Interfacial chemistry of Alq3 and LiF with reactive metals

Abstract

The electronic structure and chemistry of interfaces between tris-(8-hydroxyquinoline) aluminum $({\mathrm{Alq}}_{\mathrm{3}})$ and representative group IA and IIA metals, Al, and Al/LiF have been studied by x-ray and ultraviolet photoelectron spectroscopies. Quantum-chemical calculations at the density functional theory level predict that the ${\mathrm{Alq}}_{\mathrm{3}}$ radical anion is formed upon reaction with the alkali metals. In this case, up to three metal atoms can react with a given ${\mathrm{Alq}}_{\mathrm{3}}$ molecule to form the trivalent anion. The anion formation results in a splitting of the $\mathrm{N}\text{\hspace{0.05em}1s}$ core level and formation of a new feature in the previously forbidden energy gap. Virtually identical spectra are observed in the ${\mathrm{Al/LiF/Alq}}_{\mathrm{3}}$ system, leading to the conclusion that the radical anion is also formed when all three of these constituents are present. This is support by a simple thermodynamic model based on bulk heats of formation. In the absence of LiF or similar material, the reaction of Al with ${\mathrm{Alq}}_{\mathrm{3}}$ appears to be destructive, with the deposited Al reacting directly with the quinolate oxygen. We proposed that in those circumstances where the radical anion is formed, it and not the cathode metal are responsible for the electron injection properties. This is borne out by producing excellent injecting contacts when Ag and Au are used as the metallic component of the cathode structure.