1

Light Emitting Diode

Devansh GuptaM.Sc 1st Year

2

ContentsA. LED

Introduction Working Principle Advantages Disadvantages Applications

B. OLED Introduction Working Principle Advantages Disadvantages Application

C. Comparison Between LED &OLED

3

D. Reference

Light Emitting Diode (LED)

Introduction A light emitting diode (LED) is essentially

a PN junction opto-semiconductorthat emits a monochromatic (single color) light when operated in a forward biased direction.

LEDs convert electrical energy into light energy. They are frequently used as "pilot" lights in electronic appliances to indicate whether the circuit is closed or not.

The most important part of a light emitting diode (LED) is the semi-conductor chip located in the center of the bulb as shown at the right image.

The chip has two regions separated by a junction.

4

1. P region2. N region

The p region is dominated by positive electric charges, and the n region is dominated by negative electric charges.

The junction acts as a barrier to the flow of electrons between the p and the n regions.

When sufficient voltage is applied to the chip, the electrons which is in n region cross the junction and transfer into the p region. This results in current flow.

5

Working Principle Of LED

When a voltage is applied and the current starts to flow, electrons in the n region have sufficient energy to move across the junction into the p region.

Each time an electron recombines with a positive charge, electric potential

When sufficient voltage is applied to the chip across the leads of the LED, electrons can move easily in only one direction across the junction between the p and n regions.

6

energy is converted into electromagnetic energy.

For each recombination of a negative and a positive charge, an electromagnetic energy is emitted in the form of a photon of light. Efficiency this light depends on the semi-conductor materialwhich is usually a combination of the chemical elements gallium, arsenic and phosphorus.

Advantages Energy efficient(producemore light per watt)

Long lifetime(60,000 Hours or more)

7

Rugged(made-up of solid material, no breakage like filament)

No warm-up period(achieve full bright light in nanoseconds)

Not effected by cold temperature(used in sub zero weather)

Directional(direct the light where you want)

Environment Friendly(contains no mercury)

Controllable(brightness and colour can be controlled)

Can sustain over frequent on-off cycle

Disadvantages Very expensive than other lighting

technologies Requires accurate voltage & constant

current flow Can shift colour due to age &

temperature Cannot be used in high

temperature(Lead to device failure)

ApplicationsVehicle indicator lights and brake lights.

8

Currently Audi & BMW integrate high power LEDs.

Mobile phone flash lights.(Surface Mount Diode)

LED screens for advertising & information.Due to low power consumption, small size &

long life LEDs are used in many electrical equipments.(indicator)

Now a days airports, hotels, subways, shopping centers and some homes feature LEDs.

LED based traffic signal has been successful & is also growing rapidly.

Organic Light Emitting Diode (OLED)

Introduction OLED - Organic Light Emitting Diode

9

An OLED is any light emitting diode (LED) which emissive electroluminescent layer is composed of a film of organic compounds.

In OLED a electroluminescent layer is an organic semiconductor material which is sandwiched between two electrodes. One of these electrodes is transparent.

History The first OLED device was developed

by Eastman Kodak in 1987. In 1996, pioneer produces the

world’s first commercial PMOLED. In 2000, many companies like

Motorola, LG etc. developed various displays.

In 2001, Sony developed world’s largest full colour OLED.

In 2002, approximately 3.5 million passive matrix OLED sub-displays were sold, and over 10 million were sold in 2003.

In 2010 and 2011, many companies announced AMOLED displays.

10

Many developments had take place in the year 2012.

11

Structure Of An OLED

12

Substrate (clear plastic, glass, foil) The substrate supports the OLED.

Anode(transparent) The anode removes electrons (adds

electron "holes") when a current flows through the device.

Organic layer Itcontains two layers.

1. Conducting layer - This layer is made of organic

plastic molecules that transport holes from the anode.

One conducting polymer used in OLEDs is Polyaniline.

2. Emissive layer - This layer is made of organic plastic

molecules (different ones from the conducting layer) that transport electrons from the cathode; this is where light is made.

One polymer used in the emissive layer is Polyfluorene.

13

Cathode May or may not be transparent

depending on the type of OLED. The cathode injects electrons when a

current flows through the device.

Polyaniline (Emeraldine Salt)

Polyfluorene

14

How OLED Works

15

Advantages Faster response time than LCDs. Consume significantly less energy. Can be transparent when off. Flexible and conformal displays. Thinner display. Safer for the environment. Wider viewing angles; up to 170

degrees. OLEDs refresh almost 1,000 times

faster then LCDs. Low cost materials and fabrication

method. Less expensive than LCD due to

lesser components. Can be made using plastic screens;

LCDs require glass backing.

16

DisadvantagesOLED seems to be the perfect

technology for all types of displays, but it also has some problems.

While red and green OLED films have longer lifetimes (46,000 to 230,000 hours), blue organics currently have much shorter lifetimes (up to around 14,000 hours).

Currently, manufacturing is more expensive than LCDs.

Water can easily damage OLEDs. OLED screens are even worse than

LCD in direct sunlight. Overall luminance degradation. Limited market availability.

17

Applications Display sources. Mobile phones. Keyboards . Digital watches. Light sources.

18

OLED vs. LCD

Greater view angle. High contrast. Faster response time. Do not require

backlighting. Temperature(~50°C –

80°C).

Limited view angle. Low contrast. Slow response time. Require backlighting. Temperature(~0°C-

100°C).

OLED LCD

Reference Organic Light Emitting Devices By Joseph

Shinar-springer- Verlag, New York, 2004, Page No 150

Organic Electronic Materials By Riccardo Farchichi,g.Grosso,2000, Page No 135

Physics Of Semiconductors By Wolf Gang Brutling ,VBH Publishers, New York,1999, Page No 451

Highly Efficient OLED,S With Phosphorescent Materials By HartmutVersin, Wiley Publishers New York 2000, Page No 121

www.oled-info.com