Question

When a Metal-Isolator-Metal system is brought into contact and the two metals have different work functions (energy required to remove an electron from the solid), why does that result in an electrical field across the isolator, assuming no external voltage is applied?

As a followup that probably follows from above answer: why are electrons that are created through the absorption of a photon driven towards the metal with the lower work function in such a setup?

Answer 1

Google "metal-semiconductor junction"

Answer 2

The fermi levels need to equilibrate but the work functions of the metals can't change, so an electric field is set up. The electrons are driven by the electric field.

Answer 3

Consider the attached diagram of a metal-insulator-metal junction before and after contact. To preserve equilibrium, when two materials are intimately in contact, charge carriers will flow from one material to the other until the Fermi energies of the two materials are equal. However, when a dielectric or semiconductor is sandwiched between two metals of differing work functions, its attempt to equalise its Fermi energy with that of the two metals, results in a tilting or bending of the conduction and valence bands. Since these bands define the potential energy of the charge carriers of the material, a tilting of the bands will result in a potential difference across the dielectric, which is observed as a voltage equal in magnitude to the difference of the work functions of the two electrodes.

Answer 4

Look up Josephson Junction