Film morphology and ultrafast photoexcitation dynamics in polyfluorene

We study the effect of sample morphology on ultrafast dynamics of photoexcited species in poly(9,9-dioctyl)fluorene (PFO). We show that within 200 fs both excitons and geminate polaron pairs with distinctly different absorption bands in the visible-near infrared spectral range are photogenerated in PFO solutions, pristine films, thermal and vapor treated films. However the branching ratio between the excitons and polaron pairs, as well as the polaron pair recombination kinetics depends on film morphology; polaron pair generation is enhanced and their recombination kinetics is faster in a solid phase, called the β phase, where the polymer chains are more planar.