Origin of space-separated charges in photoexcited organic heterojunctions on ultrafast time scales

TL;DR

This paper investigates ultrafast exciton dynamics in organic heterojunctions, revealing that space-separated charges are mainly directly generated by optical excitation rather than ultrafast transfer, challenging conventional interpretations.

Contribution

It provides a theoretical analysis showing that space-separated charges form directly during photoexcitation, not solely through ultrafast exciton dissociation, refining understanding of charge generation mechanisms.

Findings

Space-separated charges are predominantly directly optically generated within 100 fs.

Ultrafast pump-probe spectra interpretations need revision based on these findings.

Theoretical model challenges the conventional view of ultrafast charge transfer in heterojunctions.

Abstract

We present a detailed investigation of ultrafast (subpicosecond) exciton dynamics in the lattice model of a donor/acceptor heterojunction. Exciton generation by means of a photoexcitation, exciton dissociation, and further charge separation are treated on equal footing. The experimentally observed presence of space-separated charges at $\lesssim 100$ fs after the photoexcitation is usually attributed to ultrafast transitions from excitons in the donor to charge transfer and charge separated states. Here, we show, however, that the space-separated charges appearing on $\lesssim 100$-fs time scales are predominantly directly optically generated. Our theoretical insights into the ultrafast pump-probe spectroscopy challenge usual interpretations of pump-probe spectra in terms of ultrafast population transfer from donor excitons to space-separated charges.