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:C60. 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.
Highlights
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
Light harvesting in CuPc:C60 is initiated through creation of an excitonic state at the chromophore (CuPc), while the desired final state consists of a separated electron–hole pair with a vacancy in the chromophore and a free electron in C60
From the open-circuit voltage of the CuPc:C60 heterojunction solar cell, which corresponds to ~0.5 eV, we can derive a boundary for the minimum energy difference between the HOMO of CuPc and the LUMO of C60
Summary
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: C60. We previously showed[10] that T1 states dominate the generation of free charges on timescales of 100’s of picoseconds to nanoseconds In contrast to these studies on Pc: C60 systems, for a number of blends of C60 with polymers or other small molecules, strong indications were found that low-energy charge-transfer excitons do contribute to free-carrier generation, even for the fully relaxed ICT excitons[13,14,15]. The direct, real-time observation of this charge generation channel, and its prevalence over competing loss channels is still outstanding
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