Organic semiconductors can be divided into two types, short chain (oligomers)
and long chain (polymers).5Typical examples for semiconducting oligomers are
pentacene, anthracene and rubrene. Some semiconducting polymers are Poly(3-
hexylthiophene) and poly(p-phenylene vinylene).
Short chain organic semiconductors are usually formed by a series of benzene
rings in which the π -bonds become delocalized to form a π -system. The gap between
occupied and empty states in these π systems becomes smaller with increasing
delocalization, leading to smaller bandgap (Figure 2).