Intrachain polaron motion and geminate combination in donor-acceptor copolymers: Effects of level offset and interfacial coupling

A model study of polaron motion and geminate combination in a molecular chain of donor-acceptor copolymers is presented. The simulations are performed within the framework of an extended version of the one-dimensional Su-Schrieffer-Heeger tight-binding model. Two effects are mainly concerned: One is level offset, and the other is interfacial coupling. A general rule associated with the ratio of level offset to polaron or exciton binding energy is obtained to contribute to optimizing donor-acceptor copolymers for optoelectronic applications. According to the rule, we identify two cases for polaron motion and four cases for geminate combination of oppositely charged polarons. It is found that an interface with weak coupling serves as an energy barrier and that with strong coupling as a well, both of which can significantly affect the intrachain process in copolymers. The effect of electron-electron interaction is briefly discussed.