W. Kaiser

Charge carrier mobility of disordered organic semiconductors with correlated energetic and spatial disorder

Low charge carrier mobility is one key factor limiting the performance and applicability of devices based on organic semiconductors. Theoretical studies on mobility using the kinetic Monte Carlo or master equation are mainly based on a Gaussian energetic disorder and regular cubic lattices. The dependence of mobility on the electric field, temperature and charge carrier density is well studied for the Gaussian disorder model. In this work, we investigate the influence of spatially correlated site energies and spatial disorder in the lattice sites on the mobility using kinetic Monte Carlo simulations. Our analysis is based on both a regular cubic and a non-cubic Voronoi lattice. The latter is used to include spatial disorder in order to study its influence on the mobility for amorphous organic materials. Our results show that charge carrier mobility is strongly influenced by correlations in the site energies. Strong correlations even invert the field dependence of the mobility as observed experimentally in semi-crystalline polymers such as P3HT. Evaluation of local currents between localized states reveals the formation of current filaments with increasing correlation. Furthermore, the influence of the electric field and the energy landscape on the transport energy is studied by evaluation of active sites. A strong correlation between the transport energy, filaments in the local currents and the charge carrier mobility is observed. Our studies on the spatial disorder model do not indicate an inversion of the field dependence as observed by other researchers. The negative field-dependence in semi-crystalline materials may be explained by a higher correlation in the site energies as shown in a strongly correlated energetic landscape.

Generalized Kinetic Monte Carlo Framework for Organic Electronics

In this paper, we present our generalized kinetic Monte Carlo (kMC) framework for the simulation of organic semiconductors and electronic devices such as solar cells (OSCs) and light-emitting diodes (OLEDs). Our model generalizes the geometrical representation of the multifaceted properties of the organic material by the use of a non-cubic, generalized Voronoi tessellation and a model that connects sites to polymer chains. Herewith, we obtain a realistic model for both amorphous and crystalline domains of small molecules and polymers. Furthermore, we generalize the excitonic processes and include triplet exciton dynamics, which allows an enhanced investigation of OSCs and OLEDs. We outline the developed methods of our generalized kMC framework and give two exemplary studies of electrical and optical properties inside an organic semiconductor.

Quantum theory of the dissipative Josephson parametric amplifier

Recent research in superconducting quantum circuits operating close to the quantum limit results in the need of a quantum mechanical treatment of losses. Of special interest is the dynamic behaviour of an open quantum system. As an example, a negative-resistance Josephson parametric amplifier is treated. The DC bias voltage is chosen such that a strong interaction between the Josephson junction and the two resonant circuits, the signal and the idler circuit, is achieved. Power exchange occurs between the two considered resonator modes and also between the resonator modes and the DC power supply. Losses in the resonators are modeled by the quantum Langevin method, which describes the losses by coupling the resonators to a heat bath representing a photon gas in thermal equilibrium. The derived dynamic behaviour does not provide signal energy saturation, like classically expected for parametric amplifiers. Introducing a phenomenological multi-photon coupling approach, saturation of the amplified signal is ensured. The time evolution of the signal and noise energy is calculated and numerically evaluated for a specific example in cases of both, the quantum Langevin method with and without the phenomenological multi-photon coupling approach. Copyright

Generalized Langevin theory for Josephson parametric amplification

Superconducting quantum circuits exhibit an extraordinary potential for future electronic applications. The most important element in superconducting quantum circuits is the Josephson junction, built by nanofabricated superconducting electrodes separated by a thin insulating layer. Superconducting devices allow mixing and parametric amplification up to the terahertz range under low energy consumption and ultra-low noise. High frequency signals at low temperatures exhibit a photon energy exceeding the thermal energy, resulting in the need of a quantum mechanical treatment of the electromagnetic field at millimeter-wave frequencies. In this work, we investigate the dynamic behaviour of a negative-resistance, dissipative DC biased Josephson parametric amplifier. The Langevin theory is used for modeling of the dissipation in the resonant circuits. We investigate Markovian dynamics for modeling the dissipative Josephson parametric amplifier in a correct manner, neglecting memory effects. We derive the equations of motions of the resonator operators and numerically evaluate the time evolution of the signal and noise energies. Applying a phenomenological multi-photon coupling approach, a correction of the Markovian assumption is achieved providing an expected saturation in the dynamical behaviour of the circuit.

Engineering the Switching Behaviour of Nanomagnets for Logic Computation Using 3-Dimensional Modeling and Simulation

In this paper, we show the feasibility of tuning both the location of domain wall nucleation and the required nucleation field by changing the geometry of nanomagnets using shadowing effects. The perpendicular anisotropy is locally reduced by shadowing effects during sputter deposition. A 3-D model based on experimental measurements of fabricated nanomagnets has been developed. Experimental data from atomic force microscopy measurements and anisotropy measurements of fabricated Co/Pt-nanomagnets allow to realistically model the anisotropy and geometry of the nanomagnets. Micromagnetic simulations based on the developed 3-D model have been performed. It is shown that increasing the undercut in the resist decreases the switching field of the nanomagnets. By varying the tip geometry of the nanomagnet, the switching field can be further decreased, and subsequently, the nucleation point can be controlled, making this technique feasible for the use in perpendicular nanomagnetic logic circuitry.

Engineering the switching behavior of nanomagnets for logic computation using 3-dimensional modeling and simulation

A 3-dimensional model of nanomagnetic devices in perpendicular Nanomagnetic Logic (pNML) has been developed based on measurements of fabricated magnets. Micromagnetic simulations for different magnet geometries (i.e. widths and gradients) in order to tailor their switching field for application in pNML have been performed.