A particular mechanism is considered for the generation of an extrinsic photocurrent in an insulating molecular crystal, for the case in which the radiation passes through an electrolyte that forms one of the electrical contacts to the crystal. It is supposed that excitons generated by absorption of photons within the solid diffuse to the insulator/electrolyte-solution interface and there react with a donor species in solution, with consequent transfer of a carrier into the solid. A kinetic analysis of this reaction is made, accounting for diffusion of the excitons, the rate of the interfacial charge-transfer reaction, and the transport within the solid of the carriers. The photocurrent is expressed as a function of the applied voltage and of the extinction coefficient of the solid.
A limiting current is predicted when the over-all rate of carrier flow is raised by the voltage to the constant rate of carrier-injection at the surface. The magnitudes of the heterogeneous chargetransfer rate constants are derived for the case of an "outer sphere" reaction, providing an indication of the dependence of the photo-injection rate upon such parameters as the band gap of the solid, the energy of the exciton and the standard potential of the redox system.