The effects of side-chain-induced disorder on the emission spectra and quantum yields of oligothiophene nanoaggregates: a combined experimental and MD-TDDFT study

Oligomeric thiophenes are commonly used components in organic electronics and solar cells. These molecules stack and/or aggregate readily under the processing conditions used to form thin films for these applications, significantly altering their optical and charge-transport properties. To determine how these effects depend on the substitution pattern of the thiophene main chains, nanoaggregates of three sexithiophene oligomers having different alkyl substitution patterns were formed using solvent-poisoning techniques and studied using steady-state and time-resolved emission spectroscopy. The results indicate the substantial role played by the side-chain substituents in determining the emissive properties of these species. Both the measured spectral changes and their dependence on substitution are well-modeled by combined quantum chemistry and molecular dynamics simulations. The simulations connect the side-chain-induced disorder, which determines the favorable chain-packing configurations within the aggregates, with their measured electronic spectra.

Electric-field-induced fluorescence quenching in polyfluorene, ladder-type polymers, and MEH-PPV: evidence for field effects on internal conversion rates in the low concentration limit

Electric field-induced fluorescence quenching has been measured for a series of conjugated polymers with applications in organic light-emitting diodes. Electrofluorescence measurements on isolated chains in a glassy matrix at 77 K show that the quenching efficiency for poly[2-methoxy-5-(2-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV) is an order of magnitude larger than that for either a ladder-type polymer (MeLPPP) or polyfluorene (PFH). This effect is explained in terms of the relatively high probability of field-enhanced internal conversion deactivation in MEH-PPV relative to either MeLPPP or PFH. These data, obtained under dilute sample conditions such that chain-chain interactions are minimal, are contrasted with the much higher quenching efficiencies observed in the corresponding polymer films, and several explanations for the differences are considered. In addition, the values of the change in dipole moment and change in polarizability on excitation (|Δμ| and tr(Δα), respectively) are reported, and trends in these values as a function of molecular structure and chain length are discussed.

Universal arrhenius temperature activated charge transport in diodes from disordered organic semiconductors

Charge transport models developed for disordered organic semiconductors predict a non-Arrhenius temperature dependence ln(mu) proportional, variant1/T(2) for the mobility mu. We demonstrate that in space-charge limited diodes the hole mobility (micro(h)) of a large variety of organic semiconductors shows a universal Arrhenius temperature dependence micro(h)(T) = micro(0)exp(-Delta/kT) at low fields, due to the presence of extrinsic carriers from the Ohmic contact. The transport in a range of organic semiconductors, with a variation in room temperature mobility of more than 6 orders of magnitude, is characterized by a universal mobility micro(0) of 30-40 cm(2)/V s. As a result, we can predict the full temperature dependence of their charge transport properties with only the mobility at one temperature known.

Electrofluorescence of MEH-PPV and Its Oligomers: Evidence for Field-Induced Fluorescence Quenching of Single Chains

Electrofluorescence (Stark) spectroscopy has been used to measure the trace of the change in polarizability (trDeltaalpha) and the absolute value of the change in dipole moment (|Deltamu|) of the electroluminescent polymer poly[2-methoxy,5-(2'-ethyl-hexoxy)-1,4-phenylene vinylene] (MEH-PPV) and several model oligomers in solvent glass matrixes. From electrofluorescence, the measured values of trDeltaalpha increase from 500 +/- 60 A(3) in OPPV-5 to 2000 +/- 200 A(3) in MEH-PPV. The good agreement found between these values and those measured by electroabsorption suggests the electronic properties do not differ strongly between absorption and emission, in contrast to earlier predictions. Evidence of electric-field-induced fluorescence quenching of MEH-PPV in dilute solvent glasses was found. When normalized to the square of the applied electric field, the magnitude of quench is comparable to that reported in the literature for thin films of MEH-PPV. In addition, fluorescence quenching was also observed in the oligomers with a magnitude that increases with increasing chain length. By using the values of trDeltaalpha measured by electrofluorescence, a model is developed to qualitatively explain the chain length dependence to the fluorescence quench observed in the oligomers as a function of exciton delocalization along the oligomer backbone. Various explanations for the origin of this quenching behavior and its chain length dependence are considered.

Space-charge limited photocurrent

In 1971 Goodman and Rose predicted the occurrence of a fundamental electrostatic limit for the photocurrent in semiconductors at high light intensities. Blends of conjugated polymers and fullerenes are an ideal model system to observe this space-charge limit experimentally, since they combine an unbalanced charge transport, long lifetimes, high charge carrier generation efficiencies, and low mobility of the slowest charge carrier. The experimental photocurrents reveal all the characteristics of a space-charge limited photocurrent: a one-half power dependence on voltage, a three-quarter power dependence on light intensity, and a one-half power scaling of the voltage at which the photocurrent switches into full saturation with light intensity.