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

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. Understanding ultrafast dynamics of photon-to-charge conversion is paramount for optimising light-harvesting systems. Here, the authors use femtosecond time-resolved X-ray photoemission spectroscopy to reveal specific charge separation sites and monitor free charge formation in a model donor-acceptor system.

Automated summary

Researchers used femtosecond time-resolved X-ray photoemission spectroscopy to investigate the ultrafast dynamics of photon-to-charge conversion in an organic light-harvesting system. They discovered a new pathway for charge generation and provided novel insights into light harvesting in organic heterojunctions.