Observation of an Ultrafast Exciton Hopping Channel in Organic Semiconducting Crystals

One of the major challenges in using organic semiconductors for photovoltaics is their extremely short exciton diffusion length. Recently, a number of studies have shown that the exciton transport range within the first few picoseconds after photoexcitation can be comparable to the exciton's diffusion length over its entire lifetime. The origin of this fast transport channel is often attributed to the large spatial coherent size of the exciton right after photoexcitation. Here we observe an ultrafast exciton hopping channel in titanyl phthalocyanine crystals even though the exciton coherent size is a few times smaller than the transport range. This channel operates only within the first few picoseconds after photoexcitation and has a hopping rate that is an order of magnitude faster than the typical Förster resonance energy transfer rate. Resonant Raman spectroscopy shows that the optically excited exciton is strongly mixed with the macrocycle vibrational mode of the phthalocyanine molecules. A hypothesis involving vibronic coherence is proposed to explain the observed transport behavior.