Why are monocrystalline wafers increasing in size?

By Mark Osborne, Senior News Editor, Photovoltaics International

The PV industry is undergoing rapid technology changes that have been driven by the well-documented swift adoption of monocrystalline wafers. Less well understood, however, is that within this wafer technology transition comes a shift to larger wafer sizes, and this includes p-type and n-type mono-Si wafers.

Effects of texture additive in large-area diamond wire cut multicrystalline silicon solar cells

By S. Saravanan, RenewSys India Pvt Ltd, Hyderabad, India; Ch.S.R. Suresh, RenewSys India Pvt Ltd, Hyderabad, India; V.V. Subraveti, RenewSys India Pvt Ltd, Hyderabad, India; K.C. Kumar, RenewSys India Pvt Ltd, Hyderabad, India; U.K. Jayaram, RenewSys India Pvt Ltd, Hyderabad, India

The silicon PV industry has predominantly used silicon wafers sliced by a steel wire, with silicon carbide particles (slurry wire – SW) as an abrasive and polyethylene glycol as a coolant. Low yield, high total thickness variation (TTV), significant material waste and short wire lifetime (and thus high downtime) of SW cutting technology have prompted the wafer slicing industry to develop an alternative technology. Researchers have developed diamond wire (DW) cutting technology for slicing the silicon and demonstrated that it overcomes the drawbacks of SW cutting technology. Although the DW cutting technology has been demonstrated for slicing wafers, the wafer surface is different after the conventional acidic texturing in a silicon solar cell process. It is therefore important to improve the existing process or to develop a new process, in order to produce a homogeneous texturization on DW-cut wafers. In this work, a systematic approach has been pursued to improve the existing process by using an additional etchant (a texture additive) in the acidic mixture. Different etch depths and the corresponding mean reflectance were studied. Optical and morphological studies on DW-cut wafers processed with and without a texture additive have been carried out and interpreted in terms of electrical performance.

Reliability of electrically conductive adhesives

By Dr. Oreski; Dr. Gabriele Eder; Lukas Neumaier; Dr. Christina Hirschl; Dr. Rita Ebner; Jörg Scheurer; Wolfgang Pranger

The application of electrically conductive adhesives (ECAs) is a promising alternative to the soldering process for cell interconnection in today’s solar module production. ECAs provide an environmentally friendly solution and offer several other advantages over the conventional solder interconnection technology, such as lower processing temperature, higher mechanical flexibility and replacement of toxic lead. When it is proposed to switch from soldering to adhesive technology in a critical process such as the production of solar cell strings, it is necessary to perform a thorough preliminary analysis of the properties of the materials involved, the material compatibilities and the long-term stability of the interconnections within the PV modules.

The opportunity for wafer-based reduction in LCOE

By Adam Lorenz, 1366 Technologies, Bedford, Massachusetts, USA; Jochen Rentsch, Fraunhofer Institute for Solar Energy Systems ISE, Freiburg, Germany; Sebastian Nold, Fraunhofer Institute for Solar Energy Systems ISE, Freiburg, Germany; Todd Templeton, 1366 Technologies, Bedford, Massachusetts, USA; Laureen Sanderson, 1366 Technologies, Bedford, Massachusetts, USA

As the PV industry strives to reach terawatt scale, addressing the last remaining cost centres of the crystalline silicon value chain will play a critical role in ensuring that the industry can continue to achieve lower systems costs, and provide the extremely low levelized cost of electricity (LCOE) required to drive the adoption of this form of energy. Wafer manufacturing remains the single largest cost driver in industrial cell production. While incremental improvements, such as diamond wire (DW) sawing, have helped to lower silicon consumption, wafer manufacturing has lacked the significant step change necessary for achieving dramatic cost reduction.

Diamond wire process monitoring

By Fabrice Coustier; Roland Riva; Mathieu Debourdeau; Nicolas Velet; Jérémy Bounan; Amal Chabli

Major progress has been made in the PV industry in the last five years as a result of the extensive use of diamond wire during silicon wafering operations. Productivity has increased and costs have fallen to the point where the price of a monocrystalline wafer cut with diamond wire is approaching the price of a multicrystalline wafer cut using slurry.

As-grown bulk lifetime: Increasingly relevant for silicon solar cell performance

By Bernhard Mitchell, University of New South Wales; Daniel Chung , is a Ph.D. student at the Australian Centre for Advanced Photovoltaics at UNSW Australia; Zhen Xiong, is the chief engineer of ingot and wafer technology at Trina Solar; Pietro P. Altermatt , is the principal scientist at the State Key Laboratory for PV Science and Technology (SKL) at Trina Solar; Peter Geelan-Small , University of New South Wales; Thorsten Trupke , is a professor at the Australian Centre for Advanced Photovoltaics at UNSW

By the end of 2017 it is expected that boron-doped multicrystalline silicon (p-type mc-Si) wafers will have been used in more than 60% of the world’s manufactured solar cells.

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.