Emission mechanism in OELD : Shift from green to blue

Organic electroluminescent devices (OELDs) with the structure of ITO/TPD/PBD/Alq 3/Al:Li were fabricated and the effects of applied voltage, thickness of organic layer and ITO anode to probe the emission mechanism was investigated. EL spectra and CIE coordinates of these devices were measured to observe emitting characteristics in various voltages and it was observed that the emission color gradually changes from yellowish-green to greenish-blue due to the shift of recombination region of holes and electrons as the driving voltage is increased in the same structure. Thickness of each organic layer and different capability of hole supplying from the anode are also among major factors to expand the recombination region in the device. Especially, the total amount of holes provided by ITO anode affects the probability of recombination to shift CIE coordinates. It is possible for OELD to recombine holes and electrons at the emitting layer (PBD) as well as the electron transport and hole transport layer to contribute an emission.