Abstract
The optical excitation of organic semiconductors not only generates charge-neutral electron-hole pairs (excitons), but also charge-separated polaron pairs with high yield. The microscopic mechanisms underlying this charge separation have been debated for many years. Here we use ultrafast two-dimensional electronic spectroscopy to study the dynamics of polaron pair formation in a prototypical polymer thin film on a sub-20-fs time scale. We observe multi-period peak oscillations persisting for up to about 1 ps as distinct signatures of vibronic quantum coherence at room temperature. The measured two-dimensional spectra show pronounced peak splittings revealing that the elementary optical excitations of this polymer are hybridized exciton-polaron-pairs, strongly coupled to a dominant underdamped vibrational mode. Coherent vibronic coupling induces ultrafast polaron pair formation, accelerates the charge separation dynamics and makes it insensitive to disorder. These findings open up new perspectives for tailoring light-to-current conversion in organic materials.
Highlights
The optical excitation of organic semiconductors generates charge-neutral electronhole pairs, and charge-separated polaron pairs with high yield
We prepare our samples by spin coating from chlorobenzene and subsequent annealing according to a recipe used for the fabrication of efficient organic solar cells based on this polymer[38]
A similar model allowing for a coherent superposition of localized Frenkel and charge transfer excitons has been proposed to model the static absorption spectrum of oligoacene crystals[47], whereas, commonly, this superposition is not considered in models of the static absorption of P3HT39
Summary
The optical excitation of organic semiconductors generates charge-neutral electronhole pairs (excitons), and charge-separated polaron pairs with high yield. We observe multi-frequency time-domain oscillations and spectral peak splittings as clear signatures of strong vibronic coupling between excitons, polaron pairs and a dominant underdamped vibrational mode of the polymer. To study the polaron pair formation in more detail, we recorded 2DES maps of the polymer thin film (Fig. 1c).
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