  The spectral breadth of conjugated polymers gives these materials a clear
advantage over other molecular compounds for organic photovoltaic applications
and is a key factor in recent efficiencies topping 10%. But why do excitonic
transitions, which are inherently narrow, lead to absorption over such a broad
range of wavelengths in the first place? Using single-molecule spectroscopy, we
address this fundamental question in a model material, poly(3-hexylthiophene).
Narrow zero-phonon lines from single chromophores are found to scatter over
200nm, an unprecedented inhomogeneous broadening which maps the ensemble. The
giant red-shift between solution and bulk films arises from energy transfer to
the lowest-energy chromophores in collapsed polymer chains which adopt a
highly-ordered morphology. We propose that the extreme energetic disorder of
chromophores is structural in origin. This structural disorder on the
single-chromophore level may actually enable the high degree of polymer chain
ordering found in bulk films: both structural order and disorder are crucial to
materials physics in devices.
