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 de vice containing the luminescent dye 2,3,7,8, 12,13,17,18-octaethyl-21H23H-p horpine platinum(II) (PtOEP) doped in a 4,4'-N,N'-dicarbazole- biphenyl (CB P) host. Due to weak overlap between excitonic states in PtOEP and CBP, eff iciency losses due to nonradiative recombination are low. However, energy t ransfer 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 excit on diffusion in CBP, improving the energy transfer to PtOEP. This technique may be applied to improve the efficiency of other electrophosphorescent de vices. (C) 1999 American Institute of Physics. [S0003-6951(99)00403-9].