Interpretation of the C1s XPS Signal in Copper Phthalocyanine for Organic Photovoltaic Device Applications
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Interpretation of the C1s XPS Signal in Copper Phthalocyanine for Organic Photovoltaic Device Applications. K. Nauka, Hou T. Ng and E.G. Hanson Hewlett-Packard Laboratories, Hewlett-Packard Company, 1501 Page Mill Road, Palo Alto, CA 94304, USA. ABSTRACT Carbon 1s signal in the photoelectron spectrum of copper phthalocyanine has been resolved into four components representing two principal carbon positions within the copper phthalocyanine macrocycle and their shake-up transitions. In addition, the contribution of organic impurities frequently found in commercial CuPc materials has been accounted for with a high degree of accuracy by adding an extra component of the C1s signal. Its magnitude has been correlated with the intensity of aliphatic C-H vibrations observed in the IR spectrum demonstrating that IR absorption measurement can be successfully used for a routine evaluation of impurities in commercial CuPc materials used to fabricate organic photovoltaic devices. INTRODUCTION Copper phthalocyanine (CuPc) molecules form planar structures consisting of porphyrinlike rings surrounded by four peripheral benzene rings and central Cu atom. Because of the Sconjugation of the center carbon atoms CuPc exhibits semiconductor behavior that has found a number of potential applications in organic electronics [1,2]. In addition, CuPc is used as a major cyan pigment in a large number of applications ranging from printing on a paper to fabric dyeing [3]. Electronic applications require high purity CuPc materials with occasional stringently controlled doping providing the desired electrical conductivity. Mass-produced commercial CuPc may contain up to several percent of mostly organic impurities in form of undesirable manufacturing residues or intentional additives used to control handling and storage of the CuPc nanocrystalline powders. Although photoelectron spectroscopy of the core CuPc electronic states employing the X-ray excitation (XPS - X-ray Photoelectron Spectroscopy) has been extensively used to characterize stoichiometric CuPc [4-6], only recently a consensus has been reached regarding the correct interpretation of the chemical shifts observed in the C1s composite XPS signal [5,6]. The presence of organic residues may complicate identification of the C1s components. This report presents a systematic XPS study of a large number of commercial CuPc materials and, in particular, the way of resolving the C1s signal into its constituents with the help of corresponding IR absorption data. EXPERIMENTAL DETAILS A number of commercial CuPc materials (20 commercial samples) were analyzed. They were obtained from different vendors in form of nanocrystalline powders. All CuPc samples
consisted of ȕ-CuPc nanocrystallites having the shape of elongated prisms with dimensions between 50 nm and 200 nm, as confirmed by the electron microscopy. The results were compared with the corresponding data obtained for a selected thin film obtained by thermal evaporation using CuPc purified by a triple-sublimation. Clean, highly c
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