Benzo[2,1-b;3,4-b′]dithiophene-Based Low-Bandgap Polymers for Photovoltaic Applications

The synthesis of four alternating copolymers using benzo[2,1-b;3,4-b']dithiophene (BDP) as the common donor unit is presented. Before the synthesis, theoretical calculations that we performed predicted that the incorporation of BDP, which consists of fused dithiophene units with a benzene ring, into these polymers would produce a low-lying highest occupied molecular orbital (HOMO) energy level. Low-lying HOMO levels are desirable to produce high open circuit voltages (V-OC) in organic bulk heterojunction (BHJ) photovoltaic devices. The polymers' structural characterization, as well as the preliminary results of their performance in BHJ devices, using (6,6)-phenyl C-61-butyric acid methyl ester as the electron acceptor, is presented. The V-OC values follow the expected trend: increasing with decreasing HOMO level of the polymer. High V-OC values of 0.81 and 0.82 V have been obtained from two polymers: PBDPBT and PBDPDPP. The initial power conversion efficiency achieved in these unoptimized devices was 1.11% because of relatively low J(SC) values. The variation observed in the J(SC) values between the four polymers is discussed. Device performance is expected to increase with optimization of processing conditions for the devices. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 701-711, 2011