  Low-gap conjugated polymers have enabled an impressive increase in the
efficiencies of organic solar cells, primarily due to their red absorption
which allows harvesting of that part of the solar spectrum. Here, we report
that the true optical gap of one prototypical material, PTB7, is in fact at
significantly higher energy than has previously been reported, indicating that
the red absorption utilized in these materials in solar cells is entirely due
to chain aggregation. Using single-molecule spectroscopy we find that PL from
isolated nanoscale aggregates consists of multiple independently emitting
chromophores. At the single-molecule level, however, straight single chains
with a high degree of emission polarization are observed. The PL is found to be
~0.4 eV higher in energy, with a longer lifetime than the red aggregates, and
is attributed to single chromophores. Our findings indicate that the impressive
light-harvesting abilities of PTB7 in the red spectral region arises solely
from chain aggregation.

Chemical Physics;Applied Physics