Charge transfer-induced horizontal orientation of organic molecules near transition metal oxide surfaces

The influence of charge transfer from organic molecules to transition metal oxide on molecular orientation characteristics of N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (α-NPD) was investigated. Absorption peaks originating from neutral and cationic states of α-NPD increased in absorbance when α-NPD was deposited on metal oxides (MoO₂, MoO₃, and WO₃). Photoluminescence from this α-NPD was directional normal to the film plane. These results indicate that α-NPD is horizontally oriented near the metal oxide surfaces so that charge transfer from α-NPD to metal oxide occurs efficiently. Such horizontal orientation of α-NPD enhanced current density of hole-only α-NPD devices because of improvement of wave function overlap and charge transfer degree at the metal oxide/α-NPD interface.