AMPHIPHILYC LOW BAND-GAP ROD-COIL BLOCK COPOLYMERS: ISSUES IN THE SYNTHESIS AND PHOTOVOLTAIC APPLICATIONS

Low band gap (LGB) polymers are characterized by a band gap below 2 eV, thus absorbing light with wavelengths longer than 620 nm.1 Their polymeric chains are constituted by the alternating electron-rich and electron-poor units. LGB were widely studied in last decades to improve the efficiency of organic photovoltaics (OPVs) due to a better overlap with the solar spectrum, leading to a maximum photon harvesting in the devices.2 Combining this intrinsic feature with the appealing capability of the block copolymers (BCPs) to nanosegregate, we obtained new rod-coil BCPs with a rigid segment constituted by a LBG polymer and a flexible one made up of poly-4vinylpyridine (P4VP) or a segmented poly(styrene-random-4-vynilpyridine). P4VP allows the interaction with commonly used acceptor materials for organic or hybrid PVs.3 Two strategies were applied to the preparation of the target polymers, combining a nitroxide-mediated radical polymerization and a Suzuki polycondensation. A step-growth like approach, consisting in the synthesis, purification, and characterization of the two blocks separately, that were then coupled through a further Suzuki reaction, and a chain-growth like method, constituted by the early synthesis of a properly activated rigid macroiniziator for the subsequent polymerization of the flexible block. The two pathways led to two series of materials differing particularly in controlling the coil block length. Due to the strong differences in chemical-physical properties of the two blocks, the purification and characterization procedures of the obtained materials have to be ad hoc tailored for a correct determination of their molecular structure, crossing the data from different techniques.4 These materials showed, together with their self-assembling capability, interesting features exploitable for photovoltaic applications. Some of the obtained materials were successfully employed as nanostructuring additive in hybrid solar cells, with inorganic semiconductor nanoparticles of CdSe, as acceptor.5 New trials are in progress for the realization of composites nanostructures with PCBM in aqueous medium, to be used as active layers in bulk heterojunctions solar cells. Preliminary results of these studies will be presented. Acknowledgments: research supported by Project AQUA-SOL - Aqueous processable polymer solar cells: from materials to photovoltaic modules (PRIN 2012A4Z2RY) 1 Bundgaard, Krebs, Solar En Mater & Solar Cells 91 (2007) 954. 2 Jayakannan, Van Hal, Janssen , J. Pol. Sci. A Pol. Chem 40 (2002) 251. 3 Di Mauro, Toscanini, Piovani, Samperi, Curri, Corricelli, Comparelli, Agostiano, Destri, Striccoli, Eur Pol J 72 (2014) 222. 4 Zappia, Mendichi, Battiato, Scavia, Mastria, Samperi, Destri, Polymer 80 (2015) 245; Samperi, Montaudo Battiato, Zappia, Destri, paper submitted. 5 Zappia, Destri, Striccoli, Curri, Di Mauro, Zoobia, Maruccio, Rizzo, Mastria, Adv. Sci. Technol. 93 (2014) 235; Zappia, Di Mauro, Mastria, Rizzo, Curri, Striccoli, Destri, accepted Eur. Pol J. (2016) doi:10.1016/j.eurpolymj.2016.03.021