Boron–oxygen-related degradation in multicrystalline silicon wafers

By Rune Søndenå, Institute for Energy Technology, Kjeller, Norway

Extended crystal defects, such as grain boundaries and dislocations, have long been considered the main factors limiting the performance of multicrystalline (mc-Si) silicon solar cells. However, because the detrimental effects of these crystal defects are reduced as a result of improvements in the solidification process as well as in the feedstock and crucible quality, the degradation caused by boron–oxygen complexes is expected to be of increasing importance. Light-induced degradation (LID) occurs in both p- and n-type crystalline silicon solar cells that contain both boron and oxygen. Because of the fundamental differences in the solidification processes, mc-Si silicon contains less oxygen than Czochralski silicon; nevertheless, the oxygen content in mc-Si silicon is still sufficient to cause degradation, although to a lesser extent than in the case of Czochralski silicon. Whereas B–O-related degradation of 0.5 to 1% abs. can be found in Czochralski cells, the degradation in conventional mc-Si cells is limited to around 0.1 to 0.2% abs.

Diamond wire sawing for PV - Short- and long-term challenges

By Hubert Seigneur, c-Si Feedstock/ Wafering Programme Manager, U.S. PVMC, Florida Solar Energy Center (FSEC), University of Central Florida (UCF); Andrew Rudack, U.S. PVMC, Operations Manager, SEMATECH; Joseph Walters, U.S. PVMC, Programme Director, Florida Solar Energy Center (FSEC), University of Central Florida (UCF); Paul Brooker, U.S. PVMC, Assistant Professor, Florida Solar Energy Center (FSEC), University of Central Florida (UCF); Kristopher Davis, c-Si Programme Manager, U.S. PVMC, Florida Solar Energy Center (FSEC), University of Central Florida (UCF); Winston V. Schoenfeld, Director of c-Si, U.S. PVMC, Director of the Solar Technologies Research, Florida Solar Energy Center (FSEC), University of Central Florida (UCF); Stephan Raithel, Managing Director, SEMI Europe; Shreyes Melkote, Associate Director, Georgia Institute of Technology; Steven Danyluk, Georgia Institute of Technology, Founder and CEO, Polaritek Systems; Thomas Newton, Director of Product Development, Polaritek Systems; Bhushan Sopori, Principal Engineer, National Renewable Energy Laboratory (NREL); Stephen Preece, Director of R&D, Process Research Products; Igor Tarasov, Researcher and Software Developer, Ultrasonic Technologies Inc.; Sergei Ostapenko, President and CEO, Ultrasonic Technologies Inc.; Atul Gupta, Director of Product Development/R&D, Suniva; Gunter Erfu, Managing Director, SolarWorld ; Bjoern Seipel, Senior Scientist, SolarWorld; Oliver Naumann, SolarWorld; Ismail Kashkoush , Vice President of Technology, Akrion Systems; Franck Genonceau, Global Product Manager, Solar Products Division, Applied Materials

A shift from free-abrasive/steel wire sawing to fixed-abrasive diamond wire sawing is expected to take place in the PV cell manufacturing industry, with 2018 being the anticipated pivotal point for market dominance. This shift is due to several key advantages of diamond wire sawing, such as higher throughput, less wire per wafer, no slurry and the possibility of kerf recycling. However, in order for diamond wire sawing to realize its promise as the next-generation workhorse for the slicing of silicon PV wafers, inherent fundamental challenges must be properly identified and successfully addressed by the PV industry. As a first step to increasing the current collective understanding of the critical needs/challenges of diamond wire sawing, the c-Si programme of the U.S. PVMC held a workshop on July 8th, 2014 in San Francisco, California. One of the key products of this workshop was an extensive list of short- and long-term challenges. This article expands on some of the most important challenges identified at the workshop through the collective discussions and dialogue among a variety of PV industry experts and stakeholders.

Five reasons to choose mono-Si

By Strategy and Planning Department, LONGi Silicon Materials Corp.

One question to emerge in recent years is whether monocrystalline silicon (mono-Si) or multicrystalline silicon (mc-Si) will become the dominant mainstream technology in the future PV industry. However, despite all the arguments, the market share of mc-Si seems barely changed, while the market share of mono-Si has not increased significantly. The reasons why mono-Si has not made progress have been extensively mentioned in the literature and will therefore not be covered here; rather, the objective of this paper is to discuss several benefits of mono-Si.

X-ray specs for solar cells

By Daniel Bright, Applications Engineer, Jordan Valley Semiconductors UK Ltd.; Richard Bytheway, Technologist, Jordan Valley Semiconductors UK Ltd.; Tamzin Lafford, Beamline Scientist, European Synchrotron Radiation Facility

Solar cell performance depends on material quality, as well as on the architecture of the cell. In the search for higher-performing cells, an ability to visualize the bulk and surface quality of the material is an advantage; to do this non-destructively, even in-line, is even better. It would be good to have X-ray vision to look inside, would it not? X-ray diffraction imaging (XRDI) does just that. Images are obtained of the distortions caused by crystal defects, and quantitative measures of the lattice deformation are available. In this paper the results obtained on a commercially available XRDI tool are compared with those from a largescale public research facility.

State-of-the-art c-Si cell manufacturing: Trends in materials, processes and products identified in the 5th edition of the ITRPV roadmap

By Markus Fischer, Director of R&D Processes, Hanwha Q CELLS GmbH; Alexander Gerlach, Senior Specialist, Hanwha Q CELLS GmbH

The crystalline silicon (c-Si) module price has been fluctuating slightly around the US$0.72/Wp level for the last 18 months. This pricing, at an estimated cumulative PV module shipment volume of 149GWp, indicates a trend change for the PV industry. C-Si module pricing appears to be currently above the production cost and should therefore yield a profit margin. However, there is still a mismatch between manufacturing capacity and future market demand. A closer look at the pricing figures reveals that there is no indication to give the allclear during the ongoing consolidation process in the PV industry. C-Si module pricing is not reflecting the increase in polysilicon and wafer prices, and therefore the pressure to reduce the cell and module conversion costs remains a looming fact. This paper describes state-of-the-art c-Si cell manufacturing solutions that are in line with identified trends in materials, processes and products recently published in the 5th edition of the International Technology Roadmap for Photovoltaic (ITRPV). Currently available c-Si cell technologies offering higher efficiencies as well as materials savings will be discussed. The need for implementing these technologies in mass production without significantly increasing the cost per piece and in the face of more complex manufacturing processes will be established. The findings of the ITRPV regarding the reduction in levelized cost of electricity (LCOE) will be discussed, leading to the conclusion that contemporary cell technology supports the long-term competitiveness of PV-based power generation.

Durable MWT PV modules made using silicone electrically conductive adhesive and an automated assembly line

By Hongfeng Lin, Vice CTO and Director of the R&D Centre, Tianwei New Energy; Kaiyan Cao, Tianwei New Energy; Zhe Qui, Member of MWT Module Technology Team, Tianwei New Energy; Liyan Zhao, R&D Engineer, Tianwei New Energy, Chengdu, P.R.C.; Wei Long, Assistant Director of the R&D Centre and Pilot Line Manager, Tianwei New Energy; Xianzhi Chen, Research Fellow, Tianwei New Energy; Brian Chislea, Dow Corning Corporation; Guy Beaucarne, R&D Group Leader, Dow Corning Europe S.A.; Peng (Jason) Wei, Dow Corning China; Adriana Zambova, Dow Corning Corporation; Yanghai Yu, Product Development Chemist, Dow Corning China; Guo Yi, Silicone Sealant Development Group Leader, Dow Corning China; Kees Broek, Solar Energy Researcher, ECN; Ian Bennett, Researcher, ECN; Jan Bakker, CTO, Eurotron; Nico van Ommen, Process Engineer, Eurotron; Egbert Fredrikze, Equipment Engineer, Eurotron

Metal wrap-through (MWT) module technology is an attractive approach for increasing module efficiency. This paper shares the results of MWT module fabrication using a silicone electrically conductive adhesive (ECA), a conductive backsheet (CBS) with a thin organic layer surface finish, and an automated module assembly line. Very low cell-to-module (CTM) power losses are observed, leading to a multicrystalline Si module power of 266W and a full-area efficiency of 16.8%. The modules are very stable in damp-heat conditions and thermal cycling, demonstrating minimal degradation after 1.5 x IEC requirements in terms of damp heat and thermal cycling, and well below 2% degradation after 2 x IEC requirements. These MWT modules have received IEC 61215 and IEC 61730 certification.

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.