  Charge carrier localization in extended atomic systems has been described
previously as being driven by disorder, point defects or distortions of the
ionic lattice. Here we show for the first time by means of first-principles
computations that charge carriers can spontaneously localize due to a purely
electronic effect in otherwise perfectly ordered structures. Optimally-tuned
range-separated density functional theory and many-body perturbation
calculations within the GW approximation reveal that in trans-polyacetylene and
polythiophene the hole density localizes on a length scale of several
nanometers. This is due to exchange-induced translational symmetry breaking of
the charge density. Ionization potentials, optical absorption peaks, excitonic
binding energies and the optimally-tuned range parameter itself all become
independent of polymer length as it exceeds the critical localization scale.
Moreover, lattice disorder and the formation of a polaron result from the
charge localization in contrast to the traditional view that lattice
distortions precede charge localization. Our results can explain experimental
findings that polarons in conjugated polymers form instantaneously after
exposure to ultrafast light pulses.

Mesoscale and Nanoscale Physics;Chemical Physics