Validity of the Einstein relation in disordered organic semiconductors

It is controversial whether energetic disorder in semiconductors is already sufficient to violate the classical Einstein relation, even in the case of thermal equilibrium. We demonstrate that the Einstein relation is violated only under nonequilibrium conditions due to deeply trapped carriers, as in diffusion-driven current measurements on organic single-carrier diodes. Removal of these deeply trapped carriers by recombination unambiguously proves the validity of the Einstein relation in disordered semiconductors in thermal (quasi)equilibrium.     

The effect of orientational disorder on hopping conductivity in disordered organic semiconductors

The model of variable range hopping is generalized to systems with extended sites. The calculation of the hopping conductivity of a disordered medium with chain structure is reduced to the percolation problem for a system of one-dimensional fragments with random distribution of lengths and orientations. The percolation problem is solved using the Monte Carlo method and the method based on the bonding criterion. The percolation threshold, which determines the dependence of the conductivity on the fragment lengths and degree of their orientational ordering (texture), is calculated. It is shown that the conductivity increases exponentially with fragment lengths and decreases with orientational ordering.     

Universal arrhenius temperature activated charge transport in diodes from disordered organic semiconductors

Charge transport models developed for disordered organic semiconductors predict a non-Arrhenius temperature dependence ln(mu) proportional, variant1/T(2) for the mobility mu. We demonstrate that in space-charge limited diodes the hole mobility (micro(h)) of a large variety of organic semiconductors shows a universal Arrhenius temperature dependence micro(h)(T) = micro(0)exp(-Delta/kT) at low fields, due to the presence of extrinsic carriers from the Ohmic contact. The transport in a range of organic semiconductors, with a variation in room temperature mobility of more than 6 orders of magnitude, is characterized by a universal mobility micro(0) of 30-40 cm(2)/V s. As a result, we can predict the full temperature dependence of their charge transport properties with only the mobility at one temperature known.     

Multiphonon hopping in disordered semiconductors

Equations of motion for the nonequilibrium single-frequency density matrix are used to obtain a balance equation describing the kinetics of electron hopping between localized states in disordered semiconductors. Conclusions are drawn about the applicability of the theory of multiphonon processes to systems with a small nonadiabaticity. The decoupling used is a generalization of one used earlier to the multiphonon case. The balance equation found in the Markov approximation for the average number of sites occupied in an electric field contains the field dependence of the multiphonon-transition probability, tending to the well-known weak-field limit.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No.2, pp.35–42, February, 1976.In conclusion the author is grateful to V. L. Bonch-Bruevich and A. G. Mironov for discussing this work.     

Exciton effects in disordered semiconductors

Experimental data on optical and photoelectric properties of molecular (polymeric) disordered semiconductors of inorganic and organic nature are analyzed. It is shown that anomalies in optical properties and in the structure of the internal photoeffect quantum-yield spectrum can be understood by assuming the existence of exciton states in disordered semiconductors.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 86–101, February, 1976.The author is grateful to Prof. S. G. Grenishin for his interest in this work.     

Electron transport in disordered semiconductors

The density matrix of an electron in an amorphous semiconductor, written as a double path integral, is averaged over random disorder. For small disorder the golden rule result is recovered. When the mean disorder energy approaches k_BT, the electronic mobility goes to zero signalling disorder induced localization.     

Thermalization and recombination in amorphous semiconductors

The fate of carriers excited in an amorphous semiconductor by a pulse of light is described. Two processes are discussed: thermalization of carriers within a distribution of localized states, and carrier recombination.     

Theory of hopping conductivity in disordered semiconductors

The bond percolation method is used to investigate the conditions under which a linear (and not exponential) temperature dependence of hopping conductivity can be observed.Translated from Izvestiya Vysshikh Uchbenykh Zavedenii, Fizika, No.2, pp. 52–62, February, 1976.     

Auger recombination in diamond-like narrow-gap semiconductors

In the calculation of the transition rate of Auger recombination in the Kane model the overlap integral between the wave functions of the conduction and heavy hole bands is equal to zero at the threshold. As a result the preexponential function has a different temperature dependence in comparison with the case of simple parabolic bands. The theoretical value of the recombination lifetime is in agreement with experimental data for InSb at 300 K. Estimates of the overlap integral given earlier are analyzed.     

Phonon-assisted auger recombination in degenerate semiconductors

Phonon-assisted Auger recombination is studied in strongly degenerate semiconductors since normal Auger recombination is negligible in this case. The results are discussed in connection with an electron-hole-plasma. There are some indications that phonon-assisted Auger recombination is an important process in this case.     

Theory of spin-dependent recombination in semiconductors

The pair model of spin-dependent recombination of carriers in semiconductors, as discussed by Kaplan, Solomon and Mott, is solved exactly in several limiting cases. The model accounts for the large influence of a saturating microwave field in ESR experiments on recombination in crystalline and amorphous silicon. Further, the effect of a static magnetic field on recombination is explored.     

Excitons in organic semiconductors

In this article, excitonic effects in organic semiconductors investigated within the framework of many-body perturbation theory are reviewed. As an example for this technologically relevant class of materials the oligoacene series is studied. The electron–hole interaction is included by solving the Bethe–Salpeter equation for the electron–hole Green's function. This approach allows for the evaluation of the exciton binding energies, which are of major interest concerning the application in organic opto-electronic devices. We start the discussion with a comparison of the Kohn–Sham band structure with recent angular resolved photo-emission data. Starting from this one-electron band structure we focus on the impact of the electron–hole interactions on the optical properties by solving the Bethe–Salpeter equation. We demonstrate the dependence of the exciton binding energy on the molecular size and emphasize the effect of the intermolecular interaction on the exciton binding energies by means of pressure investigations. To cite this article: P. Puschnig, C. Ambrosch-Draxl, C. R. Physique 10 (2009).     

Role of intermolecular coupling in the photophysics of disordered organic semiconductors: aggregate emission in regioregular polythiophene

We address the role of excitonic coupling on the nature of photoexcitations in the conjugated polymer regioregular poly(3-hexylthiophene). By means of temperature-dependent absorption and photoluminescence spectroscopy, we show that optical emission is overwhelmingly dominated by weakly coupled H aggregates. The relative absorbance of the 0-0 and 0-1 vibronic peaks provides a powerfully simple means to extract the magnitude of the intermolecular coupling energy, of approximately 5 and 30 meV for films spun from isodurene and chloroform solutions, respectively.     

Model of magnetic anisotropy in disordered semimagnetic semiconductors

We propose a model explaining the origin of cubic magnetic anisotropy in disordered semiconductor. We show that the magnetic anisotropy changes with the position of the Fermi energy in the valence band and the level of disorder in the crystal. The method is applied to Pb_1−x−ySn_yMn_xTe and Sn_1−xMn_xTe ferromagnetic semiconductor crystals.     

Deformation electron-phonon coupling in disordered semiconductors and nanostructures

We study the electron-phonon relaxation (dephasing) rate in disordered semiconductors and low-dimensional structures. The relaxation is determined by the interference of electron scattering via the deformation potential and elastic electron scattering from impurities and defects. We have found that in contrast with the destructive interference in metals, which results in the Pippard ineffectiveness condition for the electron-phonon interaction, the interference in semiconducting structures substantially enhances the effective electron-phonon coupling. The obtained results provide an explanation to energy relaxation in silicon structures.     

Auger recombination with traps in quantum-well semiconductors

Auger recombination involving traps is calculated for quantum-well semiconductors. Apart from some modifications the results are analogous to those for bulk semiconductors. In particular, the lifetime of the recombination process is proportional to the inverse carrier density and independent of temperature.     

Phonon-assisted auger recombination in quantum well semiconductors

Phonon-assisted Auger recombination (AR) is shown to be an important loss mechanism in a quantum well semiconductor in addition to the direct AR. Theoretical investigations demonstrate that it is of the same order of magnitude and has the same temperature dependence as in bulk material, just as direct AR, provided that the material parameters and the carrier concentrations are the same as in the bulk.