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Significant enhancement in the thermoelectric properties of PEDOT:PSS films through a treatment with organic solutions of inorganic salts Zeng Fan, Donghe Du, Zhimeng Yu, Pengcheng Li, Yi-Jie Xia, and Jianyong Ouyang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07234 • Publication Date (Web): 18 Aug 2016 Downloaded from http://pubs.acs.org on August 19, 2016

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ACS Applied Materials & Interfaces

Significant enhancement in the thermoelectric properties of PEDOT:PSS films through a treatment with organic solutions of inorganic salts

Zeng Fan, Donghe Du, Zhimeng Yu, Pengcheng Li, Yijie Xia, and Jianyong Ouyang*

Department of Materials Science and Engineering, National University of Singapore, Singapore 117574, Singapore Email: [email protected]

1 ACS Paragon Plus Environment


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Abstract Conducting polymers have promising thermoelectric application because they have many advantages including abundant elements, mechanical flexibility and non-toxicity. The thermoelectric properties of conducting polymers strongly depend on their chemical structure and microstructure. Here, we report a novel and facile method to significantly enhance the thermoelectric

properties

of

poly

(3,4-ethylenedioxythiophene):poly(styrenesulfonate)

(PEDOT:PSS) films through a treatment with organic solutions of inorganic salts. N,NDimethylformamide (DMF) and a common inorganic salt like zinc chloride (ZnCl2) are used as the solvent and solute of the solutions, respectively. The treatments can significantly increase both the Seebeck coefficient and electrical conductivity of the PEDOT:PSS films. The thermoelectric properties of the PEDOT:PSS films are sensitive to the experimental conditions, such as the salt concentration, treatment temperature and the cation of the salts. After treated at the optimal experimental conditions, the PEDOT:PSS films can exhibit a Seebeck coefficient of 26.1 µV/K and an electrical conductivity of over 1400 S/cm at room temperature. The corresponding power factor is 98.2 µW/(m·K2). The mechanism for the enhancement in the thermoelectric properties is attributed to the segregation of some PSSH chains from PEDOT:PSS and the conformation change of PEDOT chains as a result of the synergetic effects of inorganic salts and DMF. Keywords: PEDOT:PSS, electrical conductivity enhancement, Seebeck coefficient, organic solution, inorganic salt, thermoelectric property.


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1. Introduction Thermoelectric (TE) devices can directly convert heat into electricity. They are particularly important for the harvest of low-grade heat like waste or exhausted heat that is usually dissipated to environment.1 The thermoelectric conversion efficiency depends on the dimensionless figureof-merit (ZT), ܼܶ = ܵ ଶ ߪܶ/κ, where S is Seebeck coefficient, σ is electrical conductivity, κ is thermal conductivity, and T is absolute temperature.2-5 ZT values as high as ~2 have been reported on inorganic semiconductors and semimetals, such as Bi2Te3 and CaMnO3.6-8 However, there are severe obstacles for the practical application of these inorganic thermoelectric materials, including difficulty in materials processing, high cost, scarcity of the materials, toxicity of elements and poor mechanical flexibility.9, 10 Recently, great attention was paid to organic and polymeric

thermoelectric

materials,

such

as

conducting

polymers

like

poly(3,4-

ethylenedioxythiophene) (PEDOT) and polyaniline (PANi). Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, chemical structure as shown in Scheme 1) has attracted extensive attention due to the availability of its aqueous solution, good solution processability,11 high electrical conductivity and intrinsically low thermal conductivity.12-14 An as-prepared PEDOT:PSS film from its aqueous solution usually has an electrical conductivity of less than 1 S/cm and Seebeck coefficient of 15-18 µV/K.15-18 Both the electrical conductivity and Seebeck coefficient so as to the power factor (PF) and ZT value of PEDOT:PSS films can be greatly enhanced through a treatment.19, 20 The Seebeck coefficient can also be enhanced by chemical or electrochemical dedoping.21, 22 The electrical conductivity and Seebeck coefficient are interdependent. For instance, dedoping can increase the Seebeck coefficient but it lowers the electrical conductivity, while improving the charge carrier mobility can enhance both the electrical conductivity and the Seebeck coefficient.23



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Scheme 1 Chemical structure of PEDOT:PSS.

A couple of methods have been reported to enhance the electrical conductivity of PEDOT:PSS. They can also enhance the thermoelectric properties of PEDOT:PSS. Organic polar solvents, such as dimethyl sulfoxide (DMSO) and ethylene glycol (EG) have shown to be effective secondary dopants for the PEDOT:PSS films.16, 24-28 Pipe et al.29 found that the power factor and the ZT value of PEDOT:PSS can be enhanced to 469 µW/(m·K2) and 0.42 respectively by adding 5% DMSO into the PEDOT:PSS aqueous solution and then immersing the PEDOT:PSS film in EG bath for 2 h. Liu et al.15 observed a power factor of 37.05 µW/(m·K2) on the PEDOT:PSS films post-treated with DMSO at 120 °C. By simply adding DMSO into PEDOT:PSS, power factor of 30 µW/(m·K2) was achieved by Luo et al.18 Yi et al.30 discovered that binary secondary dopants can give rise to higher thermoelectric performance. After treated with dimethyl sulfoxide (DMSO) and poly (ethylene oxide) (PEO), the PEDOT:PSS films can exhibit a power factor of 157 µW/(m·K2). Apart from the treatment with polar solvents, a treatment of PEDOT:PSS with acids can enhance the thermoelectric performance as well.31-33 Mengistie et al.19 reported a high power factor of 80.6 µW/(m·K2) for the PEDOT:PSS treated with formic acid. Kumar et al.20 conducted sulfuric treatment on the as-prepared PEDOT:PSS and investigated the thermoelectric response of the treated film. At 460 K, a power factor of 113 µW/(m·K2) was successfully obtained. Other than the methods for electrical conductivity 4 ACS Paragon Plus Environment

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enhancements, the thermoelectric property of conducting polymers can also be improved by tuning their doping levels13, 22, 34 or incorporating with nanomaterials.31, 35-37 In this work, we report a novel and facile secondary doping method to significantly enhance the power factor of PEDOT:PSS films through a treatment with N,N-dimethylformamide (DMF) solutions of various inorganic salts. The treatment significantly enhances both the electrical conductivity and Seebeck coefficient of PEDOT:PSS. After treated at the optimal conditions, the PEDOT:PSS films can exhibit a Seebeck coefficient of 26.1 µV/K, an electrical conductivity of higher than 1400 S/cm and thus a power factor of 98.2 µW/(m·K2). This corresponds to a ZT value of 0.125 as the thermal conductivity of PEDOT:PSS is about 0.2 W/(m·K) 16, 38.

2. Experiments 2.1 Materials and treatment of PEDOT:PSS films Materials and the procedures to treat PEDOT:PSS films can refer to our previous works 39-41.

2.2 Characterization Seebeck coefficients of the PEDOT:PSS films were measured in ambient environment using a home-made system, which consisted of two Peltier devices (TEC1-19906, Beijing Geshang Electronic Pte. Ltd.) affixed on an alumina heat sink. The temperature difference (∆T) ranging from 0 to 8 K across the film was detected with two T-type thermocouples (Omega, US) which have a diameter of 25 µm, and the thermal voltage output (∆V) was measured with a Keithley 2000 multimeter. For each sample, the ∆V values were measured at 7 different ∆T values. At least 3 different Peltier heating rates were adopted for each ∆T value. The Seebeck coefficient of each film was derived through the best linear fitting of the ∆V–∆T plots. The Seebeck



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measurement system was calibrated with a pure nickel foil. The accuracy was within 10%. All other instruments used to characterize the PEDOT:PSS films were similar to our previous works 39,40

.

3. Results and discussions 3.1 Thermoelectric property of PEDOT:PSS films As-prepared PEDOT:PSS films have a low electrical conductivity of ~0.2 S/cm. A treatment of as-prepared PEDOT:PSS films with neat DMF can enhance its electrical conductivity to ~1.2 S/cm. Because the power factor is proportional the electrical conductivity, the as-prepared and DMF-treated PEDOT:PSS films have no practical thermoelectric application. As-prepared PEDOT:PSS films were treated with DMF solutions of various inorganic salts, including ZnCl2, CuCl2, InCl3, LiCl, NiCl2 and NaI. Table 1 summarizes the thermoelectric properties of the PEDOT:PSS films treated with 0.1 M DMF solutions of the salts. The electrical conductivity of PEDOT:PSS films is significantly enhanced by the treatments, and the conductivity enhancement depends on the salt used in the organic solution. The electrical conductivities of the PEDOT:PSS films treated with DMF solutions of ZnCl2, CuCl2, InCl3, LiCl, and NiCl2 are 1473, 1138, 965, 969 and 659 S/cm, respectively, while it is only 150 S/cm for the one treated with the DMF solution of NaI. These results indicate that the electrical conductivity enhancement of PEDOT:PSS is significantly affected by the inorganic cations. As presented in our early work, the electrical conductivity enhancement of PEDOT:PSS treated with aqueous solution of salts is related to the softness parameter of the metal ions in aqueous solutions.41 Interestingly, for the inorganic salts dissolved in DMF, the cations with a positive softness parameter can give rise to more significant electrical conductivity enhancement than the ones with a negative softness


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parameter. Because the softness parameter indicates the association of an ion with other ion, this suggests that the mechanism for the electrical conductivity enhancement may be related to the association of the metal cations with the PSS- polyanions of PEDOT:PSS at the presence of DMF. However, the electrical conductivity enhancement does not strictly follow the softness parameter of the cations. Presumably, this is related to the solvent used in the solutions. The soft parameters presented in Table 1 are the values for the ions in water. The softness parameters of these cations in DMF should be different from that in water, because they are solvated by the solvent molecules. The solvated molecules should affect the association of the cations with other species and thus their softness parameter. The temperature during the solution treatment affects the electrical conductivity enhancement of PEDOT:PSS. The treating temperature for the treatment with DMF solution of 0.1 M ZnCl2 was varied from room temperature to 200 oC. As shown in Figure 1(a), the optimal treating temperature for the electrical conductivity enhancement is 80 °C. The electrical conductivity increases with the increasing temperature from 40 to 80 oC and then decreases with the further increase of the treating temperature. The electrical conductivities of PEDOT:PSS films treated at room temperature and 200 oC are 790 and 550 S/cm, respectively. The optimal treating temperature is lower than that of many other treating methods like acid treatment and co-solvent treatment.42-44 Presumably, the optimal treating temperatures of different methods are related to the solvent. For instance, the optimal treating temperature is 140-160 oC when water is used as the solvent, while the electrical conductivity enhancement is insensitive to the treating temperature when PEDOT:PSS is treated with DMSO or EG.42, 45-47 The electrical conductivity enhancement of PEDOT:PSS is usually related to the segregation of PSSH chains from



PEDOT:PSS and the conformational change of the PEDOT chains, that is, the wiggling of the polymer chains of PEDOT:PSS. The wiggling of the polymer chains, which initializes the conformation change, should depend on the temperature and the solvent because they have charges and are solvated by the solvent molecules. The electrical conductivity enhancement also depends on the salt concentration in DMF. As shown in Figure 1(c), the electrical conductivity dramatically increases with the increasing ZnCl2 concentration in the concentration ranges from 10-3 to 0.1 M. But when the ZnCl2 concentration is further increased to 1 M, the electrical conductivity even decreases a little bit.

Table 1 Electrical conductivity, Seebeck coefficient and power factor of PEDOT:PSS films treated with 0.1 M solutions of various salts at 80 °C. Softness Seebeck Electrical Power factor Salt parameter of coefficient (µV/K) conductivity (S/cm) (µW/mK2) cation41, 48 ZnCl2 +0.35 22.6 1473 75.2 CuCl2 +0.38 22.8 1138 59.2 InCl3 +0.48 21.2 965 43.4 LiCl -1.02 20.3 969 39.9 -0.11 19.9 659 26.1 NiCl2 NaI -0.75 19.0 150 5.4

1400 26

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80 70 60 50 40 30 20 10 0 1E-3

Concentration (M)

0.01

0.1

1

Concentration (M)

Figure 1 Thermoelectric properties of the treated PEDOT:PSS films. (a) Variations of the electrical conductivity and Seebeck coefficient of PEDOT:PSS films with treating temperature. (b) Variation of the power factor of PEDOT:PSS films with the treating temperature. (c) Dependences of the electrical conductivity and Seebeck coefficient of PEDOT:PSS films on the ZnCl2 concentration. (d) Dependences of the power factor of PEDOT:PSS films on the ZnCl2 concentration.

The Seebeck coefficient and power factor of the PEDOT:PSS films are also displayed in Figure 1. The Seebeck coefficients of both the untreated and DMF-treated PEDOT:PSS are in the range of 15-18 µV/K (from literature for the untreated PEDOT:PSS 15-18 and measured for the DMF-treated PEDOT:PSS). We found that the noise was quite high in measuring the Seebeck coefficient of untreated PEDOT:PSS films. ∆V values with high accuracy were measured for treated PEDOT:PSS films. As shown in Table 1, although the electrical conductivity of the treated PEDOT:PSS can be different by almost one order in magnitude, their Seebeck coefficients are not too different. It is interesting to note that the dependence of the Seebeck coefficient on the treating temperature is different from that of the electrical conductivity. The Seebeck coefficient of PEDOT:PSS films treated with DMF solution of 0.1 M ZnCl2 increases with the increasing temperature during the treatment in the whole temperature range from 40 to 200 oC, consistent with the temperature effect on Seebeck coefficient of the PEDOT:PSS films post-treated by 9 ACS Paragon Plus Environment


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various solvents, such as choline chloride (ChCl),49 N-methyl-2-pyrrolidone (NMP) and DMF.50 It is 20.2 µV/K at 40 oC and increases to 25.2 at 200 oC. Figure 1(b) shows the power factors at different treating temperature. The highest power factor is 75.1 µW/(m·K2), which is at 80 oC corresponding to the optimal treatment temperature for the electrical conductivity of PEDOT:PSS. Such similar trends have also been observed for the PEDOT:PSS films treated by other inorganic salts in DMF, as shown in Figure S1. Furthermore, the Seebeck coefficient is dependent on the ZnCl2 concentration in DMF (Figure 1(c)). When the ZnCl2 concentration varies from 0.1 to 1 M, the Seebeck coefficient increases from 22.6 to 26.1 µV/K. Accordingly, the power factor increases with the increasing ZnCl2 concentration. It is 75.1 µW/(m K2) when 10-3 M ZnCl2 solution is used and increases to 98.2 µW/(m K2) when 1 M ZnCl2 solution is adopted. It agrees well with the effect of solvent additives on the Seebeck coefficient of PEDOT:PSS films.49

3.2 Mechanism of thermoelectric property enhancement of PEDOT:PSS films The PEDOT:PSS films before and after a treatment were characterized to understand the mechanism for the enhancement in the thermoelectric properties. Figure 2(a) shows the UV absorption spectra of the untreated and treated PEDOT:PSS films. As the two absorption bands located at 193 and 225 nm originate from the aromatic rings of PSS, the drops in their intensities suggest the depletion of PSSH from the PEDOT:PSS films after the treatments.44 The intensity drop of these two absorption bands is consistent with the electrical conductivity enhancement of the PEDOT:PSS films. The S2p XPS of the ZnCl2-DMF treated film further confirms the removal of PSS (Figure 2(b)).41, 42, 44 The binding energies of the sulfur atoms of PEDOT and PSS are below and above 166 eV, respectively.51, 52 The increased S2p XPS intensity of PEDOT relative


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to PSS evidences the reduced content of PSS after the treatment. Because PSS is an insulator that inhibits the charge transport, the loss of PSS can facilitate the charge transport through the PEDOT:PSS films. In addition, the S2p binding energies of PEDOT shift to red by about 0.2 eV. This can be ascribed to the change in the environment of sulfur atoms of PEDOT.

(b)

(a)

1.0 Normalized intensity

PEDOT:PSS ZnCl2-treated

Absorbance (a. u.)

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CuCl2-treated InCl3-treated LiCl-treated NiCl2-treated NaI-treated

Untreated ZnCl2-treated

0.8 0.6 0.4 0.2

200

220

240

260

280

300

0.0

172

170

Wavelength (nm)

168

166

164

162

160

Binding Energy (ev)

Figure 2 (a) UV-vis absorbance spectra of PEDOT:PSS films before and after the treatment with the DMF solutions of various inorganic salts. (b) S2p XPS spectra of untreated and ZnCl2-DMF treated PEDOT:PSS films.

To shed insight into the effect of the solution treatment on the conduction mechanism, the temperature dependences of the resistivities of the untreated and treated PEDOT:PSS films was measured from 110 K up to 350 K (Figure 3(a)). The resistance become less sensitive to temperature for the solution-treated PEDOT:PSS films. As shown in Figure 3(b), the temperature dependences of the resistance is analyzed using the one-dimensional variable range hopping (VRH) model,53, 54 ்

ଵ/ଶ

ܴሺܶሻ = ܴ଴ ݁‫ ݌ݔ‬൤ቀ ்బ ቁ

൨.

T0 = 16/kBN(EF)L//L⊥2 is the energy barrier between localized states, N(EF) is the density of the states at the Fermi level, and L//(L⊥) is the localization length in the parallel (perpendicular) direction. The T0 values dramatically decrease for the PEDOT:PSS film after a solution treatment. Compared with that (1436 K) of the untreated PEDOT:PSS film, the T0 value decreases to 801, 11 ACS Paragon Plus Environment


240, 192, 190, 139 and 128 K for the PEDOT:PSS films treated by DMF solutions of NaI, NiCl2, LiCl, InCl3, CuCl2 and ZnCl2, respectively (Figure S3). The decrease in T0 correlates well with the increased electrical conductivity, indicating the lowered energy barrier for charge transport and enlarged localization length. Nevertheless, the resistances of the treated films deviate from the linear relationship at temperatures above 220 K. Within the temperature range of 220-350 K, the insensitivity or even increase of resistances with temperature may suggest the semi-metallic behavior of the PEDOT:PSS films after treatment. As high Seebeck coefficient is usually observed on semimetals,55, 56 the increase in the Seebeck coefficient after a solution treatment may be related to the change in the charge transport mechanism from charge hopping to semimetallic behavior. (a)

(b) 1.0 1.0

0.8

Normalized resistance

Normalized resistance

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0.6 0.4

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0.05

NaI-treated

0.06

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T-1/2(K-1/2)

Temperature (K)

Figure 3 Temperature dependences of the normalized resistances of PEDOT:PSS films.(a) Normalized resistances versus temperature. (b) Analysis of resistance-temperature relationship of the treated PEDOT:PSS film with the VRH model.

The segregation of PSS chains from PEDOT:PSS can lead to the conformational change of the PEDOT chains and the morphological change of the PEDOT:PSS films. The surface morphology of the PEDOT:PSS films were studied by AFM (Figure 4). As shown in Figure 4(a), the untreated PEDPT:PSS has small grains of less than 50 nm, while after a treatment with DMF solutions of inorganic salts, grains with an elliptical shape can be observed and the domain sizes 12 ACS Paragon Plus Environment

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gradually grow larger, reaching up to ~100 nm (Figures 4(b)-(f)). In addition, nanosized fibrous structures also appear for these treated PEDOT:PSS films, indicating the change in the core/shell size of PEDOT and PSS. The morphological change is further reflected by the surface roughness. The untreated PEDOT:PSS film is quite smooth and has a RMS roughness of 1.068 nm. It then becomes rougher after the treatment. The roughness increases to 1.09, 1.264, 1.393, 1.431 and 1.533 nm for the PEDOT:PSS films treated with DMF solutions of NaI, NiCl2, LiCl, CuCl2 and ZnCl2, respectively. It can be related to the conformation change of the polymer chains during treatment. When the excess PSSH chains are removed, the PEDOT chains can change from a coil conformation to an expanded coil or linear conformation. This also facilities the charge transport across the PEDOT chains and grains. (a)

(b)

(c)

(d)

(e)

(f)

Figure 4 AFM height images of PEDOT:PSS films (a) untreated and treated with DMF solution of 01. M (b) NaI, (c) NiCl2, (d) LiCl, (e) CuCl2 and (f) ZnCl2. The unit is in µm, and all the images are 2 µm × 2 µm. The insets are the Fast Fourier Transform (FFT)-processed images.

In addition, the conformation change of PEDOT:PSS chains can lead to the change of the thiophene rings of PEDOT from the benzoid structure to quinoid structure. This conclusion is supported by the Raman spectra (Figure 5). The Raman band between 1400 and 1500 cm-1 are 13 ACS Paragon Plus Environment


due to the Cα=Cβ stretching vibrations. It appears at 1441 cm-1 for the as-prepared PEDOT:PSS film and shifts to 1434 cm-1 after the treatment with DMF solution of ZnCl2.26, 57 As reported by Crispin et al.,55, 56 PEDOT with crystalline structure can have higher Seebeck coefficient than that with amorphous structure because of the high charge carrier mobility for the former. Hence, the increase in the Seebeck coefficient of PEDOT:PSS after the solution treatment can be attributed to the increase in the charge carrier mobility, i.e. the transition from benzoid structure to quinoid structure, that is a result of the PSSH segregation and the structural change of the thiophene rings of PEDOT.

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(b) (a)

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Figure 5 Raman spectra of a PEDOT-PSS film (a) before and (b) after treated with DMF solution of 0.1 M ZnCl2.

As-prepared PEDOT:PSS has a core/shell structure with the core rich of PEDOT and the shell rich of PSS. The PSS shell can affect the electrochemical redox of the PEDOT chains. The electrochemical CVs of PEDOT:PSS films before and after a treatment are shown in Figure 6. The untreated PEDOT:PSS exhibits electrochemical activity only in the range from -0.2 to 0.8 V vs Ag/AgCl, while additional electrochemical activity appears at the potential range below -0.2


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V vs Ag/AgCl after the 0.1 M ZnCl2-DMF treatment, where a redox couple with cathodic and anodic peaks at -0.64 and -0.7 V respectively can be observed. The additional electrochemical activity can be attributed to the removal of some PSS chains and the conformational change of PEDOT chains. Similar change in the electrochemical behavior of PEDOT:PSS after treatment has also been widely observed for the films treated by various other methods.40, 42 It can be attributed to the removal of insulating PSS shells and destruction of core-shell structure after the inorganic salt-DMF solution treatment.

40 20 Current (µA)

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0 -20 Untreated ZnCl2-treated

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Figure 6 Cyclic voltammograms (CVs) of PEDOT:PSS films untreated and treated with 0.1M ZnCl2-DMF solution at 80 °C.

By such a treatment with organic solution of an inorganic salt, the PSS segregation and PEDOT conformation change are presumably results of (i) the screening effect of polar solvents weakening the Coulombic attraction and (ii) the association of inorganic ions with the negatively charged PSS.40, 41 As softness parameter is related to the association of metal ions with other species,41, 48 a cation with a positive softness parameter is more effective for the PSS removal and polymer conformation change (Table 1). Hence, the thermoelectric property enhancement is ultimately a consequence of the synergetic effect of both organic solvent and inorganic salts.



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However, the effect by the solution treatment on the Seebeck coefficient is not always consistent with that on the electrical conductivity. For the PEDOT:PSS films treated by ZnCl2 above 80 oC that is the optimal temperature for electrical conductivity enhancement (Figure 1(a)), their Seebeck coefficient further increases with the elevating temperature while the electrical conductivity decreases. In terms of the UV-vis spectra of PEDOT:PSS films treated at different temperatures (Figure S2(a)), the PSS content of the PEDOT:PSS films treated at a temperatures above 80 °C can be lower than that treated at 80 °C. Hence, the relatively low electrical conductivity may be due to possible disordered structure of PEDOT chains thermally induced or degradation of PEDOT chains. It can be revealed by the in situ UV-vis absorption spectra of electrochemically reduced PEDOT:PSS films and the AFM images of PEDOT:PSS films that were treated by DMF solution of ZnCl2 at different temperatures (Figures S2(b-e)). The intensity and shape of the absorption in the range of 400 to 1100 nm are sensitive to the PEDOT conformation. The absorption bands between 750 and 1150 nm and between 400 and 800 nm are the absorption of the polaron levels and the π-to-π* transition, respectively.55 Compared with the untreated PEDOT:PSS, the polaron absorption decreases while the intensity of the π-to-π* transition increases after a solution treatment. The changes of the two bands are also related to the treating temperature. The absorption band between 750 and 1150 nm slightly increases while that between 400 and 800 nm gradually decreases with the elevating treatment temperatures. As the intensity of the π-to-π* transition band is related to the crystallinity of conjugated polymers,41,57 the PEDOT:PSS films treated at a temperatures above 80 °C are thus speculated to possess a relatively more localized electronic structure in their polymer backbone. Differences can also be observed on the AFM images of PEDOT:PSS films treated at different temperatures. Although all three PEDOT:PSS films treated at 80, 120 and 160 °C are rougher than untreated


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PEDOT:PSS films, the grains become slightly more circulated for the PEDOT:PSS films treated at a higher temperature. Moreover, the resistance of treated PEDOT:PSS films becomes higher when they are treated at 160 oC for longer time.

4. Conclusions Treating PEDOT:PSS with DMF solution of a common inorganic salt can enhance the thermoelectric performance of PEDOT:PSS films. Both the Seebeck coefficient and electrical conductivity are enhanced, and the enhancements depend on the salt concentration, treatment temperature and the cation of salts. When the as-prepared PEDOT:PSS was treated by DMF solution of ZnCl2 at the optimal condition, its Seebeck coefficient and electrical conductivity can reach 26.1 µV/K and over 1400 S/cm, respectively. The optimal power factor is 98.2 µW/m·K2, and the ZT value is 0.125. The enhancement in the thermoelectric properties is attributed to the phase segregation of PSS from PEDOT:PSS and the conformation change of PEDOT chains induced by the synergetic effect of inorganic salt and organic solvent.

Associated content Supporting information. Seebeck coefficients of the PEDOT:PSS films treated with DMF solutions various inorganic salts; UV-vis absorption spectra, in-situ UV-vis absorption spectra, and AFM images of PEDOT:PSS films treated with DMF solution of 0.1 M ZnCl2 at various temperatures; Energy barrier T0 of the PEDOT:PSS films treated by different metal salts.

Acknowledgment



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This work was financially supported by a research grant from the Ministry of Education, Singapore (R-284-000-136-112).


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Table of Content Graph

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Power factor (µ µW/mK2)

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