Surface morphology and interdiffusion of LiF in Alq3-based organic light-emitting devices

Highly efficient organic light-emitting devices (OLEDs) have been realized by insertion of a thin insulating lithium fluoride (LiF) layer between aluminum (Al) cathode and an electron transport layer, tris-(8-hydroxyquinoline) aluminum (Alq3). In this paper, we study the surface morphology of LiF on Alq3 by synchrotron X-ray scattering and atomic force microscopy (AFM) as a function of thickness of LiF. We also study the interdiffusion of LiF into Al cathode as well as into Alq3 layer as a function of temperature. Initially, LiF molecules are distributed randomly as clusters on the Alq3 layer and then gradually form a layer as increasing LiF thickness. The interdiffusion of LiF into Al occurs more actively than into Alq3 in annealing process. LiF on Alq3 induces the ordering of Al to (1 1 1) direction strongly with increasing LiF thickness.