Gerd Willeke

Thin pyrite (FeS2) films prepared by magnetron sputtering

Abstract Thin pyrite films have been prepared by sulphur-assisted magnetron sputtering at deposition temperatures between 88 and 390 °C. The films were microcrystalline, with an average grain size of about 400 A and a preferential orientation changing from (200) to (111) on going to higher deposition temperatures. The electrical conductivity was degenerate, with σ RT = 4 Ω −1 cm −1 , p = 5 × 1018 cm−3 and μHRT = 5 cm2 V−1 s−1. A positive sign was observed in both Hall and Seebeck measurements, indicating holes to be the predominant current carrier. μH varied approximately linearly with temperature, consistent with contributions from both ionized and neutral impurity scattering.

Preparation of pyrite films by plasma‐assisted sulfurization of thin iron films

Pyrite films were prepared using the pure elements as source materials: thin iron films were evaporated on quartz substrates and exposed to a sulfur plasma. The process was controlled by a transmission measurement. X‐ray spectroscopy was used to characterize the films and preliminary optical and electrical measurements were carried out.

Quantifying the effect of metal-rich precipitates on minority carrier diffusion length in multicrystalline silicon using synchrotron-based spectrally resolved x-ray beam-induced current

Synchrotron-based, spectrally resolved x-ray beam-induced current (SR-XBIC) is introduced as a technique to locally measure the minority carrier diffusion length in semiconductor devices. Equivalence with well-established diffusion length measurement techniques is demonstrated. The strength of SR-XBIC is that it can be combined in situ with other synchrotron-based analytical techniques, such as x-ray fluorescence microscopy (μ-XRF) and x-ray absorption microspectroscopy (μ-XAS), yielding information about the distribution, elemental composition, chemical nature, and effect on minority carrier diffusion length of individual transition metal species in multicrystalline silicon. SR-XBIC, μ-XRF, and μ-XAS measurements were performed on intentionally contaminated multicrystalline silicon, revealing a strong correlation between local concentrations of copper and nickel silicide precipitates and a decrease of minority carrier diffusion length. In addition, the reduction of minority carrier diffusion length due t...

Enhanced Carrier Collection observed in Mechanically Structured Silicon with Small Diffusion Length

The diffusion length of minority charge carriers in the silicon bulk Ldiff is an important characteristic of optoelectronic devices fabricated from low cost silicon wafers. In this study computer simulations have been carried out to calculate the beneficial effects of a macroscopic surface texturization on the charge carrier generation and the collection probability. Textured solar cells should be able to collect charge carriers more effectively resulting in an increased current due to the special emitter geometry resulting from the texture, decreased reflection losses, and the inclined penetration of the light. In order to prove this expected behavior, deeply V-textured solar cells have been processed and characterized on low cost silicon reaching an Ldiff of about 25 μm. Spatially resolved high resolution measurements of the internal quantum efficiency exhibit a strongly increased signal in the texture tips which is the first experimental proof of the increased charge carrier collection probability of d...

Systematic study towards high efficiency multicrystalline silicon solar cells with mechanical surface texturization

The aim of the present work was to optimize a high efficiency process for multicrystalline silicon solar cells (including Al-gettering, oxide and hydrogen passivation, Al-BSF formation, photolithographically defined front metallization) and combine it with the mechanical texturization technique. New cell structures were created, in which only the areas between the front grid are V-grooved. Solar cells were processed on various ribbon and conventional cast silicon materials. IV-characteristics, reflectance and spectral response were measured and analysed by two dimensional device simulation. A comparison with equally processed flat cells shows an increase of cell efficiencies by more than 25% due to the reduction of optical losses before antireflection coating and by an additional 4% due to the enhanced collection probability in the V-groove volumes.

Progress on the LOPE (local point contact and shallow angle evaporation) silicon solar cell

The LOPE-Solar cell concept is based on a V-textured front surface with local point contacts which are interconnected by the SAFE (shallow angle finger evaporation) technique after a heavy phosphorus diffusion. Three dimensional computer simulations were carried out to examine the influence of the point contact distance parallel and perpendicular to the V-grooves. Additionally two different emitters, two different sheet resistances under the metal contact and a small and a larger contact area were investigated. A plating sequence for Ni-plating has been successfully implemented in the LOPE-cell process to complete the metallisation of the local openings. A detailed characterisation of the LOPE-cell is presented.