Solar Shingles: Subhendu Guha
by Martha Davidson
In 1998, Subhendu Guha and his invention were featured in the Lemelson Center's Solar Shingle Challenge, as part of the Center's year-long Inventing for the Environment programming. In May, four middle school classes in different parts of the country--the Ormond Stone Middle School in Centreville, Virginia; the Smith Middle School in Troy, Michigan; the Porter Middle School in Missoula, Montana; and the Webster Middle School in Webster, Texas -- were introduced to the technology through a video conference. Each class was given a flexible solar panel and challenged to experiment with it. In October, Guha met with students from three of the schools in another electronic field trip. The students reported on their experiments and demonstrated inventions they had made -- ranging from a nacho cooker to a Halloween ghost with eyes that lit up--all using power generated by the solar cells. Guha responded to the students' questions about solar energy and about his life as an inventor. In doing so, he also encouraged them to be innovators in their own lives.
"Don't be afraid to try new ideas," he told them, "and don't be disheartened if it doesn't work the first time. ...To put an idea into practice can take time. How long do I have to wait to know if my invention is a success? Sometimes it can take a very long time, ten or fifteen years...And there is a difference between discovery in a fundamental area and invention of a product. If you are a scientist, you may discover something which is not used in a practical way, but that is also immensely important.
"Reading is extremely important, because that helps you in making decisions. As you read more and more, you will ask questions. And you will try to solve those questions. And you get the biggest thrill when you realize you have thought of something new, and it works."
If there was a single most important influence that led Subhendu Guha to his career in science and invention, it was reading. Guha, a scientist noted for his pioneering work in amorphous silicon, is now executive vice president of United Solar Systems Corporation in Troy, Michigan, and the leading inventor of flexible solar shingles, a state-of-the-art technology for converting sunlight to electricity. As a child, however, he was not primarily interested in chemistry or electronics. Born and brought up in Calcutta, a major city in eastern India with a rich literary tradition, he was as drawn to literature as he was to science or technology. Reading was his favorite pastime; he devoured anything he could get his hands on, from textbooks to fiction. Looking back now, he realizes that it was through reading that he broadened his horizons sufficiently to make informed choices in his path of study. He grew up in a large family, with four brothers and five sisters. His father was a respected attorney and his mother, a full-time homemaker, was a brilliant woman whose innate creativity made a strong impression on her son. It was not until he began studying physics at Calcutta's prestigious Presidency College and later did graduate work at the University of Calcutta that Guha became serious about science. There he was exposed to a rigorous approach to research, which he found very exciting. "In physics, you ask really fundamental questions, and try to get answers to them," he explains. "I started working on semiconductors, which are the building blocks of most of the electronics we use today."
After earning a Ph.D. from the University of Calcutta in 1968, he joined the Tata Institute of Fundamental Research, an internationally renowned academic research institution in Bombay, India. There he began investigating certain properties of semiconductors. Semiconductors are a class of materials whose capacity for conducting electrical charges falls between those of true conductors (such as copper and other metals) and those of insulators (such as rubber or glass). "It was at that time I got interested in the use of semiconductors to convert sunlight into electricity" he recalls. "I liked it for two reasons. One is that I found the problems intellectually challenging. I also liked it because it could address societal problems."
The conversion of sunlight to electricity is known as photovoltaics. There are a number of semiconductors that can be used for photovoltaics, but Guha's concern for societal and environmental problems led him to focus on amorphous silicon, an element found in sand that can be applied as a thin film to produce a photovoltaic material. Other semiconductors that can be used for thin-film photovoltaics require the use of certain toxins in their production, but amorphous silicon presents no hazards to humans nor to the environment in its manufacture or use.
In 1974-1975, Guha did a year of post-doctoral training at the University of Sheffield in England, still pursuing his interest in amorphous silicon. Returning to the Tata Institute, he began to explore a way of producing amorphous silicon that had not been tried before. His research led him to believe that by adding hydrogen in the production process, a more useful form of amorphous silicon could be created, and he published his results in 1981. It was a major breakthrough in the field, and today most amorphous silicon produced anywhere in the world is made with hydrogen, just as Guha had described.
Although he took pride in this important achievement, he did not cease his exploration of the possibilities of this material. Up to this point, in graduate school, at the Tata Institute, and in Sheffield, he had been involved in academic research, expanding the frontiers of scientific knowledge. He enjoyed the research immensely, yet he was also curious about the practical applications of his work. He had heard about a company in the United States called Energy Conversion Devices (ECD), founded by Stanford R. Ovshinsky, a leader in the field of photovoltaics. The company was located in Michigan and promoted the use of solar energy for a variety of applications. Guha wrote to Ovshinsky and asked if he could spend a sabbatical year at ECD; Ovshinsky welcomed him, but said that Guha should make it a two-year sabbatical, because one year would not be long enough.
Guha, his wife, and their son arrived in the United States in 1982. By the time his two-year sabbatical was drawing to a close, Guha had decided to resign from the Tata Institute and continue his work with ECD. He had become hooked by the challenges of industrial research, which demanded more focused, practical results than did research in the academic realm. Gradually his responsibilities expanded beyond research and development to include administration, manufacturing, and sales as an executive of United Solar Systems, a joint venture company started in 1990 by ECD and Canon, Inc., of Japan to manufacture solar cells.
Conventional solar energy technology consisted of photovoltaic cells made with crystalline silicon. Crystalline silicon, like amorphous silicon, is a semiconductor. When sunlight falls on a semiconductor, it is absorbed to create charges (positive or negative) that are separated and then channeled to produce an electric current. Crystalline silicon has a very orderly atomic structure, whereas amorphous silicon has a random arrangement of atoms. The rigid atomic structure of the crystalline silicon actually makes absorption of light more difficult than in the looser arrangement of amorphous silicon. The conventional solar cells were therefore thick, required a rigid and heavy support, and were expensive to produce. They were not cost-effective, since it took seven or eight years of operation to recoup the energy spent in their production. ECD and United Solar Systems were interested in developing lighter, less expensive solar cells using amorphous silicon, which could be a hundred times thinner than crystalline silicon with equal capacity for light absorption.
In 1994, while giving a presentation about the company's products, Guha showed a slide of rooftop solar cells. An elderly architect in the audience exclaimed, "But it's so ugly! Who would want that on their house?" When Guha returned to his office in Troy, that comment was on his mind. He called a staff meeting and proposed that they find a way to make solar cells look more like standard roofing material. Perhaps, he suggested, the cells could even be incorporated into roof shingles rather than mounted separately on a metal frame. His team of twenty-five, including researchers and other staff members, immediately began working on the problem. They studied roof shingles and experimented with production processes. The United States Department of Energy, which was interested in the potential of solar energy, supported their efforts with a three-year, $6.26-million cost-sharing grant.
The product they developed is a photovoltaic panel, seven feet long and a foot wide, that is lightweight and flexible, yet rugged and durable; it has no moving parts and is easy to install. Produced in a brownish color with subtle variations, it blends in well with conventional roof shingles. This innovation was not in the basic technology of photovoltaics, but in the design, materials, and production process. Manufacturing begins in Michigan, with a half-mile-long, one-foot-wide roll of stainless steel that is fed through four machines in an automated system. First, the steel is washed to remove surface dirt. Then two layers of a reflective coating are applied. Next, layers of amorphous silicon and amorphous silicon-germanium alloys are applied to create a kind of triple-decker sandwich: each layer absorbs a different photon-energy or wavelength of light (blue, green, or red). The long strip is then cut into one-foot lengths, which are shipped to a plant in Mexico for final assembly. There, the pieces are joined to form seven-foot-long shingles, which are wired and encapsulated with a plastic protective coating.
The flexible panels can be mounted on a roof with nails in the same way that ordinary roof shingles are attached. The only difference is that a small hole must be drilled in the roof every seven feet so that wires can be dropped from the panels into the building. Then an electrician can hook the wires up to the building's electrical boxes to channel energy to house circuits, batteries, or appliances. Flexible solar shingles went on the market in 1997. United Solar Systems now holds approximately 160 United States patents on the design, technology, and production process and has become the world's largest manufacturer of roof-mounted solar cells.
While amorphous silicon solar cells are not yet as powerful as conventional crystalline silicon cells (they have an energy conversion rate of about ten percent, compared with seventeen percent for crystalline cells), they are actually more cost-effective, since both their materials and manufacturing costs are far lower than the production costs of standard solar cells. Moreover, their energy conversion rate is rising, as United Solar Systems continues to refine and improve the product as well as to lower its production cost. Flexible solar cells are still prohibitively expensive in communities where there is an established infrastructure for producing and distributing electrical power. But in areas of the world where power plants have not yet been built, these cells offer a low-cost alternative to diesel generators. In fact, they are already being used in many communities in Egypt, Mexico, and other developing countries. They also have great potential, as their conversion efficiency improves and their production cost decreases, to provide safe, clean, renewable energy for home use in sunny states like Arizona and California. Eventually, this source of energy could provide an important alternative to fossil fuels, reducing both pollution and dependency on foreign suppliers.
Guha and United Solar Systems are now exploring other applications for their technology, using even lighter and more flexible materials. They have produced nylon-backed cells that can be attached to a boat or tent, or folded and carried in a backpack for military use or camping. By replacing the stainless steel with a tough, very thin, extremely light plastic film, they are developing photovoltaic cells that can be used on satellites, space stations, or exploratory space vehicles. In fact, one version of this plastic-backed cell will soon be tested on the Mir space station.
The importance of Guha's innovation has already been recognized, not only by the U.S. Department of Energy, but by others who are concerned about environmental technologies. In 1996, the flexible solar shingles were named the "Best of What's New," receiving the Grand Award from Popular Science. In 1997, they were selected by a panel of environmental experts for Discover Magazine's Technology Innovation Award, which recognizes contributions that "improve the quality of our everyday life and alert us to what is next from the frontiers of human achievement and ingenuity."
Experiments with Solar Shingles
All text and images © Smithsonian Institution. Updated 26 February 1999.