Dopant emission mechanism and the effects of host materials on the behavior of doped organic light-emitting diodes

Organic light-emitting diodes made of tris-8-(hydroxyquinoline) aluminum as the electron-transport layers, N, N'-diphenyl-N, N' bis (3-methylphenyl)-1, 1'-biphenyl-4,4'-diamine (TPD) as the hole-transport layers, and 2-1, 1-dimethylethyl-62-2, 3, 6, 7-tetrahydro-1, 1, 7, 7-tetramethyl-1H, 5H-benzo(ij) quinolizin-9-yl ethenyl-4H-pyran-4-ylidene propanedinitrile (DCJTB) as the guest dopant have been studied. It is determined that (a) emission from guest DCJTB in a host transport material results primarily from separate trapping of holes and electrons, rather than the more commonly proposed Forster transfer mechanism, (b) DCJTB is a more efficient hole than electron trap, and (c) the lifetime of a doped device is longer when TPD is used as the host material.