Open Access
Add to BibSonomy
Abstract
Poly (3,4-ethylene dioxythiophene) polystyrene sulfonate (PEDOT:PSS) thin films with tunable optical properties have shown great potential in optoelectronic device applications. Here, we present a new hybrid technology for accurately determining the dielectric properties of PEDOT:PSS films upon bias. Using electrochemical method together with numerical simulation of ellipsometry measurement, significant variations of the dielectric functions in a wide spectral range (400-1690 nm) under external voltage excitation are found. The ordinary dielectric constants increase under external voltage excitation, while the extraordinary dielectric constants remain unchanged. The simulation and experimental results are in a good agreement.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Electro-tunable dielectric materials have wide applications in active optoelectronic devices [1–3]. These materials, such as liquid crystals, graphene, and electrochemical materials, possess the benefits of high optical contrast, voltage-modulated dielectric constants, as well as reversibility [4–8]. Among these materials, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is favored because of its low switching voltage, bistability, water solubility and large-area fabrication [9–11]. As a conductive polymer, its optical properties can be modulated through the oxidation/reduction by ion exchange under external voltage excitation [12,13], which is ascribed to the change in its dielectric properties. Although the dynamic dielectric constants of PEDOT:PSS have been measured in aqueous electrolytes by using the variable angle spectroscopic ellipsometer (VASE) [14], it suffers from the drawbacks of incomplete tunability and uncontrollable dissolution [15]. Since the reduction PEDOT:PSS absorbs light from the ultraviolet (UV) to the terahertz region and beyond [16–18], the dynamic dielectric properties of PEDOT:PSS cannot be well solved by using only the VASE measurement [19]. In order to accurately measure the dynamic dielectric properties of PEDOT:PSS, it would be highly desirable to develop new measurement or analysis technology.
In this work, we present a new hybrid technology to investigate the dynamic dielectric functions of the PEDOT:PSS film from the visible to near-infrared band (400-1690 nm) by combining electrochemical method, VASE and finite-difference time-domain (FDTD) simulation. The PEDOT:PSS film is prepared by the solution spin-coating method. The optical properties of the film are modulated by electrochemical methods combined with the VASE measurement. An anisotropic Drude-Lorentz model is applied to analyze the data. Finally, the FDTD simulation is carried out to ensure the accuracy of the dielectric functions.
2. Experimental
A sapphire (Al2O3) wafer pre-coated with a platinum film electrode was used as a substrate. The PEDOT:PSS thin film was prepared on the Pt electrode by using the spin coating method with a 1.3 wt% PEDOT:PSS aqueous dispersion from Heraeus (Clevios PH1000 with a 1:2.5 weight ratio of PEDOT and PSS). The film was spin-coated at a spinning rate of 600 rpm for 9 s at the beginning, followed by 3000 rpm for 60 s, then annealed on a hot plate at 120℃ for 15 min to dry completely.
The oxidation and reduction states of the PEDOT:PSS samples was carried out in an electrochemical cell via an electrochemical workstation (CHI600E). The structure of the electrochemical cell is shown in Fig. 1(a). Here, the PEDOT:PSS/Pt/Al2O3, the Pt plate and the Ag/Ag+ act as the working electrode (WE), the counter electrode (CE) and the reference electrode (RE), respectively. A solution of acetonitrile with 0.1 M NaClO4 was used as the electrolyte to provide the metal ions for the redox reactions of the PEDOT:PSS samples. The whole electrochemical system was placed in a glove box with Ar as a protective gas. The oxidation and reduction PEDOT:PSS film were achieved at potentials of +0.6 V vs. Ag/Ag+ and -1.0 V vs. Ag/Ag+, respectively, and the potential holding time is 30 s. After the redox reaction, the samples were rinsed with pure acetonitrile to clear the residual NaClO4 from its surface. Due to the bistability of the PEDOT:PSS, these samples remain in the oxidation/reduction state after leaving the electrochemical cell, which facilitates its subsequent optical characterization.
Fig. 1. Schematic device and experimental setup. (a) Three-electrode electrochemical cell for redox reactions of the PEDOT:PSS film. The oxidation/reduction of the PEDOT:PSS were driven by the voltage +0.6V/-1.0V. (b) Variable angle spectroscopic ellipsometer (VASE) for the dielectric properties of the oxidation/reduction PEDOT:PSS film. The PEDOT:PSS layer was assumed to be biaxial anisotropic with the Drude-Lorentz model for ordinary dielectric constants and Drude model for extraordinary dielectric constants.
The dielectric constants of the PEDOT:PSS thin film in oxidation and reduction states were determined by VASE (J. A. Woollam Co. M-2000DI). The VASE system is shown in Fig. 1(b). The ellipsometric spectra were recorded from 400 to 1690 nm under the incident angles of 45°, 50° and 55°, respectively. The software WVASE (J. A. Woollam Co.) was employed to fit the elliptical data (See S1 in Supplement 1 for the details on the ellipsometric date and fitting). In the VASE measurements, due to the optical anisotropy of the PEDOT:PSS [20], the εo was described by a Drude-Lorentz model, and the εe was described by a Drude model [21,22]. The models are expressed as follows
For the simulation, the PEDOT:PSS film were spin-coated on glass substrates pre-sputtered with the ITO electrodes. Here, the oxidation and reduction states of the film were obtained by the same electrochemical method as mentioned above. The model consists of a bottom glass substrate, a middle ITO layer, and the top PEDOT:PSS thin film. The dielectric constants of the glass substrate are taken from Material Library of FDTD, which is called SiO2 (Palik); the film thickness and dielectric constants of ITO layer are obtained before by VASE measurements (see S2 in Supplement 1 for details); the thickness of the PEDOT:PSS thin film is set to 84.8 nm which measured by atomic force microscopy (AFM) method (see S3 in Supplement 1 for details); the dielectric constants of PEDOT:PSS is given in by VASE, and the optical anisotropy is also taken into account. The periodic boundary condition and perfectly matched layer (PML) boundary condition are applied.
3. Rrsults and discussion
The thickness and the dielectric constants of the PEDOT:PSS film are determined by the VASE. The fitting results show that the thickness of PEDOT:PSS thin film is about 79.37 ± 0.24 nm in both oxidation and reduction states. Figure 2 shows the dielectric constants of the oxidation/reduction PEDOT:PSS film determined by the VASE. Due to the natural optical anisotropy of the PEDOT:PSS film, its dielectric properties are expressed to ordinary dielectric constants εo = ε1o + ε2o (in-plane direction of the film), and extraordinary dielectric constants εe = ε1e + ε2e (out-of-plane direction of the film). Figure 2(a) and Fig. 2(b) show the real part of the ordinary dielectric constants ε1o and the imaginary part of the ordinary dielectric constants ε2o for the PEDOT:PSS film, respectively. In general, the εo of PEDOT:PSS film exhibits typical characteristics of a semiconductor. For the real part, the ε1o in the oxidation state is close to that in the reduction state in the visible region of 400 to 800 nm. In the near-infrared region of 800 to 1690 nm, the ε1o in the oxidation state is significantly lower than that in the reduction state. For the imaginary part, the ε2o in the reduction state is significantly higher than that in the oxidation state in the range of 400 to 1200 nm, while in the range of 1200 to 1690 nm, both the ε2o in the two states are close. The variation in ε1o would imply a possible change in the semiconductor properties of PEDOT:PSS under the redox reactions, while the variation in ε2o is possibly resulted from changes in its absorption. Figure 2(c) shows the extraordinary dielectric constants εe of the PEDOT:PSS film. Both the εe of the two states are approximately close to each other. This means that the redox tuning may have little effect on its extraordinary optical properties. The PEDOT:PSS film undergoes a complete oxidation/reduction reaction under +0.6V/-1V due to the large electrochemical window of the acetonitrile electrolyte [15]. That is the reason why the dielectric constants exhibit large variations.
Fig. 2. Dielectric constants of the oxidation (ox.) and reduction (red.) PEDOT:PSS film determined by VASE. The (a) real parts and (b) imaginary parts of ordinary dielectric constants of the ox./red. PEDOT:PSS film. (c) The real parts and imaginary parts of extraordinary dielectric constants of the ox./red. PEDOT:PSS film.
The accuracy of VASE measurements of PEDOT:PSS can be confirmed by comparing the simulation and experimental results of the optical properties of the film. Using the FDTD method, we simulated the transmittance and reflectance of the oxidation and reduction PEDOT:PSS film, and the relevant experimental data is also obtained by the spectrophotometer, as shown in Fig. 3. As can be seen in Fig. 3, the simulation and experimental results are in good agreement. The slight difference between the experimental and simulation results can be evaluated by the mean relative error (MRE) as
where N is the number of experimental parameters, Si and Ei is the simulation and experimental result of transmittance or reflectance at a certain wavelength, respectively. The transmittance exhibits a low MRE of 0.6% for the oxidation state and 1.0% for the reduction state, while for the reflectance the MRE is larger, at 9.6% for the oxidation state and 11.3% for the reduction state. It is worth noting that the high MRE in reflectance is mainly brought about by the deviation of the interference peak at 400 to 800 nm, because the reflectance has a smaller MRE of 2.9% for the oxidation state and 5.7% for the reduction state from 800 to 1690 nm. This may be due to the inhomogeneity in the thickness and the surface roughness of the PEDOT:PSS film (See S3 and S4 in Supplement 1 for details) measured in the experiment, resulting in weaker interference than the ideal flat film assumed in the simulation. Besides, due to the small numerical value of reflectivity, the measurement error also become one of the main reasons for the large MRE. Overall, the simulation results agree well with the experiment, which indicating that our hybrid measurement has high accuracy.Fig. 3. The experimental (symbol) and simulated (line) optical (a) transmittance and (b) reflectance of PEDOT:PSS(ox./red.)/ITO/Glass structure. The simulations were carried out by FDTD using the dielectric constants of PEDOT:PSS (ox./red.) from Fig. 2. The match between experiments and simulations confirms the reliability of VASE results in Fig. 2.
The dielectric constants of PEDOT:PSS measured by VASE can also explain the transmission and reflection in the oxidation and reduction states. As shown in Fig. 3, the transmittance of the reduction state is much lower than that of the oxidation state in the range from 700 to 900 nm. This can be explained by the larger ε2o of the reduction state in this region, which leads to higher optical absorption. In the range from 1200 to 1690 nm, the reflectance of the reduction state is much lower than that of the oxidation state. This can be explained by the difference in the ordinary refractive indices (no) between the reduction and oxidation states. In detail, the no of the reduction state is closer to the geometric mean of the refractive index of ITO and air ($\bar{n} = \sqrt {{n_{air}}{n_{ITO}}} \approx 1.3$) than that of the oxidation state, leading to lower film reflectivity of the reduction film according to the Fresnel equations.
Table 1 shows the fitting parameters in Eq. (1) and Eq. (2) for the oxidation and reduction states of the PEDOT:PSS film. For the ordinary dielectric model, the Drude term shows a smaller AD and a larger ΓD in the reduction state than in the oxidation state. This implies that the PEDOT:PSS film in the reduction state have a lower carrier concentration, carrier mobility, and conductivity. This trend has also been observed in previous studies by electrical test methods [12,23]. The change in carrier properties can affect the NIR optical dispersion. This may be the reason that the εo of PEDOT:PSS film in the NIR region changes significantly under the electrochemical oxidation and reduction.
Table 1. Fitting parameters of VASE models for the oxidation/reduction (ox./red.) PEDOT:PSS film. “—” equals to non-existent.
For the 1st Lorentz term, the oscillator energy EL,1 is 2.10 eV for both the reduction and oxidation states, and the reduction state has a higher oscillator intensity AL,1. For the 2nd Lorentz term, the oscillator energy EL,2 is 1.41 eV for the reduction state, while the oxidation state has no 2nd Lorentz oscillator. It means that the reduction state has absorption peaks at 2.10 eV and 1.41 eV, while the oxidation state only has the weaker absorption peak at 2.10 eV. For this purpose, the absorption spectra of the oxidation/ reduction PEDOT:PSS film is measured, as shown in Fig. 4. It can be seen that the absorption peaks positioned at 2.16 eV (576 nm) and 1.36 eV (910 nm) for the reduction state, and at 2.16 eV for the oxidation state. These absorption peaks are in good agreement with the Lorentz oscillations. The change in the εo of the PEDOT:PSS film in the visible region mainly depends on the change in these Lorentz oscillations. This may be related to the change in the energy band and the structure of the PEDOT:PSS under the electrochemical oxidation and reduction [16,24]. Finally, in terms of the extraordinary dielectric model, there is little difference between the reduction and oxidation states, which clarify the reason why the extraordinary dielectric properties of the oxidation/reduction PEDOT:PSS film remain unchanged.
Fig. 4. The absorbance of ox./red. PEDOT:PSS film. Amplitude shifts at 574 nm (2.16eV) and 910 nm (1.36eV) were observed, which agree with the shifts in oscillation intensity of the Lorentz model used in the VASE measurements.
4. Conclusion
In conclusion, we have investigated the dielectric properties of the PEDOT:PSS thin film upon bias by using a hybrid technology combined with the electrochemical method, VASE measurement and FDTD simulation. The anisotropic dielectric functions in the oxidation state +0.6 V and reduction state -1.0 V was obtained by the VASE. The result shows that the ordinary dielectric constants εo increases significantly from 400 to 1690 nm, which corresponds to the variation of carrier properties and optical absorption properties of the film under external voltage excitation. The dielectric functions used in the simulation of the transmittance and reflectance of the film agree well with the experiments.
Funding
Jiangsu key R & D programs (BE2018006-3).
Acknowledgments
The authors thank the CAS Interdisciplinary Innovation Team, NANO-X Workstation and Nano Fabrication Facility of Chinese Academy of Sciences, Jiangsu Province, Suzhou City, Suzhou Industrial Park.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
References
1. X. Fu and T. J. Cui, “Recent progress on metamaterials: from effective medium model to real-time information processing system,” Prog. Quant. Electron. 67, 100223 (2019). [CrossRef]
2. H. Hajian, A. Ghobadi, B. Butun, and E. Ozbay, “Active metamaterial nearly perfect light absorbers: a review,” J. Opt. Soc. Am. B 36(8), F131–143 (2019). [CrossRef]
3. K. Xiong, D. Tordera, M. P. Jonsson, and A. B. Dahlin, “Active control of plasmonic colors: emerging display technologies,” Rep. Prog. Phys. 82(2), 024501 (2019). [CrossRef]
4. N. Jiang, X. Zhuo, and J. Wang, “Active plasmonics: principles, structures, and applications,” Chem. Rev. 118, 3054–3099 (2018). [CrossRef]
5. G. Si, Y. Zhao, E. S. P. Leong, and Y. J. Liu, “Liquid-crystal-enabled active plasmonics: a review,” Materials 7(2), 1296–1317 (2014). [CrossRef]
6. F. J. G. Abajo, “Graphene plasmonics: challenges and opportunities,” ACS Photonics 1(3), 135–152 (2014). [CrossRef]
7. C. U. Hail, A. K. U. Michel, D. Poulikakos, and H. Eghlidi, “Optical metasurfaces: evolving from passive to adaptive,” Adv. Optical Mater. 7(14), 1801786 (2019). [CrossRef]
8. J. C. Lacroix, Q. Nguyen, Y. Ai, Q. Nguyen, P. Martina, and P. C. Lacazea, “From active plasmonic devices to plasmonic molecular electronics,” Polym. Int. 68(4), 607–619 (2019). [CrossRef]
9. L. Groenendaal, F. Jonas, D. Freitag, H. Pielartzik, and J. R. Reynolds, “Poly(3,4-ethylenedioxythiophene) and its derivatives: past, present, and future,” Adv. Mater. 12(7), 481–494 (2000). [CrossRef]
10. X. Fan, W. Nie, H. Tsai, N. Wang, H. Huang, Y. Cheng, R. Wen, L. Ma, F. Yan, and Y. Xia, “PEDOT:PSS for flexible and stretchable electronics: modifications, strategies, and applications,” Adv. Sci. 6(19), 1900813 (2019). [CrossRef]
11. B. Jewłoszewicz, K. A. Bogdanowicz, W. Przybył, A. Iwan, and I. Plebankiewicz, “PEDOT:PSS in water and toluene for organic devices—technical approach,” Polymers 12(3), 565 (2020). [CrossRef]
12. M. Berggren, X. Crispin, S. Fabiano, M. P. Jonsson, D. T. Simon, E. Stavrinidou, K. Tybrandt, and I. Zozoulenko, “Ion electron–coupled functionality in materials and devices based on conjugated polymers,” Adv. Mater. 31(22), 1805813 (2019). [CrossRef]
13. W. Zhang, H. Li, E. Hopmann, and A. Y. Elezzabi, “Nanostructured inorganic electrochromic materials for light applications,” Nanophotonics 10(2), 825–850 (2020). [CrossRef]
14. G. Pathaka, Z. Krasińska-Krawetb, L. Szyk-Warszyńskab, and D. Cakara, “Doping of Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) films studied by means of electrochemical variable angle spectroscopic ellipsometry,” Thin Solid Films 651, 31–38 (2018). [CrossRef]
15. Y. Lin, Y. Zhao, Q. Xin, C. Jiang, and A. Song, “Electrical control of the optical dielectric properties of PEDOT:PSS thin films,” Opt. Mat. 108, 110435 (2020). [CrossRef]
16. S. S. Kalagia and P. S. Patilb, “Secondary electrochemical doping level effects on polaron and bipolaron bands evolution and interband transition energy from absorbance spectra of PEDOT: PSS thin films,” Synth. Met. 220, 661–666 (2016). [CrossRef]
17. M. G. Tadesse, C. Loghin, Y. Chen, L. Wang, D. Catalin, and V. Nierstrasz, “Effect of liquid immersion of PEDOT:PSS-coated polyester fabric on surface resistance and wettability,” Smart Mater. Struct. 26(6), 065016 (2017). [CrossRef]
18. Y. Du, X. Cui, L. Li, H. Tian, W. X. Yu, and Z. X. Zhou, “Dielectric properties of DMSO-doped-PEDOT:PSS at THz frequencies,” Phys. Status Solidi B 255(4), 1700547 (2018). [CrossRef]
19. H. G. Tompkins and E. A. Irene, Handbook of Ellipsometry (William Andrew, 2005).
20. A. M. Nardes, M. Kemerink, and R. A. J. Janssen, “Anisotropic hopping conduction in spin-coated PEDOT:PSS thin films,” Phys. Rev. B 76(8), 085208 (2007). [CrossRef]
21. L. A. A. Pettersson, S. Ghosh, and O. Inganäs, “Optical anisotropy in thin films of poly(3,4-ethylenedioxythiophene)-poly(4-styrenesulfonate),” Org. Electron. 3(3-4), 143–148 (2002). [CrossRef]
22. J. Bergqvist, H. Arwin, and O. Inganäs, “Uniaxial anisotropy in PEDOT:PSS electrodes enhances the photocurrent at oblique incidence in organic solar cells,” ACS Photonics 5(8), 3023–3030 (2018). [CrossRef]
23. I. Imae, M. Shi, Y. Ooyama, and Y. Harima, “Seebeck coefficients of poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) correlated with oxidation levels,” J. Phys. Chem. C 123(7), 4002–4006 (2019). [CrossRef]
24. C. Carlberg, X. Chen, and O. Inganäs, “Ionic transport and electronic structure in poly(3,4-ethylenedioxythiophene),” Solid State Ionics 85(1-4), 73–78 (1996). [CrossRef]
