title: Polarons in Narrow Band Gap Polymers Probed over the Entire Infrared Range: A Joint Experimental and Theoretical Investigation.
abstract: We investigate the photoinduced absorption (PIA) spectra of the prototypical donor-acceptor polymer [2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (C-PCPDTBT) and its silicon bridged variant Si-PCPDTBT over a spectral range from 0.07 to 1.5 eV. Comparison between time-dependent density functional theory simulations of the electronic and vibrational transitions of singlet excitons, triplet excitons, polarons, and bipolarons with the experimental results proves that the observed features are due to positive polarons delocalized on the polymer chains. We find that the more crystalline Si-bridged variant gives rise to a red-shift in the transition energies, especially in the mid-infrared (MIR) spectral range and furthermore observe that the pristine polymers' responses depend on the excitation energy. Blending with PCBM, on the other hand, leads to excitation-independent PIA spectra. By computing the response properties of molecular aggregates, we show that polarons are delocalized in not only the intra- but also the interchain direction, leading to intermolecular transitions which correspond well to experimental absorption features at the lowest energies.
authors.affiliation: Institute for Materials in Electronics and Energy Technology (i-MEET), Friedrich-Alexander University Erlangen-Nuremberg , Martensstraße 7, D-91058 Erlangen, Germany.|Photophysics and OptoElectronics, Zernike Institute of Advanced Materials, Rijksuniversiteit Groningen , Nijenborgh 4 NL-9747 AG, Groningen, The Netherlands.
authors.forename: Simon
authors.initials: S
authors.lastname: Kahmann
authors.affiliation: Max-Planck-Institut für Kohlenforschung (MPI-KOFO), Kaiser-Wilhelm-Platz 1, D-45470 Mühlheim an der Ruhr, Germany.
authors.forename: Daniele
authors.initials: D
authors.lastname: Fazzi
authors.affiliation: Institute for Materials in Electronics and Energy Technology (i-MEET), Friedrich-Alexander University Erlangen-Nuremberg , Martensstraße 7, D-91058 Erlangen, Germany.
authors.forename: Gebhard J
authors.initials: GJ
authors.lastname: Matt
authors.affiliation: Max-Planck-Institut für Kohlenforschung (MPI-KOFO), Kaiser-Wilhelm-Platz 1, D-45470 Mühlheim an der Ruhr, Germany.
authors.forename: Walter
authors.initials: W
authors.lastname: Thiel
authors.affiliation: Photophysics and OptoElectronics, Zernike Institute of Advanced Materials, Rijksuniversiteit Groningen , Nijenborgh 4 NL-9747 AG, Groningen, The Netherlands.
authors.forename: Maria A
authors.initials: MA
authors.lastname: Loi
authors.affiliation: Institute for Materials in Electronics and Energy Technology (i-MEET), Friedrich-Alexander University Erlangen-Nuremberg , Martensstraße 7, D-91058 Erlangen, Germany.|Bavarian Center for Applied Energy Research (ZAE-Bayern), Haberstraße 2a, 91058 Erlangen, Germany.
authors.forename: Christoph J
authors.initials: CJ
authors.lastname: Brabec