Gap states in organic semiconductors:  Hydrogen- and oxygen-induced states in pentacene

First-principles pseudopotential density functional calculations are reported for hydrogen- and oxygen-related defects in crystalline pentacene (C₂₂H₁₄). It is shown that defects that perturb a carbon atom so as to remove its pₙ orbital from participation in the system give rise to a state in the gap. One such defect is formed by adding a H atom to create a C₂₂H₁₅ molecule with one fourfold C atom. In the neutral-charge state a single electron occupies a orbital having reduced amplitude on the perturbed carbon atom. Charged defects correspond to addition or removal of an electron from this orbital. The possibility that these defects give rise to the bias-stress effect in pentacene-based thin film transistors is discussed.