Organic light emitting devices based on tris-8-hydroxyquinoline aluminum with an ultra-thin insulating film instead of a hole-transporting layer were investigated. Three kinds of single-layer devices were fabricated with silicon dioxide (SiO(2)), alumina (Al(2)O(3)) or polytetrafluoroethylene (Teflon(TM)) as the insulating films, and they showed better performance than devices without insulating films. The hole injection of these devices was also investigated using hole-only devices. The holes injection enhancement by tunneling effect and electrons blocking of the Ultra-thin insulating layers were experimentally proved to have yielded high performance of single-layer devices. (C) 2008 Elsevier B.V. All rights reserved.