Copper Phthalocyanine As Contact Layers For Pentacene Films Grown On Coinage Metals

The metal semiconductor interface determines the efficiency of charge carrier injection into any organic electronics device. Control of this interface, its structure, and its morphology is therefore essential for device improvement. In this study, we analyze the approach of controlling semiconductor morphology at this interface by insertion of a copper phthalocyanine (CuPc) monolayer as a primer between Ag(111), Au(111), and Cu(100) surfaces and the organic semiconductor pentacene (PEN). Controlled monolayer formation is facilitated by thermal desorption of excess multilayers, monitored via thermal desorption spectroscopy (TDS), X-ray photoelectron spectroscopy (CPS) and scanning tunneling microscopy (STM), and the growth of PEN on the resultant monolayer primers is investigated by near-edge X-ray absorption spectroscopy (NEXAFS), atomic force microscopy (AFM) and STM. While well-ordered CuPc monolayers with flat-lying molecules are formed on Au(111) and Ag(111), no long-range order is observed on Cu(100). Subsequently deposited PEN molecules initially adopt a recumbent orientation with their long axis oriented parallel to the surface, while upon further deposition this structure is metastable as molecules adopt an upright orientation beyond the bilayer and form (001) oriented films. Although the recumbent orientation of the CuPc primer layer is not transferred to thicker PEN films, which is attributed to the geometrical inequality of the two molecules, a distinct dewetting, as found for PEN films grown on bare metal surfaces, is efficiently suppressed. This effect is reproducible even for polycrystalline Au surfaces, which resemble the situation of metal contacts in devices.