PHOTOVOLTAIC EFFECT IN ASYMMETRICAL DONOR-ACCEPTOR ORGANIC SUPERLATTICES

We predict a novel photovoltaic mechanism in donor-acceptor superlatti ces. We consider an asymmetrical organic superlattice such as... D-A-N -D-A-N-D-A-N..., where D, A and N indicate layers of materials which a re, respectively, donor, acceptor and neither donor nor acceptor, all neutral in the ground state. In such a structure, the lowest-lying ele ctronic excitations are charge-transfer excitons (CTEs) at the D-A int erfaces, with electrons displaced from the donor to the acceptor side. We show that the excitation of CTEs under illumination is accompanied by the appearance of a macroscopic voltage drop along the growth dire ction. The resulting average electric field can separate and drive the photogenerated free carriers (partly coming from the ionization of th e CTEs themselves), producing a photovoltaic current. (C) 1995 America n Institute of Physics.