OLED (Organic LED) – the type of light emitting semiconductor diode that is used as a superficial light source. They are used to build flexible displays, TVs or other portable devices. Displays produced in OLED technology are extremely thin due to the lack of need for their backlighting, because they generate light by itself. OLED technology will undoubtedly „take the torch” from the LCD technology. OLED symbol looks the same as a typical LED’s.
OLED – Internal construction and manufacturing process
OLED is constructed from several very thin layers of materials put on-top of each other in a way that is „similar to the sandwich”. These layers are as follows:
- two conductive electrodes:
- anode (+) – transparent, to make the passage for the emitted light (also known as „emitter”),
- cathode (-) – not necessarily transparent (also called „conductor”),
- two organic layers:
- conductive layer – “n” -type organic semiconductor,
- emissive layer – “p”-type organic semiconductor, here applied organic material is different than on the conductive layer, the layer is so-called because it emits electromagnetic radiation in the visible spectrum. Therefore, this layer is called emissive,
- base – transparent, usually made from glass or foil.
It is possible to “add” a third organic layer for a more efficient transfer of electrons from the cathode to the emissive layer.
Technological process of OLEDs is usually done in the following order:
- The substrate is applied to the anode.
- The layer on the anode is formed especially for the transport of holes.
- Application of the conductive layer “n” -type.
- The layer is placed on-top of the conductive layer to form emissive “p” –type layer.
- Next, the transport layer for electrons is applied.
- The cathode is „placed on-top of everything”.
The thickness of the layers in the OLED technology does not exceed 500nm.
OLED – Principle of operation
The most important thing in OLED operation is applying the correct voltage to its structure for the light emission to be possible and successful. The electrons move in the direction from cathode to anode (electrons from the cathode pass into the emissive layer and holes will pass from anode, which means that anode will retrieve electrons from the conductive layer). Once the junction is forward-biased, the emissive layer is negatively charged. However, the conductive layer is positively charged because of the presence of excess holes that are a positive charge-carriers. The mutual attraction of electrons and holes in the emissive layer arises due to the occurrence of electrostatic interaction. Then the recombination phenomenon occurs – connection of a pair of particles with opposite electrical charges. The energy released during recombination causes the emission of visible electromagnetic radiation – light. Value of the voltage that supplies OLEDs (the bigger, the brighter means larger OLED power consumption) defines the brightness of the received light. The color of this light depends on the type of the organic layer of material used in an emissive layer in the OLED technology. It will be lit only when the junction is forward-biased (reverse-bias won’t work – just like in typical semiconductor diode).
OLED – Advantages and disadvantages
The use of OLED technology in the construction of any display has both advantages and disadvantages.
As OLED advantages we can put:
- the possibility of producing flexible displays or sewn-screens e.g. in clothing due to the possibility of applying the organic material on a flexible substrate,
- low weight and thickness of the display,
- reduced manufacturing costs due to the lack of backlighting, non-complex construction (small number of electronic components) and fewer layers of the display,
- OLED meaning greater brightness and color scale than in LCD technology what’s caused by spontaneous, unfiltered light emission by OLEDs,
- the backlight is not required what leads to low OLED power consumption – black color is perfectly black,
- viewing angle is almost unlimited,
- faster response time (~ 0,01ms) compared to LCD technology (2-12ms).
Among OLED disadvantages one can find:
- the lifespan of organic materials – blue OLED has the shortest lifetime among the RGB colors (Red, Green, Blue): about 5000-14000h. However, red and green OLEDs could work between 46,000-230,000h, so blue OLED challenges today’s world of scientists,
- consumption of current – scientists have observed greater power consumption when displaying white and bright elements e.g. while browsing Web, therefore it may be uneconomical for some applications,
- susceptibility to environmental factors – organic materials are particularly sensitive to environmental factors. In case of mechanical damage to the matrix display, water or humidity can easily destroy organic material. Using an additional superficial protection could be a smart idea,
- limited patents – the development of OLED technology is heavily dependent on companies that have ownership of patents (now Eastman Kodak Company), which require the payment of license-fees to allow taking any „serious, further steps” in this subject.