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We report a two-layer, blue organic light-emitting diode with a 4,4′-bis(2,2-diphenylvinyl)-1,1′-biphenyl emission layer and a LiF/Al cathode which has an external quantum efficiency of 1.4% and a maximum luminance of 3000 cd/m2. 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.

A soluble derivative of quinacridone, N,N′-di-isoamyl quinacridone (DIQA), has been synthesized and used to study the mechanisms of Förster energy transfer and charge transfer in organic light-emitting diodes (OLEDs) based on 8-hydroxyquinoline (Alq3). Quantum efficiencies and spectra were measured for both photoluminescence (PL) and electroluminescence (EL) for films of poly(9-vinylcarbazole) (PVK) doped with Alq3 and DIQA. Both PL and EL showed an efficiency enhancement in films of PVK:Alq3:DIQA compared to films of PVK:Alq3. However, the optimal DIQA doping concentration was found to be lower for EL than for PL. Examination of the spectra revealed that more emission originated from DIQA for EL than for PL at a given doping level. We conclude that Förster energy transfer from Alq3 to DIQA occurs in both cases of PL and EL, but that charge transfer to DIQA occurs in the operation of the OLED resulting in additional pathways to DIQA emission. Ultraviolet photoelectron spectroscopy measurements showed that electron transfer from Alq3 to DIQA, hole transfer from PVK to DIQA, and hole transfer from Alq3 to DIQA are all energetically favorable processes. These results suggest that charge transfer is an important mechanism in the efficiency enhancement seen in OLEDs based on a host–dopant scheme, and that both the electronic properties and the optical properties of the dopant material are important parameters for device optimization.