  Molecular materials have emerged as highly tunable materials for photovoltaic
and light-harvesting applications. The most severe challenge of this class of
materials is the trapping of charge carriers in bound electron-hole pairs,
which severely limits the free charge carrier generation. Here, we demonstrate
a significant modification of the exciton dynamics of thin films of endohedral
metallofullerene complexes upon alkali metal intercalation. For the exemplary
case of Sc$_3$N@C$_{80}$ thin films, we show that potassium intercalation
results in an additional relaxation channel for the optically excited
charge-transfer excitons that prevents the trapping of excitons in a long-lived
Frenkel exciton-like state. Instead, K intercalation leads to an ultrafast
exciton dissociation coinciding most likely with the generation of free charge
carriers. In this way, we propose alkali metal doping of molecular films as a
novel approach to enhance the light to-charge carrier conversion efficiency in
photovoltaic materials.
