Alkali metal acetates as effective electron injection layers for organic electroluminescent devices

The effect of alkali metal (Li+, Na+, K+, Rb+, and Cs+) acetates and fluorides at an aluminum/tris(8-hydroxyquinoline) aluminum (Alq3) interface on performance of organic electroluminescent (EL) devices is described. We also studied the effect of acetates with different metals, such as Mg2+ and Al3+, on the performance of the EL devices. EL characteristics of the devices with Al/alkali metal acetates were compared with those of devices with Al/alkali metal fluorides or an Al cathode itself. We found that the bilayer cathodes with alkali metal acetates exhibited better device performance than the bilayer cathodes with alkali metal fluorides. The improvement can be attributed to lowering of the work function of the Al cathode and doping of Alq3 with alkali metals that were formed by the higher reactivity of alkali metal acetates than alkali metal fluorides with hot Al atoms impinging on these salts during the vapor deposition. The enhanced electron injection resulted in more balanced charge-carrier injection and thus higher EL efficiencies.