At the high end of the cost-efficiency spectrum are PV materials like gallium arsenide and multilayer cells that are more expensive than crystalline silicon but produce more electricity from a given amount of light. These materials and designs haven't caught on because their cost more than offsets their efficiency. But researchers are working on a solution: By using mirrors to focus and amplify the amount of light hitting the PV material, "concentrating" PV systems are able to generate efficiencies that are high enough, just maybe, to more than cover the extra cost of the system. Soliant Energy, a California start-up staffed by former NASA scientists, uses "triple-junction" cells that capture a wider range of solar energy, making them 40% efficient. Acrylic lenses then concentrate incoming sunlight by a factor of 500 and direct it at the cells. The result: less PV material required for a given power output, and an overall lower cost. But concentrating solar still faces some technical hurdles. Using mirrors to concentrate light requires direct sunlight rather than the generalized light that normal solar cells utilize, which offsets some of the new materials' efficiency advantages. And both the PV materials and the mirrors are expensive, which raises the bar further. So although this technology might eventually be viable, it's further away than thin film.