David H. Dunlap

Nondispersive electron transport in Alq3

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.

Charge transport in molecular solids: dynamic and static disorder

Abstract Recent controversial issues in charge transport in molecular solids are discussed, with focus on the effects of static and dynamic disorder. The situation in ordered solids such as aromatic hydrocarbon crystals is reviewed with attention to the nature of charge carriers, and the theoretical development that led to the now accepted idea that the carriers are polaronic as a result of strong interaction with librations is described. Next, recent experiments on disordered solids (specifically molecularly doped polymers), are reviewed, and our theoretical work directed at their understanding is presented and compared with alternatives existing in the literature. Comparative comments are made on the three available approaches: the ordered polaronic, the disordered polaronic and the disordered non-polaronic. It is argued that none is in a satisfactory state at the moment. Key problems that require theoretical attention are identified.

Energetic Disorder, Spatial Correlations, and the High-Field Mobility of Injected Charge Carriers in Organic Solids

(a) Department of Physics, University of Missouri-Rolla, Rolla Missouri 65409, USA(b) Department of Physics and Astronomy and Center for Advanced Studies,University of New Mexico, Albuquerque, NM, 87131, USA(Received September 7, 1999)In a large class of disordered organic solids, the observed field and temperature dependence of themobility of photoinjected charge carriers arises from specific statistical features of the disorderedpotential energy landscape through which they move. In materials with polar constituents, energyfluctuations exhibit strong spatial correlations, with deep energetic valleys developing only overlarge length scales. We present a scaling analysis that shows how the hierarchical field-inducedflattening of fluctuations of different magnitudes gives rise to field dependent (e.g., Poole-Frenkeltype) mobilities characteristic of the spatial correlations from which they arise.

Disordered polaron transport: a theoretical description of the motion of photoinjected charges in molecularly doped polymers

Abstract We present a model for the description of the temperature and concentration dependence of the mobility of photoinjected charge carriers in molecularly doped polymers on the basis of polaron transport in disordered media. We develop an existing variable range hopping technique to incorporate excluded volume effects which can be of importance in molecularly doped polymers, use the technique in conjunction with a Gaussian distribution of site energies in the polymeric solid, apply the formalism to calculate the mobility and address recent observations on hole motion in TPD and TTA in polycarbonate matrices. We show that the competition between spatial and energetic disorder can provide a reasonable mechanism for the observed concentration dependence of the activation energy for charge transport. Our theory thus provides an alternative to a recently proposed explanation based on an adiabatic-diabatic polaron transition.

Charge-carrier transport in disordered organic materials: dipoles, quadrupoles, traps, and all that

The reason for the Poole-Frenkel (PF) mobility field dependence in dependence in disordered organic materials is believed to be a long range spatial correlation in the distribution of enough levels of transport sites. This correlation is produced by molecules with significant permanent dipole moments. However, experimental data offer a strong evidence that in transport layers containing no molecules with high dipole moment essentially the same PF dependence of transport parameters on the mean intersite separation. By means of computer simulation and analytic calculation we consider the influence of deep traps on the charge carrier mobility in organic materials.

Dispersive aspects of the high-field hopping mobility of molecularly doped solids with dipolar disorder

The time-of-flight mobility of photoinjected charges in molecularly doped polymers obeys a Poole-Frenkel law, μ ∞ exp(γ√E), which is commonly viewed as arising from hopping transport among sites with a large degree of energetic disorder. Recent theoretical investigations have focused on long-range correlations that characterize site energies when the dominant mechanism for energetic fluctuations is the interaction of charge carriers with randomly-oriented permanent dipoles of the dopant and host polymer. An exact calculation of the steady-state drift velocity v d for a one-dimensional system with correlated dipolar disorder predicts a Poole-Frenkel law similar to that observed. In order to investigate another feature commonly observed in the high-field measurements, namely, the anomalous dispersion of the current-time transients, we have performed an exact calculation of the field-dependent diffusion constant D for the same dipolar disorder model. In the bulk limit we obtain an expression D = (KT/e)∂v d /∂E that generalizes the normal Einstein relation and predicts a strongly field-dependent diffusion constant.

Influences of Exciton Diffusion and Exciton-Exciton Annihilation on Photon Emission Statistics of Carbon Nanotubes

Pump-dependent photoluminescence imaging and second-order photon correlation studies have been performed on individual single-walled carbon nanotubes (SWCNTs) at room temperature. These studies enable the extraction of both the exciton diffusion constant and the Auger recombination coefficient. A linear correlation between these parameters is attributed to the effect of environmental disorder in setting the exciton mean free path and capture-limited Auger recombination at this length scale. A suppression of photon antibunching is attributed to the creation of multiple spatially nonoverlapping excitons in SWCNTs, whose diffusion length is shorter than the laser spot size. We conclude that complete antibunching at room temperature requires an enhancement of the exciton-exciton annihilation rate that may become realizable in SWCNTs allowing for strong exciton localization.

High-Sensitivity Electric Force Microscopy of Organic Electronic Materials and Devices

Conducting and semiconducting organic materials have long been known [1], [2], but recent advances in chemical synthesis [3], [4] have enabled organic materials to begin delivering on the promise of mass-produced economical electronic devices. Organic electronic materials are better suited for constructing high-efficiency light-emitting diodes [5]–[8], solar cells [9], [10], and cheap solution-processable thin-film transistors [6], [11]–[18] than are crystalline inorganic semiconductors such as silicon and gallium arsenide. The electronic/optical properties and solubility of organic materials can be tuned independently by chemical synthesis [4]. Since they can be processed and patterned at ambient temperature, organic electronic materials are compatible with flexible large-area substrates [19].

Influence of disorder on transfer characteristics of organic electrochemical transistors

Organic electrochemical transistors (OECTs) are receiving a great deal of attention as transducers of biological signals due to their high transconductance. A ubiquitous property of these devices is the non-monotonic dependence of transconductance on gate voltage. However, this behavior is not described by existing models. Using OECTs made of materials with different chemical and electrical properties, we show that this behavior arises from the influence of disorder on the electronic transport properties of the organic semiconductor and occurs even in the absence of contact resistance. These results imply that the non-monotonic transconductance is an intrinsic property of OECTs and cannot be eliminated by device design or contact engineering. Finally, we present a model based on the physics of electronic conduction in disordered materials. This model fits experimental transconductance curves and describes strategies for rational material design to improve OECT performance in sensing applications.

Analysis of Trap Distribution Using Time-of-Flight Spectroscopy

A new analytical method for determining trap distribution from a transient photocurrent in time-of-flight (TOF) measurements has been proposed in the context of convection diffusion equation with multiple-trapping and detrapping processes. The method does not need, in principle, data on temperature dependence and any initial assumption about the form of trap distribution. A trap distribution is directly extracted from time profiles of transient photocurrents on assuming the Einstein relation between mobility and diffusion constant. To demonstrate the validity of the method, we first applied photocurrents that were prepared in advance by random walk simulation for some typical trap distributions assumed. Then, we attempt to determine a trap distribution for a particular mesophase of a liquid crystal of phenylnaphthalene derivative, for which the temperature dependence of carrier transport properties is hardly available. Indeed, we have obtained an extrinsic shallow trap distribution at about 200 meV in depth together with a tail-shaped Gaussian-type density-of-states distribution. Thus, we conclude that the method may be a powerful tool to analyze a trap distribution for a system that exhibits temperature-sensitive conformational changes and/or whose carrier transport properties are not available as a function of temperature.