Photochromic coloration of WO
3
with visible light
C. Bechinger,
a)
E. Wirth, and P. Leiderer
b)
Fakulta
¨
tfu
˙
r Physik, University of Konstanz, D-78434 Konstanz, Germany
͑Received 5 February 1996; accepted for publication 8 March 1996͒
Thin amorphous films of tungsten oxide (WO
3
) are well known to change their optical absorption
behavior upon light exposure, usually referred to as photochromic effect. Since the formation of
color centers is closely related to the optical creation of electron-hole pairs the sensitivity of the
photochromic effect in WO
3
is limited to energies above its band-gap energy of 3.25 eV,
corresponding to the near-ultraviolet range. We will demonstrate that in case of a thin cadmium
sulfide ͑CdS͒ layer underneath the tungsten oxide film the sensitivity of photochromism is shifted
into the visible range. This result is interpreted in terms of charge carrier injection from the CdS into
the WO
3
. Apart from a more detailed understanding of the photochromic effect this may have also
relevance for technical applications. © 1996 American Institute of Physics.
͓S0003-6951͑96͒01420-9͔
Due to the high potential of large area optical devices
capable to be switched between a transparent and a strong
absorptive state there is a large research activity within the
last two decades regarding transition metal oxides.
1–3
One of
the most promising candidates is tungsten oxide, often dis-
cussed in the context of ‘‘smart windows’’ in order to con-
trol the solar input of buildings or with respect to large area
displays.
2,4
For most of these purposes the so-called electro-
chromic effect is used, where coloration and bleaching of
tungsten oxide is caused by an electrochemical reaction.
Much less attention has been paid so far to the light-induced
coloration of WO
3
, i.e., photochromism, where an identical
absorption band is formed upon irradiating bare tungsten ox-
ide thin films with light.
1,5
Taking into account that the
photochromic coloration can be made completely reversible
by exposing the sample to oxygen gas,
6
the basic require-
ments for technical applications like erasable optical storage
devices are met by the photochromic effect. However, since
the formation of color centers in WO
3
requires irradiation in
the near-ultraviolet range
1,6
where no compact light sources
such as laser diodes are available at the moment, this may
impede the development of integrated storage devices based
on WO
3
considerably.
In this letter we will demonstrate that the spectral sensi-
tivity of photochromism in tungsten oxide can be shifted
from the near-UV into the visible range by the use of a thin
cadmium sulfide interlayer between the substrate and the
WO
3
film. In addition to a more detailed understanding of
the coloration mechanism, this result may also have rel-
evance for the above-mentioned applications.
It has already been demonstrated that the photochromic
effect in tungsten oxide is intimately connected to optically
excited electron (e
Ϫ
) hole ͑h
ϩ
) pairs which can decompose
water being incorporated in WO
3
to a considerable amount.
6
The light-induced decomposition of H
2
O can be written as:
7,8
H
2
Oϩ2h
ϩ
⇔Oϩ2H
ϩ
, ͑1͒
which describes the creation of protons (H
ϩ
) and metastable
oxygen radicals ͑O͒. The protons together with the optically
excited electrons finally lead to the formation of the colored
tungsten bronze HWO
3
, whereas the oxygen is assumed to
occupy vacancy sites inside the sample or escape ͑in molecu-
lar form͒ into the ambient atmosphere, respectively.
6
Due to
the high band-gap energy of WO
3
͓3.25 eV ͑Ref. 1͔͒, how-
ever, excitation of electron-hole pairs and subsequent colora-
tion is limited to the near-ultraviolet range. The aim of this
letter is to demonstrate that coloration can be already ob-
served at considerably lower energy of the incident photon
quanta if the tungsten oxide layer is in contact with a thin
CdS film.
Thin films of CdS were deposited on glass substrates
10ϫ10 mm
2
by a chemical bath deposition ͑CBD͒ process
at a temperature of 65 °C. For details concerning the prepa-
ration process we refer to the literature.
9–11
The film thick-
ness used in our experiments was determined to be about 70
nm. Figure 1 shows the absorption coefficient
␣
of the CdS
layer as a function of the incident photon energy h
in a
(
␣
h
)
2
vs h
plot.
12
The linear increase towards higher en-
ergies clearly demonstrates a direct interband transition as it
is also found for single crystals of CdS. The deviations from
a straight line below 2.6 eV can be ascribed to competing
noninterband absorption processes, possibly being due to im-
purities or internal stress. From a linear extrapolation of the
a͒
Present address: NREL, 1617 Cole Boulevard, Golden, CO 80401-3393.
b͒
Electronic mail: paul.leiderer@uni-Konstanz.de.
FIG. 1. Dependence of (
␣
h
)
2
for a 70 nm thick CdS film on the photon
energy near the fundamental absorption edge. From the abscissa intersect of
the dotted line the band-gap energy is determined to be 2.54 eV ͑Ref. 12͒.
2834 Appl. Phys. Lett. 68 (20), 13 May 1996 0003-6951/96/68(20)/2834/3/$10.00 © 1996 American Institute of Physics