For an example of extremely high-efficient solar cells at work, look no further than Mars. The rovers NASA sent to the red planet in 2003 relied on solar panels the size of a kitchen table and capable of converting about 27% of sunlight into electricity. To put that same technology to work on Earth, where the average rooftop solar panel is 15% efficient, the cost would be, well, astronomical. That could change, however, as scientists at the Department of Energy’s National Renewable Energy...