Organic Light Emitting Diodes or OLEDs are an advanced form of LEDs. They have an emissive
electroluminescent layer of films which is made up of organic molecules to emit light when
connected to an electrical source.
The invention of OLEDs happened after discovering organic semiconductors in the mid-1970s
by Alan McDiarmid, Alan Heeger and Hideki Shirakawa. They won the Nobel Prize in chemistry
in 2000. The first current efficient OLED was called “a novel electroluminescent device”, constructed
using organic materials.
In ordinary LEDs, p-n junction is powered
up with the electrical source and
the electron flow through this junction
emits light. To be simple, an ordinary
LED is a junction diode. OLEDs
use the same mechanism in a different
way. OLEDs are made up of six layers
including layers of n-type and p-type
semiconductors.
1. Seal - Glass top plate.
2. Cathode - Negatively charged electrode
which attracts cations.
3. Emissive layer - Made up of organic
molecules or polymers that transport
electrons from the cathode
layer.
4. Conductive layer - Made up of organic
molecules or polymers that
transport holes from the cathode
layer.
5. Anode layer - Positively charged
Electrode.
6. Substrate - Glass bottom plate.
Layers in OLED
To light up the OLED, an electrical current
should be applied between the anode and
the cathode. As electricity flows, energy travels
from the cathode layer to the anode layer
passing all the other layers.
This current gives electrons to the emissive
layer and removes electrons from the conductive
layer. This process produces holes in
the conductive layer. In order to fulfill those
holes with electrons (to recombine with electrons),
the holes jump from the conductive
layer to the emissive layer, which is rich with
electrons.
As electrons fill these holes, the generated
extra energy is released as a bright electroluminescent
light. This light is visible through
the outermost layer of glass (substrate). The
colour of the light that is seen in the OLED display
is affected by red, green, blue colours and
the layers attached to the substrate.
GAUGE Magazine
University of Peradeniya
Page
21