Vibrational coherence probes the mechanism of ultrafast electron transfer in polymer–fullerene blends

Abstract

The conversion of photoexcitations into charge carriers in organic solar cells is facilitated by the dissociation of excitons at the donor/acceptor interface. The ultrafast timescale of charge separation demands sophisticated theoretical models and raises questions about the role of coherence in the charge-transfer mechanism. Here, we apply two-dimensional electronic spectroscopy to study the electron transfer process in poly(3-hexylthiophene)/PCBM (P3HT/PCBM) blends. We report dynamics maps showing the pathways of charge transfer that clearly expose the significance of hot electron transfer. During this ultrafast electron transfer, vibrational coherence is directly transferred from the P3HT exciton to the P3HT hole polaron in the crystalline domain. This result reveals that the exciton converts to a hole with a similar spatial extent on a timescale far exceeding other photophysical dynamics including vibrational relaxation.

Charge transfer from delocalized excited states in a bulk heterojunction material

“…This spectrum is characterized by a single peak centered at 2.1 THz. It was recently shown that vibrational coherences can result from the charge generation process 27,28 and the population of charge separated states has a direct effect on molecular torsions. 71 The strong effect of charges on the frequency of torsional modes can be understood by noting that the contribution of quinoidal resonance forms will be affected by the charge state of the molecule; it will shift torsional mode frequencies.…”

“…This behavior should be expected by comparing the timescales of charge transfer with the lifetime of the coherent excited state; the charge transfer timescale can be as short as 25 fs, which is less than reported lifetimes of the coherent excited state. 28 …”

“…This is a critical distinction as several observations of excited state coherences have been made. 28,[53][54][55][56] Moreover, measurements that invoke charge carrier coherence [27][28][29] are predicated on the existence of excited state coherence.…”

“…52 Coherent spectroscopy has been used to investigate the excited state localization process in polymers for nearly a decade. 28,[53][54][55][56] In particular, torsional vibrations of poly(3-hexylthiophene) (P3HT) were found to be chiefly responsible for a loss of coherence between excited states in two-color three photon echo peak shift measurements. 56 More recently, it was found that vibrational coherences in the excited state are transmitted to the products of the ultrafast charge transfer reaction in organic solar cells.…”

“…56 More recently, it was found that vibrational coherences in the excited state are transmitted to the products of the ultrafast charge transfer reaction in organic solar cells. Both the electron in the fullerene 27 and the hole generated in the electron donating polymer 28 experience coherent vibrations following charge transfer. The latter work also demonstrates that the excited state is at least as large as the photogenerated hole.…”