Ultrafast Energy And Electron Transfer In Conjugated Oligomer-Fullerene Molecules
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Ultrafast Energy And Electron Transfer In Conjugated Oligomer-Fullerene Molecules P.A. van Hal,1 R.A.J. Janssen,1 G. Lanzani,2 G. Cerullo,2 M. Zavelani-Rossi,2 and S. De Silvestri2 1
Laboratory of Macromolecular and Organic Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands 2 Istituto Nazionale per la Fisica della Materia, C.E.Q.S.E.-C.N.R., Dipartimento di Fisica, Politecnico di Milano, Piazza L. Da Vinci 32, 20133 Milano, Italy
ABSTRACT The intramolecular photoinduced energy and electron transfer within a fullereneoligothiophene-fullerene triad with nine thiophene units (C60-9T-C60) and an oligo(p-phenylene vinylene)-fullerene dyad with four phenyl groups (OPV4-C60) is investigated with femtosecond pump-probe spectroscopy with sub-10 fs and 200 fs time resolution in solvents of different polarity. Photoexcitation of the π-conjugated oligomer moiety in the triad and dyad results in an ultrafast singlet-energy transfer reaction to create the fullerene singlet-excited state with a time constant of 150-190 fs, irrespective of the polarity of the medium. In a polar solvent, intramolecular electron transfer occurs from the oligomer moiety to the C60 moiety with a time constant of 10-13 ps as a secondary reaction, subsequent to the ultrafast singlet-energy transfer. The charge-separated state has a lifetime of 50-80 ps and recombines to the ground state.
INTRODUCTION Motivated by a demand for inexpensive renewable energy sources there is considerable interest for organic [1-3], polymer [4-9], and organic-inorganic heterojunction [10-13] solid-state photovoltaic cells. In polymer photovoltaic devices a polymer with donor and p-type transport characteristics is combined with a polymer or an organic molecule that has acceptor and n-type transport properties. One particular encouraging approach is the so-called bulk-heterojunction device in which the photoactive layer consists of a composite film, prepared by mixing a conjugated polymer and a fullerene derivative [5,9]. By improving the processing conditions and the device architecture, these photovoltaic cells have recently been optimized to give efficiencies of 2.5% under standard AM 1.5 conditions and external quantum efficiencies up to 50 % at the maximum absorption [14]. Further improvement can be expected from the ongoing development of low-bandgap conjugated polymers that provide an improved overlap of the absorption of the blend with the solar emission spectrum [15,16]. The underlying photophysical process for the photoinduced creation of charges in polymer/fullerene blends is a sub-picosecond forward electron transfer reaction from the polymer as a donor in the excited state to the fullerene as an acceptor [17-19]. Using sub-10 fs excitation pulses, it has recently been established that the forward electron transfer in a blend of a poly(p-phenylene vinylene) and a methanofullerene actually occurs within 50 fs [20]. In general, the backward electron transfer or recombination is much slower and may extend into the
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