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

The quantum yield and the mechanism of photogeneration of charge carri ers are investigated in bilayer assemblies consisting of a pi-conjugat ed rigid-rod polymer layer and a triarylamine doped polycarbonate laye r. The electric-field-dependent quantum efficiency of photocarrier gen eration, phi(E), in the bilayers containing a series of conjugated pol ybenzobisazoles was measured by photoinduced discharge and was found t o 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 quali tatively 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 we re excitons. However, picosecond photoinduced absorption and photolumi nescence experiments on the bilayers revealed photoinduced electron tr ansfer and exciplex formation at the bilayer interface. Thus photocarr ier generation on photoexcitation of the conjugated rigid-rod polymers in the bilayers occurs by photoinduced electron transfer, forming int ermolecular exciplexes which dissociate efficiently in an electric fie ld. This mechanistic picture of charge carrier photogeneration provide d a satisfactory account for the observed field-dependent quantum yiel ds in the series of bilayer assemblies.