Electrically switchable chromogenic materials for external glazing
Daniela Cupelli
a
, Fiore Pasquale Nicoletta
a,
Ã
, Sabrina Manfredi
b
,
Giovanni De Filpo
b
, Giuseppe Chidichimo
b
a
Dipartimento di Scienze Farmaceutiche, Universita
`
della Calabria, 87036 Rende (CS), Italy
b
Dipartimento di Chimica, Universita
`
della Calabria, 87036 Rende (CS), Italy
article info
Article history:
Received 3 July 2008
Accepted 12 November 2008
Available online 21 December 2008
Keywords:
Smart windows
Electrochromism
Liquid crystals
PDLC
Chromogenics
abstract
The development of chromogenic materials is important for their potential applications as electrically
adjustable or thermally self-adjusting light and heat filters in the external glazing of buildings. Recently,
we have reported our investigations on electrolyte–liquid crystal dispersions, which show independent
electro-optical and electrochromic properties characterized by fast bleaching times. Nevertheless, for
the external glazing of buildings, it is recommended to have a transparent off state, which turns opaque
upon application of the external field. In this paper, we present our efforts to homeotropically align by
rough surfaces fluid mixtures of a low molecular mass liquid crystal with a negative dielectric
anisotropy, a liquid crystalline monomer, and electrolyte molecules in order to obtain transparent films
after a polymerization process. Upon application of an ac electric field, the device becomes opaque and
if a dc field is added, the cell changes colour too. We have also investigated the relationship between
electro-optical/chromogenic response and physical chemical properties of the components.
& 2008 Elsevier B.V. All rights reserved.
1. Introduction
Electrically switchable chromogenic materials (ESCMs) are
glazing devices that change either their colour or transmittance
upon application of a suitable electric field. The most important
types of such devices are electrochromic windows, which change
their colour, and dispersed liquid crystal and dispersed particles
that allow controlling the scattering of the impinging light. ESCMs
can be used as ‘‘smart windows’’ for building and automotive
applications in order to control incident daylight and glare [1–3],
according to occupant comfort. ESCMs are currently limited to
side and rear-view mirrors, sunglasses, sun-roofs and, in general,
small area glazing. They are also expected to lower the demand on
precious non-renewable fuels for lighting and cooling.
Both inorganic- and organic-based electrochromic windows are
the most popular ESCMs as confirmed by large number of scientific
publications in recent years [4–8]. The major advantage of
electrochromic materials is that the low electric field must be
applied only during the switching operations. Organic electro-
chromics are based on bipyridilium systems, conducting polymers,
quinones, phthalocyanines, terephthalates, tetrathiafulvalenes,
and cyanobiphenyls, which are molecules able to change their
colour in response to oxidation/reduction processes.
The physical principle used in all liquid crystal-based systems
is the electrically driven reorientation of liquid crystal directors,
which results a change in their transmittance values [9,10].
Among them polymer dispersed liquid crystals (PDCLs) are
composite materials, which show both the advantages of a solid
polymer and a fluid liquid crystal. In fact, they are formed by
either micron-sized liquid crystal droplets embedded in a polymer
matrix, or by liquid crystal that fills the voids and crevices of a
polymer network. In the OFF state PDLCs appear translucent white
due to the light scattering. Upon application of a suitable electric
field, the molecular reorientation will change the refractive index
of liquid crystal domains and the device will appear transparent in
the ON state if the optical matching condition between the
refractive indices of liquid crystal and polymer matrix is fulfilled.
Reverse mode operation PDLCs are devices which are transparent
in the OFF state and turn opaque in the ON state. They allow
overcoming some drawbacks of direct mode PDLCs such as the
opaque unpowered state and the large values of haze. Reverse
mode shutters have been obtained by using dual frequency
addressable liquid crystals [11,12], by polymerizing nematic
emulsions [13], and by functionalizing the liquid crystal/polymer
matrix interface [14]. Recently, we proposed a reverse mode
device obtained by the photopolymerization of mixtures contain-
ing a low-molecular-mass liquid crystal with a negative dielectric
anisotropy, and a liquid crystalline diacrylate monomer home-
otropically aligned by means of rough surfaces [15]. The align-
ment is achieved through typical grooves present in the
substrates, which promote an excellent anchoring and an easy
ARTICLE IN PRESS
Contents lists available at ScienceDirect
journal homepage: www.elsevier.com/locate/solmat
Solar Energy Materials & Solar Cells
0927-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.solmat.2008.11.010
Ã
Corresponding author. Tel.: +39 0984 493194; fax: +39 0984 493298.
E-mail address: fiore.nicoletta@unical.it (F.P. Nicoletta).
Solar Energy Materials & Solar Cells 93 (2009) 329–333