M.B. Schubert

The Development of Subretinal Microphotodiodes for Replacement of Degenerated Photoreceptors

There are presently several concepts to restore vision in blind or highly visually handicapped persons by implanting electronic devices into the eye in order to partially restore vision. Here, the approach to replace retinal photoreceptors by a subretinally implanted microphotodiode array (MPDA) is summarized. A survey is given on the present state of the development of MPDAs, the possibility of in vitro and in vivo tests as well as first results on biocompatibility and histology. Additionally, electrophysiological recordings in rabbits and rats are presented which have received such subretinal implants.

Flexible solar cells for clothing

Integration of flexible solar cells into clothing can provide power for portable electronic devices. Photovoltaics is the most advanced way of providing electricity far from any mains supply, although it suffers from the limits of ambient light intensity. But the energy demand of portable devices is now low enough that clothing-integrated solar cells are able to power most mobile electronics. We introduce clothing-integrated photovoltaics, their scope and limitations, the status of flexible solar cells, charge controller and system design, as well as prototype solutions for various applications.

Amorphous and microcrystalline silicon by hot wire chemical vapor deposition

Amorphous hydrogenated silicon (a‐Si:H) was deposited by SiH4 decomposition on a hot tungsten filament. The substrate temperature was held at 400 °C for all samples, maintaining conditions where material combining a low defect density and a low hydrogen content is obtained. A systematic study of the effects of gas pressure, substrate‐to‐filament distance, and filament temperature on film properties is presented, allowing insight into the growth condition required for this material as well as the significance of secondary gas phase reactions. Material of good optoelectronic quality is obtained at high growth rates. The stability with respect to light degradation was compared to typical plasma deposited films. Conditions for the transition from amorphous to microcrystalline films, observed under gas phase dilution with hydrogen, were investigated. By in situ ellipsometry and atomic force microscopy the nucleation and film morphology were shown to be significantly different from those for plasma‐chemical vap...

Laser-scanned p-i-n photodiode (LSP) for image detection

Amorphous and microcrystalline glass/ZnO:Al/p(a-Si:H)/i(a-Si:H)/n(a-Si 1 - x C x :H)/Al imagers with different n-layer resistivities were produced by plasma-enhanced chemical vapor deposition technique (PE-CVD). The transducer is a simple, large area p-i-n photodiode; an image projected onto the sensing element leads to spatially confined depletion regions that can be readout by scanning the photodiode with a low-power modulated laser beam. The essence of the scheme is the analog readout and the absence of semiconductor arrays or electrode potential manipulations to transfer the information coming from the transducer. The effect of the image intensity on the sensor output characteristics (sensitivity, linearity, blooming, resolution, and signal-to-noise ratio) are analyzed for different material composition. The results show that the responsivity and the spatial resolution are limited by the conductivity of the doped layers. An enhancement of one order of magnitude in the image intensity and on the spatial resolution is achieved with a responsivity of 0.2 mW/cm 2 by decreasing the n-layer conductivity by the same amount. In a 4 x 4 cm 2 laser-scanned photodiode (LSP) sensor, the resolution was less than 100 μm and the signal-to-noise (S/N) ratio was about 32 dB. A physical model supported by electrical simulation gives insight into the methodology used for image representation.

Photoluminescence from silicon nitride—no quantum effect

Silicon nitride compounds emit photoluminescence all over the visible range. Recent studies ascribed this luminescence to quantum-size effects within silicon nanocrystals that were either shown or assumed to form inside the silicon nitride matrix; the luminescence of the matrix itself was ignored. In contrast, observing the same luminescence even without the presence of silicon crystallites, our work identifies the silicon nitride matrix itself as responsible for the photoluminescence. All experimental observations are well explained by band tail luminescence from the silicon matrix. In contrast to the silicon nanocrystal approach, our model explains all aspects of the luminescence. As a consequence, we conclude that silicon nitride films are inappropriate if one aims at investigating photoluminescence from silicon nanocrystals within such a matrix.

Three-color sensor based on amorphous n-i-p-i-n layer sequence

We present an amorphous silicon-based n-i-p-i-n three-color sensor with a layer sequence of substrate/metal/n-i-p-i-n/transparent contact. The color sensitivity (red, green and blue) is realized by applying small bias voltages within /spl plusmn/2 V. For the first time, this structure offers the possibility to integrate a color sensor on top of an ASIC chip, where pixel-based signal processing can be performed. Film thickness and optical band gap of the individual layers have been optimized to achieve maximum color separation. The surface of the metal-back contact is found to be critical for the performance of the sensor. A rough surface of the metal is responsible for short circuits at the common p-contact of the back-to-back p-i-n junction diodes. The steady-state I-V characteristics of the dark and photo currents have been studied. The dynamic range of the sensor is already suitable for optoelectronic applications. Sensor performance is not affected by metastable effects after prolonged light soaking. Speed limitations have been evaluated from switching experiments in the voltage mode. >

Silicon thin film solar cells deposited under 80°C

We deposited silicon thin films by plasma-enhanced chemical vapor deposition (PECVD) at very low substrate temperatures of 75 and 40°C. Even at these low deposition temperatures, the protocrystalline Si (pc-Si:H) exhibits a high photosensitivity and remarkably enhanced stability against light saturation. This material grows at the borderline between amorphous (a-Si:H) and nanocrystalline (nc-Si:H) phases in the deposition parameter space. Structural and optical characterization revealed a small fraction of crystallites embedded in an amorphous matrix. Thickness-dependent morphology of silicon films was revealed by absolute constant photocurrent method (CPM). We demonstrated the effect of the amorphous-to-nanocrystalline transition on the solar cell performance. The cells with a protocrystalline absorber layer showed an improved fill factor.

Subretinales Mikrophotodioden-Array als Ersatz für degenerierte Photorezeptoren?

ZusammenfassungEs wird eine Übersicht über den Stand der Entwicklungen eines subretinalen, elektronischen Mikrophotodioden-Arrays gegeben, mit dem degenerierte Photorezeptoren ersetzt werden können. Verschiedene Prototypen wurden entwickelt, getestet und bei verschiedenen Versuchstieren bis zur Dauer von 18 Monaten implantiert. Die Tatsache, dass mit subretinalen Elektroden elektrische Antworten vom visuellen Kortex von Schweinen abgeleitet werden können und Antworten auch in vitro in Netzhäuten von Ratten mit Netzhautdegenerationen nachweisbar sind, zeigt die Sinnhaftigkeit dieses Ansatzes. Allerdings sind noch wichtige Fragen der Biokompatibilität, der Langzeitstabilität und der Art des vermittelbaren Bildeindrucks zu bearbeiten, bevor an einen Einsatz beim Menschen zu denken ist.AbstractA survey is given on the status of developments, concerning a subretinal electronic microphotodiode array that aims at replacing degenerated photoreceptors. Various prototypes have been developed, tested, and implanted in various experimental animals up to 18 months. The fact that electrical responses were recorded from the visual cortex of pigs after electrical stimulation by subretinal electrodes and the fact that responses are also recorded in-vitro in degenerated rat retinae, shows the feasibility of this approach. However, there are a number of open questions concerning the biocompatibility, the long-time stability, and the type of transmitted image to be solved before application in patients can be considered.

Electrical properties of a light-addressable microelectrode chip with high electrode density for extracellular stimulation and recording of excitable cells

A light-addressable microelectrode chip with 3600 TiN electrodes was fabricated. Amorphous silicon (a-Si:H) serves as a photo conductor. The electrodes on the chip are addressed by a laser spot and electrical properties of the system are determined. DC measurements show a dark to bright dynamic of 10(6)-10(7). The AC impedance dynamic @ 1 kHz/100 mV and thus the signal-to-noise-ratio is determined to 60. This value is quite sufficient for electrophysiological measurements. For the first time, recordings from cardiac myocytes are reported using the principle of light-addressing. Measurements were done with a standard laser scan microscope (Zeiss LSM 410).

Nanocrystalline silicon from hot-wire deposition : a photovoltaic material?

Abstract Nanocrystalline silicon (nc-Si:H) attracts a great deal of attention due to the hope for more efficient and stable solar cells, as well as better thin-film transistors and optical sensors. In this study, we report on improvements in the structural and electronic qualities of intrinsic nc-Si:H grown from hot-wire chemical vapour deposition. For examining a wide range of deposition parameters, we use a design-of-experiments approach. In contrast to our previous films obtained from tungsten and tantalum filaments, a novel type of filament greatly enhances preferential growth in the 〈110〉 direction over a wide range of deposition conditions. General considerations on the orientation and electronic activity of grain boundaries in polycrystalline silicon explain why the electronic quality of this 〈110〉-oriented film is remarkably higher than the one previously grown, mixed-phase nanocrystalline silicon. Mobility–lifetime products of films from our novel filaments are two orders of magnitude higher than those of samples from Ta wires. In photoluminescence spectra, no band tail contributions occur, and the amorphous and defect peaks are greatly reduced. Moreover, the transverse optical Raman signal is red-shifted, and thereby indicates a reduction in mechanical strain in our novel nanocrystalline silicon films.