Dye-Sensitized Solar to Go

The first commercial product to incorporate dye-sensitized thin-film solar cells will soon be on the market. Backpacks coated with the cheap, lightweight, and flexible solar cell, for on-the-go recharging of portable gadgets, were unveiled at the Hong Kong Electronics Fair last week.

The solar cells, made by Cardiff, U.K.-based G24 Innovations, are based on technology invented by Michael Grätzel, a chemistry professor at the École Polytechnique Fédérale de Lausanne in Switzerland.

In this type of solar cell, dye-coated semiconductor nanocrystals are sandwiched between glass panels or embedded in plastic along with an electrolyte. The dye absorbs light and creates electrons, which are transferred to the semiconductor and then out into a circuit. Dye-sensitized cells have lower light-to-electricity conversion efficiencies than the best thin-film solar cells, but they are considerably cheaper to manufacture and can also be printed on flexible surfaces.

Grätzel says that dye-sensitized solar cells have further practical advantages over other thin-film solar technologies. Amorphous silicon thin-film cells degrade in sunlight over time, and their efficiency also goes down if sunlight hits them at an angle. Dye-based cells work well at wide angles and are longer lasting. Plus, they work more efficiently in indoor light, because the dye absorbs diffuse sunlight and fluorescent lighting well.

G24 Innovations says that it uses a low-cost, roll-to-roll process to make its flexible solar modules, which produce 0.5 watts of power under direct sunlight. Last week, the company shipped its first solar-module shipment to Hong Kong-based company Mascotte Industrial Associates, which makes the new bags. G24 uses ruthenium dyes coated on titanium dioxide nanocrystals and an iodide-containing nonvolatile electrolyte. The company's cells are over 12 percent efficient at converting light into electricity.

G24 plans to market modules that could be patched on clothing, tents, and awnings. The modules could also be cheaply incorporated into power-generating windows and billboards. "It's definitely a great moment for us," Grätzel says. "There has been talk of when the first commercial product will be coming out, and this has happened now."

Only one other company, Dyesol, is close to making commercial dye-sensitized solar-cell products. In October 2008, Dyesol opened a factory in Queanbeyan, Australia, to make tiles that can be integrated into building facades. Electronics giant Sony is also conducting research on dye-sensitized solar cells and announced last year that it had reached efficiencies of 10 percent--a level necessary for commercial products. The electronics maker showcased conceptual lamps based on these cells, but it doesn't have commercial products in the pipeline yet.

The solar cells may have a small niche in the market right now. But, says Michael McGehee, materials science and engineering professor at Stanford University, "in the future we may see this technology compete with the more traditional thin-film solar technologies based on amorphous silicon, cadmium telluride, and cadmium indium gallium arsenide if the combination of efficiency, cost, and durability improves."

"It's an exciting time for dye-sensitized solar cells," Grätzel says, adding that he expects to see many more products on the market soon.

Other researchers, meanwhile, are working on advanced versions of the cells using nonvolatile electrolytes and organic dyes, which could make them even cheaper and more robust.