Evidence for the Formation of Unoccupied States in Poly(2,3-Diphenylphenylene Vinylene) Following the Deposition of Metal

Near-edge x-ray-absorption spectroscopy was used to study the interaction of Ca and Al with poly(2,3-diphenylphenylene vinylene) (DP-PPV). Al does not significantly modify the ${\mathrm{C}}_K$ edge during interface formation while Ca does in a coverage dependent fashion. The evolution of the ${\mathrm{C}}_K$ edge points to the creation of new intragap states. In addition, we see a strong resonance at higher photon energies. The creation of new unoccupied states in DP-PPV following the deposition of Ca may in part explain the enhanced charge injection observed when Ca is used as the cathode material in polymer-based light emitting diodes.