Inorganic oxide solid state electrochromic devices

Materials Science and Engineering, B26 (1994) 157-161

157

Inorganic oxide solid state electrochromic devices M. P. Cantfio*, A. Louren~o, A. Gorenstein, S. I. Cdrdoba de Torresi and R. M. Torresi Instituto de Fisica "Gleb Wataghin" Universidade de Campinas, CP 6165, 13083-970 Campinas, SP (Brazil) (Received September 6, 1993)

Abstract In this work, all-solid-state electrochromic devices are studied. The devices are multilayer systems, in which all layers are metallic oxides grown by r.f. sputtering. Two materials were investigated as the anodic coloring layer (NiO x and CoOx). The cathodic coloring material was WO 3. The electrolyte was either T i O / o r TaO x. Reflectance changes as large as 90% were obtained. Time responses and open-circuit behavior are also presented in this work.

I. Introduction

Electrochromic devices are systems with dynamic optical properties that can be tuned on to optimize the desired application [1-3]. These devices are potentially interesting in the energy efficiency control field (electrochromic windows or walls), in the image field (electrochromic displays) and in the automotive industry (car glasses or mirrors, the latter being already commercially available). The device is normally a multilayer system. In the "rocking-chair" concept [4], two layers are optically complementary. The main cause of electrochromism in inorganic oxides is an oxidation-reduction process of the cations (metal ions) in the oxide matrix [2]; a third layer should behave as the electrolyte. If the device is to operate in the transmissive mode, the optically active layers should be transparent at a defined oxidation-reduction state. The optically inactive layers such as the electrolyte, the electrical conductive layers and the top and bottom layers should also be transparent. In the reflective mode, one of the outer layers, normally an electrically conductive layer, should be a metal with high reflectivity. The whole system should be electrochemically compatible. Also, the different layers should be adherent, and good contact between the different interfaces should exist. Different systems have been proposed using allorganic layers [5], all-inorganic layers [1] or mixed systems [6]. By far the most used electrochromic layer is WO3, which is a cathodically coloring material, transparent in the oxidized state and deep blue in the *Permanent address: Physics Department, Universidade Estadual de Londrina, Londrina, Pr, Brazil.

SSD10921-5107(94)01065-P

reduced state [1, 6-10]. In inorganic devices, either an optically passive counterelectrode (such as transparent tin-doped indium oxide films [11]) or an anodically coloring material (such as NiO x films [6, 12, 13]) have been proposed. A variety of materials, such as the polymers polyl(ethylene oxide) and polyl(methyl methacrylate) [6, 14], RbAg4I 5 [15], MgF 2 [16] and the oxides Ta205 [1, 11] and C r 2 0 9 [17], have been proposed as solid electrolytes. In this work, we investigate the electrochromic behavior of all-solid-state devices, in the reflective mode. All layers, except the bottom and top layers, are inorganic oxides, grown by reactive r.f. sputtering. The reflective layer was an aluminum film. A new optically active material CoOx (an anodically coloring material [2, 18]) and a new electrolyte TiOx were studied. The electrochromic properties of a device with NiO~ as the anodically coloring layer and Ta20 5 as the electrolyte were also investigated. The cathodically coloring material in all devices was WO3.

2. Experimental details

The films were grown by r.f. reactive sputtering, using a BAE 250 machine (Balzers). The substrate was glass covered with transparent conductive tin-doped indium oxide (ITO) films. Table 1 shows the deposition conditions of the ITO films. ITO films grown under these conditions have a transmittance value T = 87% ( 2 = 5 5 0 nm) and a square resistivity R ~ = 2 0 Q, as measured by the four-point probe method. The other layers were grown following the conditions shown in Table 2. Devices A and B were ITO/ NiOx/TaOx/WO3/AI multilayer systems; devices C and Elsevier Science S.A.

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Inorganic' oxide solid state electrochromic devices

D were ITO/CoO~/TiOx/WO3/AI and I T O / N i O J TiOx/WO3/Al multilayers respectively. Suitable masks were used in order to avoid short circuits, on the one hand, and to facilitate electrical contacts and thickness measurements on the other hand. Figure 1 shows the configuration of the final device. Thickness measurements were made by profilometry (Alpha-step 220, Tencor Instruments). After the deposition of each layer, the deposition chamber was opened, and the target and masks were changed. Consequently, each layer was exposed to moisture for different times. In order to investigate the electrochromic behavior, a potential difference was applied between the ITO and the aluminum layers. Two kinds of experiment were performed. In the first, consecutive potential steps of 0.25 V were applied, beginning with the potential difference corresponding to the fully bleached device state. After current stabilization for each potential step, the device reflectance changes as a function of wavelength were recorded. The stabilized current corre-

T A B L E 1. Deposition conditions for tin-doped indium oxide Substrate

C o m i n g 7059 glass

Target Target-to-sample distance R.f. power Substrate temperature Atmosphere Oxygen flux Total pressure Thickness

90% In; 10% Sn 210 mm 50 W 400 °C Ar-O 2 1.7 standard cm3min- 1 7.0 x 10 -3 mbar 2000 A

sponds to a few microamperes because of not only an ohmic drop in the electrolyte but also internal short circuits probably caused by inhomogeneities in the electrolyte film thickness. In the second kind of experiment, the device was initially fully colored (or initially fully bleached), and a potential step to potentials corresponding to the fully bleached (or to the fully colored) state was applied; the monochromatic reflectance changes were then recorded during the bleaching (or during the coloring) process, as a function of time. In these experiments, a PAR 173 potentiostat and a Perkin-Elmer Lambda-9 spectrophotometer were used. All reflectance changes are relative to the fully bleached state of each device. Also, the open-circuit behavior of fully colored devices was analyzed.

3. R e s u l t s and d i s c u s s i o n

The in-situ reflectance changes were measured as described in Section 2. Figure 2 shows these results as functions of both wavelength (visible range) and applied potential, for device A (ITO/NiOx/TaOx/WO3/ AI multilayer). Figures 3, 4 and 5 show the same results for device B (ITO/NiOx/TaOJWO3/A1 multilayer), C ( I T O / C o O J T i O J W O 3 / A I multilayer) and D (ITO/ NiOx/TiOr/WO 3/AI multilayer) respectively. The measurements were performed by connecting the anodically coloring material (WO3) in the potentiostat working electrode plug. Since in all spectra the bleached device state (strongest reflectance) was considered to be the 100% value, the reflectance change decreases in the direction of negative potential values.

T A B L E 2. Deposition conditions (atmosphere, A r - O 2 for oxide films, A r for AI deposition; total pressure: 7 × 1 0 - ~ m b a r (all layers)) Device

Layer

R.f. power (W)

Oxygen flux (standard cm 3 m i n - a )

Deposition time (min)

Thickness (nm)

A A A A

NiO x TaO x WO 3 AI

100 100 100 250

2.0 2.0 5.0 --

10 15 3 6

80 480 100 400

B B B B

NiO x TaO x WO 3 A1

100 100 100 250

2.0 2.0 5.0 --

14 15 4 6

100 480 170 400

C C C C

CoO x TiOx WO 3 AI

100 300 100 250

2.0 1.8 5.0 --

6 25 3 6

90 280 100 400

D D D D

NiO x TiO~ WO 3 AI

100 300 300 300

1.8 2.0 1.8 --

20 180 10 13

170 400 350 700

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Inorganic oxide solid state electrochrornic devices

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Fig. 4. Device C (ITO/CoOx/TiOx/WO3/Al multilayer): reflectance change as a function of wavelength and applied potential difference.

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Fig. 3. Device B (ITO/NiO~/TaO~/WO3/AI multilayer): reflectance change as a function of wavelength and applied potential

difference. The spectral reflectance of a multilayer system depends on both the optical constants and the thickness of each layer, and the optical constants of the optically active layers depend on the applied potential.

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Fig. 5. Device D (ITO/NiOx/TiOx/WO3/AI multilayer): reflectance change as a function of wavelength and applied potential difference.

So, the maxima and minima as functions of wavelength in Figs. 2-5 should be attributed to optical interference effects, since none of the different layer materials has pronounced absorption maxima in the wavelength range analyzed. The maximum reflectance changes were observed for device A, which attained a AR value of 90% in the wavelength range 600-700 nm, in the fully coloured state ( E = - 2 . 0 V). Device B, which differs from device A only in the thickness of the optically active layers, showed in the same wavelength range a AR value of 70% (fully colored state). For this device, the maximum AR value was 80%, at ;t -- 530 nm and E = - 2.0 V. The fully colored state for device C (ITO/CoOx/ TiOx/WO3/AI multilayer (Fig. 4)) was attained after the largest potential step (i.e. a larger potential difference between the aluminum and ITO contacts), showing the highest ionic resistance of the TiO x electrolyte ._in comparison with the TaOx electrolyte used in

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Inorganic: oxide solid state electrochromic devices

devices A and B (and in spite of the lowest thickness of the TiO x layer). The ionic conductivity of this kind of electrolyte has been attributed to water incorporation during or after the deposition process [1]; with an applied potential, water dissociation gives rise to H + and O H - ions, which provides ionic conduction. The maximum AR was 40% in the wavelength range 700-800 nm. This value is much lower than that observed for devices A and B and should be attributed to the low optical contrast of cobalt oxide electrodes, in comparison with nickel oxide electrodes, since it has been shown that r.f. sputtered cobalt oxide films in the stabilized state have a maximum contrast of 20% for a film 100 nm thick [18]. Also, the reflectance changes are significant only for wavelengths larger than 500 nm and potentials more negative than - 0.4 V. The higher contrast of nickel oxide films, in comparison with those cobalt oxide films, is also evident from the reflectance changes obtained for device D (ITO/NiOJTiO~/WO3/A1 multilayer (Fig. 5)). In this case, the maximum AR was 60% for all wavelengths. Also, as the thickness of the TiO Xelectrolyte was much larger than in device C, a larger potential difference was needed in order to attain fully bleached and fully colored device states. Figure 6(a) shows the time responses of devices A and B for bleaching, and Fig. 6(b) those for coloring. The fully bleached or the fully colored state is attained after 700 s (device A). For device B, 1100 s are needed to attain the fully bleached or the fully colored state. These different time responses should be attributed to the existence of internal short circuits produced by inhomogeneities in the electrolyte film thickness, rather than to the different thicknesses of the optically active layers (WO 3 and NiOx) since in both devices the electrolyte has the same thickness. The presence of internal short circuits implies that not all the current is consumed by the electrochromic reaction, leading to larger time responses. Finally, Fig. 7 shows the open-circuit behavior of device D. The device was initially polarized at bleaching (or at coloring) potential for 10 min; at this stage, the voltage supply was stopped and the reflectance changes in the open-circuit conditions were followed as a function of time. No changes were observed if the initial state was the bleached state. If, however, the circuit was opened with the device at the fully colored state, a transition to the bleached state was observed. This phenomenon is a consequence of the fact that the bleached states of WO3 and NiO~ electrodes are thermodynamically stable. As the devices are not encapsulated, chemical reactions with oxygen from the atmosphere could take place, leading to bleaching of the system. Another reason that could explain the loss of coloration could be the presence of internal short

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circuits which provide electrical contact between both active layers, producing the oxidation of WO3 (deintercalation) and the reduction of NiOx (intercalation) so that bleaching of the multilayer system occurs.

4. Conclusions

In this work, the electrochromic behaviors of different devices were analyzed. The devices were multilayered systems, with layers made of metal oxides grown by the same technique (r.f. sputtering), which provide chemical compatibility and adherence. Reflec-

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Inorganic oxide solid state electrochromic devices

tance changes as large as 90% were obtained. R e s p o n s e times were d e p e n d e n t not only on the thickness of the active layers but also on inhomogeneities in the electrolyte film thickness. Work is in progress in order to optimize the devices.

Acknowledgments Financial support was provided by Fundaqfio de A m p a r o e Pesquisa do Estado de Silo Paulo, Conselho Nacional de Desenvolvimento Cientifico e Tecnol6gico and Financiadora de Estudos e Projetos (Brazil).

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