Charge Transport in Polythiophenes

To understand how charges are transported is of fundamental importance in the materials science along with studying structures and morphologies of the materials. 

We add, however, that special care must be taken in determining from the experimental data which transport mechanism is likely. This is because resistivity (p) often exhibits exponential dependence on temperature T i.e. p oc exp[(7 /7 o) ], on the basis of which the transport mechanism is inferred. The exponents x= j2, 1/3, and 1/4 frequently arise in the non-crystalline or disordered media of semiconductors. When X = 1/3 happens, for instance, it is usually difficult to distinguish various conduction mechanisms including two-dimensional VRH, tunnehng of carriers, interchain conduction, etc. [97]. 

Polythiophene and its derivatives are characterized as p-type semiconductors, and so they are expected to form a Schottky-like junction with a metal having low work function [98]. In this case the charge injection over this metal/polymer interface plays an important role. The charge injection mechanism can be studied by measuring temperature and voltage dependence of current. 

The interaction between the polythiophene and metals was recently investigated using quantum chemical calculations [104]. Such approaches will be important to improve the electrical contact between the polymer materials and electrodes. 

X-ray diffraction measurements, in turn, show that the thin films of the oligothio-phene compounds consist of regular molecular layered structure [106,107]. Regarding an a,a -dimethylsubstituted quaterthiophene, for instance. Fig. 9 [106] indicates a very sharp and intense primary diffraction spacing (of 18.1 A) together with 

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