Gottfried H. Bauer

Subgrain size inhomogeneities in the luminescence spectra of thin film chalcopyrites

We report near-field photoluminescence (PL) spectra of Cu(InGa)Se2 thin films recorded with a lateral optical resolution of ≈200 nm and simultaneous detection of the sample topography. Our results reveal significant local variations in the PL spectra, specifically the PL yield, on length scales of 0.2–1.5 μm. Local variations in both the splitting of quasi-Fermi levels μ and the band gap energy are quantitatively extracted from the PL spectra by applying Planck’s generalized law. We show pronounced fluctuations of μ and the band gap on length scales below the grain size. These fluctuations are only weakly correlated with the topographic film structure.

Nanoparticle ring formation in evaporating micron-size droplets

Self-assembly process of CoPt3 particles into a ring pattern (ring diameter ranging from 0.6 to 1.5 μm, particle diameter 6 nm) results from phase separation in the thin film of a binary mixture, giving rise to a bilayer structure and subsequent decomposition of the top layer into droplets. Evaporation of the droplet leads to a shrinking of its contact line, and the particles located at the contact line follow its motion and self-assemble along the line.

Defects and transport in a-Si:H/c-Si heterojunctions

Abstract We have investigated amorphous silicon/crystalline silicon np and pn heterojunction devices with transport and capacitance measurement techniques. The devices consist of doped n-type or p-type hydrogenated amorphous silicon deposited on p-type or n-type float-zone crystalline silicon wafers. Low temperature (100 K) photocurrent measurements in conjunction with numerical simulations indicate a large conduction band offset at the interface between the two materials. We show experimentally and with numerical simulation that defects at the heterojunction interface affect the low frequency capacitance and the electronic transport in the devices.

What is the useful energy of a photon

The fundamental upper bound on the efficiency of photovoltaic conversion continues to attract interest of the research community. By considering the conversion efficiency of a monochromatic photon gas at constant pressure, we show that this limit is equal to the availability (or exergy), as defined in textbooks on classical thermodynamics. The application of this result to the full spectrum of black-body radiation yields the Petela-Press-Landsberg efficiency. Generalization to include entropy generation on account of the kinetic nature of the conversion process, by drawing a parallel with the efficiency of an infinite tandem converter, yields a theoretical efficiency limit of 85.2%.

Electronic properties of microcrystalline SiGe-thin films by Hall-experiments and photo- and dark-transport

Abstract Transport properties of microcrystalline Si1−xGex thin films (0⩽x⩽0.56) deposited in a VHFPE-CVD process from disilane–germane mixtures under strong H2 dilution have been analyzed by dark conductivity, photoconductivity, and by Hall-experiments. Mobilities of electrons and holes at room temperature amount to μ nRT =1 cm 2 V −1 s −1 , and μ pRT =0.25 cm 2 V −1 s −1 and (μτ)n- and (μτ)p-products decrease by increasing Ge-content from 10 −7 m 2 V −1 by a factor of 30, and from 4×10 −10 cm 2 V −1 by a factor of 2, respectively. We see a transition from n-type to p-type conduction at x≈0.56. Within the temperature regime of traditional operation of opto-electronic devices (2⩽103/T⩽4) electron and hole transport is dominated by low mobility states such as tail states or by grain boundary barriers.

Self-assembled patterns from evaporating layered fluids

We studied the formation of tree-like patterns of polymer aggregates and rings of nanoparticles during evaporation from a fluid film. We utilize phase separation between two immiscible fluids to generate a double-layer film which dries up in a sequential manner. Both fluid layers may contain a solute, polymer aggregates or nanoparticles. During evaporation of the top layer, instabilities may occur and direct a self-assembly process of the solute which may be further affected by an instability of the bottom layer at a later stage. We present two cases where, after evaporation of the top fluid layer, the solute was adsorbed on the surface of the bottom fluid layer. In comparison to dewetting of a single fluid layer on a solid substrate, the advantage of our double-layer approach lies in the deposition of the solute on the surface of the bottom fluid layer. The relatively high mobility of the solute on such a fluid surface favors the formation of ordered patterns, driven by an instability of the bottom layer.

Treelike branched structures formed in dewetting thin films of a binary solution

The authors present evidence for treelike patterns which developed during solvent evaporation from a phase separated bilayer resulting from a binary polymer solution spin coated onto a solid substrate. Initially, a bilayer structure containing a poly(isobutyl methacrylate) (BMA) solution layer on top of a nitrocellulose (NC) solution layer. During subsequent solvent evaporation, the top BMA solution layer becomes unstable and transforms into short ridges. Finally, solvent evaporation from the NC solution layer connects the BMA ridges to treelike patterns.

An Approach to High Quality a-Ge:H by VHF Deposition

Amorphous hydrogenated germanium a-Ge:H was deposited by very high frequency glow discharge (VHF-GD) at frequencies between 25 and 220MHz, low pressure (2.5Pa) and high deposition rates (≤4A /s) on both electrodes of a parallel plate reactor. The films are comparable to material deposited on the powered electrode of a conventional RF-GD. Mobility-lifetime products ηµτ for photogenerated charge carriers around 10-7cm2/V and a Fermi level position at E C -E F ≈400meV indicate good opto-electronic properties. The IR spectra show that the samples are free of oxydization and incorporate slightly more voids than the RF material. Measurements of ion energy E i and flux Ф i in the substrate plane show that “hard&” preparation conditions are obtained comparable to the deposition on the powered electrode of an RF-GD.

Note: Controllable positioning of a single particle in between nanogap electrodes.

We have developed a relatively simple procedure capable to position a single nanoparticle in between nanogap electrodes for nanoelectronic applications. The well-defined particle positioning was achieved via pinning the edge of a dispersion droplet over the nanogap. Pinning time, substrate temperature, and its displacement velocity are excellent control parameters.

Non-uniformities of opto-electronic properties in Cu(In,Ga)Se2 thin films and their influence on cell performance studied with confocal photoluminescence

We have studied a series of high quality Cu(In,Ga)Se2 thin film solar cells (efficiencies 15% - 18%) with spectrally resolved confocal photoluminescence (PL) which allows for a spatial resolution of ¿ 0.8 ¿m. The recorded PL data were analyzed by application of Plancks generalized radiation law, which allows for quantification of values for the local splitting of quasi-Fermi levels and of the local threshold energy of optical absorption for each scan-pixel, corresponding to the local open circuit voltage (Voc) in a solar cell and the optical band gap energy of the absorber, respectively. We quantify representative values for spatial variations of these parameters for each cell and discuss qualitatively as well as quantitatively the influence of local inhomogeneities on the resulting total cell quality in terms of Voc and efficiency.