White light from blue: white emitting organic LEDs based on spin coated blends of blue-emitting molecules

White light emission can be obtained from spin-coated blends of two different blue-emitting organic molecules, due to the combination of exciplex formation and residual blue emission. Light-emitting diodes using a blend of 90% N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine and 10% 2,5-bis(trimethylsilyl thiophene)-1,1-dioxide emits white light with Commision Internationale l'Eclairage co-ordinates of x=0.34,y=0.38, close to the equal energy point. We show that exciplex formation does not occur in all relative concentrations of the two components, and we relate this to phase separation in spin coated films, as measured using atomic force microscopy. We suggest that a limit to exciplex formation may be structural, i.e. where the structure and/or morphology of the solid state blend does not allow significant spacial overlap between the HOMO/LUMO of the two components.