Regensburg 2013 – scientific programme

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DS: Fachverband Dünne Schichten

DS 31: Organic thin films II

DS 31.4: Talk

Thursday, March 14, 2013, 15:30–15:45, H8

Molecular Tilt Angle Determination of CuPc Deposited under Different Vacuum Conditions — •Falko Seidel, Ovidiu D. Gordan, and Dietrich R. T. Zahn — Semiconductor Physics, Chemnitz University of Technology, D-09107 Chemnitz, Germany

Thin films of organic molecules are used in applications such as organic photovoltaic cells and organic field effect transistors. Nevertheless, ring structured planar molecules have a high intrinsic anisotropy due to the delocalised highest occupied molecular orbital parallel to the molecular plane. Therefore, the electrical respectively optical properties of these devices depend on the orientation of molecules within the film, which is accessible via the complex dielectric function. Variable angle spectroscopic ellipsometry (VASE) measurements of copper phthalocyanine (CuPc) films enable to quantitatively evaluate the anisotropic complex dielectric function. Considering the ratio of the out of plane optical extinction coefficient to the in plane one (substrate plane) of a uniaxial film, one can determine the average molecular tilt angle with respect to the substrate. CuPc film growth parameters such as vacuum pressure and substrate type have a strong influence on the molecular orientation. Hence, in this work in total 26 differently prepared CuPc films deposited on hydrogen passivated silicon respectively natively oxidized silicon were investigated using VASE. A critical vacuum pressure could be estimated, below which a clear influence of the substrate on the molecular orientation was visible. Moreover, the mathematical limitations are determined taking into account the optical properties of amorphous CuPc film deposited on liquid nitrogen cooled substrates.