Structure modification and annealing effect of polymer bulk heterojunction solar cells based on polyfluorene derivatives

Abstract

Three low bandgap polyfluorene copolymers containing a donor–acceptor–donor moiety have been synthesized via Suzuki and Stille polymerization reactions. Their bandgaps and molecular energy levels (highest occupied molecular orbital and lowest unoccupied molecular orbital) varied with different polymerization methods. The molecular weight of the copolymer increased significantly through copolymerizing with a monomer having a long alkyl side chain. In order to investigate their photovoltaic properties, polymer solar cell devices based on the copolymers were fabricated with a structure of indium tin oxide/poly(styrene sulfonic acid)-doped poly(ethylene dioxythiophene)/copolymers:[6,6]-phenyl-C61-butyric acid methyl ester (PCBM)/LiF/Al under the illumination of AM 1.5G, 100 mW/cm2. We found that the annealing temperature had a profound effect on the power conversion efficiency (PCE) of the devices with a blend of poly[9,9-didodecylfluorene-alt-(bis-thienylene) benzothiadiazole] (PF12-TBT) and PCBM. The PCE of the solar cell based on PF12-TBT/PCBM (1:4) annealing at 70 °C for 20 min was 4.13% with an open-circuit voltage (Voc) of 1.02 V, fill factor of 55.9%, and a short-circuit current (Jsc) of 7.24 mA/cm2. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011