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

The mechanisms of enhanced electron injection into the electron transport layer of ${\mathrm{Alq}}_3$ [tris(8-hydroxyquinoline)-aluminum] via LiF interlayers are studied by means of $\mathrm{I–V}$ characteristics, secondary ion mass spectroscopy (SIMS), and Kelvin probe measurements. Devices for single carrier injection were prepared using aluminum electrodes, ${\mathrm{Alq}}_3$ electron transport layers, and thin intermediate layers of LiF. It was found that devices deposited in the order ${\mathrm{Alq}}_3$ -LiF-aluminum have a much higher electron injection capability than structures deposited in the order aluminum-LiF-${\mathrm{Alq}}_3.$ SIMS depth profile analysis reveals that the evaporation of Al on LiF leads to a spatial separation of Li and F probably induced by a chemical reaction of Al with LiF. Simple thermodynamic calculations support the energetic feasibility of such a reaction. Titanium cathodes in the same layer sequence also exhibit electron injection enhancement, probably due to their similar chemical reactivity. However, electron injection from Ag electrodes is not significantly improved by the introduction of a LiF interlayer.