How OLED works ?

Organic light-emitting diodes (OLEDs) have a huge benefit: they provide brighter displays and consume less power than traditional LEDs (light-emitting diodes) or LCDs (liquid crystal displays).

This article explains how OLED technology works and what type of displays are possible.

An organic light-emitting diode is a solid-state semiconductor device. It is usually 100 to 500 nanometers thick. There are designs with two or with three layers of organic compounds. The most common is the two layer version, therefore we will focus on this version in this article.

The five main parts of OLED are substrate, anode, cathode and two organic layers. The substrate should support the OLED and can consist of glass, foil or clear plastic. The anode is usually transparent and removes electrons, the cathode does exactly the opposite, it injects electrons. The cathode can be transparent or not – depending on the type of OLED used.

The organic layers are a conducting layer and an emissive layer. The conductive layer is made of organic plastic molecules, the emissive layer is different to those molecules from the conducting layer. These molecules transport electrons from the cathode. This is the part where light is emitted.

The process that is emitting light is called electrophosphorescence, it is similir to the process to traditional LEDs. Current is flowing from the cathode to the anode through the organic layers. By this, electrons are removed from the conducting layer and are given to the emissive layer. Holes are left to the conductive layer at the removing process. The holes are jumping over to the emissive layer and recombine with the electrons. While the electrons drop into the holes they release extra energy as light.

There are different types of OLEDs. The most common are active-matrix OLED and passive-matrix OLED. Active-matrix OLEDs are called AMOLED. They consume less power than passive-matrix OLEDs and are well suited for portable devices powered by battery.

An active-matrix consists of pixels that have been integrated onto a thin film transistor array (called TFT) to form a matrix. The thin-film transistor acts as an array of switches to control the amount of electrical current flowing through each pixel of the OLED.

Passive-matrix OLEDs are called PMOLED. They are well suited for small display devices like cellular phones or audio players, because they consume more power than active-matrix OLEDs. But the power consumption is still lower than those of traditional LCDs.

A passive matrix consists of rows and columns formed to a two-dimensional array. An element of this arrray is called a pixel. Electrical current is applied to the thin film of organic material and light is emitted. The brightness of each pixel demands on the amount of current applied.

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