Liberation of charge carriers by optical pumping excitons in poly(3-hexylthiophene) aggregates

Abstract

In conjugated polymers used in photovoltaics, charges may be produced on ultrafast time scales without requiring exciton diffusion to a donor–acceptor interface. To investigate the role of high-energy, delocalized exciton states in charge generation within polymer domains, we apply a pump–push–probe transient absorption technique to pristine poly(3-hexylthiophene) nanoparticles. The near-infrared push pulse induces exciton dissociation through the S3 ← S1 electronic transition, which is predicted to show intramolecular charge-transfer character. We suggest that the spatial extent of the high-energy exciton, which induces electron–hole separation, is sufficient to overcome the intrinsic Coulombic attraction of the electron–hole pair. We observe that ∼10% of the pushed excitons undergo dissociation to form free charges. The kinetics of charge recombination indicate that the electron and hole are separated by a distance of ∼3 nm across the polymer domains.