  The performance of organic semiconductor devices is linked to highly-ordered
nanostructures of self-assembled molecules and polymers. We employ many-body
perturbation theory and study the excited states in bulk compolymers. We
discover that acceptors in the polymer scaffold introduce a, hitherto
unrecognized, conduction impurity band. The donor units are surrounded by
conjugated bands which are only mildly perturbed by the presence of acceptors.
Along the polymer axis, mutual interactions among copolymer strands hinder
efficient band transport, which is, however, strongly enhanced across
individual chains. We find that holes are most effectively transported in the
$\pi-\pi$ stacking while electrons in the impurity band follow the edge-to-edge
directions. The copolymers exhibit regions with inverted transport polarity, in
which electrons and holes are efficiently transported in mutually orthogonal
directions.
