Charge excitations in polythiophenes

The charge excitations in the conducting polymers are quite different from those of usual inorganic semiconductors such as crystalline silicon, even though the 

Meanwhile, the charge excitations couple with the lattice vibrations and allow some symmetrical vibrational modes (Raman-active modes) to become infrared active by breaking the local symmetry [75]. This was first recognized in the doping and photoexcitation studies of rran -polyacetylene [62, 76, 77]. Moreover, the amplitude mode formalism developed by Horovitz [78] has been successful in explaining the one-to-one correspondence between the photoinduced and the doping-induced IR-active vibrational modes and their relationship to the Raman modes of the pristine polymer. 

Equation (2) applies only to the case of the polaron. Consequently, to examine whether actually observed results satisfy those relationships would be an intuitive criterion for guaranteeing the validity of the theory. The details on this issue will be presented in section 2.3.4.1 along with the results of other spectroscopic studies including IR and electron spin resonance (ESR). 

Prior to the recently developed theories on the conducting polymers, Holstein [79] described how a single excess charge (polaron) behaves in a one-dimensional molecular crystal. Note that approaches of the recent theories are somewhat related 

Usually the / -type doping predominantly occurs in the case of polythiophenes. Occasionally, however, does the -type doping take place for them [91]. A pair of charges both positive (hole) and negative (electron) can be introduced at once through photoexcitation [92]. Other special techniques of doping are presented in subsequent sections in connection with the properties of the charged states. 

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