Multi-color emission of PVCz-based multi-layered electroluminescent devices

Electroluminescence (EL) of the multi-layered PVCz EL devices has been studied in order to reduce the energy transfer between dye molecules in the PVCz host simultaneously doped with three kinds of molecules. Single and triple layer devices have been fabricated, such as ITO/PVCz/PVCz:Nile Red,Coumarin 6,TPB/Mg-Ag and ITO/PVCz:Nile Red/PVCz:Coumarin 6/PVCz:TPB/Mg-Ag. Long-wavelength emission due to Nile Red dominated the spectrum in the case of the single layer device which was doped with three kinds of dye molecules. In contrast to this, the short-wavelength blue emission due to TPB was strongly enhanced in the three-layer structure in which only PVCz:TPB layer was coated next to the Mg-Ag electrode for electron injection. The emission from the molecules with the higher HOMO-LUMO energy separation could be effectively excited in the multi-layer structure. The effect of host excitation, impurity excitation and also the energy transfer between the molecules have been discussed for multi-color devices.