Here comes the sun

Thin-film solar cells on glass or stainless-steel substrates already have a 7 percent worldwide market share. They are less efficient than silicon cells, at 5 to 8 percent, but correspondingly cheaper. Shell Solar just sold its mono- and polycrystalline silicon solar cell unit to Germany's solar powerhouse, Solarwolrd AG, in order to concentrate exclusively on thin-film solar cells deposited on inexpensive glass substrates.

Since wafers account for up to 50 percent of the cost of silicon solar cells, these nonsilicon cells slash costs by just switching to inexpensive glass, stainless steel or even flexible polymer (plastic) substrates. Formulations include thin films of amorphous silicon, nanocrystalline silicon, cadmium telluride, copper indium (gallium) diselenide and other inorganic alloys.

Inorganic thin-film startups are following the public-funding route to finance their research and development. Nanosolar Inc. (Palo Alto, Calif.), for one, has garnered more than $10 million in government contracts and $48 million in venture capital to develop a cheap roll-to-roll solar cell manufacturing process on which it is collaborating with Lawrence Berkeley and Sandia National Laboratories. Its copper indium gallium diselenide cells use a low-cost substrate that can be processed without the need for vacuum deposition, creating what Nanosolar claims is the world's most cost-efficient solar cell (most watts for the low price).

Nanosys Inc. (Palo Alto) is also developing thin-film solar cells from inorganic nanostructures made from silicon, silicon germanium, cadmium selenide, gallium arsenide, gallium nitride and indium phosphide. By using lightweight, flexible plastic substrates, Nanosys too aims to provide low-cost solar cells using roll- to-roll manufacturing techniques. Last month Nanosys announced that it had secured new government funding amounting to $4.6 million.

The DOE's National Renewable Energy Laboratory reported last year that HelioVolt Corp. (Austin, Texas) had succeeded with its copper indium gallium selenide-based thin-film photovoltaic cells. HelioVolt says that its patented intra-absorber junction depends on depositing two films and forming a semiconducting junction between them by using a heated template to "print" the crystalline lattice structure into the thin film. Using a flash-heating technique similar to anodic wafer bonding, the technique forces selenium into the crystalline lattice with a reusable template capable of mass-producing the material.

Poised over the horizon with the promise of large-area solar cells are dye-stabilized and organic-polymer (plastic) cells. However, organic materials have lower carrier mobility and lower current-carrying capabilities than traditional inorganic materials. Moreover, they can't match today's silicon cell longevity of 25 years.

"Plastic devices using inkjet-style manufacturing hold great promise, but in general suffer from relatively low charge-carrier mobility today," said Tully of Gartner Dataquest. "That's certainly improving, but it represents a disadvantage in the short term." Tully believes that initially, plastic solar cells "will be restricted to applications with relatively low-frequency and low-power uses. But even so, there could still be significant opportunities for low-power personal electronic devices."

Konarka Technologies Inc. (Lowell, Mass.) is treading that decade-long path to organic solar cells. Konarka recently received $6 million from Darpa (on top of $60 million in venture capital it already had) to develop organic solar cells on flexible plastic substrates. "We are printing organic chemical solar cells onto a flexible polymer substrate that can be either transparent or opaque, depending on the formulation," said chief marketing officer Daniel McGahn. The company uses a low-temperature roll-to-roll manufacturing process similar to processing photographic film, and envisions its first products as small, light-activated, flexible "rechargers" that can absorb any kind of light (even from a desk lamp) to recharge cell phones, laptop computers and similar small electronic devices. For the military, Konarka is also developing a portable electric-generating tent. A flexible polymer coating on the outside turns the tent into an electricity generator for the equipment housed within it.

"When the spectral response of these plastic solar cells is widened to include infrared, they could potentially be woven into clothing to generate electrical energy from body heat in the dark and from sunlight during the day," said analyst Tully.

Konarka has developed formulations that cover different parts of the spectrum, concentrating on two basic types of solar cells--dye-sensitized cells with chemical cores and those with nanoparticle "grains" embedded in a polymer-matrix-like film.

"We are printing organic-chemical solar cells onto a flexible polymer substrate that can be either transparent or opaque, depending on the formulation," said McGahn. "Our material is very similar to photographic-film negative material, but our stack of materials is even simpler than photographic film." The material can also be screen-printed for camouflage.