Engadget reports today that scientists have discovered a way to produce solar cells that can capture 100% of the energy contained in the sunlight that reached them:
Researchers over at the National Renewable Energy Lab have reportedly made the first solar cell with an external quantum efficiency over 100 percent. Quantum efficiency relates to the number of electrons-per-second flowing in a solar cell circuit, divided by the number of photons from the energy entering. The NREL team recorded an efficiency topping out at 114 percent, by creating the first working multiple exciton generation (MEG) cell. Using MEG, a single high energy photon 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.
From the original article:
Multiple exciton generation (MEG) is a process that can occur in semiconductor nanocrystals, or quantum dots (QDs), whereby absorption of a photon bearing at least twice the bandgap energy produces two or more electron-hole pairs. Here, we report on photocurrent enhancement arising from MEG in lead selenide (PbSe) QD-based solar cells, as manifested by an external quantum efficiency (the spectrally resolved ratio of collected charge carriers to incident photons) that peaked at 114 ± 1% in the best device measured. The associated internal quantum efficiency (corrected for reflection and absorption losses) was 130%. We compare our results with transient absorption measurements of MEG in isolated PbSe QDs and find reasonable agreement. Our findings demonstrate that MEG charge carriers can be collected in suitably designed QD solar cells, providing ample incentive to better understand MEG within isolated and coupled QDs as a research path to enhancing the efficiency of solar light harvesting technologies.