  Control of chain length and morphology in combination with single-molecule
spectroscopy techniques provide a comprehensive photophysical picture of
excited-state losses in the prototypical conjugated polymer
poly(3-hexylthiophene) (P3HT). A universal self-quenching mechanism is
revealed, based on singlet-triplet exciton annihilation, which accounts for the
dramatic loss in fluorescence quantum yield of a single P3HT chain between its
solution (unfolded) and bulk-like (folded) state. Triplet excitons
fundamentally limit the fluorescence of organic photovoltaic materials, which
impacts on the conversion of singlet excitons to separated charge carriers,
decreasing the efficiency of energy harvesting at high excitation densities.
Interexcitonic interactions are so effective that a single P3HT chain of >100
kDa weight behaves like a two-level system, exhibiting perfect
photon-antibunching.
