Study of electronic structure of 3, 4, 9, 10-perylenetetracarboxylic bisimidazole/Ag interface by photoemission

Synchrotron radiation photoemission was used to investigate the interface between the organic semiconductor (BZP) and polycrystalline silver film. Before complete coverage by one monolayer, the BZP molecules interacted weekly with silver and a gap state appeared at 0.9eV below Fermi level. After the coverage is complete, the growth of BZP is inferred to be in the 3D-island mode and the interaction with the substrate gradually diminishes. Because of the final state effect, the highest occupied molecular orbital (HOMO) level shifts to higher binding energy and reaches ultimately 2.3eV of the HOMO level in the bulk. The deposition of BZP on silver surface resulted in the decrease of sample work function, which demonstrats interfacial dipoles forming on the BZP/Ag contact (Δ=0.3eV) and indicats the electrons being deviated from the organic molecules to the metal substrate. In addition, the effect of annealing and exposure to oxygen on the BZP/Ag interface has been taken into account. It is found that the binding energies of the characteristic peaks of BZP and the gap state hardly change after interface annealing at 250 ℃, but after being exposed to oxygen, the gap state disappears and the valence bands shift to lower binding energies.