The bilayer organic light emitting devices (OLEDs) using two common aromatic diamines as hole transporting materials (HTMs) and BBOT (2,5-bis(5-tert-butyl-2-benzoxazolyl)thiophene) as electron transporting material have been prepared, in which the electroluminescent spectra are different from the fluorescent spectra of each of the constituent materials. The electroluminescence is mainly attributed to exciplex confirmed by photoluminescence and electroluminescence measurements, and the type of exicplex is determined 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 the structure of a molecule as well as its energy level has an effect on the exciplex formation.
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