The optimization of thermoelectric properties in a PEDOT:PSS thin film through post-treatment

Abstract

Poly(3,4-ethylenedioxylthiophene):poly(styrene sulfonate) (PEDOT:PSS) has been investigated as a thermoelectric (TE) material extensively. Post-treatment using various solvents has been used to improve its TE performance. Nevertheless, the effects of using mixed co-solvents and post-treatment temperature have hardly been systematically studied. Herein, the TE properties of PEDOT:PSS thin films after solvent post-treatment are investigated. Different ratios of ethylene glycol (EG) and dimethyl sulfoxide (DMSO) as co-solvents and different treatment temperatures are used to optimize the TE properties. It is demonstrated that DMSO post-treatment is more efficient than the co-solvent or EG single solvent at removing insulating PSS from these thin films due to its high dielectric constant. DMSO treated specimens exhibit a power factor as high as 28.95 μW mK⁻². Under room temperature post-treatment, PSS is depleted and conformational changes of PEDOT are triggered. This leads to higher electrical conductivity, without an apparent effect on the redox level of PEDOT. Under higher treatment temperature, PEDOT:PSS shows a certain degree of reduced form which leads to a higher Seebeck coefficient. Meanwhile, the electrical conductivity drops and the Seebeck coefficient increases first and then drops with increasing temperature. The reason that the Seebeck coefficient increases is mainly because of the redox level variation under high temperature. With the combination of co-solvent and temperature, the highest power factor of 37.05 μW mK⁻² is obtained for PEDOT:PSS post-treated with DMSO at 120 °C. Assuming a thermal conductivity of 0.17 W m K⁻¹, the corresponding ZT of such PEDOT:PSS film is 0.065. This demonstrates that post-treatment is an effective way to optimize the TE properties of PEDOT:PSS. Furthermore, the combined use of solvent and temperature shows the potential of effective tuning of the TE properties of PEDOT:PSS via a more simple and environmentally friendly process.

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Introduction

Thermoelectric (TE) materials can convert a temperature gradient into electricity. They have attracted much attention as energy harvesting materials.^1–3 Generally, thermoelectric materials can be used to collect waste heat to produce electricity, or vice versa as solid-state Peltier coolers.^4,5 Despite the great advantages of TE materials, their relatively low efficiency greatly limits their wide application.⁶ The performance of TE materials is described by a dimensionless figure of merit, ZT, which is qualified as

ZT = σS²T/k (1)

where σ is the electrical conductivity, S stands for Seebeck coefficient (or thermopower), k is the thermal conductivity and σS² represents the power factor.

To achieve a high ZT value, materials are required to have a high power factor and low thermal conductivity. However, it is difficult to independently enhance the electrical conductivity without depressing the Seebeck coefficient due to their competing relationship. Traditional inorganic TE materials such as bismuth telluride (Bi₂Te₃) have a relatively high ZT value of ∼1 at room temperature. However, inorganic TE materials have high thermal conductivity, making it hard to further enhance the energy conversion efficiency. Furthermore, inorganic TE materials are usually toxic, rigid, rare and hard to process, limiting their use in various applications. Therefore, it is urgent to find low cost TE materials with higher ZT.^7,8 Recently, organic conducting polymers have attracted more and more attention due to their low thermal conductivity, low density, ease of synthesis and controllable electrical conductivity. Besides, they are relatively cheap and can be processed on flexible substrates.

Among various conducting polymers having been investigated as TE materials, poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is considered to be a very promising candidate. The uniformly aqueous solution of ionic conductive PEDOT with PSS as stabilizer and counterion makes the material easy processable and flexible. However, the high charge carrier concentration and relatively low Seebeck coefficient of pristine PEDOT:PSS are not especially desirable to achieve the expected high ZT.^9,10

Generally, several approaches have been used to fabricate high performance TE materials. (1) New synthesis method.^8,11 This can make new polymer chains with different doping levels, which can better adjust the TE performance of polymer. Power factor can be as high as 1270 μW mK⁻². But the process is very complicated. (2) To fabricate organic–inorganic nanocomposites.^12,13 Electrical conductivity and Seebeck coefficient can be enhanced to some extent, but ZT is still 2 orders of magnitude lower than inorganic materials. (3) To fabricate TE device.^14,15 Multilayered TE device exhibits high Seebeck coefficient because it is the total TE effect of each part. However, intrinsic TE efficiency needs to be enhanced first so that the device can have higher ZT. (4) Post treatment.^16–22 Addition of solvent can largely enhance electrical conductivity due to the depletion of insulating PSS and the conformational changes of PEDOT,^19,20 while post-treatment is said to be more effective on enhancing electrical conductivity and Seebeck coefficient simultaneously.²³ It is a rather simple method to change the conformation and oxidation level of PEDOT to optimize ZT value. Polar solvent,¹⁹ acid,²⁴ base²⁵ or salt solutions²⁶ can reduce the interactions between PEDOT and PSS in different ways which results in easier depletion of insulating PSS. Hence, electrical conductivity is enhanced. However, co-solvent is rarely used which may have synergistic effect. Experimental conditions such as temperature have yet to be considered. Temperature may influence the solubility and conformational changes of PEDOT:PSS, which may also affect its TE properties. In this case, co-solvent between EG and DMSO, and post-treatment temperature are put into consideration to further adjust TE properties of PEDOT:PSS.

In this work, a facile approach involves the use of co-solvent between ethylene glycol (EG) and dimethyl sulfoxide (DMSO), is considered to dedope pristine PEDOT:PSS to improve TE performance. We compared effect of the two single solvent and their co-solvent. Furthermore, elevated post-treatment temperatures are also used for the first time to optimize Seebeck coefficient to explore its potential for tuning TE properties of PEDOT:PSS thin films. Finally, systematic thermoelectric, morphological and spectra microscopy characterizations are carried out to investigate the mechanism behind TE performance enhancement. The combination of co-solvent and treatment temperature on PEDOT:PSS offers a better approach to optimize thermoelectric properties of conducting polymers.

Experimental

Materials and reagents

PEDOT:PSS aqueous solution (Clevios PH 1000) was purchased from H. C. Stark. The concentration of PEDOT:PSS is 1.3 wt%. The weight ratio of PSS to PEDOT is 2.5. DMSO (99%) and EG (99%) were purchased from Bodi Chemicals (Tianjin, China). All chemicals were used without further purification.

Fabrication of PEDOT:PSS nanofilms and post treatment process

PEDOT:PSS films were fabricated by spin-coating PEDOT:PSS solution onto glass substrates (1.8 × 1.8 cm², pre-cleaned by detergent, de-ionized (DI) water, acetone and isopropanol, successively) under 3000 rpm for 30 s. PEDOT:PSS solutions were filtered through a 0.45 μm syringe filter prior to spin coating. The PEDOT:PSS films were subsequently dried in an oven at 130 °C for 15 min.

Mixtures of DMSO and EG were prepared by adding DMSO directly into EG. Post treatment was performed by immersing PEDOT:PSS films in co-solvents with different EG to DMSO ratio for 30 min. Subsequently, the films were dried at 130 °C for 30 min to remove residual solvent.

Characterization of PEDOT:PSS films

To evaluate the influence factor of overall properties, electrical conductivity, Seebeck coefficient, atomic force microscope (AFM), ultraviolet-visible light detector, X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy characterizations were conducted. Electrical conductivity was measured through sheet resistance. Sheet resistance (R_s) of polymer film was measured with a four-probe equipment (RTS-8, 4-probes Tech.). The thicknesses (d) of these films were obtained using scanning electronic microscope (SEM) and the accelerating voltage used was 20 KV. Electrical conductivity (σ) was calculated as following:

σ = 1/(R_sd) (2)

Seebeck coefficient was measured by home-made device at room temperature in ambient atmosphere. The surface morphology was characterized with AFM (NanoScope MultiMode Explore, Veeco Instruments) operated by tapping mode. Absorption spectra of the films were measured using ultraviolet-visible light detector (UV-3600 Shimadzu, Japan). Raman spectra were performed using a 532 nm laser line as an excitation source on Confocal Raman spectrometer (LabRAM HR, HORIBA JobinYvon S.A.S.). X-Ray photoelectron spectroscopy (XPS, Axis Ultra DLD, Kratos Co., UK) characterization was conducted to measure the ratio of PSS to PEDOT on the surface of the films. All freshly made samples were used for these characterizations.

Result and discussions

Post treatment of PEDOT:PSS at room temperature

The influence of DMSO and EG co-solvent post treatment on thermoelectric (TE) properties of PEDOT:PSS thin films is shown in Fig. 1. x–y in the figure represents the ratio of EG to DMSO, e.g. 3–7 denotes that the ratio of EG to DMSO in co-solvent is 3 to 7.

Fig. 1 Electrical conductivity (a), Seebeck coefficient (b) and power factor (c) of PEDOT:PSS nanofilms under different post treatment conditions.

It was reported that polar solvent, such as DMSO or EG, can largely enhance electrical conductivity while preventing the decreasing of Seebeck coefficient.^16,23 Hence, the ratio between EG and DMSO in co-solvent at which optimized power factor occurs is investigated. As shown in Fig. 1(a), EG and DMSO post treatment has apparent effect on enhancing the electrical conductivity of these PEDOT:PSS nanofilms. With increasing portion of DMSO in the co-solvents, electrical conductivity rises. The nearly linear relationship between electrical conductivity and co-solvent ratio indicates that co-solvent is not showing synergistic effect in electrical conductivity enhancement comparing to single solvents. The highest electrical conductivity reaches 952.84 S cm⁻¹ after DMSO treatment. Meanwhile, the film thickness of these nanofilms treated by DMSO, 3–7, 5–5, 7–3, EG are 50.12 nm, 57.67 nm, 78.09 nm, 84.94 nm, 92.6 nm, respectively. It is thought that polar solvents such as: DMSO and EG, are able to reduce the interactions between PEDOT and PSS. This leads to easier removal of PSS and higher electrical conductivity.^16,19,27,28 The decreasing thicknesses and increasing electrical conductivity with more DMSO demonstrates that DMSO is a good solvent for depleting PSS from PEDOT:PSS. This agrees with findings in the literatures.^1–3 As shown in Fig. 1(b), the Seebeck coefficient of films treated by DMSO is slightly higher than pristine and co-solvent treated films. The Seebeck coefficient also decreases with increasing EG concentration from 18.98 μV K⁻¹ to 14.45 μV K⁻¹. The higher electrical conductivity and Seebeck coefficient results in the highest power factor for DMSO treated PEDOT:PSS films among all. The power factor of DMSO treated nanofilms is as high as 28.95 μW mK⁻², which is much higher than EG treated samples as 9.94 μW mK⁻² and pristine PEDOT:PSS as 0.008 μW mK⁻². Thus, it is believed that DMSO post treatment is a more effective way to improve thermoelectric properties compared with co-solvent and EG single solvent.

To determine the mechanism of above observed phenomenon, surface morphology of the treated specimens is characterized with AFM. It is thought that morphology changes during evaporation of solvents, can affect electrical conductivity and Seebeck coefficient.²⁹ Fig. 2 shows the AFM images of PEDOT:PSS nanofilms before and after treated by different co-solvents. Fig. 2(a) represents height image of pristine PEDOT:PSS films without post treatment; (b)–(f) show topographic images of PEDOT:PSS treated by co-solvent with the ratio of EG to DMSO as 0–10, 3–7, 5–5, 7–3 and 10–0, respectively. (g)–(l) are phase images correspondence to the ones above. Grain-like morphology is apparently appeared after post treatment. The roughness of the surfaces are 0.888, 1.678, 1.464, 1.343, 1.435 and 1.303 nm, respectively. It is well-known that addition of DMSO and EG can deplete PSS out from PEDOT:PSS films which lead to phase separation between PEDOT and PSS.¹⁸ DMSO treated films have highest roughness, which demonstrates that excess PSS is effectively removed during post treatment process. In the phase images in the lower row, bright region (positive) corresponding to PEDOT-rich grains while dark region (negative) corresponding to PSS-rich grains. Apparent grains are formed in these films which indicates obvious phase separation between PEDOT and PSS after post treatment. It also indicates that DMSO may have the best effect on removing PSS and cause phase separation. The removal of insulating PSS leads to shorter distances between conducting PEDOT chains, which causes electrical conductivity to rise from 0.3 S cm⁻¹ to 952.84 S cm⁻¹. Thus, DMSO possesses the highest electrical conductivity due to depletion of PSS and formation of large grains. It should be noted that the morphology of PEDOT can also be smooth, but only with other small ions instead of PSS.³⁰

Fig. 2 AFM images of co-solvent post treated samples. (a)–(f) are topographic images of pristine, 0–10, 3–7, 5–5, 7–3 and 10–0, respectively. (g)–(l) are phase images correspondence to the ones above. Scanning area: 2 μm × 2 μm.

To further confirm the amount of the removal of PSS, ultraviolet-visible light spectra study was carried out. Fig. 3(a) shows the UV spectra of these films before (pristine) and after post treatment. The two absorption bands between wavelength of 190 nm and 250 nm originate from the aromatic rings of PSS are observed. The reduction in PSS content can be indicated by decreased intensity in these two bands. As shown in Fig. 3(a), significant decrease in these two bands is observed. In particular, DMSO-treated specimen exhibits the lowest peak intensity, which indicates its depletion of PSS is the most among all. This agrees with the above AFM observations. Meanwhile, the peak intensity for 3–7, 5–5, 7–3 and EG is slightly higher than that of DMSO, but the difference among them is quite small. Together with AFM and UV results, it is demonstrated that DMSO-treatment has the best effect on depleting PSS to afford higher electrical conductivity.

Fig. 3 UV absorption spectra of PEDOT:PSS films using co-solvents. (a) Wavelength between 190–300 nm; (b) wavelength between 450–1100 nm.

It is well known that polar solvents with higher dielectric constants induce a stronger screening effect between counter ions and charge carriers, which reduces the Coulomb interactions between positively charged PEDOT and negatively charged PSS dopants.³¹ The relative dielectric constant of DMSO and EG are 46.6 and 37.7 at 20 °C, respectively.³² DMSO has higher dielectric constant which can have stronger screening effect to reduce interactions between PEDOT and PSS. Thus, PSS can be depleted more easily after DMSO post treatment. With more insulating PSS depleted from the surface of PEDOT:PSS films, the amount of conducting PEDOT increases. So, these samples have higher electrical conductivity. This leads to the highest Seebeck coefficient for DMSO treated sample. Therefore, DMSO treated samples have better thermoelectric properties.

For conducting polymers, charge carrier concentration in polymer matrix can largely influence electrical conductivity and Seebeck coefficient.^25,33 PEDOT has three redox levels as shown in Sketch 1. Pristine PEDOT:PSS has a broad absorption band in the near infrared region that indicates the existence of PEDOT²⁺. After reduction, the main peak moves to 900 nm for polaron PEDOT⁺ and to 600 nm for neutral PEDOT⁰. With oxidation level changes from bipolaron to neutral, Seebeck coefficient increases due to lower charge carrier concentration.²³ However, there is no apparent peak appearing after co-solvent post treatment which demonstrates that DMSO and EG co-solvent post treatment does not have any effect on changing the redox level of PEDOT:PSS (Fig. 3(b)).

Chart 1 Different redox levels of PEDOT.²³

To further confirm that PSS is depleted successfully from PEDOT:PSS films, XPS characterization is used. Fig. 4 represents the calculated results of PSS/PEDOT by the ratio of PSS and PEDOT bands area. The ratios decrease much from 2.84 to 1.321 after DMSO post treatment. With EG increasing, ratio of PSS/PEDOT increases from 1.321 to 1.70. This further confirms that more PSS are depleted instead of PEDOT and DMSO has the best effect on washing PSS.

Fig. 4 Bar plot result of PSS/PEDOT by XPS characterization.

Besides morphology and redox level, the conformation of PEDOT is also reported to have important influence on TE properties.³⁴ To study this issue, Raman spectroscopy studies are carried out. Fig. 5 shows the Raman spectras of these PEDOT:PSS films. The main peak for pristine PEDOT:PSS appears at 1435 cm⁻¹, the peak grows and becomes more narrow and shifted to red region to 1430 cm⁻¹ after post treatment. It is well-known that 1425 cm⁻¹ and 1453 cm⁻¹ are assigned to the Raman peak of symmetric stretching mode of Cα–Cβ of quinoid and Cα=Cβ of benzoid thiophene ring, respectively.^35–37 Thus, the red-shifted peak, enhanced intensity and narrow band indicate the morphology change of PEDOT:PSS from benzoid structure to a more conducting quinoid structure. PEDOT in benzoid structure tends to have a coil morphology, while quinoid structure tend to have linear morphology. Thus, post treatment can change the PEDOT:PSS chain from coil structure to linear structure, which is more helpful for charge carriers to move and cause higher electrical conductivity. Therefore, it is demonstrated that EG-treatment tends to have better effect on transferring PEDOT conformations.

Fig. 5 Raman spectra of the prepared PEDOT:PSS using different solvent.

Overall, electrical conductivity and Seebeck coefficient of PEDOT:PSS increase a lot after co-solvent post treatment at room temperature. Co-solvents only have physical effect on removal of PSS but have no effect on changing the redox level of PEDOT. DMSO treated specimen exhibits the highest power factor mainly due to the stronger screening effect on reducing interactions between PEDOT and PSS caused by higher dielectric constant. Thus, DMSO treatment was used for following investigation regarding treatment at elevated temperature.

Post treatment of PEDOT:PSS at elevated temperature

To further investigate the effect of experiment conditions on thermoelectric properties of PEDOT:PSS nanofilms, post treatment at different temperatures was conducted. As shown in Fig. 6(a), the electrical conductivity of PEDOT:PSS films decreases from 952.84 S cm⁻¹ to below 600 S cm⁻¹ under higher temperatures. However, electrical conductivity varies little when temperature changes from 80 °C to 140 °C. The drop in electrical conductivity under high temperature might be caused by the combined effect between removal of insulating PSS and the change of oxidation level in PEDOT. As for Seebeck coefficient in Fig. 6(b), it increases from 18.98 μV K⁻¹ to 24.22 μV K⁻¹ from room temperature (25 °C) to 120 °C, and fall to 20.29 μV K⁻¹ when temperature rises further. Thus, power factor decreases at first because of low electrical conductivity at elevated temperature, and then increases due to the enhancement in Seebeck coefficient, and at last decreases caused by the low Seebeck coefficient under 140 °C. The highest power factor reaches 37.05 μW mK⁻² for PEDOT:PSS under 120 °C. Thus, the power factor of PEDOT:PSS films increases 4600 times by combination of post treatment and temperature.

Fig. 6 Electrical conductivity (a), Seebeck coefficient (b) and power factor (c) of DMSO treated PEDOT:PSS films under high temperature.

To investigate the mechanism of electrical conductivity reduction observed above, AFM characterization is used. Fig. 7 shows AFM images of DMSO treated samples under evaluated temperature. (a) to (e) represent topographic images with treating temperature as 25 °C, 80 °C, 100 °C, 120 °C and 140 °C. While (f) to (g) indicate phase images corresponding with the samples above. The roughness of these samples are 1.678, 1.632, 1.478, 1.471 and 1.536 nm, respectively. Room temperature samples possess the highest roughness, which means that their PSS have been depleted the most. However, for the rest of these samples, the roughness varies little, which is accorded with the little change in electrical conductivity. Moreover, Fig. 2(f) has the largest PEDOT grains, which is much easier for charge carrier hopping. While in the other images much smaller grains are observed, which decreases the mobility of charge carrier.

Fig. 7 AFM images of co-solvent post treated samples. (a)–(e) are topographic images of DMSO treated samples under room temperature, 80 °C, 100 °C, 120 °C and 140 °C, respectively. (f)–(j) are phase images correspondence to the ones above. Scanning area: 2 μm × 2 μm.

To investigate the mechanism of Seebeck coefficient enhancement, UV spectra is conducted. Fig. 8 shows the UV spectra of these PEDOT:PSS films treated with DMSO under different temperatures. Decrease in the two peaks originates from PSS is observed after DMSO post-treatment. The lowest peaks intensity is obtained for room temperature specimens, while for higher temperature specimens, the intensity changes little, which is accorded with electrical conductivity variation. Meanwhile, as shown in Fig. 8(b), peaks appearing at about 900 nm demonstrate the oxidation level varies from PEDOT²⁺ to PEDOT⁺ at elevated temperatures. This is different from post treatment at room temperature. It is reported that the oxidation level of PEDOT largely influences the Seebeck coefficient because the charge carrier concentration is altered in polymer matrix at different oxidation levels.^25,33,38 In this case, the variation of Seebeck coefficient can be attributed to the change of oxidation level of PEDOT from state of bipolaron to polaron.

Fig. 8 UV absorption of prepared PEDOT:PSS under high temperature. (a) Wavelength between 190–300 nm. (b) Wavelength between 450–1100 nm.

To further confirm the oxidation level changes, Raman spectras of these films are collected. Fig. 9 shows the Raman spectras of PEDOT:PSS post treatment at different temperatures. Apparent peak intensity enhancement at 1440 cm⁻¹ for high temperature samples evidently indicate the transformation from high conducting quinoid PEDOT structure to a low conducting benzoid one, corresponding to the dedoping of these PEDOT:PSS films.^39–41 The growing peaks at elevated treatment temperature also indicate the change of oxidation level in PEDOT, which further confirms the result shown in UV absorption. Thus, this is the main reason for Seebeck coefficient enhancement. From thermodynamics point of view, Seebeck coefficient is related to the charge carrier concentration and mobility in polymer matrix. It is reported that solvent post-treatment method can help form PEDOT nanocrystals, which may be good for conductivity enhancement.⁴² Formation of PEDOT nanocrystal may benefit from better carrier mobility. Crystallization of PEDOT exhibit a maximum temperature. So varying temperature may change the level of nanocrystal formation, which in turn change the Seebeck coefficient and electrical conductivity.

Fig. 9 Raman spectroscopy of PEDOT:PSS films under evaluated temperature.

Fig. 10 shows the comparison of this work with other investigations in literature involving post-treatment method. It can be observed that our results are comparable to some of the power factors reported in literatures. The chart indicates our work can indeed tune the TE properties of PEDOT:PSS to some extent. And treatment temperature has important influence on the final TE properties. However, the overall TE performance of our materials still needs to be further improved.

Fig. 10 Comparison of this work with other investigations in literature.^{18,23,25,43,44}

Conclusions

In conclusion, DMSO and EG co-solvents show physical interaction without synergistic effect on enhancing thermoelectric properties of PEDOT:PSS nanofilms. DMSO treated specimens have the best thermoelectric performance due to the most depletion of PSS. Room temperature post treatment only removes insulating PSS and cause conformational changes. It has no apparent effect on the redox level of PEDOT. In contrast, high temperature treated samples demonstrate higher Seebeck coefficient and lower electrical conductivity due to dedoping. With optimized solvent and temperature, power factor can be enhanced from 0.008 μW mK⁻² for pristine films to 37.05 μW mK⁻² for DMSO treatment under 120 °C. Assuming a thermal conductivity of 0.17 W m K⁻¹ (measured by Olga B. et al.⁸), this film shows a ZT of ∼0.065. This work demonstrates that temperature is indeed an important parameter during post-treatment, and a facile way of tuning oxidation level of PEDOT:PSS involving different parameters including temperature could be found which can provide new approaches to optimize thermoelectric performances of PEDOT:PSS films.

Acknowledgements

We express our sincere thanks to the National Natural Science Foundation of China for financial support (51273117 and 51121001). H. Deng would like to thank the Ministry of Education (Program for New Century Excellent Talents in University, NCET-13-0383), the Innovation Team Program of Science & Technology Department of Sichuan Province (2013TD0013) and Sichuan Province for financial support (2013JQ0008).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra09147g

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