NREL certifies the efficiency of all solar cells and produces this handy reference chart on the efficiencies of all types of solar cell technologies. The measurements are usually made on cells the are a couple of cm square and are the best examples that a particular technology can produce. Production cells are usually less efficient due to imperfections introduced by the manufacturing process. Further loss are introduced when the cells are built into modules. I recommend two excellent books on photovoltaics:

• Clean Electricity from Photovoltaics by Archer and Hill
• The Physics of Solar Cells by Nelson

The chart was created by NREL and captured from the solar cell page on wikipedia.