A photodiode converts incident light to electric current more effectively in reverse bias conditions. Actually, in a forward bias, the depletion region between the PN junction is very thin because it allows current to flow. But in reverse bias the depletion region becomes thick. Now, The incident light creates electron-hole pairs in the depletion region. Greater the area of depletion region, more are the electron-hole pairs that can be created by the incident light which is directly proportional to the amount of current produced. That's why photodiode is connected in reverse bias.
