| All diodes are sensitive to light. Photodiodes are just doped so that they are more sensitive. When a photon hits the pn junction, it causes an electron to jump the gap. Do this with enough photons and enough photodiodes, and you can run a pocket calculator.
When you don't want to harness the photodiode for energy, but want to use it as a light detector, you should consider using the more sensitive phototransistor instead. | 
