title: Optical properties of low band gap alternating copolyfluorenes for photovoltaic devices.
abstract: In a joint experimental and theoretical work the optical response and excited-state character of two novel conjugated polymers for photovoltaic applications are studied. The polymers, alternating polyfluorene (APFO) Green 1 and APFO Green 2, are both copolymers of fluorene, thiophene, and electron accepting groups. The band gaps are extended into the red and near infrared with onsets of 780 and 1000 nm, respectively, due to alternating donor and acceptor moieties along the polymer chain. Spectroscopic ellipsometry and subsequent modeling made it possible to extract the dielectric function in the range of 260-1200 nm. Semiempirical quantum chemical calculations (ZINDO) revealed the character of the main electronic transitions in the studied spectral region. The spectral band just above 400 nm was assigned to a delocalized pi-pi* transition for both polymers. The red band lying at 622 and 767 nm in the two polymers corresponds to an electronic state mainly occupying the acceptor units and having a strong charge-transfer character. We show that the ZINDO transition energies are valuable input to the application of Lorentz oscillators in modeling of the dielectric function of the polymer material.
authors.affiliation: Biomolecular and Organic Electronics, Department of Physics and Measurement Technology, Linköpings Universitet, SE-581 83 Linköping, Sweden. nilpe@ifm.liu.se
authors.forename: Nils-Krister
authors.initials: NK
authors.lastname: Persson
authors.forename: Mengtao
authors.initials: M
authors.lastname: Sun
authors.forename: Pär
authors.initials: P
authors.lastname: Kjellberg
authors.forename: Tönu
authors.initials: T
authors.lastname: Pullerits
authors.forename: Olle
authors.initials: O
authors.lastname: Inganäs
mesh_terms: D002210:Caprylates / Q000737:chemistry*; D002627:Chemistry, Physical / Q000379:methods*; D004583:Electrons; D005449:Fluorenes / Q000737:chemistry; D005466:Fluorocarbons / Q000737:chemistry*; D008956:Models, Chemical; D008958:Models, Molecular; D015233:Models, Statistical; D008968:Molecular Conformation; D009991:Oscillometry; D010777:Photochemistry / Q000295:instrumentation* / Q000379:methods*; D011108:Polymers / Q000737:chemistry; D011789:Quantum Theory; D013056:Spectrophotometry, Ultraviolet; D019265:Spectroscopy, Near-Infrared
chemical_list: D002210:Caprylates; D005449:Fluorenes; D005466:Fluorocarbons; D011108:Polymers; C041509:fluorene; C023036:perfluorooctanoic acid