Interfaces between 8-hydroxyquinoline aluminum and cesium as affected by their deposition sequences

The electronic structures of tris(8-hydroxyquinoline) aluminum (Alq3) deposited on clean or Cs pre-covered Ag substrates have been studied by ultraviolet photoelectron spectroscopy. Interface of Cs deposited on Alq3 has also been prepared for comparison. For a low coverage of Cs on Ag, deposition of Alq3 on top of the Cs cannot induce any new electronic features. The low work function of the Cs reduces the barrier height of electron injection at the Alq3/Cs/Ag contact to 0.3 eV, as compared to 1.6 eV for the Alq3/Ag contact. For high Cs coverage, the Cs may diffuse as neutral atoms and undergo oxidation into the Alq3 layer and form a new gap state at 0.9 eV above the Alq3 highest occupied state, which is the same as that of Cs deposited on the Alq3.