Pumping up feeble photons

Your average solar panel has a bit of a Goldilocks problem: photons need to have just the right amount of energy to be converted into free electrons. Too little energy and the photon will pass right through. Too much, and the excess energy disappears as heat. University of Groningen chemists have attacked the feeble photon problem, and published the result in Nature Photonics.

“This problem limits the efficiency of solar panels”, explains professor of organic chemistry Kees Hummelen. The plastic solar cells he works with mainly absorb visible light. “But the spectrum of the sun is much broader. About half of the sunlight that reaches the earth is near-infrared, which plastic solar cells can’t absorb because these photons have too little energy.”

The solution lies in a process called ‘upconversion’. “Basically, you add up two low energy photon to make one with more energy.” There are already compounds that can perform this trick, but they have two weak points: “The conversion process has a very low efficiency of just a few percent, and these compounds only absorb a tiny fraction of the infrared spectrum.”

Hummelen decided to tackle the latter problem. “We made nanocrystals of a rare earth compound that can perform this upconversion and added antenna’s to them.” These antenna’s catch photons that are not normally absorbed by the nanocrystals. “But once they’re caught and transported to the nanocrytsal, they can be upconverted.” Hummelen took his inspiration from nature; plants use antenna-like light harvesting molecules to transfer photons to an active center where photosynthesis takes place.

It took a year to conceive and synthesize the right antenna. “We had to find a molecule that was stable, and could be attached to the nanocrystal.” And then more time was needed to assess how well the antenna does the job. The work was done by PhD student Wenqiang Zou, post doc Cindy Visser and Jeremio Maduro, student of the Master in Education and Communication at the Faculty of Science and Engineering (formerly known as the Faculty of Mathematics and Natural Sciences). Hummelen supervised the work, together with his colleague Maxim Pchenitchnikov. The result is more than satisfying: with antenna’s, the amount of energy the crystals can harvest increased by a factor of 3300. The results were published 15 July on the website of the prestigious journal Nature Photonics.

Although the increase in energy harvesting seems spectacular, it wouldn’t yet increase the overall energy production of plastic solar cells by any significant amount. “We still only harvest a part of the infrared photons. And the efficiency of upconversion in these nanocrystals is less than one percent. We need to boost efficiency ten to one hundred fold to really make an impact.”

The upconversion process is at present beyond the scope of the Hummelen group. But he is trying to harvest a larger part of the infrared photons. And there are a few other lines of inquiry. Upconversion is not just important for solar cells, but also in telecommunications (fiber optics) or medical imaging. “Infrared light can penetrate tissue much better than visible light. By adding upconverting compounds to for example the blood, you can produce an image that way.” And the upconversion process has to be tested in real solar cells. “We have provided a German group with our materials, and they will make them into an actual plastic solar.”

To be continued…

This research was done in the Stratingh Institute for Chemistry and the Zernike Institute for Advanced Materials of the RUG.

Reference: Broadband Dye-Sensitized Upconversion of Near-IR Light , Wenqiang Zou¹, Cindy Visser¹, Jeremio A.Maduro¹, Maxim S. Pshenichnikov² and Jan C. Hummelen^1,2*; ¹Stratingh Institute for Chemistry, University of Groningen, ²Zernike Institute for Advanced Materials, University of Groningen, The Netherlands; Nature Photonics, DOI 10.1038/nphoton.2012.158

Nice to know

The nanocrystals contain the rare earth elements Yttrium, Eterbium and Erbium. You may wonder why these elements have such similar names. That’s because they were all discovered in a quarry near the Swedish village of Ytterby. That same quarry also produced Terbium, Holmium, Thulium and Gadolinium.