title: Impacts of side chain and excess energy on the charge photogeneration dynamics of low-bandgap copolymer-fullerene blends.
abstract: Primary charge photogeneration dynamics in neat and fullerene-blended films of a pair of alternating benzo[1,2-b:4,5-b(')]dithiophene (BDT) and thieno[3,4-b]thiophene (TT) copolymers are comparatively studied by using near-infrared, time-resolved absorption (TA) spectroscopy under low excitation photon fluence. PBDTTT-E and PBDTTT-C, differed merely in the respective TT-substituents of ester (-E) and carbonyl (-C), show distinctly different charge photogeneration dynamics. The pair of neat PBDTTT films show exciton lifetimes of ∼0.1 ns and fluorescence quantum yields below 0.2%, as well as prominent excess-energy enhanced exciton dissociation. In addition, PBDTTT-C gives rise to >50% higher P(•+) yield than PBDTTT-E does irrespective to the excitation photon energy. Both PBDTTT-E:PC61BM and PBDTTT-C:PC61BM blends show subpicosecond exciton lifetimes and nearly unitary fluorescence quenching efficiency and, with respect to the former blend, the latter one shows substantially higher branching ratio of charge separated (CS) state over interfacial charge transfer (ICT) state, and hence more efficient exciton-to-CS conversion. For PBDTTT-C:PC61BM, the ultrafast charge dynamics clearly show the processes of ICT-CS interconversion and P(•+) migration, which are possibly influenced by the ICT excess energy. However, such processes are relatively indistinctive in the case of PBDTTT-E:PC61BM. The results strongly prove the importance of ICT dissociation in yielding free charges, and are discussed in terms of the film morphology and the precursory solution-phase macromolecular conformation.
authors.affiliation: Center for Condensed Matter Science and Technology, Department of Physics, Harbin Institute of Technology, Harbin 150001, China.
authors.forename: Ming-Ming
authors.initials: MM
authors.lastname: Huo
authors.affiliation: Department of Chemistry, Renmin University of China, Beijing 100872, China.
authors.forename: Rong
authors.initials: R
authors.lastname: Hu
authors.affiliation: Department of Chemistry, Renmin University of China, Beijing 100872, China.
authors.forename: Ya-Dong
authors.initials: YD
authors.lastname: Xing
authors.affiliation: Department of Chemistry, Renmin University of China, Beijing 100872, China.
authors.forename: Yu-Chen
authors.initials: YC
authors.lastname: Liu
authors.affiliation: Department of Chemistry, Renmin University of China, Beijing 100872, China.
authors.forename: Xi-Cheng
authors.initials: XC
authors.lastname: Ai
authors.affiliation: Center for Condensed Matter Science and Technology, Department of Physics, Harbin Institute of Technology, Harbin 150001, China.
authors.forename: Jian-Ping
authors.initials: JP
authors.lastname: Zhang
authors.affiliation: State Key Laboratory of Polymer Physics and Chemistry, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China.
authors.forename: Jian-Hui
authors.initials: JH
authors.lastname: Hou