title: Charge photogeneration in donor-acceptor conjugated materials: influence of excess excitation energy and chain length.
abstract: We investigate the role of excess excitation energy on the nature of photoexcitations in donor-acceptor π-conjugated materials. We compare the polymer poly(2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[1,2-b;3,4-b']dithiophene)-4,7-benzo[2,1,3]thiadiazole) (PCPDTBT) and a short oligomer with identical constituents at different excitation wavelengths, from the near-infrared up to the ultraviolet spectral region. Ultrafast spectroscopic measurements clearly show an increased polaron pair yield for higher excess energies directly after photoexcitation when compared to the exciton population. This effect, already observable in the polymer, is even more pronounced for the shorter oligomer. Supported by quantum chemical simulations, we show that excitation in high-energy states generates electron and hole wave functions with reduced overlap, which likely act as precursors for the polaron pairs. Interestingly, in the oligomer we observe a lifetime of polaron pairs which is one order of magnitude longer. We suggest that this behavior results from the intermolecular nature of polaron pairs in oligomers. The study excludes the presence of carrier multiplication in these materials and highlights new aspects in the photophysics of donor-acceptor small molecules when compared to polymers. The former are identified as promising materials for efficient organic photovoltaics.
authors: Photonics and Optoelectronics Group, Department of Physics and CeNS, Ludwig-Maximilians-Universität München, Amalienstr. 54, 80799 Munich, Germany.
authors: Raphael
authors: R
authors: Tautz
authors: Enrico
authors: E
authors: Da Como
authors: Christian
authors: C
authors: Wiebeler
authors: Giancarlo
authors: G
authors: Soavi
authors: Ines
authors: I
authors: Dumsch
authors: Nils
authors: N
authors: Fröhlich
authors: Giulia
authors: G
authors: Grancini
authors: Sybille
authors: S
authors: Allard
authors: Ullrich
authors: U
authors: Scherf
authors: Giulio
authors: G
authors: Cerullo
authors: Stefan
authors: S
authors: Schumacher
authors: Jochen
authors: J
authors: Feldmann