Ultraviolet photoelectron spectroscopy investigation of interface formation in an indium-tin oxide/fluorocarbon/organic semiconductor contact

It has been demonstrated that hole-injection in organic light-emitting devices (OLEDs) can be enhanced by inserting a UV-illuminated fluorocarbon (CFx) layer between indium-tin oxide (ITO) and organic hole-transporting layer (HTL). In this work, the process of interface formation and electronic properties of the ITO/CFx/HTL interface were investigated with ultraviolet photoelectron spectroscopy. It was found that UV-illuminated fluorocarbon layer decreases the hole-injection barrier from ITO to α-napthylphenylbiphenyl diamine (NPB). Energy level diagrams deduced from the ultraviolet photoelectron spectroscopy (UPS) spectra show that the hole-injection barrier in ITO/UV-treated CFx/NPB is the smallest (0.46 eV), compared to that in the ITO/untreated CFx/NPB (0.60 eV) and the standard ITO/NPB interface (0.68 eV). The improved current density-voltage (I-V) characteristics in the UV-treated CFx-coated ITO contact are consistent with its smallest barrier height.