The bilayer organic electroluminescent diodes using two common aromatic diamines TPD (N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine) and NPB (N,N'-bis(1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine) as hole transporting materials (HTM) and BBOT (2,5-bis(5-tert-butyl-2-benzoxazolyl)-thiophene) as electron transporting material have been fabricated. Redshifted and broad electroluminescent spectra relative to the fluorescent spectra of constituent materials were obtained. This electroluminescence was originated mainly from exciplexes evidenced by photoluminescence measurements, and the type of which were interpreted as the complex formed by the interaction between the excited state of BBOT (BBOT *) and ground state of the HTM in terms of the energy level diagram of the bilayer devices. By comparing the molecular structures and energy levels of TPD and NPB, it is demonstrated that exciplex formation can more easily take place for a molecule with favorable geometry and orientation than the one with appropriate energy level.

Key words: organic electroluminescence, exciplex, color tuning, molecular structure