Relationship Between Stopping Voltage and Photocurrent Online, I found a graph of photocurrent vs. stopping voltage:

And I can’t figure out how photocurrent and stopping voltage have such a relationship. Stopping voltage is proportional to maximum KE and photon frequency, but neither of those factors really affect photocurrent as long as the threshold is reached. I would appreciate some input.
 A: If you are asking why this is not a step function as soon as the potential is lower than the stopping potential, the reason is that emitted electrons are not all leaving perpendicular to the plate as they are ejected.  The plate is still attractive to electrons as they are ejected.  If the velocity perpendicular to the plate is insufficient to overcome the potential barrier going towards the collector plate, it will turn around and head back into the source.  In steady state, these electrons will register as 0 current, even though the rate of emission of electrons is fixed by intensity of the incoming light.
For the shape of the curve, I would imagine that it would be determined by the geometry of the electron emission trajectories.  When the stopping potential is lowered, a higher solid angle of trajectories can now overcome the potential barrier to be collected.  When the stopping potential crosses zero and goes negative, all of the electrons should be able to make it to the detector, and the photocurrent should saturate. 
