In the past decade

electroluminescent devices based on organic materials are of considerable interest to their attractive characteristics and potential applications. The study and application of organic light emitting have made significant progress. Organic light emitting devices (OLEDs) have many advantages such as their thinness

self light emission

broad viewing angle

quick response

high efficiency

easy fabrication

low driving voltage

low energy consumption

low cost

etc. It is therefore very possible that they will become the next generation of panel displays. White organic light emitting devices have been attracting particular attention nowadays due to their potential applications as full color displays

backlights for liquid crystal displays and even paper thin next generation of light sources. The emitting materials used in OLEDs can be divided into fluorescent and phosphorescent materials. Phosphorescence is distinguished from fluorescence by the speed of the electronic transition that generates luminescence. Both processes require the relaxation of an excited state to the ground state

but in phosphorescence the transition is "forbidden" and as a consequence it is slower than fluorescence

which arises from allowed transitions. Indeed

phosphorescence may persist for several seconds after a phosphorescent material was excited

whereas fluorescent lifetimes are typically on the order of nanoseconds. Interest in phosphorescence

and the phosphorescence of organic materials in particular

arises from the application of these materials to organic light emitting devices (OLEDs)

where it is found that the luminous efficiency may be improved by up to a factor of four over that obtained using fluorescence. This increase is fundamental to organic materials and arises during the formation of an excited state (or excitons) from the combination of electrons and holes. To achieve white emission in OLEDs

an additive mixture of the three primary colors or two complementary colors are required. Various methods have been demonstrated to produce white light emission from organic OLEDs

including mixing of the three primary colors (red

green and blue

RGB) from respective layers in a multilayer structure

doping a single host emissive layer with RGB emitting materials

or utilizing of broad exciplex or excimer emission. White emission could also be achieved by mixing a blue host with an appropriate amount of yellow or orange dopant in a single emissive layer. The development of white organic light emitting diodes has been described and discussed on the materials

the device structures and the light emitting mechanisms have also been given.