The influence of surface treatments on cathode formation and stability in polymer light emitting diodes

Abstract

We studied the stability of metal/polymer interfaces by measuring the diffusion of calcium into a polymer (OC 1 C 10 PPV) layer during and after deposition of the metal using low energy ion scattering (LEIS) and X-ray photoelectron spectroscopy (XPS). During deposition the calcium diffusion depth in the PPV was found to be comparable for untreated samples and samples prepared in oxygen ambient (10 -7 mbar). In both cases diffusion depths up to ∼7 nm were observed. For PPV layers treated with atomic oxygen, the diffusion depth during deposition was slightly smaller. After deposition, it was observed that calcium diffusion in OC 1 C 10 PPV continues for several hours. When oxygen was present during calcium deposition or during the spin coating of the PPV, the diffusion coefficient for calcium in PPV was decreased considerably. In these cases accumulation of oxygen (adsorbed in the PPV during deposition or spin coating) at the calcium/PPV interface continued for several hours after deposition. Treatment of the PPV with atomic oxygen before calcium deposition resulted in a strong decrease of the calcium diffusion coefficient after deposition. From XPS measurements it was observed that calcium interacts with the chemically bonded oxygen in the PPV and also with the oxygen absorbed in the PPV layer. It can be concluded that oxygen, either chemically bonded to the PPV chain or adsorbed in the film, reduces the calcium diffusion coefficient. The initial performance of PLEDs with atomic oxygen treated PPV layers and PLEDs with calcium deposited in oxygen ambient was worse than the performance of untreated devices, but the stability in the life-test was better. © 2004 Elsevier B.V. All rights reserved.