[Surface and interface analysis for copper phthalocyanine (CuPc) and indium-tin-oxide (ITO) using X-ray photoelectron...

Indium-tin-oxide (ITO) coated glass has been widely used as a hole injection electrode for organic light-emitting devices (OLEDs), but the work function of ITO film usually mismatches the highest occupied molecular orbital (HOMO) of the hole transport materials. Copper phthalocyanine (CuPc) has been used as a hole injection buffer to enhance the hole injection from ITO to the hole transport layer. A thin CuPc layer was thermally evaporated onto the ITO-coated glass substrate, and the surface and interface electron states of the CuPc/ITO close contact were measured and analyzed by X-ray photoelectron spectroscopy (XPS) technology. Results show that, in CuPc molecule, copper atom has a valence of +2 and interacts with nitrogen atoms through coordinate bonds. There are two kinds of carbon atoms: eight carbon atoms bonding with two nitrogen atoms and other 24 carbon atoms with an aromatic hydrocarbon character. The nitrogen atoms are also in two kinds of chemical environment: four nitrogen atoms only bond with two carbon atoms forming C-N=C bonds, and other four nitrogen atoms not only bond with carbon atoms but also bond with copper atom through coordinate bonds. Argon ion beam sputtering was used to study the interface characteristics of the CuPc/ITO contact. As sputtering time increases, the peaks of C 1s and N 1s spectra gradually become weaker, the peaks of Cu 2p, O 1s, In 3d and Sn 3d spectra get stronger. The core-levels of C 1s, N 1s, O 1s, In 3d and Sn 3d spectra all have chemical shifts towards higher or lower binding energy, but their behavior are different.