A new breed of materials for solar cells burst into the limelight this year. Known as perovskites, they are cheap, easy to make, and already capable of converting 15% of the energy in sunlight to electricity. While that remains below the efficiency of commercial silicon solar cells, perovskites are improving fast. One particularly promising feature is that they can be layered on top of silicon solar-cell material to harness a range of wavelengths that neither could capture alone. (Science, Top 10 Breakthroughs of 2013, http://news.science mag.org/people-events/2013/12/sciences-top-10-breakthroughs-2013)