OLEDs

Bipin BabyII Msc PhysicsCUSAT

Discovery of OLED Electroluminescence has been observed from single crystals of

anthracene in 1960

in 1987 Tang and van Slyke from Kodak reported efficient and low voltage OLEDs from thin films organic materials.

They used a hole-transporting molecular film of aromatic diamine as emitting layer

electron-transporting metal chelate tris-hydroxyquinoline aluminium(Alq3) between the transparent Indium Tin Oxide (ITO) anode and cathode

aluminium and magnesium as the cathode

consists of a thin film of organic material sandwiched between two electrodes.

The anode is transparent and is usually made of Indium Tin Oxide (ITO) while the cathode is reflective and is made of metal(Ca or Mg)

The thickness of the organic layer is very thin (typically between 100 nm and150 nm thick) to avoid the need of high working voltage because the organic material is almost an insulator

OLED Structure

The OLED structure is similar to inorganic LEDs: an emitting layer between an anode and a cathode

Holes and electrons are injected from the anode and cathode; when the charge carriers annihilate in the middle organic layer, a photon is emitted

The process of electroluminescence involves• charge injection• charge transport• carriers recombination• radiative decay

When a voltage is applied between the electrodes, charges are injected in the organic material

holes from the anode and electrons from the cathode Then, the charges move inside the material, generally by

hopping processes and then recombine to form excitons

The colour of the photon is a function of the energy difference between the highest occupied molecular orbital (HOMO)and the lowest unoccupied molecular orbital (LUMO) levels of the electroluminescent molecule

OLED Operating Mechanism

Classification of OLEDs

OLED devices can be divided into two classes: depending on the type of organics used

• Small molecule devices (SMOLED)• Organic polymer devices (PLED or LEP).

SMOLED

Small short-chain organic semiconductor molecules are formed by a string of benzene rings

In these rings, the π-bonds are delocalized to form a π-system. With increasing delocalization, the breach between occupied and

empty states in these π-systems squeezes, leading to smaller bandgap

Structure of SMOLED

Polymer OLED

Polymer OLED molecules have π-bonds that are delocalized along the chain forming a one-dimensional system

Delocalization takes place by forming a conjugated vertebral column of continuous overlapping orbitals composed of alternating single and double carbon–carbon bonds

Polymer OLED Structure

PEDOT:PSS or poly(3,4-ethylenedioxythiophene) polystyrene sulfonate is used as a transparent, conductive polymer

Polyaniline can be used

SMOLED Vs Polymer OLED

SMOLED emitts bright light but cost of production is expensive,uses manufacturing method called vaccum deposition

Polymer LED is manufactured by Spin Coating and Inkjet printing POLED is more suitable large area displays

Manufacturing Methodes

1. Spin coating 2. Vaccum evapouration3. Ink-jet Printing

Spin coating

organic material is deposited in liquid form on a substrate in excess

The substrate is rotated at high speed causing the liquid to spread out across the substance

The liquid will form a thin layer and solidify as it evaporates

Films produced this way tend to have inconsistent thickness as well as poor surface smoothness

Vaccum evaporation

organic materials are evaporated or sublimed in high vacuum to form molecular beams towards the substrate

organic materials are deposited onto a substrate through a thin metal stencil, also known as a "shadow mask”

This process is problematic, as a significant amount of the material is wasted because it disperses all over the mask

Applications

OLEDs are is used for making Displays Lighting

Advantages

Thinner, lighter and more flexible Can be made to larger sizes Brighter. High resolution, <5μm pixel size Large fields of view, about 170 degrees Faster response time

Disadvantages

Expensive emitter is sensitive to oxidation and humidity Lifetime. While red and green OLED films have long

lifetimes(10,000 to 40,000 hours), blue organics currently have much shorter lifetimes (only about 1000 hours

References

• Vinod kumar Khanna,Fundamentals of solid state lighting,2014• Yi-Lu chung,Efficient organic Light-Emitting diodes,CRC

Press,2016• Takatoshi Tsujimura,OLED Display fundamentals and

applications,Wiley,2012• https://en.Wikipedia.org/wiki/OLED