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.

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► Charge transfer occurs from α-NPD molecules to transition metal oxide.
► Absorbance increased when α-NPD was deposited on the metal oxides.
► Photoluminescence from this α-NPD was directional normal to the film plane.
► The charge transfer induced a horizontal orientation of α-NPD near the metal oxides.
► Such horizontal orientation enhanced current density of hole-only devices.