Solar cells that work in low light could charge devices indoors
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.
The problem with DSSCs is that the holes don’t move through the electrolyte very quickly. As a result, holes tend to pile up near the dye and TiO2 particles. If an excited electron ends up bumping into a hole, they merge, generating heat instead of electricity.
To get around this, researchers have tried to make their electrolyte layers thin, so that the holes don’t have to travel very far to reach their goal. But any imperfections in those thin layers can cause the devices to short, a fatal blow that kills the whole solar cell. Now, Graetzel and his colleagues have now come up with a possible solution. They designed a combination of dye and hole-conducting molecules that wrap themselves tightly around TiO2 particles, creating tight-fitting layers without any imperfections. That means slow-moving holes have less distance to travel before reaching the negative electrode. The tight layers, they report today in Joule, increase the diffuse light efficiency of their DSSCs to 32%, near the theoretical maximum.
“It’s really a nice advance,” says Michael Wasielewski, a chemist at Northwestern University in Evanston, Illinois. The new devices still only convert 13.1% of direct sunlight to electricity. But he notes that because the diffuse light efficiency is nearly 20% higher, it raises hopes that new ways might be found to boost the efficiency of the devices under full sunlight. And because DSSCs are far cheaper to produce than silicon solar cells, if they can approach silicon’s efficiency at a lower cost, that should be a winning formula. Until then, diffuse light DSSCs can at least help us power a host of devices without cords, plugs, or external power. Numerous companies are already working to outfit building interiors with an earlier generation of DSSCs. And Graetzel says he believes the new and improved cells will only speed up the adoption of the technology.