Internal Field Distribution in Organic Light Emitting Diodes with Double Layer Structure

Double layer organic light emitting devices (OLED) are constructed by evaporating tris(8 -hydroxy) quinoline aluminum (Alq3) on a spin cast thin film of a methyl substituted ladder type poly -para -phenylene (m -LPPP). A thick layer of Mg:Ag is used as the cathode material. These organic materials are very suitable for application in OLEDs both, as transporting materials as well as active layers. Alq3 predominantly transports electrons while m - LPPP is a conjugated polymer having higher hole mobilities. Due to these transport properties the formation and radiative recombination of the excitons in ITO/m -LPPP/Alq3/Mg:Ag devices occur close to the m -LPPP/Alq3 interface. We compare the device performance of OLEDs with varying Alq3 layer thickness (0, 50, 150, 300, 500Å) and constant m -LPPP layer thickness (900Å). A difference in the device parameters and performance as a function of the Alq3 layer thickness is observed. We analyze these results with respect to the internal electric field distribution of the double layer devices derived from electroabsorption measurements.