Organic Thin-Film Solar Cells Using Electron-Donating Perylene Tetracarboxylic Acid Derivatives

Various electron-donating amino groups were introduced into the perylene core of perylene tetracarboxylic acid derivatives (PTCs) to address the potential use in organic solar cells. Broad absorptions of the PTC solutions in the visible to near-infrared (NIR) region suggest that PTCs are promising light-harvesting molecules for solar cells. Electrochemical measurements reveal that the introduction of the electron-donating amino groups makes the energy level of the highest occupied molecular orbital (HOMO) shallow, imparting electron-rich characteristics to the PTCs. The electronic properties of the PTCs are studied on the basis of quantum chemical calculations. The results show that the variation of the amino groups has a significant influence on the HOMO level, while the lowest unoccupied molecular orbital (LUMO) level is relatively sensitive to the electronegativity of the PTC terminal atoms. The PTC thin films exhibit broad absorption bands in the visible to NIR region, as for the PTC solutions, and possess relatively shallow HOMO and LUMO energies that are higher or comparable to those of fullerene C60. These excellent properties encouraged us to employ amine-substituted PTCs as electron donors in a thin film solar cell with C60 as an n-type semiconductor. Photovoltaic devices with a structure of indium tin oxide (ITO)/PTCs/C60/bathocuproine (BCP)/Al were fabricated by spin-coating PTCs on an ITO electrode. The devices with perylene tetracarboxylic acid diimides (PTCDIs), bearing highly basic amino groups (Py-PTCDI, Me2N-PTCDI, and Ph2N-PTCDI), exhibit the higher power conversion efficiencies than those with carbazoyl PTCDI (Cz-PTCDI) and perylene tetracarboxylic acid dianhydrides (PTCDAs). The higher device performance originates from the efficient electron transfer from the PTCs to C60 as the results of the relatively shallow HOMO and LUMO levels of the PTCs bearing the highly basic amino groups. The dependence of the device performance on the PTC film thickness indicates that the introduction of diphenylamino groups on the perylene core suppresses the nonradiative decay of the exciton without decreasing the hole mobility. These results will provide important information for the molecular design of post PTC derivatives

Electron-Donating Perylene Tetracarboxylic Acids for Dye-Sensitized Solar Cells

Novel perylene imide derivatives with both electron-donating and bulky substituents have been synthesized for dye-sensitized solar cells. The power conversion efficiency reached 2.6%, which is the highest value among perylene-sensitized TiO2 solar cells

Conjugated “Molecular Wire” for Excitons

We have synthesized new conjugated, rigid rod oligomers of fluorene, F_<i>n</i>(C₆₀)₂, <i>n</i> = 4, 8, 12, and 16. These pure compounds have F_<i>n</i> chains up to 140 Å long. The C₆₀ groups covalently attached at both ends serve as traps for excitons created in the F_<i>n</i> chains. Excitons created in the chains by photoexcitation reacted rapidly with the C₆₀ groups with decays described well by the sum of two exponentials. Mean reaction times were 2.3, 5.5, and 10.4 ps for <i>n</i> = 8, 12, and 16. In F₁₆(C₆₀)₂, the 10.4 ps reaction time was 40 times faster than that found in earlier reports on molecules of slightly longer length. The simplest possible model, that of one-dimensional diffusion of excitonic polarons that react whenever they encounter the end of a chain, fits the results to obtain diffusion coefficients. Deviations of those fits from the data may point to the need for alternative pictures or may just indicate that diffusion is not ideal. The definite lengths of these molecules enable a stringent test for theories. These results reveal that exciton transport can be much faster than previously believed, a finding that could, along with appropriate nanoassembly, enable new kinds of high-efficiency organic photovoltaics

Time-Resolved EPR Characterization of a Folded Conformation of Photoinduced Charge-Separated State in Porphyrin−Fullerene Dyad Bridged by Diphenyldisilane

Time-Resolved EPR Characterization of a Folded Conformation of Photoinduced Charge-Separated State in Porphyrin−Fullerene Dyad Bridged by Diphenyldisilan

Oligosilane Chain-Length Dependence of Electron Transfer of Zinc Porphyrin−Oligosilane−Fullerene Molecules

A new series of zinc porphyrin−fullerenes bridged by flexible oligosilane chains ZnP−[Sin]−C60 (n = 1−5) was synthesized, and the photophysical properties of these molecules were investigated using steady-state and time-resolved spectroscopic methods. The spectral observations can be well explained by assuming the coexistence of extended conformers and folded conformers, that is, the observed emissions are from the extended conformers while the folded conformers form very short lifetime nonfluorescent excited-state charge-transfer (CT) complexes. Time-resolved transient absorption spectra suggest the generation of intramolecular radical-ion pairs that have sub-microsecond lifetimes. With the number of silicon atoms of the bridged oligosilane, the lifetimes of the radical-ion pairs do not vary regularly, indicating that intramolecular collision of the radical-cation moiety with the radical-anion moiety controls the charge-recombination rate. The attenuation factor of the electron transfer of the silicon chain was evaluated by the bridge-length dependence of charge-separation rate to be 0.16 Å-1 on the basis of the oligosilane chain-length dependence of fluorescence lifetimes. This is the first evaluation of the attenuation factor for the one-dimensional Si−Si chain to the best of our knowledge

Synthesis and Photophysical Properties of Electron-Rich Perylenediimide-Fullerene Dyad

An electron-rich perylenediimide-C60 dyad has been prepared to explore a new type of donor−acceptor system. Time-resolved absorption measurements in benzonitrile revealed unambiguous evidence for the formation of a charge-separated state consisting of perylene diimide radical cation and C60 radical anion via photoinduced electron transfer, showing a new class of artificial photosynthetic models in terms of charge separation

Large Reorganization Energy of Pyrrolidine-Substituted Perylenediimide in Electron Transfer

Excited-state dynamics of an electron-donating, pyrrolidine-substituted perylenediimide-C60 linked dyad have been investigated by means of time-resolved transient absorption spectroscopy and fluorescence lifetime measurements. By the picosecond transient absorption measurements at a selective excitation of the perylenediimide moiety, a charge-separated state has been successfully detected in polar solvents (i.e., benzonitrile, pyridine, and o-dichlorobenzene), demonstrating the occurrence of photoinduced electron transfer from the perylenediimide to the C60 moiety. In contrast, in nonpolar solvents (i.e., toluene), singlet−singlet energy transfer takes place from the perylenediimide to the C60, followed by intersystem crossing to the C60 excited triplet state and subsequent triplet−triplet energy transfer to yield the perylenediimide excited triplet state. Rate constants of the charge recombination in the polar solvents are found to be comparable to or even larger than those of the charge separation, which is in sharp contrast with electron transfer behavior in typical donor-C60 linked systems. A reorganization energy (0.86 eV) of the perylenediimide-C60 linked dyad obtained in the polar solvents is significantly larger than those of similar porphyrin-C60 linked dyads (0.51−0.66 eV) in which both have comparable edge-to-edge distances between donor and acceptor. The large reorganization energy of the perylenediimide-C60 linked dyad relative to the porphyrin-C60 linked dyads results from a relatively large conformational change in the pyrrolidine groups at the perylenediimide moiety accompanied by one-electron oxidation. This agrees with the fact that charge recombination to the ground state rather than the excited triplet state of the perylenediimide moiety is predominant in benzonitrile, irrespective of the lower energy level of the excited triplet state than that of the charge-separated state

A Photoelectrochemical Device with a Nanostructured SnO₂ Electrode Modified with Composite Clusters of Porphyrin-Modified Silica Nanoparticle and Fullerene

A silica nanoparticle has been successfully employed as a nanoscaffold to self-organize porphyrin and C60 molecules on a nanostructured SnO2 electrode. The quenching of the porphyrin excited singlet state on the silica nanoparticle is suppressed significantly, showing that silica nanoparticles are promising scaffolds for organizing photoactive molecules three-dimensionally in nanometer scale. Marked enhancement of the photocurrent generation was achieved in the present system compared with the reference system, where a gold core was employed as a scaffold of porphyrins instead of a silica nanoparticle. The rather small incident photon-to-current efficiency relative to a similar photoelectrochemical device using a silica microparticle may result from poor electron and hole mobility in the composite film due to poor connection between the composite clusters of a porphyrin-modified silica nanoparticle and C60 in micrometer scale

Mobility of Holes in Oligo- and Polyfluorenes of Defined Lengths

The high-frequency mobility of positive charges (holes) moving along the backbones of an extensive data set of 7 oligomers (<i>n</i> = 2–16) and 6 polymers (⟨<i>n</i>⟩ = 26–138) of fluorene was measured using pulse-radiolysis time-resolved microwave conductivity (PR-TRMC) in benzene. As expected, at 8.9 GHz, the measured isotropic ac mobility, μ_ac,meas^iso, was observed to be strongly dependent on the lengths of the chains due to the charges encountering chain ends during one microwave cycle. Values of the measured mobility, μ_ac,meas^iso, ranged from 5 × 10^–4 cm²/(V s) for an <i>n</i> = 2 repeat unit oligomer to 0.18 cm²/(V s) for a polymer with an average length of ⟨<i>n</i>⟩ = 86 repeat units. Global fits to the entire set of lengths extracted the chain-length-independent intramolecular mobility, μ_ac^intra, using the Kubo formula, assuming normal diffusion along the contour of the molecule with reflecting boundary conditions at the ends. The effects of chain conformation, chain defects, polymer length distributions, and a finite polaron length on μ_ac,meas^iso were considered quantitatively. The best fit to the whole data set, taking into account the polymer length distributions, suggests μ_ac^intra = 1.1 cm²/(V s). The fit was improved slightly by implementing randomly spaced barriers to transport along the chain with an average spacing of ∼40 repeat units, although at this spacing, the estimate of μ_ac^intra was not affected. These barriers could represent defects in the polymer or dihedral angles between repeat units, giving poor electronic coupling that persists for times greater than the 50 ps period of the microwaves. In this defect model, the best fit for their average spacing depended strongly on the chain conformation used, while the predicted intramolecular mobility did not. This estimate of the frequency of defects, a new aspect of this work, is less accurate because coiling of chains and defects have similar effects on the measured mobilities, and each can mask the effect of the other. The present data set also shows that there are few, if any, traps having depths substantially greater than thermal energy for holes on polyfluorene. The value of 1.1 cm²/(V s) is tightly constrained by the mobility of charges on the oligomers. Should the diffusion model break down for short chains and only be applicable to longer polymers, the intrachain mobility may be able to take both larger and smaller values (∼0.7–3 cm²/(V s)), although the data points to this being reasonably unlikely. It is shown that measurements of the imaginary part of the conductivity could be used in the future to clarify the mobility if this is the case