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Article

The Investigation of High Quality PEDOT:PSS Film by Multilayer-Processing and Acid Treatment

by
Po-Wen Sze
1,
Kuan-Wei Lee
2,
Pin-Chiao Huang
3,
Dei-Wei Chou
4,
Bing-Siang Kao
5 and
Chien-Jung Huang
5,*
1
Department of Electrical Engineering, Kao-Yuan University, Kaohsiung 821, Taiwan
2
Department of Electronic Engineering, I-Shou University, Kaohsiung 84008, Taiwan
3
Department of Electrical Engineering, National Chung Hsing University, Taichung 40227, Taiwan
4
Department of Aviation & Communication Electronics, Air Force Institute of Technology, Kaohsiung 820, Taiwan
5
Department of Applied Physics, National University of Kaohsiung, Kaohsiung 81148, Taiwan
*
Author to whom correspondence should be addressed.
Energies 2017, 10(5), 716; https://doi.org/10.3390/en10050716
Submission received: 29 March 2017 / Revised: 27 April 2017 / Accepted: 15 May 2017 / Published: 18 May 2017
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Abstract

:
In this study, we have investigated the performance of multilayer films of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) treated with one of the perfluorinated carboxylic acids, named trifluoroacetic acid (TFA). According to the increased density of the PEDOT chain under unit area conditions, the sheet resistance (Rsq) has improved from 300 to 65 Ω/sq through additional processing of PEDOT:PSS from single layer to multilayer. After the further treatment with TFA, however, the Rsq of the multilayer PEDOT:PSS was enhanced to 45 Ω/sq, leading to the decline of film thickness from 400 to 270 nm. Both conductivity and work function based on X-ray photoelectron spectroscopy results have built a breakthrough by double-processing because of the higher density of conductive PEDOT chains and the increase of 0.4 eV alternatives to typical indium tin oxide substrate, respectively. This improvement is contributed to the development of more effective transparent electrodes.

1. Introduction

In recent years, flexible optoelectronic devices such as organic light-emitting diodes, organic solar cells, perovskite solar cells, touch panel displays, and electronic paper have drawn considerable research attention. Developments in these fields are particularly important for next generation displays, which may offer optoelectronic devices fabricated by low cost roll-to-roll process that are lightweight and mechanically flexible [1,2]. Indium tin oxide (ITO) has been widely used as a transparent electrode in optoelectronic devices. However, ITO has several drawbacks, including its high brittleness, which makes it unsuitable for flexible electronic devices. Furthermore, the scarcity of indium and high demand for its compounds has resulted in the increase of its cost. The ITO components of polymer solar cells comprise a large portion of their overall cost [3]. Additionally, ITO has some inherent problems such as poor transparency on short wavelengths of visible light (400 to 500 nm), a need for high temperature processing, and the potential for oxygen and indium to diffuse into the organic layer [4]. To address these problems, academia and industry are actively investigating new transparent conductive materials to replace ITO.
Many materials have been investigated as transparent electrodes; however, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is considered to be one of the best transparent conductor candidates. PEDOT:PSS is a conductive composite of two polymers (PEDOT and PSS). PEDOT is insoluble in water and based on the 3,4-ethylenedioxythiophene (EDOT) monomer unit. PEDOT can form an aqueous suspension when combined with PSS, improving its process ability. PEDOT:PSS is superior to other materials because of its workability, mechanical flexibility, and optical properties [5]. Despite these advantages, the low conductivity of PEDOT:PSS restricts its application in optoelectronic devices [6,7,8,9,10,11]. However, the carrier transport (buffer) layer of anode and cathode has played an important role for solar cells, such as Metal oxide material (MoO3, WO3) and PEDOT:PSS. Therefore, many researchers have studied ways to enhance the conductivity of PEDOT:PSS. Several techniques have been proposed to increase the conductivity of PEDOT:PSS films by adding organic compounds, such as sorbitol [12,13,14,15], dimethyl sulfoxide (DMSO) [15,16,17,18], glycerol [15,19,20], ethylene glycol (EG) [21,22,23], polyethylene glycol, and boric acid [22,23,24]. Nevertheless, the conductivity of such modified PEDOT:PSS films remains very low for practical applications. In this study, the conductivity of PEDOT:PSS is considerably enhanced by forming multilayered PEDOT:PSS films and using an organic acid treatment. The mechanism by which the PEDOT:PSS film conductivity was optimized is also studied.

2. Experimental Methods

The ITO glass substrate is obtained by a commercial source with 7 Ω/sq to become the basis for comparison. The PEDOT:PSS (Clevios PH 1000, Uni-onward corp., Taipei, Taiwan) doped with sorbitol (98%, Uni-onward corp., Taipei, Taiwan) was used as the solution for the preparation of the stacked multilayer film. Glass substrates with an area of 1.5 × 1.5 cm2 were precleaned with acetone, methanol, and deionized water (DI) water in an ultrasonic for 10 min each time, sequentially. The cleaned glass substrates were treated with UV/ozone for 60 s prior to spin coating. The PEDOT:PSS solution, which filtered through a syringe filter (0.45 mm pore size), was spin-coated on the clean glass substrate. The spin-coating was performed at a different rotation rate for 30 s. The PEDOT:PSS film was heated at 150 °C for 20 min on a hot plate in ambient conditions. Afterward, thicker PEDOT:PSS films were prepared by spin coating multiple times, and annealing and film treatment was done after each layer.
In addition, the acid treatment was carried out by dropping an acid solution of 100 μL on a PEDOT:PSS film. The PEDOT:PSS film is slightly rinsed by immersion in the DI water. Trifluoroacetic acid (TFA, Uni-onward corp., Taipei, Taiwan) is an organofluorine compound with the chemical formula CF3CO2H. We have researched the effect of TFA on PEDOT:PSS because it is a colorless liquid with a sharp odor similar to vinegar but stronger in acidity. TFA is an analogue of acetic acid with the three hydrogen atoms replaced by three fluorine atoms. The acidity of TFA is approximately 34,000 times stronger than that of acetic acid due to the electronegativity of the trifluoromethyl group. At the same time, TFA is widely used in organic chemistry for various purposes. Subsequently, the PEDOT:PSS film is dried at 150 °C. The Rsq of double treated PEDOT:PSS films were measured with a four-point sheet resistivity meter (SRM103, Solar Energy Tech., Taiwan). The film thickness was measured using alpha step surface profiler (Surfcorder ET400M, Tainan, Taiwan). The X-Ray photoelectron spectroscopy (XPS) was measured by an X-ray photoelectron spectrometer (JEOL, JAMP-9500F, Kaohsiung, Taiwan). The surface morphology and roughness of the PEDOT:PSS films were measured by atomic force microscopy (AFM, Park Systems, XE-70, Suwon, Korea), and the transmittance of the PEDOT:PSS films was measured by a UV/visible spectrometer (UV-3900, Hitachi, Tokyo, Japan). The work function of the PEDOT:PSS films was measured as the surface potential (VCPD) of the films by scanning Kelvin probe microscopy (SKPM, Park Systems, XE-100).

3. Results and Discussion

The PEDOS:PSS shows the chemical molecular structure and general arrangement under solution in Figure 1a. The chains of PEDOT are obviously much shorter than those of PSS, and the combination of both of them is due to the Columbic attraction. It is believed that the conductivity of the thin film of PEDOT:PSS can be enhanced after a series of material processes in Figure 1b,c, including heating and the addition of sorbital dopant. The sorbitol additive interacts with PEDOT and PSS, causing their chains to separate in solution in Figure 1b. At the same time, this treatment causes the coiled PEDOT and PSS chains to rearrange into linear chains [25,26,27]. In addition, Figure 1c has shown the schematic of heating PEDOT:PSS with sorbitol. The evaporation of the solution during film baking allows the PEDOT:PSS chains to be much closer in order to rearrange, in comparison to the situation with sorbitol. The rearrangement of PEDOT:PSS leads to the two polymers being closer to each other. This reduces the hopping distance between the PEDOT chains and enhances the conductivity of the PEDOT:PSS film.
The effects on transmittance, Rsq, and the films’ thickness are investigated in detail after spin-coating fabrication. Figure 2 shows the transmittance, Rsq, and film thickness of a single PEDOT:PSS layer processed under different spin coating conditions. Transmittance increased at higher spin-coating speeds. It is likely that the distribution of PEDOT:PSS chains with wider sizes is more discretely arranged in the thin film, resulting in higher transmittance. Conversely, the Rsq decreased in the films spun at lower spin speeds. This indicates a narrower size distribution of the arranged PEDOT:PSS chains, contributing to lower sheet resistance and film thickness. Notably, the Rsq is related to film thickness. However, the conductivity of the single layer PEDOT:PSS film remained very low for practical applications.
The transmittance of the different multilayer PEDOT:PSS film in the visible wavelength is shown in Figure 3a, and the three layers (PEDOT:PSS films) remained of a higher value than typical ITO film from 400 to 500 nm. The transmittance values in the whole visible spectrum of one, three, and five layers of PEDOT:PSS were 97.1%, 90.1%, and 81.1%, respectively. Figure 3b shows the variation of Rsq and transmittance with film thickness for the multilayer PEDOT:PSS films. The Rsq for the films was 283, 100, and 69 Ω/sq with thicknesses of 86, 250, and 400 nm, respectively, whereas the transmittances were 97%, 90%, and 79%, respectively. The layering process increased the PEDOT chain content per unit area, thus reducing the resistivity of the PEDOT:PSS films. The Rsq of a single layer was around 283 Ω/sq but decreased to 100 Ω/sq in the multilayered films. To meet the requirements of a functional electrode, different experimental parameters, including film thickness, Rsq, and transmittance, were adjusted, and the results are shown in Table 1. To obtain smoother films, a higher spin-coating speed was used. The first three layers of the PEDOT:PSS films were spin-coated at a rotation rate of 4000 rpm. The other two layers were spun at 6000 rpm to obtain the optimal Rsq of PEDOT:PSS films. Additionally, the thickness of processed PEDOT:PSS is increased under the condition of the change of ten counts.
A schematic of PEDOT:PSS films treated with trifluoroacetic acid (TFA) is shown in Figure 4. After forming the PEDOT:PSS film on a glass substrate, the surface of the film was treated with TFA. The mechanism of conductivity enhancement is related to the H+ transfer from the acid to the PSS− groups of PEDOT:PSS. This can be expressed as CF3CO2H + PSS− → C2F3O2− + PSSH. C2F3O2− forms an ionic bond with PEDOT, and the PSS− groups capture protons, neutralizing their charge. This reduces the Coulombic attraction between the PEDOT+ and PSS− chains [28]. However, PSS− can not be completely converted to PSSH, and some of the PEDOT chains retain PSS− chains. The hydrophilic PSS chains can be easily removed from the surface of PEDOT:PSS films by rinsing with deionized water. This lowers the Rsq; first, by increasing the relative proportion of PEDOT chains at the film surface; second, the TFA behaves as a dehydrating agent as the solution vaporizes. Thus, the energy barriers for inter-chain and inter-domain charge hopping are lowered, allowing for better charge transfer along the PEDOT chains.
The spin-coating speed, film thickness, Rsq, and transmittance values of untreated and acid treated PEDOT:PSS multilayers are shown in Table 1. The Rsq of a PEDOT:PSS film as an anode depends on its thickness. The thickness of the acid-treated films was 325 nm, which was lower than that of untreated pristine multilayer films (400 nm). Under the same conditions, the treated film had a Rsq of 45 Ω/sq. Since the film thickness decreased, the transmittance of the treated PEDOT:PSS film also slightly increased from 79% to 82%.
The conformational change of the PEDOT chains after the double-processing method was further studied by Raman spectroscopy. Figure 5 shows the Raman spectra of the pristine, the sorbitol added PEODT:PSS solution, and the TFA treated PEDOT:PSS films. The strongest band of Raman between 1400 and 1500 cm−1 corresponds to the stretching vibration of the Cα = Cβ bonds of PEDOT chains [29,30]. Furthermore, there are two peaks at the benzenoid vibration of 1440 cm−1 and quinoid vibration of 1415 cm−1. The benzoid structure is the coil conformation. The quinoid structure is the expanded-coil or the linear conformation [29,31]. The peak value shifts from 1440 to 1415 cm−1 after the post-treatment, indicating that the resonant structure of the PEDOT chains changes from the benzenoid structure to the quinoid structure.
Figure 6a shows the X-ray photoelectron spectroscopy (XPS) of single layer and multilayer PEDOT:PSS films. The XPS signal peaks between 162 and 166 eV correspond to the sulfur atom of PEDOT [32,33,34]. The signal near 169 eV corresponds to the sulfur atom of PSS. The PEDOT:PSS ratio was calculated from the areas under each peak using integral calculus. The PEDOT:PSS ratio increased from 0.355 before film treatment to 0.422 after film treatment. Figure 6b shows the XPS spectra of PEDOT:PSS multilayer films before and after acid treatment. The ratio increased further to 0.588 after dropping TFA on the film. However, the PEDOT:PSS ratio of the surface region of the multilayer films increased to a value 56% greater than that of the single layer film. Increasing the PEDOT-rich chains was expected to enhance the conductivity of the PEDOT:PSS film by forming better connecting pathways with other PEDOT chains.
Figure 7 shows that the film morphology changed after the multilayer and TFA treatments. In the AFM images, the bright and dark phase regions correspond to the PEDOT and PSS grains, respectively [35,36,37]. The roughness of a single layer of untreated PEDOT:PSS was 1.141 nm. The phase image is homogeneous, indicating weak phase separation between PEDOT and PSS and low surface roughness. The roughness of the untreated multilayer PEDOT:PSS film with five layers was higher than that of the single layer at 1.873 nm. After TFA treatment of the same PEDOT:PSS film, the surface roughness increased to 2.795 nm. The decreased Rsq measured for the multilayer PEDOT:PSS film subjected to both multilayer and acid treatments is consistent with our previous findings. These findings suggested that the polymer nanoparticles swell and aggregate together after the TFA treatment of the PEDOT:PSS films. However, the connectivity of the PEDOT:PSS film is improved between conductive regions, with the result that the compact and fiber-like PEDOT-rich chains facilitate charge transport. Incidentally, the surface roughness of the TFA-treated film remained considerably lower than that of a typical ITO substrate (4.961 nm).
PEDOT:PSS is the most widely used anode material that has a high work function, ensuring its effectiveness in transporting holes to the anode [38]. The work functions of PEDOT:PSS films fabricated by different processes are shown in Table 2. These include films with sorbitol additives and multilayer and acid-treated films. To determine the work function of acid-treated multilayer PEDOT:PSS films, we measured the surface potential (VCPD) by scanning Kelvin probe microscopy. The surface potential relates to the work function as VCPD = (Φtip − Φsample)/q, where Φtip and Φsample are the work functions of the tip and sample, respectively [39]. Taking 4.6 eV as the work function of highly oriented pyrolytic graphite (HOPG) was typically used as reference of SKPM measurement [40]. The measured work functions of the acid-treated multilayer PEDOT:PSS thin films ranged from 4.8 to 5.1 eV, compared with 4.7 eV for ITO.
Figure 8 shows the sign of the work function change for double-processing methods. In the situation without this dipole layer (Figure 8a), the work function is explained in the sorbitol added to the PEDOT: PPS films [13,41]. The shift Δ in work function is consistent with the rich surface layer that is present in PEDOT: PPS films. In Figure 8b,c, the increased the work function leads to the fact that the PSS-rich and TFA ion-rich top layers result in an inward directed surface potential dipole. The variation of the work function is related to the magnitude of the shift Δ depending on the surface layer thickness. Therefore, upon an increase of the surface layer thickness by double-processing methods, the work function will be enhanced.

4. Conclusions

We have fabricated the acid-treated multilayer PEDOT:PSS films exhibiting low Rsq and transmittance. It is found that the Rsq is enhanced from 283 to 69 Ω/sq in films comprising multiple PEDOT:PSS layers. Additionally, the Rsq of stacked PEDOT:PSS increases to 45 Ω/sq after treatment with TFA. The transmittance of the PEDOT:PSS films subjected to both treatments was 82% in the visible wavelength range from 400 to 700 nm. The measured work function of the treated PEDOT:PSS thin film is 5.1 eV compared with 4.7 eV for a typical ITO sample. The mechanism of conductivity enhancement of the PEDOT:PSS film is investigated. There is a model for optimal PEDOT:PSS film processing. Based on this model, we have determined four important factors for improving the quality of PEDOT:PSS films: phase separation between PEDOT and PSS; increasing film thickness; enhancing PEDOT chain content per unit area; and the application of an organic acid treatment. Various techniques, including physical, chemical, and electrical characterizations, have showed that acid treated multilayer PEDOT:PSS films are stable and of good quality. Both the high transparency and low Rsq of these PEDOT:PSS films have showed potential for their use as transparent conductive electrodes in optoelectronic devices.

Acknowledgments

This work was partially supported by the Ministry of Science and Technology (MOST) of Taiwan under contract No MOST 105-2221-E-390-027.

Author Contributions

Po-Wen Sze analyzed the materials; Po-Wen Sze, Kuan-Wei Lee, and Bing-Siang Kao designed the experiments; Po-Wen Sze, Kuan-Wei Lee, Dei-Wei Chou, and Bing-Siang Kao performed the experiments and analyzed the results; Pin-Chiao Huang and Chien-Jung Huang gave some useful information suggestions for this work and contributed the analysis tools; Po-Wen Sze, Kuan-Wei Lee, and Bing-Siang Kao drafted the manuscript; and Po-Wen Sze and Bing-Siang Kao finalized the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. The investigation includes the (a) chemical structure of PEDOT:PSS; (b) the rearrangement of PEDOT:PSS with sorbitol; and (c) the schematic illustration of the heating of PEDOT:PSS with sorbitol.
Figure 1. The investigation includes the (a) chemical structure of PEDOT:PSS; (b) the rearrangement of PEDOT:PSS with sorbitol; and (c) the schematic illustration of the heating of PEDOT:PSS with sorbitol.
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Figure 2. (a) Transmittance and (b) sheet resistance (Rsq) and film thickness for PEDOT:PSS of single layer films with different rotational speeds.
Figure 2. (a) Transmittance and (b) sheet resistance (Rsq) and film thickness for PEDOT:PSS of single layer films with different rotational speeds.
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Figure 3. (a) Transmittance of one, three, and five layers of PEDOT:PSS films; (b) Variation of Rsq and the transmittance of average value in the visible wavelength with film thickness for PEDOT:PSS.
Figure 3. (a) Transmittance of one, three, and five layers of PEDOT:PSS films; (b) Variation of Rsq and the transmittance of average value in the visible wavelength with film thickness for PEDOT:PSS.
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Figure 4. The schematic illustration of the mechanism for conductivity enhancement of PEDOT:PSS films by treatment with organic acid (TFA).
Figure 4. The schematic illustration of the mechanism for conductivity enhancement of PEDOT:PSS films by treatment with organic acid (TFA).
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Figure 5. The Raman spectra of the pristine, the Sorbitol added PEODT: PSS solution, and the TFA treated PEDOT:PSS films.
Figure 5. The Raman spectra of the pristine, the Sorbitol added PEODT: PSS solution, and the TFA treated PEDOT:PSS films.
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Figure 6. The S (2p) XPS spectra of (a) single layer and multilayer film and (b) untreated and acid treated film.
Figure 6. The S (2p) XPS spectra of (a) single layer and multilayer film and (b) untreated and acid treated film.
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Figure 7. The atomic force microscopy (AFM) images of (a) single layer PEDOT:PSS films; (b) multilayer PEDOT:PSS films; (c) multilayer PEDOT:PSS films treated with organic acid; and (d) Indium tin oxide (ITO).
Figure 7. The atomic force microscopy (AFM) images of (a) single layer PEDOT:PSS films; (b) multilayer PEDOT:PSS films; (c) multilayer PEDOT:PSS films treated with organic acid; and (d) Indium tin oxide (ITO).
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Figure 8. Schematic representation of the band diagrams during scanning Kelvin probe microscopy (SKPM) on PEDOT:PSS thin films. (a) aorbitol-treated PEDOT:PSS without PSS surface layer; (b) multilayer PEDOT:PSS with a PSS-rich surface layer; and (c) acid-treated PEDOT:PSS with a TFA ion-rich surface layer. ϕ s 0 , ϕ s and ϕ tips , are, respectively, the work functions of sorbitol-treated PEDOT:PSS thin film, multilayer “bulk” PEDOT:PSS, and the tip. Δmulti and Δacid are the surface dipole due to the PSS-rich and TFA ion-rich surface layer, which effectively enhances the film work function to ϕ s + Δmulti and ϕ s + Δmulti + Δacid, respectively.
Figure 8. Schematic representation of the band diagrams during scanning Kelvin probe microscopy (SKPM) on PEDOT:PSS thin films. (a) aorbitol-treated PEDOT:PSS without PSS surface layer; (b) multilayer PEDOT:PSS with a PSS-rich surface layer; and (c) acid-treated PEDOT:PSS with a TFA ion-rich surface layer. ϕ s 0 , ϕ s and ϕ tips , are, respectively, the work functions of sorbitol-treated PEDOT:PSS thin film, multilayer “bulk” PEDOT:PSS, and the tip. Δmulti and Δacid are the surface dipole due to the PSS-rich and TFA ion-rich surface layer, which effectively enhances the film work function to ϕ s + Δmulti and ϕ s + Δmulti + Δacid, respectively.
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Table
Table 1. The film thickness (d), sheet resistance (Rsq), and transmittance (T) of average value in the visible wavelength for PEDOT:PSS films (PH1000: 4 wt% Sorbitol), which are spin-coated at both 4000 and 6000 rpm, is compared to the result of trifluoroacetic acid (TFA) treatment.
Table 1. The film thickness (d), sheet resistance (Rsq), and transmittance (T) of average value in the visible wavelength for PEDOT:PSS films (PH1000: 4 wt% Sorbitol), which are spin-coated at both 4000 and 6000 rpm, is compared to the result of trifluoroacetic acid (TFA) treatment.
Layer (s)d (nm)Rsq (Ω/sq)T (%)
18628397
32509090
54006979
5 with TFA treatment3254582
Table
Table 2. Work function of PEDOT:PSS films spin cast with different processes.
Table 2. Work function of PEDOT:PSS films spin cast with different processes.
ProcessWork Function (eV)
PEDOT:PSS filmes with sorbitol4.87
Multilayer of PEDOT:PSS films5.08
Acid-treated PEDOT:PSS films5.12
Typical ITO sample4.78

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MDPI and ACS Style

Sze, P.-W.; Lee, K.-W.; Huang, P.-C.; Chou, D.-W.; Kao, B.-S.; Huang, C.-J. The Investigation of High Quality PEDOT:PSS Film by Multilayer-Processing and Acid Treatment. Energies 2017, 10, 716. https://doi.org/10.3390/en10050716

AMA Style

Sze P-W, Lee K-W, Huang P-C, Chou D-W, Kao B-S, Huang C-J. The Investigation of High Quality PEDOT:PSS Film by Multilayer-Processing and Acid Treatment. Energies. 2017; 10(5):716. https://doi.org/10.3390/en10050716

Chicago/Turabian Style

Sze, Po-Wen, Kuan-Wei Lee, Pin-Chiao Huang, Dei-Wei Chou, Bing-Siang Kao, and Chien-Jung Huang. 2017. "The Investigation of High Quality PEDOT:PSS Film by Multilayer-Processing and Acid Treatment" Energies 10, no. 5: 716. https://doi.org/10.3390/en10050716

APA Style

Sze, P.-W., Lee, K.-W., Huang, P.-C., Chou, D.-W., Kao, B.-S., & Huang, C.-J. (2017). The Investigation of High Quality PEDOT:PSS Film by Multilayer-Processing and Acid Treatment. Energies, 10(5), 716. https://doi.org/10.3390/en10050716

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