Introduction:-
OLED is a
new technology for thin, efficient and bright displays. There are two types of
OLEDs: Passive-Matrix (PMOLED) and
Active-Matrix (AMOLED).
OLED is a new technology that can
make thin, efficient and bright displays. OLEDs are made from organic
light-emitting materials, and do not require any backlight and filtering
systems that are used in LCDs.
Samsung Galaxy S (AMOLED)
There are two types of OLED displays
- PMOLED and AMOLED. The difference is in the
driving electronics - it can be either Passive Matrix (PM) or Active Matrix
(AM).
A PMOLED MP3 player
A PMOLED display uses a
simple control scheme in which you control each row (or line) in the display
sequentially (one at a time). PMOLED electronics do not contain a storage
capacitor and so the pixels in each line are actually off most of the time.
To compensate for this you need to
use more voltage to make them brighter. If you have 10 lines, for example, you
have to make the one line that is on 10 times as bright (the real number is less
then 10, but that's the general idea).
So while PMOLEDs are easy (and
cheap) to fabricate, they are not efficient and the OLED materials suffer from
lower lifetime (due to the high voltage needed). PMOLED displays are also
restricted in resolution and size (the more lines you have, the more voltage
you have to use). PMOLED displays are usually small (up to 3"
typically) and are used to display character data or small icons: they are
being used in MP3 players, mobile phone sub displays, etc.
An AMOLED (Active-Matrix OLED) is
driven by a TFT which contains a storage capacitor that maintains the line
pixel states, and so enables large size (and large resolution) displays.
AMOLEDs can be made much larger than PMOLED and have no restriction on size or
resolution.
The first OLED products in the
market used PMOLEDs - these were MP3 players, sub-displays on cellphones and
radio decks for automobiles. The displays were small and usually with just one
or two colors. When AMOLED panels started to emerge in 2007 and 2008 we have
seen these larger displays in mobile video players, digital cameras, mobile
phones main displays and even OLED TV sets.
LG AMOLED TV prototype
Today there are several companies that
are working on technologies that actually close the gap between PMOLEDs and
AMOLEDs - offering a sort of hybrid system. The promise is that these displays
will be both easy to make and allow power efficient larger displays. We still
have to wait and see whether these technologies actually work on commercial
displays.
AMOLED
OLED displays are made from organic
(carbon based) materials that emit light when electricity is applied. OLEDs can
be used to create displays - and these are bright and efficient with a fast
response time and a wide viewing angle. OLED display can be made very thin (the
thinnest prototype is 50 microns...) and even transparent or flexible. The
possibilities are almost endless...
Samsung Transparent AMOLED prototype
The term AMOLED means Active-Matrix OLED. The 'active-matrix' part
refers to the driving electronics, or the TFT layer. When you display an image,
you actually display it line by line (sequentially) as you can only change one
line at a time. An AMOLED uses a TFT which contains a storage capacitor which
maintains the line pixel states, and so enables large size (and large
resolution) displays
A PMOLED uses a simpler kind of driver
electronics - without a storage capacitor. This means that each line is turned
off when you move to the next line. So let's say you have 10 rows in your
display - each row will only be on 1/10 of the time.
The
brightness of each row has to be 10 times the brightness you'd get in an
AMOLED. So you use more voltage which shortens the lifetime of the OLED
materials and also results in a less efficient display. So while PMOLEDs are
cheaper to make than AMOLEDs they are limited in size and resolution (the
largest PMOLED is only 5", and most of them are around 1" to
3"). Most PMOLEDs are used for character display, and not to show photos
or videos.
OSD 2 color 0.96-inch PMOLED
These terms relate to the driving method
of the OLED display. A PMOLED (Passive-Matrix OLED) is limited in size and
resolution, but is cheaper and easier to make than an AMOLED (which uses an
Active-Matrix). An AMOLED uses an active-matrix TFT array and storage
capacitors. While these displays are more efficient and can be made large, they
are also more complicated to make.
PMOLED displays are used in mp3 players or
secondary displays on cell phones while AMOLEDs are used in Smartphone
displays, digital cameras and TVs.
Samsung is the clear leader in AMOLED
production. Samsung are actually using the term AMOLED to brand these kinds of
displays. Samsung is making 2" to 5" panels today, used in many
mobile phones, digital cameras and other mobile devices. Samsung is also showing
prototypes of larger (14" up to 42") AMOLED panels, but these aren't
produced yet.
Samsung Galaxy S
Samsung's Super-AMOLED displays are AMOLED
displays with an integrated touch function. Samsung has placed a touch-sensor
(on-cell) over the display and made it evaporate. The thickness of the touch
sensor is just 0.001mm and this allows the screen to provide better images and
to have great visibility even in direct sunlight. Super-AMOLEDs also have
better lifetime than regular AMOLED and are very responsive to touch. In
January 2011 Samsung announced the 2nd-generation Super AMOLED Plus displays
which offer more sub-pixels (they no longer use the PenTile matrix) and are
also thinner, brighter and more efficient (by 18%) than the old Super AMOLED
displays.
All OLED TV panels will actually
be AMOLED TVs... Sony has been the first to make such a TV, the XEL-1 (back in
2007). Since then they have stopped production and marketing in Japan. The
AMOLED TV was more of a technology demo than anything else. Even though it
costs around $2500 for a 11" display, they were losing money on each set.
Sony XEL-1
LG are the second company to introduce an
AMOLED TV, the EL9500 which is a 15" TV that is also very expensive at
$2500, and currently sold only in Korea and Europe
LG 15-inch OLED TVs
OLED technology
OLEDs are made from organic (carbon based)
materials that emit light when electricity is applied. Because OLEDs do not
require a backlight and filters (unlike LCD displays), they are more efficient,
simpler to make, and much thinner. OLEDs have a great picture quality -
brilliant colors, fast response rate and a wide viewing angle.
LG 15-inch OLED
prototype
OLEDs can also be used to make OLED
Lighting - thin, efficient and without any bad metals.
OLLA White Light
Prototype
OLED materials have been discovered back in 1960, but only in the past
20 years or so have researchers started to actually work on the technology.
The basic structure of an OLED is a
cathode (which injects electrons), an emissive layer and an anode (which
removes electrons). Modern OLED devices use many more layers in order to make
them more efficient, but the basic functionality remains the same.
Making an OLED involves several steps:
taking a substrate, cleaning it, making the backplane (the switching and
driving circuitry), depositing and patterning the organic layers and finally
encapsulation the whole thing to prevent dust, oxygen and moisture damage.
There are several ways to deposit and
pattern the organic layers. Currently all OLED displays are made using vacuum
evaporation, using a Shadow Mask (FMM, Fine Metal Mask) to pattern. This is a
relatively simple method but it is inefficient and very difficult to scale up
to large substrates. There are several alternatives for next-gen deposition
techniques, including laser annealing and inkjet printing. These methods will
be scalable and more efficient than vacuum deposition.
There are several types of OLED materials.
The most basic division is between small-molecule OLEDs and large molecule ones
(called Polymer OLEDs, or P-OLEDs). Almost all OLEDs made today are SM-OLED based.
These materials are evaporable and far more advanced than P-OLEDs. P-OLEDs had
great promise and are solution processable (and so can be used in InkJet
printing and spin-coating fabrication methods). Intensive research is being
performed to develop efficient solution-processable SM-OLEDs.
Another interesting division is between
Fluorescent and Phosphorescent materials. Fluorescent materials last longer
(and were discovered first) but are much less efficient than Phosphorescent
materials. Most people agree that the future of OLEDs (especially in large-area
displays and lighting panels) lie with Phosphorescent materials, although there
are still challenges in developing a long-lasing blue Phosphorescent OLED. It
is possible to combine these materials though, and today Samsung for example
use a red PHOLED together with Fluorescent green and blue. Universal Display
Corporation is pioneering PHOLED research, holding basic patents in this area.
The two major challenges facing the OLED
industry is the lifetime of the panels (OLED panels still lag behind plasma and
LCD displays) and production scaling beyond Gen-5.5.
Today OLED displays are used mainly in
small (2" to 5") displays for mobile devices such as phones, cameras
and MP3 players. OLED displays carry a price premium over LCDs, but offer
brighter pictures and better power efficiency - making it ideal for battery
powered gadgets.
Making larger OLEDs is possible, but
difficult and expensive. There are some OLED TVs available, but these are
expensive. Sony has announced the XEL-1 11quot; OLED TV back in 2007 - at about
$2,500 (they aren't producing it anymore and now focus on professional OLED
monitors). LG is also offering an OLED TV (the 15" EL9500) which is also expensive
and isn't being mass produced. Mass production of price-competitive OLED TV
sets will probably begin towards the end of 2012 or early 2013.
In the OLED lighting market, several
companies (such as Philips, OSRAM and Lumiotec) are already shipping OLED panels,
but these are small and very expensive, mostly used in premium lighting
fixtures and as experimental design kits.
In the future, companies will be able to
produce flexible and transparent OLED panels. This will open up a whole world
of exciting applications, such as:
ITRI 4.1 Flexible AMOLED prototype
Flexible OLEDs require that the entire
device is flexible - including the electronics and the encapsulation layer.
Several companies are working on this technology, using either plastic or metal
based displays (it's also possible to use very thin flexible glass).
Transparent OLEDs are also difficult to make, although these are already in
production: since May 2011 TDK are mass producing transparent PMOLED displays
for mobile phones and other applications.
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