Solar power visionaries have long dreamed of solar panels that are cheap, efficient, and hardy enough to withstand the relentless heat of the midsummer sun. Although various solar cells meet one or two of the requirements, nailing all three has proven elusive. Now, researchers report an improved cell that just might do the trick.
Twelve years ago, chemist Michael Grätzel of the Swiss Federal Institute of Technology in Lausanne and colleagues introduced "dye-sensitized" solar cells. At their core is a network of titanium dioxide (TiO2) particles, each just a couple of tens of nanometers across, coated with a film of organic dye. The dye molecules absorb light and then quickly pass energized electrons to a neighboring TiO2 particle, where the electrons race through the particle network to an electrode. Another electrode then feeds low-energy electrons to a solution of ions, which ferry the charges back to the dye molecules, allowing the cycle to repeat. The best such cells can convert more than 10% of the energy in photons into electricity, an impressive rate.
But when the cells warm up, things can quickly go wrong. Dye molecules can fall off the TiO2 particles and dissolve into tiny pockets of water that have seeped into the cell. Conventional electrolytic solutions break down and release carbon dioxide gas, which can build up and burst the cells.
Grätzel's team looked to solve both problems simultaneously. First, the researchers tagged each molecule with a pair of long hydrocarbon chains. The oily appendages made the dyes extremely hydrophobic and therefore less likely to dissolve in hot water. And for the electrolyte, Grätzel's team turned to an "ionic liquid" that is stable up to 170°C. Together, the changes produced cells with a respectable 6% efficiency and helped them endure at 80°C for 1000 hours while losing only a tiny fraction of their performance, the team reports in the 18 May issue of Nature Materials.
If the technology proves robust and reliable, it could produce solar cells at just one-fifth the cost of conventional silicon panels, says Russell Gaudiana, research chief at Konarka Technologies of Lowell, Massachusetts. That still leaves solar power more expensive than electricity from a large coal plant, but it would help the industry close the gap with wind turbines and other rival sources of clean power.