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.
Bibliographical noteFunding Information:
This work was supported by the research project of the National Research Foundation of Korea (Grant No. 2013R1A1A1004778 and 2012M3A7B4049801 ) and Samsung Display Company .
All Science Journal Classification (ASJC) codes
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics
- Materials Chemistry
- Electrical and Electronic Engineering