Mechanisms of injection enhancement in organic light-emitting diodes through an Al/LiF electrode

The mechanisms of enhanced electron injection into the electron transport l ayer of Alq(3) [tris(8-hydroxyquinoline)-aluminum] via LiF interlayers are studied by means of I-V characteristics, secondary ion mass spectroscopy (S IMS), and Kelvin probe measurements. Devices for single carrier injection w ere prepared using aluminum electrodes, Alq(3) electron transport layers, a nd thin intermediate layers of LiF. It was found that devices deposited in the order Alq(3)-LiF-aluminum have a much higher electron injection capabil ity than structures deposited in the order aluminum-LiF-Alq(3). SIMS depth profile analysis reveals that the evaporation of Al on LiF leads to a spati al separation of Li and F probably induced by a chemical reaction of Al wit h LiF. Simple thermodynamic calculations support the energetic feasibility of such a reaction. Titanium cathodes in the same layer sequence also exhib it electron injection enhancement, probably due to their similar chemical r eactivity. However, electron injection from Ag electrodes is not significan tly improved by the introduction of a LiF interlayer. (C) 2001 American Ins titute of Physics.