OLED PLED technologies electroluminescent polymers

OLED/PLED

Electroluminescence phenomena of some materials were shown by A. Bernanose and Co. at Nancy (France) university in 1950. To do so, they showed the electroluminescence of a crystalline film composed of small molecules (similar to Fluorescein or quinacrine) by applying a high voltage a.c. on it.

Progress in electroluminescent device will rapidly increase in the 60’s and polymeric material became more present with some like Polyaniline, Polypyrrole or the most conductive one, Polyacethylene.

R. Partridge at the National Physical Laboratory (united kingdom) made the first attempt to create an organic LED (OLED) in 1975. Then, this device increases in interest and a lot of major companies such as Kodak, Sony, Samsung or also Siemens contributed to the advancement of this research orientation.

How do they work?

Classical OLED or PLED are made of one conducting material called light-emitting (e.g. π-conjugated polymer or dye) which is able to transport electrons, and two electrodes with one of them which must be transparent to let the light through. But in fact, the use of two other materials is necessary, notably for improving electrode contact on the organic layer, there should be a good compatibility. To do so, it is common to use PEDOT-PSS mixed polymers which have a good affinity with both electrode and active plastic. The other material is a hole transport again called donor material, to help the charges (positive and negative) to be separated and have more efficiency luminescence. Here is an animation of how works a PLED :

In this animation, we can easily understand that when a potential difference is applied between the two electrodes, an electron movement is made and it induces a separation of both electron and hole. Then, the light-emitting layer is negatively charged (excited electron) and conducting layer is positively charged, leading to the recombination causing a drop in the energy levels of the electrons, accompanied by an emission of light.

Which materials are used to make OLED/PLED ?

The most commonly used polymers as light emitting layer are MEH-PPV, P3HT and also polyfluorene derivative. But we also find a lot of small molecules such as Alq3, the meso-tetraphenylporphine or the N,N'-bis(3-methylphenyl)-N,N'-diphenyl-benzidine.

meh_ppv

For conductive layer, the most used are PCBM and perylene derivatives.

Anode is usually ITO because it is transparent to visible light and has a high work function, which promotes injection of holes into the polymer layer. Instead of cathode, which has to get reflection capability and electron donating ability, then, it is usually Aluminum or Calcium

Where could you find these OLED/PLED technology?

Currently, the industrialization of this kind of luminescence behavior is limited in some small displays such as cell phone or little screen display like the one you have on mp3 players and others. Just a few big companies have already made several large screens like Sony, Epson or Samsung, which made TV screens. Here are some pictures of these displays:

A big advantage of OLED/PLED technology is that it allows a flexible display like this one:

With a high color resolution and no lost of light even with high slant degree.

Here are some pictures of OLED in their basic application : light emission

The power to use this technology as a light source is not enough currently, but, if we consider the very fast progress made by R&D laboratories in the past decade, we could expect a new light-source coming from this technology in a near future.

For more information about this great technology, just have a look at our link page.