Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Transparent Conductive Dielectric−Metal−Dielectric Structures for Electrochromic Applications Fabricated by High-Power Impulse Magnetron Sputtering Hamed Najafi-Ashtiani,†,‡,⊥ Behnam Akhavan,*,†,§,⊥ Fengjuan Jing,∥ and Marcela M. Bilek†,§
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School of Physics and §School of Aerospace, Mechanical and Mechatronic Engineering, University of Sydney, Sydney, New South Wales 2006, Australia ‡ Department of Physics, Faculty of Science, Velayat University, Iranshahr 99111-31411, Iran ∥ Key Laboratory for Advanced Technologies of Materials, Ministry of Education, School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu 610031, China S Supporting Information *
ABSTRACT: The growing applications of electrochromic (EC) devices have generated great interest in bifunctional materials that can serve as both transparent conductive (TC) and EC coatings. WO3/Ag/WO3 (WAW) heterostructures, in principle, facilitate this extension of EC technology without reliance on an indium tin oxide (ITO) substrate. However, these structures synthesized using traditional methods have shown significant performance deficiencies. Thermally evaporated WAW structures show weak adhesion to the substrate with rapid degradation of coloration efficiency. Improved EC durability can be obtained using magnetron sputtering deposition, but this requires the insertion of an extra tungsten (W) sacrificial layer beneath the external WO3 layer to prevent oxidation and associated loss of conductivity of the silver film. Here, we demonstrate for the first time that a new method, known as high-power impulse magnetron sputtering (HiPIMS), can produce trilayer bifunctional EC and TC devices, eliminating the need for the additional protective layer. X-ray photoelectron spectroscopy and X-ray diffraction data provided evidence that oxidation of the silver layer can be avoided, whilst stoichiometric WO3 structures are achieved. To achieve optimum WAW structures, we tuned the partial pressure of oxygen in the HiPIMS atmosphere applied for the deposition of WO3 layers. Our optimized WO3 (30 nm)/Ag (10 nm)/WO3 (50 nm) structure had a sheet resistance of 23.0 ± 0.4 Ω/□ and a luminous transmittance of 80.33 ± 0.07%. The HiPIMS coatings exhibited excellent long-term cycling stability for at least 2500 cycles, decent switching times (bleaching: 22.4 s, coloring: 15 s), and luminescence transmittance modulation (ΔT) of 34.5%. The HiPIMS strategy for the fabrication of ITO-free EC coatings for smart windows holds great promise to be extended to producing other metal−dielectric composite coatings for modern applications such as organic light-emitting diodes (OLEDs), liquid crystals, and wearable displays. KEYWORDS: ITO-free, electrochromic, WO3/Ag/WO3, HiPIMS, smart windows, DMD voltage.8,9 This function is achieved by the intercalation/ extraction of cations (Li+, H+, Na+) and electrons from the external circuit in a reversible manner.10,11 EC materials can also conserve the vacation mode on bleached and colored states for long times without excessive energy usage.12,13 Tungsten oxide (WO3) thin films have been of prime interest among the EC materials because of their chemical stability, strong adherence to various substrates, and high coloration efficiency (CE).13,14 Compared to other transition metal oxides such as V2O5, MoO3, and TiO2, tungsten oxide exhibits superior
1. INTRODUCTION The rise of energy consumption in recent decades has led to a continuously increasing demand for renewable energy and energy-saving systems.1,2 Despite the development of new technologies and the use of renewable energy, global energy demand still heavily depends on fossil fuels.3,4 Such dependence will undoubtedly lead to global warming and escalation of energy crisis.5 On the other hand, the renewable energy sources are unsustainable and are not available anywhere at any time. Therefore, strategies to decrease the energy consumption, such as the application of smart windows, have become increasingly popular in the past few decades.6,7 Smart or switchable windows, coated with a thin layer of electrochromic (EC) material, are able to reversibly tune their light transmission properties by the application of an external © XXXX American Chemical Society
Received: January 4, 2019 Accepted: March 15, 2019
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DOI: 10.1021/acsami.9b00191 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. Various strategies for the fabrication of EC structures through magnetron sputtering methods. HiPIMS allows for the development of sputtered (rather than evaporated) EC WO3/Ag/WO3 structures composed of only three layers. (a) EC coating (WO3) deposited on an ITO−FTOcoated glass substrate. (b) Transparent, conductive DMD structure (WO3/Ag/WO3) deposited on the glass substrate. In this approach, the transparent, conductive electrode layer (ITO or FTO) is eliminated, but a tungsten (W)-protective layer has to be deposited on the metallic silver layer (Ag) to prevent its oxidation during the reactive sputtering of the external WO3 layer. (c) DMD structure (ITO−FTO free) fabricated by HiPIMS without the requirement of a protective layer.
with a significantly thinner external WO3 layer (50 nm) are stable up to 3000 cycles without significant degradation in EC performance.31 However, the mechanism behind such apparent increase in durability that disagrees with previous studies has not been explained. Reactive magnetron sputtering is a well-established, industrial-scale technology that offers large-scale uniform coatings with better adhesion to the substrate compared to those deposited by thermal evaporation methods. Another distinct advantage of reactive sputtering is its unique capability to precisely tune the chemistry of the coatings, thus also their optical properties.35−37 Despite these crucially important features, oxidation of the middle silver layer during the reactive sputter deposition of WO3 has limited the application of dc sputtering in the fabrication of ITO-free EC coatings. Lee et al. deposited WO3/Ag/WO3 structures by reactive sputtering, and they reported an average transmittance of 74.2% and a nonconductive state for the coatings.38 The insulating nature of this DMD structure is due to the oxidation of the silver layer during the deposition of the external WO3 layer in a reactive oxygen atmosphere. To mitigate this problem, most recently, Yin et al. reported a strategy that involves the deposition of an ultrathin tungsten (W) sacrificial layer before the deposition of the external WO3 layer (Figure 1b).25 Although this approach eliminates the oxidation of silver and film loss of conductivity, it introduces a new level of complexity to the deposition process as the structure consists of four layers, unlike in conventional DMD structures that are made of only three layers. The application of high-power impulse magnetron sputtering (HiPIMS), instead of conventional dc or RF magnetron sputtering methods,25 might present an opportunity to generate such structures without the deposition of the fourth sacrificial layer. In HiPIMS, high-power pulses are applied to the magnetron target at a low duty cycle to maintain a manageable average output power.39−42 The high instantaneous power produces a high-density plasma of the sputtered target material with a high degree of ionization. These conditions are favorable for the growth of stoichiometric WO3 at lower partial pressure of oxygen compared to that required in conventional reactive sputtering. Reactive HiPIMS deposition of the external WO3 layer at a lower partial pressure of oxygen is extremely advantageous, as it reduces the oxidation of the silver layer. In addition, HiPIMS coatings are of greater density because of their more energetic, ion-assisted deposition compared to those deposited by dc sputtering or thermal evaporation. The dense, external WO3 layer, therefore, protects the sensitive Ag film from
EC properties such as long-term durability and excellent contrast ratio.15 All practically applicable EC coatings must be deposited on optically transparent and electrically conductive substrates. The substrate must be conductive to allow transport of electrons to and from the EC layer and also to provide distributed electric field where ions are transferred.16 Heavily doped wide-band gap semiconductors, such as indium tin oxide (ITO) and fluorine tin oxide (FTO), have been typically used as transparent conductive (TC) electrodes (Figure 1a). However, the demand for ITOfree EC devices has been rapidly increasing for the past few years17,18 because ITO contains expensive rare elements, shows poor mechanical flexibility, and its fabrication requires precise stoichiometry control.16,19 Various alternative materials, such as aluminum-doped ZnO, silver nanowire films, and dielectric−metal−dielectric (DMD) structures have been investigated instead of ITO-based devices.20−22 Among these materials, DMD structures exhibit competitive properties because of their high transparency, low sheet resistance, and the ability of reflecting IR radiation and transmitting visible light through their middle reflective metal and internal/external antireflective dielectric layers.23−25 However, the presence of the middle metallic layer, for example, silver (Ag), reduces the optical transmittance of the DMD structure, partly because of an impedance inconformity between the metallic layer and the external media.26 The transmittance of the metallic layer can be enhanced by adding appropriate impedance-conforming dielectric layers to both of its sides.27,28 Thus, the application of WO3 as the dielectric layers in a DMD structure provides an excellent opportunity to fabricate bifunctional TC and EC DMD structures. WO3 dielectric layers not only enhance the optical transmittance of the structure but also simultaneously provide EC properties.29 DMD structures have been typically fabricated using thermal evaporation methods.30−32 Leftheriotis et al. reported the fabrication of ITO-free, EC WO3/Ag/WO3 coatings using electron beam thermal evaporation. They showed that with a relatively thick external WO3 layer (389 nm), the DMD structure was stable up to 500 continuous voltammetric coloring−bleaching cycles, while structures with a thin external WO3 layer (155 nm) failed after only 200 cycles.33 Their results are in agreement with another study34 demonstrating that thick external layers of WO3 are required to protect the sensitive silver layer from oxidative reactions with the liquid electrolyte. Recently, Li et al. reported that WO3/Ag/WO3 films also deposited using the e-beam thermal evaporation method and B
DOI: 10.1021/acsami.9b00191 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
magnet was 10% weaker than that of the outer magnets. Prior to each deposition run, a base pressure below 5 × 10−7 Torr was achieved using a turbomolecular pump. Tungsten oxide depositions were carried out in a reactive atmosphere of ultrahigh purity (99.9995%) O2 and Ar (BOC Australia) gases with various flow rate ratios, while the gas pressure was kept constant at 2.8 mTorr. The magnetron was energized by an RUP7 pulsed power supply (GBS Electronik GmbH), delivering voltage pulses of 900 V for the reactive HiPIMS of tungsten. For the deposition of WOx (0 < x < 3) layers, the pulse length and frequency were kept unchanged at 75 μs and 150 Hz, respectively. A constant bias voltage of −50 V was applied to the substrate holder for all the samples. The internal WO3 layers for all samples were deposited using Ar and O2 flow rates of 4 and 16 sccm, respectively, corresponding to 80% oxygen [O2/ (O2 + Ar)]. The middle silver layer was subsequently deposited on top of the WOx films using dc magnetron sputtering with a power of 65 W, argon flow rate of 15 sccm, and a gas pressure of 3.4 mTorr. To ensure that minimum residual oxygen atoms from the WO3 deposition atmosphere are present in the chamber, sputtering of silver was carried out without breaking the vacuum and once a base pressure below 5 × 10−7 Torr was achieved. The deposition of external WOx layers onto the silver layer was carried out at varied oxygen to argon flow rate ratios (10−80%, total flow rate = 20 sccm). No arcing was observed during the deposition. 2.3. X-ray Photoelectron Spectroscopy. The chemical compositions of the coatings were examined using a SPECS (FlexMode) spectrometer equipped with an Al Kα source (1486.7 eV) operated at a power of 200 W (10 kV and 20 mA). The survey spectra (0−1000 eV) of samples were acquired with a pass energy of 30 eV and a resolution of 0.5 eV. The take-off angle was 90° with respect to the sample surface. The tungsten and silver high-resolution spectra were recorded at a pass energy of 20 eV and a resolution of 0.1 eV. The measurements were conducted at pressures less than 5.0 × 10−8 mbar. Atomic concentration calculations from the survey spectra and curve fitting of high-resolution spectra were carried out using the CasaXPS software (version 2.3.1). A linear background and equal full-width at half-maximum (fwhm) peaks with a Gaussian (70%)−Lorentzian (30%) line shape were used for curve fitting. 2.4. Spectroscopic Ellipsometry. The thickness and optical properties of the films deposited on silicon wafers were determined using a spectroscopic ellipsometer (2000D, JA Woollam Co.) equipped with a XLS-100 light source and a control module (EC-400) run by WVASE32 software. Data acquisition was conducted in the wavelength range of 200−1000 nm (5 nm steps) and at 65°, 70°, and 75° angles of incidence. A Cauchy model was used to fit the obtained data using the WVASE32 software. The reported values are the average of at least three measurements. 2.5. X-ray Diffraction. The structural properties of the films were characterized by X-ray diffraction (XRD) measurements (PHILIPS model PW 3040, grazing angle = 0.5°) with Cu Kα radiation operated at 30 kV and 30 mA. 2.6. UV−Vis Spectroscopy. The transmission of coatings was measured by a Cary 5E UV−vis spectrometer (Varian) in the wavelength range of 300−800 nm. To measure the transmittance in colored and bleached states and the colored-bleached response times, the optical transmittance of the coatings were recorded, while the voltage applied on the coatings was turned on and off periodically (−1.0 to +1.0 V) using a dc power supply (PowerTech, N287) in 1 M LiClO4−PC solution. 2.7. EC Evaluation. The electrochemical measurements were carried out using an electrochemical workstation (eDAQ Pty Ltd model ER466). The EC properties of the coatings were investigated using a standard three-electrode system, with the DMD coating on the glass substrate as the working electrode, a platinum wire as the counter electrode, and Ag/AgCl (1 M KCl) as the reference electrode. The electrolyte was 1 M LiClO4 dissolved in PC solution. The applied voltage sweep rate was maintained at 100 mV/s for all of the cyclic voltammetry (CV) measurements. The same potential range as used for the UV−vis optical transmittance measurements (−1.0 to +1.0 V) was applied for all samples to investigate the optical transmittance in bleached and colored states.
degradation upon exposure to the liquid electrolyte, enhancing the durability of the EC device even without a sacrificial layer. Here, we report the fabrication of a three-layer DMD structure, that is, WO3/Ag/WO3, for EC applications (Figure 1c). We demonstrate that HiPIMS, unlike conventional dc or RF magnetron sputtering methods, is a highly promising technique to generate such structures without the deposition of an extra sacrificial, protective layer. We tuned the composition of the reactive sputtering atmosphere to achieve 3-layer DMD structures with maximum optical transparency, electrical conductivity, and EC property. HiPIMS technology has been shown to be a promising approach for the fabrication of transparent, conductive DMD structures applied in EC devices such as smart windows and automotive rear-view mirrors.
2. EXPERIMENTAL SECTION 2.1. Materials. Lithium perchlorate (LiClO4; Sigma-Aldrich, ≥95.0), propylene carbonate (PC; Merck, ≥99.7), ethanol, and acetone were purchased and used without further purification. Ultrahigh purity oxygen and argon gases were obtained from BOC Australia. Commercially available soda-lime glass with a thickness of 1.2 mm and silicon wafers (Addison, USA) were used as substrates. The substrates were ultrasonically cleaned in acetone and ethanol for 10 min and then rinsed in deionized water. The substrates were then dried with high-purity nitrogen gas. Silver and tungsten sputtering targets (purity = 99.995%) of 75.6 mm diameter and 6.35 mm thickness were obtained from Kurt J. Lesker. 2.2. High-Power Impulse Magnetron Sputtering. All coatings were deposited using a vacuum deposition system (AJA International 1800 F) schematically illustrated in Figure 2. An unbalanced magnetron gun was utilized in which the magnetic field strength of the central
Figure 2. Schematic diagram of the HiPIMS system used for the deposition of WO3/Ag/WO3 DMD structures. The internal and external WO3 layers of the DMD structures are deposited using HiPIMS, while the middle silver layer is deposited in the same chamber using conventional dc magnetron sputtering. In the HiPIMS process, high degrees of ionization of the sputtered material is achieved (40− 80% of the sputtered target material is ionized). Such high degree of ionization is favorable for producing WO3 at a lower partial pressure of oxygen compared to conventional reactive sputtering methods. Therefore, compared to the conventional RF and dc magnetron sputtering methods, a WO3 stoichiometric structure can be fabricated with a lower oxygen concentration in the reactive atmosphere. C
DOI: 10.1021/acsami.9b00191 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 3. XPS surface chemistry characterization of WOx films deposited at various O2/(O2 + Ar) flow rate ratios. (a) Chemical stoichiometry for WOx films as a function of O2/(O2 + Ar) ratio obtained from XPS survey spectra. (b) W 4f high resolution spectra of WOx films deposited at various O2/(O2 + Ar) ratios. The black solid lines are the recorded spectra and the purple dashed lines represent the fitted envelope. The composite spectra are fitted using doublets with W-oxidation states of W6+, W5+, and W0 and a W 4f7/2−W 4f5/2 spin−orbit separation of 2.18 eV with an intensity ratio of 0.75. (c) Area percentage for metallic tungsten (red), WOx sub oxide (black) components fitted in W 4f high resolution spectra as a function of the O2/(O2 + Ar) ratio. 2.8. Field-Emission Scanning Electron Microscopy. Fieldemission scanning electron microscopy (FESEM) (MIRA3 TESCAN), operated at 20 kV, was used to obtain cross-sectional images of the optimized DMD coating.
3. RESULTS AND DISCUSSION 3.1. Deposition of Tungsten Oxide Films with Controlled Stoichiometry. The stoichiometry of the tungsten D
DOI: 10.1021/acsami.9b00191 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 4. Tuning the chemistry of HiPIMS WOx films by changing the oxygen to argon flow rate ratio in the reactive atmosphere. (a) Deposition rate of WOx films as a function of O2/(O2 + Ar) ratio HiPIMS sputtering gas measured by spectroscopic ellipsometry and stylus profilometry. (b) Refractive indices of WOx films for the wavelength in the range of λ = 300−1000 nm. (c) % transmittance of WOx films averaged for wavelengths 400−700 nm and their calculated optical band gaps as a function of the O2/(O2 + Ar) ratio in the HiPIMS sputtering atmosphere.
oxide (WOx) films plays a crucial role in their transparency and EC properties.43−46 To modulate the stoichiometry of WOx films, we tuned the ratio of oxygen to argon flow rate [O2/(O2 + Ar)] in the HiPIMS atmosphere and employed X-ray photoelectron spectroscopy (XPS) to investigate the chemical composition and tungsten oxidation states of the films. Figure 3a shows the chemical stoichiometry of WOx films as a function of the O2/(O2 + Ar) flow rate ratio obtained from XPS survey spectra. By increasing the O2/(O2 + Ar) flow rate ratio from 0 to 80%, the chemical stoichiometry increases from WO0.6 to WO3. The increase of oxygen concentration in the WOx films deposited at higher O2/Ar ratios is simply because of the availability of more oxygen atoms in the HiPIMS working gas atmosphere. W 4f high resolution spectra (Figure 3b), obtained from WOx films deposited at various O2/(O2 + Ar) flow rate ratios, provide further insight into the chemistry of the WO3 films. XPS high resolution spectra provide a higher signal to noise ratio compared to survey spectra and are therefore suitable for the evaluation of minor changes in surfaces chemistry that are not detectable in survey scans. The W 4f is a doublet because of the spin−orbit splitting of W 4f7/2 and W 4f5/2 with a separation energy of 2.18 eV [ΔE = E(W 4f5/2) − E(W 4f7/2)].47,48 W 4f7/2 peaks for tungsten in a neutral environment (W), tungsten suboxides (WOx with 0 < x < 3), and WO3 were fitted at binding energies of 31.8, 34.7, and 36.1 eV, respectively.46,49 The corresponding 5/2 spins were also fitted, giving a total of six peaks. Figure 3c exhibits the area percentage of components fitted in the W 4f high-resolution spectra for various oxygen flow rate ratios. By an initial increase of the O2 flow rate from 0 to 20%, the concentration of tungsten in the metallic state decreases from 91.2 to 0.6%, while those of WOx (suboxides) and WO3 (stochiometric oxide) increase from 8.8 and 0% to 33.0 and 66.4%, respectively. By further increasing the O2 flow rate to values greater than 60%, the concentration of WOx decreases to less than 2.1% and that of WO3 increases to more than 97.9%. Such changes in surface chemistry are again explained by the greater availability of oxygen per sputtered tungsten atom to form the stochiometric WO3 structure. These variations in surface chemistry indicate that at relatively low O2/ O2 + Ar ratios (
