An OLED (organic light-emitting diode) is a thin film light source that emits light when an electric current is applied. OLEDs are made up of organic layers sandwiched between an anode and cathode. When voltage is applied, electrons from the cathode recombine with electron holes from the anode in the emissive organic layer, releasing energy in the form of photons. OLEDs are thinner, more efficient, and provide better contrast than LCDs. There are different types including passive-matrix, active-matrix, transparent, and flexible OLEDs. OLEDs have advantages like lower costs potential, lightweight flexible designs, and better power efficiency compared to other display technologies, but also have

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1. INTRODUCTION
1.1 What is an OLED?
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.
1.2 The Basic Principal
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
With the discovery of Electroluminescence it became possible to convert electrical energy
directly into light energy without producing heat. So in case of OLED, most of the energy is
converted to light energy which makes it power efficient.
(Figure 1) Radiative Recombination Process

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1.3 History

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2. LITERATURE REVIEW
2.1 OLED Structure
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.
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.
 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.
 Anode - The anode removes electrons when a current flows through the device. The
material used for anode must possess low work function such that electron can be
removed easily. In order to observe the emitted light anode is kept transparent and is
usually made of indium tin oxide (ITO).
 Substrate - The main function of substrate is to supports the OLED structure and
provide mechanical strength to the thin organic layers. It is usually made up of clear
plastic sheet, glass or metallic foil.
(Figure 2) OLED Structure

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2.2 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,
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.
(Figure 5) OLED Light emission process

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2.3 Types of OLED
There are several types of OLEDs:
 Passive-matrix OLED
 Active-matrix OLED
 Transparent OLED
 Top-emitting OLED
 Foldable OLED
 White OLED
 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.
(Figure 6) Passive-matrix OLED

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 Active-matrix OLED (AMOLED)
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
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.
 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.
.
(Figure 7) Active-matrix OLED
(Figure 8) Early prototype of flexible display

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 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.
2.4 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.
(Figure 9) White OLED panel

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2.5 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.
2.6 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
low end) - $25,000
$9,000 - $15,000
Viewing angle The brightness and color on
LCD TVs shift noticeably
over the screen and
depending on viewing angle
170 degree viewing angle
Backlight Yes No
Contrast Ratio 10,000,000:1 100,000,000:1
Weight Lighter compared to plasma
TV
Lighter compared to LED
TV
Brightness and color Brighter than plasma or
OLED
Not as bright as LED
Screen Thickness Thinner than LCD, plasma Even thinner than LED
(hence other TVs)
Energy Use Less for dynamically backlit
LCD TVs, about as much for
statically backlit ones.
Less than LED TVs
Picture Quality Better than most TVs, but not
as good as OLED
Better than LED TV and
other TVs

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4. CONCLUSION AND FUTURE SCOPE
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.
References
[1] http://electronics.howstuffworks.com/oled.html
[2] http://mashable.com/2012/10/03/oled-flexible/
[3] http://www.diffen.com/difference/LED_TV_vs_OLED_TV
[4] http://www.electronicproducts.com/images2/fajb_fundamentals_oled_02_jan2012-.gif
[5] http://www.oled-info.com/history
[6] http://www.docstoc.com/docs/74013021/An-Overview-of-OLED-.
Display-Technology
[7] https://www.egi.eu/export/sites/egi/images/OLED_EarlyProduct.JPG
[8] http://files.spogel.com/enewsline/p-00639--amoled_display_architecture.png
[9] http://upload.wikimedia.org/wikipedia/commons/f/f1/PMOLED.png