  The high flexibility of organic molecules offers great potential for
designing the optical properties of light-active materials for the next
generation of optoelectronic and photonic applications. However, despite
successful implementations of molecular materials in todays' display and
photovoltaic technology, many fundamental aspects of the light-to-charge
conversion have still to be uncovered. Here, we focus on the ultrafast dynamics
of optically excited excitons in C$_{60}$ thin films depending on the molecular
coverage and the light-polarization of the optical excitons. Using time- and
momentum-resolved photoemission with fs-XUV radiation, we follow the
depopulation dynamics in the excited states while simultaneously monitoring the
signatures of the excitonic charge character in the molecular valence states.
Optical excitation with visible light results in the instantaneous formation of
charge-transfer (CT) excitons, which transform stepwise into energetically
lower Frenkel-like excitons. While the number and energetic position of energy
levels within this cascade process are independent of the molecular coverage
and the light polarization of the optical excitation, we find quantitative
differences in the depopulation times and the optical excitation efficiency.
Our comprehensive study reveals the crucial role of CT excitons for the excited
state dynamics of homo-molecular fullerene materials and thin films.
