why we cannot measure potential barrier of depletion layer using sensitive voltmeter in absene of biasing?

There is difference between Voltage and electrostatic potential. Voltage arises when there is difference between fermi energy level of two points on the surface. But in a pn junction that is in dark and is under no biasing, there is no difference in frmi level on the junction of pn junction. So any voltmeter, no matter how sensetive will not be able to measure the potential barriar.