Solar cells are one technology considered by some to be a great source for clean energy, but despite our knowledge of the photoelectric effect dating back to the 19th century it is still a relatively undeveloped technology. Many of the solar cells you see today utilize single-junction design, which has an upper-efficiency limit of roughly 33%, but a multi-junction design can surpass that. Researchers at the Naval Research Laboratory have recently developed a new multi-junction design which should be able to exceed 50% efficiency.
The junctions within a semiconductor solar cell are the areas which actually collect the solar energy. A single junction is only able to absorb a small window of frequencies, so adding more junctions allows a cell to absorb more light and generate more electricity. (Theoretically an infinite-junction cell would achieve 87% efficiency.) What the researchers have done is discovered a semiconductor that can be grown with a specific structure and have band gaps from 0.7 to 1.8 electron volts. The band gaps of a material directly relate to what frequencies of light it can absorb.
Currently it is just a new design that has been created, and not an actual solar cell with better than 50% efficiency. Hopefully one will be created soon so we can see exactly how efficient it is and get to work at setting a new record.