Electrochemical potential is the general term for the combined driving forces of charge mobility:

1. Electrostatic potential (often expresed in Volts/V or electron volts/eV)
2. Chemical potential (often expressed in Joules/J)

So there are two potentials that make the holes and electrons "want" to drift and diffuse into each other. First is an electrostatic potential ( "opposites attract", so there is electrostatic drift of charges). Second is a high concentration gradient at the interface that drives one to pour into the other like two waterfalls (the diffusion from high chemical potentials).

Using a solid/solid heterojunction as a model, there are a few things to keep in mind:

1. Each material at an interface can be a reservoir for a certain dominant species of charge carriers.
2. If the respective dominant charge carriers are oppositely charged on both sides of the junction (one is positive and one is negative, makng a p-n interface or junction), the chemical potential contribution will be overwhelmed by a prevailing electrostatic potential. In this case, one may observe the charge carriers drift in response to an (intstanteous and temporary) electrostatic potential. But eventually the drift will attenuate as charges recombine in the middle of the junction. This leads to the development of a insulating space charge layer, or depletion layer.
3. In response to this insulating layer, a lasting electrostatic potential is maintained across the junction (0.6-0.7 V in silicon). This works just like a dam to hold back the waterfall event. When light of the appropriate energy level hits the p-n junction and is absorbed, electron-hole pairs are generated. What happens to these pairs? Well, if they have nowhere to go (no potential to separate them) they just recombine. But because the electric field is present, the charges feel a pull to pour over the "dam" and flow down the circuit.
4. In a p-n junction the dominant component is the stable electrostatic potential. This is not true for a vast majority of electrochemical interfaces.

The electric field from this potential is a cornerstone of First Generation Photovoltaics. In that particular technology, if you don't have a "field" you cannot harvest the photovoltaic effect, because there is no driving force to separate photogenerated charges. But remember that the full scope of driving forces are described as the electrochemical potential, not just the electrostatic potential. Newer devices like Advanced Photovoltaics (and very efficient organisms called plants, algae, and photosynthetic bacteria) take advantage of chemical potentials as well.

Note: We use the basis of plasma physics rather than the basis of electrochemistry in to define the double layer, because the definition of the double layer in electrochemistry assumes a fixed surface charge or surface charge layer. This model works well enough for an oxide particle in aqueous solution, but is limiting in trying to explain mirrored effects of charge attenuation for solid/solid semiconductor interactions.