Performance and stability of poly(phenylene vinylene) based polymer light emitting diodes with caesium carbonate cathode

We have studied the phenomena resulting from the combination of a hole-conducting poly(phenylene vinylene) (PPV) based light emitting polymer with a highly efficient electron injection layer of caesium carbonate (Cs2CO3) in light emitting diodes. A strong dependence between the thickness of the applied Cs2CO3 and the electro-optical performance of the diodes is detected and already with ultrathin Cs2CO3 layers high efficiency diodes are achieved. The Cs2CO3 is shown to diffuse into the polymer layer leading to an increased electron density but also quenching of both electro and photoluminescence when the amount of applied Cs2CO3 is increased. During electrical stressing the electron density decreases assumably through degradation of the n-doping and quenching Cs2CO3 species inducing an unusual increasing luminescence behavior.