By Robert F. Service for Science Magazine
Imagine never having to charge your phone, e-reader, or tablet again. Researchers report that they have created solar cells that work at a record efficiency for making electricity from the low-intensity diffuse light that is present inside buildings and outside on cloudy days. The solar cells could one day lead to device covers that continually recharge gadgets without ever having to plug them in.
Diffuse light solar cells aren’t new—but the best ones relied on expensive semiconductors. In 1991, chemist Michael Graetzel of the Swiss Federal Institute of Technology in Lausanne invented so-called dye-sensitized solar cells (DSSCs) that work best in dim light and are cheaper than the standard semiconductors. Yet under full sun, the best DSSCs convert only 14% of the energy in sunlight to electricity—versus about 24% for standard solar cells—essentially because the energy comes too fast for DSSCs to handle. When the energy comes at a slower pace, as it does with low-intensity indoor light, Graetzel’s DSSCs could convert up to 28% of the light energy they absorb into electricity.
DSSCs also work a bit differently from standard silicon solar cells. In standard cells, absorbed sunlight kicks electrons on silicon atoms up to a higher energy level, allowing them to skip across neighboring atoms towards a positively charged electrode. There they are collected and shunted into an electrical circuit where they can do work. The departed electrons leave behind vacancies in the atoms called holes that, oddly enough, can also move around. Over time, the holes travel to the negatively charged electrode where they are filled with electrons from the external circuit. This rebalances the charges in the solar cell’s silicon atoms, allowing it to continue to generate electricity.
DSSCs take things up a notch. They still have two electrodes that collect negative and positive charges. But in the middle, instead of just silicon, they have a different electron conductor, typically a collection of titanium dioxide (TiO2) particles. TiO2 is a poor light absorber, however. So, researchers coat the particles with organic dye molecules that are exceptional light absorbers. Absorbed photons of light excite electrons and holes on these dye molecules, just as in the silicon. The dyes immediately hand off excited electrons to the TiO2 particles, which zip them along to the positive electrode. The holes, meanwhile, are dumped into a charge-conducting liquid called an electrolyte, where they percolate through to the negatively charged electrode.
