A solar cell that duplicates renewable energies

In the quest for abundant and renewable alternatives to fossil fuels, scientists have sought to harness the sun's energy by "splitting water," an artificial photosynthetic technique that uses sunlight to generate hydrogen from the water. However, the water separation devices are not yet up to their potential, because there is still no design for materials with the right mix of optical, electronic and chemical properties necessary to their effectiveness.

Researchers at the Lawrence Berkeley National Laboratory (National Laboratory) of the United States Department of Energy and the Joint Center for Artificial Photosynthesis (JCAP), a hub of DOE's energy innovation, have put point a new recipe for renewable fuels that could bypass current limitations. Materials: an artificial photosynthesis device called "photoelectrochemical and voltaic hybrid cell (HPEV)" that transforms sunlight and water into one, but into two types of energy – hydrogen and electricity.

Find a way out of the electrons

Most water separators consist of a stack of light absorbing materials. Depending on its composition, each layer absorbs different parts or "wavelengths" of the solar spectrum, ranging from less energetic wavelengths of infrared light to more energetic wavelengths of visible light. ultraviolet.

When each layer absorbs light, it generates an electrical voltage. These individual voltages combine in sufficient voltage to divide the water into oxygen and hydrogen. According to Gideon Segev, a postdoctoral researcher in the JCAP Department of the JCAP Chemical Sciences Division at Berkeley Lab, the problem with this configuration is that, even though silicon solar cells can produce electricity very close to their limit, their high performance potential is compromised when they are part of a water separator device.

The current flowing through the device is limited by other battery materials that do not work as well as silicon. The system therefore produces much less power than it could. The less power it generates, the less solar fuel it produces. it can produce.

"It's like we're always using a car in first gear," Segev said. "It's energy you could reap, but since silicon does not work at its peak power point, most of the excited electrons in silicon have nowhere where so they lose their energy before being used to doing useful work. "

Get out of first gear

So Segev and co-authors – Jeffrey W. Beeman, JCAP researcher at Berkeley Lab's Chemical Sciences Division, and former Berkeley Lab and JCAP researchers, Jeffery Greenblatt, who now heads the technology consulting firm Emerging Futures LLC, based in the Bay Area, and Ian Sharp, now Professor of Experimental Physics Semiconductors at the Technical University of Munich in Germany, has proposed a surprisingly simple solution to a complex problem.

"We thought … And if we just let out the electrons?" Said Segev.

In water splitting devices, the front surface is usually dedicated to the production of solar fuels and the back surface serves as a power outlet. To circumvent the limits of the conventional system, they added an additional electrical contact to the back surface of the silicon component, which allowed for the creation of a HPEV device with two contacts at the back instead of a single one. . The additional rear outlet would divide the current in half, so that part of the current contributes to the production of solar fuels and the rest can be extracted in the form of electrical energy.

When what you see is what you get

After running a simulation to predict if the HPEC would work as expected, they made a prototype to test their theory. "And to our surprise, it worked!" Said Segev. "In science, you're never really sure that everything will work, even if your computer simulations predict it, but that's also what makes things fun." It was great to see our experiences validate the predictions of our simulations. "

According to their calculations, a traditional solar hydrogen generator using a combination of silicon and bismuth vanadate, a material widely studied for the dissociation of water by the sun, would produce hydrogen with a hydrogen yield of 6.8%. In other words, of all incident solar energy reaching the surface of a cell, 6.8% will be stored as hydrogen, and the rest will be lost.

In contrast, HPEV cells collect the remaining electrons that do not contribute to fuel generation. These residual electrons are instead used to generate electrical energy, resulting in a dramatic increase in overall solar energy conversion efficiency, said Segev. For example, according to the same calculations, the same 6.8% of solar energy can be stored as hydrogen in a HPEV cell consisting of bismuth vanadate and silicon, and 13.4% of Solar energy can be converted into electricity. This allows a combined efficiency of 20.2%, three times higher than conventional solar hydrogen cells.

The researchers plan to continue their collaboration to consider using the HPEV concept for other applications, such as reducing carbon dioxide emissions. "It was really a group effort where people with a lot of experience could contribute," added Segev. "After a year and a half of working together on a rather tedious process, we were delighted to see our experiences come to fruition."