Light

Emitting Diode

Presented byDevansh Gupta

M.Sc Polymer ScienceSemester 1

ContentsA. LED

IntroductionWorking PrincipleAdvantagesDisadvantagesApplications

B. OLEDIntroductionWorking PrincipleAdvantagesDisadvantagesApplication

C. Comparison Between LED & OLEDD. Reference

Light Emitting Diode (LED) Introduction

A light emitting diode (LED) is essentially a PN junction opto-semiconductor that 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. 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.

Working Principle Of LED

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.

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 energy is converted into electromagnetic energy.

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

Forward Bias (Current Flow)

+ -

P region(Hole) N region(Electron)

Electron Path

Current (HOLE) Path

Depletion LayerResistors are used to maintain voltage.

AdvantagesEnergy efficient(produce more light per watt)

Long lifetime(60,000 Hours or more)

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 & temperatureCan not be used in high temperature(Lead to

device failure)

ApplicationsVehicle indicator lights and brake lights.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 signals has been successful & is also growing rapidly.

Organic Light Emitting Diode (OLED) Introduction

OLED - Organic Light Emitting DiodeAn 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.

HistoryThe 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.

Many developments had take place in the year 2012.

Structure Of An OLED

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: It contains two layers. Conducting layer - This layer is made of organic plastic molecules that transport holes

from the anode. One conducting polymer used in OLEDs is Polyaniline.

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.

Polyaniline (Emeraldine Salt)

Polyfluorene

Cathode May or may not be transparent depending on the

type of OLED.The cathode injects electrons when a current

flows through the device.

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.

Disadvantages OLED 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.

 

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

Optimus Maximus Keyboard

Each key can be programmed to perform a series of functions

Keys can be linked to applications such as display notes, numerals, special symbols, etc...

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 Hartmut Versin, Wiley Publishers New York 2000, Page No 121

www.oled-info.com