Orientation-controlled charge transfer at $CuPc/F16CuPc$ interfaces

Molecular orientation-controlled charge transfer has been observed at the organic-organic heterojunction interfaces of copper-hexadecafluoro-phthalocyanine $(F16CuPc)$ or copper(II) phthalocyanine (CuPc) on both standing-up and lying-down CuPc or $F16CuPc$ thin films. In situ synchrotron-based photoemission spectroscopy reveals that the charge transfer at the standing $F16CuPc/CuPc$ or $CuPc/F16CuPc$ interface is much larger than that at the lying $F16CuPc/CuPc$ or $CuPc/F16CuPc$ interface. This can be explained by the orientation-dependent ionization potentials of well-ordered organic thin films, which place the highest-occupied molecular orbital of the standing CuPc film much closer to the lowest-unoccupied molecular orbital of the standing $F16CuPc$ film, facilitating stronger charge transfer as compared to that at the lying OOH interfaces. Our results suggest the possibility of manipulating interfacial electronic structures of organic heterojunctions by controlling the molecular orientation, in particular for applications in ambipolar organic field transistors and organic photovoltaics.