The bilayer organic light-emitting diodes using green-fluorescent tris(8-quinolinolato)aluminum (Alq3) as an emitting material and hole-transport materials with low ionization potentials, 1,3,5-tris(3-methylphenylphenylamino)triphenylamine and 4,4′,4″-tris[bis(4-tert-buthylbiphenyl4-yl)amino]triphenylamine, emitted bright yellow light instead of green light. The yellow emission is attributed to exciplex formation at the solid interface between Alq3 and the hole-transport material. The exciplex formation was evidenced by the measurement of the photoluminescence spectra and lifetimes of the mixture of an equimolar amount of Alq3 and each of the hole-transport materials. The emission color can be tuned by varying the applied voltage. © 1998 American Institute of Physics.