Interface formation between metal and poly-dialkoxy-p-phenylene vinylene

In this work we address the dynamics and stability of calcium/PPV and barium/PPV interfaces during and after deposition of the metal. Diffusion of calcium and barium into OC₁C₁₀ PPV is studied with low energy ion scattering (LEIS) and x-ray photoelectron spectroscopy (XPS). During metal deposition the diffusivity is found to be orders of magnitude higher than after deposition and the diffusion coefficient was found to be dependent on the metal concentration in the PPV. Furthermore, the amount of metal inside the polymer films was found to depend on the deposition rate. These observations were explained in a two-stage diffusion model. In the first stage atoms land on the surface and diffuse fast into the polymer and in the second stage metal ionizes and is trapped and diffusion is strongly decreased. The diffusion coefficient of barium into PPV at T=298 K is found to be almost an order of magnitude lower than the diffusion coefficient of calcium into PPV [(0.35±0.05)×10⁻²³m²∕s and (2.7±0.4)×10⁻²³ m²∕s, respectively]. Furthermore, the activation energy of the diffusion process of barium into PPV (0.75±0.07eV) is significantly higher than the activation energy of the diffusion process of calcium into PPV (0.62±0.05 eV). The difference in diffusion coefficient and activation energy between calcium and barium are discussed in terms of an Arrhenius law of diffusion. Finally, polymer LED performance was studied as a function of the amount of metal diffused into the polymer layer. It was observed that the light output and the efficiency decreased as the amount of metal in the PPV increased. This indicates that the metal ions form charge carrier traps and exciton quenching sites in the PPV.