Electronic structures of organic/organic heterojunctions: From vacuum level alignment to Fermi level pinning

Electronic structures of organic/organic (O/O) heterojunctions have been studied by photoemission spectroscopy. We showed that vacuum level alignment is only valid for certain O/O heterojunctions rather than a general rule for organic junctions. The mode of energy level alignment is found to depend on the Fermi level position in the organic energy gap. In general, when the Fermi level is near the midgap position, vacuum level alignment at the O/O heterojunction is observed, whereas when the Fermi level is close to the edge of the lowest unoccupied or highest occupied molecular orbital level, Fermi level pinning accompanied by molecular orbital level bending is observed at the O/O heterojunction. (c) 2007 American Institute of Physics.