New cell growth. (Innovative Technologies).

Nearly 90% of electricity-generating solar panels sold today are based on the original practical solar cell technology of the 1970s, which employs crystalline silicon. But these cells are expensive to fashion. Other, newer technologies may prove to be cheaper to make and more efficient at converting sunlight to electricity, with the added bonus of being less environmentally damaging to produce and use. Newest among these technologies is the dye-sensitized "titania" solar cell.

Just over a decade ago, Swiss scientist Michael Gratzel discovered a way to construct an efficient solar cell that relies on the interaction between a molecule-thick layer of an organometallic dye and titanium dioxide, or titania. Now, in an important step for what some call a promising technology, a small company called Sustainable Technologies International, with the aid of an Australian government grant of A$1 million (about US$526,000), has just opened the world's first manufacturing facility for a product that uses titania cells.

The company's first product is an electricity generating, translucent, reddish-brown exterior wall panel. Its efficiency at converting sunlight to electricity is about 5%, well under the 10-16% efficiency of crystalline silicon technologies, and thus would need to be improved to really compete with those cells.

Titania cells work through an elegant "artificial photosynthesis" process. Sunlight hits the dye that coats the inner surface of a porous matrix of titania, which excites electrons. These electrons pass rapidly through the titania matrix, the spaces of which are filled by an electrolyte that supplies electrons to replace the ones lost by the dye. In turn, the electrons lost by the electrolyte are supplied by a second electrode. An electrical load connected between the two electrodes completes the circuit.

The primary advantage of titania cells, says Sylvia Tulloch, a materials scientist and executive director of Sustainable Technologies International, is that they are easier to make and thus cheaper to produce than other types of cells. Rather than using various types of semiconductors that require energy-guzzling machinery and clean room conditions, their artificial photosynthesis generating structures can be built with simple screen-printing equipment. As a result, she says, devices that incorporate these cells will cost less in the long run to make than those that use established technologies.

Titania cells may also prove to be safer for the environment than other technologies. Some of the materials used in silicon cell manufacture, such as carbon tetrachloride, are toxic. "They also use high temperatures and high vacuum, and so use a considerably amount of energy to manufacture," Tulloch says. However, titania cell manufacturing processes and the cells themselves are relatively benign, she says. Titania is nontoxic; in fact, it is used in many brands of toothpaste. The ruthenium dye that Sustainable Technologies International's first solar panels use, as well as the other materials, also present a negligible risk, says Tulloch. According to the draft National Renewable Energy Laboratory report Assessment of the Dye-Sensitized Solar Cell, the amount of toxic materials in a titania cell is just 25% of that in a crystalline silicon cell.

These differences, however, may not be particularly meaningful, according to Vasilis Fthenakis, a senior chemical engineer at Brookhaven National Laboratory who specializes in the potential environmental impacts of solar cells. "There are no significant environmental and safety hazards with any of [the types of solar cells] to the scale that they are manufactured today," he explains. And although there are some hazardous materials used, such as silane gas, cadmium, carbon tetrafluoride, and lead, he says, "if you look at the quantities in relation to their use in other industries, they are very, very small." But these risks will become more significant as the industry grows, he adds. And, according to Steve Hester, technical director of the utility-industry Solar Electric Power Association, current demand for solar cells is outstripping production, and the industry is expected to expand by as much as 40% per year into the foreseeable future.

By no means are titania cells the only potential alternative to crystallin silicon solar cells, however. Other developing technologies include amorphous silicon, cadmium telluride, and copper indium selenide. These others have a head start in both development and the market, says Robert McConnell, who manages research programs in nonconventional solar electric technologies for the National Renewable Energy Laboratory and wrote the lab's Assessment. "[Titania cells are]one of many promising solar electric technologies, and we don't have enough money to develop them all. And frankly they all have a fighting chance--this is a real horse race."