Electromodulation of charge-transfer photoexcitations in pristine andC60-doped conjugated polymers

We study the effect of an electric field of up to 100 kV/cm on photoexcitations in pristine and ${\mathrm{C}}_{60}$-doped $\pi$-conjugated polymers by electromodulated photoinduced absorption (EPA). In EPA, we measure changes in the photoinduced absorption of a polymer film induced by an electric field. The dominant effect is a change in the photoexcitation recombination kinetics, which leads to reduction or enhancement of the PA bands. We found that the electric field increases the decay rate of charge-transfer excitations; exciton dissociation near defects and impurities was also enhanced. A Stark shift of the photoexcitation energy levels for ${\mathrm{C}}_{60}$-doped poly(para-phenylene) was also detected. From the EPA we calculate that the polarizability of the charge-transfer excitations in this blend is of order $3\times {10}^7(\mathrm{\AA }\mathrm{}{)}^3.$ This is two orders of magnitude larger than the polarizability of the ${1B}_u$ exciton in most luminescent conjugated polymers.