Asymmetric brightness of transparent organic emitting device with passivation layer by annealing

Abstract

We studied transparent organic light emitting diode (TOLED) with semitransparent cathode, Ca-Ag and optical enhanced passivation layer by annealing. The 8-hydroxyquinolatolithium (Liq) passivation layer improves the stability of Ca/Ag double layer, resulting in substantial increase of the efficiency by the enhanced transmittance and change brightness as electrode. Fabricated device was composed as followed structure, glass/ITO/2TNATA/α-NPD/Alq3:C545T (1%)/Bebq2/Ca-Ag. The passivation layer, Liq (40nm) was deposited on the device without broken vacuum before encapsulation with transparent glass sheets. Thermal annealing was carried out in oven at 80oC after encapsulation. At the device without passivation layer, annealing process made device to be deteriorated, such as to increase driving voltage from 6.8 V to 9.0 V at 10 mA/cm2 and to drop efficiency 14.89 cd/A to 10.81 cd/A as bottom view. This drop of properties was caused by oxidation of cathode. However, the organic passivation layer played enough roles to protect oxidation of cathode. The efficiency as bottom side view of device with passivation layer, it can be observed that the efficiency and driving voltage were improved from 13.95 cd/A (@ 6.45 V) to 15.32 cd/A (@ 4.60 V) at 10 mA/cm2 by annealing. Annealing process, in the device with passivation layer, made to increase the asymmetry of brightness through anode and cathode. So the ratio BBot/BTop of the emitted light through each electrode was changed from 2.26 to 4.33.