Electronic structure near the Fermi level of the organic semiconductor copper phthalocyanine

The electronic structure of thin films of the prototypical organic semiconductor copper phthalocyanine (CuPc) has been measured using resonant soft X-ray emission spectroscopy. We report the observation of two discrete states near EF. This differs from published photoemission results, but is in excellent agreement with density functional calculations. The implications of this result for the use of resonant soft X-ray emission (SXE) in the study of organic semiconductors are discussed. We also compare our data to published X-ray emission results, and show that the latter display clear evidence of beam damage. 2004 Elsevier B.V. All rights reserved. Organic semiconductors are the subject of intense study due to the challenge they pose to our understanding of the physical properties of complex solids and due to technological interest in developing carbonbased electronic devices [1,2]. Copper phthalocyanine (CuPc) is a prototypical organic semiconductor. The phthalocyanine ligand has a complex electronic structure which is further complicated by the introduction of a transition metal ion into the system, such as Cu2þ. The overlap of the metal 3d electronic states with ligand 2p states within the molecule produces a combination of both localized and delocalized states near the Fermi level (EF). Accurate determination of this electronic structure near EF is of general importance in understanding the properties of organic semiconductors. Many photoemission studies have been undertaken to measure this electronic structure, but it is important to appreciate that photoemission spectroscopy measures the electronic structure of an ionized system. * Corresponding author. Fax: +61-735-393-93. E-mail address: [email protected] (K.E. Smith). 1 School of Physical Sciences, Dublin City University, Dublin 9,