Epitaxial Si lift-off technology: Current status and challenges

By Hariharsudan Sivaramakrishnan Radhakrishnan, (Hari) pursued his Ph.D research at KU Leuven and imec, Belgium, on porous silicon-based gettering and epitaxial Si lift-off and solar cells.; Valérie Depauw, is a research engineer at imec, involved in several EU projects. She started playing with lif t-off techniques at the end of 2004, at the start of her Ph.D, and has not stopped since then. She has built expertise around making “holes” inside silicon, by tailoring porous silicon for kerfless wafering and exploring advanced nanostructures for light trapping in (ultra-) thin crystalline-silicon solar cells.; Kris Van Nieuwenhuysen, Kris Van Nieuwenhuysen obtained her degree in engineering in 2000. She then joined the Si solar cell group of imec, where she has been the main expert in Si epitaxial CVD processes for solar cell fabrication.; Ivan Gordon, obtained his Ph.D from the University of Leuven, Belgium in February 2002. He started to work at imec in June 2003, where he is currently leading the Silicon Photovoltaics group, working on c-Si wafer-based solar cells, thin-film silicon solar cells, and advanced module concepts for ultra-thin c-Si wafer-based cells.; Jef Poortmans, received his Ph.D from the KU Leuven, Belgium, in June 1993. Afterwards he joined the photovoltaics group and, in 2003, became the PV Department Director.

The drive towards better Si utilization (g/Wp) has been an obsession in the Si PV industry over the last few decades as one of the key aspects in making photovoltaic energy production competitive. With the cost of Si still making up a third of the final Si solar module cost, there is continued interest in reducing the cost of Si by producing thinner wafers and reducing kerf losses.

Supply of low-cost and high-efficiency multi-GW mono wafers

By Yichun Wang, Yichun Wang received a B . S . i n e l e c t r i c a l engineering in 2007 from Northwestern University, China, and an M.S. in electrical engineering in 2010 from the University of Kentucky, USA. She joined LONGi Green Energy Technology Co., Ltd. in 2014, and is currently the application engineering and customer service manager in the silicon wafer business group, where her responsibilities include technical/ product quality support and supervising technical collaboration projects with global institutes and corporations.; Tian Xie, Tian Xie received his Ph.D. in physics in 2004 f r o m H i r o s h i m a University, Japan. He is the director of the quality management department at LONGi Green Energy Technology Co., Ltd., where his primary responsibility is overseeing the quality management, customer service, product design and sales groups at the company

In the Chinese PV market, multi crystalline silicon firmly holds a large market share compared with monocrystalline silicon, entirely as a result of the development of the Chinese PV industry.

HPM silicon: The next generation of multicrystalline silicon for PV

By Matthias Trempa, Iven Kupka, Christian Kranert, Christian Reimann & Jochen Friedrich, Fraunhofer IISB, Fraunhofer THM

High-performance multicrystalline (HPM) silicon, achieved by nucleation on special seed layers at the crucible bottom, is now increasingly replacing conventional multicrystalline (mc) silicon, which is solidified on the standard silicon nitride coating. The HPM material is characterized by a very fine initial grain structure consisting of small, regularly shaped grains surrounded by a large number of random-angle grain boundaries. These grain structure properties, which differ significantly from those of conventional multicrystalline silicon, lead to a much lower dislocation content in the material, and therefore result in higher efficiencies of the silicon solar cells produced. This paper gives a rough overview of the worldwide R&D activities on HPM silicon in recent years, supplemented by several research results obtained at Fraunhofer IISB/THM. The focus is on the different seeding methods, the grain structure properties and the development of the grain and defect structure over the ingot height, as well as on the main challenges for further improvements in material quality and production costs.

Polysilicon spot prices in China hit US$20/kg

By Mark Osborne, senior news editor, Photovoltaics International

Polysilicon spot prices had bottomed in January, 2016 at just above US$13/kg on overcapacity, notably due to US producers selling excess inventory as access to the China market had been severely curtailed due to new import duties applied as part of the trade war with the US.

n-type multicrystalline silicon for highefficiency solar cells

By Stephan Riepe, Patricia Krenckel, Florian Schindler, Martin C. Schubert & Jan Benick, Fraunhofer Institute for Solar Energy Systems (ISE), Freiburg, Germany

High-efficiency silicon solar cells require silicon wafers of high electrical quality as the base material. One advantage of n-type compared with p-type doped silicon is the smaller impact of many metal impurities on the electrical material quality. This applies especially to n-type multicrystalline silicon ingots produced by the directional solidification process, with dissolved metal impurities typically introduced by the crucible system.

Oxygen-defect characterization for improving R&D relevance and Cz-Si solar cell efficiency

By Jordi Veirman, Benoît Martel, Nicolas Enjalbert & Sébastien Dubois, CEA Tech-INES, Le Bourget du Lac, & Catherine Picoulet & Pierre Bonnard, AET Technologies, Meylan, France

Most high-efficiency solar cells are fabricated from monocrystalline Czochralski (Cz) silicon (Si) wafers because of the high quality of the material. Despite the considerable heritage from microelectronics, the Cz-Si substrate quality can still limit cell performance.

Metrology at the ingot level: Addressing the growing importance of bulk material quality

By Bernhard Mitchell, Daniel Chung, Jürgen Weber & Thorsten Trupke University of New South Wales (UNSW), Sydney; BT Imaging Pty Ltd, Sydney, Australia

With the PV industry continually pushing for ever-higher silicon solar cell efficiencies, the requirements on the electronic quality of the bulk material are becoming more stringent. Advanced characterization of silicon ingots after cutting into bricks allows early quality control and immediate feedback in crystal growth, thereby facilitating shorter R&D cycles, higher yield, lower cost and higher product quality in mass manufacturing.

Inline diamond wire inspection based on resonant vibrations

By Hubert Seigneur & Winston V. Schoenfeld, USPVMC; Sergei Ostapenko, Igor Tarasov & Chad Rodrigues , Ultrasonic Technologies; Yuriy Zaikin, PetroBeam; Kevin Morrow, Niabraze

Because the wire itself is the dominant cost in diamond wire sawing, economics dictate that the wire life must be prolonged. This paper presents recent progress made in real-time non-contact monitoring of diamond wire using the resonant vibration (RV) characteristics of the wire. Additionally, a theoretical framework is presented which shows that the characteristics of the resonance curve do not change at speeds above 500m/s. As a result, this technology is expected to be able to meet the increasing demands of monitoring diamond wire wear during sawing as the wire speed continues to increase in the coming years.

Polysilicon vs. upgraded metallurgicalgrade silicon (UMG-Si): Technology, quality and costs

By Eduardo Forniés, Laura Méndez & Marta Tojeiro, Aurinka PV Group

During the severe plummet of PV prices that took place during 2008–2012 as a result of overcapacity, the polysilicon sector suffered a major adjustment of costs and capacity to face the reduction in prices and the mismatch between demand and supply. In 2012 that significant drop in prices provoked the bankruptcy of many polysilicon producers, with only the large and efficient players still surviving. However, there was also an impact on the (at that time) promising and immature industry of metallurgical purification of metal silicon, also known as upgraded metallurgical-grade silicon (UMG-Si). The strong selling point of UMG-Si producers – the production costs – was no longer an asset, leaving UMG-Si with nothing but its weakness – the quality. The generation costs for solar energy are currently comparable to those for conventional fuels. The solar industry is self-sustaining and is not dependent on government subsidies. In this current situation, the industry requires an updated comparison between the two main routes of silicon purification and their products, which is the aim of this paper.

Inline quality rating of multicrystalline wafers – Relevance, approach and performance of Al-BSF and PERC processes

By Matthias Demant, Theresa Strauch, Jonas Haunschild & Stefan Rein, Fraunhofer ISE; Kirsten Sunder & Oliver Anspach, PV Crystalox Solar Silicon

With the transition of the cell structure from aluminium back-surface field (Al-BSF) to passivated emitter and rear cell (PERC), the efficiency of multicrystalline silicon solar cells becomes more and more sensitive to variations in electrical material quality. Moreover, the variety of multicrystalline materials has increased with the introduction of high-performance multicrystalline silicon. For these reasons, a reliable and verifiable assessment of the electrical material quality of multicrystalline wafers gains importance: to this end, a rating procedure based on photoluminescence imaging has been developed.

Comprehensive analysis of strength and reliability of silicon wafers and solar cells regarding their manufacturing processes

By Felix Kaule, Fraunhofer Center for Silicon Photovoltaics CSP, Halle (Saale); Marcus Oswald, Fraunhofer Center for Silicon Photovoltaics CSP, Halle (Saale); Ringo Koepge, Fraunhofer Center for Silicon Photovoltaics CSP, Halle (Saale); Carola Klute, Fraunhofer Center for Silicon Photovoltaics CSP, Halle (Saale); Stephan Schoenfelder, Fraunhofer Center for Silicon Photovoltaics CSP, Halle (Saale); Leipzig University of Applied Science, Germany

The mechanical strength of monocrystalline and multicrystalline silicon wafers is mainly dictated by the cracks induced during the wire-sawing process. Different sawing technologies, such as diamond-wire- or slurry-based processes, lead to different strength behaviours of as-cut wafers. Furthermore, the strength is strongly influenced by texturization, and at this stage can be interpreted as the basic strength of a solar cell. The metallization and firing processes determine the final strength and reliability of a solar cell, with the metallization contacts being the root cause of breakage of solar cells, depending on the particular cell concept. This paper gives a comprehensive overview of the typical ranges of strength for as-cut wafers, textured wafers and solar cells, for the two different sawing technologies. Around 100 batches with 4,253 samples were evaluated in the study.

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.