Optical properties of low band gap alternating copolyfluorenes for photovoltaic devices

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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 –* 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.


Research areas and keywords

Subject classification (UKÄ) – MANDATORY

  • Atom and Molecular Physics and Optics
Original languageEnglish
Pages (from-to)204718-204718-9
JournalJournal of Chemical Physics
Issue number20
Publication statusPublished - 2005
Publication categoryResearch

Bibliographic note

The information about affiliations in this record was updated in December 2015. The record was previously connected to the following departments: Chemical Physics (S) (011001060)