Hole injection enhancements of a CoPc and CoPc:NPB mixed layer in organic light-emitting devices

The hole injection enhancement in organic light-emitting devices with the insertion of a cobalt phthalocyanine (CoPc) hole injection layer (HIL) between the indium tin oxide (ITO) anode and the N,N′-bis(1-naphthyl)-N,N′- diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) hole transport layer (HTL) was demonstrated through current density-voltage-luminance measurements, in situ photoelectron spectroscopy experiments, and theoretical calculations. The CoPc HIL significantly reduces the hole injection barrier (HIB) and thus serves as an efficient HIL like the conventional copper phthalocyanine HIL. This commonality originates from their similar configurations of the highest occupied molecular orbital (HOMO), which consists of conducting macrocycle isoindole ligands, not related to the central metal. However, as the CoPc:NPB mixed HIL is inserted, the hole injection enhancements are inferior to that of a single CoPc HIL. This is due to the electron transfer from NPB to CoPc, which pulls the HOMO level of the mixed HIL down to the deeper position. The reduced hole injection with the mixed layer implies directly that the HIB between ITO and HIL dominates device performance as the so-called ladder effect of HILs.