Effect of majority carrier space charges on minority carrier injection in dye doped polymer light-emitting devices

Abstract

By blending suitable dyes into poly(N-vinylcarbazole) we have fabricated devices which emit light in the whole visible spectrum. Their current-voltage characteristics can be described by space-charge limited currents with effective trapping of the charge carriers by the dye molecules, while the light intensity shows a Fowler-Nordheim-like behavior as a function of the external electric field. However, the anodic space charge changes the field distribution inside the device and leads to a cathodic field enhancement which has to be considered in the Fowler-Nordheim equation. We were able to model the electroluminescence characteristics by assuming tunneling of the minority charge carriers through a triangular barrier. The obtained barrier heights showed a strong dependence on the dye molecules, suggesting that the injection of minority charge carriers takes place directly into the lowest unoccupied molecular orbital of the chromophors. Using poly(p-phenylenevinylene) and an oxadiazole starburst molecule as additional hole and electron transport layers, respectively, luminance of 250 cd/m2 and external quantum efficiency of 1% could be achieved. © 1998 American Institute of Physics.