**3. Organic light emitting diodes (OLEDs)**

Using organic material for light emitting diodes (LEDs) is fascinating due to their vast variety and relative ease of controlling their composition to tune their properties by chemical means. For example, by applying an electric field to an anthrancene single crystal, Pope *et al.* in 1965 observed blue electroluminescence (EL) [20]. Soon after alternating current EL was also achieved using an emissive polymer [21].The observation of efficient bright EL, defined as the number of photons emitted from the face of the device per injected electron or hole, the investigation on the organic optoelectronic device commenced to investigate and developed slowly until Tang and Vanslyke demonstrated efficient green electroluminescence [22] from a vapor deposited organic compound in 1987. Till now, OLEDs are the best flat light emitting source.
