OLEDs

ORGANIC LIGHT EMITTING DIODE
1. INTRODUCTION

Imagine having a high defination television that is 80 inches wide and less than a quarter-inch thick, consumes less power than most televisions on the market today and can be rolled up when you're not using it. What if you could have a "heads up" display in your car? How about a display monitor built into your clothing? These devices may be possible in the near future with the help of a technology called organic light-emitting diodes (OLEDs).
OLEDs are solid-state devices composed of thin films of organic molecules that create light with the application of electricity. OLEDs can provide brighter, crisper displays on electronic devices and use less power than conventional light emiting diodes(LEDs) or liquid crystal display(LCDs) used today.

2. COMPONENTS OF OLEDS
Like an LED, an OLED is a solid-state semiconductor device that is 100 to 500 nanometers thick or about 200 times smaller than a human hair. OLEDs can have either two layers or three layers of organic material. In the latter design, the third layer helps transport electrons from the cathode to the emissive layer. In this article, we'll be focusing on the two-layer design.

An OLED consists of the following parts:

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

2.2 Anode (transparent) - The anode removes electrons (adds electron "holes") when a current flows through the device. Indium tin oxide is commonly used as the anode material. It is transparent to visible light and has a high work function which promotes injection of holes into the polymer layer.

2.3 Organic layers - These layers are made of organic molecules or polymers.
Conducting layer - This layer is made of organic plastic molecules that transport "holes" from the anode. One conducting polymer used in OLEDs is polyaniline. Aluminium and calcium are often used for the cathode as they have low work functionswhich promote injection of electrons into the polymer layer

3. WORKING PRINCIPLE
OLEDs emit light in a similar manner to LEDs, through a process called electrophosphorescence.
The process is as follows:

1. The battery or power supply of the device containing the OLED applies a voltage across the OLED.

2. An electrical current flows from the cathode to the anode through the organic layers (an electrical current is a flow of electrons).
 The cathode gives electrons to the emissive layer of organic molecules.
 The anode removes electrons from the conductive layer of organic molecules. (This is the equivalent to giving electron holes to the conductive layer.)

2.4 Cathode (may or may not be transparent depending on the type of OLED) - The cathode injects electrons when a current flows through the device.

3. At the boundary between the emissive and the conductive layers, electrons find electron holes.
 When an electron finds an electron hole, the electron fills the hole (it falls into an energy level of the atoms that's missing an electron).
 When this happens, the electron gives up energy in the form of a photon of light
4. The OLED emits light.

5. The color of the light depends on the type of organic molecule in the emissive layer. Manufacturers place several types of organic films on the same OLED to make color displays.

6. The intensity or brightness of the light depends on the amount of electrical current applied: the more current, the brighter the light.
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.