In the sometimes strange world of nanoscale materials, unexpected things can happen. This is exactly what scientists at the National Renewable Energy Laboratory (NREL) have discovered while exploring quantum dots (QDs). These semiconductor nanocrystals typically have diameters from about 2 to 10 nanometers (nm, or one billionth of a meter) and contain only hundreds to thousands of atoms. But they could do great things when it comes to generating electricity.
Semiconductor quantum dots used in so-called “third-generation” solar cells have the potential to dramatically increase—in some cases even double—the efficiency of converting sunlight to electricity. The conversion process works via “multiple exciton generation (MEG).”
In this process, when a single photon of light of sufficient energy is absorbed by the quantum dot, it produces more than one bound electron-hole pair, or exciton. NREL scientists were the first to predict this important unusual MEG effect in QDs, which contrasts with conventional photovoltaic (PV) cells having much larger crystals and many more atoms and in which one photon produces only one electron-hole pair. The electronic process is also very fast, occurring within 200 femtoseconds—or 200 million billionths (10-15) of a second.
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