  We address the role of excitonic coulping on the nature of photoexcitations
in the conjugated polymer regioregular poly(3-hexylthiophene). By means of
temperature-dependent absorption and photoluminescence spectroscopy, we show
that optical emission is overwhelmingly dominated by weakly coupled
H-aggregates. The relative absorbance of the 0-0 and 0-1 vibronic peaks
provides a powerfully simple means to extract the magnitude of the
intermolecular coupling energy, approximately 5 and 30 meV for films spun from
isodurene and chloroform solutions respectively.
