Evguenia Emelianova

Charge carrier transport and recombination at the interface between disordered organic dielectrics

An analytic model of charge carrier hopping at the interface separating two disordered organic materials in organic light-emitting diodes (OLEDs) is formulated. It is shown that the rate of charge carrier injection across such an interface is much lower than the rate of first interfacial jumps, most of which are followed by return carrier jumps back into initially occupied sites. In OLEDs, the rate of injection across the interface determines the leakage current while the rate of radiative recombination is proportional to the first-jump rate. Therefore, the marked difference between the injection rate and the rate of first carrier jumps across the interface implies a much higher efficiency of carrier recombination at the interface than predicted by the conventional Langevin theory of recombination.

Equilibrium carrier mobility in disordered hopping systems

Abstract It is shown that the charge carrier mobility in a positionally and energetically disordered hopping system can be evaluated by averaging either the hopping rates or the hopping times over the thermal equilibrium energy distribution of localized carriers. However, at variance with averaging the hopping rates, averaging the hopping times can be correct only if the energy dependence of the carrier energy relaxation time is also taken into consideration. The temperature and concentration dependences of the equilibrium carrier mobility were calculated by averaging the hopping rates. The consideration was based on the variable-range hopping approach with full account for the interplay between jump distance and energy difference. However, the obtained results prove that, in good quantitative agreement with both Monte Carlo simulations and experimental data, the mobility can be approximated as a product of two functions. The first function depends almost solely upon the temperature and reveals only a weak concentration dependence while the second mainly governs the concentration dependence of the mobility and is almost independent of the temperature.

A tandem mechanism of charge-carrier photogeneration in disordered organic materials

A model of tandem charge-carrier photogeneration in disordered organic materials at low excess photon energies above the absorption edge is formulated. It suggests that relaxed singlet excitons dissociate into strongly bound metastable short off-chain geminate pairs. In parallel, triplet excitons can be generated from both primary singlets and short geminate pairs. Owing to a long intrinsic lifetime of triplets, most of them are quenched by geminate pairs and the latter can gain additional energy that facilitates transformation of short pairs into longer ones. This reduces the Coulomb binding energy such that field-assisted full dissociation into free carriers becomes feasible.

Influence of the Distribution of Tail States in a-Si:H on the Field Dependence of Carrier Drift Mobilities

Exponential distributions of tail states have been able, within the framework of a multiple-trapping transport model, to account rather well for the time-of-flight photoconductivity transients that are measured with ‘standard’ a-Si:H, i.e. material prepared by plasma-enhanced chemical vapor deposition at ∼250°C. A field-dependent carrier mobility in the dispersive transport regime is part of the observations. However, samples prepared in an expanding thermal plasma, although still exhibiting the dispersive transients, fail to show this field dependence. The presence of a Gaussian component in the density of valence-band tail states can account for such behavior for the hole transients. Nanoscale ordered inclusions in the amorphous matrix are thought to be responsible for the Gaussian density of states contribution.