Photodiodes: the "generation" mechanism behind photocurrent
Actually, there is an electric field in the depletion region in a diode with zero bias.  Depending on what problem you're trying to solve at the moment, it's either the cause of the depletion region, or a necessary effect of the cause of the depletion region.  Either way, it sweeps the depletion region free of carriers.

So when a photon smacks into a diode junction and succeeds at creating a hole/electron pair, the usual consequence is that the hole is swept into the cathode, the electron is swept into the anode.  Thus, light drives current.

In fact, a pretty good model for a photodiode of any kind, whether it's an itty bitty super-fast diode for receiving laser pulses, or a component of a solar cell, is a plain old diode in parallel with a current source.  The actual photocurrent is fairly constant (I'm sure the actual number of electron/hole pairs generated per photon varies somewhat, but to a 1st-order approximation it ain't much).  Most of the differences in photodiode behavior as a function of operating point have to do with the behavior of that virtual "dark diode" that's in parallel with the current source.







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