title: Intrinsic Properties of Two Benzodithiophene-Based Donor--Acceptor Copolymers Used in Organic Solar Cells: A Quantum-Chemical Approach.
abstract: Conjugated donor-acceptor (D-A) copolymers show tremendous promise as active components in thin-film organic bulk heterojunction solar cells and transistors, as appropriate combinations of D-A units enable regulation of the intrinsic electronic and optical properties of the polymer. Here, the structural, electronic, and optical properties of two D-A copolymers that make use of thieno[3,4-c]pyrrole-4,6-dione as the acceptor and differ by their donor unit-benzo[1,2-b:4,5-b']dithiophene (BDT) vs the ladder-type heptacyclic benzodi(cyclopentadithiophene)-are compared using density functional theory methods. Our calculations predict some general similarities, although the differences in the donor structures lead also to clear differences. The extended conjugation of the stiff ladder-type donor destabilizes both the highest occupied and lowest unoccupied molecular orbital energies of the ladder copolymer and results in smaller gap energies compared to its smaller counterpart. However, more significant charge transfer nature is predicted for the smaller BDT-based copolymer by natural transition orbitals than for the ladder copolymer. That is, the influence of the acceptor on the copolymer properties is "diluted" to some extent by the already extended conjugation of the ladder-type donor. Thus, the use of stronger acceptor units with the ladder-type donors would benefit the future design of new D-A copolymers.
authors: Department of Chemistry and Bioengineering, Tampere University of Technology , P.O. Box 541, FI-33101 Tampere, Finland.
authors: Tuuva
authors: T
authors: Kastinen
authors: Department of Chemistry and Bioengineering, Tampere University of Technology , P.O. Box 541, FI-33101 Tampere, Finland.|Department of Physics, Imperial College London , South Kensington Campus, London SW7 2AZ, U.K.
authors: Mika
authors: M
authors: Niskanen
authors: Department of Chemistry and Center for Applied Energy Research, University of Kentucky , Lexington, Kentucky 40511, United States.
authors: Chad
authors: C
authors: Risko
authors: Department of Physics, Tampere University of Technology , P.O. Box 692, FI-33101 Tampere, Finland.
authors: Oana
authors: O
authors: Cramariuc
authors: Department of Chemistry and Bioengineering, Tampere University of Technology , P.O. Box 541, FI-33101 Tampere, Finland.
authors: Terttu I
authors: TI
authors: Hukka