The
power conversion efficiency of organic photovoltaics is strongly
limited by relatively large energy loss, which is partially due to
the disordered nature of organic semiconductors. This disordered nature
not only hinders the rational design of molecules with excellent photophysical
properties but also prevents a more thorough understanding of the
inherent link between microscopic parameters and physical phenomena.
In this Perspective, we demonstrate that the injection-dependent emission
line-shape in organic semiconductors is primarily associated with
a state-filling effect, where the extent of spectral blue-shift can
be a strong indicator for energetic disorder. Molecular geometry with
rigidity and coplanarity not only promotes preferential face-on stacking
that narrows the energetic distribution of subgap states but also
impedes torsional deformations of the conjugated backbone away from
planarity, thereby facilitating larger π-electron delocalization.
These structural characteristics explain the seemingly contradictory
high radiative efficiency of low-bandgap nonfullerene molecules, providing
promising molecular design strategies to realize high-efficiency organic
photovoltaics.