CuPc buffer layer role in OLED performance: a study of the interfacial band energies

We present a UV photoemission (UPS) and topographic study of the indium–tin-oxide (ITO) anode used in organic light emitting diodes (OLEDs). The performance of these devices has been shown to be improved by introducing a thin layer of copper phthalocyanine (CuPc) between the anode and the hole-transport layer (HTL). While the device current at constant driving voltage increases with increasing CuPc thickness, the efficiency is optimized at 12–18 nm. In this article we address the issue of the charge (hole) injection process at the anode interface and demonstrate the importance of directly measuring the vacuum levels in quantitative study of the energy levels of OLED interfaces. As a result of this insight, many calculations relating to the relative energies of the bands at OLED interfaces may have to be revised and corrected. The interface morphologies were also studied using AFM to determine any changes with film growth.