  The optoelectronic properties of macromolecular semiconductors depend
fundamentally on their solid-state microstructure. For example, the
molecular-weight distribution influences polymeric- semiconductor properties
via diverse microstructures; polymers of low weight-average molecular weight
(Mw) form unconnected, extended-chain crystals, usually of a paraffinic
structure. Because of the non-entangled nature of the relatively short-chain
macromolecules, this leads to a polycrystalline, one-phase morphology. In
contrast, with high-Mw materials, where average chain lengths are longer than
the length between entanglements, two-phase morphologies, comprised of
crystalline moieties embedded in largely unordered (amorphous) regions, are
obtained. We investigate charge photogeneration processes in neat regioregular
poly(3-hexylthiophene) (P3HT) of varying Mw by means of time-resolved
photoluminescence (PL) spectroscopy. At 10 K, PL originating from recombination
of long-lived charge pairs decays over microsecond timescales. Both the
amplitude and decay rate distribution depend strongly on Mw. In films with
dominant one-phase chain-extended microstructures, the delayed PL is suppressed
as a result of a diminished yield of photoinduced charges, and its decay is
significantly faster than in two-phase microstructures. However, independent of
Mw, charge recombination regenerates singlet excitons in torsionally disordered
chains forming more strongly coupled photophysical aggregates than those in the
steady-state ensemble, with delayed PL lineshape reminiscent of that in
paraffinic morphologies at steady state. We conclude that highly delocalized
excitons in disordered regions between crystalline and amorphous phases
dissociate extrinsically with yield and spatial distribution that depend
intimately upon microstructure.
