2636 World Renewable Energy Congress VI (WREC2000)

© 2000 Elsevier Science Ltd. All rights reserved. Editor: A.A.M. Sayigh

MONOLITHIC ELECTROCHROMIC DEVICES: TANTALUM OXIDE THIN FILM IONIC CONDUCTORS

M G Hutchins, N S Butt and A J Topping Solar Energy Materials Research Laboratory, School of Engineering,

Oxford Brookes University, Oxford OX30BP, United Kingdom

Abstract

Tantalum oxide thin films prepared by reactive magnetron sputtering are investigated for their potential use as the ionic conducting layer in all solid state monolithic electrochromic devices.

Introduction

The first generation of commercially available electrochromic devices, developed for use as a variable transmission glazing in buildings, are laminated and employ polymer electrolytes as the ion conducting medium 1'2. Such a device normally employs 2 glass substrates and the device is assembled from the 2 respective halves: one employing the active electrochromic layer, e.g. a-WO3, and the second the counter electrode, or ion storage layer.

An all solid state monolithic structure, built upon a transparent substrate would

1. Confer low emittance properties to the device through the final layer deposition of a transparent conducting thin film, such as indium tin oxide (ITO)

2. Raise the overall transmittance of the device in its bleached state by eliminating the need for a 2 nd glass substrate and increase dynamic range.

In this paper we presents results for thin films of tantalum oxide (Ta205) prepared by reactive magnetron sputtering which form part of an ongoing programme to develop monolithic electrochromic devices. The paper focuses on the influence of Ta205 coatings on the electrochromic response of amorphous tungsten oxide (a-WO3), the ionic conductivity of Ta205 and the mechanism of charging, and associated charge losses, during monolithic device fabrication.

Experimental

Thin films of tantalum oxide were prepared by reactive rf magnetron sputtering using techniques described previously 3. Ta205 coatings of thickness 5 0 - 160 nm were deposited on (a) Pilkington KGLASS and (b) a- WO3 films deposited on KGLASS by reactive sputtering 4. Representative deposition conditions are shown in Table 1.

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Ar/O2

%

Pressu re

mTorr

Power

W/cm 2

Thickness

am

a-WO3 5 10 1.3 250

Ta205 10 10 1.1 100

Table 1. Deposition conditions for the sputtering of a-WO3 and Ta205 films on Pilkington KGLASS.

The electrochemical response of a-WO3 and a-WO3/Ta205 films deposited on K GLASS was studied by cycling the films in a 0.1 M H2SO4 electrolyte versus a SCE 5. In situ optical properties were recorded using an optoelectrochemical cell accessory developed for use with a Bruker IFS 66 spectrometer 5.

Results

Fig. 1 compares the cyclic voltammograms recorded for a- WO3 and a- WO3/Ta205. The corresponding change in spectral transmittance of the 2 cases is shown in Fig. 2. In the case of a-WO3 -16 mC/cm 2 charge is inserted and for a-WO3/Ta205---15 mC/cm 2. The Ta205 overcoating does not impair the electrochromic properties of a-WO3; the charge capacity and charged state visible transmittance are both very similar.

°6 f Q4

-Q2

c~ 436

418-

I ' I ' I ' I ' I

, ~ K ~ a - V k Q 3 // . . . . . %

-1.0 ' -Q5 ' ({0 ' (~5 ' 1~.0 ' 1~.5 Vo~nm 6/versus SeE)

Figure 1. Cyclic voltammograms of K GLASS /a-WO3 and K GLASS/a- WO3/Ta205, cycled in 0.1 M H2804 versus SCE.

The K GLASS/a-WO3 and K GLASS/a-WO3/Ta205 samples are charged at constant current (1 mA) in 0.1M H2SO4 and voltage and resistance were measured at different charge insertion levels. The dc resistance of the 2 samples during charge insertion is compared in Fig. 3. The effective ionic

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conductivities of a-WO3 and a-WO3/ma205, calculated from these data are shown in Table 2.

Q8

Q6

I- Q4

02

Q0

. . ' " . - . . . . . . . . . . . . .

/ , / . . . . . To (~vv~ / ~ ........ T~(a-V~

// . - - . . ....... Tas (a-V~Ta205) 1"," ' ......... "-'..;'-, - ........ Tc (aVkO~a205)

i. ..... .-..,:.,.,.,.:. ....... Q4 Q5 Q6 Q7 Q8

V ~ v e l ~ 0zn)

Figure 2. Visible transmittance modulation of K GLASS/a-WO3 and KGLASS/a-WO3/Ta205.

I " I ' I I ' I I " I

1800-

~,~_ 1400-

.~ l:a:).

830.

830-

• K GLASS/a-W33 • K GLASS/a-V~Tra20 5

i 0 ~ ~ ~ ~0 ~ 40 Time (sec)

Figure 3. Resistance of the K GLASS/a-WO3 and K GLASS/a-WO3/Ta205 samples during charge insertion at constant current.

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Q (mC/cm 2)

10 20 30

KGLASS /a-WO} R (~) 1050 1260 1400

40 Table 2.

1520

(3"

(S.cm 1) 0.24 x 10 -7 0.20 x 10 .7 0.18 x 10 -7 0.16 x 10 -7

K GLASS / a-WO3 / Ta205 Q (mC/cm 2)

10 20 30 40

R (~) 1180 1420 1580 1720

(:3"

(S.cm -1) 0.29 x 10 "7 0.25 x 10 -7 0.22 x 10 .7 0.20 x 10 .7

Ionic conductivities of a-WO3 and a-WO3/ma205.

To estimate the ionic conductivity of the Ta205 films, the average resistance difference of a-WO3 and a-WO3/ma205 is used. The ionic conductivity of Ta205 is estimated to be 0.59 x 10 .7 S.cm -1. This value is close to the conductivities of reported 6 Ta205 films made by several techniques (1.5 x 10 -5 to 3 x 10 °6 S.cml).

For monolithic device construction, the films are charged with cations and returned to the vacuum chamber for subsequent deposition of the remaining component layers. During subsequent pump-down some of this charge is lost. If the charge is inserted into a-WO3 and the films returned to the vacuum chamber for deposition of the Ta205 and subsequent layers, the charge loss is much greater than when the charge is initially inserted through the Ta205 layer. Fig. 4 shows that whereas more than half of the chrage inserted into an a-WO3 film is lost within one hour of pump-down, this figure is reduced to less than 10% if the charging is performed following the deposition of the ion conducting layer. Thus the Ta205 passivates the chemical reactions associated with charge loss.

80.

6 0 =

0 40-

~ao.tm atlaired (~ Torr)

36 25 15 9 I ' I ' I • ' I

I I • a-V~3 / TazO ~

20-

110 ' I • I • I • I

lime (rnin)

Figure 4. Comparison of percentage charge lost in vacuum as a function of time for charged a-W03 and a-WO3/Ta205 coatings when the charge is inserted through the last deposited layer.

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Figs. 5 and 6 investigate the influence of the thickness of the Ta205 coating on the electrochromic properties of a-WO3. Ta205 films in the thickness range 50 - 160 nm were prepared. Fig. 5 shows the cyclic voltammograms of 3 Ta205 coated a-WO3 films. The thickness of the ion conducting layer does not impair the electrochromic response. Similarly, it can be seen from Fig. 6 that the charge loss during pumpdown is also essentially independent of the Ta205 film thickness. The results give valuable insight into monolithic device design.

I • I • I • i i

Q 6 - • . / ,x. , \

Q4

02

QO

'~_,, -Q2 . (56nm)

- ..~'~ ,7' ....... ~.~-wo~o~(~l -Q4 .i;~ .~ .... ~'ssav~rra2°~(~Onrn)

U -Q6

-Q8

-1 .0 ' ' ' ' ' ' ' ' ' ' -1.0 -£15 Q0 Q5 10. 15.

V~ag~vesu~ S(::E

Figure 5. Comparison of the voltammetric response of a - W O 3 overcoated with respectively 56 nm, 105 nm and 160 nm Ta205.

K (3.A~ I a-V~ I Taz O 5

In d I cas~s ~acuLrn reEct'ed d : ~ 106 To'r

14 • , , , , , , • , • ,

&

12- &

10-

8 - •

~" 6-

0 4-

• kept Lrda" vaoJ.mfor lhr 2 • I~l:t Ln~ vmJ.m # 1hr&40min

& i ~ t Lr i~vactumf~ 2ITs

0

- 2 , i , i , i , i • i , i

40 60 80 100 120 140 160

T a 2 0 5 th ickness (nm)

Figure 6. Comparison of influence of Ta205 film thickness on charge lost in a-W03 films held in vacuum for different periods of time.

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Finally we note that for monolithic device construction the final transparent conducting layer is required to be deposited "cold" to complete the stack. In Fig. 7 the infrared reflectance of a cold sputtered indium tin oxide film with sheet resistance Rs = 36 ~/ and thermal emittance ~ = 0.40.

0.7

0.6

~ 0.5

~ 0.4 m

0.3

{ ~ 0.2

0.1

0 2 4 6 8 10 12 14

Wavelength (microns)

Figure 7. Infrared spectral reflectance of cold sputtered indium tin oxide employed as the front surface transparent conductor in monolithic electrochromic devices.

Conclusions

Tantalum oxide films have been prepared by reactive rf magnetron sputtering and exhibit properties suitable for use as the ionic conducting layer in monolithic electrochromic devices. The dependence of the electrochromic response and related charge loss processes have been studied for Ta205 films of different thicknesses prepared on a-WO3 electrochromic films. In addition indium tin oxide films suitable for use as the top layer transparent conducting oxide have been successfully prepared by "cold" sputtering.

References

1. Hutchins M G, Smart Windows, International Energy Agency Solar Heating & Cooling Programme, Annual Report pp7-18, 1995.

2. Gallego J, Hutchins M G, Owen J and Anderson S, The Development and Use of Variable Transmission Electrochromic Glazings, 4th European Conference Solar Energy in Architecture and Urban Planning pp545-548, 1996.

3. Butt N S, Hutchins M G, and Topping A J, Electrochromic Materials And Devices For Solar Gain Control In Buildings, Proc. UK International Solar Energy Society 25th Jubilee Congress, Brighton, May 1999, pp 289-292.

4. KGLASS, Registered Trademark, Pilkington, UK 5. M G Hutchins, N S Butt, and A J Topping, Electrochromic Tungsten Oxide Thin Films:

Temperature Dependence of the Charge Capacity and Refractive Index, World Renewable Energy Congress 2000, Brighton, UK, July 2000.

6. C.G. Granqvist, Handbook of Inorganic Electrochromic Materials, Chapter 26, Elsevier, Amsterdam, 1995