Researchers create working solar cell that harvests additional electrons

MEG solar cell
Researchers have created the first solar cells capable of harvesting more than one electron for every photon of light that hits it. Credit: Science/AAAS
Solar cell researchers have long known that much of the energy in sunlight gets wasted instead of converted to energy when it hits a solar cell.   This happens because the different wavelengths of sunlight contain different amount of energy, and that some of the higher energy photons (for example from the violet and ultraviolet portion of the spectrum) actually have too much energy, and instead of generating electrons, the excess energy just gets lost as heat.    These researchers also knew that these high energy photons could actually produce more than one electron if they hit nanometer sized particles called quantum dots, through a process called multiple exciton generation (MEG).

But up until now, while labs were able to generate these additional electrons using lead sulfide quantum dots on top of another semiconductor, nobody could figure out how to collect them to make a workable solar cell.    This week, however, a team led by Arthur Nozik at the National Renewable Energy Laboratory (NREL), reported that they’ve created the first working MEG solar cell.   The team figured out how to make clusters of lead and selenium about 5 nanometers in diameter separated by short-chain organic molecules that let the electrons flow easily to a nearby electrode.

Their multi-layered working solar cell consisted of a layer of antireflection glass with a thin layer of a transparent conductor, a nanostructured layer of zinc oxide, the quantum dot layer of lead selenide, and a thin layer of gold making up the top electrode.

While the overall efficiency of the cell is still pretty low (about 5%), the cells are collecting about 30% more electrical charges than the number of photons that hit the cell – demonstrating success as the first true MEG solar cells.

Analysis of the cells shows that the extra electrons are generated when the energy in the sunlight is 2.5 to 3 times that amount that’s normally needed to generate electrons in non-MEG cells.    The team is working on getting this level down to about 2 times the normal energy level.   If Nozik and his team can do that, it should dramatically increase the efficiency of their solar cells.  The team also feels that this type of solar cell could be made using an inexpensive roll-to-roll manufacturing process.

via: Science 


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