I. P. Zvyagin

Percolation Approach to Hopping Transport in Organic Disordered Solids

Percolation approach is used to study the dc hopping conductivity and thermopower in systems with a Gaussian density of localized states typical for disordered organic materials. It is shown that the theoretical methods developed earlier for the description of hopping transport in disordered inorganic solids, such as amorphous semiconductors, can also be successfully applied to description of hopping transport in organic disordered solids, such as conjugated or molecularly doped polymers. Calculations within the percolation approach give results in excellent agreement with those obtained by using a more transparent, though less rigorous approach based on the concept of the transport energy.

Electronic transport in disordered organic and inorganic semiconductors

It is shown that theoretical methods developed for a description of hopping transport in disordered inorganic solids can be also successfully applied to a description of hopping transport in organic disordered materials, such as conjugated polymers, molecularly doped polymers and organic glasses. While the density of localized states (DOS) in inorganic materials is believed to be exponential, a Gaussian DOS has been suggested for organic disordered materials. The most powerful and rigorous theoretical method for the description of variable-range hopping (VRH) in the exponential DOS is based on percolation theory. We show that this percolation approach is also valid for systems with a Gaussian DOS, thus being universal for inorganic and organic disordered materials.

Conduction in Granular Metals by Virtual Tunneling on the Fractal Percolation Cluster

We discuss the calculation of the dc conductivity of granular metals on the insulating side of the metal-insulator transition. The granular structure is represented by a fractal percolation cluster and the problem of virtual tunneling-assisted conductivity is shown to be related to estimating the number of minimal paths for the problem of chemical distance metric on the fractal. The resulting conductivity temperature dependence has the form ln σ const - (T 0 /T) x , where x = ζ/(D B + ζ), D B is the fractal dimensionality of the backbone cluster and ζ is the superlocalization exponent. This gives the value of x close to 0.4 both in two and three dimensions that agrees fairly well with the experimental value x 1/2 for many granular conductors.

Specific features of the frequency dependence of the conductivity of disordered semiconductors in the region of the crossover between transport mechanisms

The specific features of the frequency dependence of the impedance of disordered semiconductors at low temperatures for the hopping conduction mechanism are analyzed. Emphasis is placed on the observed discrepancies between the experimental data and the available theory for the frequency dependence of the real part of the conductivity σ1(ω) in the region of crossover from a linear dependence to a quadratic dependence (the kink in the dependence of lnσ1(ω) on lnω) and for anomalously large values of cotγ (γ is the dielectric loss angle) measured experimentally. These discrepancies can be described on the basis of an approach that takes into account both the phonon and resonance contributions to the conductivity as well as the transition to the fixed-range hopping conduction at frequencies higher than the crossover frequency.

Some aspects of the transport phenomena near the mobility edge in disordered semiconductors

The critical behaviour of transport coefficients in the region near the mobility edge is discussed. It is shown that the temperature dependence of the thermopower and its variation with the Fermi level position can be used to evaluate the critical indices and the role of the correlation effects.

Anomalous relaxation of light-induced states of a-Si:H

Abstract For a large variety of B, P doped and undoped a-Si:H films, we observed the non-monotonic kinetics of structural changes at elevated temperatures (above 100°C), indicating the coexistence of two processes described by stretched exponentials. While the faster process has characteristics corresponding to the conventional metastable dangling bond creation, the characteristics of the slow process are shown to be anomalous (the activation energy for defect creation larger than that for the normal process and the parameter, β, decreasing with temperature). To describe the anomalous process, we propose a phenomenological model based on a three-level configuration–coordinate diagram with correlated activation energies for the light-induced metastable state creation and annealing. The model accounts for the observed features of the anomalous process. A possible underlying microscopic mechanism of structural changes is discussed.

Hopping in Quasi-One-Dimensional Disordered Solids: Beyond the Nearest-Neighbor Approximation

The theory of hopping transport in quasi-one-dimensional disordered organic solids with Gaussian distribution of localized state energies is generalized to account for distant-neighbor transitions. The former theories predicted a temperature dependence of the resistivity of the form ln ρ C(σ/kT) 2 , where C is a constant (C 1) and a is the disorder parameter. It is shown that the onset of second-nearest neighbor hopping leads to a decrease of C with decreasing temperature to the limiting value C = 3/4. At lower temperatures, where the hopping range extends beyond second-nearest neighbors, C is expected to decrease further. The analytical results are in fair agreement with the results of Monte Carlo simulation for one-dimensional variable-range hopping.

Non-monotone kinetics of persistent photoconductivity in compensated a-Si:H films

A detailed study of the illumination time dependence of the persistent photoconductivity as well as that of relaxation after a preliminary illumination in compensated a-Si:H films is carried out. The interpretation of the results is based on a new model of light-induced metastable defects creation. The model assumes the creation of dangling bond-impurity complexes without invoking the conventional idea of the decrease of boron doping efficiency.

Dangling-bond recombination and photoconductivity of a-Si:H

The temperature quenching of the photoconductivity of P-doped glow-discharge a-Si:H films was observed after the prolonged preliminary illumination. To explain it, a recombination model is discussed which takes into account the broadening of the distributions of energies of the dangling-bond states in the mobility gap and the effect of temperature on the optically induced repopulation of these states.

Frequency Dependence of the Conductivity of Disordered Semiconductors in the Region of the Transition to the Fixed-Range Hopping Regime

The effect of hybridization of electron states on the high-frequency conductivity of disordered semiconductors is studied. It is shown that the dependence of the pre-exponential factor of the resonance integral on the intercenter separation in a pair determines the abruptness of the change in conductivity mechanisms near the transition of the frequency dependence of the real part of the conductivity σ1(ω) from sublinear to quadratic. The abruptness of the change of the conductivity regimes is associated with a rapid decrease in hopping distance with increasing frequency near the transition, which leads to a substantial relative decrease in the contribution from the phononless conductivity component in the variable-range hopping regime with increasing frequency and transition to the fixed-range hopping conductivity regime.