Versatile hole injection of VO2: Energy level alignment at N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine/VO2/fluorine-doped tin oxide

• The electronic structures of VO2 were studied with photoemission spectroscopy.
• VO2 reduces hole injection barrier due to its high work function.
• The conduction band of VO2 is very close to the Fermi level.
• Thus the direct electron extraction from hole transport layer could occur.

Energy level alignments at the interface of N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB)/VO2/fluorine-doped tin oxide (FTO) were studied by photoemission spectroscopy. The overall hole injection barrier between FTO and NPB was reduced from 1.38 to 0.59 eV with the insertion of a VO2 hole injection layer. This could allow direct hole injection from FTO to NPB through a shallow valence band of VO2. Surprisingly, VO2 can also act as a charge generation layer due to its small band gap of 0.80 eV. That is, its conduction band is quite close to the Fermi level, and thus electrons can be extracted from the highest occupied molecular orbital (HOMO) of NPB, which is equivalent to hole injection into the NPB HOMO.

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