  The energetic landscape at the interface between electron donating and
accepting molecular materials favors efficient conversion of intermolecular
charge-transfer states (CTS) into free charge carriers in high-performance
organic solar cells. Here, we elucidate how interfacial energetics, charge
generation and radiative recombination are affected by structural ordering. We
experimentally determine the CTS binding energy of a series of model, small
molecule donor-acceptor blends, where the used acceptors (B2PYMPM, B3PYMPM and
B4PYMPM) differ only in the nitrogen position of their lateral pyridine rings.
We find that the formation of an ordered, face-on molecular packing in B4PYMPM
is beneficial to efficient, field-independent charge separation, leading to
fill factors over 70% in photovoltaic devices. This is rationalized by a
comprehensive computational protocol showing that, compared to the more
amorphous and isotropically oriented B2PYMPM, the higher order of the B4PYMPM
molecules provides more delocalized CTS. Furthermore, we find no correlation
between the quantum efficiency of radiative free charge carrier recombination
and the bound or unbound nature of the CTS. This work highlights the importance
of structural ordering at donor-acceptor interfaces for efficient free carrier
generation and shows that more ordering and less bound CT states do not
preclude efficient radiative recombination.
