Photocells based on a simple optical transition 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 presenting a host of candidate molecules meeting the requirements for realising such a system.