WO3-Based Hybrid Smart Windows with Thermochromic and

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VO2/WO3–based Hybrid Smart Windows with Thermochromic and Electrochromic Properties Sang Jin Lee, Dong Soo Choi, So Hee Kang, Woo Seok Yang, Sahn Nahm, Seung Ho Han, and TaeYoung Kim ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00052 • Publication Date (Web): 14 Feb 2019 Downloaded from http://pubs.acs.org on February 16, 2019

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ACS Sustainable Chemistry & Engineering

VO2/WO3–based Hybrid Smart Windows with Thermochromic and Electrochromic Properties Sang Jin Leea,b†, Dong Soo Choic†, So Hee Kanga, Woo Seok Yanga, Sahn Nahmb, Seung Ho Hana* and Tae Young Kimd* aElectronic

Convergence Materials and Device Research Center, Korea Electronics

Technology Institute, 1342 Seongnamdaero, Seongnam 13509, Republic of Korea bDepartment

of Materials Science and Engineering, Korea University, 145 Anamro, Seoul 02841, Republic of Korea cDepartment

of Materials Physics, Dong-A University, 37 Nakdongdaero, Busan 49315, Republic of Korea dDepartment

of Bionanotechnology, Gachon University, 1342 Seongnamdaero, Seongnam

13012, Republic of Korea Corresponding Author *Email

address: [email protected], [email protected]

†These

authors contributed equally.

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ABSTRACT

The integration of electrochromic (EC) and thermochromic (TC) systems is important for the realization of multifunctional smart windows, which can adaptively control light transmittance and solar energy in response to diverse external stimuli. Here, we report all-solid-state multifunctional smart windows where tungsten oxide (WO3)-based EC and vanadium oxide (VO2)-based TC cells are integrated into a single device. In a hybrid smart window, WO3-based EC layer modulates optical transmission in response to electrical voltage and VO2–based TC layer regulates solar energy transmission responding to the surrounding temperature. Therefore, it is capable of controlling optical transmission simultaneously or independently in response to an electric stimulus and temperature change, allowing for a selective modulation of light in visible and near infrared region. We demonstrated the applicability of such an integrated smart window system by varying optical transmission in four different optical states based on EC reaction and TC behavior. The concept for the integration of EC and TC cells into a single device can open the way for the next-generation multifunctional smart window systems.

KEYWORDS: Electrochromic material, Thermochromic material, Hybrid, Smart window applications, Optical transmittance


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INTRODUCTION With growing concerns on energy crisis, energy conservation has been on the top of global development agenda and researches have been directed towards sustainable development of energy-efficient materials.1 The progress in the research aiming to reduce heating/cooling load and save energy in building has led to the development of “smart windows” that are capable of regulating solar energy that pass through the windows of building. The smart windows are designed to modulate the amount of transmitted light in response to an external stimulus such as light irradiation (photochromism), electric field (electrochromism), environmental temperatures (thermochromism).4, 7-10 These windows can minimize energy losses and provide energy efficiency with indoor comfort in building. Smart windows are classified mainly into electrochromic (EC) and thermochromic (TC) windows. Electrochromic (EC) smart windows are the active-type smart windows that can change their optical properties by the application of an electrical voltage. The EC windows operate on the reversible electrochemical intercalation of positive ions (e.g., H+, Li+, Na+) and accompanying insertion of charge balancing electrons into the multivalent transition metal oxides (e.g., WO3, NiO, IrO, MoO3, V2O5).2,

11-13

Among the transition metal oxides,

amorphous tungsten oxide (WO3) films have been extensively studied since they provide the highest coloration efficiency (CE) in the visible region of light spectrum. Amorphous WO3 films change from optically transparent to deep blue with the insertion of positive ions and return to their transparent state by the extraction of the ions. For practical applications, EC systems based on proton (H+) intercalation are preferred owing to fast diffusion of proton into oxide layer and rapid response time. However, proton intercalation into oxide layer often suffer from rapid degradation of EC films, which limits a long-term durability of the devices. In addition, EC devices are constructed in complicated structures and generally coupled with 3 ACS Paragon Plus Environment


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additional low-emissivity (low-E) coating for thermal insulation, which increase the cost and hamper their widespread adoption. In contrast, thermochromic (TC) windows are the passive-type smart windows that exhibit selfregulating features to control the amount of transmitted light with temperature change. The TC smart windows can respond to the environmental temperature and intelligently control the optical transmittance in near infrared region (NIR) region of the solar spectrum, which carries approximately 50% of the solar energy.14-16 No additional energy is required to perform their smart functions and cost can be further reduced due to the simplicity in their structure. Vanadium dioxide (VO2) is the most extensively studied TC material, because of its near-roomtemperature metal-insulator transition (MIT) that is accompanied by a drastic change of the optical transmittance in the NIR region. VO2 undergoes a reversible phase transition between semiconducting monoclinic phase, VO2(M) and metallic rutile phase, VO2(R) at a critical temperature of τc = 68 °C.16-18 A semiconducting VO2(M) is transparent to NIR light at temperatures below τc, while the metallic VO2(R) is translucent above τc. These properties make VO2 a promising material for use in TC windows. However, low transmittance in the visible region originating from their strong absorption and high reflectance and difficulties associated with the high quality VO2 synthesis due to the polymorphism limits the application of VO2-based smart windows. Since there are issues associated with practical use of EC and TC windows, it is desirable to develop a smart window that combines the best attributes of EC and TC systems. To this end, we attempted to take advantage of hybrid system where WO3-based EC window and VO2based TC window are coupled into a single device and fabricated in a thin and compact design. This hybrid window allows for dual mode operation of EC and TC smart windows in a response to electrical voltage and/or temperature change, which result in an adaptive modulation of light 4 ACS Paragon Plus Environment

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in visible and NIR region. The integration of EC and TC components into a single device was carefully designed to overcome the limitations associated with simple combination of two separate devices, such as low transmittance values and low switching efficiency. Moreover, a proton-conducting solid electrolyte was used to ensure the stability and durability of the device. As the conventional liquid or gel electrolytes suffer from several issues such as low chemical stability, low mechanical strength, electrolyte leakage, and safety concern, we used tantalum oxide (Ta2O5) as solid electrolyte to fabricate all-solid-state device. In light of these considerations, all-solid-state EC/TC hybrid systems were fabricated according to the requirement of stability, durability, performance and ease of processing. We demonstrated the applicability of such an integrated smart window system, based on the optical switching performance and operational stability.

EXPERIMENTAL SECTION The thin films were deposited by reactive DC or radio frequency (RF) magnetron sputtering using a 100-mm-diameter target. After initial pump down to < 1.5×10-7 Torr, Ar sputter gas with 99.998% purity and O2 reactive gas with 99.998% purity were introduced into the chamber via mass-flow-controlled inlets to create working pressure of 6.5×10-3 Torr. The EC–TC hybrid device was fabricated as follows. First, amorphous VOx thin films were deposited on ITOcoated glass substrates (sheet resistance: 10 Ω/sq.) by reactive DC magnetron sputtering at room temperature (RT) under Ar (97 sccm) and oxygen (3 sccm). Post-annealing of VOx thin films was then performed at 500 °C for 30 min under high vacuum of 10-5 Pa with 50 sccm O2 flowing. Next, a Ta2O5 solid electrolyte layer was deposited on the VO2 film by reactive RF sputtering at RT using a Ta2O5 ceramic target. The sputtering was performed under the condition of 500 W and Ar/O2 ratio of 99:1. The ITO/VO2/Ta2O5 film was then taken out and 5 ACS Paragon Plus Environment


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immersed in 0.5 M sulfuric acid for proton insertion. Protons were injected across Ta2O5 thin film electrochemically under application voltage of 1.5 V. Following that, an amorphous WO3 film was deposited by reactive DC magnetron sputtering under the condition of 500 W and Ar/O2 ratio of 70:30. Finally, an ITO film was deposited by DC sputtering as a transparent top electrode. The thicknesses of the VO2, Ta2O5, and WO3 films measured using surface profiler (P-10, KLA-Tencor) were 40, 700, and 700 nm, respectively. Phase identification of the VO2 and WO3 thin films deposited on the ITO glass was performed using X-ray diffraction (XRD) (Empyrean, PANalytical) with Cu-Kα radiation. Raman spectroscopy (Renishaw spectrometer, wavelength: 514 nm) and X-ray photoelectron spectroscopy (XPS), (Sigma Probe Thermo VG, monochromatic Al Kα radiation) spectra were employed to measure the characteristic of the VO2 thin film in the H+-intercalated and H+-deintercalated states. The changes in the optical transmittance of WO3/ITO, VO2/ITO, and EC–TC hybrid devices at the colored and bleached states were measured using an ultraviolet UV–visible–NIR spectrophotometer (V-670, JASCO). The TC switching behavior was also monitored by a UV– visible–NIR spectrophotometer equipped with a heating block in the wavelength range 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 is lum or sol, 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 standing 37° above the horizon). Cyclic voltammetry (CV) was performed to evaluate the electrochemical properties of the WO3 and VO2 films using 6 ACS Paragon Plus Environment

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a potentiostat–galvanostat (PGSTAT 30, AUTOLAB) with a scan rate of 50 mV/s. Pt wire and Ag/AgCl were used as counter and reference electrodes, respectively.

RESULTS AND DISCUSSION Figure 1a illustrates the device structure of all-solid-state EC-TC hybrid smart windows. The hybrid smart windows consist of all-solid thin films and have a multilayer structure of ITO/VO2/Ta2O5/WO3/ITO on a glass substrate. Each layer serves as a transparent conductor, a thermochromic (TC) layer, a solid electrolyte, an electrochromic (EC) layer, and a transparent conductor, respectively.

Figure 1. (a) schematic view of the integrated EC/TC hybrid device layout architecture (b) cross-sectional SEM image of the device

Since the WO3-based EC and VO2-based TC components were integrated into a single device, this integrated device provides a dual function of switching the optical properties in response 7 ACS Paragon Plus Environment


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to both electric stimulus and environmental temperature. The thin films were deposited sequentially on ITO/glass substrate by sputtering with controlled thickness as shown in Figure 1b. Figure 2 shows the XRD patterns of the VO2 and WO3 thin films deposited on ITO glass substrate. The XRD pattern of the blank ITO glass is also shown for reference. It can be seen that the diffraction peak in the XRD pattern of VO2 could be indexed to the (011) peak of the monoclinic VO2(M) phase, indicating the crystalline feature of VO2 film.16, 18-19 After insertion of proton into VO2 film, the (011) diffraction peak was shifted towards the low-angle side as shown in right panel of Figure 2. This indicates that the crystal lattice spacing of VO2 film can slightly expanded by the insertion of proton. In case of WO3 film deposited on ITO/glass substrate, no diffraction peak was found other than that corresponding to ITO substrate. This indicates that the amorphous WO3 thin films were formed, which is known to show superior coloration efficiency (CE) than crystallized ones.20-21

Figure 2. XRD spectra of the VO2/ITO, WO3/ITO, and ITO glasses


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Figure 3a shows the optical transmittance spectra of the colored and bleached states of the WO3 film on the ITO glass in the wavelength range of 300–800 nm. The insets show photographs of the WO3/ITO glass in the colored and bleached states.

Figure 3. (a) Optical transmittance spectra of the as-deposited, colored, and bleached states of the WO3 film on the ITO glass. The inset images are photographs of the bleached and colored states of the WO3 film on the ITO glass. (b) Cyclic voltammograms of the WO3 film in 0.5 M H2SO4. (c) Optical transmittance spectra of the H+-intercalated and H+-deintercalated states of the VO2 film on the ITO glass measured at RT and 80 °C, respectively. The inset images are photographs of the H+-intercalated and H+-deintercalated states of the VO2 film on the ITO glass measured at RT and 80 °C, respectively. (d) Cyclic voltammograms of the VO2 film in 0.5 M H2SO4. 9 ACS Paragon Plus Environment


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Under an applied voltage of −1 V, the color of the WO3 film turned into opaque dark blue and then returned to a transparent color at +1 V. The Tlum values of the bleached (Tb) and colored (Tc) states were 76.24% and 0.66%, respectively, and these values remained almost unchanged after 100 CV cycles. To assess the performance of the device, we measured the coloration efficiency (CE) (represented in cm2/C) from the change of optical density (ΔOD), which is given as follows:3, 13, 22

ΔOD = log CE(λ) =

( ) 𝑇𝑏(𝜆)

(2)

𝑇𝑐(𝜆)

ΔOD 𝑄

(3) where Q is the charge inserted into or extracted from the WO3 layer per unit area, Tb and Tc refer to the transmission of bleached and colored states, respectively. The ΔOD and CE values of the WO3 film after 100 CV cycles slightly decreased from 2.06 and 32.65 cm2/C to 1.83 and 28.90 cm2/C, respectively. Figure 3b shows the cyclic voltammograms obtained during continuous potential cycling of the WO3 film. By applying electric potential from +1 to −1 V, a cathodic electronic current appeared when H+ ions flew into the film. The cyclic voltammograms obtained during continuous potential cycling of the WO3 film remained unaltered after 100 CV cycles, which is consistent with the optical transmission data in Figure 3a. Figure 3c shows the changes in the optical transmittance of the H+-deintercalated and H+intercalated VO2 films on the ITO glass measured at τ < τc (RT) and τ > τc (80 °C) to confirm the TC behavior of the VO2 films after electrochemical proton insertion. The optical spectra of the H+-deintercalated VO2 film at RT and 80 °C clearly showed the typical TC behavior of a high transmittance change in the NIR region. Although the VO2 thin film at the H+-intercalated state after applying -1.5 V exhibited a smaller change in the NIR transmittance compared with 10 ACS Paragon Plus Environment

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that at the H+-deintercalated one, we can clearly confirm the TC behavior as shown by the red solid and dotted lines in Figure 3c. The modulation ability of Tsol is defined as ΔTsol = Tsol(τ < τc) – Tsol(τ > τc). The ΔTsol values of the H+-deintercalated and H+-intercalated VO2 thin film were 6.3% and 3.5%, respectively. As a result, four different optical states of the hybrid devices were obtained with the combination of EC reaction and TC behavior. The cyclic voltammograms obtained during continuous potential cycling of the VO2 film exhibited good electrochemical stability during 100 CV cycles without a significant change in the shape, as shown in Figure 3d. For structural characterization of the H+-deintercalated and H+-intercalated states of the VO2 film, the chemical compositions were examined using XPS (Figure 4). The XPS core-level spectra of V2p and O1s of the H+-deintercalated and H+-intercalated states of the VO2 film show doublet peaks of V2p3/2 and V2p1/2, corresponding to 3/2 and 1/2 spin-orbital splitting photoelectrons of vanadium ions, respectively. Because the binding energy of the V2p3/2 core level depends on the oxidation state of vanadium, curve fitting of the V2p3/2 XPS signal was done to determine oxidation state of VO2 23-26.

Figure 4. XPS spectra of the O1s and V2p core states of the VO2 films at the (a) H+deintercalated and (b) H+-intercalated states. 11 ACS Paragon Plus Environment


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The H+-deintercalated and H+-intercalated VO2 films exhibited similar V2p3/2 spectra with maximum peaks at 515.4 and 515.5 eV, respectively, indicating oxidation state of vanadium is 4+.23-26 The XPS data analysis for the H+-deintercalated and H+-intercalated VO2 film suggests that the proton intercalation was unlikely to influence the oxidation state of the VO2 films. To obtain more insights on the structural phase transition of the VO2 film in the H+deintercalated and H+-intercalated states, the evolution of the Raman peaks was monitored with respect to temperature. In Figure 5, both VO2 films exhibited several peaks corresponding to the well-defined typical Raman spectra of the VO2(M) phase with mainly Ag vibrational bands in the range of 194, 226, 310, 389, and 615 cm-1.23,

26-28

With increasing temperature, the

characteristic Raman peaks of the H+-deintercalated VO2 film gradually decreased through the intermediate phase-coexistence regimes and then completely disappeared at ~60 °C, as shown in Figure 5a. This result indicated a structural phase transition from the low-temperature VO2(M) phase to high-temperature VO2(R) phase because the Raman peaks of the metallic VO2(R) phase were very weak and broad due to the damping of phonons by the electron– phonon coupling.6, 17, 29-30 With decreasing temperature, the Raman peaks of the VO2(M) phase appeared again, indicating a reversed phase transition from the VO2(R) phase to the VO2(M) phase. In the case of the H+-intercalated VO2 film, intensities of the Raman peaks were found to be smaller than those of the H+-deintercalated VO2 film and those characteristic peaks almost disappeared at a lower temperature of ~50 °C, as shown in Figure 5b. This suggests that proton intercalation might have induced partial phase transition of the VO2(M) phase into the VO2(R) phase, which is consistent with relatively smaller TC behavior of the H+-intercalated VO2 film than that of the H+-deintercalated VO2 film, as shown in Figure 3c. Despite the reduced TC behavior of the H+-intercalated VO2 film, four different optical states were obtained from the VO2 film on the ITO glass as a result of the selective control of the TC behavior and EC reaction. 12 ACS Paragon Plus Environment

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Figure 5. Sequential Raman spectra of VO2 at (a) H+-deintercalated and (b) H+-intercalated states during the heating and cooling cycle. Figure 6a and 6b show the schemes of the four different optical states of the EC–TC hybrid device and the corresponding photographs. When a constant voltage of −2 V was applied, the color of the device changed from transparent (bleached state) to deep blue (colored state) because of the

EC reaction. When the colored device was heated to 80 °C, the transmittance

further decreased because of the synergetic effect of the EC reaction and TC behavior. When a reversed constant voltage of +2 V was applied, the transmittance increased because of the TC behavior of the device. Finally, the EC–TC hybrid device returned to its original bleached state when the temperature decreased to RT. Figure 6c shows the optical transmittance spectra of the four different optical states based on the EC–TC hybrid structure and the Tlum and Tsol at each state are summarized in Table 1. The Tlum of the initial bleached state decreased dramatically from 73.57% to 8.33% because of the synergetic effect of the EC reaction and TC behavior. In addition, ΔTsol of the initial bleached state significantly decreased from 63.49% to



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5.24%. Figure 6d shows the in situ transmittance changes at 550 nm obtained during the pulse potential cycling between 2 and −2 V for the EC–TC hybrid device.

Figure 6. Characterization of the EC–TC hybrid device. (a) Scheme, (b) photographs, and (c) optical spectra of the four different optical states based on the EC–TC hybrid structure. (d) In situ transmittance changes at 550 nm during the pulse potential cycling between 2 and −2 V. Table 1. Luminous and solar transmittances of the four different optical states of the EC–TC hybrid device.

Bleached

Tlum at RT (%)

Tlum at 80 ºC (%)

Tsol at RT (%)

Tsol at 80 ºC (%)

73.57

44.69

63.49

39.21


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Colored

28.62

8.33

18.37

5.24

The device became stable after 20 cycles as the electrochemical reaction of protons are stabilized. TheTa2O5 which is an electrochemically stable proton-conducting solid electrolyte also contributed to the cycling stability of the device.11, 31 The bleaching and coloring response times of the EC–TC hybrid device was 5 and 30 s, respectively, as shown at the right side in Figure 6d. This indicates that our our EC-TC hybrid device exhibited a good interfacial charge transfer behavior despite the use of solid electrolyte which shows rather poor ion conductivity. For the practical application of smart windows , the thermal emissivity of the window should be as low as possible. Since our EC–TC hybrid device can block the light over visible and NIR region by EC reaction and TC behavior, the solar heat gains and daylight admission through the windows can be minimized at high surrounding temperature and by applying a small electric voltage. Therefore, the energy loss in a building can be minimized, and users can be provided indoor visual comfort.

CONCLUSIONS In this work, an all-solid-state EC–TC hybrid smart window device consisting of ITO/VO2/Ta2O5/WO3/ITO was successfully fabricated on a single glass substrate. This multifunctional device integrating EC and TC cells into a single device can adaptively modulate the optical transmission in response to electric stimulus and temperature change, allowing for the simultaneous or independent control of solar radiation and heat. The amorphous WO3 films serve as EC layer which allows for the optical modulation with high 15 ACS Paragon Plus Environment


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coloration efficiency under application of electrical voltage, while the crystalline VO2 films function as TC layer regulating solar energy transmission depending on the surrounding temperature. We demonstrated the applicability of such an integrated smart window system by varying optical transmission in four different optical states based on EC reaction and TC behavior. The device described here could represent a step toward the development of new ECTC hybrid smart windows with multiple functionalities and therefore can open the way for the next-generation energy-saving devices.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Seung Ho Han), [email protected] (TaeYoung Kim)

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) under Grant and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20173030014180, No. 20182020109430) and National Research Foundation of Korea (NRF) grant funded by the Korea government(MSIP) (No. 2015R1C1A2A01054433). This research was supported by the Nano Material Technology Development Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT and Future Planning (NRF-2016M3A7B4027712).


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For Table of Contents Use Only

Hybrid smart windows capable of controlling the optical transmission simultaneously or independently in response to an electric stimulus and temperature change is reported.


WO3-Based Hybrid Smart Windows with Thermochromic and

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