Defects in Amorphous and Organic Semiconductors

Abstract

Amorphous and organic semiconductors have strong topological irregularities with respect to specific ideal structures, which depend on the particular class of such semiconductors. Most of these defects are rather gradual displace- ments from an ideal surrounding. The disorder leads to defects levels with a broad energy distribution which extends as band tails into the bandgap. Instead of a sharp band edge known from crystalline solids a mobility edge exists separating between extended states in the bands and localized states in the band tails. Amorphous semiconductors, also referred to as semiconducting glasses, com- prise the classes of amorphous chalcogenides and tetrahedrally bonded amor- phous semiconductors. Amorphous chalcogenides are structurally floppy solids with low average coordination numbers and pronounced pinning of the Fermi level near midgap energy. The more rigid tetrahedrally bonded amorphous semi- conductors have larger coordination numbers. They may be well doped p-type and n-type much like crystalline semiconductors. Organic semiconductors comprise small-molecule crystals and polymers. Both have weak intermolecular bonds favoring deviations from ideal alignment. In small-molecule semiconductors the structure of thin films grown on substrates usually deviates from the structure of bulk crystals, with a substantially different molecule ordering at the interface and a strong dependence on the dielectric properties of the substrate. Polymers consist oflong chain-like molecules packed largely uniformly in crystalline domains separated by amorphous regions with tangled polymer chains. Besides chemical structure of the chains crystallinity depends on the molecular length.