P-type Doping of Organic Charge Transport Materials with Tungsten Oxide

Abstract

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.

Enabling highly-efficient OLED with solution-processed nanocrystalline copper phthalocyanine hole-injection/transport layer

“…Cao et al reported p-type doped organic charge transport materials with tungsten oxide for OLEDs [25]. Nevertheless, the organic HTLs reveals some disadvantages such as low solubility in organic solvents and an unsuitable work-function along with high aggregation of materials while thin film formation [25,26]. Therefore, the solution processable HTL materials performance should be enhanced to attain a highlyefficient OLED.…”