Passivation of black silicon boron emitters with atomic layer deposited aluminum oxide

The nanostructured surface – also called black silicon (b-Si) – is a promising texture for solar cells because of its extremely low reflectance combined with low surface recombination obtained with atomic layer deposited (ALD) thin films. However, the challenges in keeping the excellent optical properties and passivation in further processing have not been addressed before. Here we study especially the applicability of the ALD passivation on highly boron doped emitters that is present in crystalline silicon solar cells. The results show that the nanostructured boron emitters can be passivated efficiently using ALD Al2O3 reaching emitter saturation current densities as low as 51 fA/cm2. Furthermore, reflectance values less than 0.5% after processing show that the different process steps are not detrimental for the low reflectance of b-Si.Peer reviewe

Micro-spectroscopy on silicon wafers and solar cells

Micro-Raman (μRS) and micro-photoluminescence spectroscopy (μPLS) are demonstrated as valuable characterization techniques for fundamental research on silicon as well as for technological issues in the photovoltaic production. We measure the quantitative carrier recombination lifetime and the doping density with submicron resolution by μPLS and μRS. μPLS utilizes the carrier diffusion from a point excitation source and μRS the hole density-dependent Fano resonances of the first order Raman peak. This is demonstrated on micro defects in multicrystalline silicon. In comparison with the stress measurement by μRS, these measurements reveal the influence of stress on the recombination activity of metal precipitates. This can be attributed to the strong stress dependence of the carrier mobility (piezoresistance) of silicon. With the aim of evaluating technological process steps, Fano resonances in μRS measurements are analyzed for the determination of the doping density and the carrier lifetime in selective emitters, laser fired doping structures, and back surface fields, while μPLS can show the micron-sized damage induced by the respective processes

Association of obesity with disease outcome in multiple sclerosis

BackgroundObesity reportedly increases the risk for developing multiple sclerosis (MS), but little is known about its association with disability accumulation.MethodsThis nationwide longitudinal cohort study included 1066 individuals with newly diagnosed MS from the German National MS cohort. Expanded Disability Status Scale (EDSS) scores, relapse rates, MRI findings and choice of immunotherapy were compared at baseline and at years 2, 4 and 6 between obese (body mass index, BMI ≥30 kg/m2) and non-obese (BMI <30 kg/m2) patients and correlated with individual BMI values.ResultsPresence of obesity at disease onset was associated with higher disability at baseline and at 2, 4 and 6 years of follow-up (p<0.001). Median time to reach EDSS 3 was 0.99 years for patients with BMI ≥30 kg/m2 and 1.46 years for non-obese patients. Risk to reach EDSS 3 over 6 years was significantly increased in patients with BMI ≥30 kg/m2 compared with patients with BMI <30 kg/m2 after adjustment for sex, age, smoking (HR 1.87; 95% CI 1.3 to 2.6; log-rank test p<0.001) and independent of disease-modifying therapies. Obesity was not significantly associated with higher relapse rates, increased number of contrast-enhancing MRI lesions or higher MRI T2 lesion burden over 6 years of follow-up.ConclusionsObesity in newly diagnosed patients with MS is associated with higher disease severity and poorer outcome. Obesity management could improve clinical outcome of MS

Contactless transient carrier spectroscopy and imaging technique using lock-in free carrier emission and absorption

In this paper we present a contactless transient carrier spectroscopy and imaging technique for traps in silicon. At each pixel, we fit the transient decay of the trap emission which allows us to obtain both the trap time constant and trap concentration. Here we show that this technique allows for high-resolution images. Furthermore, this technique also allows to discriminate between the presence of thermal donors or oxygen precipitates in as-grown wafers, without requiring a thermal donor killing step

Analysis of solar cell cross sections with micro-light beam induced current (µLBIC)

A highly resolving micro-light beam-induced current (µLBIC)-system is presented in this work. Based on the laser excitation via an optical microscope, current values can be measured with sub-micron precision. We show, that this non-destructive, light-based approach delivers superior results to a reference electron microscope based electron beam induced current method concerning contrast and robustness towards reflection differences, whereas no vacuum is needed, no charging effects can occur and equal resolution is achieved. µLBIC allows therefore mapping of pn-junctions at silicon solar cell cross sections. By combination of µLBIC with other measurement methods in the same setup, such as micro-Raman spectroscopy, complementary microscopic information about material stress or crystallinity and electronic properties at the same region of interest on the sample is revealed. By applying µLBIC for analyzing silicon solar cross sections, two characterization examples of current technological relevance are presented: enhanced dopant diffusion along grain boundaries between grains with different orientations is quantified and the impact of a nickel silicide spike on local charge collection quality is studied

Microscopic origin of the aluminium assisted spiking effects in n-type silicon solar cells

Contact formation with silver (Ag) thick film pastes on boron emitters of n-type crystalline silicon (Si) solar cells is a nontrivial technological task. Low contact resistances are up to present only achieved with the addition of aluminium (Al) to the paste. During contact formation, Al assisted spiking from the paste into the silicon emitter and bulk occurs, thus leading to a low contact resistance but also to a deterioration of other cell parameters. Both effects are coupled and can be adjusted by choosing proper Al contents of the paste and temperatures for contact formation. In this work the microscopic electric properties of single spikes are presented. These microscopic results, i.e. alterations of the local emitter doping density, the pronounced local recombination activity at the interface between spikes and Si and its influence on the charge collection efficiency, are used to explain the observed dependencies of global cell parameters on the Al content of contact pastes

An Open Source Based Repository for Defects in Silicon

Silicon is the most studied semiconductor, having almost every aspect of it being investigated. All this information is spread over a large set of publications, review articles and textbooks and cannot be found in a single location. Furthermore, the available data is not always consistent and depends on the techniques and samples used. This problem even exists for more specialised areas such as the study of defects in silicon photovoltaics, which is the focus of this paper. Currently, if a signature of a defect is experimentally determined a literature search must then be performed through texts going back decades in the hope to find a defect with similar properties. This paper addresses this time consuming activity by introducing an open source text based repository, which anyone can access or contribute to, and that provides clearly arranged information about defects in silicon.Part of this work was funded by the Australian Government through the Australian Centre for Advanced Photovoltaics (ACAP) and the Australian Renewable Energy Agency (ARENA), as well as the Germany Government of Education and Research (BMBF) under the CCPV project

An open source based repository for defects in silicon

Silicon is the most studied semiconductor, having almost every aspect of it being investigated. All this information is spread over a large set of publications, review articles and textbooks and cannot be found in a single location. Furthermore, the available data is not always consistent and depends on the techniques and samples used. This problem even exists for more specialised areas such as the study of defects in silicon photovoltaics, which is the focus of this paper. Currently, if a signature of a defect is experimentally determined a literature search must then be performed through texts going back decades in the hope to find a defect with similar properties. This paper addresses this time consuming activity by introducing an open source text based repository, which anyone can access or contribute to, and that provides clearly arranged information about defects in silicon

High performance multicrystalline silicon: Grain structure and iron precipitation

A study of the spatial occurrence of iron precipitation in a high performance multicrystalline silicon (HPMC-Si) sample is presented. The separated effects of grain-boundaries, sparse intra-granular dislocations, and dislocation clusters are investigated by combining the Fei imaging method with glow discharge mass spectroscopy, electron backscatter diffraction, and two iron precipitation models. While the area-averaged precipitation at grain boundaries is relatively minor, almost the whole iron precipitation occurs within the grains, despite the very low intra-granular dislocation density. The fraction of non-precipitated iron in the studied HPMC-Si material was found to be one to two orders of magnitude higher than that reported previously for standard materials