Role of oxygen vacancies in TiOx films in electronic structure at interface with an α-NPD layer

Abstract

Metal oxide buffer layers are a promising method for improving the efficiency and stability of organic photovoltaic cells and organic light-emitting diodes. To design organic electronic devices intelligently, it is necessary to understand the electronic structure at organic/metal-oxide interfaces. In this study, the electronic structure of films of a well-known hole-transporting material, N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (α-NPD) on TiOx underlayers was studied via ultraviolet photoemission spectroscopy and inverse photoemission spectroscopy. New electronic states were found near the Fermi level (EF) upon adsorption of α-NPD when the TiOx underlayer was highly oxygen defective. The observed spectra were not fully explained by density functional theory (DFT) calculations for simple ionic α-NPD molecules. In addition, the vacuum level shift at the interfaces was also different from the shift predicted by several theoretical models that do not consider chemical reactions at the interface. These results suggest that the adsorbate–substrate interaction is complicated when a highly defective metal oxide is used. The stoichiometry of the metal oxide buffer layer may have a large effect on carrier injection or extraction properties at the interface.