Ultrafast Exciton Dynamics in Poly(3-hexylthiophene) Probed with Time Resolved X-ray Absorption Spectroscopy at the Carbon K-edge

π -conjugated semiconducting polymers have the potential to provide low cost, flexible and thin optoelectronic devices such as light-emitting diodes, field effect transistors and solar cells. The ultrafast dynamics of tightly bound, Frenkel excitons in these materials are crucial for the performance of these devices [1] , [2] . In this work we demonstrate the first application of soft X-ray transient absorption spectroscopy to study exciton dynamics in an organic semiconducting polymer, poly(3-hexylthiophene) (P3HT). The samples are pumped with a ∼ 15 fs optical pulse resonant with the π → π * transition at 2.25 eV, relevant to solar energy conversion. The probe is a soft X-ray supercontinuum extending to 350 eV with attosecond pulse duration generated via high harmonic generation [3] . Figure 1a shows the transient absorption spectrum of P3HT in the vicinity of the carbon K edge. At all positive time delays a blue shift in the absorption edge is observed giving rise to the strong increase in absorption at 286.25 eV. At short time delays this is accompanied by a weaker positive differential absorption feature 1.2 eV below the edge. The low energy feature is highly transient and decays with a lifetime of 16 8 fs. With the support of TDDFT simulations ( Fig. 1b ), we associate the spectral blue shift with the localised ± singlet exciton and the low energy transient feature as a direct spectroscopic signature of delocalisation of the exciton between polymer chains. Our results suggest that the photoexcited H-aggregate type state undergoes rapid localisation on a sub 50 fs timescale consistent with theoretical predictions for this polymer [4] .