In the recent paper, Gratzel and his colleagues describe making a dye-sensitized solar cell that combines these two material advances. In their prototype cell, they use an ionic liquid as the electrolyte and a dye based on the organic compound indoline. The solar cells convert light to electricity with an efficiency of 7.2 percent. Ruthenium-based dyes get efficiencies of about 11 percent, says Gerald Meyer, a chemistry professor at Johns Hopkins University. But, he says, “to my knowledge, these are the highest efficiencies with organic [dyes].”
In a dye-sensitized solar cell, electrons go to the titanium dioxide layer, while the holes go to the electrolyte. This separates the charges so that they do not recombine and reduce the current generated by the cell. Keeping the charges separated is the challenge with organic dyes. Gratzel and his colleagues attach long hydrocarbon chains to one end of the indoline-based dye molecule. These hydrocarbon chains, which do not conduct electrons, act as barriers between the titanium dioxide layer and the electrolyte. “It is like a molecular insulator that stops electrons from coming out and recombining with the positive charges in the ionic liquid,” Gratzel says.
With this charge barrier in place, the researchers can make the titanium dioxide layer thinner. That shortens the distance that the electrons have to travel to get to the external circuit, increasing the cell’s efficiency.
Parkinson cautions, though, that work on organic-dye solar cells is still at a very early stage. Going from a laboratory prototype to a commercial module typically reduces efficiencies significantly. To capture a larger share of the solar-power market, dye-sensitized solar cells will require some more improvements. “We really need a breakthrough to get up to 15 percent efficiency in the lab,” Parkinson says.