Civil Engineering Reference
In-Depth Information
There are three different kinds of layered materials in the device: The
electrolyte is a pure ionic conductor and separates the two electrochromic
fi lms (or separates a single such fi lm from an optically inactive ion storage
fi lm). The electrochromic fi lms are mixed conductors of ions and electrons,
whereas the transparent conductors conduct nothing but electrons. Optical
absorption sets in when electrons are inserted into the electrochromic
fi lm(s) together with the ions from the electrolyte and are localized on
metal ions. The valence of these ions is then changed, and when the 'extra'
electrons interact with the incident light they can acquire enough energy to
jump across a potential barrier to a neighbouring metal ion site. The absorp-
tion mechanism is conventionally referred to as 'polaron absorption' in
physics and as 'intervalency absorption' in chemistry (a somewhat more
detailed explanation is given in Section 11.2.3 below). This simplistic expla-
nation of how the electrochromic devices work indicates that they can be
viewed as thin-fi lm batteries with a charging state that corresponds to the
intensity of the optical absorption.
It is possible, already at this point, to introduce a number of interesting
properties of electrochromic glazings which make them highly relevant to
eco-effi cient building technology (Granqvist, 2012):
• The devices have open circuit memory, like batteries, which means that
they can keep their optical and charging properties for extended periods
of time without drawing any current, depending on the quality of the
electrical insulation of the electrolyte.
• The optical absorption can be set at any level between two extreme
values.
• The optical changes are gradual and occur over times ranging from
seconds to tens of minutes, depending on the size of the device; this time
can be compared with the eyes' ability to adapt to changes in light, which
takes minutes.
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The optical properties are founded on processes at the atomic scale, and
hence an electrochromic glazing can be free of haze, which is an essential
feature for most building-related applications.
By combining two different electrochromic fi lms it is possible to adjust
the overall optical transmittance and achieve better colour neutrality
than with a single electrochromic fi lm.
If the electrolyte is a solid and adhesive bulk-like polymer, the electro-
chromic glazing can combine its optical function with spall shielding,
burglar protection, acoustic damping, near-infrared absorption, etc.
The electrochromic technology is not an easy one - which explains why
it has taken so long to mature - and several more or less non-standard
technologies must be mastered (Granqvist, 2008). Six challenges stand out
for practical electrochromic glazings as listed below:
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