N. J. Vollmers

Paramagnetic signature of microcrystalline silicon carbide

The most important challenge on the way to optimized solar cells is to make the thickness of the individual layers smaller than the diffusion length of the charge carriers, in or- der to keep the collection efficiency close to unity. Here, we propose s-SiC microcrystals grown by a sol-gel based process as a promising acceptor material. The samples are character- ized by optical spectroscopy and electron paramagnetic resonance (EPR). With the help of band structures for selected surface states calculated in the framework of density functional theory (DFT) a possible scenario for the observed acceptor process is discussed.

Solving the Scattering Problem for the P3HT On-Chain Charge Transport

The effect of oxygen impurities and structural imperfections on the coherent on-chain quantum conductance of poly(3-hexylthiophene) is calculated from first principles by solving the scattering problem for molecular structures obtained within density functional theory. It is found that the conductance drops substantially for polymer kinks with curvature radii smaller than 17 A and rotations in excess of about 60∘. Oxidation of thiophene group carbon atoms drastically reduces the conductance, whereas the oxidation of the molecular sulfur barely changes the coherent transport properties. Also isomer defects in the coupling along the chain direction are of minor importance for the intrachain transmission.

Submonolayer Rare Earth Silicide Thin Films on the Si(111) Surface

Rare earth induced silicide phases of submonolayer height and 5 × 2 periodicity on the Si(111) surface are investigated by density functional theory and ab initio thermodynamics. The most stable silicide thin film consists of alternating Si Seiwatz and honeycomb chains aligned along the [1\(\overline{1}\) 0] direction, with rare earth atoms in between. This thermodynamically favored model is characterized by a minor band gap reduction compared to bulk Si and explains nicely the measured scanning tunneling microscopy images.

Surface Charge of Clean LiNbO3 Z-Cut Surfaces

The geometry of the polar LiNbO3 (0001) surface is strongly temperature dependent. In this work the surface charge associated to various surface terminations is estimated from first-principles calculations. All stable terminations are found to lower the polarization charge, showing that the surface charge compensation is a major driving force for surface reconstruction.

Lithium Niobate Dielectric Function and Second-Order Polarizability Tensor From Massively Parallel Ab Initio Calculations

The frequency-dependent dielectric function and the second-order polarizability tensor of ferroelectric LiNbO3 are calculated from first principles. The calculations are based on the electronic structure obtained from density-functional theory. The subsequent application of the GW approximation to account for quasiparticle effects and the solution of the Bethe–Salpeter equation yield a dielectric function for the stoichiometric material that slightly overestimates the absorption onset and the oscillator strength in comparison with experimental measurements. Calculations at the level of the independent-particle approximation indicate that these deficiencies are at least partially related to the neglect of intrinsic defects typical for the congruent material. The second-order polarizability calculated within the independent-particle approximation predicts strong nonlinear coefficients for photon energies above 1.5 eV. The comparison with measured data suggests that self-energy effects improve the agreement between experiment and theory. The intrinsic defects of congruent samples reduce the optical nonlinearities, in particular for the 21 and 31 tensor components, further improving the agreement with measured data.

Polarization Dependent Water Adsorption on the Lithium Niobate Z-Cut Surfaces

The effect of ferroelectric poling on the water adsorption characteristics of lithium niobate Z-cut surfaces is investigated by ab initio calculations. Thereby we model the adsorption of H2O monomers, small water clusters and water thin films. The adsorption configuration and energy are determined as a function of the surface coverage on both the positive and negative LiNbO3(0001) surfaces. Thereby polarization-dependent adsorption energies, geometries and equilibrium coverages are found. The different affinity of water to the two surfaces is explained in terms of different bonding scenarios as well as the electrostatic interactions between the substrate and the polar molecules. Surface phase diagrams for the Z-cuts in equilibrium with water are predicted from atomistic thermodynamics.

Surface Magnetism: Relativistic Effects at Semiconductor Interfaces and Solar Cells

Ab initio calculations of the electronic g-tensor of paramagnetic states at surfaces and solar cells are presented, whereby special emphasis is given onto the influence of relativistic effects. After discussing the numerical requirements for such calculations, we show that for silicon surfaces the g-tensor varies critically with the hydrogen coverage, and provides an exceptionally characteristic property. This holds also in the case of powder spectra where only the isotropic part g av is available from experiments. Extending our calculations onto microcrystalline 3C-SiC, our study explains why sol-gel grown undoped material can serve as an excellent acceptor material for an effective charge separation in organic solar cells: Due to an auto-doping mechanism by surface-induced states it fits excellently into the energy level scheme of this kind of solar cell and has the potential to replace the usually used rather expensive fullerenes.