Toward Cheaper, Robust Solar Cells

Cheap and easy-to-make dye-sensitized solar cells are still in the early stages of commercial production. Meanwhile, their inventor, Michael Gratzel, is working on more advanced versions of them. In a paper published in the online edition of Angewandte Chemie, Gratzel, a chemistry professor at the École Polytechnique Fédérale de Lausanne in Switzerland, presents a version of dye-sensitized cells that could be more robust and even cheaper to make than current versions.

Dye-sensitized solar cells consist of titanium oxide nanocrystals that are coated with light-absorbing dye molecules and immersed in an electrolyte solution, which is sandwiched between two glass plates or embedded in plastic. Light striking the dye frees electrons and creates “holes”–the areas of positive charge that result when electrons are lost. The semiconducting titanium dioxide particles collect the electrons and transfer them to an external circuit, producing an electric current.

These solar cells are cheaper to make than conventional silicon photovoltaic panels. In principle, they could be used to make power-generating windows and building facades, and they could even be incorporated into clothing. (See “Window Power” and “Solar Cells for Cheap.”) A Lowell, MA-based company called Konarka is manufacturing dye-sensitized solar cells in a limited quantity. But the technology still has room for improvement.

In existing versions of the solar cells, the electrolyte solution uses organic solvents. When the solar cells reach high temperatures, the solvent can evaporate and start to leak out. Researchers are now looking at a type of material that may make a better electrolyte: ionic liquids, which are currently used as industrial solvents. These liquids do not evaporate at solar-cell operating temperatures. “Ionic liquids are less volatile and more robust,” says Bruce Parkinson, a chemistry professor at Colorado State University.

New dyes are also being investigated. In commercial cells, the dyes are made of the precious metal ruthenium. But researchers have recently started to consider organic molecules as an alternative. “Organic dyes will become important because they can be cheaply made,” Gratzel says. In the long run, they might also be more abundant than ruthenium.

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.