Impact of silver powder grain size and inorganic additives on the performance of front-side pastes

By Kathrin Reinhardt, Thick-Film Technology and Photovoltaics Group, Fraunhofer IKTS; Markus Eberstein, Manager of the Thick-Film Technology and Photovoltaics Group, Fraunhofer IKTS; Stefan Körner, Thick-Film Technology and Photovoltaics Group, Fraunhofer IKTS; Uwe Partsch, Head of the Hybrid Microsystems Department, Fraunhofer IKTS

This paper presents the results of a study of the influence of silver powder particle size and inorganic additives on sintering and electrical performance of a PV front-side metallization paste. Three different silver powder grain sizes were used in sample front-side pastes. Also examined is the effect of using four different inorganic additives determined by their redox potential. Solar cells produced using the sample pastes were electrically characterized, and selective etch-backs and FESEM investigations were performed to correlate electrical performance with the glassy interface between the metallization and the silicon wafer. In the absence of additives, the highest efficiencies were obtained with the medium silver grain size. If the inorganic species has an oxidizing nature, the mass transport of silver in the glass phase can be enhanced. However, the etch process at the wafer surface is also improved by a greater quantity of silver oxide in the flowing glass. It is shown that if the oxidizing capacity of the additive is too powerful, the electrical performance is negatively influenced. Moreover, the impact of additives is highly dependent on the silver particle size.

The importance of backsheet quality for PV module longevity

By Carrie Xiao, PV Tech China

Certain PV modules have begun showing signs of yellowing, a consequence of backsheet deterioration. This phenomenon can impact on power plant performance and safety, and is emerging as a potential problem waiting to happen with low-cost modules. This paper explores the key attributes of backsheets and assesses the relative benefits of the different types of backsheet on the market and the materials used in them. The different tests undertaken for backsheets are reviewed, and arguments are put forward for the requirement of a standardized testing regime for this crucial module component.

Electrically conductive adhesives: An emerging interconnection technology for high-efficiency solar modules

By Torsten Geipel, Photovoltaic Modules Group, Fraunhofer ISE; Ulrich Eitner, Head of Photovoltaic Modules Group, Fraunhofer ISE

Electrically conductive adhesives (ECAs) are an alternative interconnection technology especially suited to high-efficiency cell concepts with new contact structures. This paper describes the basic principles of this emerging interconnection technology and discusses the different material types on the market. Mechanical and electrical characterization methods for conductive adhesives are also presented. Results are included from peel tests, volume and contact resistivity measurements, metallographic investigations, dynamic mechanical analysis and differential scanning calorimetry. Finally, a novel simulation approach for the cure kinetics of ECAs and arbitrary temperature profiles is briefly described and demonstrated by an example of an epoxy adhesive cure.

Progress in n-type monocrystalline silicon for high efficiency solar cells

By Bo Li, General Manager of Solar Cells, SunEdison, Inc.; Joel Kearns, Vice President for Solar R&D, SunEdison, Inc.

Future high efficiency silicon solar cells are expected to be based on n-type monocrystalline wafers. Cell and module photovoltaic conversion efficiency increases are required to contribute to lower cost per watt peak and to reduce balance of systems cost. Past barriers to adoption of n-type silicon cells by a broad base of cell and module suppliers include the higher cost to manufacture a p-type emitter junction and the higher cost of the n-type mono silicon crystal. Technologies to reduce the cost of manufacturing the p-type emitter by diffusion or implantation of boron are being developed in the industry. To reduce the cost and improve further the quality of n-type mono silicon crystal, SunEdison has developed a continuous Czochralski (CCZ) crystal pulling process, based on the technology of Solaicx, acquired in 2010. This CCZ technique allows production of a crystal with much greater resistivity uniformity, with a lower incorporation rate of lifetimereducing metals impurities, and allows crystal oxygen to be selected independent of production batch size. CCZ is expected to reduce n-type crystal cost below that of current p-type mono crystal.

Potential-induced degradation (PID) and its correlation with experience in the field

By Juliane Berghold, Head of R&D, PI-Berlin; Simon Koch, PI-Berlin; Anja Böttcher, Project Manager of Outdoor and Field Projects in R&D, PI-Berlin; Asier Ukar, Project Manager of Outdoor and System-Integrated Projects, PI-Berlin; Mathias Leers, Project Manager, PI-Berlin; Paul Grunow, Member of the Board and Senior Consultant, PI-Berlin

Statistical data on potential-induced degradation (PID) testing at the panel level are discussed in terms of their field relevance and the actual occurrence of PID in the field, since the latter is strongly dependent on both the specific climate and the weather conditions at a certain location as well as on the system configuration realized in a specific power plant. The correlation of outdoor conditions and leakage current is also considered with regard to a suitable standard test for solar panels. Real outdoor data are shown for PID-affected power plants. Indoor and outdoor recovery is demonstrated for PID in real solar plants as well as in lab and outdoor set-ups. Apart from ‘measuring’ PID in suitable tests and in the field, approaches are also presented for the mitigation of PID at the panel and system level.

Cost-effective n-Pasha solar cells with efficiency above 20%

By Astrid Gutjahr, ECN Solar Energy; Delislava Saynova, ECN Solar Energy; Eric Kossen, ECN Solar Energy; John Anker, ECN Solar Energy; Ingrid Romijn, ECN Solar Energy; Kees Tool, ECN Solar Energy

This paper presents recent developments of ECN’s n-Pasha (passivated on all sides H-pattern) solar cell technology. The n-Pasha cell, currently being produced on an industrial scale by Yingli Solar, is a solar cell fabricated on n-type Cz material with homogeneous diffusions, dielectric passivation and printed metallization on both sides. The metallization is applied in an open H-pattern to both sides, which makes it suitable for bifacial applications. In order to improve both cell performance and the cost of ownership of n-Pasha solar cells, the ECN R&D team has focused on several aspects of the device design and processing. By reducing metal coverage and improving the quality of the front-side metallization, tuning the back-surface field (BSF) doping and improving the front- and rear-surface passivation, it has been possible to obtain an average efficiency of 20%, with top efficiencies of 20.2%. At the same time, the amount of silver used for metallization has been decreased by over 50% and is now similar to that used for p-type solar cells. Furthermore, it is shown that with the ECN n-Pasha cell concept, wafers from the full resistivity range of n-Cz ingots can be used to make cells without losses in efficiency. Combining the improved efficiency and the reduction in cost makes the n-Pasha cell concept a very cost effective solution for manufacturing highly efficient solar cells and modules.

Module materials overview report 2013

By Mark Thirsk, Managing Partner and Co-Founder, Linx Consulting LLC

Module assembly drives as much as a third of the total module cost and can have a significant impact on overall module performance in terms of efficiency and module lifetime. This paper reviews some of the newest moduling material trends, and the outlook for the module market.

Cavities observed in PV modules induced by the tabbing and stringing process

By Eric Pilat, CEA-INES, LMPV (Module Laboratory); Manuel Hidalgo, Senior Research Scientist, ARKEMA Sollia Laboratory, LMPV; Dominque Thil, ARKEMA Sollia Laboratory, LMPV; Marion Vite, CEA-INES, LMPV (Module Laboratory)

A major cause of failure in PV modules is related to the penetration of the module by moisture and its retention within. The presence of moisture results in corrosion of metallic contacts or accelerates the molecular degradation of the encapsulant, causing a loss of transparency and in some cases the development of yellowing. The moisture penetration may be intrinsic to the resin itself, but most often it will occur at the interfaces. As a consequence, the adhesion of the resin to glass, metallization, cell and backsheet surfaces may be affected. Engineers involved in the assembly of PV modules used to link adhesion degradation issues to poor conditions for storing polymeric materials, especially the encapsulation resin and the backsheet. In this paper another cause, which has not yet been studied by specialists, is discussed. It is shown that the welding of copper strips can induce residues which prevent the satisfactory adhesion of the resin, resulting in elamination. This phenomenon is identified by ‘spots’ along the busbars after lamination. The study highlights the possible consequences of these defects for a module’s performance, after consecutive thermal cycling, damp-heat and humidity-freeze testing. Recommendations are also given for choosing a suitable solder flux and optimizing the soldering process, in order to maintain satisfactory control over potential delamination problems.

Achieving higher efficiencies with a low-cost etch for in-line-diffused silicon wafer cells

By Bram Hoex, Director and Group Leader in the Silicon PV Cluster, SERIS, National University of Singapore; Debajyoti Sarangi, Senior Research Scientist, SERIS, National University of Singapore; Edwin Carmona, Process Engineer in the PV Production Group, SERIS, National University of Singapore; Jason Avancena, Process Engineer in the PV Production Group, SERIS, National University of Singapore; Jessen Cunnusamy, Engineer on Crystalline Silicon Solar Cells, SERIS, National University of Singapore; Kishan Devappa Shetty, Research Engineer on Crystalline Silicon Solar Cells, SERIS, National University of Singapore; Matthew Boreland, Director of the Silicon PV Cluster, SERIS, National University of Singapore; Prabir Kanti Basu, Senior Research Scientist in the Silicon PV Cluster, SERIS, National University of Singapore; Sandipan Chakradorty, Research Fellow in the Silicon PV Cluster, SERIS, National University of Singapore; Ziv Hameiri, Research Fellow in the Silicon PV Cluster, SERIS, National University of Singapore

Emitter formation is one of the most critical processes in the fabrication of silicon wafer solar cells. The process for standard emitter formation adopted in the photovoltaic industry is tube-based diffusion, using phosphorus oxychloride as the dopant source. A potentially low-cost alternative that typically results in lower solar cell efficiencies is in-line diffusion, using phosphoric acid as the dopant source. The Solar Energy Research Institute of Singapore (SERIS) recently developed a technique called the ‘SERIS etch’, a non-acidic etch-back process technology that provides a controllable, uniform and substantially conformal etch-back suitable for solar cell processing. By using the SERIS etch, efficiencies of up to 18.7% have been demonstrated for omogeneous-emitter silicon wafer cells; a 0.4%abs efficiency improvement has also been achieved for a unique selective-emitter approach exploiting this novel etch. All work was carried out on industrial-grade p-type Cz wafers with conventional screen-printed metallization and a full-area aluminium back-surface field (Al-BSF). With Al local BSF (LBSF) homogeneous-emitter solar cells, efficiencies of 19.0% were achieved using in-line emitter diffusion and the SERIS etch, a 0.7%abs efficiency increase over the baseline efficiency at the time. To the authors’ knowledge, these are the highest solar cell efficiencies ever reported for in-line-diffused silicon solar cells. Moreover, the SERIS etch is a costeffective alternative to generating pyramid-textured surfaces without using conventional metal-assisted siliconetching processes.

Potential for mono-cast material to achieve high efficiencies in mass production

By Milica Mrcarica, Senior R&D Engineer, Photovoltech NV

Despite the drop in price of silicon wafers, they are still one of the main factors influencing the cost and performance of Si-based solar cells. These two consequences have initiated a growing commercial interest in mono-cast (castmono, mono-like or quasi-mono) Si wafers, supported by R&D in the areas of material characterization, correlation with cell efficiencies, and mono-cast material use in advanced cell technologies. This paper gives a broad overview and comparison of commercially available grades of mono-cast material from different suppliers. The performance of the material from production in high-throughput screen-printing lines, as well as an analysis of the main material characteristics influencing these results, is presented. A characterization using a lifetime tester and a photoluminescence (PL) imaging tool has shown that not only grain boundaries but also dislocations could cause a drop in cell Voc of more than 15mV. Wafers with large surface areas of <100> Si lattice planes, when processed with anisotropic texturing, could yield an increase in Isc greater than 400mA for 6" substrates, as compared to the isotropic-textured equivalents. Furthermore, when a high-grade mono-cast material processed in anisotropic texturing was compared with CZ mono material from the same supplier and of the same resistivity, light-induced degradation (LID), presented as combined Voc and Isc degradation, was only one-third of that in CZ material. However, although mono-cast material has the potential to increase cell line performance to the same level as that gained by important process and technological improvements, it imposes very high requirements for better material sorting in order to achieve stable cell electrical performance and module aesthetics acceptable to the market.

Polysilicon production technologies in a volatile market

By Johannes Bernreuter, Founder and Head, Bernreuter Research

A record-low spot price in the wake of oversupply and the aggressive cost-reduction roadmap of the PV industry are putting polysilicon producers under pressure to bring down their manufacturing costs. With the dominant Siemens process approaching a limit for further cost cuts, technologies based on the deposition from monosilane (SiH4) have now become the focus of attention.

Challenges for the PV materials supply chain

By Mark Thirsk, Managing Partner and Co-Founder, Linx Consulting LLC

The cost of PV modules manufactured and sold in 2012 is highly reliant on the materials used in the construction. A significant part of the market price is driven by the bill of materials, while other direct costs and depreciation form a small proportion of the total cost. Changes within the supply chain, and in the cost of the materials needed and used, are extremely important influences on the module cost and the end market price. In 2012 we have seen a slowdown in growth in the installation of both commercial and residential PV, despite dramatic falls in module costs. Some of the trends and effects of these changes on the materials supply chain for PV modules will be examined in this paper.

Slurry grit evolution

By Halvor Dalaker, Research Scientist, SINTEF Materials and Chemistry; Shawn Wilson, Senior Engineer in the Metallurgy Department, SINTEF Materials and Chemistry

In slurry-based wafering of silicon bricks using multi-wire saws, the slurry is subject to significant evolution with time as the grits become worn and the silicon kerf accumulates. A good understanding of this evolution would allow wafer producers to make better-informed decisions on when and how to replenish slurry during wafering. This paper summarizes certain important slurry properties and presents some experimental results regarding their evolution. Sampling the slurry at the front and rear of silicon bricks during wafering has allowed the effect of a single pass through the sawing channel to be studied. The wear on the slurry grit is interpreted in terms of identifying what portion of the particle-size distribution plays the most critical role in wafering, and how this critical region changes as the slurry ages. It is found that in a relatively fresh slurry, the particles around the median size and slightly larger are the most active, while particles more than a few μm below the median play only a small part. As the slurry ages, the active region of the particle-size distribution becomes narrower, and shifts towards larger particles even though there are fewer such particles available. This leads to less slurry–brick interaction and poorer material removal properties.

Solar-grade silicon - is ‘Siemens’ the only answer?

By Jan Ove Odden, R&D Engineer, Elkem Solar; Anne Jans Faber, Elkem Solar; Erik Enebakk, Elkem Solar; Steinar Braathen, Elkem Solar; Bjorn Sandberg, Elkem Solar; Kenneth Friestad, Elkem Solar

Solar-grade silicon (SoG-Si) based on metallurgical refining processes, often called upgraded metallurgical-grade silicon (UMG-Si), is expected to play an important role in achieving the solar industry’s necessary cost targets per Wp in order to compete with other energy sources. The broad term ‘UMG-Si’ currently embraces types of silicon feedstock that differ quite substantially in product quality and performance. This paper presents a summary of the work carried out by Elkem on low-cost production of silicon feedstock via a flexible, recycling metallurgical processing route with the lowest carbon footprint on the market. Results are given that qualify Elkem Solar Silicon® (ESS™) as a SoG-Si, with comparable efficiencies to polysilicon (poly-Si) from the traditional Siemens process. The latest results on the performance of modules based on ESS are reported. An indication of the stability of older modules based on SoG-Si feedstock from Elkem is also considered. On the basis of the results, there is no reason to expect modules based on ESS to differ from other commercial modules based on poly-Si. ESS is therefore shown to be a viable alternative to conventional poly-Si, but with the additional benefit of lowering specific energy use and cost per Wp.

Market dynamics of materials supply for PV in China

By Mark Thirsk, Managing Partner and Co-founder, Linx Consulting LLC

China has become the largest manufacturing base for crystalline silicon modules in the world, and is becoming increasingly reliant on a domestic supply base. This article discusses the emergence of local supply chains and the strategic responses of global suppliers to this domestic competition. It proceeds to review a set of conclusions from four case studies of formulated material supply within China that can apply to supply chain participants in the PV industry, concluding with some strategic considerations for suppliers on the cusp of entering the Chinese market.