When the pn junction terminals are left open-circuited, the voltage measured between them will be zero.
That is, the voltage Vo across the depletion region does not appear between the junction terminals.
This is because of the contact voltage existing at the metal semiconductor junction at the terminals, which counter and exactly balance the barrier voltage.
If this were not the case, we would have been able to draw energy from the isolated pn junction, which would clearly violate the principle of conservation of energy.