Abstract
The effects of p-type doping of wide bandgap ambipolar 4,4′-N,N′-dicarbazolebiphenyl (CBP) with WO3 were investigated through detailed electrical device characterization. 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 10nm thick. A large downward shift of the Fermi level in WO3-doped CBP causes band bending and depletion at the doped/undoped CBP interface, resulting in an additional energy barrier which hampers hole transport. Simplified green phosphorescent organic light-emitting diodes (PhOLEDs) with CBP as the hole transport and host material were fabricated. With a WO3-doped hole transport layer, the PhOLEDs attained brightness of 11,163cd/m2 at 20mA/cm2, and exhibited an improved reliability under constant-current stressing as compared to undoped PhOLEDs.
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