A Flexible Electrochromic Device based on PEDOT: PSS Working in Visible and Infrared Band

Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), a novel conducting polymer, has gained significant attention in the realm of electrochromism. In this study, we present the fabrication of a simple double-layer device utilizing PEDOT:PSS, which successfully achieves precise control of emissivity in the infrared band. Through comprehensive experimentation, we observe an impressive regulation amplitude of 0.25 and a temperature difference of 10°C, relative to a background temperature of 100°C. Additionally, the device exhibits remarkable flexibility compared to other existing devices. These findings demonstrate the potential of PEDOT:PSS-based double-layer devices as versatile tools for infrared emissivity modulation, holding promise for diverse applications in fields such as thermal management and energy-efficient technologies.


Introduction
In the past few years, electrochromic devices have emerged as a focal point of research, attracting growing attention.Electrochromism, a phenomenon wherein materials exhibit reversible colour changes upon the application of an external voltage [1][2][3][4], has opened up new avenues for manipulating the transmission, reflection, and absorption properties of devices.While extensive studies have explored the visible to near-infrared (Vis-NIR) characteristics and applications of electrochromic devices, the vast potential of electrochromism in the infrared band has remained largely untapped [5][6][7].This article sheds light on the underexplored landscape of infrared band applications for electrochromic devices.
Electrochromic materials play a crucial role in the development of dynamic optical and electronic devices.Various materials, including transition metal oxides such as tungsten oxide (WO 3 ), conducting polymers like polyaniline and polypyrrole, and viologen derivatives, have demonstrated electrochromic capabilities [1].Notably, Poly(3,4-ethylenedioxythiophene):Poly(styrene sulfonate) (PEDOT:PSS) stands out as a superior electrochromic material, showcasing exceptional characteristics such as stability, reversible cyclicity, flexibility, rapid response, simple fabrication process, and low operating voltage.The color-changing mechanism of PEDOT:PSS relies on the reversible exchange of ions with the electrolyte, resulting in alterations to its electronic states upon the application of positive or negative voltages [8].Capitalizing on these remarkable properties, PEDOT:PSS has found widespread applications in optical displays, thermal control systems, smart windows, photovoltaic devices, and photon detection, among other areas.This review highlights the exceptional attributes of PEDOT:PSS and explores its diverse range of applications, underscoring its prominence in the realm of electrochromic materials.

Fabrication of PEDOT:PSS Film
The PET substrate was cut into 1.5cm×1.5cmpieces and cleaned with deionized water and acetone by sonicated for 15 minutes.Liquid remaining on the surface was blowed away with nitrogen gas.After cleaned completely, PET was exposed to a UV-ozone generator as pretreatment.Then PEDOT:PSS was spin-coated with a spin coater(MODEL KW-4A from SIYOUYEN Electronic Technology Co., Ltd) onto PET substrate at 1500rpm for 60s.Finally, the PEDOT:PSS/PET film was dried at 120℃ on a hot stage for 10 minutes [9].For PEDOT:PSS/ITO sample, omitting pretreatment and spinning coat directly on cleaned ITO pieces is ok [10][11][12].

Devices Assembly
The working electrode is PEDOT:PSS/PET made before.It was clamped by a platinum electrode clamp.0.1mol/L TBAPF 6 /Acetonitrile was used as electrolyte.A piece of platinum was used as counter electrode.Both electrodes were immersed in electrolyte and fixed by holes of the beaker cover.A power supply connected two electrodes to provide voltage.The experiment setup is shown in figure 1.

Results
The permittivity characteristics of Poly (3,4-ethylenedioxythiophene):Poly(styrene sulfonate) (PEDOT:PSS) can be effectively captured using the Drude-Lorentz model, owing to its electrical conductivity as a polymer material.While the Drude model is commonly employed to describe the behavior of conventional conducting materials, such as metals and inorganic conductors, the peculiarities of PEDOT:PSS necessitate the inclusion of vibrational resonances originating from bond vibrations and stretching.These resonances can be adequately represented using Lorentz oscillators.By incorporating both the Drude and Lorentz components, the comprehensive Drude-Lorentz model can accurately capture the permittivity characteristics of PEDOT:PSS, accounting for its unique electrical and vibrational properties.Combined both Drude model and Lorentz oscillators, following formula can be obtained: Where  is the angular frequency,  ∞ is the high frequency offset,  is the momentum-averaged scattering time,  is the imaginary unit,   is the plasma frequency,  0 is the vacuum permittivity.  ,   and   are amplitude, resonance angular frequency and broadening for the j th Lorentz oscillator [3,13,14].
Previous research has provided evidence supporting the optical anisotropy of PEDOT:PSS, which extends from UV-Vis-NIR to MIR wavelengths.Therefore, it is essential to incorporate different dielectric functions for the in-plane and out-of-plane directions.Utilizing the provided formula, we performed Finite-Difference Time-Domain (FDTD) simulations to calculate the transmission, reflectance, and absorption characteristics of a 150nm PEDOT:PSS film on a PET substrate.The results are illustrated in figure 2. In our experimental observations, the bleached state of PEDOT:PSS appeared nearly transparent in the visible band, while the colored state exhibited a dark blue appearance, as depicted in figure 3. To obtain the transmission and reflectance values, Fourier Transform Infrared Spectroscopy (FTIR) measurements were conducted, and the absorption was calculated using the equation 1 -T -R.The results are presented in figure 4. Within the 8-14μm wavelength range, the absorption of the bleached state was approximately 0.35, whereas that of the colored state was around 0.55.Consequently, the absorption contrast reached a substantial value of 0.2, and the transmission contrast also achieved a value of 0.2.However, the reflectance of the device remained nearly unchanged due to the low reflectance properties of PET.
The variation in absorption and transmission arises from the fact that the bleached state of PEDOT:PSS exhibits metallic characteristics, leading to increased reflectance compared to the colored state.In other words, the coloured state appears more transparent, resulting in higher absorption compared to the bleached state due to the relatively high absorption of the underlying PET substrate.This phenomenon was further confirmed by depositing PEDOT:PSS onto ITO glass, as shown in figure 5.The low thermal radiation exhibited by the device, regardless of its state, is a clear illustration of this behaviour.Additionally, the coloured PEDOT:PSS/PET structure displayed lower thermal  As illustrated in figure 1, the working electrode structure employed in our study consists of Pt/PEDOT:PSS/PET.In this configuration, the PEDOT:PSS film serves the dual role of an electron transport layer and an electrochromic layer, leading to a simplified device structure compared to most comparable devices.Moreover, the use of PET as the substrate ensures excellent flexibility, and the PEDOT:PSS film maintains its integrity without cracking even when the device is subjected to bending.This design choice enables the realization of a flexible electrochromic device with enhanced mechanical durability.

Conclusion
In conclusion, this study successfully fabricated an electrochromic device using PEDOT:PSS, showcasing its versatile properties and performance.In the visible band, the device exhibited high transparency in the bleached state and a dark blue colour in the coloured state.In the infrared band, it demonstrated enhanced emissivity in the coloured state, with an amplitude of change reaching approximately 0.25.Notably, when observed with an infrared camera, the device exhibited a significant temperature difference of 10℃ when the hot stage temperature was 100℃.
In contrast to previous studies focused on the visible and near-infrared properties of PEDOT:PSS, this work highlights its characteristics and performance in the infrared band.Moreover, the device meets the demand for flexibility, ensuring its potential for applications requiring flexible devices.The fabrication process is relatively straightforward due to the simple double-layer structure.Collectively, these findings emphasize the comprehensive potential and versatility of PEDOT:PSS-based electrochromic devices, paving the way for advancements in various applications.

Figure 4 .Figure 5 .
Figure 4. Reflectance, transmission and absorption of PEDOT:PSS/PET measured by FTIR Kim B et al. 2015Patternable PEDOT nanofilms with grid electrodes for transparent electrochromic devices targeting thermal camouflage Nano Convergence 2, 7 [9] Mannayil J, Raman S M, Sankaran J, Raman R and Ezhuthachan J M K 2018 Solution Processable PEDOT:PSS/Multiwalled Carbon Nanotube Composite Films for Flexible Electrode Applications Physica Status Solidi a-Applications and Materials Science 215, 10 Lai M K, Wu J R, Yeh, J M and Chang S H 2019 Unraveling the Modified PEDOT:PSS Thin Films Based Near-Infrared Solar-Heat Shields by Using Broadband Transmittance and Raman Scattering Spectrometers Physica Status Solidi a-Applications and Materials Science 216, 5 [11] Rahimzadeh Z, Naghib S M, Zare Y and Rhee K Y 2020 An overview on the synthesis and recent applications of conducting poly(3,4-ethylenedioxythiophene) (PEDOT) in industry and biomedicine Journal of Materials Science 55, 7575-611 [12] Shi Y, Zhou Y, Shen R, Liu F and Zhou Y 2021 Solution-based synthesis of PEDOT:PSS films with electrical conductivity over 6300 S/cm Journal of Industrial and Engineering Chemistry 101, 414-22 [13] Karki A et al. 2022 Electrical Tuning of Plasmonic Conducting Polymer Nanoantennas Advanced Materials 34, 2 [14] Chen S Z et al. 2019 On the anomalous optical conductivity dispersion of electrically conducting polymers: ultra-wide spectral range ellipsometry combined with a Drude-Lorentz model Journal of Materials Chemistry.C 7, 4350-62