Researchers combine two types of photovoltaic material to make a cell that harnesses more sunlight

http://newsoffice.mit.edu/2015/tandem-solar-cell-0324

"In this initial version, the efficiency is 13.7 percent, but the researchers say they have identified low-cost ways of improving this to about 30 percent — a substantial improvement over today’s commercial silicon-based solar cells — and they say this technology could ultimately achieve a power efficiency of more than 35 percent."