two hetero aromatic rings each fused to a common aromatic group (Ar1) of either of the repeating unit structures (b1 or b2): wherein N is a nitrogen atom and B is an oxygen, sulfur, or NR group, wherein R is hydrogen or a substituted or unsubstituted alkyl or aryl attached to N. The number of repeating unit structures represented by structures (a), (b1), and (b2) is not critical.