Electrochromic coatings for “smart windows”

Solar Energy Materials 12 (1985) 391-402 North-Holland, Amsterdam

391

ELECTROCHROMIC COATINGS FOR "SMART WINDOWS" J.S.E.M. SVENSSON and C.G. GRANQVIST Physics Department, Chalmers University of Technology, S-41296 Gothenburg, Sweden We introduce electrochromic coatings for use on "smart windows" with dynamic control of radiant energy. Applications, operating principles, materials options, and device configurations are briefly reviewed. We then focus on electrochromic WO 3 and investigate crystalline films by computations aimed at assessing performance limits, and amorphous films with regard to practical performance in contact with liquid or solid electrolytes. It appears that there are several approaches to achieving a gradual and reversible modulation of the radiative performance in a manner consistent with uses on "smart windows".

1. Introduction We report on electrochromic coatings whose optical properties can be varied gradually and reversibly within wide limits. Their potential application in "smart windows" with dynamic control of radiant energy is introduced and treated theoretically as well as experimentally with regard to different sample configurations. Coated window glass is being used on an increasing scale for gaining energy efficiency. In particular, much work has been devoted to transparent infrared-reflectors of different kinds [1,2]. If the onset of high reflectance occurs at a wavelength --0.7 ~m, such coatings can be used for decreasing the inflow of infrared solar radiation thereby diminishing the need for air conditioning in a warm climate. If the onset of high reflectance is instead at X ~ 3 Fro, they provide low thermal emittance and hence the improved thermal isolation desired in a cold climate. Transparent infrared-reflectors are now beginning to reach a high degree of perfection, and the theoretical limits are being approached for some laboratory-type specimens [3-5]. It must be remembered, though, that the transparent infrared-reflectors have a basic limitation in their properties being static. It may then be that - for a temperate climate - a low emittance coating is appropriate in the winter while it causes severe overheating in the summer. It is obvious that much is to be gained if dynamic optical properties can be created. This leads us to consider optical switching materials, which in principle can be of different kinds such as photochromic (properties depending on irradiation), thermochromic (properties depending on temperature), and electrochromic (properties depending on strength and direction of an applied electric field). In this paper we focus on electrochromic materials. Their operating principles are outlined in sect. 2 below. In sect. 3 we then consider coatings consisting of crystalline WO 3 and compute the performance limits for this material by use of a novel theoretical model. Sect. 4 is devoted to experimental studies of samples which involve a layer of amorphous WO 3 as the active component. We present experimen0165-1633/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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tal data for configurations including liquid as well as solid electrolytes. It i.~ demonstrated that the optical properties are of the kind desired for dynamic radiation control in a "smart window". Sect. 5, finally, summarizes the main results and gives some concluding remarks.

2. Operating principles for electrochromic window coatings An electrochromic coating for a "smart window" must encompass several materials. Fig. 1 sketches a basic configuration which is convenient for discussing the operating principles. It comprises two transparent conducting layers, required for applying the electric field, and intervening materials serving as active electrochromic layer, ion conductor, and ion storage. We first regard the electrochromic layer and note that electrochromism occurs in many materials [6-8]. The most well known are inorganic partially hydrated transition metal oxides (based on WO3, MOO3, IrO,, etc.), organic substances (viologens, phthalocyanine lanthanides, etc.), and intercalated compounds. In this paper we only consider WO 3, which is the most widely studied electrochromic material and definitely one of the most promising candidates for device applications. Coatings of WO 3 can be prepared by several different techniques, and the resulting material can be amorphous or crystalline. In either case, the variation in the optical properties can be understood as the result of a d o u b l e i n j e c t i o n of positive ions (M ÷) and electrons ( e - ) according to the overall reaction [6,9] x M + + x e - + WO3_y ~ M~WO3_~,,

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(1)

J.S.E.M. Svensson, CG. Granqvist / Coatingsfor "smart windows"

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where 0 < x < 0.5 and y ~<0.03. Coloration is associated with the formation of MxWO 3_y; the actual coloration mechanism is different for amorphous and crystalline specimens, which we return to shortly. The ions needed in the electrochromic reaction are provided by the ion storage and are injected into, or withdrawn from, the electrochromic layer via the ion conductor. The detailed design can be of different types: for devices using liquid electrolytes the electrolyte can serve both as storage and conductor for the ions, while in all-solid-state devices the ion conductor can be an appropriate dielectric and the ion storage can be another electrochromic layer (preferably anodic if the base electrochromic layer is cathodic, or vice versa). One may also combine the conductor and storage media into one layer. Further it is possible to exclude the ion storage medium and instead rely [10-12] on a replenishment of H + originating from the dissociation of water molecules diffusing in from an ambience with controlled humidity; obviously, this requires a substantial atomic permeability of the outer electrode. The two transparent electrodes offer comparatively small problems. The electrode at the glass interface can be any high-performance transparent infrared-reflector [1-3], such as films of the doped oxide semiconductors In203 : Sn, SnO 2 : F, SnO 2 : Sb, or CdzSnO 4. There are several standard techniques for producing these coatings. The outer electrode can be of different kinds: for liquid electrolytes it can in principle be a bulk material provided that it is sufficiently small so that the overall transparency is not severely impaired, while for an all-solid-state device it must be a transparent infrared-reflector which can be produced without deterioration of the electrochromic property for the underlying layers. The later condition may require that semi-transparent noble-metal films [13] are used, or that doped oxide semiconductor coatings are produced by non-standard techniques. The radiative properties of a "smart window" are switchable between different states. The nature of this switching can be different depending on the basic requirement on the window, i.e. whether the main goal is to achieve control of the energy flowing through the window aperture, or if daylighting and glare control are of prime importance. Under certain conditions it may also be possible to have variable thermal insulation. In fig. 2 we illustrate idealized cases for energy control. For this application it is important to remember that the solar spectrum extends over the 0.3-3-~m range, whereas the eye is sensitive only in the 0.4-0.7-~tm interval. The pertinent spectra are shown by the shaded areas in fig. 2: they indicate the photopic luminous efficiency of the eye [14] and the solar spectrum [15] AM1 (corresponding to clear weather and the sun at zenith). Almost 50% of the total solar energy comes as infrared radiation, and hence it is possible - at least in principle - to change the energy throughput within rather wide limits without affecting the luminous transparency. The changed infrared transmittance can be achieved in two ways: by modulating the infrared reflectance as indicated in the upper part of fig. 2, or by modulating the infrared absorptance as indicated in the lower part. A gradual decrease in the transmitted energy is obtained as the reflectance edge moves towards shorter wavelength, or as the absorptance goes up. Both types of modulation occur in electrochromic WO 3 as we will see shortly: the properties of crystalline WO 3 films

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T h e p u r p o s e o f this section is to c o m p u t e the p e r f o r m a n c e limits for coatings i n c o r p o r a t i n g a layer of crystalline e l e c t r o c h r o m i c W O 3. This will be d o n e b y using

J.S,E.M. Svensson, C.G. Granqvist / Coatingsfor "smart windows"

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the double injection model, summarized in eq. (1), together with recent results [4,16,17] on ionized impurity scattering in heavily doped semiconductors. It is known that doped crystalline WO3 films display an approximate free-electron behavior [18,19], and hence we can treat the reflectance modulation within the framework earlier used by us [4] to explain the optical properties of transparent infrared-reflecting In 203 : Sn films. Our theoretical model for the optical properties is built on the bandstructure [20,21] of WO 3, which shows conduction bands derived from the tungsten 5d orbitals well separated from the oxygen 2p valence bands. It is known [22] that direct bandgaps lie at 3.52 and 3.74 eV, and that an indirect bandgap lies at 2.62 eV. Double injection of electrons and ions gives an electron density n e and an equal ion density N i. When n~ exceeds the Mott critical density [23] the electrons are assumed to occupy the WO 3 conduction band in the form of an electron gas. Damping of the free electrons occurs, in principle, from scattering against ionized impurities, neutral point defects, dislocations, grain boundaries, film surfaces etc. For a sufficiently ideal film we neglect all but the ionized impurities - being an unavoidable consequence of the double injection model - and write the complex dielectric function according to [4] e = ew ° 3 + i / c o , 0 0 ,

(2)

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for solar irradiance and for luminous efficiency of the eye; the spectra earlier shown in fig. 2 are reproduced for convenience also in fig. 3. It is found that three ranges of ne can be distinguished; at n e ~< 10 21 cm -3 only little solar radiation is reflected; at 1021< n ~ < 6 × 10 21 cm -3 mainly the infrared part of the solar spectrum is reflected; and at n~ > 6 × 10 ~1 cm -3 an increasing a m o u n t of luminous radiation is reflected so that the films appear strongly colored. Literature data [18,19] on spectral reflectance for crystalline electrochromic W O 3 films show a much more graded onset of high reflectance than obtained from the computations. This is illustrated by the dashed curve in fig. 3, which pertains to a 0.56-~tm thick film [19]. We interpret the slow increase of reflectance as a result of poor crystallinity. We n o w consider the application of crystalline electrochromic W O 3 films to " s m a r t windows" in more detail. To this end we introduce integrated solar and luminous properties by R sol(lum) = f d X q~o~(l.~) ( X ) R ( A )/fdA¢~ol,,um)( A ).

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(5)

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reflectance is as large as - 20%, which is a consequence of the high refractive index of u n d o p e d W O 3. At n e >_ 10 21 c m - 3 , Rso] goes up and Tso] goes d o w n monotonically in a m a n n e r consistent with the altered magnitudes of ~p and t. The results for Tl, m and R lur~ show a qualitatively similar behavior. However, R lu m c a n reach a value as low as - 5% for ne = 6 × 10 21 cm -3, and r l u m c a n display a c o n c o m i t a n t weak maximum, which occurs because of destructive interference in the 0 . 4 - 0 . 7 - ~ m interval. F o r n 3 >_ 10 21 cm -3 it is f o u n d that R + T < 1, signifying that plasma absorption is important. This also proves that a strict demarcation between m o d ulated reflectance and m o d u l a t e d absorptance cannot be maintained. Fig. 4 contains a wealth of information. To take one example, we consider a 0.2-txm thick W O 3 film whose electron density is varied from a low value up to 5 × 10 21 c m -3. The ensuing radiative performance (solid curves) goes from a state with - 82% transmittance of solar energy to another state with - 35% transmittance of solar energy. The latter state implies - 63% luminous transmittance and - 5% luminous reflectance. These properties point at the highly variable energy t h r o u g h p u t one can h o p e to achieve in a " s m a r t w i n d o w " employing optimized crystalline electrochromic layers.

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4. Modulated absorptance in amorphous electrochromic WO 3 films: experimental data The mechanism of coloration is different in amorphous and crystalline WO> In the former case - which is relevant for this section coloration has been explained from a model for intervalence absorption associated with electrons localized at tungsten sites [6,9], or from a related model involving polaron absorption [18]. It appears that the understanding of the basic radiation properties is not yet sufficient for allowing us to state performance limits analogous to those for crystalline WO 3. Instead we devote this section to experimental data on electrochromic devices incorporating amorphous WO~ films in contact with either liquid or solid electrolytes. The first set of experiments [30] was designed by drawing on the extensive experience which exists on electrochromic information displays with liquid electrolytes [6,7]. The specimens were prepared by two consecutive evaporations. First we coated a plate of Corning 7059 glass with a 0.15-~m thick In203: Sn film, having a resistance/square of - 4 0 f2, using reactive e-beam deposition according to a technique developed by us [3]. In a second step, 99.9% pure granules of WO 3 were e-beam evaporated in the presence of 2 to 5 × 10 -a Torr of oxygen until a 0.3-~m thick layer was formed. The deposition rate was maintained at a constant value in the 1 to 5 nm s-1 range by use of a vibrating quartz microbalance regulating the e-gun power via feedback control. The samples were inserted in an electrochemical cell with two oppositely placed glass windows. The cell was filled with an electrolyte [31,32] of 1M dehydrated lithium perchlorate in propylene carbonate. A small Pt foil was used as counter electrode. This configuration is shown in the inset of fig. 5. The sample was connected so that it formed the working electrode in a standard three-electrode potentiostatic coupling with a saturated calomel electrode as a reference, The cell was mounted in a double-beam spectrophotometer. An analogous cell, containing electrolyte but no glass sample, was placed in the reference beam. Spectral transmittance was recorded for the 0.3-1.3-t*m range. In the combined optical and electrical measurements we first applied a voltage between working and counter electrodes until maximum transmittance was found. The WO 3 layers were then successively colored by reversing the voltage. The coloring was interrupted at several intermediate levels in the color-bleach cycle, and the corresponding transmittance spectra and values of the voltage between working and reference electrodes were determined. The main part of fig. 5 shows a typical set of curves taken by this procedure. It is seen that the transmittance can be varied gradually and reversibly between wide limits, particularly in the near-infrared, by the use of voltages less than 1 V. Our data on fully colored and fully bleached samples are consistent with earlier results on evaporated WO3 fihns [8,10,18,31-34]; we are not aware of any earlier results for partially bleached films. The time constants associated with coloring and bleaching are of the order of seconds for the small specimens studied so far; they are expected to be considerably longer in a large window [35]. An evaluation of the integrated solar transmittance according to eq. (5) showed that this entity lay between 86 and 12%. Liquid electrolytes are awkward for window applications, and therefore we have

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recently turned to an alternative all-solid-state electrochromic configuration. The samples consist of glass coated with four layers: 0.15 t~m of In203 : Sn; 0.15 t~m of amorphous WO3; 0.05 to 0.1 t~m of MgF2; and - 15 nm of Au. The WO 3 film was made as described above. The MgF 2 layer was produced by evaporation from a resistive source at a rate of 0.5 nm s -1 in the presence of 5 x 10 .4 Torr of air. The resistively evaporated gold film is sufficiently thin to be semi-transparent. This structure is shown in the inset of fig. 6. Optical measurements indicate that the Au and MgF 2 films form a mixed layer at their common interface. A similar behavior has been reported [11] earlier for the boundary between Au and SiO 2. When a voltage is applied between the external electrodes, the specimens can undergo either coloration or bleaching. Coloration may be due to [11,12] a dissociation of water molecules and subsequent reversible formation of HxWO3_y in accordance with eq. (1). The samples were mounted in the spectrophotometer and transmittance and reflectance were recorded in the 0.35-2.2-1xm interval as a function of voltage. Main part of fig. 6 shows typical data. The transmittance is seen to go from moderate to low as the voltage is increased, whereas the reflectance is changed only marginally. The integrated solar transmittance lies between 25 and 3%. It appears that amorphous W O 3 films display pronounced modulated absorptance (rather than modulated reflectance). The reason for the undesirable low transmittance in the bleached state is to be found mainly in the reflectance from the gold electrode. An improved outer electrode would yield superior overall performance.

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5. Summary and remarks In this paper we have introduced electrochromic coatings for use on "smart windows" with dynamic control of radiant energy. Several options exist with regard to application (energy efficiency, daylighting, etc.), operating principle (modulated reflectance or absorptance), useful materials, and device configuration. All-solid-state designs are clearly preferable for windows, but apart from this guideline it is our feeling that it is premature to focus on any particular system as being definitely superior to others. In the work reported above we have studied crystalline electrochromic WO 3 films by computations aimed at assessing performance limits, and amorphous electrochromic WO 3 films with the purpose of obtaining empirical data on the degree of solar transmittance modulation. In both cases the results are encouraging, and it appears that a gradual and reversible modulation of the optical performance in a manner consistent with applications on "smart windows" can be achieved. Electrochromic materials have been studied for several years in the context of high-contrast non-emissive information displays. Thus a large body of useful results are present [6-8] in the scientific and technical literature as well as in patent specifications. The investigated designs have usually been non-transparent, and furthermore the demands on window coatings are quite different from those for displays. Therefore the development of electrochromic window coatings is not to "reinvent the wheel" but must encompass innovative and original work. It is

J.S.E.M. Svensson, C.G. Granqvist / Coatingsfor "smart windows"

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interesting to find that research p r o g r a m s whose u l t i m a t e goal is the" smart w i n d o w " are a p p e a r i n g in different parts of the world. It is o u r belief that electrochromic coatings a n d " s m a r t windows" are going to be i m p o r t a n t subjects i n the research o n materials for energy efficiency a n d solar applications for several years to come.

Acknowledgement T h e c o m p u t e r p r o g r a m used for evaluating the dielectric f u n c t i o n of crystalline electrochromic W O 3 in sect. 3 was written b y I. H a m b e r g . This work was financially s u p p o r t e d b y grants from the Swedish N a t u r a l Science Research C o u n c i l a n d the N a t i o n a l Swedish Board for Technical D e v e l o p m e n t .

References [1] C.G. Granqvist, Appl. Opt. 20 (1981) 2606; Proc. SPIE 401 (1983) 330; The Physics Teacher 22 (198a) 372. [2] C.M. Lampert, Solar Energy Mater. 6 (1981) 1; Energy Research 7 (1983) 359. [3] I. Hamberg, A Hjortsberg and C.G. Granqvist, Appl. Phys. Lett. 40 (1982) 362; Proc. SPIE 324 (1982) 31; I. Hamberg and C.G. Granqvist, Appl. Opt. 22 (1983) 609. [4] I. Hamberg and C.G. Granqvist, Proc. SPIE 428 (1983) 2; Appl. Phys. Lett. 44 (1984) 721. [5] I. Hamberg, C.G. Granqvist, K.-F. Berggren, B.E. Sernelius and L. EngstrtSm,Phys. Rev. B 30 (1984) 3240; Proc. SPIE 502 (1984) 2. [6] B.W. Faughnan and R.S. Crandall, in: Display Devices, ed. J.I. Pankove, Topics In Applied Physics, vol. 40 (Springer-Verlag,Berlin, Heidelberg, 1980) p. 181. [7] G. Beni and J.L. Shay, Adv. Image Pickup and Display 5 (1982) 83. [8] C.M. Lampert, Solar Energy Mater. 11 (1984) 1. [9] B.W. Faughnan, R.S. Crandall and P.M. Heyman, RCA Rev. 36 (1975) 177. [10] S.K. Deb, Philos. Mag. 27 (1973) 810. Ill] A.R. Lusis, Ya.K. Klyavin, Ya.Ya. Kleperis, Ya.Ya. Pinnis and O.A. Rode, Electrokhimiya 18 (1982) 1538; English translation: Soviet Electrochem. 18 (1983) 1372. [12] T. Yoshimura, M. Watanabe, K. Kiyota and M. Tanaka, Japan. J. Appl. Phys. 21 (1982) 128; T. Yoshimura, M. Watanabe, Y. Koike, K. Kiyota and M. Tanaka, Thin Solid Films 101 (1983) 141. [13] E. Valkonen, B. Karlsson and C.-G. Ribbing, Solar Energy 32 (1984) 211. [14] G. Wyszecki, in: Handbook of Optics, eds, W.G. Driscoll and W. Vaughan (McGraw-Hill, New York, 1978) sect. 9. [15] M.P. Thekaekara, in: Solid Energy Engineering, ed. A.A.M. Sayigh (Academic, New York, 1977) p. 37. [16] M.G. Calkin and P.J. Nicholson, Rev. Mod. Phys. 39 (1967) 361. [17] E. Gerlach and P. Grosse, FestkiSrperprobleme17 (1977) 157. [18] O.F. Shirmer, V. Wittwer, G. Baur and G. Brandt, J. Electrochem. Soc. 124 (1977) 749. [19] R.B. Goldner and R.D. Rauh, Proc. SPIE 428 (1983) 38; R.B. Goldner, D.H. Mendelsohn, J. Alexander, W.R. Henderson, D. Fitzpatrick, T.E. Haas, H.H. Sample, R.D. Rauh, M.A. Parker and T.L. Rose, Appl. Phys. Lett. 43 (1983) 1093; D.H. Mendelsohn and R.B. Goldner, J. Electrochem. Soc. 131 (1984) 857. [20] L. Kopp, B.N. Harmon and S.H. Liu, Solid State Commun. 22 (1977) 677. [21] D.W. Bullett, J. Phys. C 16 (1983) 2197. [22] F.P. Koffyberg, K. Dwight and A. Wold, Solid State Commun. 30 (1979) 433. [23] N.F. Mott, Metal-lnsulator Transitions (Taylor and Francis, London, 1974). [24] See, for example, G.D. Mahan, Many-Particle Physics (Plenum, New York, 1981).

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JIF. Owen, K.J. Teegarden and H.R. Shanks, Phys. Rev. B18 (1978) 3827, ,I.L. Ord, J. Electrochem. Soc. 129 (1982) 767. K. Miyake. H. Kaneko, N. Suedomi and S. Nishimoto, J. Appl. Phys. 54 (1983) 5256. P.G. Dickens, R.M.P. Quilliam and M.W. Whittingham, Mater. Res. Bull. 3 (1968) 94. M. Born and E. Wolf, Principles of Optics, 6th ed. (Pergamon, New York, 1980). J.S.E.M. Svensson and CG. Granqvist, Solar Energy Mater. 11 (1984) 29. K. Matsuhiro and Y. Masuda, Proc. SID 21 (1980) I01. J. Nagai and T. Kamimori, Japan. J. Appl. Phys. 22 (1983} 681: T. Kamimori, J. Nagai and M. Mizuhashi, Proc. SP1E 428 (1983) 51. [331 A. Nakamura and S. Yamada, Appl. Phys. 24 (1981) 55. [34] A.I. Gavrilyuk. V.G. Prokhvatilov and F.A. Chudnovskii, Fiz. Tverd. Tela 24 (1982) 982; English translation: Soviet Phys. Solid State 24 (1982) 558, [35] R. Viennet, J.-P. Randm and I.D. Raistrick, J. Electrochem. Soc, 129 (1982) 2451.