Electromodulation of charge-transfer photoexcitations in pristine and C-60-doped conjugated polymers

We study the effect of an electric field of up to 100 kV/cm on photoexcitat ions in pristine and C-60-doped ir-conjugated polymers by electromodulated photoinduced absorption (EPA). In EPA, we measure changes in the photoinduc ed absorption of a polymer film induced by an electric field. The dominant effect is a change in the photoexcitation recombination kinetics, which lea ds to reduction or enhancement of the PA bands. We found that the electric field increases the decay rate of charge-transfer excitations; exciton diss ociation near defects and impurities was also enhanced. A Stark shift of th e photoexcitation energy levels for C-60-doped poly(para-phenylene) was als o detected. From the EPA we calculate that the polarizability of the charge -transfer excitations in this blend is of order 3 x 10(7) (Angstrom)(3). Th is is two orders of magnitude larger than the polarizability of the 1B(u) e xciton in most luminescent conjugated polymers. [S0163-1829(99)10715-X].