First Solar Cell With Over 100 Percent Quantum Efficiency

Researchers from the National Renewable Energy Laboratory (NREL) have reported the first solar cell that produces a photocurrent that has an external quantum efficiency greater than 100 percent when photoexcited with photons from the high energy region of the solar spectrum. A cell’s external quantum efficiency is the number of electrons flowing per second in its external circuit, divided by the number of photons per second entering it, and is different at different wavelengths. The best result for the NREL solar cell was 114 percent. According to researchers it means that solar energy has a competitive future, making it possibly cheaper than energy from fossil or nuclear fuels. 

The team used a process called Multiple Exciton Generation (MEG), whereby a single absorbed photon of appropriately high energy can produce more than one electron-hole pair per absorbed photon. The extra efficiency comes from quantum dots ‘harvesting’ energy that would otherwise be lost as heat. The cell itself uses anti-reflection coating on a transparent conductor, layered with zinc oxide, lead selenide, and gold. NREL scientist Arthur J. Nozik predicted as far back as 2001 that MEG would do the job, but it’s taken until now for the concept to leap over from theory.