In the quest for renewable energy, an increasing amount of attention is devoted to the use of organic conjugated materials as active components in photovoltaic devices. Because the primary photoexcitations in conjugated molecules and polymers are strongly bound electron-hole pairs (excitons), efficient charge generation only takes place at the heterojunction between a low-ionization-potential (electron-donor) material and a high-electron-affinity (electron-acceptor) material in multicomponent architectures. This provides the energy mismatch between the frontier molecular orbitals required to overcome the exciton binding energy, which is on the order of 0.4 eV in conjugated polymers. [1][2][3][4] The highest quantum yields for charge generation in organic cells to date have been reported for polymer-fullerene blends. [5,6] C 60 is an efficient electron acceptor but features reduced absorption cross section in the visible spectral region due to symmetry-forbidden optical transitions (note that these symmetry constraints are slightly relaxed in the soluble C 60 derivatives such as [6,6]-phenyl C 61 -butyric acid methyl ester (PCBM) [7] ). Thus, in the simplest scenario, photoinduced charge generation in polymer-fullerene cells involves the formation of a local excited state on the conjugated polymer, which acts both as electron donor and sensitizer, followed by electron transfer to C 60 . A different picture was proposed on the basis of time-resolved transient [8] and photoluminescence quenching [9] experiments performed in solutions of model dyad and triad compounds including oligo(phenylenevinylene) (OPV n ) as donor and N-methylfulleropyrrolidine (MPC 60 ) as acceptor. [8][9][10] The solution data suggest a two-step mechanism with resonant energy transfer (RET) from the photoexcited conjugated chain to the covalently linked fullerene derivative prior to hole migration from MPC 60 to the OPV n moiety. In films, the direct photoinduced electron-transfer reaction is much faster than in solution as a result of more favorable donor-acceptor interactions, and the dynamics of direct electron transfer versus RET can no longer be distinguished using femtosecond pump-probe spectroscopy. [11,12] Thus, the two competitive channels (electron transfer versus RET followed by hole transfer) likely contribute to charge generation in solid-state photovoltaic devices. We note that energy transfer has also been demonstrated to compete with charge generation in other C 60 -based molecular dyads. [13,14] Two questions arise at this stage. First, can one take advantage of RET to design more efficient polymer-based photovoltaic cells? A threefold increase in photocurrent has been recently reported by McGehee and co-workers on inserting a thin layer of a low-bandgap polymer at the interface between a polythiophene derivative (donor) and TiO 2 (acceptor). [15] The enhanced charge generation quantum yield was ascribed to RET to the low-bandgap material and the resulting improved harvesting of electronic excitations at the donor-acceptor he...

Spectroscopic properties of a ground state nonbonded porphine-buckminsterfullerene (H2P...C60) complex are studied in several different relative orientations of C60 with respect to the porphine plane by using the density functional (DFT) and time-dependent density functional (TDDFT) theories. The geometries and electronic structures of the ground states are optimized with the B3LYP and PBE functionals and a SVP basis set. Excitation energies and oscillator strengths are obtained from the TDDFT calculations. The relative orientation of C60 is found to affect the equilibrium distance between H2P and C60 especially in the case of the PBE functional. The excitation energies of different H2P...C60 complexes are found to be practically the same for the same excitations when the B3LYP functional is used but to differ notably when PBE is used in calculations. Existence of the states related to a photoinduced electron transfer within a porphyrin-fullerene dyad is also studied. All calculations predict a formation of an excited charge-transfer complex state, a locally excited donor (porphine) state, as well as a locally excited acceptor (fullerene) state in the investigated H2P...C60 complexes.

The optical transitions of three different size oligo(p-phenylenevinylene)-fullerene dyads (OPV(n)-MPC(60); n = 2-4) and of the corresponding separate molecules are studied using density functional theory (DFT) and time-dependent density functional theory. The DFT is used to determine the geometries and the electronic structures of the ground states. Transition energies and excited-state structures are obtained from the TDDFT calculations. Resonant energy transfer from OPV(n) to MPC(60) is also studied and the Fermi golden rule is used, along with two simple models to describe the electronic coupling to calculate the energy transfer rates. The hybrid-type PBE0 functional is used with a split-valence basis set augmented with a polarization function (SV(P)) in calculations and the calculated results are compared to the corresponding experimental results. The calculated PBE0 spectra of the OPV(n)-MPC(60) dyads correspond to the experimental spectra very well and are approximately sums of the absorption spectra of the separate OPV(n) and MPC(60) molecules. Also, the absorption energies of OPV(n) and MPC(60) and the emission energies of OPV(n) are predicted well with the PBE0 functional. The PBE0 calculated resonant energy transfer rates are in a good agreement with the experimental rates and show the existence of many possible pathways for energy transfer from the first excited singlet states of the OPV(n) molecules to the MPC(60) molecule.

No abstract

No abstract