GIXRD characterization of porphyrin thin films

Porphyrin derivative complexes are ubiquitous molecules in nature. This pervasiveness arises from their special planar structure with a metal atom bound with 4 N atoms in the macrocycle and from a stable ? conjugated system. Among the plethora of applications, porphyrins are nowadays used as a photosensitized donor material in organic photovoltaic cells (OPV), because of their large absorbance in the visible region. In organic semiconductors the solid aggregation induces unique optical and electronic-magnetic properties, which often differ from those of single molecules [1, 2]. Differences in the optical absorption have been measured when comparing films with different degree of ordering [3]. To fabricate OPV cells with high power conversion efficiency, ?E, it is crucial to control the crystal structure, crystallinity, and molecular orientation of the thin films used, because the diffusion length of excitons and/or charge carriers, electrons and holes in their crystalline films becomes longer than in their amorphous form. Zinc octaethyl porphyrin (ZnOEP) is the best candidate as an electron donor for the OPV cells, because Zn owns higher photoabsorbance in the visible light, and good efficiencies have been achieved with dyes which usewhere porphyrin macrocycles act as ?-bridges [3]. In this work, we present a Grazing Incidence X-ray diffraction (GIXRD) characterization of ZnOEP thin films grown onto Silicon and Indium Tin oxide (ITO) substrates in Ultra High Vacuum and presenting different degrees of ordering. GIXRD ex situ measurements were performed at the X-ray Diffraction beamline 5.2 (XRD1) at the Synchrotron Radiation Facility Elettra in Trieste (Italy). Data show a slight progressive increase in cell volume passing from the less to the more ordered samples, even though hardly exceeding the standard errors. Preliminary peak profile analysis confirms the presence of middle-range order. Simulation of GIXRD patterns is on the way in order to evaluate the prevalent orientation of crystallites even in presence of the additional complexity due to polymorphism. References [1]M. Marsili , P. Umari, G. Di Santo, M. Caputo, M. Panighel, A. Goldoni, M. Kumar, M. Pedio, PCCP 16, 27104-11, (2014). [2]J. Bartolomé, et al. Magnetism of Metal Phthalocyanines, Ch. 9 in J. Bartolomé et al. eds., Molecular Magnets, Springer Heidelberg 2014. ISBN 9783642406096 [3]S. Ryuzaki, T. Hasegawa, J. Onoe J. Appl. Phys. 105, 113529 (2009) and refs therein.