All-conjugated block copolymers bring together distinct pi-conjugated polymers and represent promising materials for the development of organic photovoltaic devices. Recent work from our group has demonstrated all-conjugated block copolymer OPVs with nearly 3% power conversion efficiencies. Here, we explore the role of linking chemistry on performance and charge separation by studying two all-conjugated block copolymers with identical composition and molecular weights but which differ only in the linking group between donor and acceptor polymer blocks. We analyze the same system that has shown near 3% power conversion efficiencies, comprised of a poly(3-hexylthiophene) (P3HT) donor polymer and poly(2,7-(9’,9’,-dioctylfluorene)-alt-5,5-(4’,7’-di-2-thienyl-2’,1’,3’,-benzothiadiazole) (PFTBT) polymeric acceptor. The linking group is comprised of a fully conjugated push-pull monomer, oriented with the electron rich segment either adjacent to P3HT or the PFTBT block. Photovoltaic device studies, X-ray scattering, and transient absorption measurements provide information on the microstructure and electronic properties of donor-acceptor all-conjugated block copolymer films. We show that changing the orientation of the linking group can decrease power conversion efficiencies and photocurrents by more than an order of magnitude. In the optimal configuration of the linking group, transient absorption measurements reveal charge separated states (free charges) in block copolymers with fully conjugated linking groups. These measurements also reveal ultrafast charge recombination (&lt; 1 ns) for even the most efficient block copolymer OPVs, with power conversion efficiencies greater than 2%. This suggests that further optimization of all-conjugated block copolymer may be available through tailoring the linking group to increase charge-separation while suppressing charge recombiation. All-conjugated block copolymers may also serve as a useful model system for studying charge separation and charge-transfer states at organic donor-acceptor interfaces

Role of linking chemistry on charge separation and photovoltaic performance of all-conjugated block copolymers

249th National Meeting (2015)

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