Multiple exciton generation

In a typical semiconductor photovoltaic cell, each incoming photon with energy greater than the bandgap generates one charge-carrier pair at the bandgap energy, and any excess energy is lost as heat. Now, researchers led by Arthur Nozik and Matthew Beard (both at the National Renewable Energy Laboratory, NREL, in Golden, Colorado) have created solar cells, based on lead selenide nanocrystals, that can produce a photocurrent with more than one carrier pair per incident high-energy photon—a possible path toward higher efficiency. The key process, multiple exciton generation (MEG), had been observed spectroscopically, but to make use of it in a solar cell, it’s necessary to get the charge carriers off the nanocrystals and into an external circuit. Toward that end, the NREL researchers treated their nanocrystals with ethanedithiol and hydrazine, two small organic molecules that replaced the long-chain hydrocarbons on the nanocrystals’ surfaces. As a result, the nanocrystals could pack more closely together, and charge carriers could jump from one to another. The cells aren’t breaking any efficiency records yet: MEG was observed over just a small photon energy range, and it was not nearly as efficient as it could be. The figure shows the results for several of the devices: At most 1.14 carrier pairs were collected per photon with energy more than four times the bandgap. The NREL researchers are now working on optimizing the cells to increase their efficiency. (O. E. Semonin et al., Science 334, 1530, 2011.) —Johanna Miller