Interfacial chemistry of Alq(3) and LiF with reactive metals

The electronic structure and chemistry of interfaces between tris-(8-hydrox yquinoline) aluminum (Alq(3)) and representative group IA and IIA metals, A l, and Al/LiF have been studied by x-ray and ultraviolet photoelectron spec troscopies. Quantum-chemical calculations at the density functional theory level predict that the Alq(3) radical anion is formed upon reaction with th e alkali metals. In this case, up to three metal atoms can react with a giv en Alq(3) molecule to form the trivalent anion. The anion formation results in a splitting of the N 1s core level and formation of a new feature in th e previously forbidden energy gap. Virtually identical spectra are observed in the Al/LiF/Alq(3) system, leading to the conclusion that the radical an ion is also formed when all three of these constituents are present. This i s 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 Alq(3) appears to be destructive, with the deposited Al reacting directly with the quinolate oxygen. We proposed that in those circumstances where the radica l anion is formed, it and not the cathode metal are responsible for the ele ctron injection properties. This is borne out by producing excellent inject ing contacts when Ag and Au are used as the metallic component of the catho de structure. (C) 2001 American Institute of Physics.