Chemical Engineering Journal 416 (2021) 129028

Available online 3 March 20211385-8947/© 2021 Elsevier B.V. All rights reserved.

Flexible electrochromic and thermochromic hybrid smart window based on a highly durable ITO/graphene transparent electrode

Sang Jin Lee a,b, Sun Hee Lee a,c, Hyung Won Kang a, Sahn Nahm b, Bong Hoon Kim c, Hyeongkeun Kim a,*, Seung Ho Han a,*

a Electronic Convergence Materials and Device Research Center, Korea Electronics Technology Institute, Seongnam 13509, Republic of Korea b Department of Materials Science and Engineering, Korea University, Seoul 02841, Republic of Korea c Department of Organic Materials and Fiber Engineering, Soongsil University, Seoul 06978, Republic of Korea

A R T I C L E I N F O

Keywords: Flexible smart windows Electrochromic materials Thermochromic materials Transparent electrodes Atomic layer deposition Graphene

A B S T R A C T

A flexible smart window with electrochromic (EC) and thermochromic (TC) properties is fabricated on a highly durable transparent indium-tin oxide (ITO)/graphene/polyethylene terephthalate (PET) electrode. An amor-phous WO3 film and crystalline VO2 film are used as the EC and TC material, respectively, in the device. The ITO film deposited via atomic layer deposition acts as a protective layer and suppresses the plasma-induced damage of graphene, which typically occurs during the sputtering process. In this device, the amorphous WO3 film un-dergoes cathodic electrochromic reactions with high coloration efficiency, while the crystalline VO2 film shows characteristic thermochromic behavior. Thus, the flexible EC–TC hybrid smart window shows the controlled transmission of optical and solar energies according to the surrounding temperature and the applied voltage, allowing selective modulation of the visible and near-infrared light. The flexible EC–TC hybrid device is also very stable under mechanical bending, owing to the robust adhesion of the ITO film on the graphene support and the reduced plasma-induced damage of graphene. The fabricated high-performance, durable ITO/graphene/PET transparent electrode can pave the way for next-generation flexible optoelectronic devices as well as multi-functional flexible smart windows.

1. Introduction

Research on smart windows has gained significant attention following the worldwide concern about global warming and energy depletion, and the establishment of national policies and international regulations [1–7]. Smart windows based on various chromogenic tech-nologies are useful for effectively controlling the transmission of solar energy and visible light because they change their optical properties in response to an external stimulus such as electric field, heat, light, or a gas [5,8]. Recently, a tunable window device based on surface rough-ness control by an electric field also have been attracted increasing attention for its low-cost manufacturing [9,10]. Such dynamic window technologies can minimize the energy loss from buildings and provide indoor visual comfort to users by optimizing the solar heat gain and daylight admission through a glaze. Smart windows are largely classi-fied into active and passive types. A typical example of an active smart window is the electrochromic (EC) window, whose optical trans-mittance can be artificially controlled by applying an electric field. A

representative passive smart window is the thermochromic (TC) win-dow that automatically transmits or blocks near-infrared (NIR) light according to the surrounding temperature.

The EC window also allows reversible changes in the optical trans-mittance upon the intercalation or deintercalation of positive ions (e.g., H+, Li+, Na+) [2,11–15]. Transition metal oxides such as WO3, NiO, IrO, MoO3, and V2O5 are the representative inorganic EC materials. Among these, amorphous WO3 is the most widely studied and applied active material of EC windows owing to its high optical modulation and cycling stability. WO3 thin films can transition from a transparent state to a deep blue state with the intercalation of small ions and return to their original transparent state with the deintercalation of the same ions. The TC smart window helps to increase the energy efficiency of buildings because it selectively controls the NIR region of the solar spectrum, which consti-tutes ~50% of the solar energy, in response to environmental temper-ature [16–18]. The most widely used TC material is VO2, which undergoes a reversible metal–insulator transition at its critical temper-ature τc (68 ◦C for bulk VO2), accompanied by a structural phase

* Corresponding authors. E-mail addresses: faithkim99@keti.re.kr (H. Kim), shhan@keti.re.kr (S.H. Han).

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Chemical Engineering Journal

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https://doi.org/10.1016/j.cej.2021.129028 Received 6 December 2020; Received in revised form 24 January 2021; Accepted 15 February 2021

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transition between the monoclinic VO2(M) and rutile VO2(R) phases [18–20]. VO2(R) is metallic and reflects a wide range of solar radiation, whereas VO2(M) is a semiconductor that transmits solar energy.

Although EC windows allow control over the transmittance of visible and NIR radiation with a small applied voltage, the heat transfer through these windows does not change according to the environmental condi-tions such as the surrounding temperature. Until now, EC windows have been coupled with additional low-emissivity coatings to form insulated glass units for thermally insulating buildings [1,2]. However, a low- emissive glass could increase the cost of the window system. In addi-tion, it does not adapt to the change in the surrounding temperature. Therefore, it is desirable to develop a smart window that combines the advantages of EC and TC systems [21].

Recently, flexible substrates have gained considerable attention with regard to research on EC windows owing to the easy installation of the films [22–24]. However, flexible EC windows have not been commer-cialized because of several unsolved problems. One of them is related to the low flexibility and high sheet resistance of the transparent electrode on plastic substrates. In addition, flexible EC windows are vulnerable to moisture and oxygen because they are based on polymeric materials such as gel-type electrolytes that commonly suffer from low chemical stability. Graphene deposited via chemical vapor deposition (CVD) has become a popular option for water and gas barrier films as well as flexible transparent electrodes in various flexible devices because of its high transmittance, mechanical flexibility, and impermeability to any gas or liquid. However, various defects of CVD-grown graphene have hindered its practical application [25–27]. The healing of damaged graphene via atomic layer deposition (ALD) preserves the intrinsic electrical and mechanical properties of graphene and ensures long-term stability in flexible devices [28–30]. Furthermore, ALD is also known to yield ultrathin, conformal, and pinhole-free films with an almost featureless structure; therefore, it can be applied to deposit high-quality moisture and oxygen diffusion barriers [31–33]. Consequently, a transparent electrode with excellent performance and durability can be obtained when the ALD thin film layer is applied to the graphene layer.

In our previous works, an amorphous WO3-based EC device was fabricated using graphene as a transparent conducting electrode, but the high sheet resistance of graphene transferred onto the polyethylene terephthalate (PET) substrate resulted in relatively low ion exchange and consequently low optical transmittance modulation [3]. A crystal-line VO2-based TC film with excellent mechanical flexibility and TC behavior was also fabricated by employing graphene as an atomically thin two-dimensional support, but the EC reaction of the VO2 thin film was not evaluated [7]. Most recently, an all-solid-state EC-TC hybrid smart window device with a multilayer structure of ITO/VO2/Ta2O5/ WO3/ITO was fabricated on a rigid glass substrate [21]. However, flexible EC-TC hybrid smart windows with highly durable transparent conducting electrodes have not been reported yet. In this study, we first deposited indium tin oxide (ITO) via ALD (ITO(A)) on a graphene/Cu substrate for use as a transparent electrode as well as a moisture barrier. Then, ITO was continuously sputter-deposited (ITO(S)) on ITO(A) and heat-treated at 250 ◦C to further enhance the electrical conductivity of ITO(A). The prepared ITO(S)/ITO(A)/graphene was then transferred onto a flexible PET substrate for use as a robust and flexible transparent electrode of EC–TC hybrid devices. A flexible EC-TC hybrid smart win-dow was fabricated by laminating WO3-based EC film and VO2-based TC film deposited on highly durable transparent ITO(S)/ITO(A)/graphene/ PET substrate, respectively. Finally, the smart window characteristics of this EC–TC hybrid device are demonstrated by evaluating the optical switching performance and operational stability.

2. Experimental section

2.1. Preparation of the EC-TC hybrid smart window

Single-layer graphene was grown on a copper foil (99.8%) via the

rapid thermal chemical vapor deposition (RTCVD) process [7,34]. The substrate, Cu-foil, was rapidly heated to approximately 1025 ◦C under an Ar flow of 30 sccm at 600 mTorr. Once the temperature reached approximately 1025 ◦C, the Cu-foil was annealed for 10 min. Subse-quently, 20 sccm of CH4 was supplied to the chamber for 15 min and the temperature was maintained for graphene growth. After the graphene growth, the chamber was cooled to room temperature (RT). A 15 nm thick ITO film was first deposited via ALD on the graphene/Cu foil substrate at 150 ◦C using trimethylindium (TMI) and tetrakis(dimethy-lamino)tin(IV) (TDMA Sn) as precursors. This precursor system has been shown to yield acceptable growth per cycle (GPC) and saturating ALD behavior at a temperature of 150 ◦C. High-purity Ar flowing at 100 sccm was used to purge the reactor during and after precursor pulses. Each precursor pulse was set at 0.1 s. Details of the ALD process sequence and deposition conditions of the ITO(A) are illustrated and summarized in Fig. S1 and Table S1, respectively. Subsequently, a 60 nm thick ITO layer was deposited on the ITO(A)/graphene/Cu foil using a DC magnetron sputtering system (SUKWON T-504). A 4-inch disk of ceramic In2O2–SnO2 (Sn 10 wt%) was used as the sputtering target for the ITO deposition. The deposition conditions were as follows: a DC power of 500 W; an Ar:O2 ratio of 30:0.5; and a working pressure of ~2.5 × 10− 6

Torr. The ITO(S)/ITO(A)/graphene/Cu foil was then annealed at 250 ◦C for 60 min under a high vacuum of 10− 5 Pa with 50 sccm O2 flow to crystallize the ITO thin films. The sheet resistance decreased drastically from ~25 kΩ sq− 1 to 44.2 Ω sq− 1 after the deposition of the ITO(S) layer on ITO(A) owing to the increased total thickness and crystallization of ITO by post-annealing; this was measured with a four-point probe (FPP- RS9, DASOLENG Co., Ltd., Korea). For the fabrication of the WO3-based EC film, the ITO(S)/ITO(A)/graphene layer was transferred onto a PET substrate, as shown in Fig. S2 and in a previous study [35]. An amor-phous 500 nm thin WO3 film was subsequently deposited on the ITO(S)/ ITO(A)/graphene/PET substrate via reactive DC magnetron sputtering at RT under an O2/Ar ratio of 30% using a 4 in. disk W metal target. Meanwhile, amorphous 45 nm thin VOx films were deposited on the other ITO(S)/ITO(A)/graphene/Cu foil via reactive DC magnetron sputtering at RT under Ar (97 sccm) and oxygen (3 sccm) flow. The VOx thin films were then annealed at 550 ◦C for 60 min under a high vacuum of 10− 5 Pa under the flow of 50 sccm of O2 for VO2 single-phase for-mation. Thereafter, VO2/ITO(S)/ITO(A)/graphene was transferred onto a PET substrate in the same way as that described for ITO(S)/ITO(A)/ graphene. Finally, flexible EC–TC hybrid smart windows were fabricated by laminating the WO3/ITO(S)/ITO(A)/graphene/PET and VO2/ITO (S)/ITO(A)/graphene/PET samples using a Li-based polymeric electro-lyte (Soulbrain, SWOPE). Li-based gel electrolyte was drop cast onto the VO2/ITO(S)/ITO(A)/graphene/PET surface. Another WO3/ITO(S)/ITO (A)/graphene/PET was then placed on top of the gel. The device was then placed inside a UV crosslinker to cure the gel electrolyte under 395 nm UV light for 1 min at 80 W. The fabrication procedure for the flexible EC film and TC film, and the schematic of the integrated EC–TC hybrid device are outlined in Fig. 1.

2.2. Characterization

The crystal structures of WO3, VO2, and ITO thin films were deter-mined by X-ray diffraction (XRD, Seifert XRD3000 with Cu Kα radiation, λ = 1.54056 Å). A surface profiler (DektakXT-A, Bruker) was used to measure the actual thickness of the deposited films. The deposited film thickness of WO3, VO2, and ITO was linearly proportional to the time of sputtering, at measured deposition rates of 26.5, 15.2, and 25.7 nm/min, respectively. The cross-sectional morphology was examined using a transmission electron microscope (FEI Titan G2 60-300). The specimens for the cross-sectional TEM analysis were prepared by the focused ion beam method. The structure of graphene was characterized by a Raman spectroscope (Renishaw in Via Raman microscope) operated at 2.41 eV using an excitation wavelength of 514 nm. The WVTR was measured using a water vapor transmission testing system (AQUATRAN Model 2,

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Mocon Inc., USA) at 37.8 ◦C and 90% RH. The changes in the in-line optical transmittance of WO3/ITO(S)/ITO

(A)/graphene/PET, VO2/ITO(S)/ITO(A)/graphene/PET, and the EC-TC hybrid devices in the colored and bleached states were investigated using a UV–visible-NIR spectrophotometer (JASCO, V-670). A schematic of the optical transmission measurement setup is outlined in Fig. S3. CV was used to evaluate the electrochemical properties of the WO3 and VO2 films using a potentiostat-galvanostat (AUTOLAB, PGSTAT 30) with a scan rate of 10 mV/s. A Pt foil and Ag/AgCl were used as the counter and reference electrodes, respectively, and 1 M LiClO4 in propylene car-bonate (Li-PC) was used as the electrolyte. The TC switching behavior of VO2/ITO(S)/ITO(A)/graphene/PET and flexible EC-TC hybrid device were monitored by the UV–visible-NIR spectrophotometer equipped with a heating block in the wavelength of 300–2500 nm. The integrated luminous (lum) and solar (sol) irradiation transmittances were obtained from the following equation:

Ti(τ) =∫

φi(λ)T(λ, τ)dλ/∫

φi(λ)dλ (1)

where T(λ) is the spectral transmittance, τ is the temperature, i refers to lum or sol condition, φlum is the standard luminous efficiency function of the photopic vision, and φsol is the solar irradiance spectrum for air mass 1.5 (corresponding to the sun aligned 37◦ above the horizon).

3. Results and discussion

3.1. Structures of the WO3/ITO(S)/ITO(A)/graphene and VO2(ITO(S)/ ITO(A)/graphene layers

The crystal structure of each layer of the EC–TC hybrid smart win-dow was analyzed by X-ray diffraction (XRD) (Fig. 2). The formation of crystalline ITO on the graphene/Cu layer was confirmed by the diffraction peaks at 30.3◦ and 35.2◦, corresponding to the (222) and (400) planes of ITO, respectively, consistent with our previous results [34]. The WO3 film deposited on ITO(S)/ITO(A)/graphene/Cu provided no peak other than that corresponding to the ITO, indicating the for-mation of an amorphous WO3 thin film. Further, the diffraction peak in the XRD pattern of VO2 could be indexed to the (011) peak of the monoclinic VO2 phase (JCPDS card no. 82-0661) which indicated the crystalline feature of the VO2 film [17–19]. To confirm the trans-ferability of thin films, each layer of the EC–TC hybrid device was transferred onto a Si wafer instead of PET. Note that the XRD peak of the VO2(M) phase could be hardly identified owing to the broad diffraction peak of the PET substrate between 20◦ and 30◦. As shown in Fig. 2b, the peaks of all the constituent films of the EC–TC hybrid device appeared as observed in the diffraction pattern obtained from the graphene/Cu substrate, indicating that all the layers were successfully transferred onto the Si substrate. After the insertion of Li+ ions into the VO2 film, the (011) diffraction peak shifted to a lower angle, as shown in the right panel of Fig. 2b. This peak shift indicates that the crystal lattice spacing of the VO2 film increased slightly upon the insertion of Li+ ions.

The microstructures of WO3/ITO(S)/ITO(A)/graphene and VO2/ITO

Fig. 1. Schematic of the fabrication of (a) WO3-based EC film and (b) VO2-based TC film. (c) Schematic of the integrated EC–TC hybrid device; EC film of WO3/ITO (S)/ITO(A)/graphene/PET and TC film of VO2/ITO(S)/ITO(A)/graphene/PET film were laminated using a Li-based polymeric solid-state electrolyte.

Fig. 2. XRD patterns of ITO, WO3, and VO2 thin films (a) deposited on graphene/Cu and (b) transferred onto a Si substrate.

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(S)/ITO(A)/graphene layers on Si wafers were examined by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (TEM) (Fig. 3). As shown in Fig. 3a and d, the SEM images show different morphologies with an amorphous and dense surface for WO3 thin film and crystalline and compact surface for VO2 thin film, respectively. The cross-sectional TEM images and the corresponding fast Fourier transform (FFT) patterns of WO3 (Fig. 3b) and VO2 (Fig. 3e) confirm their amorphous and crystalline structures, respectively. The thicknesses of the WO3, VO2, ITO(S), and ITO(A) were found to be 500, 45, 60, and 15 nm, respectively. The magnified TEM images (Fig. 3c, f and insets) clearly show smooth interfaces between the graphene layer and ITO layers, and the presence of 1–3 layers of graphene. During the thin film deposition via physical vapor deposition (such as magnetron sputtering), the energetic ions and electrons in the plasma are known to damage the graphene surface [7,14,36].

To investigate the effects of the ALD ITO layer growth on graphene, Raman spectra were obtained from ITO(S)/graphene, ITO(A)/graphene, WO3/ITO(S)/ITO(A)/graphene, and VO2/ITO(S)/ITO(A)/graphene to determine if the graphene layer was damaged. As shown in Fig. 4, the G and 2D bands appeared at approximately 1580 and 2680 cm− 1, respectively, for the as-transferred monolayer graphene, indicating that the graphene was grown well on the Cu foil and then transferred well onto the Si wafer. For the as-transferred monolayer graphene, the D band emerged, whereas the 2D band intensity decreased with the deposition of the ITO(S) thin film on it; this is probably due to the plasma-induced damage of the graphene layer [34,37,38]. However, the Raman spectrum of ITO(A)/graphene remained similar to that of monolayer graphene, indicating that the damage caused to graphene during the ITO deposition via ALD is negligible. With the increase in the number of ALD cycles, the ITO nucleated around various defect sites of the CVD graphene, and the electrical and mechanical properties of graphene were improved as a consequence [28–30]. In addition, the high-density continuous ITO(A) film effectively acted as a protective layer against the plasma-induced damage of graphene during the deposition of ITO, WO3, and VO2 through sputtering. As a result, no significant changes were observed in the Raman spectra of WO3/ITO(S)/ ITO(A)/graphene and VO2/ITO(S)/ITO(A)/graphene, compared with those of monolayer graphene and ITO(A)/graphene.

3.2. EC and TC properties of the WO3 and VO2 on ITO(S)/ITO(A)/ graphene /PET substrate

Fig. 5a presents the optical transmittance spectra of the colored and bleached states of the WO3/ITO(S)/ITO(A)/graphene film in the range of 300–800 nm. Upon applying a voltage of − 1 V, the WO3 film turned opaque dark blue and returned to its transparent state at +1 V. The Tlum values of the bleached and colored states are 82.06% and 12.52% respectively, and these values remained almost unchanged after 100 cyclic voltammetry (CV) cycles. To assess the electrochromic properties of the WO3 films, the coloration efficiency (CE, represented in cm2/C) was determined using the change in the optical density (ΔOD), which is calculated as follows [3,15,21]:

ΔOD = log(

Tb(λ)Tc(λ)

)

(2)

Fig. 3. Surface SEM images of (a) WO3 and (d) VO2 thin films deposited on ITO(S)/ITO(A)/graphene/Si, respectively. Cross-sectional TEM images of (b) WO3/ITO (S)/ITO(A)/graphene/Si and (e) VO2/ITO(S)/ITO(A)/graphene/Si; the corresponding FFT images are shown in the insets. (c) and (f) are enlarged versions of (b) and (e), respectively; the partially enlarged figures of the graphene layers are shown in the insets.

Fig. 4. Raman spectra of ITO(S)/graphene, ITO(A)/graphene, WO3/ITO(S)/ ITO(A)/graphene, and VO2/ITO(S)/ITO(A)/graphene transferred onto Si wafers.

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CE(λ) =ΔOD

Q(3)

where Q is the charge inserted into or extracted from the WO3 layer per unit area and Tb and Tc are the optical transmittances of the bleached and colored states, respectively. The ΔOD and CE values of the WO3 film after 100 CV cycles slightly increased from 0.82 and 19.92 cm2/C to 0.88 and 21.51 cm2/C, respectively. Fig. 5b shows the CV curves obtained under continuous potential cycling for WO3/ITO(S)/ITO(A)/graphene in Li-PC. Upon applying a potential between +1.0 and − 1.0 V, a cathodic electronic current arises when Li+ ions flow into the film [39,40]. In addition, the WO3 film exhibits superior electrochemical stability during 100 CV cycles, which is consistent with the optical transmission results presented in Fig. 5a. Fig. 5c shows the changes in the optical transmittance of the Li+-deintercalated and Li+-intercalated states of the VO2/ITO(S)/ITO(A)/graphene film measured at τ < τc (RT) and τ > τc (80 ◦C) to investigate the TC behavior of the VO2 film after electrochemical Li+ intercalation. When the temperature of the film was increased from RT to 80 ◦C, the typical TC behavior, i.e., decreased transmittance in the NIR region was observed with both Li+-dein-tercalated and Li+-intercalated VO2.

The solar modulation ability is defined as follows:

ΔTsol = Tsol(τ < τc) − Tsol(τ > τc) (4)

The ΔTsol values of the Li+-deintercalated and Li+-intercalated VO2 thin films were calculated to be 5.26 and 4.10%, respectively. The temperature-dependent optical transmission (at the wavelength of 2500 nm) and thermal hysteresis loop of the Li+-deintercalated and Li+- intercalated VO2 thin films are presented in Fig. S4. Both the VO2 thin films exhibit characteristic hysteresis behavior during heating and cooling cycles. However, the switching efficiencies at λ = 2500 nm, ΔTr,

of the Li+-deintercalated and the Li+-intercalated VO2 thin films are 11.0 and 9.2%, respectively, indicating a slight decrease in TC response after Li+-intercalation. A slight decrease in the transmittance of visible and infrared light was observed after Li+-intercalation at both RT and 80 ◦C owing to the cathodic EC reaction of VO2 [41,42]. Therefore, four different optical states were observed in the VO2 thin film owing to the combination of its EC reaction and TC behavior. The CV curves obtained during continuous potential cycling for the VO2 film indicate its superior electrochemical stability over 100 CV cycles, as shown in Fig. 5d. The VO2 nanomaterial have been reported to exhibit an oxide suppercapacitor-like behavior where charge is absorbed via surface ef-fects [43]. As a result, the dense and thin layer of the VO2 film resulted in a lower current density compared to the WO3 film during the CV cycles, as shown in Fig. 5b and d.

The evolution of the Raman peaks with respect to temperature was examined for the VO2/ITO(S)/ITO(A)/graphene films to investigate the structural phase transitions of Li+-deintercalated and Li+-intercalated VO2 films. As shown in Fig. 6, both Li+-deintercalated and Li+-interca-lated VO2 exhibit several peaks corresponding to the typical Raman spectra of VO2(M) [7,19,21]. The characteristic Raman peaks of the Li+- deintercalated VO2 film gradually decreased with an increase in tem-perature, and then completely disappeared at ~60 ◦C, indicating a structural transition from the low-temperature VO2(M) phase to the high-temperature VO2(R) phase. With a decrease in temperature, the Raman peaks of VO2(M) reappeared, indicating the reverse transition from the VO2(R) phase to the VO2(M) one. The Raman peaks were less intense in the case of the Li+-intercalated VO2 film than in the case of the Li+-deintercalated VO2 film even below 60 ◦C. Therefore, it can be stated that Li+ intercalation induced a partial phase transition from the VO2(M) phase to the VO2(R) phase; this was also confirmed from the XRD data (Fig. 2b). These results explain the inferior TC behavior of the Li+-

Fig. 5. (a) Optical transmittance spectra of the WO3 film in as-deposited, colored, and bleached states. (b) Cyclic voltammograms of the WO3 film in 1 M Li-PC. (c) Optical transmittance spectra of the Li-intercalated and Li-deintercalated states of the VO2 film recorded at RT and 80 ◦C, respectively. (d) Cyclic voltammograms of the VO2 film in 1 M Li-PC.

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intercalated VO2 film compared to that of the Li+-deintercalated VO2 film, as shown in Figs. 5c and S3. In our previous report, the H+-inter-calated VO2 film showed relatively poor TC behavior compared with that of the H+-deintercalated VO2 film [21], which is consistent with the current results.

3.3. EC and TC properties of the EC-TC hybrid smart window

Fig. 7a shows the photographs of the flexible EC–TC hybrid smart window in four different optical states. When a constant voltage of − 1.5 V was applied, the device transitioned from a transparent (bleached) to a deep blue (colored) state owing to the EC reaction. The transmittance decreased further when the colored device was heated to 80 ◦C because the phase transition from the VO2(M) phase to the VO2(R) phase induced the TC behavior. When an opposite constant voltage of +1.5 V was applied with the maintenance of the temperature, the device recovered its transmittance. Finally, the flexible EC–TC device returned to its original bleached state upon decreasing the temperature to RT. Fig. 7b shows the optical transmittance spectra of the four different optical states of the flexible EC–TC hybrid device, and the Tlum and Tsol values of each state are summarized in Table 1. Tlum and Tsol of the initial bleached state decreased drastically from 52.44 to 2.42% and from 49.17 to 2.32%, respectively, owing to both the EC reaction as well as the TC behavior of the device. Our flexible EC–TC hybrid device can control the solar heat gain and daylight admission simultaneously or independently, depending on the surrounding temperature and applied electric field, allowing the selective modulation of light in the visible and NIR regions. Fig. 7c shows the in situ transmittance modulation at 550 nm of the flexible EC–TC hybrid device during continuous pulse potential cycling (100 cycles, +1.5 to − 1.5 V, duration 60 s). After the initial stabilization period of the electrochemical reaction of Li+ ions, the device showed stable transmittance modulation during 100 cycles. As shown in Fig. 7d, the response times during the coloring/bleaching processes were quantitatively calculated by normalized transmittance of the trans-mittance changes of the pulse potential cycling data in Fig. 7c. The response times were calculated at the level of a 90% of their full trans-mittance change. The response times for coloring and bleaching of the flexible EC–TC hybrid device were 36 and <8 s, respectively, almost similar to those of the EC–TC hybrid device fabriced on a glass substrate [21].

3.4. Highly durable transparent electrode of ITO(S)/ITO(A)/graphene/ PET

In our previous study, a flexible ITO(S)/graphene film on PET transferred from Cu foil exhibited excellent bending stability, despite the plasma-induced damage of graphene during the ITO sputtering process [34]. The graphene support effectively enhanced the flexibility of the film because the atomically controlled CVD graphene acted as me-chanically robust support. As shown in Fig. 7e, the flexible EC–TC hybrid device developed in this study was also very stable against mechanical bending owing to the robust adhesion of the ITO films on the graphene support and the reduced plasma damage of the graphene owing to the deposition of ITO(A) between ITO(S) and graphene. To further evaluate the long-term mechanical flexibility of the flexible EC–TC hybrid device, the bending cycling experiment (at a bending radius of 21.5 mm) was carried out. As shown in Fig. 7f, no obvious change in the colored/ bleached transmittance spectra was observed after the 1000 times cyclic bending tests. The bending performance of the flexible EC–TC hybrid device can be further improved by enhancing flexibility and interfacial adhesion force of the gel electrolyte.

ITO(A) also plays an important role in improving the durability of the device by preventing moisture and oxygen permeation. Fig. S5 shows the water vapor transmission rate (WVTR) of the ITO(A) film grown directly on a polyethylene naphthalate (PEN) substrate. Because the growth of ITO(A) film directly on a PET was impossible due to PET’s lack of thermal robustness above 100 ◦C, the ITO(A) was grown on a PEN substrate. When the WVTR of the bare PEN film was 1.29 g/(m2

day), the WVTR of ITO(A)/PEN was ~2.64 × 10− 3 g/(m2 day); these values are in good agreement with those of ALD-grown Al2O3 [33]. However, the WVTR values of the transferred ITO(A)/graphene/PET and ITO(S)/ITO(A)/graphene/PET could not be measured, probably owing to various damages, including wrinkles and residues, caused during the transfer process. If the transfer process is optimized, a highly durable and flexible transparent electrode can be obtained for various next-generation flexible optoelectronic devices.

4. Conclusion

In summary, a flexible EC–TC hybrid smart window was successfully fabricated on an ITO(S)/ITO(A)/graphene/PET substrate. ITO(A) effectively acted as a protective layer against the plasma-induced damage of graphene during the sputtering process. The amorphous WO3 film showed a cathodic EC reaction with high coloration efficiency and exhibited good electrochemical stability. The crystalline VO2 film

Fig. 6. Sequential Raman spectra of the VO2 film in (a) Li+-deintercalated and (b) Li+-intercalated states during the heating and cooling cycles.

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showed the characteristic TC behavior of regulating the solar energy transmittance in response to the change in the surrounding temperature. We have demonstrated the smart window characteristics of the flexible EC–TC hybrid device by changing its optical transmittance in four different states in response to the temperature change and applied electric field, and this allows the adaptive modulation of light in the visible and NIR regions. The fabricated ITO(S)/ITO(A)/graphene/PET substrate with high performance and durability is a promising flexible transparent electrode material for a variety of next-generation flexible

optoelectronic devices as well as multifunctional smart windows.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the Korea Institute of Energy Technol-ogy Evaluation and Planning (KETEP) and the Ministry of Trade, In-dustry & Energy (MOTIE) of the Republic of Korea (No. 20182020109430), and by Creative Materials Discovery Program and Material Innovation Leading Project through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (2020M3D1A2101136 and 2020M3H4A3081879).

Fig. 7. Characterization of the flexible EC–TC hybrid device. (a) Photographs, (b) optical spectra of the four different optical states based on the EC–TC hybrid structure. (c) In situ transmittance changes at 550 nm during the pulse potential cycling at 550 nm between +1.5 and − 1.5 V. (d) Normalized transmittance during the pulse potential cycling. (e) Photographs of the flexible EC–TC hybrid smart window in its bleached and colored states. (f) The colored/bleached transmittance spectra of the flexible EC–TC hybrid smart window before and after the 1000 times cyclic bending tests.

Table 1 Luminous (Tlum) and solar transmittances (Tsol) of the four different optical states of the flexible EC–TC hybrid device.

Tlum at RT Tlum at 80 ◦C Tsol at RT Tsol at 80 ◦C

Bleach (%) 52.44 41.95 49.17 39.22 Color (%) 6.41 2.42 5.38 2.32

S.J. Lee et al.

Chemical Engineering Journal 416 (2021) 129028

8

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi. org/10.1016/j.cej.2021.129028.

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