Direct observation of ultrafast exciton localization in an organic semiconductor with soft X-ray transient absorption spectroscopy
Douglas Garratt, Lukas Misiekis, David Wood, Esben W. Larsen, Mary Matthews, Oliver Alexander, Peng Ye, Sebastian Jarosch, Clément Ferchaud, Christian Strüber, Allan S. Johnson, Artem A. Bakulin, Thomas J. Penfold, J. P. Marangos
Abstract
Abstract The localization dynamics of excitons in organic semiconductors influence the efficiency of charge transfer and separation in these materials. Here we apply time-resolved X-ray absorption spectroscopy to track photoinduced dynamics of a paradigmatic crystalline conjugated polymer: poly(3-hexylthiophene) (P3HT) commonly used in solar cell devices. The π→π * transition, the first step of solar energy conversion, is pumped with a 15 fs optical pulse and the dynamics are probed by an attosecond soft X-ray pulse at the carbon K-edge. We observe X-ray spectroscopic signatures of the initially hot excitonic state, indicating that it is delocalized over multiple polymer chains. This undergoes a rapid evolution on a sub 50 fs timescale which can be directly associated with cooling and localization to form either a localized exciton or polaron pair.