The bilayer organic electroluminescent diodes using two novel pyrazoline derivatives as hole transporting materials (HTM) and BBOT [2,5-bis(5-tert-butyl-2-benzoxazolyl)thiophene] as electron transporting material have been constructed, in which the electroluminescent spectra were red-shifted and broadened relative to the fluorescent spectra of constituent materials. Formation of exciplexes at the interfaces of the pyrazoline derivatives and BBOT layers were determined by photoluminescence spectral studies on the HTM/BBOT bilayers and HTM∶BBOT mixed evaporated films as well as electroluminescence measurements. The type of exciplexes formed by the interaction between the excited state of BBOT (BBOT⁽) and ground state of the HTM were confirmed in light of the energy level diagram of these bilayer devices. The maximum luminance and efficiency of the HTM∶BBOT mixed emissive layer devices in which the interface of exciplex formation was enlarged, have been improved by about a factor of 2 in comparing with the corresponding bilayer devices.

Key words: electroluminescence, exciplex, mixed emissive layer, color tuning