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 S-3 <- S-1 electronic transition, which is predicted to show intra-molecular 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 similar to 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 similar to 3 nm across the polymer domains.