Doping, mechanisms

The incorporation of phosphorus yields fourfold-coordinated P atoms, which are positively charged, as phosphorus normally is threefold coordinated. This substitutional doping mechanism was described by Street [52], thereby resolving the apparent discrepancy with the so-called S N rule, with N the number of valence electrons, as originally proposed by Mott [53]. In addition, the incorporation mechanism, because charge neutrality must be preserved, leads to the formation of deep defects (dangling bonds). This increase in defect density as a result of doping explains the fact that a-Si H photovoltaic devices are not simple p-n diodes (as with crystalline materials) an intrinsic layer, with low defect density, must be introduced between the p- and n-doped layers. 

However, it remains to be understood how this type of doping process can induce enhancement in the electronic transport of the polymers. Indeed, the clarification of the doping mechanism of conducting polymers and of the associated electronic band evolution is of fundamental importance for the comprehension of the operational behaviour of these compounds as novel electrode materials. 

Fig. 18 Doping mechanisms for molecular p-type doping (top) and for n-type doping (bottom). P-type (n-type) doping is achieved when the molecular dopant acts as acceptor (donor). After [107]...

At the molecular level, studies of the doping mechanism in the poly(heterocycles) reveal that very short lived radical cations ("polarons") immediately decay (combine) to form dications ("bi-... 

Impurity addition, however, is not the only doping mechanism. Nonstoichiometry in compound semiconductors such as CdTe (Table 1) also gives rise to n- or p-type behavior depending on whether Cd or Te is in slight excess, respectively. The defect chemistry in these solid chalcogenides controls their conductivity and doping in a complex manner, a discussion of which is beyond the scope of this chapter. Excellent treatises are available on this topic and on the solid-state chemistry of semiconductors in general [16-22]. 

Polyaniline provides the prototypical example of a chemically distinct doping mechanism [33,34], Protonation by acid-base chemistry leads to an internal redox reaction and the conversion from semiconductor (the emeraldine base) to metal (the emeraldine salt). The doping mechanism is shown schematically in Fig. II-2. The chemical structure of the semiconducting emeraldine base form of polyaniline is that of an alternating copolymer, denoted as [(1A)(2A)] , with... 

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