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Article

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

Wei Chen,* Han Huang, Shi Chen, Xing Yu Gao, and Andrew Thye Shen Wee*

Department of Physics, National University of Singapore, 2 Science Drive 3, 117542, Singapore

J. Phys. Chem. C, 2008, 112 (13), pp 5036–5042

DOI: 10.1021/jp710722s

Publication Date (Web): March 7, 2008

Corresponding authors. E-mail: (W.C.) phycw@nus.edu.sg; (A.T.S.W.) phyweets@nus.edu.sg.

Abstract

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-perylene-tetracarboxylic-dianhydride (PTCDA). It is found that CuPc molecules adsorb flat on lying-down PTCDA thin films with their π-conjugated molecular plane parallel to the substrate surfaces. The preferential orientation of CuPc thin films is determined by the directional π-π interaction at the CuPc/PTCDA interface, which also gives rise to the lying-down configuration of CuPc thin films on highly ordered pyrolytic graphite.

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