F. Jansson

On the efficiency of exciton dissociation at the interface between a conjugated polymer and an electron acceptor

It is a matter of controversy why excitons can efficiently dissociate into free carriers at an intrinsic polymer/fullerene interface, despite the strong Coulomb interaction between the charges provided by the very low dielectric constant in organic materials. The effect has been ascribed to the presence of intrinsic dipoles on the polymer/fullerene interface, though assuming an unrealistically small carrier effective mass necessary for exciton dissociation. We improve the model showing that it allows realistic carrier effective masses. The dissociation probability is calculated as a function of electric field acting on the dissociating electron-hole pairs.

Effect of electric field on diffusion in disordered materials. II. Two- and three-dimensional hopping transport

In the previous paper [Nenashev et al., arXiv:0912.3161] an analytical theory confirmed by numerical simulations has been developed for the field-dependent hopping diffusion coefficient D(F) in one-dimensional systems with Gaussian disorder. The main result of that paper is the linear, non-analytic field dependence of the diffusion coefficient at low electric fields. In the current paper, an analytical theory is developed for the field-dependent diffusion coefficient in three- and two-dimensional Gaussian disordered systems in the hopping transport regime. The theory predicts a smooth parabolic field dependence for the diffusion coefficient at low fields. The result is supported by Monte Carlo computer simulations. In spite of the smooth field dependences for the mobility and for the longitudinal diffusivity, the traditional Einstein form of the relation between these transport coefficients is shown to be violated even at very low electric fields.

Role of diffusion in two-dimensional bimolecular recombination

Experiments on carrier recombination in two-dimensional organic structures are often interpreted in the frame of the Langevin model with taking into account only the drift of the charge carriers in their mutual electric field. While this approach is well justified for three-dimensional systems, it is in general not valid for two-dimensional structures where the contribution of diffusion can play a dominant role. We study the two-dimensional Langevin recombination theoretically and find the critical concentration below which diffusion cannot be neglected. For typical experimental conditions, neglecting the diffusion leads to an underestimation of the recombination rate by several times.

Hopping conduction in strong electric fields: Negative differential conductivity

Department of Physics and Material Sciences Center, Philipps-University, 35032 Marburg, Germany(Dated: September 23, 2008)Effects of strong electric fields on hopping conductivity are studied theoretically. Monte-Carlocomputer simulations show that the analytical theory of Nguyen and Shklovskii [Solid State Com-mun. 38, 99 (1981)] provides an accurate description of hopping transport in the limit of veryhigh electric fields and low concentrations of charge carriers as compared to the concentration oflocalization sites and also at the relative concentration of carriers equal to 0.5. At intermediateconcentrations of carriers between 0.1 and 0.5 computer simulations evidence essential deviationsfrom the results of the existing analytical theories.The theory of Nguyen and Shklovskii also predicts a negative differential hopping conductivityat high electric fields. Our numerical calculations confirm this prediction qualitatively. Howeverthe field dependence of the drift velocity of charge carriers obtained numerically differs essentiallyfrom the one predicted so far. Analytical theory is further developed so that its agreement withnumerical results is essentially improved.

Large positive magnetoresistance effects in the dilute magnetic semiconductor (Zn,Mn)Se in the regime of electron hopping

Magnetoresistance in dilute magnetic semiconductors is studied in the hopping transport regime. Measurements performed on Cl-doped Zn1–xMnxSe with x < 8% are compared with simulation results obtained by a hopping transport model. The energy levels of the Cl donors are affected by the magnetization of Mn atoms in their vicinity via the s-d exchange interaction. Compositional disorder, in particular, the random distribution of magnetic atoms, leads to a magnetic-field induced broadening of the donor energy distribution. As the energy distribution broadens, the electron transport is hindered and a large positive contribution to the magnetoresistance arises. This broadening of the donor energy distribution is largely sufficient to account for the experimentally observed magnetoresistance effects in n-type (Zn,Mn)Se with donor concentrations below the metal–insulator transition.