Identification of Ultrafast Photophysical Pathways in Photoexcited Organic Heterojunctions

TL;DR

This study uses a quantum approach to reveal ultrafast photophysical pathways in organic heterojunctions, highlighting the role of higher orbitals and bridge states in exciton dissociation and charge separation.

Contribution

It introduces a fully quantum model to identify and analyze ultrafast pathways involving higher orbitals and exciton mixing in organic heterojunctions, advancing understanding of charge separation mechanisms.

Findings

Higher-than-LUMO orbitals create photon-absorbing charge-bridging states.

Bridge states facilitate multiple photophysical pathways for charge separation.

Pathway efficiency depends on energy alignment, excitation frequency, and carrier-phonon interactions.

Abstract

The exciton dissociation and charge separation occurring on subpicosecond time scales following the photoexcitation are studied in a model donor/acceptor heterojunction using a fully quantum approach. Higher-than-LUMO acceptor orbitals which are energetically aligned with the donor LUMO orbital participate in the ultrafast interfacial dynamics by creating photon-absorbing charge-bridging states in which charges are spatially separated and which can be directly photoexcited. Along with the states brought about by single-particle resonances, the two-particle (exciton) mixing gives rise to bridge states in which charges are delocalized. Bridge states open up a number of photophysical pathways that indirectly connect the initial donor states with states of spatially separated charges and compete with the efficient progressive deexcitation within the manifold of donor states. The diversity and efficiency of these photophysical pathways depend on a number of factors, such as the precise energy alignment of exciton states, the central frequency of the excitation, and the strength of carrier-phonon interaction.