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New Research Delivers the
Blue Light Special of Solar Power

by Michael Byrne
Motherboard, November 18, 2012

How the solar spectrum breaks down:  Irradiance (Watts/meter squared) as a function of wavelength. Solar energy is actually quite wasteful. Perhaps not in the same sense of wasteful as burning coal -- in which a limited material is destroyed forever -- but current solar cells don't make the very best use of sunlight because of a very fundamental feature of electricity. A particular trade-off needs to be made between current and voltage, the product of which gives us the total amount of energy that can be carried by a solar cell. It works like this: blue light and wavelengths near blue light have the highest energy photons, but those wavelengths have fewer total photons within them than infrared light. Meanwhile, infrared light has more photons to crash into your solar cell, but those photons have less energy than blue light photons. What that means is that the highest energy blue light goes unused by most solar cells because it lacks the current needed to make its harvest worthwhile.

Cambridge researcher Brian Walker and his team have devised a possible way around the problem -- described in a study out today in Nature Chemistry -- using one of the many quirks of quantum mechanics. He uses a waterwheel analogy for the problem. Imagine dripping water from a very high height onto a waterwheel. Those drops, analogous to blue light photons, hit the wheel with a tremendous amount of energy, but there just aren't enough of them to make the wheel do much. Meanwhile, you have a river pushing directly against the wheel (or falling just a couple of feet), full of water drops -- each without much energy but with a ton of friends sharing the work. In electrical terms, "You can't win by just collecting UV and blue light in a solar cell," Walker told me yesterday. "There's just not enough [photons]."

So, we're stuck with the river doing our solar cell work and being rather wasteful at it. "If you only collect the infrared [photons]," Walker continued, "the problem is that when the solar cell absorbs that light, just because of the nature of the light-matter interaction, any energy that's in excess of the band gap is very quickly turned into heat. It's not useful." Note that the band gap here is referring to the range of light wavelengths at which the solar cell is able to collect light. Our blue light drips just become wasted heat. The maximum theoretical efficiency of a conventional solar cell is only 34 percent.

The Cambridge solution uses something called singlet fission. Essentially, it's a way of splitting the energy of a single photon into two particles using a quantum property known as spin. It's a brief moment of charge-sharing between two particles that allows a doubling in current for only picoseconds, but long enough to boost the total power available from blue light in a sort of two-for-one deal. The new maximum efficiency using this method would be 44 percent.

What's more, the method described here (that's also being pursued by a team at MIT, Walker noted) doesn't require a complete rebuild of current solar power technology. Part of the entire point of the research is finding a cheap technique for capturing blue light outside of the typical band gap. There are current solar cells that can capture many different wavelengths of light, but they rely on multiple layers of material and are typically prohibitively expensive. Imagine instead of just the one water wheel, you create an entire array of them, one for each of the possible energies of falling water.

"Really our goal is to have a new technology that is fully integrated with what we've already got," Walker said. "An ideal case would be a coating." A spray-on green future? One can hope.

Michael Byrne
New Research Delivers the Blue Light Special of Solar Power
Motherboard, November 18, 2012

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