Survey of Representative Experimental Results

However, in that survey Schein and Tyutnev question the intermolecular origin of cr. They compared cr values derived from studies of hole transport in 1-phenyl-3-((diethylamino)styryl)-5-(p-(diethylamino)phenyl)pyrazoline (DEASP) molecules, derivatives of pyrazoline, whose dipole moment is 4.34 D, blended with either polystyrene or polycarbonate as function of concentration. They found that cr is independent of the matrix material and that a remains constant when the concentration of DEASP increases from 10% to 70% while one would expect that cr increases as 

A textbook example for the application of the uncorrelated GDM is the recent study by Gambino et al. on a light emitting dendrimer [49]. The system consists of a bis (fluorene) core, meta-linked biphenyl dendrons, and ethylhexyloxy surface units. ToF experiments shown in Fig. 5 were performed on 300 nm thick sandwich films 

Despite the success of the disorder model concerning the interpretation of data on the temperature and field dependence of the mobility, one has to recognize that the temperature regime available for data analysis is quite restricted. Therefore it is often difficult to decide if a In p vs or rather a In p vs representation is more appropriate. This ambiguity is an inherent conceptual problem because in organic semiconductors there is, inevitably, a superposition of disorder and polaron effects whose mutual contributions depend on the kind of material. A few representative studies may suffice to illustrate the intricacies involved when analyzing experimental results. They deal with polyfluorene copolymers, arylamine-containing polyfluorene copolymers, and c-bonded polysilanes. 

The most comprehensive study is that of Khan et al. [86]. They describe ToF experiments on sandwich-type samples with films of poly(9,9-dioctyl-fluorene) 

However, one should be cautious about overinterpreting the field and temperature dependence of the mobility obtained from ToF measurements. For instance, in the analyses of the data in [86, 87], ToF signals have been considered that are dispersive. It is well known that data collected under dispersive transport conditions carry a weaker temperature dependence because the charge carriers have not yet reached quasi-equilibrium. This contributes to an apparent Arrhenius-type temperature dependence of p that might erroneously be accounted for by polaron effects. 

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