Photoexcitation dynamics in polythiophene/fullerene blends for photovoltaic applications

We used the transient and steady state photomodulation spectroscopies for studying the photoexcitations dynamics in blends of regio-regular poly(3-hexylthiophene) (RR-P3HT) and fullerene in a broad spectral range from 0.15 to 2.25 eV. We found that both localized polarons and singlet excitons are instantaneously photogenerated in the blends. However the photogeneration process of delocalized polarons which contribute to the photocurrent proceeds in two steps: first, within a couple of ps the excitons generated in the polymer domains populate the charge transfer complex states at the RR-P3HT/fullerene interfaces; this is followed by the charge transfer ionization into delocalized charge polarons in the polymer and fullerene constituents within ~20 ps. In contrast, the localized polaron dynamics are unrelated with the excitons and delocalized polarons dynamics. We also report on the occurrence of ultrafast quantum interference anti-resonances between photoinduced infrared-active vibrations and the delocalized polaron band in the blends, which shows the delocalization character of the photogenerated charges that contribute to the photocurrent.