I'm not an expert in plant physiology. I was wondering if, per square centimeter, leaves are converting more or less energy than photovoltaic systems. Can this be estimated? How?
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.
Photovoltaics made from organic sources, like Graphene, (OPVs) have the potential for efficiencies exceeding 24% in a stacked structure. A theoretical max efficiency has not been proposed, but it is expected to be less than silicon. The main benefit of OPVs is the rock-bottom price and the ability to make very tiny panels.