White-light-emitting organic electroluminescent devices based on interlayer sequential energy transfer

We demonstrate efficient, molecular organic white-light-emitting devices us ing vacuum-deposited thin films of red luminescent [2-methyl-6-[2-(2,3,6,7- tetrahydro-1H, 5H-benzo [ij] quinolizin-9-yl) ethenyl]-4H-pyran-4-ylidene] propane-dinitrile (DCM2), doped into blue-emitting 4, 4(') bis [N-1-napthyl -N-phenyl-amino]biphenyl (alpha-NPD), and green-emitting tris-(8-hydroxyqui nolinato) aluminum(III) (AlQ(3)). The luminescent layers are separated by a hole-blocking layer of 2,9-dimethyl, 4,7-diphenyl, 1,10-phenanthroline (BC P), whose thickness is on the order of a typical Forster transfer radius of 30-40 Angstrom. Excitons formed on alpha-NPD sequentially transfer their e nergy via a Forster mechanism to AlQ(3) across the BCP layer, and from AlQ( 3) to DCM2. This interlayer sequential energy transfer results in partial e xcitation of all three molecular species, thereby producing white light emi ssion. The thickness of the blocking layer and the concentration of DCM2 in alpha-NPD permit the tuning of the device spectrum to achieve a balanced w hite emission with Commission Internationale d'Eclairage chromaticity coord inates of (0.33, 0.33). The spectrum is largely insensitive to the drive cu rrent, and the devices have a maximum luminance of 13 500 cd/m(2). At a lum inance of 100 cd/m(2), the quantum and power efficiencies are 0.5% and 0.35 lm/W, respectively. (C) 1999 American Institute of Physics. [S0003-6951(99 )01133-X].