1. Wochenbericht POS512

(25.4.2017-30.4.2017

Layer selective laser ablation for local contacts to thin emitters

High efficiency solar cells require high generation and low recombination rates. High bulk lifetime, well passivated surfaces, and lowly doped thin emitters allow for low recombination rates. Thin passivated emitters should be contacted locally in order to avoid excessive contact recombination. This is common practice for front junction solar cells but is also advantageous for back junction cells. We analyze a novel layer selective laser ablation process. From a passivating stack composed of 70 nm silicon nitride that we deposit on top of 35 nm of amorphous silicon we selectively ablate the silicon nitride layer. Transmission electron microscopy investigations confirm the full ablation of the silicon nitride layer. After the ablation process, a 17 nm-thick amorphous silicon layer remains on the substrate. The crystalline silicon substrate shows no dislocations after the process. Evaporating aluminum on top of the locally ablated nitride layers forms local contacts of the aluminum to the silicon

Geochemical mapping of a paleo-subduction zone beneath the Troodos Ophiolite

Supra-subduction zone ophiolites such as the Cretaceous Troodos Ophiolite of Cyprus are fragments of oceanic crust formed by seafloor spreading close to subduction zones. Their exact tectonic setting of origin has been intensively debated. Although many supra-subduction zone ophiolites are thought to represent fore-arc crust, created during subduction initiation, others may have formed at a subducting ridge, or in a back-arc, ridge-trench-trench/transform triple junction or ‘plate edge’ setting. We carried out major and trace element analyses of 515 fresh volcanic glasses from 7 detailed sections through the Troodos lava sequence in order to determine the regional and temporal variation in the composition of Troodos magmatism, and hence reconstruct the distance and orientation of the Troodos spreading axis relative to the former subduction zone. Troodos glasses range from boninite through tholeiitic basalt and andesite to dacite. All glasses are enriched in fluid-mobile trace elements, and variably depleted in the high-field strength elements compared to Mid-Ocean Ridge Basalt (MORB). None of these glasses therefore have compositions identical to Izu-Bonin-Mariana fore-arc lavas that have been proposed to be the prime example of lavas formed during subduction initiation. Boninites are apparently restricted to the southern margin of the Troodos Ophiolite, and glasses from the southeast margin of the ophiolite are the most depleted and contain the strongest input of subduction zone fluid and melt signature. These geographic variations in glass composition indicate that the Troodos Ophiolite formed by NW-SE directed spreading (at 91 Ma) approximately 100–120 km above an eastward-dipping subducting plate. The orientation of the Troodos spreading axis relative to the former trench could be explained if the Troodos Ophiolite formed in a fore-arc position by subduction initiation at a transform fault. However, the lack of glasses with fore-arc basalt composition, and similarities between the trace element compositions of Troodos glasses and those from the Fonualei basin and northern Lau Basin in the southwest Pacific suggest that the Troodos Ophiolite formed in a ridge-trench-trench or ridge-trench-transform triple junction setting, at a back-arc spreading centre that propagated into arc and fore-arc crust.Peer reviewe

Chemical Evolution of Calc-alkaline Magmas during the Ascent through Continental Crust: Constraints from Methana, Aegean Arc

M1 - egaa036Quaternary calc-alkaline andesitic to dacitic lavas effusively erupted on top of about 30 km thick accreted continental crust at Methana peninsula in the western Aegean arc. We present new data of major and trace element concentrations as well as of Sr-Nd-Pb isotope ratios along with mineral compositions of Methana lavas and their mafic enclaves. The enclaves imply a parental basaltic magma and fractional crystallization processes with relatively little crustal assimilation in the deep part of the Methana magma system. The composition of amphibole in some mafic enclaves and lavas indicates deeper crystallization at similar to 25km depth close to the Moho compared with the evolved lavas that formed atPeer reviewe

A Comparison of the Magmatic Evolution of Pacific Intraplate Volcanoes: Constraints on Melting in Mantle Plumes

The interaction of deep mantle plumes with lithospheric plates is one fundamental concept of plate tectonics. Based on observations mainly made on the Hawaiian volcanoes the compositional evolution of hotspot volcanoes is believed to reflect the variation of partial melting and source composition as the plate moves across the different melting zones of the mantle plume. The model predicts the formation of several magmatic stages that differ in composition. In order to test this model, we compare published compositional and age data from the intraplate volcanoes of the Hawaii, Society, Marquesas and Samoa hotspots on the older part of the Pacific Plate. The compiled data indicate that most volcanoes display variations within and between several magmatic series, and in most cases the more evolved lavas are associated with the voluminous shield stage. The Hawaiian volcanoes show up to four different series ranging from tholeiites to nephelinites/melilitites, whereas the other hotspots mainly erupt two magmatic series consisting of transitional basalts and basanites. Submarine preshield stages at the Society and Marquesas hotspots resemble those observed at Hawaii. The large variation of primitive magmas in the Hawaiian plume as opposed to the other Pacific intraplate systems may reflect the higher temperatures, higher buoyancy flux, and extreme chemical heterogeneity at Hawaii. The shield stage activity at all four hotspots lasts for 1 million years indicating similar widths of the melting zone, although the temperatures of the distinct mantle plumes vary considerably. The relatively depleted shield stage magmatism typically overlaps by ~200 kyrs with the formation of the more enriched postshield magmas indicating that the two melting and magma ascent systems exist contemporaneously

On the chances and challenges of combining electron-collecting nPOLO and hole-collecting Al-p+ contacts in highly efficient p-type c-Si solar cells

ISFH is following a distinct cell development roadmap, which comprises—as a short-term concept—the combination of an n-type doped electron-collecting poly-Si on oxide (POLO) junction with an Al-alloyed p+ junction for hole collection. This combination can be integrated either in front- and back-contacted back junction cells (POLO-BJ) or in interdigitated back-contacted cells (POLO-IBC). Here, we present recent progress with these two cell concepts. We report on a certified M2-sized 22.9% efficient POLO-BJ cell with a temperature coefficient TCη of only −(0.3 ± 0.02) %rel/K and a certified 23.7% (4 cm2 d.a.) efficient POLO-IBC cell. We discuss various specific conceptual aspects of this technology and present a simulation-based sensitivity analysis for quantities related to the quality of the hole-collecting alloyed Al-p+ junction which are subject to continuous improvement and thus hard to predict exactly. We report that the measured pseudo fill factor values decrease more due to metallization than would be expected from recombination in the metallized regions with an ideality factor of one only. The gap to pseudo fill factor values that are theoretically achievable at the respective open-circuit voltages is 1.1%abs (Ga-doped wafer) for POLO-IBC and 1.4%abs (B-doped wafer) to 2%abs (Ga-doped wafer) for POLO-BJ. With an embedded blocking layer for Ag crystallites in the poly-Si, we present a concept to reduce this gap

Impact of the contacting scheme on I-V measurements of metallization-free silicon heterojunction solar cells

I-V measurements are sensitive to the number and positioning of current and voltage sensing contacts. For busbarless solar cells, measurement setups have been developed using current collection wires and separate voltage sense contacts. Placing the latter at a defined position enables a grid resistance neglecting measurement and thus I-V characteristics independent from the contacting system. This technique has been developed for solar cells having a finger grid and good conductivity in the direction of the fingers. The optimal position of the sense contact in case of finger-free silicon heterojunction solar cells has not yet been studied. Here, the lateral charge carrier transport occurs in a transparent conductive oxide layer resulting in a higher lateral resistance. We perform finite difference method simulations of HJT solar cells without front metallization to investigate the impact of high lateral resistances on the I-V measurement of solar cells. We show the high sensitivity on the number of used wires for contacting as well as the position of the sense contact for the voltage measurement. Using the simulations, we are able to explain the high difference of up to 7.5% in fill factor measurements of metal free solar cells with varying TCO sheet resistances between two measurement systems using different contacting setups. We propose a method to compensate for the contacting system to achieve a grid-resistance neglecting measurement with both systems allowing a reduction of the FF difference to below 1.5%