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Defects in Amporphous and Organic Semiconductors

Semiconductor Physics

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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 displacements 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, comprise the classes of amorphous chalcogenides and tetrahedrally bonded amorphous 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 semiconductors 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 of long 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.

Karl W. Böer is retired.

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Notes

  1. 1.

    In amorphous structures the average number of next neighbors may substantially differ from that of the corresponding crystalline counterpart, see Sect. 3.1 of chapter “The Structure of Semiconductors.”

  2. 2.

    It should be noted that the definition of a band state is related to the coherence length of an electron wave, which is essentially the same as the mean free path λ.

  3. 3.

    A third class, somewhat in between these two, contains α-P and α-As.

  4. 4.

    Bipolar devices made from such materials are capable of switching at high speed from a low to a high conducting state at a critical bias (Ovshinsky 1968).

  5. 5.

    Polycyclic aromatic hydrocarbons like pentacene have a positively charged planar backbone of atom cores and negatively charged π electrons in front and at the back of this plane, yielding a permanent quadrupole moment.

  6. 6.

    The repetition unit of fused-ring polythiophene comprises four rings like those shown in Fig. 15a with the inner two rings fused by commonly sharing a two carbon atoms with a C = C double bond (see inset Fig. 16).

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Authors and Affiliations

  1. University of Delaware, 138 Moorings Park Rd. O 301, Naples, FL, 34105, USA

    Karl W. Böer

  2. Institut für Festkörperphysik, Technische Universität Berlin, Hardenbergstr. 36, Berlin, 10623, Germany

    Udo W. Pohl

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Böer, K.W., Pohl, U.W. (2016). Defects in Amporphous and Organic Semiconductors. In: Semiconductor Physics. Springer, Cham. https://doi.org/10.1007/978-3-319-06540-3_20-1

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    Defects in Amorphous and Organic Semiconductors
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    DOI: https://doi.org/10.1007/978-3-319-06540-3_20-4

  2. Defects in Amorphous and Organic Semiconductors
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    DOI: https://doi.org/10.1007/978-3-319-06540-3_20-3

  3. Defects in Amorphous and Organic Semiconductors
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    DOI: https://doi.org/10.1007/978-3-319-06540-3_20-2

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    Defects in Amporphous and Organic Semiconductors
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    30 December 2016

    DOI: https://doi.org/10.1007/978-3-319-06540-3_20-1

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