Infrared regulating smart window based on organic materials
Citation for published version (APA):
Khandelwal, H., Schenning, A. P. H. J., & Debije, M. G. (2017). Infrared regulating smart window based on
organic materials.
Advanced Energy Materials
,
7
(14), 1-18. [1602209]. https://doi.org/10.1002/aenm.201602209
DOI:
10.1002/aenm.201602209
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Published: 19/07/2017
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Infrared Regulating Smart Window Based on
Organic Materials
Hitesh Khandelwal, Albertus P. H. J. Schenning,* and Michael G. Debije*
DOI: 10.1002/aenm.201602209
(IR), here defined as light with wavelengths
between 700 nm and 2500 nm, accounts
for around 50% of the total energy emitted
by the sun reaching Earth (Figure 1b),
[3,4]
and this light produces interior heating but
is invisible to the unaided eye.
The absorption of sunlight by building
materials and passage of IR through trans-
parent surfaces such as windows is respon-
sible for much of the interior overheating
of office rooms, automobile interiors,
greenhouses, and other similar spaces. The
use of artificial cooling and heating systems
will only increase with the continued influ-
ence of global climate change, with energy
used for cooling systems surpassing energy
used for heating around the year 2070, and
a 40 fold increase in air cooling energy use
is expected by 2100.
[5]
By controlling the
influx of radiant heat transfer, calculations
show that more than 50% of the energy
used in lighting, heating and cooling could be saved by deploying
better control systems over only 18% of available window stock.
[6]
In areas with human inhabitants employing windows,
more aspects must be considered than simply reducing the
use of energy in the room: any switchable window used in, for
example, a commercial office space has several other require-
ments that must be met before it may be installed. Among
these requirement are reasonably fast switching speeds
[7]
(although for IR control, relatively longer times compared to
visible light switching should be acceptable), good optical trans-
parency with minimum haze, an acceptable device lifetime,
[8]
and functionality over a range of exterior temperatures. Con-
trolling the excess of solar energy without compromising the
visible transparency of the window is an important considera-
tion for human health: maintaining inside/outside contact and
daylighting are vital in retaining well-being and productivity, as
well as providing economic and aesthetic gain by reducing the
need for artificial lighting systems.
[9,10]
These are challenging
goals for a window to realize.
A number of materials have been developed over the past
few decades to maintain indoor temperatures. Many of these
focus on the opaque structural building elements like walls and
roofing.
[10–13]
Other solutions target the transparent window,
employing external mechanical shutters and blinds,
[14]
phase
change materials (PCMs),
[15]
thermochromic materials,
[16]
aero-
gels,
[17]
trapped gas in fluid membranes,
[18]
and even phononic
materials,
[19]
among other options. Indeed, controlling heat
passage through the window in response to changing climate
conditions is a great challenge; ideally, one would accomplish
Windows are vital elements in the built environment that have a large impact
on the energy consumption in indoor spaces, affecting heating and cooling
and artificial lighting requirements. Moreover, they play an important role
in sustaining human health and well-being. In this review, we discuss the
next generation of smart windows based on organic materials which can
change their properties by reflecting or transmitting excess solar energy
(infrared radiation) in such a way that comfortable indoor temperatures
can be maintained throughout the year. Moreover, we place emphasis on
windows that maintain transparency in the visible region so that additional
energy is not required to retain natural illumination. We discuss a number of
ways to fabricate windows which remain as permanent infrared control ele-
ments throughout the year as well as windows which can alter transmission
properties in presence of external stimuli like electric fields, temperature and
incident light intensity. We also show the potential impact of these windows
on energy saving in different climate conditions.
H. Khandelwal, Prof. A. P. H. J. Schenning,
Dr. M. G. Debije
Functional Organic Materials and Devices
Department of Chemical Engineering and Chemistry
Eindhoven University of Technology
Den Dolech 2, 5600 MB Eindhoven, The Netherlands
E-mail: A.P.H.J.Schenning@tue.nl;
m.g.debije@tue.nl
H. Khandelwal
Dutch Polymer Institute (DPI)
P.O. Box 902, 5600 AX Eindhoven, The Netherlands
Prof. A. P. H. J. Schenning
Institute for Complex Molecular Systems (ICMS)
Eindhoven University of Technology
5600 MB Eindhoven, The Netherlands
Prof. A. P. H. J. Schenning
Laboratory of Device Integrated Responsive Materials (DIRM)
Guangzhou, China
1. Introduction
More than 50% of the total energy used in the building envelope
in the Western world is spent on cooling, heating and lighting the
interior places (Figure 1a).
[1,2]
A significant fraction of this energy
use is related to our inability to control the ingress and egress of
infrared light from the sun through windows. Near infrared light
This is an open access article under the terms of the Creative Commons
Attribution-NonCommercial License, which permits use, distribution and
reproduction in any medium, provided the original work is properly cited
and is not used for commercial purposes.
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this without compromising the influx of visible light and the
integrity of the view beyond the window.
[20]
The focus of this review is on infrared regulating windows
based on organic materials which can adjust the transmittance of
IR radiations depending on environmental conditions (Figure 2).
There are advantages to employing organic rather than inor-
ganic materials in IR window control systems: for instance, since
they are non-metallic, they do not corrode or interfere with elec-
tromagnetic waves (signals from/to radios, cell phones, GPS,
or garage door openers, for example)
[21–23]
and often are much
easier to process at lower temperatures than inorganic materials.
Cholesteric (or ‘Chiral nematic’) liquid crystalline (Ch-LC)
materials have attracted much attention for development of
infrared regulating windows. They are formed when nematic
liquid crystals are doped with chiral molecules. The chiral dopants
generate a LC organization wherein successive layers of nematic
LC are displaced by a small rotation in molecular director with
respect to their neighboring layers. The ‘twist’ generated may
be either right- or left-handed, depending on the nature of the
chiral dopant molecule. The central reflection band of Ch-LC is
determined by the pitch (P), average refractive index (n
avg
) of the
material and incident angle of light (Equation (1)). Pitch (P) of
the Ch-LC depends on the concentration (C) and helical twisting
power (HTP) of the chiral dopants (Equation (2)). Ch-LC selective
mirrors demonstrate a distinct advantage over, say, an inorganic
Bragg reflector in that the LC self-organizes into a helical struc-
ture and can be easily processed from solution. Moreover, Ch-LCs
can be made responsive to external stimuli, including tempera-
ture, electric/magnetic fields, light, pH, humidity and gasses
that makes them interesting for a variety of applications.
[24,25]
It
is important to note that since a cholesteric-based reflector has a
degree of angular dependence with respect to the incident light
(Equation (1)), a blue shift in reflection band will be observed on
deviating from the normal incident angle.
[26,27]
The bandwidth of
the light reflected by the Ch-LC is determined by the difference
between the extraordinary (n
e
) and ordinary (n
o
) refractive indices
and the pitch of the host LC (Equation (3)).
cosPn
oavg
λθ
=× ×
(1)
P
1
C HTP
=
×
(2)
()
0
nn P
e
λ
∆= −×
(3)
The maximum reflection by the cholesteric reflector layer is
limited to 50% of the incident sunlight, matching the polari-
zation of the helix: that is, a right-handed cholesteric will
reflect only right circularly-polarized light. Both left-circularly
polarized light and light outside of the cholesteric reflection
bandwidth are unaffected by the liquid crystal matrix and are
transmitted normally (Figure 3).
[25]
LC molecules can be oriented in number of ways between
two glass plates (Figure 4). The arrangements of the LC mole-
cules determine their collective optical properties. For example,
when the molecules are arranged in a helical fashion and par-
allel to the substrate, known as planar alignment, the layer
reflects light of specific wavelengths depending on the pitch and
Hitesh Khandelwal received
Integrated Bachelor and
Master degree from
Indian Institute of Science
Education and Research,
Thiruvananthapuram
(IISER-TVM), India
(2008–2013). In April 2013,
he defended his Master thesis
from the group of Dr. Mahesh
Hariharan. In May 2013,
he joined the group of
Functional Organic Materials and Devices, Eindhoven
University of Technology for Ph.D. He is currently working
on infrared reflecting smart windows based on liquid
crystal polymer under the supervision of Dr. Michal Debije,
Prof. Dick J Broer and Prof. Albert Schenning. He received
INSPIRE fellowship from year 2008–2013.
Albert Schenning received
his Ph.D. degree at the
University of Nijmegen in
1996 under the direction
of Dr. M. C. Feiters and
Prof. Dr. R. J. M. Nolte.
Thereafter he was a post-
doctoral fellow in the group
of Prof. Dr. E. W. Meijer at
Eindhoven University of
Technology, and in 1997 he
joined the group of Prof.
Dr. F. Diederich at the ETH in Zurich. From 1998 until
2002, he was a Royal Netherlands Academy of Science
(KNAW) fellow at Eindhoven University of Technology. He
is currently full professor at the Eindhoven University of
Technology. His research interests center on sitmuli-
reponsive functional organic materials and devices.
Michael Debije received
an M.Sc. degree in High-
Energy Physics from Iowa
State University, Ames,
Iowa in 1994 and a Ph.D.
in Biophysics from the
University of Rochester, NY,
USA in 2000. After completing
a postdoc at the Interfaculty
Reactor Institute at the Delft
University of Technology in
the group of John Warman
in 2003, he joined the staff of the Functional Organic
Materials and Devices group under Prof. Dick Broer at the
Eindhoven University of Technology, and is responsible for
the Energy cluster within the program.
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transparent for the rest of the wavelengths as described above.
In a focal conic alignment, which consists of aligned molecules
where the helical structure is preserved but tilted with respect
to the substrate, results in more scattering of the incident light
as the refractive index changes continuously from the top to the
bottom of cell. In the homeotropic alignment, where molecules
extend perpendicular to the substrate, the layer is transparent to
all the wavelengths of light.
Apart from LCs, we also briefly discuss other organic mate-
rials originally intended for control of visible light that could be
adapted to IR control elements. Furthermore, we also discuss
the influence of these IR managing windows on temperature
control and energy savings in the built environment. Inorganic-
based window solutions, including metallic based reflective
layers,
[28–31]
photochromic,
[3,32,33]
electrochromic,
[3,8,34–39]
and
thermochromic
[3,40–43]
systems, plasmonic nanoparticles,
[36,44–46]
aerogel glazing,
[47]
privacy windows,
[48]
thin film photo-
voltaics,
[49]
and even microfluidic
[50]
based windows have not
been discussed in this review, as they have already received con-
siderable attention and discussion. Additionally, organic based
window devices and materials primarily intended to absorb and
control visible light passage, including electro-,
[51–53]
photo-,
and thermochromic
[3]
windows, are also beyond the scope of
this review of infrared control materials: they have already been
detailed in a number of excellent review articles.
This review is separated into three parts. The first two will
describe efforts in the areas of static systems and dynamic
(adjustable) IR regulating elements. The final section will spec-
ulate as to some possible future research areas that are ripe for
exploitation.
2. Static IR Regulating Window
We define a static IR regulating window as a window whose
properties do not change with external stimuli. In other words,
the infrared control is a permanent feature of the window,
regardless of exterior conditions.
2.1. Absorption Based Technologies
The simplest IR control solution is to use a dye which is trans-
parent in the visible region and absorbs only infrared radia-
tion.
[54]
The shortcoming of absorbing based systems is the
majority of absorbed energy is eventually re-released as heat,
with approximately half the heat being radiated into the room
space. A more advanced absorption-based concept is the lumi-
nescent solar concentrator (or LSC),
[55]
illustrated in Figure 5.
The LSC uses dyes embedded in the polymer or glass plate
which functions as the window. The dyes absorb the near IR
sunlight and subsequently fluoresce at a longer wavelength. A
fraction of this re-emitted light is trapped in the higher refrac-
tive index polymer or glass panel which acts as a lightguide.
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Other
Lighting
Water Heating
34.3%
9.5%
15.9%
40.3%
Heating, Cooling
and Ventilation
5001000 1500 200
02
50
0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
UV light
Visible light
Infrared light
Solar Irradiance, W/m
2
/nm
Wavelength, nm
a)
b)
Figure 1. a) U.S. Buildings Energy End-Use in 2008.
[2]
b) Solar spectrum on Earth (Data taken from National Renewable Energy Laboratory).
Figure 2. Schematic diagram of an ideal smart window reflecting infrared radiations in warm days (left) and allowing it to enter in cold days (right),
while remaining transparent in visible region in both climate conditions.
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The trapped emission light is transported by total internal
reflection and only exits at the edges of the window, where it
may be converted to electricity via the use of attached photovol-
taic cells.
[56]
A significant fraction of absorbed light energy in
LSCs is still lost through the top and bottom surfaces,
[57]
and
thus still would contribute towards interior heating. In addi-
tion, the absorption ranges of the dyes in such devices are still
quite limited, often with significant absorption in the visible
wavelength region and thus only process a small fraction of the
total incident light.
[58]
2.2. Reflection Based Technologies
There is an enormous literature of Ch-LCs being employed
to reflect visible light for a wide variety of (display) applica-
tions.
[59]
What has not been as widely exploited are cholesterics
as IR control elements in transparent windows in buildings and
automobiles. One of the key challenges to employ Ch-LCs as
IR reflectors are their limited bandwidths when directly pro-
cessed from solution. For regular cholesterics, bandwidth is
restricted to around 100 nm in the IR due to the limited
∆
n
of the LC itself (Equation (3)), which would
have limited impact on controlling interior
temperatures.
The range of IR wavelengths reflected may
be increased by creating a broadband cho-
lesteric reflector. There are a variety of ways
in which this may be achieved. The simplest
is to simply layer narrow band cholesterics
of different pitches on top of one another.
[60]
The drawback of this is that the layers need to
be laminated together in an extra processing
step, and the number of layers necessary for
effective IR control grow rapidly: to cover
the spectrum from 750–1100 nm requires a
minimum of three cholesterics layers. How-
ever, as mentioned earlier, even this will
allow a maximum of 50% reflection (one
handedness of the incident light), so a full six
layers will be minimally required for effec-
tive IR control. The lamination of additional
layers has an added potential disadvantage of
introducing additional haze into the system,
something that an IR reflector to be employed in a window
should seek to avoid as much as possible.
A second option to obtain a broadband reflector is to create
a pitch gradient in a single layer; in other words, for the Ch-LC
to display a number of different pitches within the same film.
A number of methods have been developed to fabricate pitch
gradient broadband reflectors in the visible region,
[59]
some of
these methods have produced reflection bands in the infrared
region appropriate for window applications. The most common
method for creating broadband Ch-LC reflectors was developed
by Broer et al.,
[61]
and employed a liquid crystal mixture con-
sisting of a nematic monoacrylate and chiral diacrylate which
acts as the dopant. Exposing the acrylate mixture to a low-
intensity UV light induces polymerization in a non-uniform
manner.
[62]
The differing reaction rates of the monoacrylate and
diacrylate to the UV light results in diffusion of more reactive
(diacrylate) materials towards the illuminated side, generating
a variation of the chiral dopant concentration between the top
and bottom of the film (Figure 6a), resulting in a pitch gra-
dient. In this way, a broadband Ch-LC reflecting from 750 to
1050 nm was fabricated.
[62]
By combining two broadband reflec-
tors, either stacking a right- and left-handed film on top of one
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Figure 4. Different orientation of Ch-LC molecules in the cell and their optical behaviors. a) Planar orientation: reflecting a certain wavelength of light
depending on the pitch, b) focal conic orientation: scattering the incident light, c) homeotropic orientation: transparent for all the wavelengths of light.
Figure 3. a) Schematic diagram showing the reflection of light by Ch-LCs: reflecting circular
polarized light of same handedness. b) Typical transmission spectrum of Ch-LC.