Voltammetric study of Bphen electron-transport layer in contact with LiF∕Al cathode in organic light-emitting diodes

We report that the similarities of I-V characteristics of organic light-emitting diodes (OLEDs) incorporating Bphen (4,7-diphenyl-1,10-phenanthroline) electron-transport layer, with or without lithium doping, do not stem from the analogous compositions of the respective electron-transport layers. Voltammetric studies of OLED devices of the following general structure: anode ∣ NPB (N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine) ∣ TBADN (9,10-bis(2-naphthyl)-2-t-butylanthracene) ∣ Bphen ∣ LiF ∣ Al, indicate that the bulk of the Bphen electron-transport layer has insulating rather than semiconducting properties. A previously unknown feature is also detected by voltammetry: the presence of a macroscopic dipole comprised of a negative interfacial charge at the NPB ∣ TBADN interface and a positive charge in the Bphen layer and∕or at the TBADN ∣ Bphen interface, which could result from the diffusion of lithium ions. We also find that, in contrast to the bulk of the Bphen layer, its thin sublayer has, in fact, semiconducting properties, which also could be reasonably explained by the diffusion of lithium ions along with the migration of a countercharge, e.g., electron hopping between Bphen molecules.