Contact formation of LiF/Al cathodes in Alq-based organic light-emitting diodes

A systematic study has been carried out on the formation of an effective electron-injecting contact by depositing an LiF/Al bilayer on tris-(8-hydroxyquinoline) aluminium (Alq) in organic light-emitting diodes. Efficient electron injection is observed in both a LiF/Al bilayer cathode and an LiF-doped Al composite cathode. An analysis with ultraviolet photoelectron spectroscopy reveals a strong similarity in interface chemistry between LiF/Al and LiF-doped Al on Alq. Measurements with high-resolution electron energy loss spectroscopy show limited interfacial reaction of LiF on both Al and Alq, whereas a strong attenuation of the loss peak related to the Li-F stretch mode is observed after depositing an ultrathin Al film on Alq/LiF. The results indicate that the contact is formed as a consequence of chemical reaction with a reacted layer of 1 nm or less. Molecular orbital calculation suggests that the release of Li and subsequent reaction with Alq is thermodynamically allowed. The shallow-contact nature allows for much greater flexibility in the design of cathode structures and potential applications to various device configurations.