Direct observation of charge separation in an organic light harvesting system by femtosecond time-resolved XPS

TL;DR

This study uses femtosecond time-resolved XPS to directly observe charge separation dynamics in an organic light harvesting system, revealing a new exciton dissociation channel and insights into charge generation efficiency.

Contribution

It introduces a novel femtosecond TR-XPS technique for real-time, site-specific observation of charge separation in organic heterojunctions, uncovering a previously unobserved dissociation pathway.

Findings

Identification of a new exciton dissociation channel

Real-time measurement of charge generation timescale

Evidence of charge separation from low-energy states

Abstract

The ultrafast dynamics of photon-to-charge conversion in an organic light harvesting system is studied by femtosecond time-resolved X-ray photoemission spectroscopy (TR-XPS) at the free-electron laser FLASH. This novel experimental technique provides site-specific information about charge separation and enables the monitoring of free charge carrier generation dynamics on their natural timescale, here applied to the model donor-acceptor system CuPc:C$_{60}$. A previously unobserved channel for exciton dissociation into mobile charge carriers is identified, providing the first direct, real-time characterization of the timescale and efficiency of charge generation from low-energy charge-transfer states in an organic heterojunction. The findings give strong support to the emerging realization that charge separation even from energetically disfavored excitonic states is contributing significantly, indicating new options for light harvesting in organic heterojunctions.