Ultrafast field-induced dissociation of excitons in conjugated polymers

Photoluminescence intensity fluctuations and electric-field-induced photoluminescence quenching in individual nanoclusters of poly(phenylenevinylene)

Substantial fluctuations of the fluorescence intensity have been detected for single clusters of poly(phenylenevinylene) containing more than 75 polymer chains or 30,000 monomer units. To the best of our knowledge, this is the first time such fluctuations (which resemble the "blinking" effect in single-molecule fluorescence) have been reported for such a large molecular ensemble containing several macromolecules. Together with the distinct jumps, smooth fluctuations of the fluorescence intensity, with characteristic times from milliseconds to seconds, were observed. This fact distinguishes the fluorescence behaviour of the polymer clusters from that of other multichromophoric systems such as the single chains of conjugated polymers reported in the literature. The consecutive or simultaneous switching of one or several emitting sites from the "on" to "off" state does not explain the character of the fluctuations observed. We suggest that the quenching of the light-emitting exciton by a long-lived species, such as, for example, polarons, plays an important role in these unusual fluctuations. Electric field induced fluorescence quenching differs significantly for different clusters. It is proposed that this fluorescence was mainly quenched by polarons injected from the electrodes in the presence of an electric field. The specific behaviour of each cluster is explained by suggesting a different position of the clusters with respect to the electrodes.

Picosecond transient photoconductivity in functionalized pentacene molecular crystals probed by terahertz pulse spectroscopy

We have measured transient photoconductivity in functionalized pentacene molecular crystals using ultrafast optical pump-terahertz probe techniques. The single crystal samples were excited using 800 nm, 100 fs pulses, and the change in transmission of time-delayed, subpicosecond terahertz pulses was used to probe the photoconducting state over a temperature range from 10 to 300 K. A subpicosecond rise in photoconductivity is observed, suggesting that mobile carriers are a primary photoexcitation. At times longer than 4 ps, a power-law decay is observed consistent with dispersive transport.

Time-resolved charge carrier generation from higher lying excited states in conjugated polymers

Sub-ps three-pulse transient differential transmission spectroscopy using two excitation pulses is used to directly investigate the generation of charge carriers in ladder-type poly(para)phenyl in bulk film. The role of higher excited singlet states of both even and odd symmetry is examined and the dynamics of the major processes involved is described quantitatively. The charge generation efficiency is found to depend strongly on the delay between the two excitation pulses. This is explained by the interplay between internal conversion, excitation energy migration, and on-site vibronic relaxation.

Dynamics of the electric field-assisted charge carrier photogeneration in ladder-type poly(para-phenylene) at a low excitation intensity

Electric field-assisted charge carrier photogeneration in a ladder-type methyl-substituted poly(paraphenylene) was investigated by ultrafast absorption spectroscopy at low excitation intensity. The dissociation of excitons into electron-hole pairs occurs from the vibrationally relaxed excited state throughout its lifetime and is caused by the applied electric field, rather than by existence of special "dissociation sites." These findings are of importance for material choice in device applications.

Precursor states for charge carrier generation in conjugated polymers probed by ultrafast spectroscopy

Photocurrent experiments using two femtosecond laser pulses are performed on a photodiode using a ladder-type conjugated polymer as the active layer. With a photon energy of 3.1 eV the first pulse excites singlet excitons. A time-delayed second pulse with a photon energy of 2.49 eV leads to a decrease of the photocurrent by exciton depletion due to stimulated emission. S1 excitons being dissociated during their entire lifetime are identified as the only relevant channel for charge carrier generation. Intrachain polaron pairs are also formed on an ultrafast time scale with a yield of approximately 10%. They can be efficiently dissociated by reexcitation with photons of an energy of 1.9 eV.

Singlet exciton binding energy in poly(phenylene vinylene)

The exciton binding energy (E(b)) and the band gap energy (E(g)) of poly(phenylene vinylene) are determined by high-resolution measurements of the photoconductivity excitation profile as a function of light polarization, applied electric field, and temperature. At high applied electric fields, a peak in the photoconductivity is observed when the sample is pumped at a photon energy just below the onset of the band-to-band pi-pi* absorption. This peak is interpreted as resulting from field ionization of a weakly bound exciton with E(b) approximately 60 meV. The binding energy is obtained from the energy of the exciton peak relative to the band edge and independently from analysis of the dependence of the exciton dissociation on field and temperature.

Ultrafast spectroscopy of even-parity states in pi-conjugated polymers

Relaxation dynamics of even parity ( A(g)) states in poly( p-phenylene vinylene) derivatives are studied using a novel fsec transient spectroscopy, in which two different excitation pulses successively generate odd parity ( 1 (1)B(u)) excitons at 2.2 eV and then reexcite them to higher A(g) states. For reexcitation energies Planck's over 2piomega<1.1 eV ultrafast internal conversion back to 1 (1)B(u) takes place in accordance with Vavilov-Kasha's rule. However, for Planck's over 2piomega>1.1 eV the decay occurs in a nonemissive state identified as a polaron pair, showing that the A(g) states above 3.3 eV mediate charge transfer.