by UV−vis absorption spectra (UV−visible spectrophotometer T60). The cross-sectional morphologies and the surface morphologies of the samples were respectively observed using SEM (JEOL 7001) and an AFM (Cypher AFM from Asylum Research).

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Figure 2. . (a) Schematic diagram of PEDOT:PSS. (b) Transformation from benzoid (quinoid) character to quinoid (benzoid) character by post-treatment with H2SO4 (NaBH4). (c) XRD results for PEDOT:PSS films with different treating conditions. (d) XPS results for PEDOT:PSS films with different treating conditions with inset showing an enlarged view of XPS near 164 eV. (e) UV−vis− NIR spectra of the CH3NO-H2SO4-NaBH4-treated PEDOT:PSS samples. Here, untreated means the CH3NO-H2SO4-treated sample. (f) Cross-sectional SEM images of pristine (I), CH3NO (II), CH3NO-H2SO4 (III), and CH3NO-H2SO4-NaBH4 (IV)-treated PEDOT:PSS films.

RESULTS AND DISCUSSION During the experiment, we discovered that the TE properties of the PEDOT:PSS films are determined by four controllable factors, that is, the treating duration of the CH3NO treatment, repeated treating times of the saturated CH3NO treatment, treating duration of the concentrated H2SO4 treatment, and chemical concentration of the NaBH4 treatment. For simplicity, the sample treated only by CH3NO is labeled as CH3NO-treated sample, the sample treated by CH3NO and H2SO4 is labeled as CH3NO-H2SO4-treated sample, and the sample treated by CH3NO, H2SO4 and NaBH4 is labeled as CH3NO-H2SO4-NaBH4-treated sample. Figure 1a shows the measured σ and S of the as-prepared films with and without post-treatments at room temperature. The as-prepared drop-cast PEDOT:PSS films from a commercial dispersion solution without chemical treatments in this study exhibit very poor TE properties (σ = 2 Scm−1 and S = 11.2 μVK−1). Such a poor TE performance mainly stems from the excess amount of PSS in the dispersion solution (discussed later in this paper).27 Figure 1a shows that the σ of the CH3NO-treated sample sharply increases to ∼900 Scm−1 after being treated for only 5 min, and it saturates at ∼1500 Scm−1 when the treating duration is beyond 15 min, owing to the selective removal of excess PSS in the film. So far, we have discovered that increasing the treating duration no longer improves σ. To further increase σ, we repeat the same posttreatment several times. Figure 1b shows the TE properties measured at room temperature of CH3NO-treated samples as a function of the treating times. The σ increases from 1518 Scm−1 to 1732 Scm−1 when the treating process was repeated two times. Over that, σ stabilizes at 1732 ± 50 Scm−1. This might be attributed to the limited ability of CH3NO to preferentially remove the insulating shell of the hydrophilic PSS. Although it was reported that CH3NO post-treatment at 180 °C improved the σ of PEDOT:PSS films up to 3000

Figure 3. . AFM images of (a) pristine, (b) CH3NO-treated, (c) CH3NO-H2SO4-treated, and (d) CH3NO-H2SO4-NaBH4-treated PEDOT:PSS samples. The sizes of all images are 500 × 500 nm.

Scm−1,33 in our study, the PEDOT:PSS films were all damaged during the treatment at such a high temperature. Figure 1c shows σ of the CH3NO-H2SO4-treated film measured at room temperature from which we observed an increased σ from 1732 Scm−1 to 2974 Scm−1 when the immersing duration increases up to 10 h. After 10 h, σ remains C

DOI: 10.1021/acs.chemmater.9b01500 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials Table 1. n and μ of Three Types of Treated Samples parameter

CH3NO

CH3NO-H2SO4

CH3NO-H2SO4-NaBH4

n (1021 cm−3) μ (cm2 V−1 s−1)

1.19 ± 0.18 8.64 ± 1.3

1.35 ± 0.2 14.87 ± 2.23

0.71 ± 0.1 13.93 ± 2.1

To evaluate the mechanism of the enhanced TE performance, the pristine, CH3NO-, CH3NO-H2SO4-, and CH3NOH2SO4-NaBH4-treated samples were all characterized by X-ray diffraction (XRD) and X-ray photoemission spectroscopy (XPS). Figure 2a,b shows the schematic diagram of PEDOT:PSS and transformation from benzoid (quinoid) character to quinoid (benzoid) character by post-treatment with H2SO4 (NaBH4). Figure 2c shows the XRD patterns for PEDOT:PSS films with different treatment conditions. For the XRD pattern of the pristine sample, two diffraction peaks at low 2θ of 3.3 and 7° correspond to the lamella stackings in the (100) plane of alternate orderings of PEDOT and PSS chains,34,35 whereas the other two peaks at high 2θ of 18 and 26° are respectively attributed to the amorphous halo of PSS and the interchain planar ring stacking along the normal direction of the (010) plane.34 This observation agrees well with the reported results.25 After treatment with CH3NO, the intensity of the first diffraction peak is enhanced and slightly shifts to 2θ = ∼4°, suggesting an increased crystallinity of the sample and a narrowed alternate lamella stacking of PEDOT and PSS in their (100) planes caused by the selective removal of excess PSS between these (100) planes, according to Block’s law.25 A further treatment with H2SO4 is even more effective in removing excess PSS. As a result, this effect shifts the first two diffraction peaks toward higher 2θ of ∼7 and 12.5° and enhances their intensities. Apart from the removal of excess PSS, another reason for the greatly enhanced diffraction intensities is a molecular structure transformation of PEDOT chains from benzoid character to quinoid character induced by the H2SO4 post-treatment. In the molecular structure transformation, a C−C single bond between two monomers has been replaced by a π bond, as shown in Figure 2b. The quinoid character favors the extension of PEDOT chains, leading to an enhanced crystallized molecular structure.31 The final posttreatment with a strong reducing agent of NaBH4 solution aims to optimize the σ and S of the PEDOT:PSS sample by tuning its oxidation level. The reduction process is reversible to the oxidation process in the H2SO4 post-treatment, inducing a reversed molecular structure transformation from quinoid character to benzoid character. In its XRD pattern, a decrease in the intensities of the first two diffraction peaks and a slight shift toward higher 2θ are detected, which may originate from the reversed transformation. As a loss of quinoid character,

Figure 4. (a) Schematic diagram of the homemade flexible TE device. (b) Photograph of the homemade device (I), measurement setup for the open-circuit thermovoltage (II), and measurement setup for voltage on an external resistor framed by the red dot cycle (III). To maximize the output power, the external resistance was set to be the same as that of our device, i.e., 168 Ω, measured by a digital multimeter. (c) Measured thermovoltages using both the human arm and the heating plate as heating sources (d) Voltage on an external variable resistor as a function of resistance.

constant. Although σ of PEDOT:PSS films has been greatly enhanced after post-treatment with CH3NO and H2SO4, S generally fluctuates between 13 and 15 μVK−1, as shown in Figure 1b,c. To further enhance S, the optimization process has been carried out by immersing the CH3NO-H2SO4-treated films into the NaBH4 solution with various concentrations for 30 min at room temperature. Figure 1d shows that S increases sharply by increasing the concentration of the NaBH4 solution and then reaches a stable value when the concentration goes beyond 20 mM. In contrast, σ decreases rapidly when the films were treated with the NaBH4 solution even with a small concentration, and then it saturates when the concentration of the NaBH4 solution is higher than 40 mM. As shown in Figure 1d, the power factor is changed accordingly with a peak value reaching 141 μWm−1 K−2 at room temperature when the concentration is equals to 10 mM (where σ = 1786 Scm−1 and S = 28.1 μVK−1).

Table 2. TE Properties of Several PEDOT:PSS-Based Films and Devices materials

σ (Scm−1)

S (μVK−1)

PF (μWm−1 K−2)

ΔT (K)

voltage (mV)

PDa (μWcm−2)

ref

PEDOT:PSS PEDOT:PSS PEDOT:PSS/Te PEDOT:PSS/Bi2Te3 PEDOT:PSS/SWCNT PEDOT:PSS PEDOT:PSS

1786 2500 680 945 650 2929 510

28.1 20.6 27.5 22.2 24.1 17.4 44

141 107 51.4 47 37.8 88.4 100

12 25 13

2.9 2 2.5

∼1

this work 29 46 47 48 28 31

a

PD represents the output power density. D

DOI: 10.1021/acs.chemmater.9b01500 Chem. Mater. XXXX, XXX, XXX−XXX

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insulating PSS shells to boost the thermally induced hopping of the charge carriers between the conducting host molecules, accounting for the significantly enhanced σ of the PEDOT:PSS sample after treatments.25,42,43 The average grain height in Figure 3c is larger than that in Figure 3b, suggesting that concentrated H2SO4 is more effective in cleansing the excess dissociative PSS aggregates. As a result, the CH3NO-H2SO4treated sample presents a higher σ (refer to Figure 1a−c). Further NaBH4 treatment does not lead to a notable difference in the surface topography. Table 1 summarizes the carrier concentration n and carrier mobility μ of all three types of samples (CH3NO-, CH3NOH2SO4-, and CH3NO-H2SO4-NaBH4-treated samples). As for the pristine sample, it is difficult to obtain its n and μ using the Hall system; however, according to previous report, their values are respectively on the order of 1020 cm−3 and 0.045 cm2 V−1 s−1.44,45 The CH3NO-treated sample shows an enhanced n of 1.19 ± 0.18 × 1021 cm−3 and an increased μ value of 8.64 ± 1.3 cm2 V−1 s−1 after the CH3NO treatment. Further treatment with H2SO4 greatly improves the μ value up to 14.87 ± 2.23 cm2 V−1 s−1, which may be derived from the highly ordered microstructures of the sample. Moreover, to optimize the TE performance, NaBH4 treatment resulted in n reduction to almost a half, while μ was maintained the same. This may be attributed to the reduction reaction during the NaBH4 treatment and unchanged microstructures. The evolutions of both n and μ provide a new prospect of the influence on the thermoelectric properties of PEDOT:PSS samples by these post-treatments. To verify the application potential of our treated PEDOT:PSS films, a thermoelectric device was assembled, and a human arm was used as a heat source from which the thermovoltage was measured. Figure 4a shows the schematic drawing of the assembling process of our TE device in which polyimide was used as the substrate and copper wires served as conductive connections between the PEDOT:PSS films. Figure 4b shows the photographs of our homemade device and related measurement set-up. The size of one single piece of PEDOT:PSS film is 2 × 0.35 cm2. A variable resistor was applied in the measurement of the output power, as shown in Figure 4b (III). Figure 4c shows the measured voltages generated from the device using the human arm or the hot plate as the heating sources, respectively. In the case of using a heating plate, one end of the device was firmly attached to the heating plate (from 21 to 51 °C) and the other was exposed in the air (21 °C) so that higher thermovoltages are obtained by increasing the temperature of the heating plate. On the other hand, in the case of using human arm, the device was attached firmly to the human arm (33 °C). Since two ends of the device all contact with the human skin, its temperature difference is much lower than the corresponding temperature difference (12 °C) generated by the heating plate (set at 33 °C), resulting in a lower thermovoltage. Figure 4d shows the voltage on an external resistor as a function of its resistance. Obviously, the voltage increases with increasing external resistance. The output power is maximized when the external resistance equals the resistance of the device. Therefore, a peak output power density of ∼1 μWcm−2 can be obtained when using the human arm as a heating source. Table 2 shows TE properties of several PEDOT:PSS-based films and devices. Obviously, we have obtained a high power factor, owing to the optimized electrical conductivity and Seebeck coefficient. Moreover, our device with 14 units

coiled and more compact conformations of PEDOT chains are obtained.31,36 Figure 2d presents the S2p XPS spectra of all PEDOT:PSS films. The binding energies of sulfur atoms of PEDOT and PSS are respectively below and above ∼165.5 eV.37 According to the evolution of XPS spectra of films treated by different methods, we find that CH3NO treatment leads to a decrease in the mass ratio between PSS and PEDOT, and post-treatment with H2SO4 further reduces the mass ratio of PSS and PEDOT. The reduced mass ratio of PSS and PEDOT confirms the selective removal of excess PSS in the drop-casted PEDOT:PSS films, in agreement with the aforementioned XRD patterns. However, such a mass ratio generally remains identical before and after the NaBH4 treatment, while a little shift in the S2p binding energy of PEDOT is detected. This shifting of 0.15 eV toward a lower binding energy originates from the reduction of the average oxidation level. A similar result was reported.31 This phenomenon is also proved by the UV−vis absorption spectra of the PEDOT:PSS samples, as shown in Figure 2e. As reported,38,39 the absorptions of neutral monomers and polarons occur at ∼600 and ∼900 nm, respectively. When the CH3NO-H2SO4-treated sample was subjected to the NaBH4 treatment, the sample shows higher spectral absorption at 600 nm compared with that at 900 nm, indicating that the average oxidation level was reduced. Such a reduced average oxidation level indicates a state transition of the charge carriers from polarons to neutrons, leading to a decreased charge carrier concentration, which, in turns, results in an increased S and a decreased σ.15,40 Interestingly, once the concentration of the NaBH4 solution goes beyond 20 mM, the absorption spectra of the CH3NO-H2SO4-NaBH4-treated samples remain basically identical, corresponding to the barely changed S in that range presented in Figure 1d. Figure 2f presents the cross-sectional scanning electron microscopy (SEM) images of all treated PEDOT:PSS films. As can be seen, the film thickness decreases after the CH3NOH2SO4 treatment because of the depletion of excess PSS between their (100) planes. Moreover, the H2SO4-treated sample shows a lamella stacking microstructure distributing along the entire cross section. This highly ordered microstructure is due to the sufficient removal of the excess PSS aggregating as insulating domains between metallic grains.36,41 Without the insulating domains, the effective and direct contact between metallic grains will enable the charge carriers to flow smoothly through the PEDOT:PSS film, leading to a higher carrier mobility.41 As a result, σ is remarkably enhanced without the sacrifice of S, as shown in Figure 1c. Further treatment with NaBH4 did not make any detectable change to the microstructure of the film. Since the chemical treatments were performed on the PEDOT:PSS film surfaces, it is necessary to clarify the surface morphology of PEDOT:PSS films that have been treated differently. Figure 3 shows the atomic force microscopy (AFM) images. The contrast of AFM images can be indicative of the level of phase separation between PEDOT and PSS. Figure 3a shows the surface topography of the pristine sample. Its relatively uniform contrast indicates neglectable phase separation between PEDOT and PSS, suggesting PEDOT chains within the film are homogeneously disconnected.42 For both CH3NO- and CH3NO-H2SO4-treated samples, in Figure 3b,c, granular structures with shape contrast were also observed, where the phase separation becomes obvious. The granular structures provide evidence for the depletion of E

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produces a thermally induced voltage of 2.9 mV and a peak output power density of ∼1 μWcm−2 using human arm as the heating source.

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CONCLUSIONS A high power factor of 141 μWm−1 K−2 at 25 °C has been obtained for the CH3NO-H2SO4-NaBH4-treated PEDOT:PSS film. Such a high power factor stems from the high σ (1786 Scm−1) and S (28.1 μVK−1) after post-treatment with CH3NO, H2SO4, and NaBH4 in order. Both the CH3NO and H2SO4 treatments contribute to the selective removal of excess PSS acting as insulating domains within the films, while the H2SO4 treatment also causes a conformation change in the PEDOT chains from benzoid to quinoid character due to an oxidative dehydrogenation process. At the microstructural level, these two processes give rise to the highly ordered lamella stacking structure that ensures a high carrier mobility, resulting in an ultrahigh σ. The subsequent NaBH4 treatment serves as an optimization process for TE properties by modulating the oxidation level. Moreover, a TE device was also made with a peak output power density of ∼1 μWcm−2 using the human arm as a heating source. This work proves that the fabrication of high-performance flexible TE devices based on polymers is viable.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]; [email protected] (Z.G.C). *E-mail: [email protected] (J.Z.). ORCID

Min Hong: 0000-0002-6469-9194 Xiao-Lei Shi: 0000-0003-0905-2547 Yang Bai: 0000-0001-8481-368X Lianzhou Wang: 0000-0002-5947-306X Matthew Dargusch: 0000-0003-4336-5811 Jin Zou: 0000-0001-9435-8043 Zhi-Gang Chen: 0000-0002-9309-7993 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support provided by the Australia Research Council. Z.G.C. thanks the USQ Strategic research fund and USQ start-up grant. S. X. acknowledges the China Scholarship Council for providing the Ph.D. stipend. The Australian Microscopy and Microanalysis Research Facility is acknowledged for providing the characterization facilities.

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DOI: 10.1021/acs.chemmater.9b01500 Chem. Mater. XXXX, XXX, XXX−XXX