Chris Lord
ESC 482
12/12/08
Organic Light Emitting Diodes
Organic Light Emitting Diode technology is an emerging new form of technology used in
electronic display and lighting. This new form of technology is especially evident in new
television design. OLED televisions are becoming very popular; however, the price for an OLED
television is very expensive, at least for now. What makes OLED televisions so remarkable is
that they consume less power than televisions in today’s market and they are incredibly thin.
Sony’s first OLED television, the XEL-1 is an 11” television that is only 3mm thick. The research
and development of OLED televisions is becoming very popular today. Samsung for example,
announced that they have successfully designed a 40” OLED television that has a high definition
resolution of 1,920 X 1,080. This new type of technology is very interesting, but how does an
Organic Light Emitting Diode operate?
A light emitting diode or LED is a semiconductor device that emits light whenever an
electric current is passed through the forward biased region. The light is emitted from the pn-
junction of the semiconductor and this effect is a form of electroluminescence. The difference
between an LED and OLED is that the electroluminescence layer of the diode is composed of a
thin film of organic materials. The first OLED device was invented in the 1980’s by Dr. Ching
Tang and Steven Van Slyke at Eastman Kodak. This diode was engineered to produce light
emission at the middle of the organic material. In order to achieve this result the diode was
designed with the electron and hole layers separated. After testing the first OLED it was
concluded that an OLED requires less operating voltage and is more efficient. This result
started OLED research by many scientists and engineers.
An OLED is composed of five parts which include:
Anode- Electrode in which electrons leave the cell resulting in oxidation. Typically made
of Indium Tin Oxide which is transparent to visible light and has a high work function.
Cathode- Electrode in which electrons enter the cell resulting in reduction. Typically
made of calcium or aluminum which has a low work function.
Substrate- Supports the OLED and is made out of clear plastic, glass, or foil.
Conducting layer- Layer made up of organic plastic molecules that transport holes from
the anode. Polyaniline is one type of polymer used in forming the conducting layer.
Emissive layer- Layer made up of different organic plastic molecules that transport
electrons from the cathode. Polyfluorene is one type of polymer used in forming the
emissive layer.
The following images show the structure of a typical OLED:
One of the most difficult aspects in manufacturing OLED’s is applying the emissive and
conductive layers to the substrate. This can be done by Vacuum Deposition, Organic Vapor
Phase Deposition, or Inkjet Printing. Vacuum Deposition is a process done in a vacuum
chamber that slowly heats the organic molecules to a temperature in which they will
evaporate. Once the organic molecules evaporate they condense on the substrate creating a
thin film. Organic Vapor Phase Deposition is a process in which again the organic molecules are
heated in a low pressure, hot-walled reactor chamber until they evaporate. However, in this
process the molecules are transported onto the substrates by a carrier gas. Using a carrier gas
to transport the molecules increases the efficiency and reduces the cost of the OLED’s. The
final process is the Inkjet Printing process. This process is done exactly has it sounds, by inkjet
technology, which includes spraying the OLED’s onto the substrates via a printing process.
Think of it as an image being printed on a piece of paper. Inkjet Printing is much more cost
savvy compared to the other two described processes. Also, Inkjet Printing gives the designer
the ability to print the OLED’s on a large surface, as in large televisions or electronic displays.
The images below may help understand how these processes are carried out.
Vacuum Deposition: Organic Vapor Phase Deposition:
Inkjet Printing:
The purpose of an OLED is to of course emit light. The color of the light an OLED emits
depends on what type of organic molecule is used on the emissive layer. When trying to emit
certain colors sometimes several layers of organic film will need to be used. The brightness of
the light that the OLED produces can be regulated. The brightness or intensity of the light is
based on the amount of current applied to the OLED, and of course, more current results in
brighter light and vice versa. In an OLED light is created by a recombination process. This is
done by connecting a battery across the anode and cathode, resulting in an electric current
flowing from the cathode to the anode. In order to get to the anode the current must flow
through both the conductive and emissive layer. As the current flows through the layers it
gives electrons to the emissive layer and removes electrons from the conductive layer leaving
holes. These holes now jump to the emissive layer to recombine with electrons. As the holes
are filled with electrons from the emissive layer they release excess energy called light. This is
how OLED’s release light photons.
There are many different types of OLED’s and each type has different attributes. Some
of the types of OLED’s are as follows:
AMOLED = Active Matrix OLED device
FOLED = Flexible Organic Light Emitting Diode (UDC)
OLED = Organic Light Emitting Diode/Device/Display
PHOLED = Phosphorescent Organic Light Emitting Diode (UDC)
PLED = Polymer Light Emitting Diode (CDT)
PMOLED = Passive Matrix OLED device
POLED = Polymer Organic Light Emitting Diode (CDT)
RCOLED = Resonant Color Organic Light Emitting Diode
SMOLED = Small Molecule Organic Light Emitting Diode (Kodak)
SOLED = Stacked Organic Light Emitting Diode (UDC)
TOLED = Transparent Organic Light Emitting Diode (UDC)
Since there are so many types of OLED’s we won’t get into detail for each one.
However, a few of the above will be explained in detail. The PMOLED or Passive Matrix OLED
Device has various anode and cathode strips arranged perpendicular to each other. The
intersection points of the anode and cathode strips are the “pixels” and this is where the light is
emitted. The amount of pixels in a PMOLED device depends on how many anode and cathode
strips there are on the device. In order to turn the pixels “on” and “off” external current is
applied to certain strips of the anode and cathode. An external circuit is applied to the PMOLED
to supply the current to the certain pixels to turn them on and off. This type of OLED works
best in products with smaller screens such as cell phones and iPods. The structure of the
PMOLED is shown below to get a better understanding of how it is organized.
The AMOLED or Active Matrix OLED device are the OLED’s that are used in large screen
televisions, computer monitors, and electronic signs. They are used in these types of
applications because AMOLED devices have a faster refresher rate that works very well with
video. In this device the anode and cathode are not in strips, they are in layers just as the
emissive and conductive layers. Another difference in the AMOLED device is that the anode
overlays a thin film transistor (TFT) array that forms a matrix. This matrix array determines
which pixels get turned “on” and “off” to produce an image. The AMOLED device is more
efficient and uses less power than the PMOLED device since the AMOLED device doesn’t
depend on an external circuit to determine which pixel turns “on” and “off”. The image below
shows the structure of the AMOLED device to help better understand how this device is
organized.
OLED technology is a continuing form of research and is being seen in today’s market in
televisions, notebook computers, keyboards, display systems, and other forms of electronic
devices. As noted before OLED televisions are very expensive and cost much more than an LCD
or Plasma screen television. There is a reason that OLED devices cost much more than the
more common LCD display. When comparing an OLED television to a LCD television there are
many differences. One of the main differences is that in OLED sets there isn’t a backlight or
twisting crystals as in LCD sets. The reason for this is because the organic molecules on the
substrate emit light when current is passed through. In an LCD set a backlight is needed to
produce light through the twisting crystals. Since OLED television does not require a backlight
they require much less power to operate than the LCD television. Another major difference
between OLED and LCD sets is the picture quality.
When comparing the contrast and black levels the OLED does excel in this area. Sony
manufactures both LCD and OLED televisions and there new OLED television’s contrast level far
exceeds the level of their LCD set. The stated contrast ratio for this new television is
1,000,000:1, while the highest contrast level for the LCD is 10,000:1. One other big difference
in picture quality between the two is the black level that is displayed. For an OLED, since no
backlight is required the organic compounds obtain deep black levels fairly easily by limited
conductivity. Producing the black levels doesn’t require much power consumption because
hardly any light is needed. However, producing bright colors requires the most power
consumption; therefore more current will be needed to obtain white light as well as other
bright colors. An interesting fact between an OLED and LCD is that there power consumption
level is the opposite. For an LCD to produce bright colors it requires less power and to produce
dark colors it requires the most power. This is because the higher the voltage that passes
through the crystal the more the crystals untwist to block the incoming light from the back
light. This is a significant drawback in LCD technology as when the viewing is off axis the black
levels seem to consistently drop. OLED’s do not suffer from this problem, as OLED’s are capable
of producing black and dark matter very well.
Color quality in OLED displays is much more deep and realistic than the color LCD’s
produce. In an OLED each pixel contains red, green, and blue organic molecules that are
capable of producing millions of colors. The color that is reproduced in an OLED is much more
accurate than in an LCD. The chromaticity coordinates are also more accurate in an OLED. In a
LCD display colors are produced by subtracting colors from white light which is done by
manipulating light waves. This is a difficult approach in producing accurate and vibrant colors,
however most of the higher end LCD televisions conquer this challenge fairly well. Although,
when directly comparing an OLED to a LCD screen it will be seen that the color will be more
realistic and richer in an OLED screen. OLED sets are also said to have a “perfect” viewing angle
because the organic molecules create light rather than block light. This gives the viewer a
perfect picture to look at up to 170 degrees. In a LCD light is blocked to create the darker
images, therefore when the viewing angle is increases the contrast is significantly affected and
the image is hard to see. Overall, when looking for the best picture quality an OLED television is
much better in all aspects when compared to a LCD television.
OLED displays are also much faster than LCD displays. OLED displays are known to be
the fastest in response rate time compared to all other display technologies. This fast response
time is due to the OLED’s using TFT technology as described previously. OLED displays refresh
up to 1000x faster than LCD displays. This makes OLED displays capable of changing
information almost in “real time”. LCD displays suffer from motion blurs and the “trailer
effect”. The trailer effect is when the individual pixels are slightly out of step with the image on
the screen. When looking at the screen if it seems the picture is jumping around as the camera
is moved this is the “trailer effect”. This lag time is most notable in fast moving sports game
where as if you look closely enough you may detect this on the display.
Another advantage of an OLED television is that they are very thin in depth. As noted
before the first OLED Sony produced was only 3mm thick. It is projected that one day OLED
televisions may be flexible and large enough to cover a 9’ X 9’ wall. This is probably one of the
biggest advantages OLED technology has over LCD and Plasma. LCD and Plasma screens have
very good quality as well, but what gives OLED an edge is that if one day the screens become
flexible, this could lead to the use of OLED technology in many new applications.
With all of these comparisons it seems that the OLED televisions blow the LCD
televisions out of the water in every aspect. However, there are some drawbacks with the
OLED televisions. One of the major differences is the price. OLED’s are very expensive when
comparing to the prices to LCD televisions. The Sony XEL-1 OLED set is an 11” screen and is
priced at $2499, whereas a 15” LCD screen costs about $180. This is a major difference in price
and most people will go for the better price before quality, especially when you are paying
$2320 more for quality. Another major drawback of an OLED is there lifetime. The OLED
contains red, green, and blue organic molecules. The blue organic molecule has a lifetime of
about 7000 hours, while the red and green have longer lifetimes of about 46,000 hours. This is
a major drawback for OLED technology. Even though 7,000 hours seems like a long time it’s
really not especially when comparing to the lifetime of a standard LCD television. A standard
LCD television is rated to last, on average, 50,000-65,000 hours which is about 8x longer than
an OLED television.
Overall OLED technology is a work in progress. The benefit of OLED technology is very
remarkable when comparing to the top technology of today’s market. Work needs to be done
to try to make the blue OLED last longer and to reduce the cost of OLED displays. In today’s
market OLED displays can be found in cell phones, PDAs, digital cameras, and televisions. The
future for OLED technology is geared towards someday producing heads-up displays,
automotive dashboards, billboard displays, home-office lighting, and flexible displays. It has
even been said that OLED technology could lead to the newspaper of the future. If OLED
technology gets to the point of flexible displays, the OLED displays could update as breaking
news occurs, and when the reader is finished they will be able to fold up the display and put it
away until later. OLED technology is something that is going to be very popular in the future; at
least I think it will be.