-
Paving the way
for roll-up screens and switch-on
wallpaper
This project focused
on researching and developing the
revolutionary potential of polymeric
light-emitting diodes (PLEDs) for
light and image display screens. The
replacement of the omnipresent liquid
crystal display with a PLED display
would allow for less costly processing
and, because of the flexibility of
the material, a new range of innovative
applications. Significant progress
has already been made in the application
of this technology - Philips have
incorporated it into their top-of-the-range
'Spectra' shaver - and it may not
be long before we have roll-up TV
and computer screens and switch-on
wallpaper.
A chance observation
made in 1989 by the Cambridge group,
led to the discovery that electrical
excitation of certain semiconducting
plastics caused them to emit light.
The idea soon followed that display
screens formed from polymeric light-emitting
diodes (PLEDs) could replace the formerly
ubiquitous liquid crystal displays.
With surfaces easily coated, for example,
by ink-jet printing, there is no theoretical
limit to the size of the displays
that can be produced. The concept
is revolutionary, consisting of a
thin layer of plastic which has electrodes
on either side of it. The electrodes
injecting electric charge into the
plastic and the energy associated
with the electronic charge is used
to excite the polymer semiconductor
which then generates the image through
emitting light.
As the earlier devices
showed only moderate efficiency and
had lifetimes of just a few hours,
it was evident that the process of
delivering manufacturable technology,
initiated in Cambridge through the
formation of Cambridge Display Technology,
required a coordinated European level
of activity. EU support has enabled
a strong consortium of academic and
industrial partners.
Progress made by the
German project partner - Covion in
the synthesis of polymers has contributed
to a major increase in luminescence
efficiencies. One surprising finding
was that materials, such as the PPV
family, do not exhibit the expected
'solid-state quenching' effect, where
substances that fluoresce vibrantly
in solution lose this property in
their solid form. PLEDs can now be
produced with efficiencies comparable
to those of a tungsten light-bulb.
Therefore, future devices could be
used not only in displays, but also
for lighting applications. Another
area where results have been very
promising is that of the interfaces
between the polymers and the metallic
electrodes. Interfacial chemistry
has been assessed in Mons and measured
at Linkoping, and reveals properties
that are unexpectedly favourable.
Philips was first
to market a product derived from the
work: a pilot-scale line at its factory
in Heerlen, the Netherlands is manufacturing
monochrome passive-matrix screens
for the top-of-the-range 'Spectra'
electric shaver (which notably featured
in the 2002 James Bond film Die Another
Day). However, real commercial success
will come with the introduction of
full colour displays. The consortium
has demonstrated a small screen giving
satisfactory results at full video-rate
speeds. Viable production of the required
three primary colours is dependent
on diode durability. Lifetime (measured
as the time to ½ brightness) now exceeds
20,000 hours for the yellow/green
and red diodes, and the durability
of blue emitters has required the
most effort, though these are now
approaching similar levels of performance.
PLED screens for computers and TV
sets could be in service before the
end of the decade.
Currently, displays
are limited to deposited glass or
silicon backings, but with flexible
plastic substrates even reel-to-reel
processing would be possible. This
would effectively overcome the problem
of size limitations, while offering
the economies of scale benefits of
continuous mass production, as well
as the convenience and environmental
gains of operating under ambient conditions.
Cambridge Display Technology and Philips
are actively pursuing the use of direct
ink-jet printing. Top-end printers
already offer adequate resolution
and uniformity; it remains only to
develop the appropriate 'inks' and
matching print heads.
Contact:
Prof. Richard Friend
of the University of Cambridge (UK)
in association with researchers from
Cambridge Display Technology (UK),
Materia Nova in Mons (Belgium), Linköping
University (Sweden), Philips Electronics
Nederland in Eindhoven (the Netherlands)
and Covion Organic Semiconductors
in Frankfurt am Main (Germany).
Website: http://www-oe.phy.cam.ac.uk
