Scientific Highlight of the Month
Fraunhofer Center for Silicon Photovoltaics CSP
5/2016: Characterization of Structured Wires for Wire Sawing Processes
Compared to straight sawing wires the geometry of structured wires causes a significant positive effect on the wafering process. In order to optimize the cutting performance it is crucial to characterize their specific geometry and its mechanical behaviour under tensile load. The structured wire is formed by two periodical bending shapes (see Figure 1), which have to be characterized separately. For this purpose, Fraunhofer Center for Silicon Photovoltaics CSP provides measuring equipment and evaluation software, which enables the investigation of wire samples at different forces representing the tensile loading during the sawing process.
The wire surface geometry is detected optically with a sensitivity of 1µm and a lateral resolution of 1 µm. The surface scan includes all geometrical information of the wire based on the manufacturing process. In particular, a mathematical algorithm was developed to extract sawing process related wire parameters from the surface scan data.
Important wire parameters are:
- wire core diameter
- amplitudes and periods of the two wire bending shapes
- torsion of the wire
- angle between bending planes
- position probability of the wire center point
Thus, using results of various tensile loads allow an accurate assessment of the wire structure. Furthermore, a Finite-Element model of the wire was created in order to predict the non-linear mechanical behaviour of the wire and the change of its geometrical structure (see Figure 2). The model considers the plastic deformation (hardening) of the wire material due to bending during manufacturing as well as in tensile loading during the experiment. Thus, the experiment and the simulation model enable a systematic investigation of different wire structures and diameters in order to improve the structure design and wire parameters for optimal sawing performance.
3/2016: Microstructural analysis of the cementation process during soiling on glass surfaces in arid and semi-arid climates
To investigate the soiling behavior of solar energy systems like photovoltaics or concentrated solar power, glass samples were exposed to outdoor conditions in Doha, Qatar for one month. Soil formation on the glass was characterized at microstructural level using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Further, elemental analysis of the crust was done with energy dispersive X-ray spectroscopy (EDX). Small fibrous structures were found on the glass surface and dust particles, providing evidence of a cementation process leading to a strong adhesion of airborne dust particles. In contrast to the common perception, that cementation occurs via the precipitation of salt (sodium chloride) these needle structures were found to be mainly composed of oxides of Si, Mg and Al. This indicates that cementation processes in desert regions are enhanced by the growth of fibrous clay minerals.
2/2016: Root causes of defects on PERC solar cells
Solar cells with a full aluminum back-contact which have a dominant market share at the moment will be replaced by PERC-solar cells in the next few years. PERC solar cell technology improves significantly the efficiency of solar cells by replacing the full aluminum rear-contact by a rear side passivated design with local rear contacts. Caused by the improved design fundamental new cell defects arise and therefor new criteria for quality assessment are needed. Defects can arise during production processes like the “PERC void formation” where large cavities are formed in the metallized local rear contact. Furthermore the long term stability can be affected by new degradation mechanisms like the light-induced degradation “LID” on new multi-crystalline PERC solar cells. Electrical and microstructural characterization techniques have been combined at the Fraunhofer CSP for the root cause analysis of the defects. Thereby, new and reliable methods for a profound assessment of PERC solar cells are now available.
For the proof of „voids“ in a solar cell, fast and non-destructive tests have been successfully proven. These tests allow the quantification of voids in a solar cell and the assessment of the void related electrical losses. Another test has been developed which proves the local rear contact formation regarding the distribution of the back-surface field (BSF) over extended areas. The BSF is important for a low-loss passivation of the rear contact and can be degraded due to void formation.
Beside the production-related defects of PERC solar cells, the long-term reliability under realistic environmental conditions is crucial. At the Fraunhofer CSP a patent pending method has been developed for the fast assessment of LID. This method allows the quantification of different phases of the degradation and regeneration processes. Predictions regarding the long-term stability of solar cells can be deduced.
 S. Großer, M. Werner, C. Hagendorf; Microstructure of Void Formation Stages at Local Rear Contacts
Energy Procedia 77 (2015) 701 - 706
 S. Großer, S. Eiternick, M. Turek;Non-Destructive Characterization and Microscopic Properties of Rear Contact Voids in PERC Cells 31st European Photovoltaic Solar Energy Conference and Exhibition (2015) ISBN 3-936338-39-6
6/2015: Influence of Grain Boundaries on Strength of Multicrystalline Silicon Wafers
With a market share of nearly 66%, multicrystalline silicon (mc-Si) wafers are widely used substrates for the production of solar cells. However, besides the fact, that their production costs are lower than those for monocrystalline wafers, they also tend to have a lower mechanical strength. That reduced strength, which is substantially determined by cracks induced in the surface, leads to an earlier breakage and, consequently, to a reduced economic efficiency in manufacturing solar cells.
Grain boundaries are suspected as a possible reason for that, because they represent crystal defects and, thus, potential mechanical weak spots. Researchers of the Fraunhofer CSP have now experimentally investigated the mechanical influence of grain boundaries on crack propagation in mc-Si wafers using the micro indentation technique. Thereby, the investigation has focused on crack shapes that are typically caused during wafer processing through multi-wire sawing. Statistical studies of more than 280 indentation events in various regions (on, near and far away from grain boundaries, see Fig. 1a) showed that neither the energetically more favorable symmetrical twin boundaries nor grain boundaries between highly asymmetrically to each other oriented grains have an influence on the crack propagation in mc-Si wafers. Even cracks, which were positioned directly in grain boundaries, propagated in an unimpeded manner next to and parallel to grain boundaries or even crossed them. They did not show any affinity to propagate in grain boundaries (see Fig. 1b). Instead, the grains themselves with their anisotropic mechanical properties and crack distributions seem to reduce the strength. Thus, these results provide a further contribution in acquiring a profound mechanical knowledge of multicrystalline silicon wafers that are getting thinner and thinner.
5/2015: Finite Element Analysis of Module Substructures
The continuous price decline of solar modules, share in overall cost reduced from 2/3 to below 1/3 in some cases, leads to increased focus on other components (mounting frame, inverter, electrical work and equipment, assembly & scaffold) of a PV-System. Fraunhofer CSP with its expertise in finite element simulation offers the opportunity to analyse substructures and their interaction with the PV modules. This opens possibilities for optimization regarding cost of materials.
For the simulation models with different levels of detail are set up. The deformation of the whole mounting frame with the modules is simulated under different load scenarios, like temperature, snow or wind loads to determine the stress of the various components. Subsequent detailed simulations allow in-depth examination of e.g. mounting points and screwed connections. The objective is to find cost-effective solutions with standard parts and minimal material usage to further reduce overall costs. These models allow the investigation of loads on the modules in interaction with the substructure. In contrast to standard tests the flexibility of the mounting frame is considered, which may lead to a different load situation (e.g. by additional torsion) and can be incorporated in the analysis of cell breakage or other defects. With help of fluid dynamic simulations wind loads in the field can be further investigated. The inhomogeneous pressure distribution under real conditions might be more severe for the modules and should be considered for reliable designs.
S. Dietrich, U. Zeller, M. Pander, M. Ebert, „Evaluation of Non-Uniform Mechanical Loads on Solar Modules“, 39th IEEE, (2013), 2998-3003 DOI: 10.1109/PVSC.2013.6745093
4/2015: New insights into ultrathin oxide layers
In the course of developing modern silicon solar cell concepts towards higher efficiencies, interfacial properties and the quality of passivation layers are becoming increasingly important. In order to understand and evaluate the function of the used ultrathin oxide layers, reliable analytical methods for structural and chemical characterization are needed. Performing these analyses on nanometer scale is in particular challenging. This was realized at the Fraunhofer CSP in the Group Diagnostics of Solar Cells by various high-resolution analysis methods investigating the rear side oxide layer occurring for example in the TOPCon cell concept which was developed at the Fraunhofer ISE. Thus, among other things, various oxygen compounds were detected within the passivation layer. It was evaluated how a temperature annealing step affects the oxide layer structure and interaction with the silicon crystal lattice. The results were also convincing at the SiliconPV 2015 conference in Konstanz. From 200 contributions, the article »High-resolution Structural Investigation of passivated Interfaces of Silicon Solar Cells« written by Susanne Richter and her colleagues in cooperation with the Fraunhofer ISE has been voted among the top ten and received the SiliconPV 2015 award. These results as well as further research activities will help customers in the solar industry with new technology to benefit from these insights and to accordingly optimize the development of their cells.
 F. Feldmann, M. Bivour, C. Reichel, M. Hermle, S. W. Glunz, »Passivated rear contacts for highefficiency n-type Si solar cells providing high interface passivation quality and excellent transport characteristics«, Solar Energy Materials and Solar Cells, 120, 270-274 (2014)
 S. Richter, K. Kaufmann, V. Naumann, M. Werner, A. Graff, S. Großer, A. Moldovan, M. Zimmer, J. Rentsch, J. Bagdahn, C. Hagendorf, High-resolution structural investigation of passivated interfaces of silicon solar cells, Solar Energy Materials and Solar Cells, 2015, 10.1016/j.solmat.2015.06.051 (in print)
 A. Moldovan, F. Feldmann, K. Kaufmann, S. Richter, M. Werner, C. Hagendorf, M. Zimmer, J. Rentsch, M. Hermle, Tunnel Oxide Passivated Carrier-Selective Contacts Based on Ultra Thin SiO2 Layers, 42nd IEEE PVSC New Orleans, June, 17th 2015 (in print)
3/2015: Identification of Critical Process Parameters and Improvement of the Wafering Process by Measuring of Mechanical Load
The determination of critical process steps can be done at the Fraunhofer Center for Silicon Photovoltaic CSP by performing wafering and wire wear experiments. The goal of decreasing the process time and consumables could be reached by higher feed speed and less wire consumption. Four load sensors for forces and moments (figure at the top, Fig. 1) are used for an inline force detection in three dimensions to determine process limits. As an example the figure below (Fig. 2) is showing the load in wire movement direction of each sensor. In general, the wafering process can be separated into three parts, the cutting-in and cutting-out process and the main sawing period in the middle of process time. By the analysis of force curves an improvement of the cutting-in and cutting-out process is possible in order to decrease the amount consumables and to increase the wafer surface quality or surface damage. The variation of load during the main sawing process is almost constant.
Any changes of these homogeneous conditions could influence the wafer geometry. Furthermore, the wire wear can be observed according to the position in the wire web because of the position of forces sensors. The wire usage is increasing from sensor 1 to sensor 4. The figure shows a slightly change of forces in wire direction that are depending on a decrease of wire abrasion properties. This combination of different investigations enables an optimization of wire usage and wire consumption. Finally the influence of process parameters can be analyzed regarding the running process itself but also to the wafer specifications. This unique measurement system is used basic research like the abrasion mechanismen of silicon, as well as questions of high-volume production in industry.
2/2015: Listening to Materials – Material Characterization with Ultrasound
Background, Motivation and Objective
Reliability of a photovoltaic module is a key factor for being financially attractive for customers all over the world. The further reduction in manufacturing costs leads to increased demands on module components and their materials to maintain acceptable mechanical yields and module reliability. Thus fast, economic and preferably non-destructive material / component characterization and manufacturing process control methods come more and more into focus.
Statement of Contribution/Methods
In current research ultrasonic based approaches for material characterization of solar cell interconnectors – the typical electrical interconnection of solar cells in a photovoltaic module – are evaluated and compared to conventional electron backscatter diffraction techniques (EBSD) and destructive mechanical testing (tensile tests). The dispersion characteristics can be used to evaluate second order elastic parameters (Young’s modulus, Poisson’s ratio). Attenuation measurements allow an estimation of average grain size and yield strength. Acoustoelastic effects are applied to get more information about the microstructure. With help of these new measurement techniques a non-destructive evaluation of important mechanical cell interconnector properties can be achieved. These properties serve as valuable quality assurance characteristics for interconnector and solar module manufacturing lines.
Guided acoustic wave transmits a solar cell interconnector (Scanning Laser Doppler Vibrometry)
1/2015: Rapid and quantitate determination of organic surface contaminations using TOC and contact angle measurement
Irrespective of the semiconductor type and technology, organic contaminations on wafer surfaces have a detrimental impact on subsequent manufacturing steps and/or the device function. Therefore, it is of great interest to have a quick and reliable tool to detect those contaminations with sufficient sensitivity. Two such methods were developed recently at Fraunhofer CSP. In combination they enable the detection of absolute amount and distribution of organic impurities. By extraction the wafer surface and analysing the total organic carbon (TOC) of the extraction solution an integral quantification of surface organics as low as 0,5 ng per cm2 is possible. This may be complemented by a laterally dissolved determination of the surface energy which is realised by semi-automatic contact angel mapping of the surface. Usually two test solutions like water and diiodomethane are applied displaying different surface tensions. To scan the whole surface, the contact angle of 30 or more single droplets in a defined array are measured using an automated video based optical contact angle instrument. Both methods are rapid and sensitive and can be realised at comparably low costs. Thus, they are useful tools for process and quality control.
Meyer, S.; Timmel, S.; Hagendorf, C., Rapid determination of organic contaminations on wafer surfaces, Solid State Phenomena 219 (2014) 317-319.
10/2014: Laser-generated Microstructures in Glass for Improved Light Management in Solar Modules
Researchers at the Fraunhofer Center for Silicon Photovoltaics CSP and the Hochschule Anhalt have developed a process, which will perspectively enhance the harvesting of sunlight incident on a solar module. This so-called light management has the goal to redirect that part of light to the active areas of solar cells, which is normally being lost for power generation (e.g., by reflection on the metal contacts or absorption in the spaces between the individual solar cells). For this purpose, femtosecond laser pulses have been used, because these are able to write optically functional structures under the surface of the front glass of solar modules by a non-contact and nondestructive process.
As shown schematically in Figure 1, the laser is being focused into the glass volume such that its refractive index is changed persistently in the focus of the laser beam, while the glass surfaces remain completely unchanged. For correct choice of the laser parameters, the glass remains completely transparent (no absorption losses), and no micro-cracks occur, i.e. the mechanical stability of glass remains intact. Therefore, basically arbitrary optical structures can be generated by scanning the laser over the area to be addressed. As an example, Figure 2 shows the effect of a simple, non-optimized diffraction grating (parallel lines in 6µm distance) on blue light: about 50 % of light under normal incidence (lightest point and highest peak in the middle) are diffracted to different propagation angles.
Currently work is in progress to optimize the structures with respect to their »redirection« efficiency. Basically the procedure comprises the ability to machine even complete (and already installed) solar modules, and thus to improve their power yield: due to the very tight focus of the laser and the consumption of a significant part of the pulse energy for the structural change in the glass, a solar cell below the glass will not be damaged during creation of the light management structures.
These results have been obtained by a cooperation between the Hochschule Anhalt and the Fraunhofer Center for Silicon Photovoltaics (CSP) in the framework of a research program funded by the BMBF (Kooperatives Forschungskolleg »StrukturSolar«). This research project is being continued.
Muchow, M.; Büchner, T.; Seifert, G.: Femtosecond Laser-induced optical microstructures inside glass volume for light management in solar modules. Proc. of 29th European Photovoltaic Solar Energy Conference and Exhibition (29th EU PVSEC), 22-26 September 2014, Amsterdam, The Netherlands. DOI: 10.4229/EUPVSEC20142014-1BV.6.57
9/2014: Moth-eye Effect for Crystalline Solar Cells
A nanostructure for crystalline silicon is imitating the surface morphology of moth eyes and is decreasing optical losses in solar cells.
Scientists at Fraunhofer Center for Silicon Photovoltaic CSP and the University of Applied Science Anhalt have developed a nanostructuring for silicon solar cells which is imitating the optical characteristics of moth eyes. In nature, this structure protects the moth from predators. Goal of the technological development is an implementation in current industrial processes. Due to this technology low reflectivity values can be achieved leading to an improved efficiency of the solar cells.
The physical fundamental of moth-eye structures is a surface morphology with widths of 400 nm and smaller. The width of the texture is in the order of the visible light, making the texture works as a so-called effective medium. This effective medium causes a gradual transition of the refractive index between air and silicon. The incident light is almost entirely absorbed.
Since the moth-eye structures are formed by an isotropic plasma etching by SF6 and oxygen, this technology is also suitable for both mono-crystalline and multi-crystalline silicon. Moreover, the plasma etching process is independent of the surface characteristics of the wafer. As a result, the technology is particularly suitable for texturing of diamond wire sawn wafers and wafers from kerf-less technologies.
The next step is to investigate the influence of the moth-eye structures on subsequent process steps in the industrial manufacturing of solar cells. As an example, surface passivation and metallization must be adapted to the moth-eye structure. Therefore, the SiN surface passivation and the cleaning of the nanostructures are being optimized. The goal of this optimization is the complete abandonment of wet chemical process steps, as these represent a cost factor and safety issue in industrial production.
This development is the result of the cooperation of the University for Applied Science Anhalt and the Fraunhofer Center for Silicon Photovoltaics CSP within the framework of the research project StrukturSolar (BMBF 03SF0417A).
8/2014: High-Efficiency Silicon Float-Zone Solar-Cells from Low-Cost Feed-Material
The Float Zone (FZ) method allows the growth of silicon crystals with one to two orders of magnitude lower oxygen concentrations than any other growth method is able to. This makes FZ wafers exceptionally suited for power electronics, where low oxygen is mandatory, but also for high-efficiency solar cells, since no degradation by the formation of boron-oxygen complexes will occur. FZ growth chambers are free of any carbon based parts, reducing the carbon concentration in the grown crystals significantly, compared to e.g. Czochralski crystals. The growth rate itself is about a factor of 2 higher than it is achievable by standard pulling processes. Further, rather uniform axial resistivity profiles can be achieved by doping the ingots during the growth process by the gas phase. The lower energy consumption and the lack of any crucible or hot-zone consumables result in an attractive cost-of-operation profile.
Beside the above listed advantages, certain difficulties have to be mentioned dealing with the FZ process: the specification of the poly-silicon feed rods needed for the FZ process is very tight, the rods have to be crack-free, with a low bow, a smooth morphology, and a diameter in the same range as the anticipated FZ ingot. The market price for high-quality FZ-feed rods is significantly higher than it is the case for standard poly-silicon chunks or granular material.
An attractive alternative to poly-silicon feed rods produced by the Siemens-CVD process (the standard method for poly-silicon FZ feed rod material) is the pre-pulling of feed rods from the melt using a modified, cost-optimized pulling configuration. This approach allows the use of standard solar grade poly-silicon (e.g. broken chunks), nowadays easily available on the market.
Our investigations show both: (1) the efficiency of the resulting solar cells is comparable to commercial electronic-grade Float-Zone material (see figure) while (2) the cost structure of our FZ process based on pre-pulled feed rods is favorable compared to a standard process using poly-silicon feed rods from CVD reactors.
7/2014: Fraunhofer CSP at PVSEC, Amsterdam
On September 22-26 Fraunhofer CSP will be presenting its latest research results in the fields of thin film technologies, diagnostics of solar cells, module manufacturing and new module concepts at EUPVSEC in Amsterdam präsentieren vom auf der EUPVSEC in Amsterdam.
6/2014: Experimental Fatigue Investigation of Solar Cell Interconnectors in Module Laminates
In a crystalline solar module the cells are connected in series with solar cell interconnectors (solder coated copper ribbons). Ribbon fatigue is critical for solar modules because the interconnection has direct influence on series resistance. Thus ribbon breakage leads to power loss, locally increased temperatures and even arcing. Loading of the ribbons results from relative displacement of the solar cells due to temperature changes or mechanical loading. The amount of displacement and frequency depends among other things on climatic region as well as installation conditions and therefore influences the loading of the ribbon and long-term module performance. The opening of new markets in extreme climates (e.g. desert) and the associated different loading conditions requires an enhancement of the only sporadic empirical knowledge on ribbon fatigue in solar industry. Fraunhofer CSP deals with characterization of ribbon materials and ribbon fatigue in solar module laminates with experimental methods and numerical simulation.
The group Module Reliability has developed an assembly for cyclic mechanical testing of small modules in a four point bending test setup. The test setup allows an in-situ current measurement and regular electroluminescence (EL) images during different mechanical load steps. In this manner broken interconnectors are identified and cell breakage can be investigated. With help of finite element simulations the loading profile is designed to achieve the same cell displacement in the cell gaps as the maximum amplitude found in a full size module simulation under ±1000 Pa cyclic loading, which corresponds to the current IEC proposal.
This way ribbon fatigue based on the IEC proposal can be tested in a much faster way, which enables a much more economic comparison of different ribbon materials, geometries and their dependence on module production steps and other module components.
M. Pander, R. Meier, M. Sander, S. Dietrich, M. Ebert, „Lifetime Estimation for Solar Cell Interconnectors“, 28th European Photovoltaic Solar Energy Conference and Exhibition, (2013) 2851-2857 DOI: 10.4229/28thEUPVSEC2013-4CO.10.3
Meier, R.; Pander, M.; Ebert, M.; Mikrostrukturoptimierung von Kupfer durch Wärmebehandlung für die Anwendung in der Photovoltaik; Metall: Fachzeitschrift für Metallurgie 66 (2012) 391-394; 410/2013
5/2014: Rapid PV-Module Component Testing and Quantification of Performance Gains
The »levelized cost of electricity« (LCOE, €/kWh) of photovoltaic modules is determined by the overall costs of the components such as connectors or encapsulants – but also by their contribution to the total generated current and their reliablility.
Besides the direct current contribution of the cells, there is also a contribution of the back-sheet and the connectors to the total current due to internal back-reflection. Another example is the improved angular dependence of the light transmission of anti-reflection coatings on glas. These effects improve the »cell-to-module« power characteristics thus decreasing the LCOE. On the other side, individual componentents can also cause electrical and optical losses thus reducing the power output.
The team »Electrical Characterization«, group »Solar Cell Diagnostics«, has developed a new rapid module component test which results in a quantitative estimate of individual contributions to the module performance. This new approach complements other methods at Fraunhofer CSP that are frequently applied for the loss analysis of individual solar cells using an automated cell tester or a LED-solar-simulator with adjustable light spectrum.
4/2014: GISMO – Building Integrated Solar Module
GISMO is a special solution photovoltaic module for the integration in pitched roofs. Based on a carrier structure made of fibre reinforced polymer, the module features installation and mounting elements while also ensuring the sealing of the roof. The GISMO can be directly hooked into the roof battons. Classical roof tiles are only needed for the edge area of the roof.
Changing the cost structure of module installations through the combination of functions, improving solar aestethics with the seamless mounted modules and making installation as easy as tiling a roof, our special module solution GISMO brings innovative technology concepts into residential photovoltaic installations. The realization of such innovative module solutions is one of the aims of Fraunhofers CSPs module technology group.
GISMO was developed in the framework of the »Fraunhofer Innovation Cluster Solar Polymers« which aims to combine our longtime know how in polymer processing and the top of the line solar module manufacturing equipment to develop new solutions for photovoltaic applications.
3/2014: Directional Strength of Al-BSF Solar Cells with Continuous Busbars
The strength of full Al-BSF solar cells with continuous busbars strongly was tested systematically in 4-point bending tests. The strength or breakage stress depends on the tested surface (back or sunny side) and the direction of loading related to the busbars. On the sunny side the strength is independent of the loading direction or the presence of a busbar inside the tested area in 4-point bending. On the back side the strength is significant lower when the busbar is loaded parallel compared to the load across to the busbar. If there is no busbar inside the tested area, the strength is even higher. This difference in strength is caused by different defect types and intrinsic stresses in silicon in the metallization regions while the back side metallization with overlapping Al and Ag metallization shows the strongest influence on strength.
In other cell layouts the strength behavior can change due to differences in the metallization concept, e.g. different busbar/pad layouts or laser processing. With these detailed strength characterizations of solar cells the breakage risk can be estimated for manufacturing and PV module lifetime.
1/2013: Reduction of Optical Loss by Combining Innovative Solar Module Technologies
The individual and combined impact of anti-reflective coatings, thin glass, PVB UV+ and light harvesting strings on optical properties of solar modules is investigated. Optical measurements are used to calculate the spectral distribution of optical loss mechanisms in solar modules. Solar modules with different configurations are built and measured electrically for comparison. The combined optical improvements lead to 5%rel. increase in photo current and 1% absolute gain in module efficiency.