Some of the energy levels of the sunlight (depends on your frequency of the light wave) are absorbed by electrons that are bonded to an atom in the solar semiconductor material which bumps them to a higher energy level and releases them from their atomic bond (the move from the valence band to the conduction band). Other electrons in the semiconductor structure are pulled in to fill this displaced electron's spot. This displacement of electrons from their atoms by the sunlight energy causes a current of these electrons, and, because it is in a semiconductor device, it is designed so that it can only move in one direction much like a diode. This pushes the electrons to one area of the semiconductor, and leaves their holes in the other. This build up of electrons creates a charge potential between the collected electrons and their holes which in turn gives you a voltage across the semiconductor ends. This voltage can then be used across a circuit (flash light filament, calculator electronics, etc.) to return the displaced electrons to their holes.
So the energy of sunlight has been "collected" builds up as an energy potential (voltage) to be used as electricity. The amount of energy you get to use is proportional to how much you can pull out of the sunlight. So generally, the more electrons you can get to jump between your semiconductor p-n junction in the same amount of time, the more your power capability increases.