Optical absorption and durability of sputtered amorphous tungsten oxide films

www.elsevier.com/locate/ssiSolid State Ionics 165 (2003) 51–58

Optical absorption and durability of sputtered

amorphous tungsten oxide films

Lars Berggren*, Gunnar A. Niklasson

Department of Materials Science, The Angstrom Laboratory, Uppsala University, P.O. Box 534, SE-751 21, Uppsala, Sweden

Abstract

Amorphous tungsten oxide films were made by sputtering onto glass substrates that were coated with conductive tin doped

indium oxide (ITO). The films were deposited at different O2/Ar gas flow ratios and different substoichiometric compositions

was determined by Elastic recoil detection analysis (ERDA). Substoichiometric as-deposited tungsten oxide is transparent

above a particular oxygen content and is blue below that content. This indicates that there are at least two kinds of defects in the

substoichiometric films. The oxygen vacancies may be coupled to W5 + sites, giving rise to strong absorption, or to (W–W)10 +

complexes in the transparent films. Lithium ions were electrochemically intercalated at several charge levels. At each level the

transmittance and reflectance were measured in the wavelength range between 0.3 and 2.5 Am. We show that as-deposited blue

films and intercalated transparent films display similarly shaped optical absorption bands. Electrochromic devices were made by

laminating the tungsten oxide films with sputtered Ni–Voxide deposited on ITO-coated plastic substrates. The durability under

electrochemical cycling was best for the case of very substoichiometric WO2.63 films.

D 2003 Elsevier B.V. All rights reserved.

PACS: 78.20.Jq; 81.15.Cd

Keywords: Sputtered amorphous tungsten oxide; Lithium intercalation

1. Introduction tion of charged species. Devices can be made using

Electrochromic (EC) materials have high applica-

tion potential in the optical industry because of their

well-controlled absorptance modulation [1]. In win-

dow applications, a gradual and reversible modulation

from a transparent state to a dark neutral color, is

desirable. Most of the suitable materials are oxide

compounds; they change color upon insertion/extrac-

0167-2738/$ - see front matter D 2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.ssi.2003.08.016

* Corresponding author. Tel.: +46-18-471-6215; fax: +46-18-

50-01-31.

E-mail address: [email protected] (L. Berggren).

one EC material and a transparent ion storage material

that does not color upon ion insertion or extraction.

However, it is usually more effective to use two

complementary EC materials. In this paper we con-

centrate on EC devices for smart windows.

A smart window device consists of two comple-

mentary electrochromic materials; one cathodic (col-

ors upon positively charged ion insertion) and one

anodic (colors upon extraction of postitively charged

ions) [1,2]. Thin layers of the EC materials are depos-

ited onto transparent substrates that are covered with

transparent electron conducting films. The comple-

mentary half-cells are laminated together with an

L. Berggren, G.A. Niklasson / Solid State Ionics 165 (2003) 51–5852

ion-conducting electrolyte. Glass or plastic is often

used as substrates, tin doped indium oxide (ITO) as the

electron conducting films and an ion-conducting poly-

mer as the electrolyte for the devices. The EC materials

become colored when a suitable potential is applied

between the films and become transparent when the

potential is reversed.

Among the most studied compounds are iridium-

and nickel oxide that exhibit anodic coloration, and

tungsten oxide that exhibits cathodic coloration.

Amorphous tungsten oxide is often nonstoichiometric,

but shows very good EC properties [3] and is suitable

in most devices. It is of importance to understand the

influence of oxygen deficiency on the absorption,

cyclic lifetime and coloration/bleaching speed of devi-

ces containing a-LixWOy.

In this report we describe the preparation and

characterization of WOy in terms of deposition meth-

od, analysis of thickness, composition and disorder,

and electrochemical intercalation procedure (Section

2.1). Further on, a WOy–NiO:V device construction

is described in Section 2.2. Section 3.1 describes how

the optical measurements are carried out for WOy

films while Section 3.2 explains the measurements on

the devices. Results from optical properties and life-

time tests will be discussed in Section 4. Finally we

summarize the work in Section 5.

2. Sample preparation and characterization

2.1. WOy films

The tungsten oxide films were made by reactive dc

magnetron sputtering. A turbo molecular pump

connected to the sputter chamber gave a background

pressure of below 10� 6 Torr after 8 h baking at 120 jCbefore the depositions. The films were deposited on

45� 45 mm glass substrates that were covered with

transparent, 40 nm thick and 60V per square tin doped

In2O3 (ITO). Sputtering took place from a 5-cm-

diameter W (99.99%) target in Ar and O2 gas. The

O2/Ar gas flow ratio (C) was varied from 12% to 36%.

Both the discharge power and the total pressure were

controlled. They were set at 200 W for the power and

20 mTorr for the pressure.

For all films the thickness (d) was obtained by a

Tencor Alpha step surface profilometer. The thick-

nesses were 310F 25 nm and the deposition rate

0.2–1 nm/s.

With a Siemens D5000 diffractometer, X-ray dif-

fraction measurements were performed at angles be-

tween 10j and 80j. However, no diffraction peaks

could be observed. We conclude that the films are

amorphous in the X-ray validity range.

From Elastic Recoil Detection Analysis (ERDA)

we have determined the densities and the compositions

of the Woxide films. Iodine ions with 50 MeVenergy,

and a detection angle of 45j were used for this

purpose. The compositions have been calculated as

an average over the atomic depth profile. Fig. 1

compares the stoichiometry number ( y) in WOy as a

function of O2/Ar gas flow ratio (C) for samples in this

work with data from literature obtained by other

methods [4–6]. All our films in Fig. 1 were sputtered

at the same pressure (20 mTorr) and power (200 W).

Our composition values versus O2/Ar ratios are in

general agreement with most of the earlier works [7]. It

should be noted that X-ray photoelectron spectroscopy

(XPS) and Auger spectroscopy (AES) are surface-

sensitive techniques which give the composition of a

surface layer. The AES results appear to be of low

accuracy, as evident from the large scatter in these

data. The figure shows that the O/W ratio is slightly

below 3 at 20% oxygen relative to argon and that the

O/W ratio seems to go asymptotically towards 3 with

increasing oxygen gas flow. Our films are visually

transparent when C>16%. A larger increase in O/W

ratio occurs between O2/Ar ratios of 0% and 20%.

There is a window in the region 12%VCV 16%

where as-deposited films have blue color. The films

look dark metallic at a sufficiently low oxygen gas

flow (C < 12% in our case).

Li+ ions and charge balancing electrons were

inter/deintercalated into the films in an Ar filled

glove box with water content less than 4 ppm. A

standard three-electrode arrangement was used with

the W oxide sample as the working electrode and Li

foils serving as counter and reference electrodes.

The W oxide films were electrochemically lithiated

in 1 M lithium perchlorate (LiClO4), containing less

than 2% water, that was dissolved in propylene

carbonate (PC) ( < 0.02% water). All the electro-

chemical measurements were controlled by the use

of an ECO Chemie Autolab/GPES with electrochem-

ical interface. Precyclic voltammetry was performed

Fig. 1. The O/W ratio as a function of sputter O2/Ar gas flow ratio for the WOy films in this work and as obtained in Refs. [4–6]. The analysis

methods were Elastic Recoil Detection Analysis (ERDA), X-ray Photo electron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES).

Fig. 2. A comparison of the optical absorption coefficient as a function of energy for the as-deposited samples WO2.63 and WO2.89, and the Li-

intercalated Li0.03WO2.89 and Li0.10WO2.89.

L. Berggren, G.A. Niklasson / Solid State Ionics 165 (2003) 51–58 53


for five cycles to ensure charge reversibility before the

intercalations.

2.2. WOy–NiO:V devices

The cathodic tungsten oxide films were prepared

and characterized in the same way as explained above

and anodic vanadium doped Ni oxide was used as the

complementary EC material. This material has earlier

been shown to exhibit a very good performance for

electrochromic applications [8]. All of the W oxide

films were colored with a charge density of 12 mC/

cm2 (xc 0.14) before lamination with the NiO:V

films. The vanadium doped nickel oxide films were

sputtered onto ITO covered transparent plastic sub-

strates. The target was made of 99.95% nickel–

vanadium with a composition of 93% nickel and 7%

vanadium. During sputtering additional H2 gas was

added to the O2 and Ar. The H2/O2/Ar ratio was

1:3:10, the sputter pressure was 19 mTorr and the film

thickness approximately 200 nm. Coloration of all the

NiO:V samples was done at the same time in ozone

Fig. 3. Optical absorption coefficient of ‘‘deintercalated’’ WO2.63 as a fun

potential.

for 3 min. The charge density in the films was

electrochemically estimated by comparison with in-

tercalation in KOH electrolyte to be approximately 15

mC/cm2 [9]. Lamination with a lithium containing

polymer electrolyte was done using a desk laminator.

Sealing of the device was performed with epoxy glue

and aluminum tape was attached to the ITO to assure a

good contact.

3. Optical measurements

3.1. WOy films

A double beam Perkin-Elmer lambda 9 spectro-

photometer was used for measuring both total trans-

mittance (T) and reflectance (R) at wavelengths from

0.33 to 2.5 Am. The instrument was equipped with an

integrating sphere that was coated with highly diffuse

white Barium sulphate (BaSO4). For reflectance meas-

urements, a BaSO4 plate was used as a reference and a

black cone for zero level correction. The absorption

ction of energy. Time intervals of 500 s have been applied at each


coefficient spectra a(k) in the visible above mentioned

range can be calculated using the T(k) and R(k)spectra, and the thickness value d: [10]

aðkÞ ¼ 1

dln1� RðkÞTðkÞ

The expression is a good approximation for refrac-

tive indices between 1.3 and 2.5 of the coating

materials and in the range 1.5–1.7 of the substrates.

The estimated error is less than 10% in the absorption

coefficient. The equation does not take into account

reflections from the backside of the substrate surface.

This omission will lead to errors in (1�R)/T of at

most 4%.

3.2. WOy–NiO:V devices

Before the lifetime tests and the optical measure-

ments, the devices were bleached by short-circuiting.

Afterwards all devices were connected in parallel to a

(voltage) ramp generator. After initial testing during 30

cycles, the coloration potential (Uc) was set to � 1.4 V

Fig. 4. Transmittance in colored (b, d and f) and bleached states (a, c and e)

(a and b), 550 (c and d) and 775 nm (e and f), for devices containing NiO:V

Uc/b =� 1.4/1.0 V and the coloring/bleaching times were kept in the rang

while the bleaching potential (Ubl) was set to 1.0 V.

The aim of the test was, if possible, to keep the

transmittance of all films, at 550 nm, above 70%

during the bleaching procedure and below 40% during

the coloring procedure. In order to achieve this, the

coloration and bleaching times were repeatedly adjust-

ed within the interval 1–4.5 min. Most of the color-

ation and bleaching appears to take place up to 1.5–

2.5 min. The coloring time was kept at 1.75 min and

the bleaching time at 1.5 min during the last 1000

cycles. During 10 s, the generator switched the poten-

tials from 1.0 to � 1.4 V and from � 1.4 to 1.0 V.

In close vicinity to the generator an Ocean Optic

spectrophotometer was used for measuring transmit-

tance spectra between 0.2 and 1.0 Am as a function of

time. Time-dependent single wavelength transmittance

was measured for each sample at suitable intervals.

Three wavelengths were recorded: 400, 550 and 775

nm during a period of 5–10 cycles for each sample.

Approximately one measurement per 300 cycles was

taken during the first 2000 cycles. After that only one

measurement was taken after 3000 cycles.

, as a function of number of cycles, recorded at the wavelengths 400

laminated with WO2.63 (a). The coloring/bleaching potentials were

es 1.75–4.5 and 1–4 min, respectively.


4. Results and discussion

In Section 2.1 we showed that as-deposited tung-

sten oxide could have a blue absorption if the sputter

conditions were in a narrow range. The absorption

coefficient as a function of energy, of an as-deposited

blue film with a composition of WO2.63 is compared

with WO2.89, Li0.03 WO2.89 and Li0.10 WO2.89 films

(Fig. 2). The WOy films are the same as those in Fig. 1,

deposited with C = 12% and C = 16%, respectively. A

pronounced absorption is detectable between 0.5 and

2.5 eV already at small intercalations of Li as seen in

the Li0.03 WO2.89 film. The absorption spectra are

close to a Gaussian distribution with deviations above

f 1.5 eV [11]. The WO2.63 film has not been interca-

lated but it has nevertheless a pronounced absorption

coefficient with a shape similar to the intercalated one.

This WO2.63 absorption has also an asymmetric shape,

but with differences above 1.5 eV, compared to the

intercalated samples, that increase with energy. It is

clear that the absorption coefficient depends strongly

on the oxygen deficiency of the film especially when

Fig. 5. Transmittance in colored (b, d and f) and bleached states (a, c and

400 (a and b), 550 (c and d) and 775 nm (e and f), for devices containin

were Uc/b =� 1.4/1.0 V and the coloring/bleaching times were kept in th

films are sputtered in near-metallic regions below

C = 20% (see Fig. 1).

It is possible to bleach the WO2.63 film by a ‘‘dein-

tercalation’’ procedure as described in Section 2. Fig.

3 illustrates the absorption coefficient as a function of

energy after subsequent bleaching steps (Fig. 3).

A constant voltage from 3.5 to 4.5 V with steps of

0.25 V, was applied, during 500 s at each step. A charge

density of 12.2 mC/cm2 was measured in total after this

procedure. The film reached a transparent state be-

tween 10 and 11 mC/cm2 corresponding to the 4.25 V

step in the figure. The extracted negative charge con-

tent to obtain transparency corresponds to xc 0.12. It

is not known in detail what causes the bleaching, but it

should be pointed out that propylene carbonate decom-

poses in this potential region [12]. An oxidation of the

film is possible by this process but presence of water in

the electrolyte might also be considered.

It is known that one proposed mechanism of

coloration is due to photon induced electron transfer

(polaron) excitation from localized W5 + and probably

also from W4 + to empty W6 + sites [4,13]. The

e), as a function of number of cycles, recorded at the wavelengths

g NiO:V laminated with WO2.89. The coloring/bleaching potentials

e ranges 1.75–4.5 and 1–4 min, respectively.


absorption coefficient of intercalated W oxide, in Fig.

2, has a very similar shape as that of the as-deposited

blue W oxide. This suggests that the mechanisms

should also be similar. In the first (W5 +) case, external

electrons are trapped at empty localized states during

the lithium intercalation while in the second (W4 +)

case oxygen vacancies give rise to empty localized

states at W sites, to which electrons can be transferred.

The absorption peaks of Li intercalated films with

xc 0.10 are stronger than that of as-deposited blue

films. This indicates a higher absorption strength for

Li intercalation than for oxygen vacancies. However,

the as-deposited blue films also exhibit a higher

absorption in the band edge region above 2 eV.

The results based on Figs. 2 and 3 indicate that not

all oxygen vacancies give rise to W5 + sites that exhibit

optical absorption. Up tof 0.25 O-vacancies/W seem

to be optically inactive. It has been suggested by

bandstructure calculations that deep gap states can

occur in amorphous tungsten oxide [14]. They are

likely associated with (W–W)10 + species [14]. We

suggest that up to 0.25 O-vacancies/W give rise to

Fig. 6. Transmittance in colored (b, d and f) and bleached states (a, c and

400 (a and b), 550 (c and d) and 775 nm (e and f), for devices containin

were Uc/b =� 1.4/1.0 V and the coloring/bleaching times were kept in th

filled (W–W)10 + d-type states. Then transitions to the

conduction band will be forbidden and there will be no

optical absorption below the band gap. This is a

possible explanation of the transparency of substoi-

chiometric WOy films, like the WO2.89 one in Fig. 2.

Only a substantially higher amount of oxygen vacan-

cies will lead to W5 + sites that are associated with

strong optical absorption.

Durability under electrochemical cycling was

studied on the devices using NiO:V and –WO2.63,

–WO2.89 and –WO2.95 EC films. Figs. 4–6 shows

the transmittance in the colored and the bleached

states, at the wavelengths 400, 550 and 775 nm, as a

function of number of cycles for these devices (Figs.

4–6). The measurements were taken during f 3000

cycles. The filled marks represent maximum coloration

and the empty marks represent maximum bleaching.

The scatter in the data during the cycles is mainly

caused by the adjustment of coloration and/or bleach-

ing time. Despite of a few divergent values (at 800

cycles) there is a clear tendency. To determine wheth-

er the devices are reversible or not, we use the

e), as a function of number of cycles, recorded at the wavelengths

g NiO:V laminated with WO2.95. The coloring/bleaching potentials

e ranges 1.75–4.5 and 1–4 min, respectively.


transmittance optical contrast, i.e. the difference be-

tween colored and bleached transmittance. There is in

all cases a good optical contrast that is nearly constant

during the first 1200 cycles. After that the optical

contrast for the device with the WO2.95 film decreases

for all wavelengths as can be seen in Fig. 6 while the

contrast remains almost the same for the devices with

less oxygen in the tungsten oxide films (Figs. 4 and

5). The transmittance of the NiO:V–WO2.63 device

(Fig. 4) was maintained around 40% and 70%, at 550

nm, and around 35% and 70%, at 775 nm, in the

colored and the bleached state, respectively, while the

transmittance of the NiO:V–WO2.89 device was (Fig.

5), at 550 nm, around 50% in the colored and 70% in

the bleached state and, at 775 nm, 40% in the colored

and 70% in the bleached state. The device with the

lowest oxygen content in the W oxide film had a

slightly higher optical contrast between the bleached

and colored state, as can be seen also at 400 nm.

Degradation is visually apparent in the colored state in

all devices in the form of small transparent spots. The

number and size of these spots are smallest for the

WO2.63 film, while more than half of the WO2.89

device was covered with the spots. The WO2.95 device

was completely degraded after 3000 cycles.

5. Conclusions

Amorphous tungsten oxide films were sputter de-

posited in different O2/Ar gas flow ratios, yielding

different degrees of substoichiometry. We verify that

the WOy composition of the films is close to stoichio-

metric ( y = 3) and slightly decreasing when the oxygen

gas flow is reduced to around 20% in the O2/Ar gas

flow ratio. A large decrease in the O/Wratio is observed

when oxygen gas flow is further reduced. As-deposited

blue WO2.63 has been sputtered using 12% O2/Ar.

Slightly substoichiometric WOy films with yz 2.75

are transparent, possibly because the oxygen vacancies

are associated with filled (W–W)10 + states. Films with

yc 2.6–2.75 are blue and the absorption spectrum is

similar to that of Li intercalated films initially trans-

parent, suggesting that W5 + states are involved.

Complete EC devices containing NiO:V films lam-

inated with WO2.63, WO2.89 and WO2.95 films were

electrochemically cycled 3000 times. The device con-

taining the WO2.63 film showed the best cyclic dura-

bility and the least degradation while the device that

contained the highest oxygen content (WO2.95) had the

worst durability and showed severe degradation after

3000 cycles. Low oxygen content in the tungsten oxide

film is recommended to improve cyclic durability of

EC devices.

Acknowledgements

This work was carried out under the auspices of the

Angstrom Solar Center at Uppsala University, sup-

ported by the Swedish Foundation for Strategic

Environmental Research (MISTRA) and the National

Energy Administration of Sweden. The research

foundation of Angpanneforeningen provided for the

participation at the IME-5 conference 2002. We would

like to thank A. Azens for sputtering of the NiO:V

films and valuable discussions, and G. Gustavsson for

assistance with laminations. We also thank Y. Zhang

for carrying out the ERDA measurements.

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Optical absorption and durability of sputtered amorphous tungsten oxide films