Hauke Harms

Magnetooptical current transformer. 1: Principles.

The principles of measuring currents in high-voltage lines by magnetooptical means are described. Particular attention is payed to using an optical fiber both as a transmission line and a measuring sensor. The influence of birefringence on the measuring signal is discussed.

Advanced Stacked Elemental Layer Process for Cu(InGa)Se 2 Thin Film Photovoltaic Devices

Targeting large area and low cost processing of highly efficient thin film solar modules an advanced stacked elemental layer process for Cu(InGa)Se 2 (CIGS) thin films is presented. Key process steps are i) barrier coating of the soda lime glass substrate combined with the addition of a sodium compound to the elemental Cu/In/Ga/Se-precursor stack and ii) rapid thermal processing (RTP) to form the CIGS compound. By this strategy exact impurity control is achieved and the advantageous influence of sodium on device performance and on CIGS film formation is demonstrated unambiguously by means of electrical characterisation, XRD, SEM, TEM and SIMS. Sodium enriched and sodium free precursor stacks were heated to intermediate states (300°C–500°C) of the RTPreaction process. The experiment clearly reveals that on the reaction pathway to the chalcopyrite semiconductor increased amounts of copper-selenide are formed, if sodium is added to the precursor films. TEM-electron diffraction unambiguously identifies the CuSe-phase which is localised at the surface of the forming CIGS-film. These experimental findings propose a sodium assisted quasi liquid growth model for the CIS formation taking into account that sodium promotes the existence of CuSe at higher temperatures and its effect as a flux agent. The model contributes to a better understanding of the observed superior crystal qualitiy for sodium enriched in contrast to sodium free CIGS films. Application of these experimental findings in the technique of the optimized and controlled sodium incorporation significantly improves process reproducibility, CIGS film homogenity over larger substrate areas and shifts the average efficiency of cells and modules to a significantly higher level. This is demonstrated by a 12-cell integrated series connected minimodule with an aperture area of 51 cm 2 and a confirmed efficiency of 11.75 %.

Polarization optics of liquid-core optical fibers.

Whereas straight liquid-core optical fibers have the capability of transporting linearly polarized light with any incident polarization angle,bent fibers exhibit form birefringence with the principal axes fixed to the plane of curvature. The effect of this curvature-dependent birefringence can be compensated by either periodically altering the plane of curvature by 90 degrees or winding the fiber into a helix.

Magnetooptical properties of index-gradient optical fibers

The magnetooptical Faraday rotation of a guided He-Ne laser beam in index-gradient fibers was measured. The proportionality between the rotation of the polarization vector and the length of the light path in an applied homogeneous longitudinal magnetic field is disturbed. The measuring results can be explained by means of a theoretical model combining the Faraday effect with birefringence.

Magnetooptical current transformer. 2: Components

In this second part, the most important components of a magnetooptical current transformer are described and their suitability is discussed. Today He-Ne gas lasers and laser diodes seem to meet best the requirements of the light source. Single-mode optical fibers are capable of transmitting polarized light without depolarization. A fiber coil wound on a special coil former is used as a measuring sensor. Two P-I-N photodiodes, together with an analog electronic circuit, transform the optical into an electrical measuring signal.

Magnetooptical current transformer. 3: Measurements

A laboratory setup of a magnetooptical current transformer was established. A ingle-mode fiber acts as both a measuring sensor and a transmission line. The overall measurement accuracy at room temperature within a primary current range 50-1200 A is better than 0.24%, the SNR at 1000 A is 85 dB. Current measurements of high accuracy were also performed in the -20-+ 45 degrees C temperature range.

Investigation of the Performance of a Thin-Film Transistor-Driven Linear Image Sensor

The performance of a thin-film transistor (TFT) driven linear image sensor is analyzed. By experiment and computer simulation the charge trans-fer is investigated and the results are applied to interprete the performance of the conventional readout circuit. Its main disadvantage are the large number of crossovers between signal lines. Therefore, an alternative readout circuit with reduced number of crossovers is proposed.

a-Si:H TFTs and their application in linear image sensors

Abstract a-Si:H TFTs with SiNx as dielectric were fabricated for application as readout switches in a-Si:H linear image sensors. μFE=0.4....0.5cm2/Vs and Ron = 2..l.5Mω were achieved. The transfer of the signal from a photodiode through a TFT to a readout circuit was investigated and computer calculations for the transfer were made in order to analyze signal, timing, and switching noise.

Temperature dependence of the photocurrent in pin and nip solar cell structures made from a-Si:H

In connection with the development of solar cells made from amorphous silicon (a-Si:H), we investigated the temperature dependence of the photocurrent under reverse bias of different diodes. From the saturation behavior of the current-voltage characteristics we conclude that the reverse-bias photocurrent is of primary nature. Therefore, it is expected to be independent of temperature. A slight decrease with rising temperature might be due to increasing absorption within the p+ layer. Surprisingly, we found an increase of the photocurrent with temperature in all samples investigated. The dependence of this effect on bias voltage, illumination wavelength and intensity suggests the following picture. Using weakly absorbed light, temperature dependent absorption can explain the positive temperature coefficient (>5·10−3/K). Strongly absorbed light leads to a small temperature coefficient of the order of +10−4/K. The photo-generated carriers influence the internal field distribution and may thus cause an additional increase of the current.