Great answer by eman27_hc!
I just want to quickly add that the problem with
Newton is Nice's answer isn't that it is necessarily wrong, it just doesn't answer what the question asks for. The question says why an increase in
voltage doesn't lead to an increase in photocurrent. This is because all the available photoelectrons are ejected, so an increase in voltage (which basically makes the receiving plate more "attractive") wouldn't do anything.
However, an increase in intensity would increase the photocurrent. Remember that this is a circuit, so if you increase the intensity, the
rate of incident photons will increase, so more electrons will be ejected and hence a higher current will be achieved.
are still more available electrons on the cathode that are able to be ionized.
This is a very minor detail, but note that the electrons are not being
ionised. Ionisation would imply that the metal becomes positively charged after losing the electron. However, as the photelectric effect takes place in a circuit, the lost electrons are replenished and hence no ions are formed.
Hope this helps,
James