Solar Cell Captures Multiple Excitations

Quantum dots, a class of semiconductor nanocrystals, can serve as the basis of an efficient photovoltaic system for exploiting high-energy light’s propensity for generating multiple electronic excitations, according to a report (Science 2010, 330, 63). As photons with a minimum characteristic energy impinge on a semiconductor, they stimulate electronic excitations, which lead to electron-hole pairs and trigger the process that generates a current. Photons in the high-energy portion of the solar spectrum can generate multiple excitations, which in principle could lead to enhanced electrical output. The excess energy, however, is generally lost as heat. Justin B. Sambur and Bruce A. Parkinson of the University of Wyoming and Thomas Novet of optoelectronics maker Voxtel, in Beaverton, Ore., prepared a photovoltaic device by first coating lead sulfide quantum dots with a passivating layer of mercaptopropionic acid and then attaching the quantum dots to a titanium dioxide crystal via thiolate and carboxylic acid units. That setup, in which the PbS and TiO₂ energy levels are favorably aligned, enabled the team to measure a quantum yield—the ratio of photons in to electrons out—greater than one, signifying multiple electron excitations.