Low-temperature scanning tunneling microscopy and near-edge X-ray absorption fine structure investigations of molecular orientation of copper(II) phthalocyanine thin films at organic heterojunction interfaces

Controlling the molecular orientation in p-n organic heterostructures is one of the key challenges in organic electronics that needs to be solved to improve device performance. In situ low-temperature scanning tunneling microscopy and near-edge X-ray absorption fine structure measurements are used to investigate the molecular orientation of copper(II) phthalocyanine (CuPc) thin films at the interface of a p-n organic heterojunction comprising CuPc on top of 3,4,9,10-peryiene-tetracarboxylic-dianhydride (PTCDA). It is found that CuPc molecules adsorb flat on lying-down PTCDA thin films with their pi-conjugated molecular plane parallel to the substrate surfaces. The preferential orientation of CuPc thin films is determined by the directional pi-pi interaction at the CuPc/PTCDA interface, which also gives rise to the lying-down configuration of CuPc thin films on highly ordered pyrolytic graphite.