A. Heinzel

Subwavelength-structured antireflective surfaces on glass

Abstract The usability of porous sol–gel coatings and periodic or stochastic subwavelength surface-relief structures for low-cost broadband antireflective (AR) surfaces on glass and on plastics was studied experimentally. Porous sol–gel coatings were produced by dip-coating on glass. Large-area periodic subwavelength surface-relief master structures were manufactured by holographic exposure of photoresist and transferred into nickel by electroforming. Stochastic surface-relief master structures were produced by a PVD process. The surface-relief structures were replicated in organically modified sol–gel materials on glass and in acrylic materials by embossing. With porous sol–gel coatings and periodic subwavelength surface-relief master structures, hemispherical reflectance values of

Radiation filters and emitters for the NIR based on periodically structured metal surfaces

Abstract The spectrally selective optical properties of wavelength selective radiation emitters and filters based on periodically microstructured metal surfaces were investigated. Metal surfaces were structured by the use of a holographic mask and subsequent etching processes. Due to the microstructure, thermally excited surface plasmons couple to electromagnetic radiation. Therefore a structured tungsten surface can act as a selective radiation emitter. The calculation of the absorptance by a rigorous diffraction theory allows the prediction of the emissivity of such structures. The angle dependent emissivity of tungsten gratings with periods of 1.4 μm and 2.0 μm was measured. A peak emissivity of 70% at a wavelength of 1.6 μm was achieved. Band pass filters for the near infrared spectral range based on perforated metal films were investigated theoretically and experimentally. Filters with a grating period of 2.0 μm were produced. A peak transmittance of 80% at a wavelength 2.9 μm was achieved. The optical properties of the diffractive elements described partly show a strong angle dependence

Antireflective transparent covers for solar devices

Abstract Promising solutions for low-cost antireflective (AR) covers for solar receivers are based on the principle of mixing bulk material with air on a subwavelength scale in order to obtain very low effective refractive indices. Possibilities to achieve this are given by porous media and by periodic or stochastic subwavelength surface-relief structures. In this work, the mentioned approaches were investigated experimentally. Subwavelength surface-relief structures were embossed in organically modified sol–gel materials or in acrylic materials, and porous sol–gel coatings were produced by dip-coating. It is shown that the solar transmittance of a transparent cover can be improved by up to 6% with a porous sol–gel coating at normal incidence. This improvement of the solar transmittance increases with larger angles of incidence.

Efficiency and power density potential of combustion-driven thermophotovoltaic systems using GaSb photovoltaic cells

The renewed interest in thermophotovoltaic (TPV) energy conversion, based on recent progress in materials and photovoltaic (PV) cell technology, requires a new evaluation of the TPV efficiency and power density potential. In this paper, we address some important points in TPV system design. We proceed in three steps, analyzing 1) the thermodynamic limit, 2) an idealized, and 3) a realistic model, based on an extrapolation of the current state of technology. In a TPV system, the radiation converted to electricity is adapted to the spectral response of the PV cell. This can be achieved by different means, which are examined in detail. Broadband and selective radiators, and optical filters are considered. We focus on combustion driven systems using low bandgap GaSb PV cells. For a system containing GaSb cells and a radiator at 1500 K, we find a thermodynamic limit efficiency of 60.5% and an output power density of 3 W/cm/sup 2/. For an idealized system model, an efficiency of 34% and a power density of 2.2 W/cm/sup 2/ are determined. For a realistic system with a broadband radiator and a filter, 9% and 1.2 W/cm/sup 2/ are estimated; using a selective radiator without filter, 16% and 1 W/cm/sup 2/ are expected. Performance values of this order should be achievable with a sufficient development effort.

Glazing with very high solar transmittance

Abstract Different attempts have been made to produce anti-reflection (AR) layers for solar applications. The most promising solutions are based on the principle of mixing bulk material with air on a subwavelength scale in order to obtain the very low effective refractive indices suitable as AR coatings for glazing. Possibilities for achieving this are given by porous media and subwavelength surface-relief gratings. Subwavelength grating structures, which were embossed in organically modified sol-gel materials or acrylic materials, and porous sol-gel coatings were investigated and compared theoretically and experimentally by the Fraunhofer Institutes. It is shown, theoretically and experimentally, that an increase of solar transmittance of ≈3% per glass surface can be achieved with a porous sol-gel coating.

Antireflective submicrometer surface-relief gratings for solar applications

The optical properties of surface-relief gratings, structured on a submicrometer scale, were investigated by effective-medium approaches and by rigorous diffraction theory. Linear and crossed surface-relief gratings in photoresist were prepared on relatively large areas with a holographic process. From the photoresist gratings, nickel masters were produced and then used for an embossing process. With this process, the surface-relief gratings were transferred into an ORMOCER film on a glass pane. The angle-dependent transmittance and the reflectance of glass panes coated with structured ORMOCER films were measured. Transmittance values of more than 98% can be achieved.

Optical properties of inhomogeneous media

Abstract Inhomogeneous media such as ceramic–metal composites are used in a wide range of applications as functional optical coatings e.g. in selective solar absorber coatings. Their optical properties can be calculated by the use of effective medium theories. These theories need the optical constants of the components as input parameters. Especially for the metallic component, these parameters depend strongly on the deposition conditions and on the particle sizes. The limitation of the mean free path of the conduction electrons can be explained by the well-known Drude theory. It was found that additional size effects occur due to a size- dependent modified band structure of the metal particles. The size dependence of the band structure and the resulting effects on the optical constants are discussed for the model system of sputtered small gold particles embedded in an amorphous carbon matrix produced simultaneously by hydrogenated, plasma- activated, chemical vapour deposition. Changes in the band structure were analysed by ultraviolet and X-ray photoelectron spectroscopy (UPS/XPS). The optical constants of the cermets were determined from reflectance and transmittance measurements as well as from spectral ellipsometry. Changes in the optical constants of the gold particles were found from comparison of these measurements with effective medium theories. The size of the metal particles and the surface structure of the cermets were determined from TEM images and atomic force microscopy. The size-dependent changes of the optical constants of the metal particles were correlated with the UPS spectra.

Microstructured tungsten surfaces as selective emitters

This paper presents the optical properties of periodic microstructured tungsten surfaces used as selective emitter materials. Theoretical calculations are done using rigorous diffraction theory, and the influence of the grating period and the grating depth on the absorptance of tungsten gratings is investigated. Microstructured tungsten surfaces are fabricated using a holographic process to produce a photoresist mask. Subsequently a reactive ion etching process or a chemical etching process transfers the structure into tungsten. The absorptance of the tungsten gratings is measured and compared with the theoretical predictions of the rigorous diffraction theory.

Efficiency and power density potential of thermophotovoltaic systems using low bandgap photovoltaic cells

In this paper, an updated evaluation of the efficiency and power density potential of TPV technology is presented. We focus on combustion-driven systems using low bandgap photovoltaic (PV) cells. In a geometry-independent approach, we analyze (i) the thermodynamical limit, (ii) an idealized and (iii) a realistic model extrapolating the current state of technology. Special attention is paid to the performance of the optical system. For a system containing a propane burner, GaSb PV cells and a radiator at 1500 K, we estimate that efficiencies of the order of 10% and power densities of 1 W/cm2 are achievable with a sufficient development effort. In addition to this theoretical analysis, the 5 W TPV test facility at Fraunhofer ISE is described.

Evaluation of the optical properties of molybdenum particles in metal/ceramic composites

The optical constants of thin molybdenum layers and molybdenum/tin oxide cermets, produced by a batch sputtering deposition system, were determined by spectroscopic ellipsometry. The geometrical structure of the layers and the embedded particles was analysed with an atomic force microscope. The results were used for the investigation of a suitable effective medium theory to describe the optical constants of cermets with relatively high filling factors.