Optical constants of electrochromic films and contrast ratio of reflective electrochromic devices Cheng-Chung Jaing,1,* Chien-Jen Tang,1 Chih-Chao Chan,2 Kun-Hsien Lee,2 Chien-Cheng Kuo,2,3 Hsi-Chao Chen,4 and Cheng-Chung Lee2 1
Department of Optoelectronic System Engineering, Minghsin University of Science and Technology, Hsin-Chu 304, Taiwan 2
Department of Optics and Photonics/Thin Film Technology Center, National Central University, Chung-Li 320, Taiwan
3
Graduate Institute of Energy Engineering/Thin Film Technology Center, National Central University, Chung-Li 320, Taiwan 4
Graduate School of Optoelectronics, National Yunlin University of Science and Technology, Yunlin 640, Taiwan *Corresponding author: [email protected] Received 30 August 2013; revised 5 November 2013; accepted 6 November 2013; posted 7 November 2013 (Doc. ID 196651); published 9 December 2013
This study investigates the optical constants of WO3 electrochromic films and NiO ion-storage films in bleached and colored states and that of a Ta2 O5 film used as an ion conductor. These thin films were all prepared by electron-beam evaporation and characterized using a spectroscopic ellipsometer. The spectra obtained using a spectrophotometer and those calculated from the optical constants agreed closely. An all-solid thin-film reflective electrochromic device was fabricated and discussed. Its mean contrast ratio of reflectance in the range of 400–700 nm was 37.91. © 2013 Optical Society of America OCIS codes: (310.6860) Thin films, optical properties; (310.6845) Thin film devices and applications. http://dx.doi.org/10.1364/AO.53.00A154
1. Introduction
Electrochromic materials used in display devices [1–5] and smart windows [6–10] with variable light transmission have been studied. Electrochromism is the phenomenon that is described as a reversible color change of a material caused by applying an electric potential [11,12]. The changes in the optical properties of electrochromic materials are attributed to an oxidation-reduction process. Electrochromic materials could be divided into cathodic electrochromic materials and anodic electrochromic materials. Tungsten oxide (WO3 ) is the most widely studied cathodic electrochromic material, which is transparent in an oxidation state and deep blue in a reduction state. Nickel oxide (NiO) belongs to anodic electrochromic materials, which is transparent in a reduction state and dark brown in an oxidation state. 1559-128X/14/04A154-05$15.00/0 © 2014 Optical Society of America A154
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A typical electrochromic device is composed of a glass substrate, which is coated with a five-layered structure, TC/EC1/IC/EC2/TC, where TC is the transparent electrical conductor, EC1 and EC2 are the electrochromic layers (one of which may be an electrochromic or nonelectrochromic ion storage layer), and IC stands for the ion conductor [13]. Coloration and bleaching occur when voltage is applied across TC; ions then move from EC1 through IC to EC2 or when the polarity of the voltage is reversed. The injection or extraction of mobile ions can modify the optical constants of electrochromic films and ion storage films, changing the transmittance of the electrochromic devices [14,15]. On the other hand, a reflective electrochromic device with reversible movements of protons has been investigated [16–20]. However, few researchers have studied the high contrast ratio (CR) of reflectance of an all-solid thin-film reflective electrochromic device with reversible movements of Li ions. In this study, WO3 and NiO films were used as the electrochromic
layers EC1 and EC2. The optical constants of the WO3 electrochromic films and NiO ion storage films in bleached and colored states were determined using a spectroscopic ellipsometer with Cauchy and Drude models. The optical constant of a tantalum pentoxide (Ta2 O5 ) film, which was used as the ion conductor, IC, also was obtained using the spectroscopic ellipsometer with a Cauchy model. The optical constants of all of these films were examined by comparing the transmittance or reflectance spectra, which were measured using a spectrophotometer with those calculated from the optical constants. Finally, an all-solid thin-film reflective electrochromic device with a five-layered configuration, TC/EC1/IC/EC2/Al, was designed and fabricated to yield a high CR of reflectance, where Al is the reflective layer of aluminum, and TC is the transparent conductive ITO film. 2. Experimental Procedure
Thin films of WO3, Ta2 O5 , and NiO were deposited on ITO glass, B270 glass, and Si substrates at room temperature in an electron-beam-evaporation system. The respective starting materials were WO3 , Ta2 O5 , and NiO granules (99.99% pure, 1–3 mm). Before evaporation, the deposition system was pumped down to a base pressure of under 3 × 10−5 Torr using cryopumps. The deposition rate was controlled using a quartz monitor. The WO3 and Ta2 O5 films were prepared at three deposition pressures (2 × 10−4 , 4 × 10−4 , and 6 × 10−4 Torr) with oxygen as the reactive gas. The NiO films were prepared at three deposition rates (0.1, 0.6–0.8, and 1.3–1.5 nm∕s) without the introduction of oxygen. A cell used a WO3 or NiO film/ITO/glass substrate as a working electrode and a platinum (Pt) sheet as a counter-electrode in liquid electrolyte consisting of 1 M lithium perchlorate (LiClO4 ) in propylene carbonate (PC). DC voltage of 3 V was applied for 1 min between the two electrodes to color or bleach the WO3 or NiO film. These samples were then taken from the liquid electrolyte before measuring optical properties of films. After the LiClO4 solute and PC solvent were removed from the surface of these samples by blowing nitrogen, all of the samples were immediately measured using a Hitachi U4100 spectrophotometer in transmission or reflectance mode over a spectral range from 350 to 850 nm. The optical constants and thickness of the films were determined using a Sopra GES5E spectroscopic ellipsometer with Cauchy and Drude models. The crystalline structure of the films was elucidated using a Bruker D8 Discover x-ray diffractometer with a copper anode.
Fig. 1. Variation in the properties of WO3 samples prepared at three deposition pressures.
2 × 10−4 Torr, the WO3 sample exhibited the greatest variation in transmittance among the three deposition pressures because the variation in the optical constant was the largest, probably due to over stoichiometric oxygen content [21]. The extinction coefficient of the WO3 film in the bleached state was smallest from the film, which was prepared at a deposition pressure of 2 × 10−4 Torr. Moreover, the index of refraction in the bleached state decreased with an increase in the deposition pressure among the three deposition pressures. It was attributed to reduction in kinetic energy of WO3 deposition particles at higher oxygen pressures due to collisions. Figure 2 displays the refractive index and extinction coefficient of the WO3 film, which was prepared at a deposition pressure of 2 × 10−4 Torr in the bleached and colored states. Figure 3 plots measured transmittance spectra of the WO3 film in bleached and colored states and those calculated from the refractive index and extinction coefficient in Fig. 2. The strong agreement in Fig. 3 supports the refractive index and extinction coefficient in Fig. 2. Figure 4 shows the transmittance spectra of Ta2 O5 films, which were prepared on B270 glass at three deposition pressures (2 × 10−4 , 4 × 10−4 , and 6 × 10−4 Torr). Since the refractive index of the Ta2 O5 film, which was prepared at a deposition pressure of 2 × 10−4 Torr, was high due to the large
3. Results and Discussion
Figure 1 presents the difference among the refractive indices, extinction coefficients, and transmittances at 550 nm in the bleached and colored states of the WO3 films, which were prepared on the ITO glass at three deposition pressures (2 × 10−4 , 4 × 10−4 , and 6 × 10−4 Torr). When the deposition pressure was
Fig. 2. Optical constants of WO3 film prepared at a deposition pressure of 2 × 10−4 Torr in bleached and colored states. 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
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Fig. 3. Measured transmittance spectra of WO3 sample in bleached and colored states, and spectra calculated from constants in Fig. 2.
Fig. 4. Transmittance spectra of Ta2 O5 samples prepared at three deposition pressures.
peak to valley in Fig. 4, this film became so dense [22] that mobile ions had difficulty passing through it. Clearly, the absorption was strong following deposition at a pressure of 6 × 10−4 Torr due to losing kinetic energy of Ta2 O5 deposition particles from collisions. In this study, the ion conductor could not have a large absorption or be a dense film. Figure 5 demonstrates the refractive index and extinction coefficient of the Ta2 O5 film, which was prepared on an Si wafer at a deposition pressure of 4 × 10−4 Torr. Figure 6 reveals that the reflectance spectrum calculated from the refractive index and extinction coefficient in Fig. 5 was close to that obtained using the spectrophotometer. Figure 7 presents the XRD patterns of the NiO films, which were prepared on ITO glass at three deposition rates (0.1, 0.6–0.8, and 1.3–1.5 nm∕s). The poor degree of crystallinity in Fig. 7 was due to the small kinetic energy of deposition particles when the NiO film was deposited at a deposition rate of 0.1 nm∕s and due to losing kinetic energy of deposition particles from collisions when the NiO film was deposited at a deposition rate of 1.3–1.5 nm∕s [23]. The XRD peak that corresponded to the NiO (111) plane was strongest from the film deposited at a deposition rate of 0.6–0.8 nm∕s. The preferred orientation was (111), which is the most preferred crystalline orientation of an NiO film for exchanging ions A156
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Fig. 5. Optical constants of Ta2 O5 film prepared at deposition pressure of 4 × 10−4 Torr.
Fig. 6. Measured reflectance spectra of Ta2 O5 sample and spectra calculated using constants in Fig. 5.
Fig. 7. XRD patterns of NiO films prepared on ITO glass at three deposition rates.
[24,25]. Figure 8 displays the refractive index and extinction coefficient of the NiO film prepared at a deposition rate of 0.6–0.8 nm∕s in the bleached and colored states. The NiO film was modeled using the Cauchy and Drude models with inhomogeneity. Figure 9 illustrates the measured transmittance spectra of the NiO film in the bleached and colored states as well as the spectra calculated from the refractive index and extinction coefficient in Fig. 8. As seen in Fig. 9, the calculated transmittance spectrum was not close to that of measurement in the bleached
Fig. 8. Optical constants of NiO film prepared at deposition rate of 0.6–0.8 nm∕s in bleached and colored states.
Fig. 10. Measured and calculated reflectance spectra and CR of reflective electrochromic device in bleached and colored states.
state, presumably due to the anisotropic and graded NiO film with the optical axis perpendicular to the surface [26–28]. A reflective electrochromic device with a fivelayered structure glass∕ITO174 nm∕WO3 321 nm∕ Ta2 O5 80 nm∕NiO315 nm∕Al100 nm was fabricated in this study. The WO3 was used as an electrochromic layer, NiO as an ion storage layer, and Ta2 O5 as an ion conductor layer in the all-solid thinfilm electrochromic device. Using a cathodic electrochromic WO3 film and an anodic electrochromic NiO film can enhance the response of this device. The thicknesses of the WO3 film and NiO film could not be small due to the need for the charge balance and electrochromic property. In order to obtain high CR of reflectance, the thicknesses of the WO3 film and NiO film were chosen by optimization with respect to a target of zero reflectance over 400–700 nm in the colored state. After the Ta2 O5 film was deposited on a WO3 ∕ITO∕glass substrate, the sample was removed from the deposition chamber. This sample as a working electrode and a Pt sheet as a counter-electrode were immersed in liquid electrolyte consisting of 1 M LiClO4 in PC. DC voltage of 3 V was applied for 1 min between the two electrodes to inject Li ions into the WO3 film. The sample was taken from the liquid electrolyte, and the LiClO4 solute and PC solvent were removed from the surface of the sample by blowing nitrogen. This sample was then
placed in the deposition chamber again to deposit the NiO and Al films. When the process of the device was complete, the sample was immediately measured using a Hitachi U4100 spectrophotometer in reflectance mode over a spectral range from 400 to 700 nm. Figure 10 shows the measured reflectance and calculated spectra of this device in the bleached and colored states. The mean measured reflectance from 400 to 700 nm was 44.19% in the bleached state and 1.14% in the colored state. Figure 10 also plots the CR of reflectance as a function of wavelength. The mean measured CR from 400 to 700 nm was 37.91, where CR is determined as the ratio of the reflectance in the bleach state to that in the colored state. The difference of 15%– 20% in Fig. 10 between the measured reflectance and calculated spectra in the bleached state of the device was thought to result from the fact that Li ions cannot totally leave the WO3 film into the NiO film within the all-solid thin-film electrochromic device, and that water vapor entered into the WO3 and NiO films, which were exposed to the atmosphere during the process of the device.
Fig. 9. Measured transmittance spectra of NiO sample in bleached and colored states, and spectra calculated using constants in Fig. 8.
4. Conclusions
The optical constants of WO3 electrochromic films and NiO ion storage films in bleached and colored states were investigated. The optical constant of a Ta2 O5 film used as an ion conductor also was studied. The WO3 , NiO, and Ta2 O5 films were all prepared by electron-beam evaporation and characterized using a spectroscopic ellipsometer. The spectra obtained using a spectrophotometer and those calculated from optical constants agreed closely. When the deposition pressure was 2 × 10−4 Torr, the WO3 electrochromic film exhibited the greatest variation in transmittance due to over-stoichiometric oxygen content. When the deposition rate was 0.6–0.8 nm∕s, the NiO ion storage film had the strongest (111) plane, which is the most preferred crystalline orientation of an NiO film for exchanging ions. When the deposition pressure was 4 × 10−4 Torr, the Ta2 O5 ion conductor film could not have a large absorption or be a dense film. Therefore mobile ions can easily pass through the Ta2 O5 film. An all-solid thinfilm reflective electrochromic device with a fivelayered structure (glass∕ITO∕WO3 ∕Ta2 O5 ∕NiO∕Al) 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
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was fabricated and discussed. The mean CR of reflectance over 400–700 nm was 37.91. The authors would like to thank the National Science Council of Taiwan for financially supporting this research under contract nos. NSC 101-2221E-159-015 and NSC 101-2221-E-159-016. References 1. M. Grätzel, “Ultrafast colour displays,” Nature 409, 575–576 (2001). 2. C. G. Granqvist, “Electrochromic tungsten oxide films: review of progress 1993-1998,” Sol. Energy Mater. Sol. Cells 60, 201–250 (2000). 3. D. Corr, U. Bach, D. Fay, M. Kinsella, C. McAtamney, F. O’Reilly, S. N. Rao, and N. Stobie, “Coloured electrochromic ‘paper-quality’ displays based on modified mesoporous electrodes,” Solid State Ionics 165, 315–321 (2003). 4. A. A. Argun and J. R. Reynolds, “Line patterning for flexible and laterally configured electrochromic devices,” J. Mater. Chem. 15, 1793–1800 (2005). 5. F. G. K. Baucke, “Electrochromic mirrors with variable reflectance,” Sol. Energy Mater. 16, 67–77 (1987). 6. C. M. Lampert, “Towards large-area photovoltaic nanocells: experiences learned from smart window technology,” Sol. Energy Mater. Sol. Cells 32, 307–321 (1994). 7. C. G. Granqvist, A. Azens, A. Hjelm, L. Kullman, G. A. Niklasson, D. Ronnow, M. S. Mattsson, M. Veszelei, and G. Vaivars, “Recent advances in electrochromics for smart windows applications,” Sol. Energy 63, 199–216 (1998). 8. G. Macrelli, “Electrochromic windows,” Renew. Energy 15, 306–311 (1998). 9. D. Cummins, G. Boschloo, M. Ryan, D. Corr, S. N. Rao, and D. Fitzmaurice, “Ultrafast electrochromic windows based on redox-chromophore modified nanostructured semiconducting and conducting films,” J. Phys. Chem. B 104, 11449–11459 (2000). 10. H. N. Cui, M. F. Costa, V. Teixeira, I. Porqueras, and E. Bertran, “Electrochromic coatings for smart windows,” Surf. Sci. 532–535, 1127–1131 (2003). 11. C. G. Granqvist, Handbook of Inorganic Electrochromic Materials (Elsevier, 2002). 12. P. M. S. Monk, R. J. Mortimer, and D. R. Rosseinsky, Electrochromism and Electrochromic Devices (Cambridge, 2007). 13. P. K. Shen and A. C. C. Tseung, “In situ monitoring of electrode polarisation during the operation of an electrochromic device based on WO3,” J. Electroanal. Chem. 389, 219–222 (1995).
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14. C. R. Ottermann, A. Temmink, and K. Bange, “Comparison of tungsten and nickel oxide electrochromism in single films and in all-solid-state devices,” Thin Solid Films 193–194, 409–417 (1990). 15. R. Ghosh, M. B. Baker, and R. Lopez, “Optical properties and aging of gasochromic WO3,” Thin Solid Films 518, 2247–2249 (2010). 16. A. Subrahmanyam, C. S. Kumar, and K. M. Karuppasamy, “A note on fast protonic solid state electrochromic device: NiOx / Ta2O5/WO3−x,” Sol. Energy Mater. Sol. Cells 91, 62–66 (2007). 17. K. Bange, C. R. Ottermann, W. Wanger, and F. Rauch, “Investigation of reflective electrochromic all-solid-state devices by nuclear reaction analysis,” Proc. SPIE 1272, 122–128 (1990). 18. C. R. Ottermann, J. G. Segner, and K. Bange, “PVD materials for electrochromic all-solid-state devices,” Proc. SPIE 1728, 211–222 (1992). 19. F. G. K. Baucke, K. Bange, and T. Gambke, “Reflecting electrochromic devices,” Displays 9, 179–187 (1988). 20. K. Bange, “Colouration of tungsten oxide films: a model for optically active coatings,” Sol. Energy Mater. Sol. Cells 58, 1–131 (1999). 21. A. I. Inamdar, Y. S. Kim, B. U. Jang, H. Im, W. Jung, D. Y. Kim, and H. Kim, “Effects of oxygen stoichiometry on electrochromic properties in amorphous tungsten oxide films,” Thin Solid Films 520, 5367–5371 (2012). 22. H. A. Macleod, Thin-Film Optical Filters (Academic, 2001). 23. J. A. Thornton, “High rate thick film growth,” Annu. Rev. Mater. Sci. 7, 239–260 (1977). 24. F. F. Ferreira, M. H. Tabacniks, M. C. A. Fantini, I. C. Faria, and A. Gorenstein, “Electrochromic nickel oxide thin films deposited under different sputtering conditions,” Solid State Ionics 86–88, 971–976 (1996). 25. D. A. Wruck and M. Rubin, “Structure and electronic properties of electrochromic NiO films,” J. Electrochem. Soc. 140, 1097–1104 (1993). 26. I. Valyukh, S. Green, H. Arwin, G. A. Niklasson, E. Wäckelgård, and C. G. Granqvist, “Spectroscopic ellipsometry characterization of electrochromic tungsten oxide and nickel oxide thin films made by sputter deposition,” Sol. Energy Mater. Sol. Cells 94, 724–732 (2010). 27. M. Montecchi, R. M. Montereali, and E. Nichelattic, “Reflectance and transmittance of a slightly inhomogeneous thin film bounded by rough, unparallel interfaces,” Thin Solid Films 396, 264–275 (2001). 28. A. V. Tikhonravov, M. K. Trubetskov, B. T. Sullivan, and J. A. Dobrowolski, “Influence of small inhomogeneities on the spectral characteristics of single thin films,” Appl. Opt. 36, 7188–7198 (1997).
Department of Optoelectronic System Engineering, Minghsin University of Science and Technology, Hsin-Chu 304, Taiwan 2
Department of Optics and Photonics/Thin Film Technology Center, National Central University, Chung-Li 320, Taiwan
3
Graduate Institute of Energy Engineering/Thin Film Technology Center, National Central University, Chung-Li 320, Taiwan 4
Graduate School of Optoelectronics, National Yunlin University of Science and Technology, Yunlin 640, Taiwan *Corresponding author: [email protected] Received 30 August 2013; revised 5 November 2013; accepted 6 November 2013; posted 7 November 2013 (Doc. ID 196651); published 9 December 2013
This study investigates the optical constants of WO3 electrochromic films and NiO ion-storage films in bleached and colored states and that of a Ta2 O5 film used as an ion conductor. These thin films were all prepared by electron-beam evaporation and characterized using a spectroscopic ellipsometer. The spectra obtained using a spectrophotometer and those calculated from the optical constants agreed closely. An all-solid thin-film reflective electrochromic device was fabricated and discussed. Its mean contrast ratio of reflectance in the range of 400–700 nm was 37.91. © 2013 Optical Society of America OCIS codes: (310.6860) Thin films, optical properties; (310.6845) Thin film devices and applications. http://dx.doi.org/10.1364/AO.53.00A154
1. Introduction
Electrochromic materials used in display devices [1–5] and smart windows [6–10] with variable light transmission have been studied. Electrochromism is the phenomenon that is described as a reversible color change of a material caused by applying an electric potential [11,12]. The changes in the optical properties of electrochromic materials are attributed to an oxidation-reduction process. Electrochromic materials could be divided into cathodic electrochromic materials and anodic electrochromic materials. Tungsten oxide (WO3 ) is the most widely studied cathodic electrochromic material, which is transparent in an oxidation state and deep blue in a reduction state. Nickel oxide (NiO) belongs to anodic electrochromic materials, which is transparent in a reduction state and dark brown in an oxidation state. 1559-128X/14/04A154-05$15.00/0 © 2014 Optical Society of America A154
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A typical electrochromic device is composed of a glass substrate, which is coated with a five-layered structure, TC/EC1/IC/EC2/TC, where TC is the transparent electrical conductor, EC1 and EC2 are the electrochromic layers (one of which may be an electrochromic or nonelectrochromic ion storage layer), and IC stands for the ion conductor [13]. Coloration and bleaching occur when voltage is applied across TC; ions then move from EC1 through IC to EC2 or when the polarity of the voltage is reversed. The injection or extraction of mobile ions can modify the optical constants of electrochromic films and ion storage films, changing the transmittance of the electrochromic devices [14,15]. On the other hand, a reflective electrochromic device with reversible movements of protons has been investigated [16–20]. However, few researchers have studied the high contrast ratio (CR) of reflectance of an all-solid thin-film reflective electrochromic device with reversible movements of Li ions. In this study, WO3 and NiO films were used as the electrochromic
layers EC1 and EC2. The optical constants of the WO3 electrochromic films and NiO ion storage films in bleached and colored states were determined using a spectroscopic ellipsometer with Cauchy and Drude models. The optical constant of a tantalum pentoxide (Ta2 O5 ) film, which was used as the ion conductor, IC, also was obtained using the spectroscopic ellipsometer with a Cauchy model. The optical constants of all of these films were examined by comparing the transmittance or reflectance spectra, which were measured using a spectrophotometer with those calculated from the optical constants. Finally, an all-solid thin-film reflective electrochromic device with a five-layered configuration, TC/EC1/IC/EC2/Al, was designed and fabricated to yield a high CR of reflectance, where Al is the reflective layer of aluminum, and TC is the transparent conductive ITO film. 2. Experimental Procedure
Thin films of WO3, Ta2 O5 , and NiO were deposited on ITO glass, B270 glass, and Si substrates at room temperature in an electron-beam-evaporation system. The respective starting materials were WO3 , Ta2 O5 , and NiO granules (99.99% pure, 1–3 mm). Before evaporation, the deposition system was pumped down to a base pressure of under 3 × 10−5 Torr using cryopumps. The deposition rate was controlled using a quartz monitor. The WO3 and Ta2 O5 films were prepared at three deposition pressures (2 × 10−4 , 4 × 10−4 , and 6 × 10−4 Torr) with oxygen as the reactive gas. The NiO films were prepared at three deposition rates (0.1, 0.6–0.8, and 1.3–1.5 nm∕s) without the introduction of oxygen. A cell used a WO3 or NiO film/ITO/glass substrate as a working electrode and a platinum (Pt) sheet as a counter-electrode in liquid electrolyte consisting of 1 M lithium perchlorate (LiClO4 ) in propylene carbonate (PC). DC voltage of 3 V was applied for 1 min between the two electrodes to color or bleach the WO3 or NiO film. These samples were then taken from the liquid electrolyte before measuring optical properties of films. After the LiClO4 solute and PC solvent were removed from the surface of these samples by blowing nitrogen, all of the samples were immediately measured using a Hitachi U4100 spectrophotometer in transmission or reflectance mode over a spectral range from 350 to 850 nm. The optical constants and thickness of the films were determined using a Sopra GES5E spectroscopic ellipsometer with Cauchy and Drude models. The crystalline structure of the films was elucidated using a Bruker D8 Discover x-ray diffractometer with a copper anode.
Fig. 1. Variation in the properties of WO3 samples prepared at three deposition pressures.
2 × 10−4 Torr, the WO3 sample exhibited the greatest variation in transmittance among the three deposition pressures because the variation in the optical constant was the largest, probably due to over stoichiometric oxygen content [21]. The extinction coefficient of the WO3 film in the bleached state was smallest from the film, which was prepared at a deposition pressure of 2 × 10−4 Torr. Moreover, the index of refraction in the bleached state decreased with an increase in the deposition pressure among the three deposition pressures. It was attributed to reduction in kinetic energy of WO3 deposition particles at higher oxygen pressures due to collisions. Figure 2 displays the refractive index and extinction coefficient of the WO3 film, which was prepared at a deposition pressure of 2 × 10−4 Torr in the bleached and colored states. Figure 3 plots measured transmittance spectra of the WO3 film in bleached and colored states and those calculated from the refractive index and extinction coefficient in Fig. 2. The strong agreement in Fig. 3 supports the refractive index and extinction coefficient in Fig. 2. Figure 4 shows the transmittance spectra of Ta2 O5 films, which were prepared on B270 glass at three deposition pressures (2 × 10−4 , 4 × 10−4 , and 6 × 10−4 Torr). Since the refractive index of the Ta2 O5 film, which was prepared at a deposition pressure of 2 × 10−4 Torr, was high due to the large
3. Results and Discussion
Figure 1 presents the difference among the refractive indices, extinction coefficients, and transmittances at 550 nm in the bleached and colored states of the WO3 films, which were prepared on the ITO glass at three deposition pressures (2 × 10−4 , 4 × 10−4 , and 6 × 10−4 Torr). When the deposition pressure was
Fig. 2. Optical constants of WO3 film prepared at a deposition pressure of 2 × 10−4 Torr in bleached and colored states. 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
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Fig. 3. Measured transmittance spectra of WO3 sample in bleached and colored states, and spectra calculated from constants in Fig. 2.
Fig. 4. Transmittance spectra of Ta2 O5 samples prepared at three deposition pressures.
peak to valley in Fig. 4, this film became so dense [22] that mobile ions had difficulty passing through it. Clearly, the absorption was strong following deposition at a pressure of 6 × 10−4 Torr due to losing kinetic energy of Ta2 O5 deposition particles from collisions. In this study, the ion conductor could not have a large absorption or be a dense film. Figure 5 demonstrates the refractive index and extinction coefficient of the Ta2 O5 film, which was prepared on an Si wafer at a deposition pressure of 4 × 10−4 Torr. Figure 6 reveals that the reflectance spectrum calculated from the refractive index and extinction coefficient in Fig. 5 was close to that obtained using the spectrophotometer. Figure 7 presents the XRD patterns of the NiO films, which were prepared on ITO glass at three deposition rates (0.1, 0.6–0.8, and 1.3–1.5 nm∕s). The poor degree of crystallinity in Fig. 7 was due to the small kinetic energy of deposition particles when the NiO film was deposited at a deposition rate of 0.1 nm∕s and due to losing kinetic energy of deposition particles from collisions when the NiO film was deposited at a deposition rate of 1.3–1.5 nm∕s [23]. The XRD peak that corresponded to the NiO (111) plane was strongest from the film deposited at a deposition rate of 0.6–0.8 nm∕s. The preferred orientation was (111), which is the most preferred crystalline orientation of an NiO film for exchanging ions A156
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Fig. 5. Optical constants of Ta2 O5 film prepared at deposition pressure of 4 × 10−4 Torr.
Fig. 6. Measured reflectance spectra of Ta2 O5 sample and spectra calculated using constants in Fig. 5.
Fig. 7. XRD patterns of NiO films prepared on ITO glass at three deposition rates.
[24,25]. Figure 8 displays the refractive index and extinction coefficient of the NiO film prepared at a deposition rate of 0.6–0.8 nm∕s in the bleached and colored states. The NiO film was modeled using the Cauchy and Drude models with inhomogeneity. Figure 9 illustrates the measured transmittance spectra of the NiO film in the bleached and colored states as well as the spectra calculated from the refractive index and extinction coefficient in Fig. 8. As seen in Fig. 9, the calculated transmittance spectrum was not close to that of measurement in the bleached
Fig. 8. Optical constants of NiO film prepared at deposition rate of 0.6–0.8 nm∕s in bleached and colored states.
Fig. 10. Measured and calculated reflectance spectra and CR of reflective electrochromic device in bleached and colored states.
state, presumably due to the anisotropic and graded NiO film with the optical axis perpendicular to the surface [26–28]. A reflective electrochromic device with a fivelayered structure glass∕ITO174 nm∕WO3 321 nm∕ Ta2 O5 80 nm∕NiO315 nm∕Al100 nm was fabricated in this study. The WO3 was used as an electrochromic layer, NiO as an ion storage layer, and Ta2 O5 as an ion conductor layer in the all-solid thinfilm electrochromic device. Using a cathodic electrochromic WO3 film and an anodic electrochromic NiO film can enhance the response of this device. The thicknesses of the WO3 film and NiO film could not be small due to the need for the charge balance and electrochromic property. In order to obtain high CR of reflectance, the thicknesses of the WO3 film and NiO film were chosen by optimization with respect to a target of zero reflectance over 400–700 nm in the colored state. After the Ta2 O5 film was deposited on a WO3 ∕ITO∕glass substrate, the sample was removed from the deposition chamber. This sample as a working electrode and a Pt sheet as a counter-electrode were immersed in liquid electrolyte consisting of 1 M LiClO4 in PC. DC voltage of 3 V was applied for 1 min between the two electrodes to inject Li ions into the WO3 film. The sample was taken from the liquid electrolyte, and the LiClO4 solute and PC solvent were removed from the surface of the sample by blowing nitrogen. This sample was then
placed in the deposition chamber again to deposit the NiO and Al films. When the process of the device was complete, the sample was immediately measured using a Hitachi U4100 spectrophotometer in reflectance mode over a spectral range from 400 to 700 nm. Figure 10 shows the measured reflectance and calculated spectra of this device in the bleached and colored states. The mean measured reflectance from 400 to 700 nm was 44.19% in the bleached state and 1.14% in the colored state. Figure 10 also plots the CR of reflectance as a function of wavelength. The mean measured CR from 400 to 700 nm was 37.91, where CR is determined as the ratio of the reflectance in the bleach state to that in the colored state. The difference of 15%– 20% in Fig. 10 between the measured reflectance and calculated spectra in the bleached state of the device was thought to result from the fact that Li ions cannot totally leave the WO3 film into the NiO film within the all-solid thin-film electrochromic device, and that water vapor entered into the WO3 and NiO films, which were exposed to the atmosphere during the process of the device.
Fig. 9. Measured transmittance spectra of NiO sample in bleached and colored states, and spectra calculated using constants in Fig. 8.
4. Conclusions
The optical constants of WO3 electrochromic films and NiO ion storage films in bleached and colored states were investigated. The optical constant of a Ta2 O5 film used as an ion conductor also was studied. The WO3 , NiO, and Ta2 O5 films were all prepared by electron-beam evaporation and characterized using a spectroscopic ellipsometer. The spectra obtained using a spectrophotometer and those calculated from optical constants agreed closely. When the deposition pressure was 2 × 10−4 Torr, the WO3 electrochromic film exhibited the greatest variation in transmittance due to over-stoichiometric oxygen content. When the deposition rate was 0.6–0.8 nm∕s, the NiO ion storage film had the strongest (111) plane, which is the most preferred crystalline orientation of an NiO film for exchanging ions. When the deposition pressure was 4 × 10−4 Torr, the Ta2 O5 ion conductor film could not have a large absorption or be a dense film. Therefore mobile ions can easily pass through the Ta2 O5 film. An all-solid thinfilm reflective electrochromic device with a fivelayered structure (glass∕ITO∕WO3 ∕Ta2 O5 ∕NiO∕Al) 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
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was fabricated and discussed. The mean CR of reflectance over 400–700 nm was 37.91. The authors would like to thank the National Science Council of Taiwan for financially supporting this research under contract nos. NSC 101-2221E-159-015 and NSC 101-2221-E-159-016. References 1. M. Grätzel, “Ultrafast colour displays,” Nature 409, 575–576 (2001). 2. C. G. Granqvist, “Electrochromic tungsten oxide films: review of progress 1993-1998,” Sol. Energy Mater. Sol. Cells 60, 201–250 (2000). 3. D. Corr, U. Bach, D. Fay, M. Kinsella, C. McAtamney, F. O’Reilly, S. N. Rao, and N. Stobie, “Coloured electrochromic ‘paper-quality’ displays based on modified mesoporous electrodes,” Solid State Ionics 165, 315–321 (2003). 4. A. A. Argun and J. R. Reynolds, “Line patterning for flexible and laterally configured electrochromic devices,” J. Mater. Chem. 15, 1793–1800 (2005). 5. F. G. K. Baucke, “Electrochromic mirrors with variable reflectance,” Sol. Energy Mater. 16, 67–77 (1987). 6. C. M. Lampert, “Towards large-area photovoltaic nanocells: experiences learned from smart window technology,” Sol. Energy Mater. Sol. Cells 32, 307–321 (1994). 7. C. G. Granqvist, A. Azens, A. Hjelm, L. Kullman, G. A. Niklasson, D. Ronnow, M. S. Mattsson, M. Veszelei, and G. Vaivars, “Recent advances in electrochromics for smart windows applications,” Sol. Energy 63, 199–216 (1998). 8. G. Macrelli, “Electrochromic windows,” Renew. Energy 15, 306–311 (1998). 9. D. Cummins, G. Boschloo, M. Ryan, D. Corr, S. N. Rao, and D. Fitzmaurice, “Ultrafast electrochromic windows based on redox-chromophore modified nanostructured semiconducting and conducting films,” J. Phys. Chem. B 104, 11449–11459 (2000). 10. H. N. Cui, M. F. Costa, V. Teixeira, I. Porqueras, and E. Bertran, “Electrochromic coatings for smart windows,” Surf. Sci. 532–535, 1127–1131 (2003). 11. C. G. Granqvist, Handbook of Inorganic Electrochromic Materials (Elsevier, 2002). 12. P. M. S. Monk, R. J. Mortimer, and D. R. Rosseinsky, Electrochromism and Electrochromic Devices (Cambridge, 2007). 13. P. K. Shen and A. C. C. Tseung, “In situ monitoring of electrode polarisation during the operation of an electrochromic device based on WO3,” J. Electroanal. Chem. 389, 219–222 (1995).
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