Failure of polymeric light emitting diodes by controlled exposure of the polymer-cathode interface to oxygen

The influence of the controlled exposure to oxygen of the calcium cathode of polymeric light emitting diodes (pLEDs) is investigated. The LEDs are fabricated with ITO as anode, OC1C10-PPV as electroluminescent polymer, calcium as cathode and aluminium as protecting layer. The polymer layers of the LEDs are spincoated in a dry nitrogen atmosphere and transported directly into an UHV chamber where the metal electrodes are deposited by evaporation. In order to investigate the influence of the exposure to oxygen of the calcium cathode, the deposition of the calcium layer was interrupted in some cases and the samples were exposed to 30-1000 mbar of oxygen. We determined the amount of oxygen in the different layers of the I-V-light characterized pLEDs with elastic recoil detection analysis and correlated it with the characteristics of the devices. Exposing a part of the calcium layer to oxygen at layer thicknesses equal to or less than 10 nm leads to a total loss of the brightness, while exposing thicker layers or the pristine PPV does not affect the LEDs significantly.