Slicing solar power costs

September 15, 2008

SALT LAKE CITY - University of Utah engineers devised a new way to slice thin wafers of the chemical element germanium for use in the most efficient type of solar power cells. They say the new method should lower the cost of such cells by reducing the waste and breakage of the brittle semiconductor.

The expensive solar cells now are used mainly on spacecraft, but with the improved wafer-slicing method, "the idea is to make germanium-based, high-efficiency solar cells for uses where cost now is a factor," particularly for solar power on Earth, says Eberhard "Ebbe" Bamberg, an assistant professor of mechanical engineering. "You want to do it on your roof."

Dinesh Rakwal, a doctoral student in mechanical engineering, adds: "We're coming up with a more efficient way of making germanium wafers for solar cells - to reduce the cost and weight of these solar cells and make them defect-free."

Bamberg and Rakwal are publishing their findings in the Journal of Materials Processing Technology. Their study has been accepted, and a final version will be published online late this month or in early October, and in print in 2009.

Brass-coated, steel-wire saws now are used to slice round wafers of germanium from cylindrical single-crystal ingots. But the brittle chemical element cracks easily, requiring broken pieces to be recycled, and the width of the saws means a significant amount of germanium is lost during the cutting process. The sawing method was developed for silicon wafers, which are roughly 100 times stronger.

The new method for slicing solar cell wafers - known as wire electrical discharge machining (WEDM) - wastes less germanium and produces more wafers by cutting even thinner wafers with less waste and cracking. The method uses an extremely thin molybdenum wire with an electrical current running through it. It has been used previously for machining metals during tool-making.

Germanium serves as the bottom layer of the most efficient existing type of solar cell, but is used primarily on NASA, military and commercial satellites because of the high expense - raw germanium costs about $680 per pound. Four-inch-wide wafers used in solar cells cost $80 to $100 each, and the new cutting method may reduce the cost by more than 10 percent, says Grant Fines, chief technology officer for germanium wafer-maker Sylarus Technologies in St. George, Utah.

"Anything that can be done to lower this cost ultimately will lower the cost of solar power per kilowatt-hour, which is beneficial," and will encourage wider use of solar power, he adds. "That's why this technology Ebbe has come up with is very intriguing."

Sylarus is considering using the new method, but must determine if it can be scaled up so wafers can be mass-produced in a commercially viable manner, Fines says.

Bamberg's method would "reduce the amount we have to recycle and increase the yield," he adds. "It has the potential to give good savings, which helps enable this technology here on Earth."

A patent is pending on a way of using the new method so that multiple, parallel electrically charged wires are used to cut germanium wafers - a mass-production method Bamberg compares with an egg slicer.

Bringing High-Efficiency Solar Cells Down to Earth

Germanium is a semiconductor at the bottom of "multijunction" solar cells. Above it are layers of gallium-indium-arsenide and gallium-indium-phosphide. The layers work together to capture different wavelengths of sunlight, and the germanium also serves as the substrate upon which the solar cell is "grown."

When sunlight hits a solar cell, the energy is converted to a flow of electrons in the cell, namely, electricity.

Silicon-based solar cells on Earth have maximum efficiency of 20 percent, Fines says. In space, germanium solar cells typically convert 28 percent of sunlight into electricity, but on Earth where solar concentrators are used, they can convert more than 40 percent of sunlight into electricity, and their efficiency theoretically exceeds 50 percent, he adds.

Despite the greater efficiency of germanium-based solar cells, a 2005 survey found that 94 percent of solar cells made for non-space uses were silicon-based because silicon is much cheaper and less fragile than germanium, the Utah researchers say.

Bamberg says germanium-based solar cells are used on most spacecraft because they are more efficient and lighter than silicon-based solar cells. By making it more attractive economically to use efficient germanium solar cells on rooftops, the weight and size of solar panels can be reduced "so it doesn't bother you aesthetically," he adds.

The new method may make germanium-based solar cells competitive with less efficient but less expensive silicon-based solar cells for uses on Earth, says Bamberg.

Less Waste, More Wafers

In the new method, the molybdenum wire essentially is an electrode, and it is connected to a pulsed power supply that charges the wire during the cutting process.

A cylinder-shaped germanium ingot rests on a horizontal support, and the wire is lowered into the ingot as new wire is pulled continually from a supply spool to replace the cutting wire as it wears. Thin, synthetic oil is injected along the wire, both to increase the electrical charge on the wire and to flush away material that melts during the cutting process.

The process is slow. Wire electrical discharge machining takes 14 hours to cut a single wafer. Bamberg says the electrified wire method has to be done gently to avoid cracking the germanium, but he hopes to increase the speed to the six hours it now takes to cut a wafer using a wire saw.

Wire saws made of brass-coated steel have a thickness of about 170 or 180 microns (millionths of a meter). The Utah researchers used molybdenum wire 75 to 100 microns thick, a bit thicker than a human hair. Less germanium is wasted during the slicing process because the electrified cutting wire is thinner.

The study found that a 100-micron-thick electrified wire significantly reduced the waste and increased the number of wafers that could be made from a germanium ingot, but a thinner 75-micron-wide wire did even better.

"At the current standard wafer thickness of 300 microns, you can produce up to 30 percent more wafers using our method" with a 75-micron-wide wire, Bamberg says. "Since we produce them crack free, we can also make them thinner than standard techniques. So if you go down to a 100-micron-thick wafer, you can make up to 57 percent more wafers [from the same germanium ingot]. That's a huge number."

Making the wafers thinner will reduce their cost because more can be made from the same ingot, he adds.

The new study found that the "kerf" - which is the amount of germanium wasted during the slicing process - was 22 percent less when a 75-micron diameter electrified wire was used to cut the wafers, compared with the conventional wire saw method. The researchers cut 2.6-inch-diameter wafers with a thickness of 350 microns.

The study also showed less germanium was wasted not only using the smaller wire size, but also if the charge on the electrified wire was lower.

University of Utah

---

---

	Solar Cells: Materials, Manufacture and Operation by Tom Markvart (Author), Luis Castaner (Author) The capture and use of solar energy has been growing for many years, but only in recent times have advances in design and manufacture allowed us to see the incorporation of solar energy as a significant player in the renewable energy arena. Solar cells are at the heart of any photovoltaic system and in this book the various types are described and their characteristics reviewed. Going beyond materials, design and function, 'Solar Cells' also covers their testing, monitoring and calibration thus providing a comprehensive account of current activity in this important field of research and industry. 'Solar Cells' has been abstracted from the recent 'Practical Handbook of Photovoltaics' by the same editors (ISBN 185617 3909. 2003: Elsevier) ...
	The Physics of Solar Cells (Properties of Semiconductor Materials) by Jenny Nelson (Author) This book provides a comprehensive introduction to the physics of the photovoltaic cell. It is suitable for undergraduates, graduate students, and researchers new to the field. It covers: basic physics of semiconductors in photovoltaic devices; physical models of solar cell operation; characteristics and design of common types of solar cell; and approaches to increasing solar cell efficiency. The text explains the terms and concepts of solar cell device physics and shows the reader how to formulate and solve relevant physical problems. Exercises and worked solutions are included. Contents: Photons In, Electrons Out: Basic Principles of PV; Electrons and Holes in Semiconductors; Generation and Recombination; Junctions; Analysis of the p n Junction; Monocrystalline Solar Cells; Thin Film...
	Physics of Solar Cells: From Basic Principles to Advanced Concepts (Physics Textbook) by Peter Würfel (Author) Based on the highly regarded and extremely successful first edition, this thoroughly revised, updated and expanded edition contains the latest knowledge on the mechanisms of solar energy conversion. The textbook describes in detail all aspects of solar cell function, the physics behind every single step, as well as all the issues to be considered when improving solar cells and their efficiency. Requiring no more than standard physics knowledge, the book enables both students and researchers to understand the factors driving conversion efficiency and to apply this knowledge to their own solar cell development. New exercises after each chapter help students to consolidate their freshly acquired knowledge, while the book also serves as a reference for researchers already working in...
	Practical Photovoltaics: Electricity from Solar Cells by Richard J. Komp (Author), John Perlin (Introduction) Practical Photovoltaics, the now-classic reference on solar electricity, offers a unique combination of technical discussion and practical advice. Physicist, lecturer, and solar-home dweller Richard Komp explains the "how" and the "how-to" of PV, while providing valuable information on the industry, new developments, and the future. The book is a comprehensive guide to the theory and reality of solar electricity, as well as a detailed installation and maintenance manual. A well-illustrated appendix offers step-by-step instructions for constructing your own solar module, a creative approach to demystifying the technology. Presented in a clear, concise, and understandable style, Dr. Komp's contribution to PV literature has been called the "best single reference available," "the easiest and...
	Photovoltaic Design and Installation For Dummies (For Dummies (Math & Science)) by Ryan Mayfield (Author) The fun and easy way to get a grip on photovoltaic design and installationDesigning and installing solar panel systems is a trend that continues to grow. With 'green collar' jobs on the rise and homeowners looking for earth-friendly ways to stretch their dollars and lesson their carbon imprint, understanding photovoltaic design and installation is on the rise.Photovoltaic Design & Installation For Dummies gives you a comprehensive overview of the history, physics, design, installation, and operation of home-scale solar-panel systems. You'll also get an introduction to the foundational mathematic and electrical concepts you need to understand and work with photovoltaic systems.Covers all aspects of home-scale solar-power systemsViable resource for professionals, students, and technical...
	Dye-sensitized Solar Cells by Kuppuswamy Kalyanasundaram (Author) Several forms of thin-film solar cells are being examined as alternatives to silicon-solar cells—one of the most promising technologies is the dye-sensitized solar cell (DSC), with proven efficiencies that approach 11%. This book, which provides a comprehensive look at this promising technology, aims to provide both a graduate level text that brings together the fundamentals of DSC from three perspectives (materials, performance, and mechanistic aspects), as well as to serve as an advanced monograph that summarizes the key advances and lists the technical challenges remaining to be solved.
	Solar Electricity Handbook - 2012 Edition: A Simple Practical Guide to Solar Energy - Designing and Installing Photovoltaic Solar Electric Systems by Michael Boxwell (Author) The Solar Electricity Handbook is a practical and straightforward guide to using electric solar panels. Assuming no previous knowledge of solar panels, the book explains how solar panels work, how they can be used and explains the steps you need to take to successfully design and install a solar electric system from scratch using photovoltaic solar panels. This is an up to date 2012 Edition of the book with even more diagrams, details and up-to-the-minute information on this exciting technology. Accompanying this book is a solar resource website containing lots of useful information, lists of suppliers and on-line solar energy calculators that will simplify the cost analysis and design processes. Why buy the Solar Electricity Handbook? The Handbook is a...
	Solar Cell Device Physics, Second Edition by Stephen Fonash (Author) There has been an enormous infusion of new ideas in the field of solar cells over the last 15 years; discourse on energy transfer has gotten much richer, and nanostructures and nanomaterials have revolutionized the possibilities for new technological developments. However, solar energy cannot become ubiquitous in the world's power markets unless it can become economically competitive with legacy generation methods such as fossil fuels. The new edition of Dr. Stephen Fonash's definitive text points the way toward greater efficiency and cheaper production by adding coverage of cutting-edge topics in plasmonics, multi-exiton generation processes, nanostructures and nanomaterials such as quantum dots. The book's new structure improves readability by shifting many detailed equations to...
	How to Build a Solar Panel Power System for less than $150.00 (The Debt Killer) The wonderful world of Photovoltaic’s, otherwise known as solar power, doesn’t have to be restricted to the rich. You can build a simple solar power set-up that can run all the lights in your home, a TV, stereo, computer, or similar electrical device. All you need is 1 solar panel, 1 battery, 110 volt inverter and a set of car jumper cables. All for no more than a Friday night out on the town ($130). You can now run your laptop, Blackberry, Ipod, Iphone or any other electronic, wherever you are. Take them camping or to the beach, you’ll never be far from a plug again. This is a simple system to run some basic electronics that anyone can put together.
	Build Your Own Solar Panel: Generate Electricity from the Sun. by Phillip Hurley (Author) Whether you’re trying to get off the grid, or you just like to experiment, Build Your Own Solar Panel has all the information you need to build your own photovoltaic panel to generate electricity from the sun. Now available for the first time in print, this revised and expanded edition has easy-to-follow directions, and over 150 detailed photos and illustrations. Lists of materials, tools, and suppliers of PV cells are included. Every-day tools are all that you need to complete these projects. Build Your Own Solar Panel will show you how to: Design and build PV panels, Customize panel output, Make tab and bus ribbon, Solder cell connections, Wire a photovoltaic panel, Purchase solar cells, Test and rate PV cells, Repair damaged solar cells, Work with broken cells, Encapsulate solar...