Universality of AC conductivity: random site-energy model with Fermi statistics

The universality of the frequency-dependent (AC) conduction of many disordered solids in the extreme-disorder limit has been demonstrated exptl. Theor., this universality has been established with different techniques and for various models. A popular model that has been extensively investigated and for which AC universality was established is the sym. random-barrier model without Fermi statistics. However, for the more realistic model of random site-energies and Fermi statistics AC universality has never been rigorously established. In the present work we perform a numerical study of the latter model for a regular lattice in two dimensions. In addn., we allow for variable-range hopping. Our main conclusion is that AC universality appears to hold for this realistic model. The obtained master curve for the cond. and the one obtained for the random-barrier model in two dimensions appear to be the same. [on SciFinder (R)

Universal size-dependent conductance fluctuations in disordered organic semiconductors

Numerically exact results of hopping charge transport in disordered organic semiconductors show for uncorrelated and dipole-correlated Gaussian energy disorder a universal, power-law, and non-power-law dependence, respectively, of the relative conductance fluctuations on the size of the considered region. Data collapse occurs upon scaling with a characteristic length having a power-law temperature dependence. Below this length, which can be as high as 100 nm for correlated disorder in a realistic case, fluctuations dominate and a continuum description of charge transport breaks down

Energy-band structure of SiC polytypes by interface matching of electronic wave functions

We interpret SiC polytypes as natural superlattices, consisting of mutually twisted cubic layers. A method is presented to calculate the electron band structure of any polytype, based on an empirical pseudopotential description of cubic SiC. Bloch and evanescent waves belonging to cubic layers are matched at interfaces in order to make up the wave functions of the respective polytypes. Band gaps of hexagonal and rhombohedral modifications are in excellent agreement with experimental data such that the nearly linear relationship between the indirect gap and the hexagonal nature is reproduced. A simple explanation of this relationship is given in terms of a Kronig-Penney-like mode

Energy-band structure of SiC polytypes by interface matching of electronic wave functions

We interpret SiC polytypes as natural superlattices, consisting of mutually twisted cubic layers. A method is presented to calculate the electron band structure of any polytype, based on an empirical pseudopotential description of cubic SiC. Bloch and evanescent waves belonging to cubic layers are matched at interfaces in order to make up the wave functions of the respective polytypes. Band gaps of hexagonal and rhombohedral modifications are in excellent agreement with experimental data such that the nearly linear relationship between the indirect gap and the hexagonal nature is reproduced. A simple explanation of this relationship is given in terms of a Kronig-Penney-like mode

Exchange-correlation energy of a hole gas including valence band coupling

We have calculated an accurate exchange-correlation energy of a hole gas, including the complexities related to the valence band coupling as occurring in semiconductors like GaAs, but excluding the band warping. A parametrization for the dependence on the density and the ratio between light- and heavy-hole masses is given. We apply our results to a hole gas in an AlxGa1-xAs/GaAs/AlxGa1-xAs quantum well and calculate the two-dimensional band structure and the band-gap renormalization. The inclusion of the valence band coupling in the calculation of the exchange-correlation potentials for holes and electrons leads to a much better agreement between theoretical and experimental data than when it is omitted

Effect of Coulomb scattering from trapped charges on the mobility in an organic field-effect transistor

We investigate the effect of Coulomb scattering from trapped charges on the mobility in the two-dimensional channel of an organic field-effect transistor. The number of trapped charges can be tuned by applying a prolonged gate bias. Surprisingly, after increasing the number of trapped charges to a level where strong Coulomb scattering is expected, the mobility has decreased only slightly. Simulations show that this can be explained by assuming that the trapped charges are located in the gate dielectric at a significant distance from the channel instead of in or very close to the channel. The effect of Coulomb scattering is then strongly reduced

Influence of the semiconductor oxidation potential on the operational stability of organic field-effect transistors

During prolonged application of a gate bias, organic field-effect transistors show a gradual shift of the threshold voltage towards the applied gate bias voltage. The shift follows a stretched-exponential time dependence governed by a relaxation time. Here, we show that a thermodynamic analysis reproduces the observed exponential dependence of the relaxation time on the oxidation potential of the semiconductor. The good fit with the experimental data validates the underlying assumptions. It demonstrates that this operational instability is a straightforward thermodynamically driven process that can only be eliminated by eliminating water from the transistor

Effects of energy correlations and superexchange on charge transport and exciton formation in amorphous molecular semiconductors:an ab initio study

In this study, we investigate on the basis of ab initio calculations how the morphology, molecular on-site energies, reorganization energies, and charge transfer integral distribution affect the hopping charge transport and the exciton formation process in disordered organic semiconductors. We focus on three materials applied frequently in organic light-emitting diodes: α-NPD, TCTA, and Spiro-DPVBi. Spatially correlated disorder and, more importantly, superexchange contributions to the transfer integrals, are found to give rise to a significant increase of the electric field dependence of the electron and hole mobility. Furthermore, a material-specific correlation is found between the HOMO and LUMO energy on each specific molecular site. For α-NPD and TCTA, we find a positive correlation between the HOMO and LUMO energies, dominated by a Coulombic contribution to the energies. In contrast, Spiro-DPVBi shows a negative correlation, dominated by a conformational contribution. The size and sign of this correlation have a strong influence on the exciton formation rate

Ab initio modeling of steady-state and time-dependent charge transport in hole-only α-NPD devices

We present an ab initio modeling study of steady-state and time-dependent charge transport in hole-only devices of the amorphous molecular semiconductor α–NPD [N,N ′ −Di(1–naphthyl)−N,N ′ −diphenyl−(1,1 ′ −biphenyl)−4,4 ′ −diamine] α–NPD [N,N′-Di(1–naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine]. The study is based on the microscopic information obtained from atomistic simulations of the morphology and density functional theory calculations of the molecular hole energies, reorganization energies, and transfer integrals. Using stochastic approaches, the microscopic information obtained in simulation boxes at a length scale of ∼10 nm is expanded and employed in one-dimensional (1D) and three-dimensional (3D) master-equation modeling of the charge transport at the device scale of ∼100 nm. Without any fit parameter, predicted current density-voltage and impedance spectroscopy data obtained with the 3D modeling are in very good agreement with measured data on devices with different α-NPD layer thicknesses in a wide range of temperatures, bias voltages, and frequencies. Similarly good results are obtained with the computationally much more efficient 1D modeling after optimizing a hopping prefacto