Indium diffusion in model polymer light-emitting diodes

The diffusion of indium into poly-(phenylenevinylene) (PPV) in model polymer light-emitting diodes (p-LEDs) was studied with Rutherford backscattering spectrometry (RBS), X-ray photoelectron spectroscopy (XPS), low energy ion scattering spectroscopy (LEIS) and particle induced X-ray emission (PIXE). The model p-LEDs consisted of a glass substrate, an indium-tin-oxide (ITO) electrode, a PPV layer obtained by thermal conversion of sulfonium precursor PPV, and a patterned aluminum electrode. From RBS measurements it was concluded that about 0.01 at.% indium was present in the PPV, homogeneously distributed in depth. Annealing at 230 °C for 19 h caused the amount of In in the PPV layer to increase by roughly an order of magnitude. Under the patterned aluminum electrode, the annealing treatment resulted in accumulation of In at the PPV/Al interface, whereas the depth distribution of In remained homogeneous in the uncovered region of the model LEDs. XPS spectra on annealed model LEDs show that In was present in the near surface region of the PPV films, although LEIS analysis showed that In was not situated in the outermost atomic layer. LEIS measurements on as-prepared model LEDs showed that the patterned Al electrode had caused surface contamination of the uncovered PPV film with Al, which can have impact on the diffusion of In to the outermost surface during annealing treatments.