How high is the energy yield of photovoltaics compared to photosynthesis?
According to wikipedia, plants typically convert around 5% of the energy of the sunlight that hit the leaves into energy usable by the plant.  Sugarcane seems to be the best, it converts up to 8% of the energy into actual biomass.

The best solar panels on the market, according to the Independent, convert 21% of energy from sunlight into usable electricity.  Experimental prototypes do much better, and there are some that get over 40%.  Wikipedia has a nice graph of the most efficient solar cell prototypes.