Seebeck Coefficients of Poly(3,4-ethylenedioxythiophene):Poly

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Seebeck Coefficients of Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) Correlated with Oxidation Levels Ichiro Imae, Mengyan Shi, Yousuke Ooyama, and Yutaka Harima J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10956 • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 27, 2019

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The Journal of Physical Chemistry

Seebeck Coefficients of Poly(3,4-ethylenedioxythiophene):Poly(styrene sulfonate) (PEDOT:PSS) Correlated with Oxidation Levels

Ichiro Imae*, Mengyan Shi, Yousuke Ooyama, Yutaka Harima

Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University,

1-4-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8527, Japan.

E-mail: [email protected]

ABSTRACT

Oxidation states of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) were controlled by electrode potential and their thermoelectric properties were studied elaborately as a function of oxidation level measured by a potential-step chronocoulometry. With an increase in 1

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oxidation level from 0.07 to 21%, electrical conductivities (σ) were increased from 0.37 × 10-3 to 1.1 S cm-1, accompanying a slight decrease in Seebeck coefficient (S) from 79 to 28 V K-1. Addition of ethylene glycol or dimethyl sulfoxide to PEDOT:PSS enhanced σ values, but led to a decrease of S. Both σ and S values of the solvent-added PEDOT:PSS films became sensitive to oxidation level and double logarithmic plots of S and oxidation level were found to fit a straight line with a slope of -0.25.

INTRODUCTION An increasing number of studies have been devoted to the development of thermoelectric (TE) devices based on organic materials, since relatively high TE performances close to those of inorganic TE materials were found with some electrically conducting polymers.1-5 However, the TE performances are still low because of the short research history of conducting polymers as TE materials. The energy conversion efficiency of TE devices depends on a dimensionless figure-of-merit (ZT) or a power factor (PF) defined, respectively, by ZT = σS2T/κ or PF = σS2, where σ is the electrical conductivity, S is the Seebeck coefficient, T is the absolute temperature, and κ is the thermal conductivity. To improve TE performances of conducting polymers, many researchers are focusing on the improvement of σ values by tuning their nano-structures and/or morphologies. On the other hand, S values are of greater importance to enhance TE performances 2


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since both ZT and PF values are proportional to the square of S. Consequently, in order to realize a practical use of organic TE devices, to gain a deep insight into Seebeck effect in organic materials is of a great importance. Very recently, Seebeck coefficients of regioregular poly(3-hexylthiophene) (P3HT) were studied as a function of doping level measured by a potential-step chronocoulometry (PSC), a useful and reliable electrochemical technique based on the measurements of electric charges passing through an electrode upon doping/dedoping processes.6 It was found that double logarithmic plots of S and doping level for the P3HT films fit a straight line with a slope of -1 over a wide range of doping level between 1 and 20%. The result is of a great interest in view of the fact that the equation, S = (8π2kB2/3eh2)m*T(π/3n)2/3, derived for metals and degenerate semiconductors predicts a straight line with a slope of -2/3 in a plot of log S vs. log n, where n is the density of charge carriers, kB is the Boltzmann constant, h is the Planck constant, and m* is the effective mass of charge carriers.7,8 In the present study, poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS, Figure 1), which is one of the most promising candidates for organic TE materials, is investigated to reveal a possible relationship between S and oxidation level9 by using the PSC technique applied first to the P3HT films.6 It is well-known that the solvent additive with high-boiling points can modify the crystallinity of PEDOT10-12 and enhance the electrical conductivity of the PEDOT:PSS film.13-16 PEDOT:PSS films prepared from PEDOT:PSS solution by adding 5 wt% of ethylene

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glycol (EG) and DMSO are also examined with a special interest in relations between S and oxidation level.

Figure 1. Chemical structure of PEDOT:PSS.

EXPERIMENTAL PEDOT:PSS films with or without solvent additives were prepared on ITO electrodes as described below. A 60 μL of PEDOT:PSS dispersion (Clevios, PH1000) was drop-casted on an ITO glass substrate (Kuramoto Co., Ltd., ITO thickness ~150 nm, sheet resistance ~8 Ω/sq) using a silicone-formwork with a diameter of 1 cm and then dried at 60 °C for 30 min in air. For the preparation of PEDOT:PSS films containing DMSO or EG, DMSO or EG was mixed with the PEDOT:PSS solution (5% by volume) and the mixed solution was stirred continuously for 24 h at room temperature. The mixed solutions were casted on ITO electrodes similarly to the case of pristine PEDOT:PSS and dried at 60 °C for 60 min in vacuum to obtain the films. These films are

4


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named as PEDOT:PSS-EG and PEDOT:PSS-DMSO. Thicknesses of these films evaluated by a 3D laser microscope (Keyence, VK-9700) were around 1 μm. Thus prepared PEDOT:PSS films were transferred to acetonitrile (AN) containing 0.1 M tetrabutylammonium perchlorate (TBAP) for electrochemical studies of the PEDOT:PSS films by the three-electrode system with polymer-coated ITO electrode, Pt-wire, and Ag/Ag+ as working, counter, and reference electrodes, respectively. Physicochemical properties of PEDOT:PSS films are well known to be sensitive to polar solvents.13-16 AN was chosen in this study as the most appropriate electrochemical medium because of a negligible influence of AN on σ and S of PEDOT:PSS films (Supporting information, Section 1). The rest potentials of these films were ca. 0.12 V vs. Ag/Ag+. Cyclic voltammograms (CV) were obtained with an automatic polarization system (Hokuto Denko Corp., HSV-110). For the control of oxidation state and the determination of oxidation levels for PEDOT:PSS, a potentiostat/galvanostat (Hokuto Denko Corp., HAB-151) with an X-Y recorder (Riken Denshi Co., Ltd., F-57) and a coulometer (homemade) were used. At first, the potential of the PEDOT:PSS film was set at -1.4 V to completely reduce the film to a neutral state, which was confirmed by the absorption spectra (Supporting information, Section 2). Then, the potential was stepped from -1.4 V to a certain potential (Ea) to control the oxidation state of the PEDOT:PSS film. The amount of electricity (Q) passing through the electrode during keeping the potential at Ea was recorded by the coulometer and was used to determine the oxidation level by the following equation: 5



(𝑄𝐹) Oxidation level (%) = 𝑊 × 100 (𝑀)

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(1)

where F, W, and M denote the Faraday constant, the weight of PEDOT:PSS film, and the sum of molecular weights of monomer units of PEDOT and PSS, respectively. Measurements of oxidation levels were repeated in the same solution by changing Ea to determine a potential dependence of oxidation level. Oxidation states of PEDOT:PSS films on ITO were controlled by potential in AN and the respective films were peeled off from the ITO electrodes after being removed from AN and dried in air. Measurements of TE properties were performed immediately after preparation of the free-standing PEDOT:PSS films as in the case of P3HT films.6 The σ values were measured using a four-probe method with a resistivity meter (Loresta-GP MCP-T610, Mitsubishi Chemical Corp.). To evaluate S of the film, a custom-made setup composed of thermocouples and Peltier devices was used (Supporting information, Section 3).17 UV-Vis-NIR absorption spectra were taken on a spectrophotometer (Shimadzu, UV-3150,). All these measurements were made at room temperature.

RESULTS AND DISCUSSION Figure 2 depicts CV curves of pristine PEDOT:PSS, PEDOT:PSS-EG, and PEDOT:PSS-DMSO films, along with a CV curve of P3HT film for a comparison. We note first that the CV curves of PEDOT:PSS-EG and -DMSO films are shifted slightly to a cathodic direction relative to that of a pristine PEDOT:PSS film. More importantly, oxidation of the three PEDOT:PSS films occurs at 6


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potentials much more negative than that of P3HT used in our previous study.6 The remarkable negative shift of oxidation potential for the PEDOT:PSS films is ascribed to the introduction of electron-donating alkoxy groups at β-position of thiophene ring18-20 and it implies that low oxidation states of PEDOT:PSS films can be more easily oxidized than that of P3HT and, consequently, oxidation levels of PEDOT:PSS films controlled in AN are likely to be changed (increased) during measurements of σ and S in air. Actually, however, time course of absorption spectra of a lightly oxidized PEDOT:PSS-EG film measured in air (Supporting information, Section 4) demonstrates that an influence of air oxidation for the PEDOT:PSS films is negligible for determinations of σ and S.

Figure 2. Cyclic voltammograms of polymer films deposited on ITO electrodes in TBAP (0.1 M)/AN. (red: pristine PEDOT:PSS, blue: PEDOT:PSS-EG, green: PEDOT:PSS-DMSO, black: P3HT)

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Oxidation levels for the three sorts of PEDOT:PSS films determined by PSC as a function of potential are shown in Figure 3. These semilogarithmic plots are similar except for in the low potential region where a slight difference is seen in their CV curves of Figure 2. Logarithms of oxidation levels increase linearly with an increase in potential, deviate downward from the straight lines, and finally tend to level off at oxidation levels of around 20 %, corresponding to one positive charge on every five thiophene rings.

Figure 3. Oxidation levels of PEDOT:PSS films plotted against potential, measured in TBAP (0.1 M)/AN. (red: pristine PEDOT:PSS, blue: PEDOT:PSS-EG, green: PEDOT:PSS-DMSO)

Electrical conductivities and Seebeck coefficients were measured for the free-standing films of pristine PEDOT:PSS, PEDOT:PSS-EG, and PEDOT:PSS-DMSO, whose oxidation states were precisely controlled by potential in AN, and in Figures 4 (a) and (b) are plotted the observed σ and S values, respectively, against oxidation level determined from Figure 3. In Figure 4 (a), we note a

8


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large difference in σ (more than two orders of magnitude) between a pristine PEDOT:PSS and solvent-added PEDOT:PSS films at high oxidation levels, in accord with a drastic enhancement of σ due to addition of EG and DMSO to a pristine PEDOT:PSS film as has been reported so far. We note further that the double logarithmic plots for the three polymer films have slope values greater than unity, although the slope for the pristine PEDOT:PSS is not much greater than unity. The large slope values exceeding unity have been found for common conducting polymers including P3HT and the findings have been discussed in detail in our previous studies.21-23 As shown in Figure 4 (b), Seebeck coefficients of the three PEDOT:PSS films were decreased with an increasing oxidation level. This and an increasing trend of σ with oxidation level shown in Figure 4 (a) for the PEDOT:PSS films demonstrate a trade-off relation between σ and S commonly observed for organic as well as inorganic TE materials. We also note that in a clear contrast to the case of P3HT,6 where the double logarithmic plot between S and oxidation level fits a single straight line with a slope of -1, the decrease of S with oxidation level for the pristine PEDOT:PSS is obviously small (S decreases only from 79 to 28 V K-1 for the increase of oxidation level from 0.07 to 21%). In addition, the plot bends downwards slightly and more sharply at higher oxidation levels. As to the PEDOT:PSS-EG and -DMSO films, on the other hand, the S values were much smaller than those for the pristine PEDOT:PSS over entire oxidation levels studied and both of the plots fit a straight line with a slope of -0.25 except those at oxidation levels higher than ca. 12%, corresponding to the rest potential of 0.12 V vs. Ag/Ag+ for the PEDOT:PSS films in AN. The 9



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deviation from the linearity for the PEDOT:PSS-EG and -DMSO films and the marked decrease of S for the pristine PEDOT:PSS at higher oxidation levels were ascribed to the overoxidation of the PEDOT:PSS films.24 Plots of log σ vs. S are empirically known to show a linear relationship,25 and thus the same log σ vs. S plots were made using the data of Figure 4 obtained with the pristine PEDOT:PSS, PEDOT:PSS-EG, and PEDOT:PSS-DMSO. It is seen from Figure 5 that all the plots deviate from straight lines at higher oxidation levels where marked decreases of S due to overoxidation of the PEDOT:PSS films are noticed in Figure 4 (b), indicating that an irreversible change of the PEDOT:PSS films caused by overoxidation leads to a failure of the empirical rule between σ and S. Crispin et al. have reported changes of σ and S with oxidation level for a pristine PEDOT:PSS film by using a set-up of organic electrochemical transistor (OECT).26,27 The observed dependence of σ and S on oxidation level are considerably different from those obtained in the present study (Supporting information, Section 5). In their study, chronocoulometric determinations of oxidation levels were made in the two-electrode system (application of voltage between the gate electrode and the source/drain electrode), while in the present study the three-electrode system is used to precisely control the oxidation state of a PEDOT:PSS film by applying potential to PEDOT:PSS film with respect to the reference Ag/Ag+ electrode. Another possible reason for the difference in σ and S between the studies of Crispin et al. and us may lie in the difference of the electrolyte cations (H+ and TBA+, respectively) which are incorporated into the polymer network upon reduction of the 10


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polymer films. In our preceding study with P3HT, an influence of electrolyte anion on the relation between log S and oxidation level was investigated and a negligible influence was found between the two sorts of anions, ClO4- and PF6-.6 These two bulky anions have similar ionic sizes and are unlikely to affect physicochemical properties of conducting polymers. We recall also that the data of Crispin’s group are obtained at oxidation levels as large as 10-30 %, where the overoxidation of PEDOT:PSS is likely to occur.

(a)

(b)

Figure 4 (a) Electrical conductivities (σ) and (b) Seebeck coefficients (S) of PEDOT:PSS films plotted against oxidation level (red: pristine PEDOT:PSS, blue: PEDOT:PSS-EG, green: PEDOT:PSS-DMSO, filled circle: region of overoxidation, black dashed line in (a): guide for eyes to show y = x)

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Figure 5. Semi-log plots of electrical conductivities against Seebeck coefficients for PEDOT:PSS films at various oxidation levels. (red: pristine PEDOT:PSS, blue: PEDOT:PSS-EG, and green: PEDOT:PSS-DMSO, filled circle: region of overoxidation)

CONCLUSION By taking account of overoxidation of the PEDOT:PSS films, Seebeck coefficients (S) and electrical conductivities () of pristine, and EG- and DMSO-added PEDOT:PSS films were correlated reasonably with oxidation level precisely controlled and determined by a potential-step chronocoulometry (PSC). A clear difference in the dependence of S on oxidation level was found between the PEDOT:PSS films studied here and the poly(3-hexylthiopene) (P3HT) film reported earlier. In contrast to a large slope value of -1 in the double logarithmic plot of S and oxidation level for P3HT, a pristine PEDOT:PSS film exhibited only a slight decrease of S (79 to 28 V K-1) by increasing oxidation level from 0.07 to 21%. Addition of EG or DMSO to PEDOT:PSS enhanced σ values, but led to a decrease of S. Both σ and S values of the solvent-added PEDOT:PSS films

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became sensitive to oxidation level and double logarithmic plots of S and oxidation level were found to fit a straight line with a slope of -0.25.

SUPPORTING INFORMATION

Optimization of solvents for TE measurements, absorption spectra of the PEDOT:PSS film applied various electrode potentials, set-up for the measurement of Seebeck coefficients, confirmation of reliability of TE measurements, comparison of our result with Crispin’s report, and estimation of power factors

ACKNOWLEDGEMENTS

This research was financially supported in part by JSPS KAKENHI Grant Numbers 16K05920 (I. I.) and 17K05965 (Y. H.), and a grant from SEI Group CSR Foundation (I. I.).

CONFLICTS OF INTEREST There are no conflicts of interest to declare.

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TOC Graphic

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Seebeck Coefficients of Poly(3,4-ethylenedioxythiophene):Poly

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