I have some questions about the open circuit voltage of the photodiodes. Physically, what is the origin of the open circuit voltage? In the book "Physics of Solar cells": the open circuit voltage Voc defines the difference between the Fermi energies at which the total recombination rate in the cell is equal to the total generation rate given by the absorbed photocurrent. However, what always confusing me is why there is no open circuit voltage of a photodiode when no light illuminate on the device, but actually there is a p-n junction or Schottky barrier (built in potential) exists. But when light is illuminates on the diode, we can measure the open circuit voltage. So how to determine the Voc in a band diagram and what is the theoretically limit of the Voc? I appreciate any advice on it.