Electrochromism of nickel-based sputtered coatings

Solar Energy Materials 16 (1987) 19-26 North-Holland, A m s t e r d a m

19

E L E C T R O C H R O M I S M OF NICKEL-BASED S P U T I ' E R E D C O A T I N G S J.S.E.M. SVENSSON and C.G. GRANQVIST Physics Department, Chalmers University of Technology, $412 96 Gothenburg, Sweden Electrochromic hydrated nickel oxide films were produced by reactive rf-magnetron sputtering followed by electrochemical treatment in KOH. Spectrophotometry was used to study the modulation of luminous and solar transmittance and to assess the durability, lSN nuclear reaction analysis indicated that coloration occurred upon hydrogen extraction. The investigated coatings appear to have very good properties for smart window applications.

1. Introduction and summary

This paper discusses electrochromism of hydrated nickel oxide coatings prepared by sputter deposition. We report on production technology, optical properties, durability, and a model for the electrochromism. The main result is the demonstration that the studied coatings seem to have excellent properties for applications on smart windows. Electrochromism has been studied for several years in the context of high-contrast nonemissive displays [1-3]. Currently [4-23], there are also vigorous research efforts to develop electrochromic coatings specifically for applications on smart windows [20], characterized by their ability to achieve energy efficiency by dynamic control of the radiant energy throughput. The work cited above [5-23] is confined to electrochromic tungsten-oxide based coatings which color under ion insertion (i.e., cathodically) according to the general relation [1] color

xM~+xe-+WO3-y

~

bleach

MxWO3 v,

(1)

with M + denoting ions and e electrons, and with 0 < x ~< 0.5 and y _< 0.03. Iridium oxide is another electrochromic material [2,3], with superior durability, which has been investigated in detail. The mentioned electrochromic materials may have problems for large-area and long-time applications in that tungsten oxide tends to dissolve slowly in aqueous electrolytes while iridium is very expensive, and there seems to be a need for other materials to be used on smart windows. One such alternative is hydrated nickel oxide, whose electrochromic property has been documented [24-26], whereas the adequate production technology, the optimum device performance, and the pertinent coloration mechanism are poorly known and understood. These issues are addressed below. Section 2 is devoted to the production of NiO x coatings by reactive rf-magnetron sputtering onto transparent and electrically conducting substrates. Electrochromic coloring and bleaching in a K O H electrolyte is studied by spectrophotometry in 0165-1633/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

J.S.E.M. Svensson, C.G. Granqvist / Nickel-based sputtered coatings

20

section 3, where we also demonstrate the excellent durability by extended color-bleach cycling. Section 4 reports on the luminous and solar transmittance versus the amount of charge injected into or withdrawn from the sample. It is found that the luminous transmittance can be altered gradually and reversibly from -~ 75% to _< 10%, while the solar transmittance goes from - 75% to _< 20%. The very large modulation of the optical performance is noteworthy. An earlier proposed [26] coloration reaction is Ni(OH)2

color

~

NiOOH + H ~+e

,

(2)

bleach

stating that hydrated nickel oxide colors by ion extraction (i.e., anodically). This reaction is given support by 15N nuclear reaction analysis as discussed in section 5

2. Thin film fabrication Thin films of NiO x were made by reactive rf-magnetron sputtering in a versatile deposition unit designed for research and demonstration purposes. The unit is described elsewhere [27,28]. The sputter system was evacuated to < 4 × 10 6 Tort by cryopumping. Oxygen (99.998% purity) was then introduced and kept al at constant pressure of 10 mTorr. Reactive sputtering was conducted from a nickel target (10 cm diameter; 99.5% purity). The rf power was 100 W at 13.56 MHz. Presputtering was performed for 5 min prior to thin film deposition onto unheated substrates positioned 5 cm below the target. Typically, sputtering was carried out for - 20 min in order to produce a - 0.2 ~ m thick NiOx coating. The substrates were 2.5 × 2.5 cm 2 Corning 7059 glass plates precoated with transparent and electrically conducting In 203 : Sn. The latter coating was prepared by reactive dc-magnetron sputtering from an I n - S n target according to a procedure

TGALVANOSTATT ..Ox.. pt . ~

~ ~

In203:Sn glass

COUNTER ELECTRODE KOH~ ELECTROLYTE Fig. 1. Arrangement for coloring and bleaching of an electrochromic hydrated nickel oxide film on In203: Sn-coated glass. The sample is mounted so that it forms the working electrode in an electr~x'hem ical cell containing K O H and a Pt counter electrode.

J.S.E.M. Svensson, C.G. Granqvist / Nickel-based sputtered coatings

21

developed in our laboratory [27]. The properties of these sputtered In203 : Sn films were similar to those of e-beam evaporated In203 : Sn, earlier studied by us [29-31], except that the luminous absorptance was somewhat larger. Normally, the sputtered In203 : Sn films were 0.07 ~m thick and had a resistance/square of - 40 ~2. Film thicknesses were recorded by a mechanical stylus instrument. Fig. 1 illustrates a typical sample configuration. Coloring and bleaching was accomplished in an electrochemical cell containing 1M KOH, as shown in fig. 1. The coated glass substrate formed the working electrode with an active area of 4.25 cm 2. A platinum sheet, 1.5 × 2.0 cm 2 in size, was used as counter electrode. A galvanostat was connected to the electrodes. After an initial bleaching of the sample by charge injection, it was colored to successively darker appearance by extracting charge at a constant current of 0.1 mA. When a desired charge had been extracted, the sample was removed from the electrolyte, rinsed in distilled water, and blown dry with filtered air. It was then ready for optical measurements. The samples could be put back into the cells repeatedly in order to extract more charge.

3. Spectrophotometry and durability tests Transmittance T and reflectance R were measured for the coated substrates as a function of wavelength. We used a Beckman ACTA M V I I double-beam spectrophotometer with reflectance attachment. Fig. 2 shows transmittance for normal incidence and reflectance for 10 ° off-normal incidence in the 0.35 < ?~ < 2.5 ~tm range

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Numberof cycles

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~2x104

~ T _ _

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,,1 ~ - - ~ . . . . . . . . . //

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1 1.5 WAVELENGTH (pm)

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Fig. 2. Spectral transmittance T and reflectance R for an electrochromic hydrated mckel oxide fihn on [n203: Sn-coated glass (cf. fig. 1). The curves refer to different magnitudes of extracted charge and number of color-bleach cycles.

22

J.S.E.M. Svensson, C.G. Granqvist / Nickel-based sputtered coatings

for the sample sketched in fig. 1. The oscillations are indicative of optical interference. The drop at X _< 0.4 Fm is due to absorption in the glass. The dotted curves refer to the initial bleached film. It is seen that the transmittance is moderately high while the reflectance is moderately low. In the mid-luminous range (~ = 0.55 p.m) we have T = 75% and R = 15%. The durability of the coated glass is of obvious importance for practical window applications. One of the salient properties is the ability to survive extended color-bleach cycling. We tested this property on samples, immersed in electrolyte in the electrochemical cell, by connecting them to a function generator giving ~-1.35 V during coloring and - 0 . 9 5 V during bleaching relative to the Pt counter electrode. The cycle time was 30 s. Optical measurements of transmittance and reflectance were performed after - 1 0 4 and - 2 × 104 cycles. The solid curves in fig. 2 refer to a bleached sample after - 104 cycles. The optical data are in good agreement with those measured before the cycling (dotted curves), which point at the excellent durability. We also left samples in the spectrophotometer for hours without noting any sign of deterioration. Coloring was accomplished by charge extraction. The dashed curves in fig. 2 were obtained after heavy coloration by extraction of 150 mC. The transmittance is very low at short wavelengths and increases monotonically so that it reaches - 40% at X ~ 1.3 Fm. Beyond this wavelength it remains rather constant. The reflectance is also altered by the charge extraction. In the mid-luminous range we find T - - 12% and R -- 6%.

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Symbol

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50

100

EXTRACTED

CHARGE ( m e )

1

150

J

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200

Fig. 3. Integrated luminous (open symbols) and solar (filled symbols) transmittance for electrochromJc hydrated nickel oxide films on In203: Sn-coated glass (cf. fig. 1). Squares, triangles and circles refer to 0, 104 and - 2 × 104 color-bleach cycles, respectively. The curves are drawn only as a guide to the eye.

J. S.E.M. Svensson, C.G. Granqvist / Nickel-based sputtered coatings

23

4. Luminous and solar transmittance In order to assess the performance of the hydrated nickel oxide coating on windows, we must introduce suitably averaged transmittance values. The needed quantities are the integrated luminous (lum) and solar (sol) transmittance, defined by

Tlum(sol) =

fdXq,,um
(3)

where ~lum is the standard luminous efficiency function for photopic vision [32] and ~sol is the AM1 solar spectrum [33] (referring to the sun at zenith). Spectral data on transmittance, analogous to the curve in fig. 2, where used to plot Tlum and T~oI as a function of the amount of extracted charge. Fig. 3 shows that Tinm can be varied between - 7 5 % and - 2 0 % . The transmittance goes down monotonically with increasing charge extraction and the stated minimum values pertain to 200 mC. The rate of change for Tlum and T~oI is largest at small amounts of extracted charge but is still not zero at 200 inC. In fact, a further lowering of the transmittance was observed at 500 inC.

5. Electrochromism: A model and its support by nuclear reaction analysis Hydrated nickel oxide electrodes are known to have a remarkable cycling durability in alkaline electrolytes. This has been interpreted on the premise that a highly reversible redox transfer takes place between well defined active species. A general reaction scheme, originally proposed by Bode [34], reads [~-Ni(OH)2 ~- I3-Ni,OOH + H + + e ,

\

c~-Ni(OH)2"t:~

,{-NiOOH + H + + e ,

(4)

where dashed arrows indicate partial reactions. Clearly, reaction (4) is a more detailed version of reaction (2). A thorough discussion of the chemistry and electrochemistry of hydrated nickel oxide is outside the scope of this paper; it can be found in several reviews [35-41]. These also discuss the crystal structures of the cx, 13 and y phases indicated in reaction (4). In order to support the above mechanism for the electrochromism by experimental evidence, we employed 15N nuclear reaction analysis [42] by using facilities of the Uppsala Tandem Accelerator Laboratory. This analysis is based on the nuclear reaction 15N + 1 H ---~12 C* + 4 H e L, 12 c + ,{(4.43 MeV).

(5)

Clearly the amount of hydrogen is proportional to the intensity of the 4.43 MeV y radiation. The nuclear reaction is resonant, which makes it possible to obtain the hydrogen depth profile [42]. In the experiment, indicated in fig. 4, the sample was

24

J.S.E.M. Scensson, C.G. Granqvist / Nickel-based sputtered coatings

15N+ 7 MeV NiOxHy ~ It0.095 pm In203:Snl I ,~O.07pm glass ~!i~ii~] 4.43MeV

I

I "a' detector

Fig. 4. Experimental configuration for tSN nuclear reaction analysis of an electrochromic hydrated nickel oxide film on InyO3: Sn-coated glass

bombarded with - 7 MeV 15N ions and the emitted y-quanta were counted on a NaI detector. Fig. 5 shows the hydrogen concentration versus depth below the surface of a 0.095 lxm thick electrochromic hydrated nickel oxide film in the bleached state and after heavy coloration by extraction of 200 mC. The concentration is -. 2 ><. 10 22 a t o m s / c m 3 in the bleached state and - 1.2 × 1022 a t o m s / c m 3 in the colored state over the cross-section of the film. The underlying I n 2 0 3 : S n film contains little hydrogen, which is to be expected. The given hydrogen concentrations are consistent

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COLORED SAMPLE

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1

z

~ 1.0 z o

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u

I

z ~0.5 o

•1- 0 0

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40

60 80 100 DEPTH (nm)

Fig. 5. Hydrogen density versus depth for a bleached (open circles) and heavily colored (filled circles) electrochromic film of hydrated nickel oxide (cf. fig. 4).

J.S.E.M. Svensson, C.G. Granqvist / Nickel-based sputtered coatings

25

with reactions (2) and (4) and support the view that coloring occurs via hydrogen extraction. However, a fully quantitative test is hampered by the widths of the error bars on the hydrogen profiles, the uncertainty in the calibration of the concentration scale, and the lack of knowledge of precise coating density and structure. Furthermore, nuclear reaction analysis takes place with the sample in vacuum, and hence a loss of water - which may be different for colored and bleached samples - adds uncertainty.

Acknowledgements The nuclear reaction analysis was performed by Mr. Tore Eriksson, Physics Department, Uppsala University, Uppsala, Sweden. We are grateful to Dr. V. Wittwer, Freiburg, FRG, for supplying us with ref. [25], and to Dr. C.M. Lampert, Berkeley, USA, for a preprint of ref. [26]. Financial support was received from the Swedish Natural Science Research Council and the National Swedish Board for Technical Development.

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J.S.E.M. Svensson, C. G. Granqvist / Nickel-based sputtered coatings

[23] A.P. Schuster, D. Nguyen and O. Caporaletti, Solar Energy Mater. 13 (1986) 153. [24] J.D.E. Mclntyre, W.F. Peck and G.P. Schwartz, Abstracts of the 21st Electronic Materials ConL (Boulder, Colorado, 1979) paper D-4. [25] P. Schlotter, L. Pickelmann, Ch. Vogel, H. Beibl and H. Strinitz, Bundesministerium ftir Forschung and Technologie (FRG), Forschungsbericht BMFT-FB-T 84-301 (December 1984). [26] C.M. Lampert, T.R. Omstead and P.C. Yu, Solar Energy Mater. 14 (1986) 161. [27] S.-J. Jiang and C.G. Granqvist, Proc. SPIE 562 (1985) 129. [28] T.S. Eriksson and C.G. Granqvist, J. Appl, Phys. 60 (1986) 2081. [29] I. Hamberg, A. Hjortsberg and C.G. Granqvist, Appl. Phys. Lett. 40 (1982) 362. [30] 1. Hamberg and C.G. Granqvist, Proc. SPIE 428 (1983) 2; Appl. Phys. Lett. 44 (1984) 72! [31] 1. Hamberg and C.G. Granqvist, J. Appl. Phys. 60 (1986) R123. [32] G. Wyszecki and W.S. Stiles, in: Color Science, 2nd ed. (Wiley, New York, 19821 p. 25~, [33] M.P. Thekaekara, in: Solar Energy Engineering, ed. A.A.M. Sayigh (Academic, New York. 19771 p. 37. [34] H. Bode, K. Dehmelt and J. Witte, Electrochim. Acta 11 (1966) 1079. [35] P.C. Milner and U.B. Thomas, in: Advances in Electrochemistry and Electrochemical fmgmeenng, ed. C.W. Tobias (Interscience, New York, 1967) Vol. 5, p. t. [36] J.P. Hoare, The Electrochemistry of Oxygen (lnterscience, New York, 1968) p. 271 [37] S.U. Falk and A.J. Salkind, Alkaline Storage Batteries (Wiley, New York, 1969). [38] G.W.D. Briggs, Chem. Soc. Spec. Period. Rep., Electrochem. 4 (1974) 33. [39] A.J. Bard, editor, Encyclopedia of Electrochemistry of the Elements (Dekker, New York~ ! 975) Vol. 3, p. 349. [40] U. Falk, in: Electrochemical Power Sources. ed. M. Barak (Peter Peregrinus Ltd.. Stevenagc, 1980) p. 324. [41] P. Oliva, J. Leonardi, J.F. Laurent, C. Delmas, J.J. Braconnier, M. Figlarz, F. Fievet ',rod A. de Guibert, J. Power Sources 8 (1982) 229. [42] J.W. Mayer and E. Rimini, Ion Beam Handbook for Material Analysis (Academic, New York. 1977).