University of Delaware researchers just achieved an all-time record of solar cell efficiency, with an efficiency of 42.8%. (The previous record was 40.7%, set by Boeing in 2006; the U of D team is working towards 50%.) The 42.8% is notable not only for the relatively high efficiency, but also for its practicality: the device was developed in less than two years, is only one centimeter thick, and has no moving parts. (In contrast, the machine that set the 40.7% record was a foot thick--hardly portable!)
The device is called the VHESC (Very High Efficiency Solar Cell), and its creators are hoping to see it used commercially. First, though, they're looking at military implementation: the U.S. soldier apparently carries 20 pounds of batteries in a standard pack.
For comparison, ecotality blog points out that usual solar cells--the rooftop variety--peak out at about 17%.