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Investigation of all-solid-state electrochromic devices with durability enhanced tungsten-doped nickel oxide as a counter electrode
Journal of Alloys and Compounds 815 (2020) 152399
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Investigation of all-solid-state electrochromic devices with durability enhanced tungsten-doped nickel oxide as a counter electrode Sang Jin Lee a, b, Tae-Gon Lee b, Sahn Nahm b, Dong Hun Kim c, Dae Jin Yang d, **, Seung Ho Han a, * a
Electronic Convergence Materials and Device Research Center, Korea Electronics Technology Institute (KETI), Seongnam, 13509, Republic of Korea Department of Materials Science and Engineering, Korea University, Seoul, 02841, Republic of Korea Department of Materials Science and Engineering, Myongji University, Yongin, 17058, Republic of Korea d Samsung Electronics, 466-712, Yongin, Republic of Korea b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 June 2019 Received in revised form 20 September 2019 Accepted 21 September 2019 Available online 23 September 2019
Films of tungsten-doped nickel oxide, denoted Ni1-xWx oxide as counter electrode in electrochromic devices were deposited onto indium tin oxide coated glass substrates by reactive dc magnetron sputtering using tungsten-doped nickel alloy target. Electrochromic properties of NixW1-x oxides were evaluated in lithium perchlorate in propylene carbonate. Enhanced cycle stability with moderate optical modulation was obtained for Ni1-xWx oxide film with x ¼ 0.024. Significant degradation in pure NiO film during extended electrochemical cycling caused by microstructural deep trap sites was minimized by W doping. The EC device fabricated with WO3 thin film and Ni1-xWx oxide thin film with x ¼ 0.024 showed stable optical modulation of about 2% reduction from 100 to 1000 cycles. Flexible EC device was also fabricated with WO3 thin film and Ni1-xWx oxide thin film with x ¼ 0.024 deposited on c-ITO/graphene/ PET electrode and showed stable cycling performance with maintaining optical modulation of DT z 40%. © 2019 Elsevier B.V. All rights reserved.
Keywords: Electrochromic device Tungsten-doped nickel oxide Counter electrode Flexible smart window
1. Introduction Electrochromic (EC) devices have been attracted increasing attention because of their interesting characteristics of modulating optical properties by an applied voltage. EC devices have many commercial applications such as smart windows of green buildings, information display devices and reflectance mirrors [1e10]. Conventional EC device is composed of TCO/EC/IC/CE/TCO layers, where TCO, IC and CE are transparent conducting oxide, ion conducting layer (electrolyte) and counter electrode, respectively [11,12]. The oxides of nickel and tungsten are the well-known electrochromic materials of anodic and cathodic coloration, respectively. In an EC device, nickel oxide is generally used as a counter electrode material with enhancing coloration efficiency of whole device with a pair of tungsten oxide [13,14]. However, electrochromism in NiO films is rather complicated and is still debated, although it is generally accepted that the transition from a colored to a bleached state is related to a multiple valances of Ni ions in Ni oxide due to
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (D.J. Yang), [email protected] (S.H. Han). https://doi.org/10.1016/j.jallcom.2019.152399 0925-8388/© 2019 Elsevier B.V. All rights reserved.
the variation of Ni3d electrons. Especially, owing to complicated reaction with Liþ-conducting electrolytes on cycling, the decay of the charge exchange, and hence of the optical modulation span of NiO films still limits the commercialization of EC devices [15e18]. Recently, it is accepted that the enhancements of cycling stability in EC materials can be achieved by optimizing their composition with various additives such as lithium, carbon, nitrogen, tungsten, iridium, (Li,W), (Li,Al), (Li,Zr), and others [19e22]. Among the representative candidates, adding modest amount of Ir into NiO significantly enhanced the cycling durability in a Liþ-conducting electrolyte. However, very high cost of Ir prevents it from being used as a counter electrode of EC devices. The incorporation of Li into NiO is also hindered due to the difficulties in fabrication of Li or Li-metal alloy target for the reactive DC magnetron sputtering. Although several researches on Li doping in NiO have been conducted by RF magnetron sputtering using various Li-based oxide target, low deposition rate of RF magnetron sputtering makes it unsuitable for mass production of EC devices. In this work, we report the enhancement of cycling stability of EC device with excellent EC properties using films of tungstendoped nickel oxide, denoted Ni1-xWx oxide as a counter electrode. We confirmed the effect of compositional modulation by means of
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specific electrochemical tests and structural characterization using cyclic voltammetry (CV), X-ray diffraction (XRD), and scanning electron microscope (SEM). And we proposed a reaction mechanism accounting from the observed degradation in the EC performances of optimized Ni1-xWx oxide films. Finally, we demonstrated flexible EC device using graphene-supported transparent conducting electrode which offers requirements for practical applications, that is, high-contrast optical modulation, good electrical conductivity and mechanical flexibility. Fabrication strategy of EC devices using highly stable Ni1-xWx oxide film on flexible electrode provide a general framework for designing optically and mechanically high-performance EC devices. 2. Experimental section The thin films were deposited by reactive DC magnetron sputtering using 4-inch-diameter target. After initial pump down to <1.5 107 Torr, sputter gas of 99.998% purity Ar and reactive gas of 99.998% purity O2 were introduced into the chamber via massflow-controlled inlets to form working pressure of 3.5 103 Torr. The DC power output was set at a constant value of 500 W Ni1-xWx oxide thin films were deposited on ITO coated glass substrates (sheet resistance, 10 U/sq.) at room temperature (RT) under Ar (90 sccm) and oxygen (10 sccm). The composition was varied by changing the W concentration of the Ni-W metal alloy target from 0 to 20 at%. Amorphous WO3 thin film was also deposited on ITO glass at RT under O2/Ar ratio of 30%. Finally, all-slid-state EC devices were fabricated by laminating two coated substrates with Li-based polymeric solid state electrolyte (Soulbrain, SWOPE). The thicknesses of the Ni1-xWx oxide, gel-electrolyte and WO3 films measured by a surface profiler (P-10, KLA-Tencor) were 400 nm, 1 mm and 700 nm, respectively. The elemental composition of the Ni1-xWx oxide was evaluated by X-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo Fisher Scientific). Phase identification of Ni1xWx oxide thin films deposited on ITO glass was performed using Xray diffraction (XRD, Empyrean, PANalytical) with Cu-Ka radiation. Surface and cross-section microstructure of the films were investigated using scanning electron microscopy (SEM; Hitachi S-4800, Hitachi High-Tech, Tokyo, Japan). Cyclic voltammetry (CV) was performed to evaluate the electrochemical properties of Ni1-xWx oxide films using a potentiostat-galvanostat (PGSTAT 30, AUTOLAB) between 1.0 and 1.5 V with a scan rate of 50 mV/s. Pt foil and Ag/ AgCl were used as the counter electrode and reference electrode, respectively and 1 M LiClO4 in propylene carbonate (Li-PC) was used as the electrolyte. The optical transmittance of Ni1-xWx oxide was simultaneously measured in situ during continuous CV cycling tests. A He/Ne laser (633 nm) and a power meter were used for the in situ transmittance measurement. The optical transmittance changes of all-solid-state EC devices at their colored and bleached states were measured in the wavelength range of 380e780 nm using UVevisible spectrophotometer (clay-60, Agilent). Continuous potential cycling of all-solid-state EC devices were also carried out for up to 1000 cycles in the range of 1.25e1.25 V by kinetic mode of UVevisible spectrophotometer at a wavelength of 550 nm. Flexible and crystallized ITO thin films were prepared on plastic PET substrate with monolayered graphene as a platform. Singlelayered graphene was grown on Cu foil (99.8%) by rapid thermal chemical vapor deposition (RTCVD) [23]. The highly crystalline ITO (c-ITO) was fabricated on graphene/Cu foil by DC magnetron sputtering at RT and subsequent post-annealing process at 250 C with a thickness of 60 nm and it was continuously transferred onto a PET substrate, as described elsewhere [24]. The c-ITO/graphene/ PET electrode showed a sheet resistance of ~45 U/sq. and a transmittance of ~92% at a wavelength of 550 nm. Finally, the flexible EC devices were fabricated by laminating the WO3 film and optimized
Ni1-xWx oxide film deposited on c-ITO/graphene/PET substrate, respectively using Li-based polymeric solid state electrolyte. 3. Results and discussion Fig. 1 shows XPS spectra of Ni1-xWx oxide thin films deposited using Ni-W metal alloy target with a W concentration of 5e20 at%. When the W concentrations of the Ni-W alloy targets are 5, 10, and 20 at%, the magnitude of x is determined to be 0.024, 0.079 and 0.268, respectively. Fig. 2 (a) shows XRD patterns of Ni1-xWx oxide thin films. The pure Ni oxide thin film exhibits a crystalline phase with a strong (111) diffraction peak, indicating the preferred orientation of the film surface. The (111) peak intensity decreases upon the increase of W content. Previously, amorphization upon W addition has been reported elsewhere [25,26], and it is associated with weakening the long-range order in the Ni-O matrix. W ions of high oxidation state (6þ) would induce a large perturbation of the lattice around a W cation inserted in a Ni site due to the large difference between the stable valence states of Ni (2þ) and W (6þ) [25,26]. As a result, the overall crystal structure of polycrystalline Ni oxide turns less dense state when the W content increases. W addition also influenced on the microstructure of the Ni1-xWx oxide thin films, as shown in Fig. 2(b). With increase in W content, a columnar nanostructure becomes gradually dense due to the decreased crystallinity. The formation of columnar features are generally believed to be in favor of ion transportation because of increased active sites, however they simultaneously cause reduction in stability and electrical properties as well as adhesion to substrates [27,28]. Fig. 3 shows CV data of Ni1-xWx oxide thin films with addition of W. The initial CV curve of pure Ni oxide film is broader than those of other W-doped films however, significant decay of the charge exchange is observed upon electrochemical cycling. This degradation is associated with the decreased ion insertion/extraction and subsequent decrease of electrochromism, which was already reported in sputter-deposited pure Ni oxide films in Li-PC [29,30]. Although the charge exchange was decreased with increase in W addition, the decay of charge density during electrochemical cycling was significantly decreased with Ni1-xWx oxide film with x ¼ 0.024 and 0.079. The Ni1-xWx oxide film with x ¼ 0.268 shows very low charge insertion and extraction. The results of CV data in Fig. 3(a)-(d) were numerically quantified as the charge density (mC/cm2) value during 1000 cycles of Ni1-xWx oxide film, as shown in Fig. 4. Initial charge density drop during the first 100 cycles of Ni1-xWx oxide films with x ¼ 0, 0.024, and 0.079 can be understood as an effect of activation by Refs. [31,32]. NiOx þ yLiþ þye 4 LiyNiOx
(1)
After that, Li-containing active materials on the surface can be further modulated by Liþ insertion and extraction associated with reversible charge-transfer process between two different phases of the Li-Ni oxide, as described by Refs. [31,32]: LiyNiOx (bleached) 4 Li(y-z)NiOx þ zLiþ þ ze (colored)
(2)
A gradual charge density decline of the pure Ni oxide upon extended voltammetric cycling indicates the decreased Li ion insertion and extraction corresponding to irreversible electrochemical reactions between electrolyte and thin film. Although the charge density of the Ni1-xWx oxide films with x ¼ 0.024 and 0.079 slightly decreased, stable cycling performance was obtained indicating enhanced durability. For Ni1-xWx oxide film with x ¼ 0.268, very low Li ion insertion and extraction took place during extended cycling.
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Fig. 1. XPS spectra of Ni1-xWx oxide thin films using Ni-W metal alloy target with a W concentration of 5e20 at%.
Fig. 2. (a) XRD patterns and (b) SEM images of Ni1-xWx oxide thin films deposited on a Si wafer.
Fig. 5 shows transmittance modulation of Ni1-xWx oxide films measured in situ during continuous 1000 CV cycling tests. The color of the Ni1-xWx oxide films with x ¼ 0, 0.024, and 0.079 changed from dark gray (colored state) to yellowish gray (bleached state) reversibly, as shown in inset image of Fig. 5. As the W content increased, the transmittance decreased, which is consistent with previous reports that the band gap of pure Ni oxide was reduced upon W addition [33,34]. The initial optical modulation of pure Ni oxide was large, but gradually decreased upon electrochemical cycling. As the cycle number increased, the transmittance gradually increased, but the transmittance in the colored state of pure Ni oxide was further increased compared to the bleached state. As the W content increased, the initial optical modulation was reduced but remained almost constant upon electrochemical cycling. Furthermore, the increase in the transmittance with increasing cycle number decreased in both colored and bleached states, and the optical modulation became nearly flat at the Ni1-xWx oxide film with x ¼ 0.079. The Ni1-xWx oxide film with x ¼ 0.268 was almost optically passive. The long-term degradation mechanism of Ni oxide is still a matter of controversy. According to our experimental results with
Ni-W oxide system, while preliminary, we suggest that the stability problem of pure-NiO film is related to the microstructure, that is, columnar nanostructure. Although the columnar nanostructure is advantageous in ionic diffusion and ion insertion kinetics, iontrapping-induced degradation of electrochromic properties can be easily happened due to the existence of microstructural deep trap sites [29,30]. Further increase in colored state transmittance of pure Ni oxide upon cycling compared to bleached state transmittance corresponds to higher amount of Li ion insertion than extraction, which might be originated from the Li ions trapping at the deep trap sites. As we demonstrated in SEM images of the Ni1xWx oxide films as shown in Fig. 2(b), a columnar nanostructure becomes gradually dense with increasing W content, which may result in reduced modulation but increased stability due to the decreased deep trap sites. Ni1-xWx oxide films with x ¼ 0.024 and 0.079 were considered to be optimal condition, however, Ni1-xWx oxide films with x ¼ 0.079 shows relatively low transmittance at bleached state and low optical modulation for application to EC devices. As Ni1-xWx oxide film with x ¼ 0.268 showed very low Li ion insertion and extraction during extended electrochemical cycling, which means that electrochromic properties disappear
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Fig. 3. Cyclic voltammetry of the Ni1-xWx oxide films immersed in the LiClO4-PC solution with (a) x ¼ 0, (b) x ¼ 0.024, (c) x ¼ 0.079 and (d) x ¼ 0.268.
Fig. 4. Inserted and extracted charge density during 1000 CV cycles of Ni1-xWx oxide films with (a) x ¼ 0, (b) x ¼ 0.024, (c) x ¼ 0.079 and (d) x ¼ 0.268.
above a certain amount W doping in NiO [25]. Fig. 6 shows schematic and optical images EC device at the bleached and colored states using Ni1-xWx oxide film with x ¼ 0.024
as a counter electrode. EC devices consists of an WO3 film for EC layer in contact with a Li-based polymeric solid state electrolyte adjacent to Ni1-xWx oxide film for counter electrode, and these
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Fig. 5. In situ transmittance modulation at 633 nm during 1000 CV cycles for Ni1-xWx oxide films with (a) x ¼ 0, (b) x ¼ 0.024, (c) x ¼ 0.079 and (d) x ¼ 0.268.
Fig. 6. Schematic and optical images of EC device using Ni1-xWx oxide film with x ¼ 0.024 as a counter electrode at the (a), (c) bleached state and (b), (d) colored state, respectively.
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layers are sandwiched between ITO coated glass substrates. The transmittance modulation can be controlled by the reversible electrochemical reaction induced by an external electric field, which changes from transparent (bleached state) to deep blue (colored state). Fig. 7 shows transmittance modulation during continuous pulse potential 1000 cycling (1.25 ~ 1.25 V, duration 60sec) for EC devices fabricated using Ni1-xWx oxide films as counter electrodes. The transmittance modulation of EC device fabricated with pure Ni oxide film gradually decreased from 57% to 30% at 100 and 1000 cycles, respectively, which is related to Li ions trapping at the deep trap sites, as mentioned earlier. The EC device fabricated using Ni1-xWx oxide film with x ¼ 0.024 showed relatively stable transmittance modulation (about 2% reduction from 100 to 1000 cycles), which indicates that certain amount of W doping in the Ni-oxide thin film is advantageous for delaying deterioration of the counter electrode layer with minimum optical modulation loss. Although the EC device fabricated using Ni1-xWx oxide film with x ¼ 0.079 shows stable transmittance modulation, low bleach state transmittance of below 60% and low optical modulation of about 30% hindered it for practical application to EC devices. Very low optical modulation of EC device fabricated using Ni1-xWx oxide film with x ¼ 0.268 is related to the very low Li ion insertion and extraction of Ni1-xWx oxide film with x ¼ 0.268 during extended cycling. In addition, we fabricated flexible EC devices by laminating WO3 thin film and Ni1-xWx oxide thin film with x ¼ 0.024 deposited on cITO/graphene/PET electrode, respectively using a Li-based polymeric solid state electrolyte, as shown in Fig. 8. During 1000 pulse potential cycling, flexible EC device in our work shows very stable cycling performance with maintaining optical modulation of DT z 40% after initial activation process. The color of the flexible device changed from light yellow (1.5 V) to deep blue (þ1.5 V), even at the bended state. By comparison with rigid device
fabricated on ITO glass, the c-ITO on graphene/PET having a relatively large sheet resistance of ~45 U/sq (ITO glass: 10 U/sq) needs higher operating voltage, but electrolyte is normally dissociated at high voltage [35]. Therefore, applied voltage of ±1.5 V must be insufficient to make full transmittance change in the flexible device. In spite of the low transmittance modulation of flexible device compared to rigid device, it is enough to be used for electrochromic display, which has attractive merit of flexible application. The graphene support film showed significant enhancement in flexibility because the atomically controlled monolayered graphene acted as a mechanically robust support [24]. Our flexible EC device was very stable against mechanical bending, which provides the possibility of potential application to flexible EC device.
4. Conclusions W-doped Ni oxide counter electrode layers were prepared by reactive DC magnetron sputtering using Ni-W metal alloy target with a W concentration of 5e20 at%. With increase in W content, the crystal structure of polycrystalline Ni oxide gradually turns into amorphous state and a columnar nanostructure becomes dense microstructure. Degradation of charge density and optical modulation upon extended electrochemical cycling was enhanced by W addition. Ni1-xWx oxide films with x ¼ 0.024 and 0.079 showed stable cycling stability, but Ni1-xWx oxide films with x ¼ 0.079 showed relatively low transmittance at bleached state and low optical modulation. Reduced modulation but increased stability with increasing W content in Ni oxide might be due to the decreased deep trap sites. The Ni1-xWx oxide film with x ¼ 0.268 showed very low Li ion insertion/extraction, and consequently resulted in almost optically passive. EC devices fabricated using Ni1xWx oxide with x ¼ 0.024 showed stable transmittance modulation until 1000 cycles. Finally, flexible EC device was fabricated with
Fig. 7. In situ transmittance modulation at 550 nm upon continuous potential cycling (þ1.25 ~ 1.25 V, duration - 60sec) for EC devices fabricated using Ni1-xWx oxide counter electrodes with (a) x ¼ 0, (b) x ¼ 0.024, (c) x ¼ 0.079 and (d) x ¼ 0.268.
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Fig. 8. Schematic diagrams and optical images of flexible EC devices using c-ITO/graphene/PET transparent electrode at the (a) bleached state and (b) colored state. (c) In situ transmittance modulation at 550 nm upon continuous potential cycling (þ1.5 ~ 1.5 V) and (d) transmittance spectra for flexible EC devices.
WO3 thin film and Ni1-xWx oxide thin film with x ¼ 0.024 deposited on c-ITO/graphene/PET electrode and showed good cycle stability. These results suggest that Ni1-xWx oxide thin film with x ¼ 0.024 is a promising material for a counter electrode of EC devices.
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Acknowledgements [9]
This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) under the Ministry of Trade, Industry and Energy (MOTIE) of the Republic of Korea (No. 20173030014180, No. 20182020109430).
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Investigation of all-solid-state electrochromic devices with durability enhanced tungsten-doped nickel oxide as a counter electrode Sang Jin Lee a, b, Tae-Gon Lee b, Sahn Nahm b, Dong Hun Kim c, Dae Jin Yang d, **, Seung Ho Han a, * a
Electronic Convergence Materials and Device Research Center, Korea Electronics Technology Institute (KETI), Seongnam, 13509, Republic of Korea Department of Materials Science and Engineering, Korea University, Seoul, 02841, Republic of Korea Department of Materials Science and Engineering, Myongji University, Yongin, 17058, Republic of Korea d Samsung Electronics, 466-712, Yongin, Republic of Korea b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 June 2019 Received in revised form 20 September 2019 Accepted 21 September 2019 Available online 23 September 2019
Films of tungsten-doped nickel oxide, denoted Ni1-xWx oxide as counter electrode in electrochromic devices were deposited onto indium tin oxide coated glass substrates by reactive dc magnetron sputtering using tungsten-doped nickel alloy target. Electrochromic properties of NixW1-x oxides were evaluated in lithium perchlorate in propylene carbonate. Enhanced cycle stability with moderate optical modulation was obtained for Ni1-xWx oxide film with x ¼ 0.024. Significant degradation in pure NiO film during extended electrochemical cycling caused by microstructural deep trap sites was minimized by W doping. The EC device fabricated with WO3 thin film and Ni1-xWx oxide thin film with x ¼ 0.024 showed stable optical modulation of about 2% reduction from 100 to 1000 cycles. Flexible EC device was also fabricated with WO3 thin film and Ni1-xWx oxide thin film with x ¼ 0.024 deposited on c-ITO/graphene/ PET electrode and showed stable cycling performance with maintaining optical modulation of DT z 40%. © 2019 Elsevier B.V. All rights reserved.
Keywords: Electrochromic device Tungsten-doped nickel oxide Counter electrode Flexible smart window
1. Introduction Electrochromic (EC) devices have been attracted increasing attention because of their interesting characteristics of modulating optical properties by an applied voltage. EC devices have many commercial applications such as smart windows of green buildings, information display devices and reflectance mirrors [1e10]. Conventional EC device is composed of TCO/EC/IC/CE/TCO layers, where TCO, IC and CE are transparent conducting oxide, ion conducting layer (electrolyte) and counter electrode, respectively [11,12]. The oxides of nickel and tungsten are the well-known electrochromic materials of anodic and cathodic coloration, respectively. In an EC device, nickel oxide is generally used as a counter electrode material with enhancing coloration efficiency of whole device with a pair of tungsten oxide [13,14]. However, electrochromism in NiO films is rather complicated and is still debated, although it is generally accepted that the transition from a colored to a bleached state is related to a multiple valances of Ni ions in Ni oxide due to
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (D.J. Yang), [email protected] (S.H. Han). https://doi.org/10.1016/j.jallcom.2019.152399 0925-8388/© 2019 Elsevier B.V. All rights reserved.
the variation of Ni3d electrons. Especially, owing to complicated reaction with Liþ-conducting electrolytes on cycling, the decay of the charge exchange, and hence of the optical modulation span of NiO films still limits the commercialization of EC devices [15e18]. Recently, it is accepted that the enhancements of cycling stability in EC materials can be achieved by optimizing their composition with various additives such as lithium, carbon, nitrogen, tungsten, iridium, (Li,W), (Li,Al), (Li,Zr), and others [19e22]. Among the representative candidates, adding modest amount of Ir into NiO significantly enhanced the cycling durability in a Liþ-conducting electrolyte. However, very high cost of Ir prevents it from being used as a counter electrode of EC devices. The incorporation of Li into NiO is also hindered due to the difficulties in fabrication of Li or Li-metal alloy target for the reactive DC magnetron sputtering. Although several researches on Li doping in NiO have been conducted by RF magnetron sputtering using various Li-based oxide target, low deposition rate of RF magnetron sputtering makes it unsuitable for mass production of EC devices. In this work, we report the enhancement of cycling stability of EC device with excellent EC properties using films of tungstendoped nickel oxide, denoted Ni1-xWx oxide as a counter electrode. We confirmed the effect of compositional modulation by means of
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specific electrochemical tests and structural characterization using cyclic voltammetry (CV), X-ray diffraction (XRD), and scanning electron microscope (SEM). And we proposed a reaction mechanism accounting from the observed degradation in the EC performances of optimized Ni1-xWx oxide films. Finally, we demonstrated flexible EC device using graphene-supported transparent conducting electrode which offers requirements for practical applications, that is, high-contrast optical modulation, good electrical conductivity and mechanical flexibility. Fabrication strategy of EC devices using highly stable Ni1-xWx oxide film on flexible electrode provide a general framework for designing optically and mechanically high-performance EC devices. 2. Experimental section The thin films were deposited by reactive DC magnetron sputtering using 4-inch-diameter target. After initial pump down to <1.5 107 Torr, sputter gas of 99.998% purity Ar and reactive gas of 99.998% purity O2 were introduced into the chamber via massflow-controlled inlets to form working pressure of 3.5 103 Torr. The DC power output was set at a constant value of 500 W Ni1-xWx oxide thin films were deposited on ITO coated glass substrates (sheet resistance, 10 U/sq.) at room temperature (RT) under Ar (90 sccm) and oxygen (10 sccm). The composition was varied by changing the W concentration of the Ni-W metal alloy target from 0 to 20 at%. Amorphous WO3 thin film was also deposited on ITO glass at RT under O2/Ar ratio of 30%. Finally, all-slid-state EC devices were fabricated by laminating two coated substrates with Li-based polymeric solid state electrolyte (Soulbrain, SWOPE). The thicknesses of the Ni1-xWx oxide, gel-electrolyte and WO3 films measured by a surface profiler (P-10, KLA-Tencor) were 400 nm, 1 mm and 700 nm, respectively. The elemental composition of the Ni1-xWx oxide was evaluated by X-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo Fisher Scientific). Phase identification of Ni1xWx oxide thin films deposited on ITO glass was performed using Xray diffraction (XRD, Empyrean, PANalytical) with Cu-Ka radiation. Surface and cross-section microstructure of the films were investigated using scanning electron microscopy (SEM; Hitachi S-4800, Hitachi High-Tech, Tokyo, Japan). Cyclic voltammetry (CV) was performed to evaluate the electrochemical properties of Ni1-xWx oxide films using a potentiostat-galvanostat (PGSTAT 30, AUTOLAB) between 1.0 and 1.5 V with a scan rate of 50 mV/s. Pt foil and Ag/ AgCl were used as the counter electrode and reference electrode, respectively and 1 M LiClO4 in propylene carbonate (Li-PC) was used as the electrolyte. The optical transmittance of Ni1-xWx oxide was simultaneously measured in situ during continuous CV cycling tests. A He/Ne laser (633 nm) and a power meter were used for the in situ transmittance measurement. The optical transmittance changes of all-solid-state EC devices at their colored and bleached states were measured in the wavelength range of 380e780 nm using UVevisible spectrophotometer (clay-60, Agilent). Continuous potential cycling of all-solid-state EC devices were also carried out for up to 1000 cycles in the range of 1.25e1.25 V by kinetic mode of UVevisible spectrophotometer at a wavelength of 550 nm. Flexible and crystallized ITO thin films were prepared on plastic PET substrate with monolayered graphene as a platform. Singlelayered graphene was grown on Cu foil (99.8%) by rapid thermal chemical vapor deposition (RTCVD) [23]. The highly crystalline ITO (c-ITO) was fabricated on graphene/Cu foil by DC magnetron sputtering at RT and subsequent post-annealing process at 250 C with a thickness of 60 nm and it was continuously transferred onto a PET substrate, as described elsewhere [24]. The c-ITO/graphene/ PET electrode showed a sheet resistance of ~45 U/sq. and a transmittance of ~92% at a wavelength of 550 nm. Finally, the flexible EC devices were fabricated by laminating the WO3 film and optimized
Ni1-xWx oxide film deposited on c-ITO/graphene/PET substrate, respectively using Li-based polymeric solid state electrolyte. 3. Results and discussion Fig. 1 shows XPS spectra of Ni1-xWx oxide thin films deposited using Ni-W metal alloy target with a W concentration of 5e20 at%. When the W concentrations of the Ni-W alloy targets are 5, 10, and 20 at%, the magnitude of x is determined to be 0.024, 0.079 and 0.268, respectively. Fig. 2 (a) shows XRD patterns of Ni1-xWx oxide thin films. The pure Ni oxide thin film exhibits a crystalline phase with a strong (111) diffraction peak, indicating the preferred orientation of the film surface. The (111) peak intensity decreases upon the increase of W content. Previously, amorphization upon W addition has been reported elsewhere [25,26], and it is associated with weakening the long-range order in the Ni-O matrix. W ions of high oxidation state (6þ) would induce a large perturbation of the lattice around a W cation inserted in a Ni site due to the large difference between the stable valence states of Ni (2þ) and W (6þ) [25,26]. As a result, the overall crystal structure of polycrystalline Ni oxide turns less dense state when the W content increases. W addition also influenced on the microstructure of the Ni1-xWx oxide thin films, as shown in Fig. 2(b). With increase in W content, a columnar nanostructure becomes gradually dense due to the decreased crystallinity. The formation of columnar features are generally believed to be in favor of ion transportation because of increased active sites, however they simultaneously cause reduction in stability and electrical properties as well as adhesion to substrates [27,28]. Fig. 3 shows CV data of Ni1-xWx oxide thin films with addition of W. The initial CV curve of pure Ni oxide film is broader than those of other W-doped films however, significant decay of the charge exchange is observed upon electrochemical cycling. This degradation is associated with the decreased ion insertion/extraction and subsequent decrease of electrochromism, which was already reported in sputter-deposited pure Ni oxide films in Li-PC [29,30]. Although the charge exchange was decreased with increase in W addition, the decay of charge density during electrochemical cycling was significantly decreased with Ni1-xWx oxide film with x ¼ 0.024 and 0.079. The Ni1-xWx oxide film with x ¼ 0.268 shows very low charge insertion and extraction. The results of CV data in Fig. 3(a)-(d) were numerically quantified as the charge density (mC/cm2) value during 1000 cycles of Ni1-xWx oxide film, as shown in Fig. 4. Initial charge density drop during the first 100 cycles of Ni1-xWx oxide films with x ¼ 0, 0.024, and 0.079 can be understood as an effect of activation by Refs. [31,32]. NiOx þ yLiþ þye 4 LiyNiOx
(1)
After that, Li-containing active materials on the surface can be further modulated by Liþ insertion and extraction associated with reversible charge-transfer process between two different phases of the Li-Ni oxide, as described by Refs. [31,32]: LiyNiOx (bleached) 4 Li(y-z)NiOx þ zLiþ þ ze (colored)
(2)
A gradual charge density decline of the pure Ni oxide upon extended voltammetric cycling indicates the decreased Li ion insertion and extraction corresponding to irreversible electrochemical reactions between electrolyte and thin film. Although the charge density of the Ni1-xWx oxide films with x ¼ 0.024 and 0.079 slightly decreased, stable cycling performance was obtained indicating enhanced durability. For Ni1-xWx oxide film with x ¼ 0.268, very low Li ion insertion and extraction took place during extended cycling.
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Fig. 1. XPS spectra of Ni1-xWx oxide thin films using Ni-W metal alloy target with a W concentration of 5e20 at%.
Fig. 2. (a) XRD patterns and (b) SEM images of Ni1-xWx oxide thin films deposited on a Si wafer.
Fig. 5 shows transmittance modulation of Ni1-xWx oxide films measured in situ during continuous 1000 CV cycling tests. The color of the Ni1-xWx oxide films with x ¼ 0, 0.024, and 0.079 changed from dark gray (colored state) to yellowish gray (bleached state) reversibly, as shown in inset image of Fig. 5. As the W content increased, the transmittance decreased, which is consistent with previous reports that the band gap of pure Ni oxide was reduced upon W addition [33,34]. The initial optical modulation of pure Ni oxide was large, but gradually decreased upon electrochemical cycling. As the cycle number increased, the transmittance gradually increased, but the transmittance in the colored state of pure Ni oxide was further increased compared to the bleached state. As the W content increased, the initial optical modulation was reduced but remained almost constant upon electrochemical cycling. Furthermore, the increase in the transmittance with increasing cycle number decreased in both colored and bleached states, and the optical modulation became nearly flat at the Ni1-xWx oxide film with x ¼ 0.079. The Ni1-xWx oxide film with x ¼ 0.268 was almost optically passive. The long-term degradation mechanism of Ni oxide is still a matter of controversy. According to our experimental results with
Ni-W oxide system, while preliminary, we suggest that the stability problem of pure-NiO film is related to the microstructure, that is, columnar nanostructure. Although the columnar nanostructure is advantageous in ionic diffusion and ion insertion kinetics, iontrapping-induced degradation of electrochromic properties can be easily happened due to the existence of microstructural deep trap sites [29,30]. Further increase in colored state transmittance of pure Ni oxide upon cycling compared to bleached state transmittance corresponds to higher amount of Li ion insertion than extraction, which might be originated from the Li ions trapping at the deep trap sites. As we demonstrated in SEM images of the Ni1xWx oxide films as shown in Fig. 2(b), a columnar nanostructure becomes gradually dense with increasing W content, which may result in reduced modulation but increased stability due to the decreased deep trap sites. Ni1-xWx oxide films with x ¼ 0.024 and 0.079 were considered to be optimal condition, however, Ni1-xWx oxide films with x ¼ 0.079 shows relatively low transmittance at bleached state and low optical modulation for application to EC devices. As Ni1-xWx oxide film with x ¼ 0.268 showed very low Li ion insertion and extraction during extended electrochemical cycling, which means that electrochromic properties disappear
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Fig. 3. Cyclic voltammetry of the Ni1-xWx oxide films immersed in the LiClO4-PC solution with (a) x ¼ 0, (b) x ¼ 0.024, (c) x ¼ 0.079 and (d) x ¼ 0.268.
Fig. 4. Inserted and extracted charge density during 1000 CV cycles of Ni1-xWx oxide films with (a) x ¼ 0, (b) x ¼ 0.024, (c) x ¼ 0.079 and (d) x ¼ 0.268.
above a certain amount W doping in NiO [25]. Fig. 6 shows schematic and optical images EC device at the bleached and colored states using Ni1-xWx oxide film with x ¼ 0.024
as a counter electrode. EC devices consists of an WO3 film for EC layer in contact with a Li-based polymeric solid state electrolyte adjacent to Ni1-xWx oxide film for counter electrode, and these
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Fig. 5. In situ transmittance modulation at 633 nm during 1000 CV cycles for Ni1-xWx oxide films with (a) x ¼ 0, (b) x ¼ 0.024, (c) x ¼ 0.079 and (d) x ¼ 0.268.
Fig. 6. Schematic and optical images of EC device using Ni1-xWx oxide film with x ¼ 0.024 as a counter electrode at the (a), (c) bleached state and (b), (d) colored state, respectively.
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layers are sandwiched between ITO coated glass substrates. The transmittance modulation can be controlled by the reversible electrochemical reaction induced by an external electric field, which changes from transparent (bleached state) to deep blue (colored state). Fig. 7 shows transmittance modulation during continuous pulse potential 1000 cycling (1.25 ~ 1.25 V, duration 60sec) for EC devices fabricated using Ni1-xWx oxide films as counter electrodes. The transmittance modulation of EC device fabricated with pure Ni oxide film gradually decreased from 57% to 30% at 100 and 1000 cycles, respectively, which is related to Li ions trapping at the deep trap sites, as mentioned earlier. The EC device fabricated using Ni1-xWx oxide film with x ¼ 0.024 showed relatively stable transmittance modulation (about 2% reduction from 100 to 1000 cycles), which indicates that certain amount of W doping in the Ni-oxide thin film is advantageous for delaying deterioration of the counter electrode layer with minimum optical modulation loss. Although the EC device fabricated using Ni1-xWx oxide film with x ¼ 0.079 shows stable transmittance modulation, low bleach state transmittance of below 60% and low optical modulation of about 30% hindered it for practical application to EC devices. Very low optical modulation of EC device fabricated using Ni1-xWx oxide film with x ¼ 0.268 is related to the very low Li ion insertion and extraction of Ni1-xWx oxide film with x ¼ 0.268 during extended cycling. In addition, we fabricated flexible EC devices by laminating WO3 thin film and Ni1-xWx oxide thin film with x ¼ 0.024 deposited on cITO/graphene/PET electrode, respectively using a Li-based polymeric solid state electrolyte, as shown in Fig. 8. During 1000 pulse potential cycling, flexible EC device in our work shows very stable cycling performance with maintaining optical modulation of DT z 40% after initial activation process. The color of the flexible device changed from light yellow (1.5 V) to deep blue (þ1.5 V), even at the bended state. By comparison with rigid device
fabricated on ITO glass, the c-ITO on graphene/PET having a relatively large sheet resistance of ~45 U/sq (ITO glass: 10 U/sq) needs higher operating voltage, but electrolyte is normally dissociated at high voltage [35]. Therefore, applied voltage of ±1.5 V must be insufficient to make full transmittance change in the flexible device. In spite of the low transmittance modulation of flexible device compared to rigid device, it is enough to be used for electrochromic display, which has attractive merit of flexible application. The graphene support film showed significant enhancement in flexibility because the atomically controlled monolayered graphene acted as a mechanically robust support [24]. Our flexible EC device was very stable against mechanical bending, which provides the possibility of potential application to flexible EC device.
4. Conclusions W-doped Ni oxide counter electrode layers were prepared by reactive DC magnetron sputtering using Ni-W metal alloy target with a W concentration of 5e20 at%. With increase in W content, the crystal structure of polycrystalline Ni oxide gradually turns into amorphous state and a columnar nanostructure becomes dense microstructure. Degradation of charge density and optical modulation upon extended electrochemical cycling was enhanced by W addition. Ni1-xWx oxide films with x ¼ 0.024 and 0.079 showed stable cycling stability, but Ni1-xWx oxide films with x ¼ 0.079 showed relatively low transmittance at bleached state and low optical modulation. Reduced modulation but increased stability with increasing W content in Ni oxide might be due to the decreased deep trap sites. The Ni1-xWx oxide film with x ¼ 0.268 showed very low Li ion insertion/extraction, and consequently resulted in almost optically passive. EC devices fabricated using Ni1xWx oxide with x ¼ 0.024 showed stable transmittance modulation until 1000 cycles. Finally, flexible EC device was fabricated with
Fig. 7. In situ transmittance modulation at 550 nm upon continuous potential cycling (þ1.25 ~ 1.25 V, duration - 60sec) for EC devices fabricated using Ni1-xWx oxide counter electrodes with (a) x ¼ 0, (b) x ¼ 0.024, (c) x ¼ 0.079 and (d) x ¼ 0.268.
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Fig. 8. Schematic diagrams and optical images of flexible EC devices using c-ITO/graphene/PET transparent electrode at the (a) bleached state and (b) colored state. (c) In situ transmittance modulation at 550 nm upon continuous potential cycling (þ1.5 ~ 1.5 V) and (d) transmittance spectra for flexible EC devices.
WO3 thin film and Ni1-xWx oxide thin film with x ¼ 0.024 deposited on c-ITO/graphene/PET electrode and showed good cycle stability. These results suggest that Ni1-xWx oxide thin film with x ¼ 0.024 is a promising material for a counter electrode of EC devices.
[8]
Acknowledgements [9]
This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) under the Ministry of Trade, Industry and Energy (MOTIE) of the Republic of Korea (No. 20173030014180, No. 20182020109430).
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