EFFECT OF MAJORITY CARRIER SPACE CHARGES ON MINORITY-CARRIER INJECTION IN DYE-DOPED POLYMER LIGHT-EMITTING DEVICES

By blending suitable dyes into poly(N-vinylcarbazole) we have fabricat ed devices which emit light in the whole visible spectrum. Their curre nt-voltage characteristics can be described by space-charge limited cu rrents with effective trapping of the charge carriers by the dye molec ules, while the light intensity shows a Fowler-Nordheim-like behavior as a function of the external electric field. However, the anodic spac e 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 char acteristics by assuming tunneling of the minority charge carriers thro ugh a triangular barrier. The obtained barrier heights showed a strong dependence on the dye molecules, suggesting that the injection of min ority 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 t ransport layers, respectively, luminance of 250 cd/m(2) and external q uantum efficiency of 1% could be achieved. (C) 1998 American Institute of Physics.