Light Absorption in Organic Thin Films: The Importance of Oriented Molecules

O Guskova and C Schunemann and KJ Eichhorn and K Walzer and M Levichkova and S Grundrnann and JU Sommer, JOURNAL OF PHYSICAL CHEMISTRY C, 117, 17285-17293 (2013).

DOI: 10.1021/jp4048083

In this work we apply a joint experimental and theoretical approach to investigate thin films of side chain substituted dicyanovinyl quaterthiophenes (DCV4T-Et2) and DCV4T-Et2:C60 blends, prototypic absorbers for small molecule organic solar cells. Structural characterization of the morphology of thin films thermally deposited at different substrate temperatures on a silica surface was performed by variable angle spectroscopic ellipsometry, grazing incidence X-ray diffraction, and atomic force microscopy measurements. These methods, combined with full-atomistic molecular dynamic (MD) simulation, provide detailed information about thin film morphology, namely about molecular orientation, absorption, phase separation, and crystallinity, i.e., factors that affect the efficiency of organic solar cells. Using molecular dynamics simulation, we can constitute why the DCV4T-Et2 molecules arrange strongly tilted in pristine (69 degrees to 70 degrees tilt angle to the substrate normal) and DCV4T-Et2:C60 blend films (tilt angle of 65 degrees to 69 degrees).

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