Improved energy transfer in electrophosphorescent devices

External quantum efficiencies of up to (5.6+/-0.1)% at low brightness and (2.2+/-0.1)% at 100 cd/m(2) are obtained from a red electrophosphorescent device containing the luminescent dye 2,3,7,8, 12,13,17,18-octaethyl-21H23H-phorpine platinum(II) (PtOEP) doped in a 4,4'-N,N'-dicarbazole- biphenyl (CBP) host. Due to weak overlap between excitonic states in PtOEP and CBP, efficiency losses due to nonradiative recombination are low. However, energy transfer between the species is also poor. In compensation, a thin layer of 2,9-dimethyl-4,7 diphenyl-1,10-phenanthroline is used as a barrier to exciton diffusion in CBP, improving the energy transfer to PtOEP. This technique may be applied to improve the efficiency of other electrophosphorescent devices. (C) 1999 American Institute of Physics. [S0003-6951(99)00403-9].