This is only a preview of the August 2003 issue of Silicon Chip. You can view 31 of the 104 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Items relevant to "PC Infrared Remote Receiver":
Items relevant to "Digital Instrument Display For Cars, Pt.1":
Items relevant to "Home-Brew Weatherproof 2.4GHz WiFi Antennas":
Items relevant to "Fitting A Wireless Microphone To The PortaPAL":
Items relevant to "Jazzy Heart Electronic Jewellery":
Articles in this series:
Purchase a printed copy of this issue for $10.00. |
OLED DISPLAYS –
Better than Plasma
or LCD!
By PETER SMITH
Flat panel displays come in two types: LCD and Plasma. Right? No, there’s
a third type just being introduced – the Organic LED, or OLED display. It is
brighter, has much better contrast, wider viewing angle, uses less power and
has faster response time. It looks set to take over as the flat panel display of
choice, for small appliances, computer monitors and large TV sets.
8 Silicon Chip
www.siliconchip.com.au
I
n 2002, OLED displays began to appear in small
consumer appliances like cameras and mobile phones.
The superiority of this new technology will ensure
that it replaces LCDs in many more applications within
the next few years. And that might just be the beginning!
What is an OLED?
Scientists have long known about the electrolumin-escence of organic crystals. Early attempts at generating light
with organic electroluminescent (EL) devices were not
developed past the experimental stage, as they required
high excitation voltages (upwards of 100V) and were very
power inefficient.
An important step in the evolutionary process began
with the use of thin-film organic layers. The first EL thinfilm device used a single organic layer sandwiched between
two injecting electrodes (Fig.1).
Operation of these single-layer devices is relatively straightforward.
When a voltage is applied
across the electrodes,
holes are injected from
the anode and electrons
from the cathode. These
carriers migrate through
the organic layer until
they meet and recombine
Fig.1: the first EL thin-film
to form an exciton. Redevice used a single
laxation from the excited
organic layer sandwiched
to ground states then
between two injecting
occurs, causing emission
electrodes.
of light.
Single-layer EL devices are impractical because of the
extremely accurate matching required between the electrodes and the organic material. Essentially, mismatching
results in carriers crossing the structure without combining
with an opposite sign, thus wasting energy.
In the latest James Bond movie
thriller, Die Another Day, the
hero shaves with a PhilipsNorelco Sensotec. This razor
has a Polymer-based OLED
display showing battery life
and shave-sensitivity settings.
When switched off, it acts as a
mirror! Photos: Philips
Technology breakthrough
K o d a k s c i e n t i s t s C h i n g T a n g a n d
Steve Van Slyke demonstrated an efficient, low-voltage
OLED for the first time in 1987. Their device used two
layers of organic thin-film material.
In the two-layer EL device, one layer is optimised for
hole injection and transport while the other is optimised
for electron injection & transport. In this way, each sign of
charge is blocked at the interface between layers, in effect
“waiting” until a partner is found.
Tang and Van Slyke also improved on the composition
Kodak’s EasyShare LS633 zoom digital camera, available in Australia this year, sports an AM550L 2.2" activematrix display. Kodak boasts that the display is so good that you don’t need a PC to own one! Photo: Kodak
www.siliconchip.com.au
August 2003 9
OK, so Kodak like the model! LCD
versus OLED: the advantages of
having a wide viewing angle are
clearly demonstrated in this shot.
Photo: Kodak
and construction of the EL cell, resulting in a bright, efficient device that operates on less than 10V.
Due to the monopolar nature of the organic layers, EL
devices conduct current in one direction only; in other
words, they behave like diodes, hence the common name
“OLEDs”.
In one and two-layer devices, the organic compounds
must perform two major functions. They must be luminescent as well as hole/electron transporters. By incorporating a third organic layer chosen specifically for
its luminescent qualities, researchers have been able to
further improve efficiencies by optimising each layer for
a specific function.
OLED structure
Fig.2 shows the physical structure of an RGB OLED
cell. A conductive, transparent anode material such at
indium-tin-oxide (ITO) is first deposited on a transparent
substrate. Next, the organic layers are added. Lastly, a
reflective metal cathode of magnesium-silver alloy or
lithium-aluminium completes the structure. Incredibly,
the thickness of the structure, minus the substrate, is only
about 300nm. This means that most of the total weight and
thickness is due to the substrate itself.
OLED types
To date, OLEDs can be divided into two groups, de-
Fig.2: the physical
structure of an
RGB OLED cell.
10 Silicon Chip
pending on the processes used to apply the thin-film
organic layers during manufacture. Small Molecular
OLEDs (SMOLEDs) use organic material with very small
molecular structures. This allows the layers to be built
using sophisticated vacuum vapour deposition.
On the other hand, Polymer OLEDs (Poly-OLEDs) utilise
organic polymers, which consist of much larger molecular
structures. These are commonly applied with simpler
solution processing (spin coating) methods.
Recent advances in chemical-resistant polymers have
also enabled traditional photolithography technichques
to be brought to bear. Inkjet printing methods have also
proved popular due to their high resolution and “on-thefly” design versatility.
OLEDs in colour
Using fluorescent dopants in the luminescent layers,
manufacturers have been able to produce OLEDs in many
colours, including the three primaries (red, green & blue).
White OLEDs are realised with the use of dual emitting
layers of complementary colours. By individual control
of the drive level to each layer, hue can be adjusted from
pale yellow to light blue.
OLED displays versus LCDs
Because of their small size and relatively high efficiency,
OLEDs are ideally suited for use in flat-panel displays.
Fig.3: passive-matrix OLED display
panel concept.
www.siliconchip.com.au
Liquid crystal display (LCD) technology is the current
leader in this area. So how do OLEDs stack up?
As you’ve probably guessed, OLED displays offer significant advantages over LCDs. Being self-luminous, they
require no backlighting. By contrast, LCDs require either
an external light source (reflective type) or a fluorescent or
LED backlight (transflective type). No backlighting means
OLED displays are smaller in size, use less power, weigh
less and cost less.
Their self-luminous nature is also responsible for two
other important advantages. First, they have a virtually
unlimited viewing angle (165°). LCDs, on the other hand,
are limited by the “aperture” effect. In addition, they
have very high brightness and contrast (>100:1). This is
something that LCDs can’t hope to match. A backlit LCD
typically looks “washed out” under bright light.
Equally importantly, OLED displays have almost instantaneous update speed. The response time of LCDs
has always been a problem, particularly when displaying
real-time video. The microsecond switching speeds of the
OLED has entirely eliminated this issue!
In summary, OLED displays have:
* High brightness and contrast
* Ultra-wide viewing angle
* No backlight required
* Thin, compact form factor
* Fast response time
* Low power consumption
Display types
In common with their LCD counterparts, OLED displays
are currently being manufactured in both active and passive types.
Passive-matrix display panels are typically created by
depositing the electrode material in a matrix of rows and
columns (Fig.3). An OLED is formed at the intersection
of each row and column line. Display electronics can
illuminate any OLED (pixel) in the array by driving the
appropriate row line and column line. A video image is
created by sequentially scanning through all rows and
columns, briefly switching on the pixels needed to display
a particular image. An entire display screen is scanned
(“refreshed”) in about 1/60 second.
Active-matrix displays use TFT (thin-film transistor)
technology. Every OLED cell is controlled by at least two
transistors. All transistors in the array are individually
addressable in a row/column format. However, unlike the
passive-matrix display, the transistor circuits retain the
state (on/off) and level (intensity) information programmed
by the display electronics. Therefore, the light output of
every pixel is controlled continuously, rather than being
“pulsed” with high currents just once per refresh cycle.
Active-matrix displays are considerably more expensive
than passive displays, but they boast brighter, sharper
images and use less power.
Monochrome (single colour) displays are generally of
the passive type. Full-colour displays may be either active
or passive. Similarly to other display technologies, the full
colour spectrum is generated by modulating individual
red, blue and green OLED cells positioned side-by-side
in a “triad” arrangement.
Universal Display Corp. has recently announced a different architecture for full-colour display. In their Stacked
www.siliconchip.com.au
Prototype of a highresolution, fullcolour passivematrix PolyLED
display, fabricated
with inkjet printing
techniques. Photo:
Philips Research.
OLED (SOLED), they stack red, green and blue sub-pixels
on top of one another instead of next to one another. This
provides a three-fold increase in display resolution and
enhances picture quality.
Availability
Researchers still have a lot of work to do before OLED
displays are ready for the majority of mainstream applications. Of particular concern is the longevity and intensity
of the light-emitting layers. In addition, manufacturing
methods need to be improved in order to produce high
yields at low costs.
Small passive-matrix OLED displays can already be
found in many consumer items, such as mobile phones,
hand-held games, music systems and in-car instrumentation.
Kodak and Sanyo Electric Co., Ltd., produced the first
full-colour 2.4" active-matrix OLED display in 1999. Less
than a year later, they produced a larger, 5.5" model, and
in 2002 demonstrated a 15" display. Since then, at least
one manufacturer has demonstrated a 19" full-colour
display.
The first commercially available active-matrix display
is to be found in Kodak’s new EasyShare LS633 zoom
digital camera, available in Australia this year (see
photos).
Where to next?
According to some sources, more than 80 companies
and universities around the world are involved in OLED
research. Clearly, there is a great deal of interest and much
potential in this new technology.
For example, several companies have recently demonstrated highly flexible display panels fabricated on plastic
substrates. Apart from making panels much more robust,
this breakthrough could also allow very cheap mass production, where displays are produced in a roll-to-roll,
printed medium style.
Yet another discovery involves the use of non-metallic
transparent anodes. Manufacturers will soon be able to
make OLED panels that are over 85% transparent (when
not active). The applications are mind-boggling!
More reading
This web page has a list of useful links to OLED researchers and manufacturers: www.chipcenter.com/eexpert/
SC
dbraun/main.html
August 2003 11
|