Theory of photoinduced charge transfer in a molecularly doped conjugated polymer

In a molecularly doped conjugated polymer, it is possible for an exciton, photoexcited on the polymer, to decay into a free charge in the polymer's valence or conduction band and a charge of opposite sign in an available level on an adjacent dopant molecule. Such pair processes, which conserve energy, can occur if the energy omega(0) of the polymer exciton lies within the continuum part of the charge-transfer electron-hole excitation spectrum of the polymer/molecule composite. We calculate the corresponding exciton decay rate 1/tau(Delta) for two types of matrix element that describe the microscopic charge-transfer event. This rate is a maximum when the energy Delta of the molecular acceptor level lies below the conduction-band edge by an amount that is a little more than the exciton's binding energy, a similar result occurring for the case of hole transfer to the molecule. The reduction of 1/tau by the Coulomb interaction between the electron and hole in the final state is important.