In a dye-sensitized solar cell, dye-coated semiconductor nanoparticles are sandwiched, along with the electrolyte, between two glass plates. The dye molecules absorb light and generate electrons, which are transferred to the semiconductor and on to the external circuit. Meanwhile, positively charged holes go to the electrolyte. The big problem with organic dyes in the past has been the difficulty of keeping the charges separated: they tend to recombine and lower currents.
The dye molecule that Wang and his colleagues designed increases the cell’s efficiency in three ways: it quickly shuttles electrons to the semiconductor particles; it keeps the electrons and holes from recombining; and it covers a broader spectrum of light, absorbing more red light than was possible before.
At least two companies are commercializing dye-sensitized solar cells. These devices are up to 11 percent efficient and use ruthenium dyes and a volatile electrolyte. In October 2008, Dyesol opened a factory in Queanbeyan, Australia, to make tiles that can be integrated into building facades. G24 Innovations, in Cardiff, U.K., is making solar chargers for mobile phones.
Wang says that his new work, published online in the journal Chemical Communications, could make the technology cheaper and open up broader applications. “At the moment, the use of toxic and volatile solvents in high-efficiency cells is a big hurdle for the large-scale application of dye-sensitized solar cells,” he says.
The researchers are working to boost efficiency even more. Wang says that will involve “mainly extending the spectral response of sensitizers to the infrared and the design of better solvent-free electrolytes.”
Meanwhile, Grätzel and his colleagues at Lausanne have set themselves a lofty goal. They plan to reach efficiencies of 14 percent by the end of next year using nonvolatile electrolytes. As for the dye, either ruthenium-based dyes or organic dyes could win the race, he says. And he is optimistic about cheaper organic dyes. “If you see how efficiencies of both have gone up, the slope is steeper for organic molecules,” Grätzel says. “If you extrapolate, it could be a year or so before they overtake.”