Scj Stefan Meskers

Effect of the Fibrillar Microstructure on the Efficiency of High Molecular Weight Diketopyrrolopyrrole‐Based Polymer Solar Cells

The nature of the solubilizing alkyl side chains has a strong effect on the performance of semiconducting diketopyrrolopyrrole polymers in organic solar cells with fullerene acceptors. The effect relates to the width of semicrystalline polymer fibrils that form in these blends. If the width of the fibril is wider than the exciton diffusion length, fewer charges form and the efficiency drops.

Electronic memory effects in diodes from a zinc oxide nanoparticle-polystyrene hybrid material

Current-voltage characteristics of diode structures with an active layer of a zinc oxide nanoparticle-polystyrene hybrid material (1:2 by weight) deposited by spin coating from solution were investigated. Aluminum and poly(3,4-ethylenedioxythiophene):polystyrene-sulfonate were used as electrodes. After a forming step, the conduction under reversed bias voltage can be raised or lowered in a gradual and reversible manner by applying forward and reverse bias voltages, respectively. Electrically induced switching between states with high and lower conductivities is possible on a time scale of 100ms and the conduction levels remain stable for over 1h.

Singlet and triplet excitations of chiral dialkoxy-p-phenylene vinylene oligomers

The excited state properties of a series of α,ω-dimethyl-oligo{2,5-bis[2-(S)-methylbutoxy]-p-phenylene vinylene}s (OPVns, with n the number phenyl rings) are investigated for n=2–7 in solution at ambient temperature, under matrix-isolated conditions at low temperature, and as nanoaggregates using absorption (time-resolved), photoluminescence, photoinduced absorption, circular dichroism, and circular polarized luminescence spectroscopies. The singlet (S1←S0) and triplet (Tn←T1) transition energies decrease with conjugation length. For the S1 state of OPVn the lifetime strongly decreases with chain length due to enhanced nonradiative decay and radiative decay. The increase in the nonradiative decay rate constant is much more pronounced, and as a result the photoluminescence quantum yield is less for longer oligomers. Studies at low temperature afforded spectra with well-resolved vibronic fine structure. Under these conditions the Stokes’ shift is very small (⩽0.04 eV). The Huang–Rhys parameter and relaxatio...

Electronic memory effects in diodes of zinc oxide nanoparticles in a matrix of polystyrene or poly(3-hexylthiophene)

Electronic memory effects in metal-insulator-metal devices with aluminum and poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) as electrodes and a solution processed active layer consisting of zinc oxide (ZnO) nanoparticles embedded in a matrix of poly(3-hexylthiophene) or polystyrene are investigated. After an initial forming process, the devices show a reversible change in conductivity. The forming process itself is interpreted in terms of desorption of molecular oxygen from the ZnO nanoparticle surface, induced by injection of holes via the PEDOT:PSS contact, leading to a higher n-type conductivity via interparticle ZnO contacts. The forming can also be induced with ultraviolet light and the process is studied with electron paramagnetic resonance, photoinduced absorption spectroscopy, and field effect measurements. Also, the composition of the active layer is varied and the memory effects can by influenced by changing the ZnO content and the polymer, allowing for data storage with lifet...

Charge recombination in a poly(para-phenylene vinylene)-fullerene derivative composite film studied by transient, nonresonant, hole-burning spectroscopy

Transient, nonresonant, hole-burning spectroscopy has been used to study the charge recombination process in poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1-4-phenylene vinylene] (MDMO-PPV): methanofullerene (PCBM) composite films. The position and intensity of the spectral hole in the absorption band of MDMO-PPV have been monitored as a function of time in the 10 ns–10 μs time range. A time-dependent redshift is observed. The intensity of the spectral hole decays with time according to a power law (∝t−α). The exponent α≈0.5 is found to be nearly independent of the excitation fluence in the range 0.05–2 mJ/cm2. The depth of the spectral hole depends sublinearly on the excitation fluence (I) and can be described by (∝Γ−β) with β∼0.5. The time-dependent redshift and the power-law type time decay can be reproduced by numerical simulations. The Monte Carlo method is used to simulate the hopping dynamics of the photoinduced charges in a lattice of energetically disordered sites before they eventually recombine at ...

Relaxation of photo-excitations in films of oligo- and poly-(para-phenylene vinylene) derivatives

Abstract The photo-luminescence from solid films of poly(para-phenylene vinylene) polymers and an oligomeric model system, consisting of seven repeat units, are investigated at low temperature (8 K) using time-resolved spectroscopic techniques. Results are compared to those for the materials in solution. In the case of the oligomer, the shape of the visible absorption band observed for the film is quite different from the band shape for the polymer in frozen solution and is characteristic of H-type aggregates. Theoretical models are presented describing the dependence of the band shape of absorption and emission spectra on intermolecular excited state interactions, electron-vibration coupling and disorder represented by distributions of the molecular excitation and intermolecular interaction energies. Using these models, it is concluded that intermolecular interactions in the film of the oligomer are strong (⩾1400 cm−1), and the disorder low, implying delocalization of the excitation over several molecules. In accordance with these models the fluorescence lifetime for the film (∼2 ns) is considerably longer than for isolated molecules in solution (0.45 ns). The emission spectra of the film, taken early after excitation, are consistent with delocalization of the excitation over several molecules. A time-dependent red shift of the fluorescence band is observed and interpreted in terms of migration of localized excitations between disorder induced trap sites, which exist in the low energy tail of the density of excited states. For the polymers, differences between the shape of the absorption bands of solid film and frozen solution are smaller than for the oligomer indicating that interchain interactions that are, on average, weaker than for the oligomer. For the polymer films, a time-dependent red shift of the emission is observed and fluorescence depolarization measurements provide direct evidence for migration of the photo-excitations between trap sites. For one polymer, a time dependent change in the band shape of the fluorescence after pulsed excitation is observed with the band shape of the long-lived emission being compatible with that expected for an excitation delocalized over at least two, nearly parallel aligned, chains. For a second polymer, the emission band shape and its time evolution indicate that the major part of the fluorescence originates from disorder induced luminescent sites. These results indicate that the spectroscopic properties of films of π-conjugated polymer critically depend on parameters such as density of defects and excited state interchain interaction energy.

Circular Polarization of the Fluorescence from Films of Poly(p‐phenylene vinylene) and Polythiophene with Chiral Side Chains

Circularly polarized photoluminescence has been observed from chiral -conjugated polymers (e.g., see Figure). It was found that the degree of circular polarization was greater in the absorption than in the emission, which is interpreted in terms of trap sites acting as luminescent centers. These results may be important for the fabrication of LEDs for producing circularly polarized light.

Time delayed collection field experiments on polymer: fullerene bulk-heterojunction solar cells

The recombination of photogenerated charge carriers in poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene]:1-(3-methoxycarbonyl)-propyl-1-phenyl-[6,6]C61 bulk-heterojunction solar cells is investigated using the time delayed collection field technique. Here the lifetime of photogenerated electrons and holes that have escaped charge recombination can be determined from current measurements using a pulsed collection voltage that is delayed with respect to the excitation pulse. At 80K, the number of long lived charge carriers decays in time according to t−α with α=0.2, practically independent of laser fluence in the range of 1–1000μJ∕cm2. For excitation density <4μJ∕cm2 the number of long lived carriers (nL) depends linearly on the fluence. At higher fluence, nL is limited by a process that occurs in the time span between generation and carrier extraction under a constant bias (−4V). Continuous background illumination reduces the lifetime of long lived carriers, probably by filling the low ener...

Intramolecular excimer formation between 3,6-Di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione chromophoric groups linked by a flexible alkyl spacer

Bichromophoric molecules containing two 3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DPP) moieties linked via aliphatic spacers of different length are synthesized. Optical absorption spectroscopy indicates that the molecules adopt an extended conformation in solution. Fluorescence spectroscopy shows that photons are emitted from the locally excited singlet state in an extended conformation. In sufficiently polar solvents, quenching of fluorescence occurs and fluorescence quantum yield (ΦF) and fluorescence lifetime (τF) measurements indicate formation of an intramolecular excimer as the quenching mechanism. The redox potentials of the molecules and the solvent polarity dependence of the quenching are consistent with significant charge-transfer character of the excimer state. Photoinduced absorption measurements show enhanced intersystem crossing to the triplet state in polar solvents. Results indicate that in donor-acceptor π-conjugated materials involving the DPP moiety, excimer-like interchain polaron pair excited states could play an important role in the photophysics because of their close proximity in energy to the lowest singlet excited state.

Near-field circular polarization probed by chiral polyfluorene

We demonstrate that a high degree of circular polarization can be delivered to the near field (NF) of an aperture at the apex of hollow-pyramid probes for scanning optical microscopy. This result is achieved by analyzing the dichroic properties of an annealed thin polymer film containing a chiral polyfluorene derivative, placed in close proximity to the optical probe. We also prove that the degree of circular polarization in the probe NF does not depend in a significant way on the shape of the aperture, at variance with the far-field behavior. These results demonstrate the feasibility of nano-optics applications exploiting circularly polarized NFs.