Organic Light-Emitting Diodes

OLEDs



OLEDs(Organic Light Emitting Diodes) is a device that converts electric energy into light by applying current to an organic material. The principle of luminescence is roughly as follows,


DC Current → Electron & hole injected into emission layer →Delocalized along pi bonding within molecule → Coupling with lattice within an emission layer →Forming polaron (carrier-lattice) →Form excitons (SX:TX=1:3) → Falling from LUMO to HOMO→ Photon emission



Trend



High-efficiency and environment-friend light source is recognized as an essential element for sustainable growth in the future. LEDs(light emitting diodes) & OLEDs(organic light emitting diodes) are currently in the spotlight as a candidate technology of these light source.


Although OLEDs technology has low light efficiency(30lm/W) and shorter lifetime than LED(10,000~15,000 hr), low cost substrate and no heat dissipation systems and diffusion plates are required and a simple fabrication such as solution process is possible. It is a next-generation lighting technology that is thin and light, and can be fabricated as flexible and natural area light source with a color index close to that of sunlight.



Applications



OLEDs are used in variety of electronic devices such as mobile phones, watches, computer peripherals, televisions industry and general lighting.


As display technology has evolved towards larger sizes with thinner and lighter designs and added new features, picture quality has remained the most important factor.



Issues



For lifetime and luminous efficiency of OLED illumination, improving the light extraction efficiency and developing hole transporting organic material having excellent hole mobility are key issues.


Light extraction technology is divided into internal light extraction technology and external light extraction technology. The one is a technique to reduce the loss due to the wave-guiding effect between the substrate and the light emitting layer of the OLEDs element. The latter is a technique to reduce the loss due to the total reflection effect between substrate and air.


In particular, the internal light extraction technology can improve the light efficiency by more than 3 times theoretically, but it is an advanced technology that must satisfy all the electrical, mechanical and chemical characteristics due to sensitive influence on the OLEDs driving characteristics.