Evidence for the changes in hole injection mechanism with a CoPc hole injection layer

• Organic hole-only devices were fabricated with and without CoPc hole injection layer.
• The hole injection layer changes transport mechanism with respect to the applied bias.
• The mechanism changes were analyzed with current-density and impedance measurement.
• Field distribution inside the device was estimated from dielectric properties of each layer.
• The dynamic energy level changes with internal field distributions were presented.

The hole injection in hole-only devices with the structures of Al/N,N′-bis(1-naphthyle)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB)/ITO and Al/NPB/cobalt phthalocyanine (CoPc)/ITO were analyzed. With the combined analysis of current density–voltage and impedance measurement, the charge injection mechanism based on the injection limited current model was investigated. The NPB single layer device shows Richardson–Schottky type thermionic emission in the entire applied bias range. On the other hand, the device with the CoPc hole injection layer shows thermionic emission until the applied bias reaches 3.7 V. Increasing the bias further, Fowler–Nordheim tunneling dominates the charge injection. The changes of hole injection mechanism were discussed by evaluating the energy level changes with internal field distributions.