1. INTRODUCTION
An OLED is any light emitting diode in which organic layers are responsible for light
emission. OLEDs are solid-state devices made up of thin films of organic molecules that
produce light when exposed to strong electrical field. This layer of organic material is
situated between two electrodes anode and cathode, all placed on a substrate. OLED can be
made extremely thin that is about 200 times smaller than a human hair. The most amazing
fact of OLED technology is that it is active display source meaning that it emits light itself
unlike the LCD technology that is dependent on external backlight unit to produce light.
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3. Working Principle
OLEDs emit light in a similar manner to LEDs, through a process that called
Electroluminescence. It is the result of recombination of electrons and holes in a material.
Electroluminescence is a process in which a material emits light in response to electrical
field applied across it. The excited electrons release their energy as photons.
Prior to recombination, electrons were at higher energy level which is also known as excited
state.As electron and holes recombines, electrons fall down to ground state. And the energy
difference is radiated as photons. The wave length of emitted light depends upon the energy
difference of the defect level by the equation E = h. f
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Structure of OLED
The basic structure of an OLED consists of two thin film of organic material
sandwiched between two electrodes anode and cathode and light is produced by
recombination of holes and electrons at the boundary of two organic layers
.Emissive layer - That transport electrons from the cathode. It is termed as emissive
layer because the organic molecules in this layer are responsible for light emission. The
organic material used here in the emissive layer is polyfluorene.
Conducting layer – As the name suggest the organic material used here is
conductive. The layer transport holes from cathode. The conducting polymer used in here is
polyaniline.
Conducting layer – As the name suggest the organic material used here is
conductive. The layer transport holes from cathode. The conducting polymer used in here is
polyaniline.
An OLED consists of the following parts:
Cathode -The cathode injects electrons to the organic layers when a current flows
through the device. It may be transparent depending upon type of OLED.
Organic layers - These layers are made of organic molecules or polymers. The
thickness of the organic layer is between 100 and 150 nm.
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How Do OLEDs emit light?
If voltage is applied across the anode and cathode. Current flows through the organic layers.
The cathode injects electrons to the emissive layer. The anode removes electrons from the
conductive layer of organic molecules. That is same as giving holes to the conductive layer.
So the emissive layer behaves as N-type material and the conductive layer behaves as P-type.
At the boundary of the two organic layers, electrons recombines with holes. When this
happens,
OLED Light emission process
the electron radiates energy in the form of a photon.
The color of the light depends on the type of organic material used in the emissive layer.
Manufacturers can arrange organic films of RGB layers to make color displays. Again, the
intensity or brightness of the light depends on the amount of current applied.
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Types of OLED
There are several types of OLEDs:
Passive-matrix OLED
Active-matrix OLED
Transparent OLED
Top-emitting OLED
Foldable OLED
White OLED
8. Passive-matrix OLED (PMOLED)
PMOLEDs have cathode strips, organic
layers and anode strips. The cathode strips
and anode strips are kept perpendicular to
each other. The intersection of cathode strips
and anode strips makes one pixel from where
light is emitted. External circuitry applies
current to selected strips of cathode and
anode, decides which pixels get
turned on and which pixels turned off. The brightness of each pixel is directly proportional to
the amount of applied current. PMOLEDs manufacturing is easy, but they consume more
power than other types of Organic-LED, because PMOLEDs need external circuitry for
power on. PMOLEDs are the best for text and icons and suited for small screens such as in
mobile phones, MP3 music players. Even if we use external circuitry than also PMOLEDs
consume less power than current used LCD displays.
AMOLEDs have full layers of cathode, organic materials and anode, but in this type of
OLEDs anode layer is overlays of a thin film transistor (TFT). The TFT array has some
controlling circuitry that determines which pixels get
Active-matrix OLED
turned on to form an image.
AMOLEDs consume less power than
PMOLEDs because the TFT array requires
less power than external circuitry used in
PMOLEDs, so they are more suitable for
large displays like computer monitor and
television screen. AMOLEDs also have faster
refresh rates suitable for video.
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9. Transparent OLED
Transparent OLEDs have only transparent components that is substrate, cathode and anode.
When the display is turned off, are up to 84 % as transparent as their substrate. When the
transparent OLED turns on it emits light in both direction. A transparent OLED display can
be either active-matrix or passive-matrix. Transparent OLED can be used for heads-up
displays in cars.
Top-emitting OLED
This types of a OLEDs have a substrate that is either opaque or reflective. They are best
suited to active-matrix design. Manufacturers used this Top-emitting OLED in smart cards.
Foldable OLED
Foldable OLEDs have substrates made of very flexible metallic foils or plastics. Foldable
OLEDs are very less weight and durable. Their use in
devices such as mobile phone and PDAs can reduce
breaking of a device, a major cause for return or repair.
Potentially, foldable OLED displays can be attached to
create "smart" clothing, such as outdoor survival
clothing with some computer chip, mobile phone, GPS.
Early prototype of flexible display
White OLED
White OLEDs emit deep white light that is brighter and
more energy efficient than that light emitted by
conventional lights. White OLEDs also have the true-color
qualities of incandescent lighting. In future OLEDs can be
replace fluorescent lights that are currently used in
homes lightning and buildings. By using this technology we reduce energy costs for lighting.
.
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10. Advantages
Lower cost in the future-
OLEDs can be printed onto any suitable substrate by an inkjet printer or even by
screen printing, fundamentally making them cheaper to produce than LCD or plasma
display. The manufacturing process of OLEDs substrate is expensive than normal
LCSs. Roll-to-roll vapor deposition methods for organic devices that allow mass
production of devices per less time for minimum amount of cost.
Lightweight and flexible plastic substrates-
By using flexible plastic substrate or foils we can make our OLED displays more
flexible and lightweight. Example of flexible display, we can use it in heads up display
in cars and making creative printed integrated circuit on clothing. As the substrate used
can be flexible such as polyethylene terephthalate (PET), so that the display can be
inexpensive. We use plastic substrate which has low shatter resistant than glass substrate.
Better power efficiency and thickness
LCDs uses backlight to produce picture on screen and that backlight passes through filter so
it’s allow small fraction of light through it. So, LCDs cannot show true deep black color.
However, an inactive OLED does not produce any kind of light or consume power, thus
OLED allowing true deep blacks. Here we are not using any backlight, so it also makes
OLEDs lighter. This allows electronics device to be manufactured pocket friendly.
Response time-
OLEDs response time is a much faster than LEDs. By Using response time compensation
technologies, LCDs can go up to 1ms response time for its fastest color transition. OLED
response time is much faster than LCDs and it is up to 1000 times faster.
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11. Disadvantages
Lifespan- The biggest problem with OLEDs is average lifetime of the organic
materials. The main reason being degradation of organic material over time. Particularly
the blue OLEDs have very short lifetime. However, R&D in the field OLED technology
improved lifetime of OLED display in last few decades.
Water damage- The organic material used in the display are vulnerable to water. That
means water can easily damage the display. Water damage limits the lifetime and
durability of the OLED displays.
Outdoor performance- We know that the OLED is self-emissive. So the organic layer
between anode and cathode radiates light. In order to observe light anode is made transparent
and cathode is made reflective up to 80%. This may lead to poor readability in bright
environment.
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12. LED Vs. OLED
LED OLED
Thickness 30 mm 4 mm
Power consumption 230 W 74 W
Screen size Up to 90 inches Up to 55 inches (yet)
Life span Around 100,000 hours Recent improvements allow
up to 43,800 hours
Cost $100 (small size and very $9,000 - $15,000
low end) - $25,000
Viewing angle The brightness and color on 170 degree viewing angle
LCD TVs shift noticeably
over the screen and
depending on viewing angle
Backlight Yes No
Contrast Ratio 10,000,000:1 100,000,000:1
Weight Lighter compared to plasma Lighter compared to LED
TV TV
Brightness and color Brighter than plasma or Not as bright as LED
OLED
Screen Thickness Thinner than LCD, plasma Even thinner than LED
(hence other TVs)
Energy Use Less for dynamically backlit Less than LED TVs
LCD TVs, about as much for
statically backlit ones.
Picture Quality Better than most TVs, but not Better than LED TV and
as good as OLED other TVs
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CONCLUSION
13. Research and development in the field of OLEDs is proceeding rapidly and may lead to
future applications such as flexible display, transparent display, Light Emitting Wall, heads-
up display. The newspaper of the future might be an OLED display that refreshes with
breaking news and like a regular newspaper, you could fold it up when you're done reading it
and stick it in your briefcase. OLED can be considered as one of the most promising
upcoming technology. The success story of OLED is yet to be written.
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