To squeeze the most energy we possibly can out of the Sun, we need to improve the efficiency of solar cells. Currently, most technologies only capture the visible light, meaning the rest of the spectrum goes to waste. Now, researchers at Lawrence Berkeley National Laboratory have developed a way to use nanoparticles coated in special organic dyes to convert near-infrared light into visible light, which could allow solar cells to harvest more of the spectrum.

On their own, the particles are known as upconverting nanoparticles (UCNPs), and they contain metal ions of lanthanides like ytterbium and erbium. The former absorbs near-infrared light and passes it to the latter, which then releases it as visible green light. Coating the UCNPs in special dyes was later found to improve that function – although figuring out why that was happening was a challenge.

"The dyes appeared to degrade almost immediately upon exposure to light, and nobody knew exactly how the dyes were interacting with the nanoparticle surface," says Emory Chan, co-lead author of the study.

Now, the Berkeley Lab scientists claim to have nailed down the mechanism, and used that knowledge to design better-functioning systems. The team found that the lanthanides in the particles cause the dyes to enter a triplet state, which allows it to merge multiple infrared photons into single visible light photons, transferring energy to the lanthanides more efficiently.

"The dyes act as molecular-scale solar concentrators, funneling energy from near-infrared photons into the nanoparticles," says P. James Schuck, co-lead author of the study.

Observing the dye's light emission and the UCNP's absorption showed that the peaks of both measurements lined up, revealing how these nanoparticles were functioning.

Based on that understanding, the researchers designed new UCNPs to better take advantage of the triplet effect. After ramping up the concentration of lanthanides in the particles, from 22 percent to 52 percent, the team found that their new dyed UCNPs were 33,000 times brighter and 100 times more efficient than they would be without the dye.

Unfortunately, the downside is that the dyes are still very unstable – for these experiments, the researchers had to keep them in nitrogen environments. Future work will focus on developing protective coatings for the UCNPs.

Once those kinks are ironed out, the team says the particles could be used to make solar cells that can harvest more of the light spectrum. Since they're transparent to visible light, a layer of them can be placed over the top of regular solar cells.

"These organic dyes capture broad swaths of near-infrared light," says Bruce Cohen, co-lead author of the study. "Since the near-infrared wavelengths of light are often unused in solar technologies that focus on visible light, and these dye-sensitized nanoparticles efficiently convert near-infrared light to visible light, they raise the possibility of capturing a good portion of the solar spectrum that otherwise goes to waste, and integrating it into existing solar technologies."

The nanoparticles could also be used in optogenetic systems and for biological imaging.

The research was published in the journal Nature Photonics, and the team describes how the UCNPs work in the video below.

Source: Berkeley Lab