E.M. Conwell

High-field hopping mobility in molecular systems with spatially correlated energetic disorder

Abstract Prior Monte Carlo simulations of hopping mobility μ versus electric field E using the Gaussian disorder model have shown, in disagreement with experiment, only a short range of fields where log μ α E 1 2 (Poole-Frenkel type behavior). We demonstrate that the introduction of correlation of the energies of sites close together can lead to field dependences similar to the Poole-Frenkel one over the wide range of fields where it is usually seen experimentally. Such correlation is physically reasonable, for example, for systems in which the fluctuations of site energy are due to the interaction of charge carriers with permanent dipoles or to molecular density fluctuations.

Electronic structure of some polyanilines

Abstract We present some photoelectron spectroscopy data relevant to the chemical and electronic structure of certain members of the polyaniline family of conducting polymers.

Molecular conformation of polyaniline oligomers: Optical absorption and photoemission of three‐phenyl molecules

To examine the conformation of polyanilines with different degrees of oxidation two three‐ring oligomers, N‐N’‐diphenyl‐1,4‐phenylenediamine (the ‘‘amine trimer’’) and N‐N’‐diphenyl‐1,4‐benzoquinodiimine (the ‘‘imine trimer’’) have been studied. The amine and imine trimers are model molecules for leucoemeraldine and pernigraniline, respectively. MNDO calculations for the cis and trans forms of the imine trimer predict the ground states to have almost identical energies. Both forms have the quinoid ring in the plane of the backbone and the torsion angle between the quinoid and benzenoid rings close to 90°. The density of valence states (DOVS) and optical absorption were calculated for different conformations of the amine and imine molecules using the CNDO/S3 model. Comparison of the calculated DOVS with photoemission data for the amine molecules indicates a conformation about the same as that predicted by MNDO for the ground state. The optically excited state for the amine, according to the comparison betw...

Field-dependent thermal injection into a disordered molecular insulator

Abstract A Monte Carlo simulation is performed of emission-limited injection into an energetically disordered molecular insulator. For thermal injection, disorder leads to the increased probability for injected carriers to return to the electrode. As a result, enhanced non-linearity of the injected current is found with respect to applied electric field compared to that caused by the image force only. Particularly, the field-dependent part of the effective activation energy is significantly increased by disorder. These effects are in sharp contrast to those found for hot injected carriers.

Polarons and their stability in poly(phenylenevinylene)

Abstract To study properties of polarons in poly(phenylenevinylene) (PPV) we set up a tight-binding Hamiltonian along the lines of the SSH Hamiltonian for polyacetylene. The parameters were determined by fitting the bandgap, valence bandwidth and vinylene geometry to better than 1%. We used the resulting Hamiltonian to calculate the geometry of the polaron and the distance of the polaron level from the closest band edge as a function of chain length. To describe interchain coupling we took into account the actual separation of each atom from the closest atom on the neighboring chain which, unlike polyacetylene, is different for each atom in the PPV monomer. It was found that interchain coupling does not destabilize the polaron in PPV.

High-field hopping mobility of polarons in disordered molecular solids. A Monte Carlo study

Abstract For a system subject to disorder of both diagonal (site energy variation) and off-diagonal (variation of intersite electron wave-function overlap) types we study the high-field mobility of charge carriers including polaronic effects. The elementary jump rate between sites is given by the classical expression due to Marcus and Holstein. We find there is a range of parameters where the dependence on field E of mobility μ is qualitatively similar to that found for the disorder model with Miller—Abrahams-type jump rate, including a region where In μ may be considered proportional to E 1 2 .

Excimers in poly(phenylene vinylene) and its derivatives

Abstract Photoinduced absorption measurements on poly(phenylene vinylene), PPV, and its derivatives have revealed that some percentage of photons past the absorption edge create a long-lived excitation rather than the expected excitons. We review the evidence that these excitations are polaron pairs, and the properties of excimers in this situation, to show that the polaron pairs are essentially excimers. We show that the reason excimers in some derivatives of PPV are emissive while in others they are not is the different separation of the chains constituting the excimer. Reasons why the percentage of photons that give rise to excimers can vary greatly from sample to sample arc presented. The origin of the different peaks in the photoinduced absorption is discussed.

Definition of exciton binding energy for conducting polymers

Abstract The exciton binding energy ɛb for a conducting polymer can only be defined unambiguously in the long chain limit. In that limit it can be taken as the difference between the bandgap or the ‘single particle bandgap’ — the former if the exciton dissociates into a free electron and hole, the latter if it dissociates into a pair of oppositely charged polarons — and the optical absorption edge.

Transport in α-sexithiophene films

Abstract The field-effect mobility of hole polarons in α-sexithiophene, measured in thin film transistors, was shown to be well fitted by Holsteins small polaron theory. Unfortunately, Holsteins formulation is based on an integral that does not converge. We show that the data are well fitted by a theory of polaron transport that was successful in accounting for mobility in molecular crystals of naphthalene.

Luminescent behavior of soluble poly(para-phenylene vinylene) copolymers

Abstract Soluble diblocks with well-defined lengths of poly(phenylene vinylene), PPV, and an inert polymer, polynorbornene, PNBE, have been made by a new type of synthesis. For dilute solutions and relatively short lengths of PPV the quantum efficiency of luminescence is over 70% and the radiative lifetimes are 3 ns. We find that with increasing concentration of PPV in solution the luminescence efficiency decreases, which we propose is due to formation of polaron pairs.