We quantify the degree of paracrystalline disorder in the π-π stacking direction of crystallites of a high performing semicrystalline semiconducting polymer with advanced x-ray line-shape analysis. Using density functional theory calculations to provide input to a simple tight-binding model, we obtain the density of states of a system of π-π stacked polymer chains with increasing amounts of paracrystalline disorder. We find that, for an aligned film of PBTTT, the paracrystalline disorder is 7.3%. This type of disorder induces a tail of trap states with a breadth of ∼100 meV as determined through calculation. This finding agrees with previous device modeling and provides physical justification for the mobility edge model. © 2011 American Physical Society.