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Continuous-wave and pulsed-current stressing was conducted to evaluate the reliability of green phosphorescent organic light-emitting diodes (OLEDs). Through modifications of the ITO anode by different pretreatments and the hole transport layer (HTL) by incorporating inorganic component or dopants, we proved that the energy level misalignment at the ITO/HTL interface leads to localized Joule heating, accelerating material deterioration, and luminescence decay. Stressing with short current pulses was employed to suppress the heating effect. For OLEDs with a large interfacial barrier of 1.2 eV, the effective half life was increased by 70% under pulsed operation, whereas in OLEDs with an interfacial barrier of 0.5 eV, only 14% improvement was obtained, indicating a minor heating role.

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The effects of WO3 doping in 4,4’-bis-9-carbozyl biphenyl (CBP) were studied through detailed electrical device characterization. A series of hole-only devices have been fabricated, where the doping level was varied from 10-40mol% and the doped CBP thickness was varied from 5-40 nm. It was found that, to achieve effective doping for improved hole injection and transport, the doping level should be greater than 20mol% and the doped layer should be at least 10 nm thick. It was also found that an energy barrier exists at the doped and undoped CBP interface, resulting in an additional voltage drop. This finding was explained by a large downward shift of the Fermi level in WO3-doped CBP, which causes band bending and depletion at the interface. Finally, simplified green phosphorescent organic light-emitting diodes (OLEDs) with CBP as the hole transport and host material were fabricated. With a WO3-doped hole transport layer, the OLEDs attained brightness above 105 cd/m2 at 20 mA/cm2, and exhibited an improved reliability under constant-current stressing as compared to undoped OLEDs.