Understanding LED Technology
In 1961, Gary Pittman and Bob Biard from Texas Instruments found that that gallium-arsenide diode emits infrared light every time it is connected to current. The same year they received patent for infrared LED. Nick Holonyak Jr., employed in General Electric, developed in 1962 first light-emitting diode that emitted light in the visible part of the frequency range. It was a red LED.
How LED work
LEDs create light by electroluminescence in a semiconductor material. Electroluminescence is the phenomenon of a material emitting light when electric current or an electric field is passed through it – this happens when electrons are sent through the material and fill electron holes. An electron hole exists where an atom lacks electrons (negatively charged) and therefore has a positive charge. Semiconductor materials like germanium or silicon can be “doped” to create and control the number of electron holes.
Doping is the adding of other elements to the semiconductor material to change its properties. By doping a semiconductor you can make two separate types of semiconductors in the same crystal. The boundary between the two types is called a p-n junction. The junction only allows current to pass through it one way, this is why they are used as diodes. LEDs are made using p-n junctions. As electrons pass through one crystal to the other they fill electron holes. They emit photons (light). This is also how the semiconductor laser works.
Then and Now : Organic LED technology
The Organic LED is made of a layer of organic
electroluminescent material with p/n junction sandwiched between to electrodes. At least one of the electrodes is transparent so the photons can escape. Similar to an EL lamp, current is passed through a semiconductor (like the phosphor in an EL lamp), however the difference is that an OLED uses a p/n junction were there is a recombination of p and n carriers. EL (TDFEL, TFEL, powder EL) technology only uses a material excited by current to make light.
The semiconductor in an OLED is organic which means it contains carbon. The OLED uses one of two kinds of compounds: polymers or ‘small molecule’.