PHOTOGENERATION OF CHARGE-CARRIERS IN BILAYER ASSEMBLIES OF CONJUGATED RIGID-ROD POLYMERS

The quantum yield and the mechanism of photogeneration of charge carriers are investigated in bilayer assemblies consisting of a pi-conjugated rigid-rod polymer layer and a triarylamine doped polycarbonate layer. The electric-field-dependent quantum efficiency of photocarrier generation, phi(E), in the bilayers containing a series of conjugated polybenzobisazoles was measured by photoinduced discharge and was found to be in the range 10(-3) to 10(-2) at low fields (<10(4) V/cm) and as high as 0.12-0.32 at similar to 10(6) V/cm. The phi(E) data were qualitatively in accord with Onsager's three-dimensional model for ion-pair dissociation. Transient (ps) absorption spectroscopy of single-layer thin films of the polymers showed that the primary photoexcitations were excitons. However, picosecond photoinduced absorption and photoluminescence experiments on the bilayers revealed photoinduced electron transfer and exciplex formation at the bilayer interface. Thus photocarrier generation on photoexcitation of the conjugated rigid-rod polymers in the bilayers occurs by photoinduced electron transfer, forming intermolecular exciplexes which dissociate efficiently in an electric field. This mechanistic picture of charge carrier photogeneration provided a satisfactory account for the observed field-dependent quantum yields in the series of bilayer assemblies.