2.2. Materials 929 2.3. Factors Influencing Charge Mobility 931 2.3.1. Molecular Packing 931 2.3.2. Disorder 932 2.3.3. Temperature 933 2.3.4. Electric Field 934 2.3.5. Impurities 934 2.3.6. Pressure 934 2.3.7. Charge-Carrier Density 934 2.3.8. Size/molecular Weight 935 3. The Charge-Transport Parameters 935 3.1. Electronic Coupling 936 3.1.1. The Energy-Splitting-in-Dimer Method 936 3.1.2. The Orthogonality Issue 937 3.1.3. Impact of the Site Energy 937 3.1.4. Electronic Coupling in Oligoacene Derivatives 938

Charge transport in organic semiconductors.

Charge Transport in Disordered Organic Photoconductors a Monte Carlo Simulation Study

The Haldane model on a honeycomb lattice is a paradigmatic example of a Hamiltonian featuring topologically distinct phases of matter. It describes a mechanism through which a quantum Hall effect can appear as an intrinsic property of a band structure, rather than being caused by an external magnetic field. Although physical implementation has been considered unlikely, the Haldane model has provided the conceptual basis for theoretical and experimental research exploring topological insulators and superconductors. Here we report the experimental realization of the Haldane model and the characterization of its topological band structure, using ultracold fermionic atoms in a periodically modulated optical honeycomb lattice. The Haldane model is based on breaking both time-reversal symmetry and inversion symmetry. To break time-reversal symmetry, we introduce complex next-nearest-neighbour tunnelling terms, which we induce through circular modulation of the lattice position. To break inversion symmetry, we create an energy offset between neighbouring sites. Breaking either of these symmetries opens a gap in the band structure, which we probe using momentum-resolved interband transitions. We explore the resulting Berry curvatures, which characterize the topology of the lowest band, by applying a constant force to the atoms and find orthogonal drifts analogous to a Hall current. The competition between the two broken symmetries gives rise to a transition between topologically distinct regimes. By identifying the vanishing gap at a single Dirac point, we map out this transition line experimentally and quantitatively compare it to calculations using Floquet theory without free parameters. We verify that our approach, which allows us to tune the topological properties dynamically, is suitable even for interacting fermionic systems. Furthermore, we propose a direct extension to realize spin-dependent topological Hamiltonians.

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Experimental realization of the topological Haldane model with ultracold fermions

Field-effect transistors based on solution-processible organic semiconductors have experienced impressive improvements in both performance and reliability in recent years, and printing-based manufacturing processes for integrated transistor circuits are being developed to realize low-cost, large-area electronic products on flexible substrates. This article reviews the materials, charge-transport, and device physics of solution-processed organic field-effect transistors, focusing in particular on the physics of the active semiconductor/dielectric interface. Issues such as the relationship between microstructure and charge transport, the critical role of the gate dielectric, the influence of polaronic relaxation and disorder effects on charge transport, charge-injection mechanisms, and the current understanding of mechanisms for charge trapping are reviewed. Many interesting questions on how the molecular and electronic structures and the presence of defects at organic/organic heterointerfaces influence the device performance and stability remain to be explored.

Device Physics of Solution‐Processed Organic Field‐Effect Transistors

A comprehensive review of the literature on electron transport materials (ETMs) used to enhance the performance of organic light-emitting diodes (OLEDs) is presented. The structure−property−performance relationships of many classes of ETMs, both small-molecule- and polymer-based, that have been widely used to improve OLED performance through control of charge injection, transport, and recombination are highlighted. The molecular architecture, electronic structure (electron affinity and ionization potential), thin film processing, thermal stability, morphology, and electron mobility of diverse organic ETMs are discussed and related to their effectiveness in improving OLED performance (efficiency, brightness, and drive voltage). Some issues relating to the experimental procedures for the estimation of relevant material properties such as electron affinity and electron mobility are discussed. The design of multifunctional electroluminescent polymers whereby light emission and electron- and hole-transport pro...

Electron Transport Materials for Organic Light-Emitting Diodes

The motion of a charged particle on a discrete lattice under the action of an electric field is studied with the help of explicit calculations of probability propagators and mean-square displacements. Exact results are presented for arbitrary time dependence of the electric field on a one-dimensional lattice. Existing results for the limiting cases of zero frequency and zero field are recovered. A new phenomenon involving the dynamic localization of the moving particle is shown to result in the case of a sinusoidally varying field: The particle is generally delocalized except for the cases when the ratio of the field magnitude and the field frequency is a root of the ordinary Bessel function of order 0. For these special cases it is found to be localized. This localization could be used, in principle, for inducing anisotropy in the transport properties of an ordinarily isotropic material.

Dynamic localization of a charged particle moving under the influence of an electric field.

The transport of photoinjected charges in disordered organic films is often interpreted using a formula based on a Gaussian disorder model (GDM) that neglects spatial correlations due to chargedipole interactions, even though such correlations have recently been shown to explain the universal electric field dependence observed in these systems. Based on extensive computer simulations of a 3D disorder model that includes such correlations, we present a new formula for analyzing experiments that accurately describes transport in these materials. [S0031-9007(98)07626-1]

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Essential Role of Correlations in Governing Charge Transport in Disordered Organic Materials

We consider here a 1D tight-binding model with two uncorrelated random site energies ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{a}}$ and ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{b}}$ and a constant nearest-neighbor matrix element V. We show that if one (or both) of the site energies is assigned at random to pairs of lattice sites (that is, two sites in succession), an initially localized particle can become delocalized. Its mean-square displacement at long times is shown to grow in time as ${\mathit{t}}^{3/2}$ provided that -2V${\mathrm{\ensuremath{\epsilon}}}_{\mathit{a}}$-${\mathrm{\ensuremath{\epsilon}}}_{\mathit{b}}$2V. Diffusion occurs if ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{a}}$-${\mathrm{\ensuremath{\epsilon}}}_{\mathit{b}}$=\ifmmode\pm\else\textpm\fi{}2V and localization otherwise. The dual of the random-dimer model is also shown to exhibit an absence of localization and is shown to be relevant to transmission resonances in Fibonacci lattices.

Absence of localization in a random-dimer model

Using the general result that the mobility $\ensuremath{\mu}$ of charge carriers driven in a spatially correlated random potential by an electric field $E$ can be expressed in terms of the Laplace transform of a particular correlation function related to the random potential, we demonstrate that the exponential dependence of $\ensuremath{\mu}$ on $\sqrt{E}$ universally observed in molecularly doped polymers arises naturally from the interaction of charge carriers with randomly distributed permanent dipoles.

Charge-Dipole Model for the Universal Field Dependence of Mobilities in Molecularly Doped Polymers

We have studied room temperature electron transport in amorphous films of tris (8-hydroxyquinolinolato) aluminum (III) (Alq3) with the time-of-flight technique. Nondispersive photocurrent transients indicate the absence of intrinsic traps in well-purified films. Exposure of the films to ambient atmosphere results in highly dispersive transport, indicating that oxygen is a likely candidate for a trapping site. The mobility was found to obey the Poole–Frenkel law. We use the correlated disorder model to determine an effective dipole moment for Alq3, and the corresponding meridional to facial isomeric ratio.

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David H. Dunlap

Papers

Dynamic localization of a charged particle moving under the influence of an electric field.

Essential Role of Correlations in Governing Charge Transport in Disordered Organic Materials

Absence of localization in a random-dimer model

Charge-Dipole Model for the Universal Field Dependence of Mobilities in Molecularly Doped Polymers

Nondispersive electron transport in Alq3