Optical measurements and modeling of an all solid state inorganic thin film electrochromic system

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Journal of Non-Crystalline Solids 218 (1997) 296-301

Optical measurements and modeling of an all solid state inorganic thin film electrochromic system G. Macrelli a,*, E. Poli a, H. Demiryont a, R. G~Stzelmann b a lsoclima SpA, R and D Dept., uia A. Volta 14, 35042 Este (PD), Italy b Leybold AG, R and D Dept., Wilhelm Rohn Strasse 25, D-63450 Hanau, Germany

Abstract Single layer optical measurements have been carried on for multilayer physical vapor deposited thin film electrochromic system. The measurements have has been performed for tungsten oxide (active electrochromic material) and vanadium pentoxide (counter electrode material) thin films and for indium tin oxide (transparent electric conductor material) thin film. The electrolyte layer was assumed dispersion free with rear refractive index n = 1.5 and imaginary part k = 0. The optical parameters, n and k, for each layer were measured in the visible and near infrared spectral regions. They have been calculated by the spectrophotometrically measured reflectance and transmittance and by the physical thickness measured using a stylus profilometer. The final five layer electrochromic system has been modelled in the colored state using a multilayer thin film calculation program. The complete electrochromic device has been spectrophotometrically described and comparison with the calculated results indicate that the single layer technique is not appropriate to describe, optically, the electrochromic coating. The optical model allows the design optimization of electrochromic systems for switchable energy saving glazings. © 1997 Published by Elsevier Science B.V.

1. Introduction Electrochromism and electrochromic devices have been studied in the literature [1-4]. Different types of devices have been proposed in the past and among them full solid-state devices can be considered promising as reversible switching optical shutters for windows. These variable transmittance devices allow modulation of luminous (mainly visible light transmission) and energetic properties (direct energy transmission and total solar energy transmission). The main functional features to be taken in account

" Corresponding author. Tel.: +39-429 55 788; fax: +39-429 600 021; e-mail: [email protected].

for building and automotive applications are the reduction of heating and cooling loads for thermal comfort and daylight control and glare reduction for visual comfort. W e will consider electrochromic systems with five layer structure based on two active materials, the electrochromic material which is a thin film of tungsten trioxide ( W O 3) and the counter electrode material which is vanadium pentoxide (V2Os), separated by an ion conductor material that we will consider optically not absorbing and not dispersive with a refractive index of 1.5. The other two materials are transparent conductors, namely thin films of indium tin oxide (ITO), that are connected to an external power supply to provide electronic negative charges to the active materials. The function of the external

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G. Macrelli et al. / Journal of Non-Crystalline Solid.; 218 (1997) 296-301

power supply is to maintain a constant potential difference between the active materials so that alkali ions, previously loaded in the structure, are forced to move from one material to the other until chemical activity (concentration) of the intercalated ions match the corresponding electrochemical potential. This coating structure is for a system in which lithium ions are intercalated and de-intercalated in tungsten oxide and in vanadium pentoxide according to the following electrochemical reactions: W O 3 + x e - + xLi + ~ Li xW03,

( 1)

Li~V205 ~'-->V205 + y e

(2)

+yLi+.

The deposition technology we have chosen is well established for small area (50 × 50 mm 2) applications (lenses for ophthalmic use or devices for precision optics). It consists of a plasma enhanced physical vapor deposition (PEPVD) technique based on thermal and electron beam evaporation sources with a plasma source that can assist the process. Lithium ions are dry loaded in the coating structure during the deposition process. We have introduced a controlled amount of lithium ions in the W O 3 layer by direct evaporation of lithium by an electron beam. In this study we will consider the optical properties of each active layer of a full solid state inorganic thin film electrochromic device. We will determine the refractive index (real and imaginary part) of the electrochromic and counter electrode material and of the transparent conductors. These data will allow the calculation of the overall spectrophotometric performances (transmittance and reflectance in the colored state) in the solar range (350 to 2500 rim). These calculations will be compared with the experimental result obtained on the complete electrochromic device.

2. Theoretical background Refractive index can be calculated for a material deposited as single thin film layer on a glass substrate, providing that normal transmittance and near normal reflectance are known and physical thickness

297

is also known [5,6], by solving the following system of two equations in two unknowns n and k:

R(n, k, d, ,q = R0x,

r(,,,

d, a) = rex.

(3)

The system can be solved iteratively starting from two estimated values (n o, k o) [5]. It is well known that this procedure has multiple solutions and generally the choice is easily constrained by other physical properties [6]. The advantage of this method is that only simple spectrophotometric and thickness measurements are needed. This method is straightforward when developing a new coating where measurements have to be carried out promptly to define material properties and accordingly deposition recipes. As a second step more sophisticated variable angle ellipsometry or variable angle radiometric techniques can be used to obtain more accurate results. In the framework of our research program it was important to verify if this simple method was suitable for the description of the five layer electrochromic coating. The calculation of the spectral transmittance and reflectance for a thin film multilayer is a straightforward matter and a computer program can be easily obtained from well known formulas [7].

3. E x p e r i m e n t a l procedure Single layer specimens were produced in 50 × 50 mm 2 size for transmittance and reflectance spectrophotometric measurements. Complete electrochromic devices were also produced in 50 × 50 mm 2 size for spectrophotometric measurements. All the coatings were deposited on glass substrates, B 270, previously cleaned by ultrasonic washing. The electrochromic coating was deposited using a commercial machine (APS 1104 Leybold AG, Hanau-Germany). The machine is basically an evaporation machine fitted with a thermal evaporation source, a multipocket (4 pockets) electron beam and a rotating crucible electron beam. A central plasma source allows plasma assistance (Argon ion bombardment of the film) during deposition, gas inlet control systems allow reactive evaporation; the ma-

G. Macrelli et al./ Journal of Non-Crystalline Solids 218 (1997) 296-301

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chine was equipped with two independent quartz monitors for rate control and one optical monitor. Detailed description of the machine and of the plasma source can be found in the literature [8-10]. Process parameters, deposition conditions and electrochemical characterization of the active materials (WO 3 and V205) are reported elsewhere [11]. The physical thickness of each layer was measured by a profilometer (Alpha Step 200 Stylus). Two spectrophotometers (Perkin-Elmer Lambda 9) were used for normal transmittance and near normal

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G. Macrelli et al. / Journal of Non-Crystalline Solids 218 (1997) 296-301 n

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In Figs. 1-4 transmittance and reflectance measurements are reported as solid lines, for ITO 297 nm, WO 3 317 nm, V205 191 nm and for the loaded Li0.2WO3 coating, respectively. The stoichiometric coefficient 0.2 corresponds to a surface density of charge for the dry loaded single layer of tungsten trioxide of 15 m C / c m 2. The surface density of charge measured on the complete electrochromic device was 19 m C / c m 2.

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reflectance measurements in the solar range (350 to 2500 nm). The loading level of WO 3 was determined electrochemically, integrating the current density measured in a potentiostatic voltage step response test.

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Table 1 Integrated photometric and energetic parameters of the complete device Parameter

Colored state

Bleached state

Tv(A ) a ~'v(D65) " An (nm) P pv(A ) a pv(D65 ) a AD (nm) P ~'E PE

13.8+0.5 14.3 +0.5 556.2 + 0.5 27.4+0.2 8.1+ 1 8.4__ 1 477.4 + 0.5 17.3+_0.4 11.7 _+0.5 9.7__+ 1

48.3+0.5 47.2+0.5 578.5 + 0.5 13.9+0.2 13.44- 1 13.8+ 1 499.8 +-0.5 8.5+_0.4 49.7 +_0.5 12.7+ 1

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L (nm) Fig. 7. Real (n) and imaginary (k) part of refractive index for V205 layer.

a Integrated luminous transmittance (~-) and reflectance ( p ) according to Illuminant A (A) and Illuminant D65 (D65).

300

G. Macrelli et aL / Journal of Non-Crystalline Solids 218 (1997) 296-301

100 I . . . . . . . . . . . . . . .

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. . . . . . .

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1000 1200 1400 1600 1800 2000 2200 2400 Wavelength (nm)

Fig. 10. Measured transmittance and reflectance for all the device in the colored and bleached state.

Figs. 5 - 8 report refractive indices (real and imaginary part) for the same coatings. From the refractive indices and thicknesses of materials, transmittance and reflectance has been calculated in the solar range for all single layers (Figs. 1-4 - dashed lines), for a complete electrochromic device without lithium in the W O 3 layer and for a loaded system (Fig. 9). Fig. 10 reports the spectral transmittance measured in the spectral solar range on the complete electrochromic device. Integrated luminous, energetic and colorimetric parameters for the complete device were calculated from the spectral curves and are reported in Table 1.

5. Discussion

From the reported results, single layer spectrophotometric measured transmittance and reflectance do not fit very well the calculated curves obtained from the complex refractive index. The only material that does not exhibit such differences is indium tin oxide. From complex refractive indices and thicknesses, spectral transmittance and reflectance for the complete ECD can be calculated (Fig. 9). In this calculation we have not assumed that loaded vanadium pentoxide is optically active. This assumption is obviously misleading for the values calculated in the bleached state, but it can be assumed for an optically passive counter electrode. The comparison of the calculated transmittance in the colored state with that measured in the same state for the complete device is not very satisfactory,

indicating again that the technique based merely on the single layer approach and resolution of system (3) is not, in this case, very appropriate [12]. This problem can be ascribed to different reasons: - single layers grown on glass substrates have generally different structures from the same layers grown on pre-existing films; - dry loading of the W O 3 single layer is actually different from the dry loading during the complete electrochromic device deposition because in the former case no electrolyte is placed on top of the loaded film and so loaded W O 3 comes directly in contact with atmosphere. We should also notice that a different surface density of charge has been measured in the two cases so that 4 m C / c m 2 seem to be 'trapped' or 'lost' in the single layer loaded WO3; - the multiple solutions problem [6,12] is a critical factor for the application of the characterization method to different materials. Recent works [13] indicates that satisfactory results can be obtained coupling variable angle ellipsometry to radiometric techniques. We think that, for future developments, these directions should be investigated.

6. Conclusion

Single layer spectrophotometric properties have been recalculated from the measured complex refractive indices and are not satisfactory. Calculated spectral transmittance and reflectance for the complete electrochromic device have been reported and compared with the corresponding measured values, again the calculated values are not satisfactory. This difference has been ascribed to several mechanisms: - the film structure may be influenced by the surface of the substrates; - dry loading of WO 3 single layer may result in a different film as compared to dry loading during the complete electrochromic device deposition; - the multiple solution problems inherent to the photometric technique used can be a critical factor when considered for different materials. Spectrophotometric characterization of the complete electrochromic device has been reported to-

G. Macrelli et al. / Journal of Non-Crystalline Solids 218 (1997) 296-301

gether with calculated integrated photometric and energetic parameters. References [1] C.M. Lampert, C.G. Granquist, eds., Large-Area Chromogenics: Materials and Devices for Transmittance Control, Vol. IS4 (SPIE Optical Engineering Press, Bellingham, WA, 1990). [2] M.K. Carpenter, D.A. Corrigan, eds., Electrochromic Materials, PV 90-2, The Electrochemical Society Proceedings Series (Electrochemical Society, Pennington, NJ, 1990). [3] K.C. Ho, D.A. MacArthur, eds., Electrochromic Materials II, PV 94-2, The Electrochemical Society Proceedings Series (Electrochemical Society, Pennington, NJ, 1994). [4] C.G. Granquist, Handbook of Inorganic Electrochromic Materials (Elsevier, Amsterdam, 1995). [5] F. Abeles, Advanced Optical Techniques (North-Holland, Amsterdam, 1967) ch. 5, p. 145.

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[6] J. Rivory, Le Vide 191 (1978) 77. [7] H.A. Macleod, Thin Film Optical Filters (Adam Hilger/J.W. Arrowsmith, Bristol, 1986). [8] K. Marl, W. Klug, A. Zoller, Mater. Sci. Eng. A140 (1991) 523. [9] S. Pongratz, A. Zoller, Ann. Rev. Mater. Sci. 22 (1992) 279. [10] S. Beil3wenger, R. Gtitzelmann, K. Matl, A. Zoller, 37th Annual Technical Conf. Proc., 1994, 1-878068-13-X - Society of Vacuum Coaters. [11] G. Macrelli, E. Poli, 'An all solid state inorganic thin film electrocbromic device: device fabrication, optical and electrochemical characterization', 190th Meeting of the Electrochemical Society, Electrochromic Materials and Their Applications III, San Antonio, TX, Oct. 1996, pp. 6-11. [12] L. Ward, The Optical Constants of Bulk Materials and Films (Adam Hilger/J.W. Arrowsmith, Bristol, 1988). [13] M. Rubin et al., 'Optical indices of lithiated electrochromic oxides', presented at SPIE's Int. Symp. on Optical Materials Technology for Energy Efficiency and Solar Energy Conversion XV, Freiburg, Germany, 1996.