The man, the myth, the legend:  Einstein.²¹          

    Einstein postulated the photoelectric effect in 1905, which states that the energy of a photon depends on its frequency and is given as E = hf where h is Planck’s constant.⁷ Surface electrons in a metal can capture a photon from incident light and if the energy of the photon exceeds the binding energy of the electron to its parent atom, the electron may escape or free flow through the metal until it recombines with an atom that is lacking an electron called a hole, upon which time a photon is re emitted.⁷  This process can be seen at work on a piece of Potassium in the diagram bellow.

    We can see then that energy could be extracted from sunlight if the exposed material had electrons of a binding energy sufficiently low so as to be exited by the incident photons.  This is due to the particle like nature of light that we mentioned earlier.  Solar cells are not made of metal, but are actually semiconductors, which are materials that have some free electrons available to conduct electricity; in this sense they are like a cross between a conductor and an insulator.  Lets briefly discuss the difference between conductors and insulators to get a better sense of this concept.