  Conventional photocells suffer a fundamental efficiency threshold imposed by
the principle of detailed balance, reflecting the fact that good absorbers must
necessarily also be fast emitters. This limitation can be overcome by `parking'
the energy of an absorbed photon in a dark state which neither absorbs nor
emits light. Here we argue that suitable dark states occur naturally as a
consequence of the dipole-dipole interaction between two proximal optical
dipoles for a wide range of realistic molecular dimers. We develop an intuitive
model of a photocell comprising two light-absorbing molecules coupled to an
idealised reaction centre, showing asymmetric dimers are capable of providing a
significant enhancement of light-to-current conversion under ambient
conditions. We conclude by describing a roadmap for identifying suitable
molecular dimers for demonstrating this effect by screening a very large set of
possible candidate molecules.
