Electrochromic devices have increasing application in display devices, switchable mirrors and smart windows. A variety of vacuum deposition technologies have been used to make electrochromic devices. The sol- gel process offers an alternative approach to the synthesis of optical quality and low cost electrochromic device layers. This study summarizes the developments in sol-gel deposited electrochromic films. The sol-gel process involves the formation of oxide networks upon hydrolysis-condensation of alkoxide precursors. In this study we cover the sol-gel deposited oxides of WO[sub 3], V[sub 2]O[sub 5], TiO[sub 2], Nb[sub 2]O[sub 5], and NiO[sub x]

Lampert, C.M.

Ozer, N.

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Sol-gel deposited electrochromic coatings

Ozer, Nilgun

Lampert, Carl M

eScholarship - University of California

Lawrence Berkeley National LaboratoryLBL PublicationsTitleSol-Gel Deposited Electrochromic CoatingsPermalinkhttps://escholarship.org/uc/item/04c0238pAuthorsOzer, NilgunLampert, Carl MPublication Date1995-06-01eScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaLBL-38527 UC-1600 ERNEST ORLANDO LAWRENCE BERKELEY NATIONAL LABORATORY Sol-Gel Deposited Electrochromic Coatings Nilgun Ozer and Carl M. Lampert Energy and Environment Division June 1995 Presented at the Optical Interference Coatings Topical Meeting, Tu~-~~.~g~,"~"'········· ... June·-5.::.,9;~:1995, and to be published <> (") 0 "C 1<: ..... r-IJl r-I w CXl l11 N -.,J DISCLAIMER This document was prepared as an account of work sponsored by the United States Government. While this document is believed to contain conect information, neither the United States Government nor any agency thereof, nor the Regents of the University of California, nor any of their employees, makes any wananty, express or implied, or assumes any legal responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by its trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof, or the Regents of the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof or the Regents of the University of California. Sol-Gel Deposited Electrochromic Coatings Nilgun Ozer and Carl M. Lampert Energy and Environment Division Ernest Orlando Lawrence Berkeley National Laboratory University of California Berkeley, California 94 720 June 1995 I LBL-38527 UC-1600 This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Building Technologies, Building Systems and Materials Division, of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098. Sol-gel Deposited Electrochromic Coatings Nilgun Ozer and Carl M. Lampert Lawrence Berkeley Laboratory, University of California, M.S.62-203, Energy and Environment Div., Berkeley, CA 94720 USA Tele: 510-486-6093, Fax: 510-486-7339 ABSTRACT FJectrochromic devices have increasing application in display devices, switchable mirrors and smart windows. A variety of vacuum deposition technologies have been used to make electrochromic devices. The sol-gel process offers an alternative approach to the synthesis of optical quality and low rost electrochromic device layers. This study summarizes the developments in sol-gel deposited electrochromic films. The sol-gel process involves the formation of oxide networks upon hydrolysis-condensation of alkoxide precursors. In this study we cover the sol-gel deposited oxides of WOj. V20s, Ti(h, N~Os. and NiOx. · l. INTRODUCTION The electrochromic properties of transition metal oxides have been extensively studied during last decade for potential use in display devices or smart windows [1-2]. Electrochromism is a reversible color change in a material induced by injected/ejected electrical charge and ions. The electrochromic material has to be deposited as a thin layer onto transparent electrode. This is usually done by vacuum evaporation [3], sputtering [4] or electrolytic deposition [5]. The sol-gel process offers a new approach to the synthesis of oxide coatings [6]. In sol-gel process molecular precursors are transf onned into an oxide network via organic polymerization reactions. The molecular precursor is usually a metal alkoxide M(OR)n , and polymerization occurs via hydrolysis and condensation [7]. The overall reaction can be described as follows: Hydrolysis : M(OR)n + nH20 -> M(OH)n + n ROH Condensation : M(OH)n -> MOo.sn + (0.5n) H20. Sol-gel derived electrochr9mic metal oxides can be made from colloidal solutions by dipping. spin coating or spying [1]. They are easy to synthesize and exhibit a rather good electrochromic behavior [6,8-10]. This paper reports a discussion about the electrocbromic properties of WO:J, Ti~. Ntn<>s. V20s and NiOx layers. It is based on published data from literature together with results from our laboratory. · 2. RESULTS AND DISCUSSION 2.1. Tungsten Oxide Tungsten oxide coatings have been deposited from a number of sol-gel precursors such as tungstic acid solutions [8,11], tungsten alkoxides based solutions [12]; and chloro-alkoxide (13]. Films prepared from choloro-alkoxide showed better uniformity than those prepared from colloidal route. Sol-gel deposited tungsten oxide shows a transparent to dark blue electrochromism with ion injection. A proposed reaction for coloration is based on a dual mechanism a) ion insertion to the film; b) electron insertion. The reaction can be interpreted as: WOJ + X A+ + X e- < > Ax WOj • (A+ = H+ or u+ ) transparent blue Fig. 1 shows a typical cyclic voltammogram of sol-gel spin coated (SG) WOj films prepared from tungsten dichloride triethoxide compared with the ones prepared by vacuum techniques. <( E -c: Ql ... 8.0 4. 0.0 8 -4.0 -8.0 -0.6 -0.4 -0.:2 0.0 __ :·SGW~ ----: SPWO . 3. ------ : .EV·Wo3. ·. 0.2 0.4 ·Potential vs SCE ( Volts ) Fig. 1 . Cyclic voltammograrns of W03 films. 0.1 H2S04 solution was used as an electrolyte with a sweep rate of 20m V /s. The film thicknesses were about 200 nm for each film. The films color under a negative potential. 2.2. Titanium Oxide Sol-gel titanium oxide films can be formed from the classical alkoxy route and by the oolloidal route [6,9,14]. They appear to be amorphous by X-ray diffraction. Upon heating the films aystallized into TiO:z anatase around 400 OC. Amorphous Ti0:2 undergoes an electrochromic reactions according to : TiO:z + Xu+ + xe- <=> Lix TiO:l or TiO:z + H+ + e- <==> TiOOH transparent gray transparent blue Titanium oxide also oolors when reduced by ion insertion, but its coloration efficiency is lower than tungsten oxide. Amorphous films change from transparent to gray or blue when reduced by u+ or H+ ions. 2.2. Niobium and Vanadium Oxides The group V-B oxides, namely Nh20s and V ~5 exhibit cathodic electrochromism like WDJ. Although the change in optical spectrum between oxidized and reduced state by ion intercalation is not as large WOj, it has been shown that these oxides have good u+ ion storage capability and reversibility [2,6,15]. The reversibility of V205 in the crystalline state is superior to that in the amorphous state. Sol-gel niobium and vanadium oxide films have been synthesized from alkoxides, oxoalkoxides or alkoxyhalides based solutions [15-17]. XPS and optical spectra of the colored V 20s and N~s coatings together with electrochemical experiments show that the electrochromic properties can be described by: Nh2Qs +xu++ x e- <=> Lix Ni>20s or Nh20s + 2 H+ + 2 e- <==> Nh20.3(0H)2 transparent brown transparent green-blue V205+ X :U+ +X e- <=> Lix V205 transparent yellow gray 2.4. Nickel Oxide Nickel oxide is presently the subject of several studies [2,17]. Nickel oxide films have been formed by dissolving or reacting nickel nitrates in alcohol and then using the product ac; precursors [18]. The oolo~tion mechanism ha<> been determined for protons by J. C. Giron (19]. 2 The reaction involves two coupled reactions: Ni(OH)2 < > NiOOH +H++e- and H+mm+ OH-electrolyte <==> H20ri1m transparent brown . Sol-gel films made from nickel methoxyethoxide precursor also showed a transparent to brown electrochromism with u+ insertion. These films colored by anodic transfer of electrons out of the film couple with anion injection from the electrolyte. Table I. summarizes some properties of the sol-gel deposited electrochromic layers. Table 1. Sol-gel electrochromic coatings Material Color oxidized or Stability Flectrode Reference neutral reduced fonn (lifetime) WOJ (C) ·1 tansparent Blue I04cycles uao4 11-13 Ti(h (C) Transparent Gray or blue 103cycles li004 6,14 Nh20s (C) Transparent Brown. gray I04cycles uao4 I, 15 V20s(A) light yellow Brown. green I@ cycles liCI04 1, 6 NiO,. (A) Light green Brown 103 cycles uao4 present work note: (C) cathodically colonn • (A) g 111 y colonng. 2.5.. Sol-gel made devices The construction of display devices or smart windows consist of typically a (1) transparent conductor (ITO). (2) electrochromic layer, (3) solid electrolyte or ion conductor, (4) ion storage layer. (5) transparent conductor. Recently, the sol-gel process has been also adopted to prepare a variety of inorganic type solid electrolytes [20, 21]. An all sol-gel electrochromic device has been realized using sol-gel coatings with the configuration glass liTO IWO) ffi~ m~~ liTO /glass [22]. The optical transmission spectra of these window is shown in fig. 2 This window has a fast optical response. but a shoit lifetime. Currently. several groups are trying to develop devices using some or all sol-gel deposited layers. 100 (0.8 V). 80 -w -Gl 60 0 c:: cu oi.J oi.J 40 ...-l 1: (/) c:: cu $.I 20 ~ 300 Wavelenght (nm) Fig. 2. Optical transmission of two electrochromic windows having the configuration : glass liTO I Hx WOj I Ti~-sg I Ce~-Ti~-sg /ITO I glass, where (-) is vacuum evaporated WOj and ( --- ) is a sol-gel (sg) WOJ . 3 4. SUMMARY The sol-gel process offers new approaches for the fabrication of electrochromic materials. In the electrochromic device construction, microstructure control and chemical composition of the layers are important since they need to have eleetron and ion transport properties in addition to the optical requirements. Major advantages of the sol-gel process are ease of stoichiometry control and low sintering temperatures . Large-area coating can be easily deposited under ambient conditions by dip coating, spin ·coating, spying or even screen printing. Sol-gel deposited electrochromic layers have similar properties to those films deposited by vacuum techniques. We expect to see further developments in sol-gel deposited films for electrochromic devices in the future. 5. ACKNOWLEDGMENT We wish to thank Dr. Phillip Baumeister for inviting this paper to the American Optical Society meeting~ This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Building Technologies, Building Systems and Materials Division of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098. 6. REFERENCES 1. C.G., Granqvist. Handbook of Inorganic Electrochromic Materials, Elsevier, . Amsterdam (1995). 2. C.M. Lampert. Solar Energy Mater.,11 (1984) 1. 3. Y. Fujita. K. Miyazaki, and C. Tatsuyama. Jpn. J. Appl. Phys., 24 (1985) 1082. 4. A. Deneuville. P. Gerard, and R. Billat. Thin Solid Films, 70 (1986) 203. 5. E. Andrukaitis, P.W.M Jacobs, and J.W. Lorimer, Solid State lonics, 27 (1988) 19. 6. M. Nabavi, S. Doeuff, C. Sanchez and J. Livage, Mater. Sci. Eng., B3 (1989)203. 7. J. Brinker, and G. W. Scherer, Sol-Gel Science, Academic Press, San Diego, 1990. 8. A. Chemseddine, R. Morineau and J. Livage, Solid State lonics, 5 (1983) 357. 9. N. Ozer, Thin Solid Films, 214 (1992) 17. 10. A. Agrawal, J.P. Cronin and R. Zhang, Sol. Energy Mater. and Sol. Cells, 31(1993)9. 11. P. Judenstein, and J. Livage, SPIE Proc, 1328 (1990) 344. 12. H. Unuma. K. Tanooka. Y. Suzuki, T. Furasaki, K. Kodaira and T. Matsushita. J. Mater. Sci. Lett., 5 (1986) 1248. 13. A~M. Habib, and D. Glueck, Sol. Energy Mater., 18 ( 1989) 127. 14. N. Ozer, D.O. Chen, and J.H. Simmons, Ceramic Tr3nsactions ,V.20 (1991} 253. 15. N. Ozer, T. Barreto, T. Buyuklimanli, and C.M Lampert. Sol. Energy Mater. and Sol. Cells, in press (1995). . 16. C. Sanchez, M. Nabavi, and F. Taulelle, in Better Ceramics Through Chemistry Ill, MRS, Reno, April 1988, 121 (1988) 93. 17. F.H. Moser,and N.R. Lynam, U.S. Patent 4,855, 61 (1989). 18. F.H. Moser, and N.R. -Lynam, U.S. Patent 4,959,247 (1990). 19. J.C. Giron and C.M. Lampert, The Electrochem. Soc. Extended Abst V. 93-2 (1993) 652 20. LC. Klein, Solid State lonics, 32-33 (1989) 639. 21. N. Ozer, Y.X. He, and C.M. Lampert. Proc. SPIE, 2255 (1994) 456. 22. M.A. Macedo, LH. Daii'Antonia. and M.A. Aegerter, Proc. SPIE, 1758 (1992) 320. 4 

Sol-gel deposited electrochromic coatings

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