Band bending of LiF/Alq3 interface in organic light-emitting diodes

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Abstract
The insertion of LiF for an interlayer material between the Alcathode and tris-(8-hydroxyquinoline) aluminum ( Alq 3 ) in the organic light-emitting diodes(OLEDs) provides an improved device performance. The highly occupied molecular orbital (HOMO) level lowering in the Alq 3 layer induced by a low-coverage LiF deposition results in the reduction of electron injection barrier height. We investigated the electronic structure of the interface between the ultrathin LiF and the Alq 3 layer, using synchrotron x-ray photoelectron emission spectroscopy. The results revealed that the major origin of the HOMO level lowering is not the chemical bonding of dissociated fluorine in the Alq 3 layer but the band bending caused by charge redistribution driven by work function difference between LiF and Alq 3 layer.