We have determined the electronic energy level alignment at the interface between 4,4′-bis(N-phenyl-1-naphthylamino)biphenyl (NPB) and 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN) using ultraviolet photoelectron spectroscopy. The highest occupied molecular orbital (HOMO) of 20 nm thick HAT-CN film was located at 3.8 eV below the Fermi level. Thus the lowest unoccupied molecular orbital (LUMO) is very close to the Fermi level. The HOMO position of NPB was only about 0.3 eV below Fermi level at NPB/HAT-CN interface. This enables an easy excitation of electrons from the NPB HOMO to the HAT-CN LUMO, creating electron-hole pairs across this organic-organic interface.
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