Efficient electron injection from bilayer cathode with aluminum as cathode

A new bilayer cathode has been developed for varieties of RGB emitting polymers to form an effective electron injector. An effective electron injection from high work function metal, such as Al cathode can be realized by incorporation of a thin layer of amino-ended alkyl-substituted polyfluorene copolymer and corresponding ammonium salt-cationic polyelectrolyte between metal cathode and emitting layer. The performance of polymer light emitting-diodes (PLEDs) fabricated with such bilayer cathodes can be enhanced to the levels comparable or even higher than that obtained by using Ca and Ba as the cathode (for red QE = 2-3%, for green 5-8%, for blue 1-2% respectively). The new electron-injection layer can be processed from environment-friendly solvents such as alcohol or water in which most of light-emitting polymers are insoluble. The mechanism of lowering the barrier height for electron injection from Al is discussed based on I-V characteristics, build-in potential, and X-ray diffraction data.