Photoelectron spectroscopy studies of the electronic structures of Al/RbF and Al/CaF₂ cathodes for Alq₃‐based organic light‐emitting devices

Abstract

The electronic structures of Al/RbF/tris‐(8‐hydroxyquinoline)aluminium (Alq₃) and Al/CaF₂/Alq₃ interfaces were investigated using x‐ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). For both systems, the UPS showed a significant valence band shift following the deposition of the thin fluoride layers on Alq₃. However, the formation of gap state in valence region and the extra peak N 1s core level spectra showed different trends, suggesting that the alkali fluoride and alkali‐earth fluoride interlayer have different reaction mechanisms at the interface between Al cathode and Alq₃. In addition, the deposition of Al has considerably less effect on the valence band shift compared to the deposition of both RbF and CaF₂. These results suggest that the charge transfer across the interface and the resulting gap state formation may have lesser effect on the enhancement of organic light‐emitting device performance than the observed valence band shift, which is thought to lower the electron injection barrier.

Keywords:

• OLED
• electron injection layer
• photoelectron spectroscopy
• energy level alignment

Notes

Member, KIDS.

Nano‐Surface Group, Korea Research Institute of Standards and Science, P.O. Box 102 Yuseong, Daejon 305–600, Korea.

Korea Basic Science Institute, Jeonju Center, Jeonju 561–756, Korea. E‐mail: [email protected] Tel: [phone] Fax: [phone]