Electronic structures at organic heterojunctions of N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamin (NPB)-based organic light emitting diodes

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Interface electronic structures of four-kinds of electron transporting or hole blocking organic materials (n-type) on a widely-used hole transporting material (p-type) in organic light emitting diodes (OLEDs), N,N′-bis (1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamin (NPB), were investigated by means of photoelectron spectroscopy (PES). 1,3-bis[5-(4-tert-butylphenyl)-1,3,4-oxadiazole-2-yl]benzene (OXD-7) and 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) overlayers show continuous energy shift of each overlayer-derived spectral components and the vacuum level proportional to the thickness. This energy shift is ascribed to a spontaneous building up of the electrostatic potential within the organic layers (giant surface potential; GSP). The energy shift of the overlayers induced by GSP as well as the interface vacuum level shift are adequately taken into account to determine the actual energy barrier heights of the hole conduction levels at the heterojunctions. 4,4′-bis(9-carbazolyl)biphenyl (CBP) and p-bis(triphenylsilyl) benzene (UGH2) induce band bending in the NPB film which presumably results from charge transfer (CT) to the n-type materials from NPB. Despite absence of a practical vacuum level shift and thickness dependent shift of the overlayer-derived electronic states, the CT-derived energy shift of NPB reduces the actual energy barrier height with respect to the nominal barrier height being simply interpreted from PES spectra of a thick overlayer of each material. The energy level diagrams across these ‘n-on-p’ organic–organic heterojunctions were finely determined based on the above interpretation of the PES spectra.