Watching the coherent birth of polaron pairs in conjugated polymers

TL;DR

This study reveals that vibronic quantum coherence in conjugated polymers facilitates ultrafast polaron pair formation, enhancing charge separation and potentially improving organic photovoltaic efficiency.

Contribution

It demonstrates the role of vibronic coherence in ultrafast polaron pair formation using two-dimensional spectroscopy, revealing hybridized exciton-polaron states at room temperature.

Findings

Observation of multi-period oscillations indicating vibronic coherence

Identification of hybridized exciton-polaron-pair states

Ultrafast charge separation driven by coherent vibronic coupling

Abstract

Organic semiconductors have the remarkable property that their optical excitation 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.