DAVID FYFE AND TERRY NICKLIN TECHNOLOGY AND INNOVATION
CAMBRIDGE DISPLAY TECHNOLOGY
A bright future
How chemicals drive the way
we display information
What are PLEDs? Benefits of PLEDs
Imagine a world where your The discovery and development of Unlike existing technologies such as
mobile phone display is soft PLEDs began life in the Cavendish LCDs, PLEDs can be fabricated
tailored into your shirt sleeve Laboratory of Cambridge University in completely on one sheet of glass or
or where your office window 1989. Although the phenomenon of plastic. Benefits include brighter, clearer
blind is instantly transformed light emission by small molecule displays with viewing angles
into a giant interactive organic systems (SMOLEDs) was approaching 180 degrees; a simpler
display screen. Science discovered a few years earlier, Jeremy construction resulting in cheaper, more
fiction? Maybe not for long. Burroughes (working in the research robust display modules; and fast
David Fyfe and Terry Nicklin group of Professor Richard Friend) response times allowing full colour
discuss how the possibilities discovered that LEDs could be made video pictures at low temperatures.
of organic polymer light- using conjugated polymers. The potential for PLED technology is
emitting diodes (PLEDs) are In particular, polyphenylene vinylene not limited to displays. Lighting
becoming a reality. (PPV) was found to emit yellow-green applications are also feasible as
light when sandwiched between a pair increased brightness can be achieved.
of electrodes. The initial device Even the reverse (photovoltaic) effect is
efficiencies were very low, but the team viable where electric power is generated
quickly realised the potential of this by light falling on a suitable polymer.
he availability of immediate
information is an essential part of discovery for manufacturing displays The optimism generated by this
our daily lives. In response, the that emitted their own light. These ‘new-to-the-world’ technology
global display industry is anticipating would offer significant advantages over generated the first ever commercial
and fuelling the dreams of product the main display technology we use ‘spin out’ from Cambridge University
designers and manufacturers for today, such as liquid-crystal display (or when Cambridge Display Technology
brighter, faster, cheaper, more versatile LCD) in which a separate light source (CDT) was formed in 1992. Since that
methods to present information, has to be filtered in several stages to time over US $100 million has been
instructions and entertainment. produce the image we see. injected through private equity funding.
TECHNOLOGY AND INNOVATION
conjugated polymer emissive layer
(PLED); and a cathode, which is
normally a low work function metal
such as calcium or barium capped with
aluminium (see Figure 1).
The self-emitting nature of PLEDs
means that, unlike LCDs, they do not
require backlights, opening up the
promise of low manufacturing costs in
terms of both capital investment and
materials. Cost reductions of
20–40 per cent are expected at maturity.
This is perceived as the most important
producer benefit of PLED displays and
has resulted in over a hundred
companies investing in research and
development, and increasingly, in the
Figure 1 PLED device structure manufacture of PLED displays.
The story does not stop there, as
PLEDs have another advantage over
even their close relatives the SMOLEDs:
they are soluble in common organic
solvents such as toluene and xylene. As
a result, PLEDs can be inkjet printed.
Inkjet Heads This offers the potential of even lower
manufacturing costs as material
utilisation is expected to be greater
Red Green Blue than 50 per cent. Conventional
techniques, such as spin coating, can
have a material wastage greater than
98 per cent. Inkjet printing has become
key to the commercialisation of PLEDs
and is discussed further below.
SMOLEDs, which cannot be solution-
processed, may be thermally evaporated
Substrate ITO Interlayer PLED Organic photo-patternable because they use organic materials with
bank structure a sufficiently small molecular weight. This
requires sophisticated and expensive
vacuum vapour deposition techniques.
Figure 2 Schematic of the inkjet printing technique for PLED materials
In order to pattern the materials, the
deposition also has to occur through
Today, much of that early optimism technology in the flat panel display metal masks. The masks limit the area
remains, with perhaps a touch more industry is forecast to rise at an 88 per over which high resolution displays may
realism and commercial reality for co- cent CAGR to US$3 billion by 2007. be manufactured due to difficulties in
operation between all supply chain maintaining alignment between the mask
members to commit to something Device architecture – how it works and the substrate, thereby limiting the
new; and not forgetting the dominance One of the attractive features of PLED cost reductions attributable to large
of the incumbent (LCD) technology, devices is their simple device substrate size production.
which in itself continues to be architecture. A device is essentially Whilst glass substrates have been a
developed for new applications. comprised of an anode (usually indium necessity for conventional displays,
No one should doubt the potential tin oxide on a glass substrate); three PLEDs can be deposited on flexible
for PLED displays to make an impact thin polymer layers comprising a plastic substrates to offer
on the overall market. The total revenue polymer hole conducting layer, an manufacturers greater versatility of
arising from the application of PLED interlayer (described later) and the display form, as well as potential for
Applications of PLED Technology
The first PLED products were launched available with a resolution of 94 × 54,
in 2002 by CDT licensees. Philips 96 × 64 or 128 × 64.
introduced an electric shaver with an Whilst the first PLED products are
orange battery gauge display and small (< 2 inches), low information
launched a secondary cellphone screen content, single colour, passive matrix
in early 2004. Delta Optoelectronics displays, Philips, Toshiba-Matsushita
used a green 16 × 64 display in an Display, DuPont, Microemissive Display
MP3 player. Dupont launched a PLED Ltd., Casio, Samsung SDI and Seiko-
display for the APED range of MP3 Epson have demonstrated full colour
players marketed by Evolution active and passive matrix displays.
Technologies. The display is marketed Toshiba-Matsushita Display exhibited an
with the brand name Olight and it is 2.1 inkjet printed, full colour, video capable,
inches diagonally with a 128 × 64 17 inch, 1280 × 768 PLED prototype
resolution. display at the SID trade show in 2002.
Figure 3 Osram Opto Semiconductors is also Dai Nippon Printing (DNP), another
Philips Electric close to launching its first product CDT licensee, recently showcased
shaver with incorporating PLED technology. It PLED displays incorporated into book
orange gauge currently offers evaluation displays kits covers and posters for dynamic
display branded as Pictiva. The displays are advertising.
further production cost reductions if the process for PLED displays has required used in its printers. Volume variations
displays are manufactured on a roll-to- significant developments of the inkjet across the head of ±2 per cent can be
roll line. Formable substrates allow the print head, the inks and the substrates achieved using DPN. In addition to
production of displays that conform to (see Figure 2). very good volume control, the head has
unique, non-planar shapes – imagine a Creating a full colour, inkjet printed been designed to give drops of ink with
‘wrap round’ car instrument module display requires the precise metering of a very small angle-of-flight variation. A
perhaps; whilst truly flexible substrates volumes in the order of 10 picolitres. 200 dots per inch (dpi) display has
open up a world of display and lighting Red, green and blue polymer solutions colour pixels only 40 microns wide; the
opportunities we can only begin to are jetted into well defined areas with latest print heads have a deviation of
imagine. an angle of flight deviation of less than less than ±5 microns when placed
PLEDs have switching speeds in the 5º. To ensure the displays have uniform 0.5 mm from the substrate. In addition
order of nanoseconds – much faster emission the film thickness has to be to the precision of the print head, the
than LCD displays, which take very uniform. For some materials and formulation of the ink is key to making
milliseconds – and so they are ideal for display applications the film thickness effective and attractive display devices.
video display applications and uniformity may have to be better than The formulation of a dry polymer
television. They also offer more vibrant, ±2 per cent. A conventional inkjet head material into an ink suitable for PLED
high contrast images with real ‘zing’ may have volume variations of up to displays requires that the inkjets
and wide viewing angles. They can do ±20 per cent from the hundred or so reliably at high frequency and that on
all of this at a wide range of nozzles that comprise the head and, in reaching the surface of the substrate,
temperatures (minus 40°C to plus the worst case, a nozzle may be forms a wet film in the correct location
70°C), making them attractive for blocked. For graphic art this variation and dries to a uniformly flat film. The
automotive and military applications. can be averaged out by multi-passing film then has to perform as a useful
with the quality to the print dependent electro-optical material. Recent
Inkjet printing on the number of passes. Although progress in ink formulation and printer
Although inkjet printing is well multi-passing could be used for PLEDs technology has allowed 400 mm
established in printing graphic images, the process would be unacceptably panels to be colour printed in under a
only now are applications emerging in slow. minute.
printing electronics materials. Recently, Spectra, the world’s largest
Approximately a dozen companies supplier of industrial inkjet heads, has
have demonstrated the use of inkjet started to manufacture heads where PLED developments
printing for PLED displays and this the drive conditions for each nozzle can Since the discovery of PLEDs in 1989,
technique is now at the forefront of be adjusted individually – so called significant effort has been directed into
developments in digital electronic drive-per-nozzle (DPN). Litrex in the the development of red, green and blue
materials deposition. However, turning USA, a subsidiary of CDT, has materials that exhibit high efficiency and
inkjet printing into a manufacturing developed software to allow DPN to be stability under normal operating
TECHNOLOGY AND INNOVATION
A recent significant development is
the discovery that inserting a thin
polymer interlayer (about 10 nm thick)
significantly improves both the device
efficiency and the lifetime of PLEDs.
With the interlayer, the external
quantum efficiency (EQE) increases by
170 per cent for a typical red-emitting
PLED and by 58 per cent for green-
emitting PLEDs. Blue PLEDs gain
around 35 per cent higher EQE. These
increases in efficiency are accompanied
by a significant increase in the lifetime
of the devices.
Figure 4 PLED test cells Equally important, this breakthrough
has made it possible to develop a new
conditions to enable integration into flat range of consumer electronic products, range of blue PLED materials that
panel display applications. a stable blue LEP has presented a require a simple barium/aluminium
For a wide range of consumer greater challenge. cathode structure – one which is also
electronic products, the ‘useful lifetime’ In the past several years, Dow compatible with red and green.
(time taken for the device luminance to Chemical, Sumitomo Chemical, CDT One of the hurdles facing any new
drop to half of its initial value) must and Covion (a subsidiary of Britain’s technology is the resistance from those
exceed 10,000 hours. Full colour Avecia) worked on developing new blue using incumbent technologies. LCD
displays typically use groups of three LEP materials and optimised device remains dominant in the display market
adjacent pixels emitting red, green and structures. CDT has now announced and continues to develop at an
blue. Although the green and red the achievement of an extrapolated impressive rate despite being thirty-year-
polymers currently available easily meet lifetime of 30,000 hours from 100 cd/m2 old technology. Nonetheless, all the
the stability specifications required for a at room temperature. current major LCD manufacturers are
Glossary (Primary source: http://www.cdtltd.co.uk)
DPN Drive-per-nozzle an inkjet printer head nozzle with drive conditions that can be adjusted individually
LCD Liquid crystal display an electrically activated material composed of a crystal with a polarising material
on each surface. The crystal twists 900 when an electrical charge is applied to it,
thus blocking light transmission.
LEP Polymer emissive layer alternative terms for PLED.
OLED Organic light-emitting devices that use organic materials to produce light through electrical stimulation.
diode The term OLED includes PLED, SMOLED and dendrimer technologies.
PLEDs Polymer light emitting the most advanced display technology, based on the use of organic polymers
diodes that emit light when stimulated electrically. Also described as polymer light
emitting device or polymer light emitting display. PLEDs are a form of OLED.
Developed after SMOLEDs, PLEDs have the major advantage of being solution
processable, and can therefore be applied to substrates using techniques such
as inkjet printing.
SMOLEDs Small molecule organic the original technology developed to exploit the light emitting property of some
light emitting diodes organic chemicals. It has been the basis of most commercial products to date, but
has the disadvantage of requiring complex and expensive production methods
such as vacuum deposition.
TECHNOLOGY AND INNOVATION
technologies, such as televisions and
computer monitors. PLEDs are
anticipated as the technology of choice
for new products including virtual reality
headsets; a wide range of thin,
lightweight, full colour portable
computing; communications and
information management products; and
conformable or flexible displays.
The exciting possibilities opened up
by these new generation materials does
not raise the question whether they will
make it to the mainstream market, but
just how big that market will be and in
what time frame. s
Figure 5 Futuristic example of how LEP technology could evolve
David Fyfe is a graduate in Natural
looking closely at PLED technologies allow product designers a much freer Sciences from Cambridge
and investing significant sums rein. The environmentally conscious will University and has a PhD from
developing them for future product warm to the absence of toxic London University. He has been
ranges. substances and lower overall material CEO of
Despite the early stage of requirements of PLEDs, and it would not Cambridge
production, one extremely encouraging be an exaggeration to say that all current Display
feature for faster commercialisation of display applications could benefit from Technology,
PLEDs is that Philips have reported the introduction of PLED technology. which holds
manufacturing yields higher than CDT sees PLED technology as fundamental
85 per cent. Products can therefore be being first applied to mobile IP rights for
expected to meet market expectations communications, small and low light
in terms of both performance and information content instrumentation, emitting
cost. The rate of progress has been and appliance displays. With the polymers,
rapid, especially in the past two years, emergence of 3G telecommunications, since July 2000. Previously he has
as the additional investment in high quality displays will be critical for held a number of similar positions
resources has paid dividends. handheld devices. PLEDs are ideal for in companies involved in rapid
the small display market as they offer strategic or technology change.
vibrant, full-colour displays in a He is a member of the UK’s
Future compact, lightweight and flexible form. Chemistry Leadership Council
The future is bright for products Within the next few years, PLEDs are recently established to advise the
incorporating PLED displays. Ultra-light, expected to make significant inroads UK Government on creating an
ultra-thin displays, with low power into markets currently dominated by the innovative, strong British chemical
consumption and excellent readability cathode ray tube and LCD display industry.
Nicklin is a
He has recently joined CDT as
Figure 6 Light emitting polymer materials