What is considered 100% efficiency when talking about photovoltaic cells?

Question

I was reading this article in Popular Science and figured that I actually don't know what is considered 100% efficiency.

Is it when every photon that hits the cell kicks out an electron that is excited enough to become free? Not every photon in the spectrum has enough energy to do that, does it?

Answer 1

Have a look at the Wikipedia article on Solar cell efficiency. The efficiency is the electrical power output divided by the total energy received across all wavelengths. As you say, this would be unlikely ever to reach 100% as some of the energy is at long wavelengths.