title: An oligomer study on small band gap polymers.
abstract: Small band gap polymers may increase the energy conversion efficiency of polymer solar cells by increased absorption of sunlight. Here we present a combined experimental and theoretical study on the optical and electrochemical properties of a series of well-defined, lengthy, small band gap oligo(5,7-bis(thiophen-2-yl)thieno[3,4-b]pyrazine)s ( E g = 1.50 eV) having alternating donor and acceptor units. The optical absorptions of the ground state, triplet excited state, radical cation, and dication are identified and found to shift to lower energy with increasing chain length. The reduction of the band gap in these alternating small band gap oligomers mainly results from an increase of the highest occupied molecular orbital (HOMO) level. The S 1-T 1 singlet-triplet splitting is reduced from approximately 0.9 eV from the trimeric monomer to -0.5 eV for the pentamer. This significant exchange energy is consistent with the fact that both the HOMO and the lowest unoccupied molecular orbital (LUMO) remain distributed over virtually all units, rather than being localized on the D and A units.
authors.affiliation: Laboratory of Macromolecular and Organic Chemistry, Eindhoven University of Technology, P.O. Box 513, NL-5600 MB Eindhoven, The Netherlands.
authors.forename: Bram P
authors.initials: BP
authors.lastname: Karsten
authors.forename: Lucas
authors.initials: L
authors.lastname: Viani
authors.forename: Johannes
authors.initials: J
authors.lastname: Gierschner
authors.forename: Jérôme
authors.initials: J
authors.lastname: Cornil
authors.forename: René A J
authors.initials: RA
authors.lastname: Janssen