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Supercharging Solar

For many decades, researchers have investigated various methods to double the power output of solar cells by making use of “hot electrons”.

Recently researchers at Boston College have provided new experimental evidence that this theory will work.

The team built solar cells that get a power boost from high-energy photons.

This boost, the researchers say, is the result of extracting hot electrons which are normally wasted in typical solar cells. The results are a step forward for solar cells that break conventional efficiency limits.

Because of the way ordinary solar cells work they can, in theory, convert at most about 35% of the energy from sunlight into electricity, the remaining energy escapes as heat. Making use of hot electrons could result in efficiencies as high as 70% says Matthew Beard, a senior scientist at the National Renewable Energy Laboratory in Golden. He feels that doubling the efficiency of solar cells could cut the cost of solar power by half.

Conventional solar cells can only efficiently convert the energy of certain wavelengths of light into electricity. For example, when a solar cell is optimized for red wavelengths of light, it absorbs photons of red light and produces electrons with energy levels similar to those of the incoming photons.

When the cell absorbs higher energy photons like the blue photon, it first produces a hot electron. But this loses much of its energy very quickly as heat before it can escape the cell to produce electricity. Conversely cells optimized for blue light do not convert red light into electricity, so they sacrifice the energy in this part of the spectrum.

The Boston College researchers made ultra-thin solar cells just 14nm thick. Because the cells were so thin, the hot electrons could be pulled out of the cell quickly before they cooled. The researchers found that the voltage output of the cells increased when they illuminated them with blue light rather than red.

“Now we’re getting the electrons from the blue light out before they lose all of their excess energy,” says Michael Naughton, a professor of physics at Boston College.

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