Organic photovoltaic devices consist of an active layer made of an electron donating species (e.g. conjugated polymer) and an electron accepting species (e.g. fullerene derivative), and the device efficiency is strongly dependent on the chemistry of the donor and acceptor material and their morphology in the active layer. Poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene), PBTTT, is one such electron donating conjugated polymer typically used in transistors and solar cells because of its high charge carrier mobility. Furthermore, in PBTTT blends with electron accepting fullerene derivatives, e.g. [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), the PCBM has been shown to intercalate between the alkyl side chains of PBTTT, a unique morphological feature missing in many other polythiophene-acceptor blends. While these PBTTT polymers are commonly used in organic electronic devices, oligothiophenes (BTTT-n) consisting of few monomer units are also of interest because they have significant charge carrier mobility, and their synthesis and film morphology are more easily controlled than PBTTT. In addition, oligothiophenes serve as useful models to study the assembly and crystalline packing of polythiophenes without the formation of amorphous domains that complicate polythiophene structural characterization. In recent work  Zhang et al. synthesized BTTT oligomers, and studied single crystals, neat polymer films, and films of 1:1 blends of BTTT oligomers with PCBM to understand the effect of oligomer length on morphology, electronic properties, and device performance. Acceptors were found to intercalate in blends with BTTT-2 oligomers, but not in the case BTTT-1. Also, BTTT-PCBM blends had higher crystallinity than neat BTTT with no PCBM, with the BTTT-2:PCBM blends forming co-crystal with well-ordered, 1D channels of acceptors. In this talk we will present molecular simulation studies using intermediate resolution coarse-grained models, where we observe morphological agreement with these experiments. Using these simulations we also link various molecular features of BTTT oligomers to the blend/composite morphology, thereby guiding synthesis of new materials for organic photovoltaic applications.
. Lei Zhang, Feng Liu, Ying Diao, Hilary, S. Marsh, Nicholas S. Colella, Arthi Jayaraman, Thomas P. Russell, Stefan C. B. Mannsfeld*, and Alejandro L. Briseno* “The Good Host: Formation of Discrete 1-D Fullerene ‘Autobahnen’ in Well-Ordered BTTT Oligomers” J. Am. Chem. Soc. 136 (52), 18120 (2014)
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