NEXAFS 분광법에 의한 Alq3/Ba과 Alq3/Au의 계면에서의 전자 천이에 관한 연구

Abstract

Tris(8-quinolinolato)aluminum(III); <TEX>$Alq_3$</TEX> 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 <TEX>$Alq_3$</TEX> layer in vacuum. And then, the behaviors of electron transition at the <TEX>$Alq_3$</TEX>/Ba and <TEX>$Alq_3$</TEX>/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 <TEX>${\pi}^*$</TEX>(LUMO, LUMO+1, LUMO+2 and LUMO+3) and <TEX>${\sigma}^*$</TEX>. And the relative intensities of these peaks were investigated. In an oxygen atom composing <TEX>$Alq_3$</TEX> molecule, the relative intensities for a transition from K-edge to LUMO+2 were largely increased as Ba coverage (<TEX>${\Theta}_{Ba}$</TEX>, 2.7 eV) with a low work function was in-situ sequentially increased on <TEX>$Alq_3$</TEX> layer. In contrast, the relative intensities for the LUMO+2 peak were reduced as Au coverage (<TEX>${\Theta}_{Au}$</TEX>, 5.1 eV) with a high work function were increased on <TEX>$Alq_3$</TEX> layer. This means that the electron transition by photon in oxygen atom which consists in the unoccupied orbitals in <TEX>$Alq_3$</TEX> molecule, largely depends on work function of a metal. Meanwhile, in the case of electron transition in a carbon atom, as <TEX>${\Theta}_{Ba}$</TEX> was increased on <TEX>$Alq_3$</TEX>, the relative intensity from K-edge to <TEX>${\pi}_1{^*}$</TEX> (LUMO and LUMO+1) was slightly decreased, and from K-edge to <TEX>${\pi}_2{^*}$</TEX> (LUMO+2 and LUMO+3) was somewhat increased. This rising of the energy state from <TEX>${\pi}_1{^*}$</TEX> to <TEX>${\pi}_2{^*}$</TEX> exhibits that electrons provided by Ba would contribute to the process of electron transition in the <TEX>$Alq_3$</TEX>/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.