Hopping charge semiconductor

Organometallic and inorganic polymers have frequently been investigated as possible conducting polymers. An early trick was to look for extended conjugation and then introduce mixed valence states. This most often led to charge-hopping type semiconductors. Thus, both poly(ferrocenylacetylene) and poly(vinyl-... 

The electronic band structure of a neutral polyacetylene is characterized by an empty band gap, like in other intrinsic semiconductors. Defect sites (solitons, polarons, bipolarons) can be regarded as electronic states within the band gap. The conduction in low-doped poly acetylene is attributed mainly to the transport of solitons within and between chains, as described by the intersoliton-hopping model (IHM) . Polarons and bipolarons are important charge carriers at higher doping levels and with polymers other than polyacetylene. 

TABLE 7.1 Electrical Conductivity and Mobility of Charge Carriers in Metals, Band-like Semiconductors, and Hopping Semiconductors... 

A hopping semiconductor such as an oxide is often best described by Eq. (7.1) rather than by classical band theory. In these materials the conductivity increases with temperature because of the exponential term, which is due to an increase in the successful number of jumps, that is, the mobility, as the temperature rises. Moreover, the concentration of charge carriers increases as the degree of nonstoichiometry increases. 

Although these materials are often best described as hopping semiconductors in which the mobility is proportional to the exponential of the energy required to liberate the charge carriers, the conduction mechanism in devices is often complex. For application, mechanisms ate sometimes neglected in favor of empirical relationships. 

Ion hopping is a familiar concept in the chemistry of solid-state conductors, e.g. in the semiconductor industry. In the fluoride electrode, fluoride vacancies in.side the solid LaF lattice allow for conduction of charge (see Figure 3.11), in turn registered by the electrode as a potential. The emf is zero if the internal and external solutions are the same because the same numbers of fluoride ion enter the crystal from either face. 

Eishchuk II, Arkhipov VI, Kadashchuk A, Heremans P, Bassler H (2007) Analytic model of hopping mobility at large charge carrier concentrations in disordered organic semiconductors polarons versus bare charge carriers. Phys Rev B 76 045210... 

In relation to this problem is the fact that as the top of a band is approached the effective mass of a carrier changes and the range of allowed k values is small. Thus, the mobility of a carrier either in a narrow band conductor or at the top of an almost filled band must inevitably be small (9). In these cases it is probably not correct to assume the mass of the carrier and an electron to be the same. Under some circumstances the transfer of charge in a narrow band semiconductor is better described as an activated hopping process. 

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