Konstanz, Germany
Konstanz, Germany

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Modanese C.,Norwegian University of Science and Technology | Di Sabatino M.,Sintef | Soiland A.-K.,Elkem ASA | Peter K.,ISC Konstanz | Arnberg L.,Norwegian University of Science and Technology
Progress in Photovoltaics: Research and Applications | Year: 2011

In this work, the effect of the concurrent presence of B and P on bulk and solar cell properties of directionally solidified multicrystalline ingots from commercially compensated solar grade silicon (SoG-Si) feedstock produced by Elkem Solar was investigated. The initial B and P content prior to the directional solidification experiment was 1260 and 762 ppba, respectively. Two reference ingots have been solidified in a silica crucible from 100% electronic grade silicon (EG-Si) feedstock, with 332 ppba of boron added. All ingots have been cast under similar process parameters. The resistivity measurements by Four Point Probe (FPP) are in good agreement with the net dopant content, i.e., NA - ND for p-type material, measured by Glow Discharge Mass Spectrometer (GDMS). Bulk lifetime measurements show a decrease in the values compared to the EG reference. Lifetime distributions show the highest values of 13 and 19 μs at approximately half ingot height, compared to 30 and 44 μs in the reference ingots. This decrease can be due to the concurrent effect of compensation and of other impurities present in the ingot. However, the content of several transition metals measured by GDMS at half ingot height was not significantly higher than that of the reference ingots. Oxygen content as measured by Fourier Transform Infra-Red (FTIR) spectroscopy shows no significant difference compared to the references. Solar cells made from the compensated ingots and processed under standard process conditions show efficiency values up to 15.5% and fill factor values up to 78%, comparable to conventional multicrystalline silicon cells. Copyright © 2010 John Wiley & Sons, Ltd.

Urrejola E.,ISC Konstanz | Peter K.,Sunways AG | Plagwitz H.,Sunways AG | Schubert G.,Sunways AG
Applied Physics Letters | Year: 2011

We show that the lateral spread of silicon in a screen-printed aluminum layer increases by (1.50±0.06) μm/°C, when increasing the peak firing temperature within an industrially applicable range. In this way, the maximum spread limit of diffused silicon in aluminum is predictable and does not depend on the contact area size but on the firing temperature. Therefore, the geometry of the rear side pattern can influence not only series resistance losses within the solar cell but the process of contact formation itself. In addition, too fast cooling lead to Kirkendall void formations instead of an eutectic layer. © 2011 American Institute of Physics.

Cabrera E.,ISC Konstanz | Olibet S.,ISC Konstanz | Rudolph D.,ISC Konstanz | Vullum P.E.,Sintef | And 5 more authors.
Progress in Photovoltaics: Research and Applications | Year: 2015

Good quality contacts between metal and silicon emitter are crucial for high crystalline solar cell efficiencies. We investigate the impact of defects originating from electrically inactive phosphorus on contact formation within silver thick film metallized silicon solar cells. For this purpose, emitters with varying sheet resistance, depth, and dead layer were metallized with silver pastes from different generations. Macroscopic contact resistivity measurements were compared with the microscopic contact configurations studied by scanning electron microscopy. The density of direct contacts between Ag crystallites grown into Si and the Ag finger bulk is essential for low contact resistivity. The presence of glass-free regions needed for such direct contacts depends on the paste composition and on the surface texture, and does not vary with the Si emitter properties. Indeed, the decrease in contact resistivity correlates with increasing density of Ag crystallites embedded in the Si surface. Furthermore, the density of Si surface-embedded Ag crystallites scales proportional to the electrically inactive P and is independent of the sheet resistance. Using the newest silver paste, the Ag crystallite density is independent of the emitter doping, but the Ag crystallite size increases as a function of the thickness of the dead layer. Transmission electron microscopy characterization of the excess P-doped Si crystal lattice shows that significant strain and Si bond weakening may play a major role for both Ag crystallite nucleation and growth. Finally, we studied Si crystal defects by metallizing nanocracks, dislocations, and grain boundaries and found that Ag crystallite nucleation is defect-property dependent. Copyright © 2013 John Wiley & Sons, Ltd.

Urrejola E.,ISC Konstanz | Peter K.,ISC Konstanz | Plagwitz H.,Sunways AG | Schubert G.,Sunways AG
Journal of Applied Physics | Year: 2010

For high efficiency silicon solar cells, the rear surface passivation by a dielectric layer has significant advantages compared to the standard fully covered Al back-contact structure. In this work the rear contact formation of the passivated emitter and rear cell device structure is analyzed. Contrary to expected views, we found that the contact resistivity of fine screen printed Al fingers alloyed on narrow p-type Si areas depends on the geometry of the Al-Si alloy formation below the contacts, and decreases by reducing the contact area, while the contact resistance remains constant. At the solar cell level, the reduction in the contact resistivity leads to a minimization of the fill factor losses. At the same time, narrow Al-Si alloy formations increased the passivated area below the contacts, improving the optical properties of the rear side, reducing the short-circuit current and open-circuit voltage losses. Scanning electron microscopy analysis of the Al-Si alloy geometry is performed, in order to understand its influence on the contact resistivity. The analysis presented in this article has application in Al-Si alloying processes and advanced solar cells concepts, like back-contact and rear passivated solar cells. © 2010 American Institute of Physics.

Butler K.T.,University of Sheffield | Vullum P.E.,Sintef | Muggerud A.M.,Norwegian University of Science and Technology | Cabrera E.,ISC Konstanz | Harding J.H.,University of Sheffield
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

We present the results of an experimental and atomistic modeling investigation of the silicon/silver (Si/Ag) interfaces found in industrial solar cells. We use small ab initio calculations to parametrize a new interatomic potential for the Si/Ag interaction. This interatomic potential is then validated against larger ab initio calculations as well as the results of previous experimental and theoretical studies of Si/Ag systems. The interatomic potential allows us to perform a large-scale search of the conformational space of Si/Ag interfaces identified from transmission electron microscopy studies. The most favorable geometries thus identified are then used as the input for more accurate ab initio calculations. We demonstrate that the two interfaces which we identify experimentally have significantly different geometric and electronic structures. We also demonstrate how these different structures result in significantly different Schottky barriers at the interfaces. © 2011 American Physical Society.

Lippold M.,TU Bergakademie Freiberg | Buchholz F.,ISC Konstanz | Gondek C.,TU Bergakademie Freiberg | Honeit F.,TU Bergakademie Freiberg | And 2 more authors.
Solar Energy Materials and Solar Cells | Year: 2014

The reactivity of HF(40 wt%)-HNO3(100 wt%)-H2SO 4(97 wt%) etching mixtures towards conventional SiC-slurry and diamond-wire sawn silicon wafers has been studied. Sulfuric acid-rich mixtures exhibit adequate etching rates (r<5 μm min-1) and generate homogenously distributed, small etching pits on both types of silicon wafers. Textured wafer surfaces were characterized by means of scanning electron microscopy (SEM), laser scanning microscopy (LSM), surface roughness analyses and reflectivity studies. The surface roughness is influenced by the etch depth and the type of saw damage. Etching in sulfuric acid-rich mixtures significantly reduces the reflection of SiC-slurry sawn wafers and, in particular, of diamond-wire sawn wafers. The reflection of etched silicon surfaces is discussed in terms of etch depth and surface roughness. Compared to the conventional HF-HNO3-H2O etching process, multicrystalline silicon (SiC-slurry and diamond-wire sawn) based solar cells texturized by sulfuric acid-rich mixtures exhibit increased efficiencies. © 2014 Elsevier B.V.

Lohmann M.,ISC Konstanz | Wefringhaus E.,ISC Konstanz
Energy Procedia | Year: 2013

In this paper, alkaline textured surfaces are described by means of statistical methods to evaluate what can be behind the term homogeneity found in literature. The evaluation of pyramid homogeneity is described in respect of the pyramid tip positions in the plane by testing against complete spatial randomness. The basic statistical methods used are described and a comparison with microscopic geometrical features of the pyramids is discussed. The statistical analysis yields appropriate classifications allowing for quantitative judgment of whether a surface has a regular (" homogeneous") distribution of pyramids or not. © 2013 The Authors.

Wilson M.,Semilab SDI LLC | Edelman P.,Semilab SDI LLC | Lagowski J.,Semilab SDI LLC | Olibet S.,ISC Konstanz | Mihailetchi V.,ISC Konstanz
Solar Energy Materials and Solar Cells | Year: 2012

Excess carrier photoconductance decay lifetime, measured under small perturbation conditions imposed on steady-state generation, offers an attractive and parameter free alternative to quasi-steady-state photoconductance, QSSPC. A recent version of this technique referred to as QSS-μPCD is based on microwave reflectance PCD monitoring. For this technique, it is critically important to maintain a mono-exponential decay over a large range of steady-state light intensity. Toward that goal we present QSS-μPCD with stringent quality of decay control, QDC. The quality of decay parameter, QD (ideally QD=1) measures the direction and magnitude of departures from an ideal exponential transient and enables tuning toward an optimal range of experimental variables, both apparatus and wafer dependent, whereby QD is within 1±Δ where Δ defines the QDC limits. Within QDC limits, the small perturbation effective decay lifetime, τ eff.d, enables accurate determination of important silicon PV parameters, up to about 25 suns, including J 0 and the steady-state lifetime, τ eff.ss. Two J 0 procedures are compared. The ingenious analytical procedure adopted from Basore and Hansen (1990) [2] enables direct determination of J 0. The second J 0 procedure uses integration of τ eff.d over illumination intensity. The results are self-consistent and they show excellent correlation with Sinton QSSPC results. © 2012 Elsevier B.V. All rights reserved.

Edler A.,ISC Konstanz | Mihailetchi V.D.,ISC Konstanz | Koduvelikulathu L.J.,ISC Konstanz | Comparotto C.,ISC Konstanz | And 2 more authors.
Progress in Photovoltaics: Research and Applications | Year: 2015

In this study, we investigate the metallization-induced recombination losses of high efficiency bifacial n-type and p-type crystalline Si solar cells. From the experimental data, we found that the most efficiency limiting parameter by the screen-printed metallization is the open-circuit voltage (VOC) of the cells. We investigated the mechanism responsible for this loss by varying the metallization fraction on either side of the cell and determined the local enhancement in the dark saturation current density beneath the metal contacts (J0(met)). Under optimum fabrication conditions, the J0(met) at metal-p+ (boron) emitter interfaces was found to be significantly higher compared with the values obtained for metal-n+ emitters. A two-dimensional simulation model was used to get further insight into the recombination mechanism leading to these VOC losses. The model assumes that metal contacts penetrate (or etch) into the diffused region following the firing process and depassivate the interface. Applying this model to our n-type solar cells with a boron p+ emitter, we demonstrated that the simple loss of passivated area beneath the metal contact cannot explain the degradation observed in the VOC of the cell without considering a significant etching or metal penetration into the emitter region. Copyright © 2014 John Wiley & Sons, Ltd.

Duran C.,ISC Konstanz | Deuser H.,Neonsee GmbH | Harney R.,ISC Konstanz | Buck T.,ISC Konstanz
Energy Procedia | Year: 2011

Being interested in bifacial and not fully covered rear contact (e. g. back contact) silicon solar cells and their rear side importance, we have studied how the sample holders add an external current mainly due to the reflectance properties of their surface and we have found that this influence can be higher than one percent in JSC. In a second approach, an innovative measurement setup configuration is presented, which includes a simultaneous front-rear illumination. For this configuration we have measured two types of bifacial solar cells which can be distinguished by their ratio of front to rear performance and we have found differences in power output of about thirty percent if the rear illumination is applied or not. Modules with different back sheets were further manufactured using these types of bifacial solar cells. Outdoor measurements for modules with transparent back sheets demonstrated an average gain in power output of up to twenty percent if the module was placed on a highly reflecting surface and scattered light penetrated the module from the rear side. A set of mini modules was also tested indoors to show how the back sheet influences the reflection as well as the spectral resolved response of the devices. © 2011 Published by Elsevier Ltd.

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