Bright blue organic light-emitting diode with improved color purity using a LiF/Al cathode

We report a two-layer, blue organic light-emitting diode with a ${\text{4,4}}^{\prime }-{\mathrm{bis(2,2-diphenylvinyl)-1,1}}^{\mathrm{\prime }}\mathrm{-biphenyl}$ emission layer and a LiF/Al cathode which has an external quantum efficiency of 1.4% and a maximum luminance of $\text{3000\hspace{0.17em}}{\mathrm{cd/m}}^{\mathrm{2}}.$ Insertion of the thin LiF layer results in a 50-fold increase in the device efficiency compared to a device with an aluminum only cathode, and eliminates the need for an electron-transporting layer, such as tris(8-hydroxyquinoline)aluminum. This results in a device with excellent color purity with an emission peak at 476 nm and a full width at half maximum of 78 nm. Using ultraviolet photoelectron spectroscopy, we find that the effective work-function of aluminum decreases dramatically with sub-monolayer amounts of LiF deposited on the surface.