AMPHIPHILYC LOW BAND-GAP ROD-COIL BLOCK COPOLYMERS: A TOOL FOR WATER-PROCESSABLE ORGANIC SOLAR CELLS

The morphology control during active layer deposition is a key point to achieve high efficiency in bulk heterojunction (BHJ) solar cells. Optimized interpenetrating networks of donor and acceptor materials, nanoscale phase separation, and vertical percolation pathways are essential for effective separation, transport and collection of charges. Usually this control is obtained using mixed organic solvents for active layer deposition (as chlorinated and/or aromatic solvents), additives or post-fabrication treatments, as thermal/solvent annealing.1 The preparation of water-processable nanoparticle (NP) dispersions of semiconducting polymers recently emerged as an elegant technique to control morphology at nanoscale. Meanwhile, it allows lowering chlorinated solvents waste, thus reducing environmental payback of energy obtained from OPVs and paving the way for industrial application.2 Rod-coil block copolymers (BCPs) can be a tool to achieve ideal morphologies for their self-assembly capability strictly connected to the length of the two blocks and to their physical-chemical properties.3 We report on the synthesis and photovoltaic application of rod-coil BCPs, PCPDTBT-b-P4VP, constituted by a low band-gap copolymer, PCPDTBT as the rigid segment, extensively studied as donor material in BHJ and hybrid solar cells,4 and tailored segments of poly-4-vinylpyridine (P4VP) able to interact with acceptor materials commonly used in OPV,5 as fullerene derivatives. Taking advantage of the hydrophilic behavior of their flexible moiety, these amphiphilic rod-coil BCPs were used to obtain NPs in blend with PCBM, through miniemulsion method in aqueous medium without use of surfactant agents. The NPs were fully morphologically and electrically characterized and then used to prepare efficient organic solar cells with water-processable active layers. 1. Polymers 2014, 6, 2832; 2. Nature Mat 2003, 2, 408; 3. Mat Sci Eng R 2008, 62, 37; 4. Chem Commun 2013, 49, 8602; 5. Eur Pol J 2014, 60, 222.