Placing two materials in atomically-close contact such that their physical properties at the interface are very different from that of the bulk material. Hence, a junction is a materials science vehicle for talking about an electrochemical interface between solids.

An unusual twist in many advanced photovoltaics is the interface between the components. This interface is dimensionally complex (>2D planar) and often makes use of interactions between two completely different materials (e.g.: organometallic dye/metal oxide, polymer/carbon fullerene/metal oxide, metal chalcogenide/metal oxide).

Cartoon example of dimensionally complex interface in ETA cell

The lynchpin in these systems is essentially taking advantage of increasing the surface area of the charge generation interface. If you have more suface area, you can stick in more sensitizer (dye, nanoparticle, polymer) and hopefully increase your photoconversion efficiency. But this has proved to be very tricky given charge recombination at the interface. So the big advantage can also be the big hinderance if the system isn't tailored just right. Not to worry, new ideas are being developed monthly in this area of conversion limitation.