Tuning the frontier molecular orbital energy levels of n-type conjugated copolymers by using angular-shaped naphthalene tetracarboxylic diimides, and their use in all-polymer solar cells with high open-circuit voltages

An angular-shaped naphthalene tetracarboxylic diimide (NDI) was designed and synthesized as a new building block for n-type conjugated polymers to tune their energy levels. Three n-type copolymers incorporating this angular-shaped NDI as the acceptor moiety were obtained by Stille coupling reactions and had number average molecular weights of 18.773.0 kDa. All-polymer bulk-heterojunction solar cells made from blends of these polymers with poly(3-hexylthiophene) gave a power conversion efficiency up to 0.32% and exhibited an open-circuit voltage (Voc) up to 0.94 V due to their relative high-lying lowest unoccupied molecular orbital energy levels. The high Voc of 0.94 V is higher than that of solar cells based on linear-shaped NDI-containing polymers (<0.6 V). The results indicate that the angular-shaped NDI is a promising building block for constructing nonfullerene polymer acceptors for solar cells with high open-circuit voltages. (c) 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013, 51, 1999-2005