Publications Search Results

Now showing 1 - 10 of 66
  • Publication
    Enabling the In-Situ Stress and Temperature Measurement by Silicon Solar Cell Integrated Stress and Temperature Sensors for Photovoltaic Modules
    ( 2020)
    Beinert, A.J.
    ;
    Imm, M.
    ;
    Benick, J.
    ;
    Becker, F.
    ;
    Seitz, S.
    ;
    Heinrich, M.
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    Paul, O.
    ;
    Glunz, S.W.
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    Aktaa, J.
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    Eitner, U.
    ;
    Neuhaus, H.
    We propose silicon solar cell integrated stress and temperature sensors as a new approach for the stress and temperature measurement in PV modules. The solar cell integrated sensors enable a direct and continuous in-situ measurement of mechanical stress and temperature of solar cells within PV modules. In this work, we present a proof of concept for stress and temperature sensors on a silicon solar cell wafer. Both sensors were tested in a conventional PV module setup. For the stress sensor, a sensitivity of (-47.41 ± 0.14) %/GPa and for the temperature sensor a sensitivity of (3.557 ± 0.008) × 10-3K-1 has been reached. These sensors can already be used in research for increased measurement accuracy of the temperature and the mechanical stress in PV modules due to the implementation at the precise location of the solar cells within a laminate stack, for process evaluation, in-situ measurements in reliability tests and the correlation with real exposure to climates.
  • Publication
    Silicon Solar Cell-Integrated Stress and Temperature Sensors for Photovoltaic Modules
    ( 2020)
    Beinert, A.J.
    ;
    Imm, M.
    ;
    Benick, J.
    ;
    Becker, F.
    ;
    Seitz, S.
    ;
    Heinrich, M.
    ;
    Paul, O.
    ;
    Glunz, S.W.
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    Aktaa, J.
    ;
    Eitner, U.
    ;
    Neuhaus, H.
    We propose silicon solar cell-integrated stress and temperature sensors as a new approach for the stress and temperature measurement in photovoltaic (PV) modules. The solar cell-integrated sensors enable a direct and continuous in situ measurement of mechanical stress and temperature of solar cells within PV modules. In this work, we present a proof of concept for stress and temperature sensors on a silicon solar cell wafer. Both sensors were tested in a conventional PV module setup. For the stress sensor, a sensitivity of (−47.41 ± 0.14)%/GPa has been reached, and for the temperature sensor, a sensitivity of (3.557 ± 0.008) × 10−3 K−1 has been reached. These sensors can already be used in research for increased measurement accuracy of the temperature and the mechanical stress in PV modules because of the implementation at the precise location of the solar cells within a laminate stack, for process evaluation, in‐situ measurements in reliability tests, and the correlation with real exposure to climates.
  • Publication
    Simultaneous Contacting of Boron and Phosphorus Doped Surfaces with a Single Screen Printing Paste
    ( 2019)
    Huyeng, J.
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    Spribille, A.
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    Garcia Prince, M.
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    Rendler, L.C.
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    Ebert, C.
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    Eitner, U.
    ;
    Clement, F.
    The contact formation by screen printed metal pastes is widely employed in standard solar cell production. To expand the use of screen printed electrodes to n-type solar cells, both boron and phosphorus doped surfaces need to be contacted. To do so with a single material has some advantages especially for IBC solar cells. In this study we test four different screen printing pastes on different boron and phosphorus dopings and in combination with different silicon nitride thicknesses. Phosphorus doping could be contacted over a wide range of sheet resistances, nitride thicknesses and fast firing conditions, leaving much freedom to target the boron contacts. Boron dopings are successfully contacted with all materials, if no capping silicon nitride layer was present. With silicon nitride capping an AgAl and an Ag paste are found to be suitable choices. The lowest contact resistivities with 100 nm SiNX capping determined in this study are C = 0.5 mΩ cm² on phosphorus (Ag) and C = 1.8 mΩ cm² on boron (Ag) doping with one single paste. These results enable highly efficient homojunction IBC cells at low cost.
  • Publication
    Thermochemical Stress in Solar Cells: Contact Pad Modeling and Reliability Analysis
    ( 2019)
    Rendler, L.
    ;
    Romer, P.
    ;
    Beinert, A.
    ;
    Stecklum, S.
    ;
    Kraft, A.
    ;
    Eitner, U.
    ;
    Wiese, S.
    This study analyses thermomechanical stresses in silicon solar cells after the soldering process by finite element modeling. An experimentally validated model shows compressive and tensile stresses, longitudinal and transversal to a busbar or a pad row on the surface of a silicon solar cell. The impact of the interconnector segments at and in between two solder pads was investigated and characteristic locations of maximum stress were identified. In addition, the influence of the layout of the contact metallization on the thermomechanical stress was identified by geometry variations to reveal design guidelines that lead to reduced thermomechanical stress in a solar cell after the soldering process. The model results reveal maxima of the tensile stress located at the outermost contacts. Furthermore, a significant influence of the distance between the outermost contact areas and the solar cell edge was determined; with decreasing distance, the compressive stress maxima are higher, but in contrast the more critical tensile stress maxima decrease. For connected pad rows tensile stress maxima are larger compared to single pad connection, which shows the influence of the interconnector segments in between the solder joints of a pad row. After several stages of thermal cycling, electroluminescence measurements showed, in compliance with the model results, contact damages, mainly at the outermost contacts. Furthermore, connected pad rows revealed a steadily growing amount of damaged contacts, whereas single solder joints showed no defects up to 400 thermal cycles.
  • Publication
    Enabling the Measurement of Thermomechanical Stress in Solar Cells and PV Modules by Confocal Micro-Raman Spectroscopy
    ( 2019)
    Beinert, A.
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    Büchler, A.
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    Romer, P.
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    Haueisen, V.
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    Rendler, L.C.
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    Schubert, M.C.
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    Heinrich, M.
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    Aktaa, J.
    ;
    Eitner, U.
    Understanding the origin of thermomechanical stress in solar cells is a key factor to extend the lifetime of photovoltaic modules. However, the methods to determine the stress are very limited. With the confocal micro-Raman spectroscopy, we present a contactless method, which is able to measure through the front glass and is well-known in the field of microelectronics. One major challenge for the measurement on crystalline silicon solar cells and modules is the surface texturization of the mono crystalline solar cell, which changes the topology from a plain (100) surface to pyramids with (111) flanks and (100) valleys. We develop a procedure to cover the challenges arising from this topology, namely the inhomogeneous stress distribution on the pyramid flanks and the different crystal planes of the phonon vibrations and the photon back scattering. By studying the procedure on a reference system, we determine a factor for the conversion of a micro-Raman peak shift to stress of e=-(833 +- 49) MPa/rel cm-1. The presented measurements show that the factor holds for uniaxial stress, biaxial stress as well as the stress states occurring from the PV module production processes. We then apply the procedure to measure the stress from soldering 156 × 156 mm2 solar cells and the lamination. We obtain (− 21 ± 2) MPa for the stress in the unsoldered solar cells, which arise from the cell production steps, like metallization. After soldering, we measure (− 26 ± 3) MPa and after lamination (− 53 ± 6) MPa. Additionally we perform a line scan along the cell diagonal and area scans of the quarter cell as well as the end of one busbar. All results match well with a simulation of stress induced by the soldering process and lamination using the finite element method.
  • Publication
    Rapid Calculation of the Backsheet Coupling Gain Using Ray Groups
    ( 2018)
    Pfreundt, A.
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    Mittag, M.
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    Heinrich, M.
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    Eitner, U.
    Internal reflections within photovoltaic modules are known to contribute to power gains from cell to module. The module rear cover, usually a white backsheet, is one module component reflecting additional light onto the solar cell. A novel approach to model the effect of backsheet reflectance on the achievable coupling gain in solar modules is presented. Using a discrete ray optics approach, results can be calculated rapidly for arbitrary reflectance distributions using a partition of the emerging rays into groups. The model is fully wavelength resolved, using measured data to model optical material properties. It is therefore suitable for arbitrary material stacks in front of and behind the solar cell with a single diffusely scattering layer. We study the impact of layer thicknesses, incidence angle and distribution function on the coupling gain using the presented approach. Comparison to measurements of the coupling gain using single cell modules shows good agreement with the calculated results.
  • Publication
    Influence of Interconnection Concepts for IBC Solar Cell Performance by Simulation
    ( 2018)
    Huyeng, J.D.
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    Spribille, A.
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    Rendler, L.C.
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    Ebert, C.
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    Eitner, U.
    ;
    Keding, R.
    This work describes interdigitated back-contact cells with a number of different rear side geometries, for different interconnection concepts and module integration, by means of numerical simulations. We show that a simple interconnection concept can be realized with copper wires as bus features and interrupted metal electrodes to avoid shunting, without severe losses compared to multilayer metallization concepts. Using Quokka3, which due to enhanced speed allows for very large simulation sizes, this enables principal investigations undescribed in previous literature. We use this to investigate the disconnection of a single (or multiple) solder joint(s) in terms of device performance, in the case of interrupted metal electrodes. Our findings show that disconnected emitter electrodes cause higher power losses than disconnected BSF electrodes (∼x4), both following a linear relationship. Nevertheless, when multiple of such defects are aligned, the losses are increasing much stronger. We accordingly derive the need to balance design choices such as BSF and emitter width in an industrial implementation, with an empirically derived disconnection probability.
  • Publication
    Analysis of Grain-Size Distribution and Yield Strength of Interconnector Ribbons and Wires at Different Streching Conditions Using Color Etching
    ( 2018)
    Walter, J.
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    Stegmaier, J.
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    Kraft, A.
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    Eitner, U.
    In this paper we analyze the microstructure of solder coated copper ribbon and wire interconnectors for silicon solar cells from different manufacturers at different stretch levels in longitudinal and cross microsections by color etching and microscopy. The used etchant colors each grain according to its crystallographic orientation. This enables the extraction of the grain size and frequency information by image and data processing tools. Furthermore we measure the yield strength of the solar cell interconnectors for strain levels of 0.5 %, 5 % and 10 % and evaluate the impact on the microstructure of the interconnector. We find a large variation in the copper microstructure, especially for wires and observe an inverse relation between yield strength and grain sizes corresponding to the Hall-Petch relation. The lowest measured yield strength for a wire interconnector is about 82 MPa (avg. grain size: 237 mm²), which is about 20 MPa higher compared to the lowest yield strength measured for ribbons (avg. grain size: 247 mm²). The wire with the highest yield strength of 148 MPa shows fine grains (avg. grain size: 29 mm²). In the ribbon analysis we find the same overall correlation between grain size and yield strength with some exceptions. This underlines that grain size distribution is not the only attribute which affects the yield strength. The analysis of copper ribbons at different stretch levels discloses a deformation or refinement of the copper grains associated with rising yield strength. In general the results show that the approach of a color etching, optimized for solar cell interconnector cross and longitudinal sections, is a suitable, fast and cost-effective solution to quantify the grain size distribution and evaluate mechanical impacts like stretching or bending on the copper microstructure.
  • Publication
    Wave-Shaped Wires Soldered on the Finger Grid of Solar Cells: Solder Joint Stability under Thermal Cycling
    ( 2018)
    Rendler, L.
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    Haryantho, A.P.
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    Walter, J.
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    Huyeng, J.
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    Kraft, A.
    ;
    Wiese, S.
    ;
    Eitner, U.
    In this work the reliability of silicon solar cells interconnected by wires soldered directly on the contact fingers of the front side grid is analyzed in detail. The interconnection of busbarless solar cells enables significant silver reduction. We use solder coated wave-shaped wires to reduce thermomechanical stress in the solder joints, which results in minimized cell bowing. Consequently, this interconnection concept is especially suitable for the interconnection of back-contact solar cells. We analyze the mechanical and electrical properties of wave-shaped wires with different amplitudes by measuring length and electrical resistance, and by performing standard tensile tests. Furthermore, we manufacture 8 one-cell modules, 4 with full scale 156 x 156 mm2 solar cells and 4 with half cells, as well as a module including a string of three half cells using semi-automatic infrared soldering and a conventional lamination process. Subsequently, the one-cell module samples undergo thermal cycling and are characterized by EL imaging and IV measurements to detect solder joint defects. After temperature cycling we determine additional solder joint failures, mainly in areas with initial defects and at the solar cell edges. This confirms our assumption that most defects occur at the cell edges, where the thermomechanical stress maxima are located. However, all module samples show a maximum relative power loss of <3% after 200 temperature cycles and except one module all samples show a power loss of <5% after 400 temperature cycles. This demonstrates the feasibility of our interconnection approach based on soldering wave-shaped wires on the finger grid or small contact pads of silicon solar cells.
  • Publication
    Optimization of electrically conductive adhesive bonds in photovoltaic modules
    ( 2018)
    Geipel, T.
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    Meinert, M.
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    Kraft, A.
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    Eitner, U.
    Gluing ribbons to silicon solar cells by using electrically conductive adhesives (ECAs) is an alternative interconnection technology for module integration to the state-of-the-art soldering process. We reveal cost reduction potentials by analyzing the influence of volume and contact resistivity, as well as the bond design of ECAs on the fill factor of photovoltaic modules. Solar cells with structured busbars are considered in the analysis. The volume resistivity is controlled by the cure temperature. We contact individual cells at different curing conditions and measure their fill factors. A volume resistivity of 1 × 10 -2 O · cm does not cause a significant fill factor reduction compared with an ECA with around 1 × 10 -4 O · cm. The contact resistivity is varied by using different ribbon coatings. Ag and Sn coatings achieve almost identical fill factors. A bare Cu surface reduces the fill factor. We lower the consumed ECA from 40 to 3 mg/cell by modifying the bond design. A design with 16 mg/cell achieves similar fill factors as with 40 mg/cell. A finite-element model is developed to study the combined influence of electrical properties and the bond design. We propose an optimized contact design for high fill factors and reduced material consumption.