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423
Session 16
Electrochromic Glazing with an Ion-Conducting
PVB Interlayer
Dr. Holger Stenzel, HT Troplast AG, R&D Trosifol, Troisdorf, Germany
Dr. Alexander Kraft, Dr. Karl-Heinz Heckner, Dr. Matthias Rottmann
, Gesimat GmbH, Berlin, Germany
Dr. Martin Steuer, Dr. Bernd Papenfuhs, Kuraray Specialities Europe, Frankfurt, Germany
Keywords
1 = Laminated safety glass 2 = Electrochromic glazing 3 = Smart windows 4 = PVB fi lm
Abstract
Smart windows with switchable light
transmittance and refl ectance are the
windows of the future. Among the
different possible technologies for
smart windows electrochromism is the
most promising candidate. We present
a new laminated electrochromic glass
consisting of two K glass panes coated
with complementary electrochromic thin
fi lms and laminated together by use of
an ion-conducting PVB sheet.
For the fi rst time ion-conducting PVB
fi lms are used as a polymer electrolyte
in electrochromic glazings. This has
many advantages compared to other
technologies for the preparation of
polymer electrolytes such as the poured
resin technology. Additionally, an
electrochromic glass which combines
the possibility of adjusting the light
transmittance with the properties
of a laminated safety glass becomes
possible. The well known technologies
for the production of conventional PVB
interlayers and for the production of
laminated safety glass can also be used
for the manufacture of electrochromic
glazing.
Introduction
Since more than 60 years plasticized
PVB is in use as an interlayer material
for laminated safety glass. This is
because of its unique properties such
as high, adjustable adhesion to glass,
high optical transparency, excellent
toughness and fl exibility, high impact
strength, light and temperature
resistance. In addition to safety glass
features, other functionalities have been
included in the last decades, among
them sound control [1], UV blocking,
colours and others. By providing a
new, ion conducting PVB sheet, the
application potential of PVB is extended
to the area of polymer electrolytes for
electrochromic devices.
Electrochromic devices offer many
advantages for the non-mechanical
transmission change of smart glazing
in future architectural and automotive
applications. An important characteristic
of electrochromic glazing is the
switching of both, the visible light
transmission and the overall energy
transmission (g value) in response to
changing weather conditions. Modern
buildings with large area glazing require
not only a low heat loss in winter (low
U value) but also protection against
overheating in summer (low g value)
even in moderate climatic zones.
However, a high g-value is necessary
during winter months to realize solar
energy gains. Because of the possibility
of user controlled or automatic change
the optical properties of electrochromic
windows such systems can match very
well these demands. Large energy
savings up to 50 % due to lowering of
the climatisation costs can be realized
[2].
Electrochromism
Several technologies are discussed for
the construction of switchable glazing
devices, among them thermochromism,
photochromism, electrochromism
and the use of electro-optical fi eld
effects in liquid crystal or so-called
suspended particle devices. Considering
all advantages and drawbacks of the
different concepts, the most advanced
technology is electrochromism.
Electrochromism is defi ned as the
reversible change of the optical
properties of a material induced by
electrochemical oxidation and reduction
due to an applied d.c. voltage.
Three different basic device
constructions for electrochromic
elements are possible:
a) electrochromic compound(s)
dissolved in a liquid or gel-type
electrolyte (used in automatic-
dimming rear view mirrors produced
e.g. by Gentex, USA)
b) electrochromic solid fi lms joint
together by an electrolyte, preferably
a solid polymer electrolyte (battery
type)
c) hybrid type devices with one solid
electrochromic fi lm and one dissolved
redox-active substance in a liquid or
gel-type electrolyte.
The fi rst and third kind of
electrochromic devices are self
bleaching. A constant current fl ow has
to be maintained to provide a constant
coloration. Only the second kind of
device has a memory effect and needs
current fl ow only during switching [3].
Therefore, and because of gel-type
or liquid electrolytes cannot be used
in large area glazing only battery-
type smart glazing can be used in
architectural applications. The electrical
and optical switching characteristics
of such a battery-type electrochromic
device are shown in fi gure 1.
Due to the application of a small
d.c. voltage a current fl ow is induced
which is accompanied by a change in
optical transmittance of the glazing
(coloration or charging). If the voltage
is switched off the glazing retains its
optical transmittance. By short circuiting
or reversing the applied voltage a
current in opposite direction fl ows and
the glazing bleaches (discharging). A
controlled gradual change of the light
transmittance is possible. Figure 2
schematically shows the change in light
transmittance of such an electrochromic
system in the coloured and bleached
state, which can be used to regulate
the amount of light and heat radiation
which enters a building through the
glazing.
However, the application potential
of electrochromic devices is not limited
Figure 1:
Electrical and optical
switching behaviour
of a battery-type elec-
trochromic device
424
Session 16
to smart windows in residential and
commercial buildings. They can also be
used for:
- glazings for means of transportation,
e.g. automatic regulation of the light
and heat transmittance of switchable
glazing panels (in vehicles, trains,
aircraft, ships) and for automatic-
dimming rear view mirrors
- large area information displays (e.g.
airports, train stations, advertising)
- fast switching sunglasses
- switchable fi lters for light and
heat (for cameras, microscopes,
spectrometers, etc).
Despite the large application
potential, the many benefi ts of
electrochromic glass and huge
research efforts until today automatic-
dimming rear-view mirrors are the
only commercial product based
on electrochromism. This can be
explained by the drawbacks of
previous development concepts for
electrochromic devices: Vacuum
deposition methods for electrochromic
fi lms are expansive and restricted to
only a low number of electrochromic
materials. Further disadvantages are the
often proposed use of tungsten oxide
as only one electrochromic fi lm coupled
with a so-called ion-storage layer, the
use of the poured resin technology
for the preparation of solid polymer
electrolytes or even the use of non-solid
electrolytes (gel-type or liquid) and many
others.
Our new concept overcomes these
drawbacks and provides a laminated
electrochromic glazing with the
potential for very high transmittance
change, safety glass properties,
moderate production costs and
durability.
Electrochromic glazing with ion
conducting PVB interlayer
For the fi rst time ion-conducting PVB
fi lms are used as a polymer electrolyte
in electrochromic glazing. This has
many advantages compared to other
technologies for the preparation of
polymer electrolytes such as the poured
resin technology. Additionally, an
electrochromic glass which combines
the possibility of adjusting the light
transmittance with the properties
of a laminated safety glass becomes
possible. The well known technologies
for the production of conventional PVB
interlayers and for the production of
laminated safety glass can also be used
for the manufacture of electrochromic
glazing.
The construction of an electrochromic
glazing is shown in Figure 3. The applied
glass can be either fl oat glass or heat
strengthened and it is coated with a
FTO (Fluorine doped Tin Oxide) – so
called K-Glass - or ITO (Indium doped
Tin Oxide) layer. This layer is responsible
to conduct the electrical current into the
electrochromic layers. The connection
to the direct-current supply (bus bar) is
realized with metallic conductors printed
directly to the edges of the glass.
The electrochromic layers themselves
are coated electrochemically on the
FTO layer. This technology implies
lower costs for equipment installation
and maintenance in comparison with
vacuum coaters. One electrochromic
layer is usually Tungsten trioxide (WO
3
),
the other can be an electrochromic
inorganic complex compound (e.g.
Prussian Blue PB: [Fe
III
Fe
II
(CN)
6
]
-
) or a
polymeric layer system (e.g. Polyaniline,
Polythiophene). The fact that a
complementary electrochromic system
is used, is a big advance: The difference
in transmittance between the bleached
and the coloured state is much bigger
and the coloration rate is faster in
comparison to existing electrochromic
systems with only one electrochromic
fi lm and a so-called ion-storage layer.
The transmittance in the visible T
L
can
be varied between 75% and 10% (in a
range from 380 to 780 nm according to
EN 410). But the shadowing does not
only work in the visible, but also in the
infrared. The energy transmission T
E
is
variable between 50% and 10% (in a
range from 300 to 2500 nm according
to EN 410). These values are valid for
a system with WO
3
/PB electrochromic
layers. Figure 4 shows the transmittance
versus the wavelength for such a system
in the fully bleached and fully coloured
situation.
The two glass panes are laminated
with a ion conducting PVB interlayer.
The ion conductivity realizes the ion
transport between the electrochromic
layers. The fi nal product is an
insulating glazing with the laminated
electrochromic glazing as external pane
and a low-e coated glass as inner pane.
The PVB fi lm for electrochromic
glazing uses a modifi ed resin in
combination with a special plasticizer
system. To get a suffi cient ion
conductivity of the fi lm, a conducting
salt is added to the formulation. This
salt is usually based on Lithium or
Potassium. The conductivity of the
fi lm should be in a range of 5 x 10
-6
Figure 2:
Schematic drawing
of the control of
the transmission
and refl ection of
electromagnetic
radiation by use of an
electrochromic device
Figure 3:
Schematic drawing of
the construction of the
electrochromic glazing
with the ion conduct-
ing PVB interlayer
Figure 4:
Transmission spectra
in the fully bleached
and darkened state
of the electrochromic
glazing with ion con-
ducting PVB interlayer
and tungsten oxide
425
Session 16
S/cm to 2 x 10
-5
S/cm to get acceptable
switch times. The switch times of an
electrochromic glazing with such a
PVB interlayer varies dependent on the
size of the glazing between 5 and 20
min., which is absolutely satisfactory for
architectural glazing. Figure 5 shows the
transmittance vs. the wavelength for
discrete time to bleach the glass, Figure
6 to darken it.
Why do we use PVB interlayer
as a polymeric electrolyte for the
electrochromic glazing ?
PVB fi lm is used to produce
laminated safety glass for more
than fi ve decades and it´s the
most experienced material for this
application. Adhesion to glass and ion
conducting function is integrated in
one material. So it is obvious to use
PVB fi lm in electrochromic glazing. It
can be laminated under almost the
same process conditions than standard
PVB interlayer. The delivery form is
also the same, either refrigerated
or PE interleaved. The mechanical
properties of the fi lm give the laminate
a satisfactory stability. The UV stability
of the PVB fi lm is excellent. There are
no effects due to secondary induced
polymerisation of free monomers or
oligomeres as compared to poured
resins.
The components for production of
electrochromic glazing can be easily
integrated in a standard laminating
line. For the lamination itself standard
operations are used (pre-niproller,
autoclave). Figure 7 shows a scheme
for the integration of electrochromic
components to a laminating line.
What tests are necessary to verify the
long term durability?
The continuous cycle test switches
the electrochromic glazing incessantly
between the bleached and coloured
state. The test indicates the stability of
the switching and possible interactions
between the PVB fi lm and the
electrochromic layers. The objective is
to reach at least 40,000 cycles, which is
equivalent to an average of fi ve switches
a day over 20 years.
This test should be also done under
more severe environmental conditions.
The UV radiation, humidity test and
high temperature storage according
to EN 12543 Part 4 simulates different
weather conditions. After the tests
the samples are evaluated in terms
of visual changes (e.g. discoloration,
bubbles, delaminations, haze) and loss
of transmittance ratio.
The only standard which is actually
valid for the testing of electrochromic
glazing is the ASTM E2141-02. This
standard describes the exposition of
electrochromic specimen to simulated
solar irradiation in a temperature- and
humidity-controlled chamber at selected
temperatures ranging from 70°C to
105°C while the specimen are cyclically
coloured and bleached. This procedure
runs for 50.000 cycles, inspecting the
samples every 4.000-10.000 cycles. The
test is passed, if the transmission in the
bleached state is more than 50% or the
ratio between the transmission of the
bleached and the coloured state is more
than 4 at room temperature.
The insulating glass units should be
tested according to prEN 1279 Part 2:
Type test on airfi lled insulating glass
units. It describes a climate changing
test of four weeks with temperatures
between -18°C and +53°C in addition
with a heat storage for another 7 weeks
at 58°C. The relative humidity goes
up to 95% including a condensing
phase. The test is followed by a
Figure 5:
Change of the
transmittance of an
electrochromic glass
with ion conducting
PVB between 380
and 780 nm during
coloration: spectra re-
corded in steps of 10
seconds between 0
and 180 seconds and
after 600 seconds
Figure 6 :
Change of the trans-
mittance of an elec-
trochromic glass with
ion conducting PVB
interlayer between
380 and 780 nm dur-
ing bleaching: spectra
recorded in steps of
10 seconds between
0 and 180 seconds
and after 600 sec-
onds
Figure 7:
Schematic drawing of the production process
for the electrochromic glass with ion conducting
PVB interlayer
426
Session 16
visuell inspection for defects and a
measurement of the transmission ratio.
To verify the adhesion between
the single layers of an electrochromic
glazing, the compressive shear
strength (CSS) test regarding patent
DE 19756274 is a common method.
Laminated samples of size 25.4 x 25.4
mm
2
are sheared under an angle of 45°
until they are destroyed. The maximum
force related to the sample area is the
CSS in N/mm
2
. So the construction will
break at the weakest connection of the
single layers. Typical values of laminates
with standard PVB are between 8 and
20 N/mm
2
. Figure 8 shows the CSS test
apparatus.
The impact strength of the system
can be determined according to EN
12600. A pendulum consisting a twin
tire of 50 kg weight swings onto the
laminate from a height of 190, 450 and
1200mm. The test counts as fulfi lled at
a certain height, when the glass does
not break or if it breaks, the opening in
the laminate is not bigger than 76 mm.
Figure 8:
Compressive shear strength test apparatus
Summary
Electrodeposition is a very cost effective
way of producing electrochromic
layers. It is much cheaper than other
technologies like sputtering. The use
of complementary electrochromic thin
fi lms allows for a bigger difference
in light transmittance between
the bleached and coloured state
Electrochromic layers can be made from
oxides, inorganic complex compounds
or polymeric layer systems. This means
bigger variety in materials and colours.
The ion conducting PVB fi lm as a
polymeric electrolyte enables the use
of standard laminating lines to produce
electrochromic glazing and adds the
benefi t of safety and security to the
system.
References
[1] U. Keller: Improved sound reduction with
laminated glass, Glass Processing Days 2001,
Conference Proceedings Book, pages 735-737
[2] H. Wittkopf: Electrochromics for architectural
glazing applications, Glass Processing Days
1997, Conference Proceedings Book, pages
299-303
[3] K.-H. Heckner, A. Kraft: Similarities between
electrochromic windows and thin fi lm batteries,
Solid State Ionics 152-153 (2002) 899-905
