  Charge-transfer excitons (CTX) at organic donor/acceptor interfaces are
considered important intermediates for charge separation in photovoltaic
devices. Crystalline model systems provide microscopic insights into the nature
of such states as they enable microscopic structure-property investigations.
Here, we use angular-resolved UV/Vis absorption spectroscopy to characterize
the CTX of crystalline pentacene:perfluoro-pentacene (PEN:PFP) films allowing
to determine the polarization of this state. This analysis is complemented by
first-principles many-body calculations, performed on the three-dimensional
PEN:PFP co-crystal, which confirm that the lowest-energy excitation is a CTX.
Analogous simulations performed on bimolecular clusters are unable to reproduce
this state. We ascribe this failure to the lack of long-range interactions and
wave-function periodicity in these cluster calculations, which appear to remain
a valid tool for modelling properties of organic materials ruled by local
intermolecular couplings.
