Applied physics Solar cells to dye for

Photovoltaic conversion of solar energy -- from photons to electrons -- has so far been dominated by solid-state devices, usually made of silicon and profiting from the expertise of the semiconductor industry. That dominance is now being challenged by new generations of photovoltaic cells. On page 616 of this issue, McFarland and Tang present an intriguing embodiment of a converter that is based on light harvesting by dye molecules on a metal surface. Contrary to expectation, their device shows a strikingly high internal quantum efficiency for electric-current generation. The silicon used in most of today's solar cells must fulfil several tasks. It must absorb sunlight, converting photons into negative- and positive-charge carriers (electrons and holes, respectively); it must transmit an electric field to separate the electrons and holes; and it must then conduct these carriers to the current collectors. To achieve all this simultaneously, materials of very high purity are needed, and consequently silicon-based solar cells are too costly to compete with conventional means of producing electric power.