1. Vertical axis is energy Emitter WE Collector
No electrical connection between emitter (cathode) and collector (anode) .
The vacuum levels are aligned
Notation:
EF Fermi energy
EFE , EFC Fermi energy of emitter and collector respectively
WE , WC Work function of emitter and collector respectively
Photon
energy hn
KEE
Vacuum level
Vertical axis is energy
Emitter WE
Collector
Kinetic energy of the electron when leaving the emitter Is: WC
EFE
EFC
Emitter and connector are connected (V=0).
Note that now the Fermi energies in the two metals are aligned
KEE
KEC
Kinetic energy of the electron when reaching the collector. Is:
Collector
WC -WE WC
Emitter WE

2. Retarding voltage Now: Collector WC Emitter WE eV
Apply a retarding voltage V between collector and emitter.
The two Fermi energies are now offset by eV where e is the electron charge (note e is negative)
KEC
Now:
KEE
Collector
eV+WC -WE WC
Emitter WE
Potential difference between collector and emitter as seen by the electron: eV
Condition of zero current corresponds to Zero Kinetic energy of the electron when reaching the collector:
PROVIDED that the photoelectric effect happened ie:

3. Accelerating voltage Emitter WE Collector WC eV
Now consider Forward bias - accelerating voltage
KEE
KEC
Potential difference between collector and emitter as seen by the electron:
Emitter WE
Collector WC eV
Now it looks like the electron velocity and therefore the current increases with V.
But the current cannot increase indefinitely it will saturate when the electron flux
equals to the incoming photon flux 𝑛 = 𝑃 ℎ𝜈 where P is the light power impinging on the emitter.

4. Saturation voltage electron is neither accelerated nor retarded
Now consider Forward bias - accelerating voltage
KEE
KEC
Emitter WE
Collector WC eV
Now it looks like the electron velocity and therefore the current increases with V.
But the current cannot increase indefinitely it will saturate when the electron flux
equals to the incoming photon flux 𝑛 = 𝑃 ℎ𝜈 where P is the light power impinging on the emitter.