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Making Cheaper Solar Cells

Startup Heliovolt could help bring high-performance thin-film solar cells to market.
September 12, 2007

Powered by $77 million in new investment, startup Heliovolt, based in Austin, TX, will build a factory next year for mass-producing a new type of solar cell that could, in much of the United States, make solar electricity as cheap as electricity from the grid. The company will be scaling up a new manufacturing technique that could produce high-performance thin-film solar cells more reliably than other methods.

Cheap power: A solar module (top) is complete with integrated electrodes (light markings) for collecting electricity. A micrograph (bottom) of the semiconductor material is at the heart of the device. Alternating regions with different electronic properties (delineated by the dotted lines) quickly guide electrons and their positive counterparts, holes, out of the material, improving efficiency.

Heliovolt is one of several startups developing a type of thin-film solar cell that converts light into electricity with a micrometers-thick layer of a copper-indium-gallium selenide (CIGS) semiconductor. Thin-film solar cells are attractive because they could produce electricity cheaper than conventional silicon solar cells. Although thin-film cells produce less electricity per square meter than conventional silicon solar cells do, they make up for this by using orders of magnitude less active material per square meter. This can result in significant savings. For example, generating one watt of electricity requires about 80 cents’ worth of silicon, but it only requires a penny’s worth of a semiconductor used in a thin-film cell, says John Benner, who manages electronic materials for photovoltaics research at the National Renewable Energy Laboratory (NREL), in Golden, CO. (Heliovolt is working with NREL to further develop its cells.)

The challenge has been to reliably make thin-film solar cells at a large scale. In the lab, CIGS solar cells have shown the highest efficiency of any thin-film cell (19.5 percent), exceeding that of some types of silicon solar panels made today. But, while no one expects to reach this level of efficiency in mass-produced cells, it has proved difficult to reliably make them with even a minimum level of efficiency needed to compete with other types of solar cells.

Heliovolt’s new manufacturing method, however, could prove more reliable than others, Benner says, by providing more control over the composition of the semiconductor film.

In a typical process, the precursor materials are printed or sputtered onto a surface, where they combine to form the final semiconductor material. This doesn’t provide much control over the chemistry and microscopic structure of the material, he says. In particular, it can allow atoms of selenium, a volatile element, to escape, altering the electronic properties of the material. Having too few selenium atoms can “kill a cell,” says Heliovolt’s CEO, Billy Stanbery. In the Heliovolt process, which Stanbery developed, the semiconductor is made in two steps. First, films of cadmium selenide and indium selenide, which are relatively easy to make reliably, are deposited on two flat plates. Then these plates are brought together and, through a combination of electromagnetic attraction and heat, fused together. Benner says that this process keeps the selenium from escaping, since it’s trapped between the two plates.

So far the company has demonstrated the process only on relatively small, 15-centimeter-wide solar modules. Its goal for early next year is to build a factory to produce, first, 30-centimeter modules, and later–most likely sometime after June–prototypes of modules that are about three-quarters of a square meter (60 by 120 centimeters). The larger size is necessary for bringing the product in line with standard module sizes, making them cheaper to install. The factory will be designed to produce enough cells per year to generate 20 megawatts of electricity at first, and eventually 40 megawatts. A typical wind turbine, in comparison, might generate two megawatts of electricity.

The first of the company’s large cells will have efficiencies of 10 to 12 percent, Stanbery estimates, or slightly better than other thin-film solar cells. In 10 years, he expects those figures to increase to 13 to 15 percent, or good enough to compete with some types of conventional silicon solar cells. Stanbery also expects to manufacture cells for less than First Solar, a Phoenix, AZ-based maker of a different type of thin-film solar cell that has seen high demand for its products. (See “Thin Film’s Time in the Sun.”)

Ultimately, the most important figure is the cost of electricity from the cells once they’ve been installed–the bottom-line cost for customers. Heliovolt is at too early a stage to provide concrete information, since this cost depends on a number of factors in addition to the cost of the modules, including the cost of installation and other hardware. But the company’s target is to reach 15 cents per kilowatt hour of electricity by 2010. That’s less than the cost of grid electricity in half of the world’s markets, according to an industry report. This goal puts the company in a race with conventional solar cells, which, according to the report, could reach similar costs by that time. Currently, the cost of solar electricity is about 25 cents per kilowatt hour. To further cut costs, Heliovolt intends to incorporate the solar modules into building materials, such as the glass facades on skyscrapers.

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