Semiconductors Models for CPs

With the above understanding of the band structure and semiconductor model for CPs, we may now turn to an elementary interpretation of the evolution of the spectra of a typical CP as a function of doping level. We first look at the spectra expected of CPs, then turn again to poly(pyrrole) as a prototypical example, following an earlier treatment first presented by Bredas, Street et al. [18]. 

Another important feature to note regarding the bipolaron levels in particular is that they are either empty (p-type doping) or fiilly occupied (n-type doping), and thus spinless. An important reason why a model different from that of conventional semiconductors was sought for CPs is that in highly doped CP samples, which had high intrinsic conductivity, there was no evidence for unpaired electrons from experiments such as electron spin resonance (esr) measurements, or correlation of conductivity with esr absorption, but rather, spinless charge carriers were indicated [16]. 

We are now in a position to consider conduction models, after which we may look at some of the experimental observations that these models attempt to predict. One of the first models to be considered for CPs, primarily because it appeared to apply to disordered "conventional" semiconductors and CPs were considered disordered. 

The PPV spectra of Fig. 16 show all the signatures of exciton absorption and emission, such as in typical molecular crystals. The existence of well-defined structure in the absorption spectrum is not so easily accounted for in a band-to-band absorption model. In semiconductor theory, the main source of structure is in the joint density of states, and none is predicted in one-dimensional band structure calculations (see below). However, CPs have high-energy phonons (molecular vibrations) which are known (see, e.g., RRS spectra) to be coupled to the electron states. The influence of these vibrations has not been included in previous theories of band-to-band transition spectra in the case of such wide bands [176,183]. For excitons, the vibronic structure is washed out in the case of very intense transitions, corresponding to very wide exciton bands, the strong-coupling case [168,170]. Does a similar effect occur for one-electron bands Further theoretical work would be useful. 

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