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Realistic prediction of molecular organizations in thin organic films

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The molecular organization of organic semiconductors (OSC), and in particular of those that present liquid crystal (LC) phases [1], has a strong influence on charge and energy transport, particularly at interfaces [2]. Predicting realistic morphologies and molecular organizations from chemical structure is, however, far from easy and has only recently proved doable by atomistic molecular dynamics [3-5]. The issue is further complicated in thin films, where the material is strongly affected by surface interactions, even if obtaining information on alignment and anchoring is essential to optimize the specific interfacial orientations required for different applications (e.g. for Field Effect Transistors, rather than Organic Solar Cells).
Here we show examples of the prediction of alignment and anchoring of organic functional materials (cyanobiphenyls in particular) at the interface with different substrates giving alignment parallel to the support surface e.g. for crystalline and glassy silica with different roughness [5] or polymers like PMMA or polystyrene [6]. We also show how hometropic orientations can be obtained coating the silica surface with suitable self assembled monolayers (SAM) of alkysilanes [7,8]. The importance of the film fabrication process on molecular alignment is also briefly discussed taking as an example the vapour deposition of sexithiophene (T6) on C60 [9] or pentacene on silica [10] While detailed atomistic simulations are on the way to providing reliable results for samples of the order of a few thousand molecules, going to significantly larger sizes comparable to those of real devices (e.g. 100nm thick) demands samples of the order of, say 106 molecules, which in turns requires giving up some details, using some form of coarse graining (CG). Ideally this CG procedure should provide reliable morphologies, albeit at molecular, rather than fully atomistic resolution, but also be capable of returning on demand the atomistic details needed for further charge transport calculations. Some examples will be presented of such a reversible CG approach based on modelling organic functional materials with collections of anisotropic Gay-Berne beads [11].

[1] H. Iino, T. Usui and J-I. Hanna, Nature Comm. 6, 6828 (2015)
[2] O.M. Roscioni, C. Zannoni, Molecular Dynamics Simulation and its Applications to Thin-Film Devices, in Unconventional Thin Film Photovoltaics, edited by E. Da Como, F. De Angelis, H. Snaith, A. B Walker, RSC (2016)
[3] J. Idé, R. Méreau, L. Ducasse, F. Castet, H. Bock, Y. Olivier, J. Cornil, D. Beljonne, G. D’Avino, O. M. Roscioni, L. Muccioli, C. Zannoni, JACS , 136, 2911 (2014)
[4] M. F. Palermo, L. Muccioli, C. Zannoni, PCCP , 17, 26149 (2015)
[5] O. M. Roscioni, L. Muccioli, R. G. Della Valle, A. Pizzirusso, M. Ricci, C. Zannoni, Langmuir, 29, 8950 (2013).
[6] M.F. Palermo, F. Bazzanini, L. Muccioli, C. Zannoni, Liq. Cryst. 44, 1764 (2017)
[7] A. Mityashin, O.M. Roscioni, L. Muccioli, C. Zannoni, V. Geskin, J. Cornil, D. Janssen, S. Steudel, J. Genoe, P. Heremans, ACS Applied Materials & Interfaces, 17, 15372 (2014)
[8] O. M. Roscioni, L. Muccioli, C. Zannoni, ACS Applied Materials & Interfaces 9, 11993 (2017).
[9] G. D'Avino, L. Muccioli and C. Zannoni, Adv. Funct. Mater. 25, 1985 (2015).
[10] O. M. Roscioni, G. D'Avino, L. Muccioli and C. Zannoni, J. Phys. Chem. Lett. 9, 6900 (2018).
[11] M. Ricci, O. M. Roscioni, L Querciagrossa, C. Zannoni. to be published (2019)

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