The authors show that a photovoltaic device composed of a -donor-bridge–acceptor-bridge- type block copolymer thin film exhibits a significant performance improvement over its corresponding donor/acceptor blend (Voc increased from 0.14to1.10V and Jsc increased from 0.017 to 0.058mA∕cm2) under identical conditions, where donor is an alkyl derivatized poly-p-phenylenevinylene (PPV) conjugated block, acceptor is a sulfone-alkyl derivatized PPV conjugated block, and bridge is a nonconjugated and flexible unit. The authors attribute such improvement to the block copolymer intrinsic nanophase separation and molecular self-assembly that results in the reduction of the exciton and carrier losses.

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A novel class of light harvesting conjugated block copolymers, with electron-donating conjugated blocks (D) connected to electron-accepting conjugated blocks (A) via non conjugated and flexible bridge chains (B), has been designed, synthesized, and characterized. Specifically, D is a decyloxy-substituted polyphenylenevinylene (C10−PPV). A 1 and A 2 are PPVs with sulfone (SO2) acceptor moieties substituted on every other phenylene unit. A 1 carries two decyloxy groups on every phenylene unit, while in A 2 , half of the phenylene units are unsubstituted. The optical energy gaps are 2.24 eV for the donor block (D), 2.33 and 2.45 eV for A 1 and A 2 acceptor blocks. LUMO level offsets are 0.24 and 0.16 eV for D/A 1 and D/A 2 pairs, respectively. Comparing the photoluminescence from both films and solutions, very large red shifts (71 and 74 nm for A 1 and A 2 respectively) were observed in the two acceptor polymers. These red shifts in the emission spectra were more than twice as much as that observed for D (31 nm). The (DBA 1 B) n and (DBA 2 B) n block copolymer films exhibited improved processability and optoelectronic properties when compared with the corresponding films composed of donor/acceptor blends. Atomic force microscopic (AFM) studies of D, A 1 , and A 2 films were also undertaken to observe the degree of aggregation in the films. The results indicate the tendency of intermolecular aggregation increases as A 2 > D > A 1 . AFM topological images revealed that large aggregates of several hundreds of nanometers formed in donor/acceptor blend films, while in block copolymer films, domain sizes were similar to individual block sizes which are 1 order of magnitude smaller than in the blend.

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