Photoemission study of a new electroluminescent material : Trimer of N-arylbenzimidazoles (TPBI)

Electronic properties of ultra-thin films of an N-arylbenzimidazoles trimer (TPBI) vacuum-deposited on Ag substrates have been studied by ultraviolet photoelectron spectroscopy (UPS). It was found that the assumption of a common vacuum level at the metal/organic interface is not valid in the TPBI-metal case. It was observed that there exists a gradual increase in energetic separation between the highest occupied molecular orbital (HOMO) of TPBI and the Fermi level, E_F, of the metal during the formation of the TPBI-metal interface. For monolayer-thick TPBI covered Ag surfaces, the barrier height for electron injection (separation between E_F and the lowest unoccupied molecular orbital (LUMO) was determined to be 0.6 eV.