A Study on the Electron Transfer at the Alq₃/Ba and Alq₃/Au Interfaces by NEXAFS Spectroscopy

Abstract

Tris(8-quinolinolato)aluminum(III); Alq 3 has been frequently used as an electron transporting layer in organic light-emitting diodes. Either Ba with a low work function or Au with a high work function was deposited on Alq 3 layer in vacuum. And then, the behaviors of electron transition at the Alq 3 /Ba and Alq 3 /Au interfaces were investigated by using the near edge x-ray absorption fine structure (NEXAFS) spectroscopy. In the each interface, the energy levels of unoccupied obitals were assigned as π * (LUMO, LUMO+1, LUMO+2 and LUMO+3) and σ * . And the relative intensities of these peaks were investigated. In an oxygen atom composing Alq 3 molecule, the relative intensities for a transition from K-edge to LUMO+2 were largely increased as Ba coverage (Θ Ba , 2.7 eV) with a low work function was in-situ sequentially increased on Alq 3 layer. In contrast, the relative intensities for the LUMO+2 peak were reduced as Au coverage (Θ Au , 5.1 eV) with a high work function were increased on Alq 3 layer. This means that the electron transition by photon in oxygen atom which consists in the unoccupied orbitals in Alq 3 molecule, largely depends on work function of a metal. Meanwhile, in the case of electron transition in a carbon atom, as Θ Ba was increased on Alq 3 , the relative intensity from K-edge to π 1 * (LUMO and LUMO+1) was slightly decreased, and from K-edge to π 2 * (LUMO+2 and LUMO+3) was somewhat increased. This rising of the energy state from π 1 * to π 2 * exhibits that electrons provided by Ba would contribute to the process of electron transition in the Alq 3 / Ba interfaces. As shown in above observation, the analyses of NEXAFS spectra in each interface could be important as a basic data to understand the process of electron transition by photon in pure organic materials.