MOLECULAR ORIENTATION AND ENERGY LEVEL ALIGNMENT AT THE CuPc/SAMs INTERFACE

This article highlights recent progress in the use of functionalized self-assembled monolayers for organic electronics, with particular emphasis on the investigation of the CuPc-SAMs interface properties, particularly the energy level alignment and molecular orientation. Synchrotron-based high-resolution photoemission spectroscopy (PES) and near-edge X-ray absorption fine structure measurements (NEXAFS) are used to address these issues. It is found that the energy level alignment at the CuPc-SAMs interface depends on the chain length of SAMs. Fermi level pinning occurs at the interface of CuPc with short chain SAMs of 4-trifluoromethyl-benzenethiol (CF3-SAM) and 4-methyl-benzenethiol (CH3-SAM), whereas the vacuum level aligns at the interface of CuPc with long chain SAMs including 1-(p-thiophenyl)-4-phenylbenzene (BBB), 4-(p-thiophenyl)-2, 2', 5, 5'-tetramethoxy-biphenyl (BOO), 1-(p-thiophenyl)-4-(2', 5'-dimethoxyphenyl)-tetrafluorobenzene (BFO) and 4-pentafluorophenyl-1-(p-thiophenyl)-2, 5-dimethoxybenzene (BOF). A significant reduction of the hole injection barrier (Δh) by up to 0.75 eV was observed after deposition of 5 nm CuPc on BOF/Au(111) as compared to the CuPc/Au(111) (Δh = 0.9 eV). Angular-dependent NEXAFS measurements reveal that CuPc molecules adopt a standing up configuration on all SAMs. This suggests that the interface charge transfer has negligible effect on the molecular orientation of CuPc on various SAMs.