An OLED includes a wide gap inert host material doped with two dopants.
One of the dopants is an emissive phosphorescent material that can
transport either electrons or holes. The other dopant is a charge
carrying material that can transport whichever of the electrons and holes
that is not transported by the phosphorescent dopant. The materials are
selected so that the lowest triplet energy level of the host material and
the lowest triplet energy level of the charge carrying dopant material
are each at a higher energy level than the lowest triplet state energy
level of the phosphorescent dopant material. The device is capable, in
particular, of efficiently emitting light in the blue region of the
visible spectrum.