  It is well known that phonons can overscreen the bare Coulomb
electron-electron repulsion, turning it into the effective attraction that
binds the Cooper pairs responsible for BCS superconductivity. Here, we use a
simple lattice model to prove that the counterpart of this is also possible,
whereby phonons overscreen the bare electron-hole attraction and may turn it
repulsive at short distances, driving exciton dissociation in certain regions
of the parameter space. We argue that this phonon-mediated short-range
screening plays an important role in the physics of organic solar cell
materials (and other materials with strong electron-phonon coupling) and could
point the way to new strategies for optimizing their efficiencies.
