ITO-Free Solution-Processed Flexible Electrochromic Devices Based

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ITO-Free Solution-Processed Flexible Electrochromic Devices Based on PEDOT:PSS as Transparent Conducting Electrode Rekha Singh,† Joseph Tharion,‡ Sengottaiyan Murugan,† and Anil Kumar*,†,‡,§ †

Department of Chemistry, ‡Centre of Excellence in Nanoelectronics, §National Centre of Excellence in Technologies for Internal Security (NCETIS), Indian Institute of Technology Bombay, Powai, Mumbai 400076, India S Supporting Information *

ABSTRACT: Electrochromic devices (ECDs) are emerging as novel technology for various applications ranging from commercialized smart window glasses, goggles, and autodimming rear view mirrors to uncommon yet more sophisticated applications such as infrared camouflage in military and thermal control in space satellites. The development of lowpower, lightweight, inexpensive, and flexible devices is the need of the hour. In this respect, utilizing PEDOT:PSS as transparent conducting electrode (TCE) to replace indium tin oxide (ITO) and metal based TCEs for ECDs is a promising solution for the aforementioned requirements. In this work we have demonstrated the performance of PEDOT:PSS films coated on flexible substrates, treated with PTSA-DMSO, as TCEs for ECD applications and their comparison with that of ITO based ECDs. The PEDOT:PSS based flexible TCEs used in this study have conductivity of 1400−1500 S·cm−1 and figure of merit (FoM) of 70−77. The process of increasing the conductivity of PEDOT:PSS films also led to the broadening of the conducting potential window (CPW), which is important for electrochemical applications of PEDOT:PSS when used as a stand-alone electrode. More than achieving a comparable electrochromic contrast, switching time, and coloration efficiency with respect to the ITO based ECDs, PEDOT:PSS devices also had the added advantage of good mechanical flexibility. These devices demonstrated superior stability during electrochemical cycling and multiple mechanical bending tests, making them an inexpensive alternative to the costly ITO based ECD technology. KEYWORDS: PEDOT:PSS, transparent conducting electrode, electrochromic device, ITO-free, conductive potential window

1. INTRODUCTION

Apart from the electrochrome, transparent conducting electrode (TCE) also plays an important role in the final performance of an ECD and generally is the most expensive part of the whole device.15,16 Indium tin oxide (ITO), fluorine doped thin oxide (FTO), metal nanowires,17−20 and metal grids21,22 are some of the examples of the most commonly used inorganic and metallic TCE materials. ITO is the most abundantly used TCE due to its excellent optical transparency and electrical properties. However, the demand for alternative materials arises due to increasing demand for raw indium and incompetence of ITO coating with the flexible substrates because of its brittle nature.23 On the other hand, the vacuum processing steps involved in ITO coated TCE formation are highly expensive and sophisticated when used in a pilot plant.24 As electrochrome, various inorganic materials such as Prussian blue, tungsten oxide (WO3), vanadium oxide (V2O5), and

Electrochromic devices (ECDs) are light modulating devices in which the modulation of light is controlled by an electric potential. Generally they are known as color changing devices and find applications in smart windows, sunglasses, displays, and autodimming rear-view mirrors, etc.1−3 However, their application is not restricted to the phenomenon of color change. They are emerging as novel devices for infrared camouflage in military applications and for satellite thermal control.4−8 Besides their interesting applications, ECDs have also attracted considerable attention due to their low power consumption.9,10 Electrochromic devices can be either transmissive/absorptive or reflective depending on the type of application. For example, in smart window and goggle applications, transmissive devices are essential. As a result, ECDs are being constructed in various design/configurations depending on their end use.11−13 Transmissive ECDs are simple electrochemical cells wherein the electrochromic layer (electrochrome) coated on a transparent conducting electrode (working electrode) is sandwiched with another transparent conducting counter electrode using a gel electrolyte as spacer.14 © XXXX American Chemical Society

Special Issue: Focus on India Received: July 30, 2016 Accepted: October 20, 2016

A

DOI: 10.1021/acsami.6b09476 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces cerium−titanium oxide (CeO2·TiO2), etc., have been used.25,26 Due to the cost, brittleness, and difficulty in getting conformal flexible coatings of these inorganic materials for large-area applications, there is a need to replace them with cheaper organic materials. Reynolds’ group has contributed significantly in the development of various organic electrochromic polymers based on 3,4-alkylenedioxythiophene.27 Though there has been tremendous success in the development of organic electrochromic materials based on conjugated polymers, progress in the development of organic TCE materials for ECDs is still in its infancy. In terms of organic TCE materials, PEDOT:PSS,28,29 graphene,30 and carbon nanotubes31,32 are the potential candidates. Since PEDOT:PSS electrodes have been reported to exhibit superior compatibility with plastic substrates and maintain high conductivity even on repeated stretching and bending, it is a potential candidate for flexible ECDs.33−35 In the past couple of years, PEDOT:PSS has emerged as a potential candidate to replace ITO as TCE material because of a significant increase in its conductivity while retaining high transparency.28,36,37 Recently, the reported values of conductivity of PEDOT:PSS have become comparable to that of ITO.36−38 In order to quantify the quality of a material as a TCE, both conductivity and transparency are coupled into one parameter (the ratio of electric and optical conductivity (σDC/ σOP)) called the figure of merit (FoM).39 In the case of ITO based TCEs, the value of the FoM has reached as high as 400 which is significantly higher than that of a low value of 30 for both graphene and carbon nanotube.34,40 Interestingly, for PEDOT:PSS based TCEs, the FoM has been shown to have a value greater than 100.41,42 Generally a FoM value of ∼35 or higher is sufficient for a material to be used as a TCE.43 In the case of high current driven applications a high value of FoM (>200) is desirable; however, for some low-current applications such as a touch screen, it can be 10.40 Replacement of ITO by PEDOT:PSS has been demonstrated for many applications, e.g., photovoltaics (bulk-heterojunction,44 DSSC,37,45 perovskite),46 OLEDs (organic light emitting diodes),47,48 and LCDs (liquid crystal displays).49 However, the potential to replace ITO by PEDOT:PSS in ECDs has met with limited success as these devices exhibit inferior device performance compared to corresponding ITO based devices. The first fully ITO-free organic ECD was demonstrated by Reynolds and co-workers,50 where they used PEDOT:PSS as a transparent electrode (SR ∼ 600 Ω·□−1, σ ∼ 120 S·cm−1, %T ≥ 75, and FoM ∼ 2) to replace ITO. Due to high SR, their devices had a longer switching time (8 s for 80% color change). Single layer ECDs, in which PEDOT:PSS act as both the TCE and the electrochrome, were also investigated;51,52 however, these devices had low contrast, were slow switching, and operated at a very high voltage (0 to +3 V). Berggren and coworkers printed the ECDs on paper by using an ordinary printing technique, in which PEDOT:PSS of a conductivity of nearly 100 S·cm−1 was used.12 To improve the color contrast, they used different polymers as electrochrome on commercially available PEDOT:PSS coated polyester sheets.11 Although they observed an improvement in the color contrast, the switching time observed was high (4.5 s for bleaching and 6.7 s for coloring).53 In all of these fully organic ECDs that are mentioned above, the longer switching time remained a major issue. Therefore, in order to improve the switching speed, ECDs were constructed using silver grids printed on flexible PET sheets.54 The major drawback of this design was the “blooming effect” which arises because of the differences in

surface resistance from the grid edge toward the center and therefore results in an uneven progression of color change.21,13 The obvious reasons for the limited success of PEDOT:PSS as TCE for ECD application were its low conductivity and high redox activity in the working potential window of the ECDs. Generally PEDOT maintains its conductivity until a high positive value of potential (≤1.6 V vs Ag wire); however on applying a negative potential (less than −0.4 V), conductivity decreases due to dedoping. Therefore, its application as a TCE in ECD application defines a window of high conductivity known as a conducting potential window (CPW), inside which it should be operated. A broader CPW (shifted toward the more negative side to prevent dedoping during potential switching) is therefore preferable for its electrochemical application as an electrode. In this work we report a fast switching flexible ECD with good electrochromic contrast and coloration efficiency (CE) based on PEDOT:PSS as TCE. In fact, our ITO-free organic ECDs have comparable performance with those based on ITO making this an economically viable process for large-area electrochromic device applications. These devices were further tested for electrochemical cyclic stability and mechanical bendability and were found to be reasonably stable both electrochemically and mechanically.

2. EXPERIMENTAL SECTION 2.1. Materials. PEDOT:PSS aqueous solution (Clevios PH1000) was purchased from Heraeus. The concentration of PEDOT:PSS was 1.3% by weight, and the weight ratio of PSS to PEDOT was 2.5 in solution. Poly(methyl methacrylate) (PMMA; Sigma-Aldrich, Mw ∼ 120,000), p-toluenesulfonic acid (PTSA; Fisher Scientific; 99%), lithium perchlorate (LiClO4; Sigma-Aldrich, ≥95.0), propylene carbonate (PC; Merck, ≥99.7), acetonitrile (MeCN; Merck, ≥99.93), dimethyl sulfoxide (DMSO; Merck; 99.9%), and isopropyl alcohol (IPA; Merck; 99%) were used without further purification. PET and ITO (∼85 Ω·□−1) coated PET sheets were purchased from Garware Polyester Ltd., Mumbai, India and SKC, Seoul, Korea, respectively. ITO coated glasses (∼20 Ω·□−1) were purchased from Delta Technologies. Poly(AcDOT-co-EDOT) (Mn = 15 kDa, PDI = 2.9) was used as the primary electrochrome which was synthesized in the laboratory according to the reported procedure.55 Fe(NO3)3·9H2O and Na4[Fe(CN)6]·10H2O were purchased from Spectrochem and used as received. 2.2. Preparation of Films and Device. Laurell spin coater (WS650MZ-23NPP/LITE) was used for making films on the substrates. PEDOT:PSS solution was spin coated on a cleaned PET surface at 2000 rpm for 30 s. Films were dried at 120 °C for 5 min before posttreatment or the next coating. Multiple coatings were done for obtaining a thicker layer of PEDOT:PSS. Post-treatment of films were performed by spreading 0.9 M PTSA solution in DMSO on the PEDOT:PSS coated PET, keeping it for 1−2 min followed by spinning off the excess reagent. After that, these films were baked at 120 °C for 5 min, washed in IPA two times for removing the excess of PTSA, and baked again at 120 °C for 10 min. Solutions of poly(AcDOT-co-EDOT) polymer were prepared in toluene (9, 18, and 22.5 mg/mL) and coated on ITO and PEDOT:PSS electrodes by spin coating at 2000 rpm for 20 s. Prussian blue (PB) powder was synthesized and dispersed in water according to a reported protocol.56 The final solution used for coating PB film had a solid content of 2.5 wt %. For improving the adhesion of PB on ITO and PEDOT:PSS coated PET sheet, polystyrenesulfonate (1 mg/mL) was added to its solution. PB solution was spin coated on electrodes at 3000 rpm for 15 s and baked at 100 °C for 10 min. A second layer of PB was prepared on top of the first at the same spinning parameters. Cyclic voltammogram and spectroelectrochemstry of poly(AcDOT-coEDOT) and PB on ITO is given in the Supporting Information (Figures S1 and S2). Gel electrolyte was prepared according to the B


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ACS Applied Materials & Interfaces reported protocol and consists of PMMA, propylene carbonate, acetonitrile and lithium perchlorate.57 Glass beads (50−100 μm) were added to the gel to maintain a distance between two electrodes while sandwiching the gel in-between. All of the electrodes were kept under vacuum for overnight, before construction of ECDs. The ECDs were dried for 24 h at room temperature. Silver paste was used for making contacts with the electrodes of the ECD. 2.3. Characterization of Films and Devices. The SR of the films was measured by a four-point probe (Lucas-signatone) with a Keithley 2400 sourcemeter. The film thickness was estimated by cross-sectional scanning electron microscopy (SEM) using JEOL JSM-7600F field emission gun-SEM. For this PEDOT:PSS films were coated on ITO surface for getting better contrast. SR of PEDOT:PSS_PD films on ITO were compared with that on PET (Table S6 in the Supporting Information). Transmittance was measured by Shimadzu UV−vis− near-IR spectrometer (UV-3600 Plus). For this, PEDOT:PSS films were coated on glass (SR was compared with that on PET) and the reference of bare glass was taken while measurement was done. Cyclic voltammetric (CV) studies of PEDOT:PSS_PD electrodes were carried out using CHI760D potentiostat in a three-electrode configuration. Pt foil and Ag/Ag+ were used as auxiliary and reference electrodes respectively and PEDOT:PSS coated platinum plate as working. A 100 mM solution of LiClO4 in dry acetonitrile was used as electrolyte. In situ conductance measurement was performed for PEDOT:PSS (pristine and PTSA-DMSO treated) coated on gold interdigitated electrodes (5 μm width and spacing; picture and setup details are given in the Supporting Information, Figure S3). CV, spectroelectrochemistry, and chronoamperometric studies of ECDs were carried out using potentiostat and Ocean Optics spectrophotometer with USB2000+ detector and LS-1 tungsten halogen lamp. Air reference was taken for the optical measurements of all the ECDs. The repetitive bending test of the ECDs was performed on an inhouse-assembled automated bending machine. The diameter of curvature was fixed to ∼14 mm while bending the device on the automated bending machine. (Figure S7 in the Supporting Information and Movie S1).

Apart from improving the FoM, the treatment of PTSADMSO also led to broadening in the CPW of the PEDOT:PSS based electrodes (Figure 1) from a value of −0.57 to −0.72 V.

Figure 1. In situ conductance of untreated and PTSA-DMSO treated PEDOT:PSS electrodes as a function of gate potential (Vg).

The first indication of broadening of CPW was observed in the cyclic voltammogram of the PEDOT:PSS_PD film where a new redox couple (3/3′) was observed at higher negative (−ve) potential (Figure S4 in Supporting Information). This additional redox active couple at higher −ve redox potential is an indication of increase in either the number of charge carriers (doping level) or redox active charge carriers.63 As per our earlier report,45 PTSA-DMSO treatment leads to an increase in the doping level; therefore we can expect an increase in the number of charge carriers. Nevertheless it is clear that the charge carriers are more electrochemically reachable due to better electron transport between the external circuit and PEDOT:PSS film and therefore redox active in the case of PTSA-DMSO treated films. This shift in the redox potential of PEDOT:PSS_PD films toward a more negative side is an indication of the broadening of CPW which was further confirmed by in situ conductance measurement (Figure 1). As can be seen from Figure 1, the CPW could be pushed further from −0.57 to −0.72 V. Therefore, PTSA-DMSO treatment of PEDOT:PSS electrode improves both the FoM as well as CPW, making these electrodes a potential candidate as TCE for ECD applications. 3.1. Flexible Electrochromic Devices Based on PEDOT:PSS. Gel based electrochromic devices were constructed using PEDOT:PSS_PD coated PET as TCEs, poly(AcDOT-co-EDOT) as primary electrochrome, and Prussian blue as complementary electrochrome, respectively. A digital image of PEDOT:PSS_PD based ECD is shown in Figure 2. The two TCEs in each ECD were prepared from PEDOT:PSS_PD coated PET sheets of the same SR, %T, and

3. RESULTS AND DISCUSSION FoM (the higher the better) and conducting potential windows (the broader the better) are the two important parameters that influence the performance of the PEDOT:PSS transparent conducting electrodes in ECDs. We targeted to improve the FoM by minimizing their sheet resistance (SR), while keeping the transparency within an acceptable range. Generally, SR can be reduced (by two to three orders) by post-treating PEDOT:PSS films with high-dielectric solvents, such as DMSO, ethylene glycol, and so on,49,58,59 or protonic acids60 or surfactants,34 or their combinations.28,45,61 Since we are using PET as the substrate, we could not use strong inorganic acids such as H2SO4 and HNO3 to improve FoM. Various mild organic acids have been used earlier for improving the SR of PEDOT:PSS.60,62 Here, we chose PTSA in combination with high-dielectric DMSO solvent.44 DMSO was chosen for this treatment as a broader CPW was demonstrated by Park et al. using a secondary treatment of PEDOT:PSS films with DMSO.63 These PTSA-DMSO treated PEDOT:PSS films are referred to as PEDOT:PSS_PD in the rest of this work. PTSADMSO treatment leads to a phenomenal decrease in the resistivity of the films from 500 kΩ·□−1 to 160 Ω·□−1 and having a transparency of 97% (Table S1 in Supporting Information). By using a double layer of PEDOT:PSS, we observed a SR of 80−85 Ω·□−1 while maintaining the transparency of 94%. The figure of merit, σDC/σOP, for these transparent conducting PEDOT:PSS films was still ∼70, which is a good number for any TCE.

Figure 2. (a) Colored and (b) bleached states of ECD prepared from PEDOT:PSS_PD(85) electrodes coated on PET sheets. C


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Figure 3. (a) Switching time vs sheet resistance of PEDOT:PSS_PD and ITO electrodes. (b) Cyclic voltammogram and optical switching response (at 550 nm) for ECDs based on (c) ITO and (d) PEDOT:PSS_PD coated TCEs. Devices are switched between −1.0 and +1.0 V with a pulse width of 20 s.

studying its spectroelectrochemstry and CV, respectively. Spectroelectrochemical measurements give information regarding the spectral changes happening in an ECD at different electric potentials (Figure S6 in Supporting Information). The ECDs based on PEDOT:PSS_PD differ significantly from those based on ITO in their spectral changes, notably during the doping phase while the potential approaches a higher value. From −1.0 V until 0.6 V, there is a continuous decrease in the absorbance due to doping of the electrochrome; however, on increasing the voltage further (>0.6 V) in the case of the PEDOT:PSS_PD based device, a slight increase in the absorbance at higher potential is observed. This increase is expected in PEDOT:PSS based ECDs, due to the coloration (dedoping) of the counter PEDOT:PSS_PD electrode, when the working electrode is at positive potential. Cyclic voltammetry studies give information regarding the redox response of electrochrome on both the electrode. The conductivity of electrode on which electrochrome is coated plays an important role in its redox response. As shown in Figure 3b, the CVs of ECD based on ITO and PEDOT:PSS_PD are similar in terms of the peak potentials and peak currents of the electrochrome (Table S3 in Supporting Information) indicating that the redox response of electrochrome is unaffected with the nature of the electrode. However, the charge capacity was more for PEDOT:PSS_PD based ECDs, which shows a capacitive contribution of PEDOT:PSS_PD electrode. On comparing the electrochromic

conductivity values. For comparison, the reference device was made up of ITO coated PET sheets. The SR of an electrode has significant effect on the performance of ECD.64 On increasing the thickness of PEDOT:PSS_PD film, the SR decreases; however, the %T of the film also decreases. This decrease in transparency can be accommodated as long as the FoM is not affected, significantly. In order to study the effect of SR on the switching speed of ECDs, three different PEDOT:PSS_PD films, referred to as PEDOT:PSS_PD(160), PEDOT:PSS_PD(120), and PEDOT:PSS_PD(85), were used for ECD fabrication. Details of these films in terms of SR, %T, and FoM is given in Table S1 of Supporting Information. We observed an improvement in the switching time of ECD on decreasing the SR of PEDOT:PSS_PD electrodes (Figure S5 and Table S2 in the Supporting Information) without affecting the color contrast (ΔA) of the device. Interestingly the CE of ECDs increases on increasing the SR of PEDOT:PSS_PD electrodes: 375 cm2·C−1 for PEDOT:PSS_PD(85), 427 cm2· C −1 for PEDOT:PSS_PD(120), and 442 cm2 ·C −1 for PEDOT:PSS_PD(160). This increase in CE can be attributed to the decrease in the capacitive charge contribution of PEDOT:PSS_PD electrode on decreasing their thicknesses. Hence PEDOT:PSS_PD(85) electrode based ECD is giving the best results, in terms of device switching time, and therefore is used for comparing it with ITO based devices. The optical and electrochemical performance of ECDs based on ITO and PEDOT:PSS_PD TCEs has been evaluated by D


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Figure 4. Optical switching response for (at 550 nm) ECDs based on ITO (black) and PEDOT:PSS_PD (green) at (a) different anodic potentials +1, +0.8, and +0.6 V while keeping cathodic potential −1.0 V in all three cases and at (b) different cathodic potentials −1.0, −0.8, and −0.6 V while keeping anodic potential +0.6 V in all three cases.

Table 1. Electrochromic Properties of PEDOT:PSS_PD and ITO Based Electrochromic Devices with Different Thicknesses of Primary Electrochrome switching timea (s)

a

TCE

electrochrome (A at 550 nm)

Δ%T

ΔA

tb

tc

coloration efficiencya (cm2·C−1)

PD1 PD2 PD3 ITO1 ITO2 ITO3

0.38 0.66 0.87 0.35 0.70 0.89

27.0 43.0 45.0 25.0 42.0 49.0

0.35 0.61 0.76 0.29 0.62 0.75

1.6 1.5 2.0 1.6 1.3 1.8

1.5 1.5 4.6 1.6 1.9 4.6

425 417 429 565 570 600

For 90% change, switching potential +0.6 to −1 V.

potential, respectively, since there is no significant change in the color contrast beyond these potential values. The abovementioned potential is applied to the working electrode [poly(AcDOT-EDOT)], with respect to the counter electrode (Prussian blue). The effects of anodic and cathodic potential on PEDOT:PSS_PD based ECD and ITO based devices were compared. In the case of PEDOT:PSS_PD based devices, while keeping the cathodic potential to −1.0 V when the anodic voltage was varied from +0.6 to +1.0 V, a decrease in ΔA (Figure 4a) was observed. This decrease in ΔA is due to coloring of the counter electrode, which undergoes dedoping when the device is in its bleached state. However, for ITO based ECD, we did not observe significant change in ΔA on increasing anodic potential (Table S4 in Supporting Information). However, a slight increase in switching time was observed on decreasing the anodic voltage from +1.0 to +0.6 V, which indicates that the switching kinetics becomes slower at lower value of voltage. On the other hand, varying the cathodic voltage from −0.6 to −1.0 V (Figure 4b, anodic potential maintained at +0.6 V) leads to an increase in ΔA in the case of both PEDOT:PSS_PD and ITO based ECDs. The absorbances of both the ECDs increase on going from −0.6 to −0.8 V due to coloration of electrochrome. However, after −0.8 V, a further increase in the absorbance for the PEDOT:PSS_PD device is probably due to slight dedoping of the working PEDOT:PSS_PD electrode (Table S5 in the Supporting Information). This additional color due to dedoping of PEDOT:PSS_PD electrode increases the electrochromic contrast of device compared to that of the ITO based device without hampering the switching speed. Higher switching speed (coloration) at −1.0 V for both devices is a result of faster redox kinetics at higher potential. Therefore, using −1.0 V for coloring the ECD is beneficial in terms of both ΔA and switching speed leading to an optimized switching range of

response (Figure 3c,d) of PEDOT:PSS_PD(85) and ITO based devices, we observed equivalent efficiency of PEDOT:PSS_PD and ITO transparent electrodes for ECD application. They show a comparable ΔA and the switching times of 0.9/(1.5 s) and 1.0/(1.6 s) (for bleaching/coloring), respectively (Table S2 in Supporting Information). The coloration efficiencies are 375 and 464 cm 2 ·C 1− for PEDOT:PSS_PD and ITO based devices, respectively. Other than their comparable electrochromic performances, an interesting feature during the optical switching of PEDOT:PSS_PD based ECDs was the appearance of a small spike on inverting the potential from anodic to cathodic (zoomed in Figure 3d). This can be attributed to the counter PEDOT:PSS_PD electrode which undergoes slight dedoping (coloring) on applying +1.0 V on the device (with respect to counter electrode) and when reverting the potential to cathodic (−1.0 V), it undergoes an instantaneous bleaching (doping) before the primary electrochrome gets coloration. This indicates that the switching potential has to be optimized for getting the best performance from PEDOT:PSS_PD based ECDs. Optimization of ECD Switching Potential. Since the conductivities of PEDOT:PSS_PD electrodes vary with the applied potential, the optimum conductance is maintained only in a certain range of potential (CPW). Hence, the polarity and extent of the switching potential applied to the device can play an important role in the device performance. In ECDs based on PEDOT:PSS_PD, each electrode may respond differently to the voltage change. Therefore, a suitable range for the switching potential of the overall device cannot be predicted based on the CPW of a single electrode and needs to be determined experimentally. For optimizing the device switching potential (anodic and cathodic), we initially fixed +1.0 and −1.0 V as an upper limit of anodic (doping) and cathodic (dedoping) E


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ACS Applied Materials & Interfaces +0.6 to −1.0 V for getting the maximum color contrast in the PEDOT:PSS_PD based device. Effect of Electrochrome Thickness. Thickness of primary electrochrome affects the electrochromic contrast (Δ%T) of an ECD. Therefore, it is important to check the performance of PEDOT:PSS_PD based device with higher electrochrome thickness and whether performance is consistent with the ITO based device of the same electrochrome thickness. We varied the concentration of primary electrochrome solution for changing the thickness of the electrochrome layer. The absorbance at the wavelength maxima of the three different thicknesses of the primary electrochrome was nearly 0.4, 0.7, and 0.9. The corresponding ΔA values for PEDOT:PSS_PD and ITO based devices have been given in Table 1. We observed that switching time is not affected significantly for ΔA ∼ 0.6 and it is comparable to that for lower electrochrome thickness (ΔA ∼ 0.35). However, for ΔA ∼ 0.8, a slight increase in switching time was observed especially in the coloration time. The same trend was observed for ITO based devices, and switching time was found to be comparable with that of PEDOT:PSS_PD based devices for different thicknesses of electrochrome. CE was not much affected by the thickness of electrochrome for both the PEDOT:PSS_PD and ITO based ECDs. These results indicate that even at higher electrochrome thickness, PEDOT:PSS_PD based ECDs have comparable performance to that of ITO based ECDs. 3.2. Effect of Mechanical Bending and Electrochemical Switching on Lifetime of ECDs. Bending Studies. The major advantage of employing PEDOT:PSS_PD transparent conducting thin films on any flexible substrate is their ability to be bent without any adverse effect on the properties of films. Therefore, these electrodes are suitable for their application in flexible electronic devices. It is notable that ITO coated flexible electrodes show the least tolerance to mechanical bending; therefore cracking of its coating and increase in the resistance are observed on multiple bendings.65,66 In order to demonstrate the mechanical flexibility of our PEDOT:PSS_PD based devices, ECDs were subjected to repeated bending cycles. The electrochemical and electrochromic properties of the ECD were monitored as a function of repetitive bending cycles. CV studies were carried out to investigate the effect of repetitive bending on degradation of electrode or electroactive species. CV studies are not only a demonstration of oxidation and reduction processes happening in an electrochemical system but also a quantitative measure of charge consumed in each process. If any inefficiency in the redox process arises, the consumed charge decreases, which is reflected in a decrease in the area of CV. This inefficiency might be due to degradation of electrode or electroactive species or both. Therefore, CV studies are most ideal for monitoring any loss in the electroactivity of ECD upon subjecting it to repetitive bending. We observed no significant change in the redox behavior of the devices, as there is no change in the number or position of redox peaks (Figure 5a). On the other hand, the total charge consumed in the redox process (doping and dedoping) also is not affected by repetitive bending. A decrease of 6%, in the consumed charge density (Figure 5b) of the device was observed upon 1000 bending cycles. This indicates the endurance of our devices with respect to mechanical bending. The color contrast (ΔA) of the device was minimally affected on bending, and a decrease of 5% was observed after 1000 bending cycles (Figure 6b). The switching response of the

Figure 5. (a) Cyclic voltammogram and (b) charge capacity (mC· cm−2) of a PEDOT:PSS_PD based device with respect to repetitive bending cycles.

device was also monitored with respect to the number of bending cycles and was barely affected on multiple bending cycles (Figure 6a).

Figure 6. (a) Optical switching response (at 550 nm) and (b) ΔA of a PEDOT:PSS_PD based device with respect to repetitive bending cycles.

Electrochemical Cyclic Stability. PEDOT is known for its high electrochemical stability; therefore using PEDOT:PSS_PD as an electrode in ECDs should have good stability of electrochemical cycling. PEDOT:PSS_PD based ECDs were electrochemically cycled for 3000−4000 double potential cycles, and ΔA along with switching time of the device was monitored. We observed a decrease of ∼5% in ΔA value after 4000 cycles (Figure 7).

Figure 7. Absorbance (at 550 nm) of ECD based on PEDOT:PSS_PD, as a function of double potential cycles when potential was switched between +0.6 V to −1.0 V. F


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ACS Applied Materials & Interfaces Table 2. Electrochromic Properties of Various ITO-Free Electrochromic Devices switching timea (s)

ΔA

tb

tc

switching potential (V)

coloration efficiency (cm ·C )

electrochemical stability (for 5% loss)

PEDOT:PSS

P(AcDOT-co-EDOT)b

24

0.31

0.9

1.5

−1.0 to +1.0

375

4,000

PEDOT:PSS

P(AcDOT-co-EDOT)b

40

0.57

1.0

1.6

−1.0 to +1.0

367 395

16 2.5 2.5

−1.2 to +1.2 0.0 to +2.0 0.0 to +3.0 −0.5 to +1.0 −0.5 to 1.0

TCE

PEDOT:PSS PEDOT:PSS PEDOT:PSS Ag grid/PEDOTe Ag grid/PEDOTh

primary ECP

c

PProDOT-Me2 PEDOT:PSS PEDOT:PSS PProDOT-EtHx2f PProDOT-EtHx2f

Δ%T

43 22−35 14 26 29

8 0.15 0.65g 0.80g

d

20 1.8 0.3

a

2

−1

36,000

310 391

ref this work this work 49 11 50 21 21

a

For 90% change. bPoly[3,4-di-(2-ethylhexyloxy)thiophene-co-2,3-dihydrothieno[3,4-b][1,4]dioxine]. cPoly[3,4-(2,2-dimethyl-propylene-1,3-dioxy)thiophene-2,5-diyl]. dFor 80% change. eFlexoprinted. fPoly[3,4-(2,2-bis(2-ethylhexyloxymethyl)propylene-1,3-dioxy)thiophene-2,5-diyl]. gCalculated from Tb and Tc values given in this work. hEmbedded.

In summary, a comparison of PEDOT:PSS_PD based ECDs prepared in this work with other reported ITO-free ECDs based on PEDOT:PSS and metal is given in Table 2. The PEDOT:PSS based ECDs reported earlier were slow switching devices49 and required higher operating voltages.50 Although the silver grid based ECDs showed an improvement in switching time, there were coloration abnormalities associated with it.21 However, without incorporating any inorganic/ metallic conductive coatings, our devices could achieve a comparable performance. Moreover, an added advantage of these PEDOT:PSS_PD based devices is their good mechanical flexibility.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +91 22-2576 7153. Fax: +91 22-2576 7152. Notes

The authors declare no competing financial interest.

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4. CONCLUSIONS PTSA-DMSO treatment of PEDOT:PSS films leads not only to an increase in figure of merit but also broadening of conducting potential window toward negative potential which is important for using these electrodes in ECDs. The key result of this work is the successful fabrication of solution processable, flexible electrochromic devices with high switching speed, good coloration efficiencies, mechanical flexibility, and electrochemical cyclic stability, without incorporating ITO or metal grids. These PEDOT:PSS_PD electrodes are, therefore, ideal transparent conductors for flexible ECDs. In principle, all those electrochromic polymers for which their redox couple lies within the CPW of the present PEDOT:PSS_PD can also be used for the fabrication of flexible ECDs based on PEDOT:PSS_PD based TCE. Upon screening various electrochromic polymers from the literature, we found electrochromic polymers such as ECP-red and ECP-orange67 and donor− acceptor polymer PBProDOT-Hx2:CN-PPV68 have their redox activities in the conductive potential window of PEDOT:PSS_PD films; therefore ECDs can also be fabricated using these polymers along with the present PEDOT:PSS_PD based TCE.

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ization of PEDOT:PSS_PD and ITO based ECDs (Tables S2−S5, Figures S5−S7), and comparison of sheet resistance of PEDOT:PSS films on PET and ITO coated glass (Table S6) (PDF) Bending device (Movie S1) (AVI) Working ECD while bending (Movie S2) (AVI)

ACKNOWLEDGMENTS We acknowledge the Department of Electronics and Information Technology (DeitY) and Ministry of New and Renewable Energy (MNRE), India, for the financial support. We also thank SAIF, IIT Bombay for availing the FEG-SEM facilities. R.S. acknowledges the financial assistance in the form of a fellowship from the University Grant Commission (UGC), India. S.M. acknowledges the financial assistance in the form of a fellowship from the CSIR India.

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ASSOCIATED CONTENT

S Supporting Information *

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b09476. Electrochemical and spectroelectrochemical characterization of electrochromes (Figures S1 and S2), fabrication and characterization of PEDOT:PSS_PD electrodes (Table S1, Figures S3 and S4), character-

ABBREVIATIONS CPW, conductive potential window DMSO, dimethyl sulfoxide ECD, electrochromic device FoM, figure of merit FTO, fluorine doped tin oxide ITO, indium tin oxide PD, PTSA-DMSO PEDOT:PSS, poly(3,4-ethylenedioxythiophene)-polystyrenesulfonate PET, polyethylene terephthalate PTSA, p-toluenesulfonic acid SEM, scanning electron microscopy TCE, transparent conducting electrode REFERENCES

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