Charge carrier transport mechanism

Researchers have used various models to explain the charge carrier transport mechanism in organic semiconductors. Two models have been used frequently, (i) the trapping model, which assumes a certain distribution of traps in the energy space and (ii) the field dependent mobility model, which assumes an exponential dependence of mobility on square root of electric field. 

The measurements of temperature dependences of conductivity are the most frequently used method aiming to determine the charge carrier transport mechanism. They are of fundamental importance however, they need to be complemented by other experiments, which employ predictions of different models for the dependence of conductivity on other factors -electric field and frequency additional verification can be done by the examination of dependence of conductivity on pressure and by the determination of temperature dependence of thermopower. 

Although different representations may facilitate interpretations of some phenomena, one should bear in mind that they contain only these information on electrical properties of the investigated material which are contained in the capacitance C(cu) and in the conductance G(cu). The measurements of the frequency dependence of conductivity performed at different temperatures provide a very important means to distinguish the charge carrier transport mechanisms. 

Novel composite carbazole polymer films containing silver and gold nanoparticles and being thiol-stabilized have been fabricated. Optical properties of PEPC films are more sensitive to the nanoparticles incorporation than the properties of PVC films. This can be provided by the formation of additional interaction of particles and -CH2-O-CH2- group presented in the polymer chain. High values of the current density measured and the character of the current-voltage dependence in the composite films can be the consequence of several mechanisms of charge carrier transport in both composite PEPC and PVC films. 

An alternative interpretation of the phenomenon of metal-support interactions induced by doping of semiconductive carriers with aliovalent cations is based on the theory of electrochemical promotion or the NEMCA effect. According to this interpretation, the charge carriers transported from the carrier to the metal particles are oxygen ions, which diffuse to the surface of the metal particles, thus altering the surface work function and, subsequently, chemisorptive and catalytic parameters. Work is currently in progress to elucidate the mechanism of induction of metal-support interactions by carrier doping. 

Figure 2 Scheme of a bottom-contact OFET illustration of the current research topics including the stmcture and morphology of organic semiconductor films as well as charge carrier transport and injection mechanism, which are the subject of this book. 

Mechanism of charge carrier transport in amorphous polymers... 

Our research was aimed to identify the sources of fluctuations in the dielectric layer prepared by anodic oxidation and to find the method for self-healing kinetics study. Charge carrier transport in thin isolating layer creates excess noise, which is the superposition of 1 /f and G-R noise. It has been observed, that samples with the same DC current have different noise spectral densities. We suppose that DC current is a sum of at least two independent current flow mechanisms, which have not the same noise intensity. 

The mechanism of charge carrier transport in oxidized conjugated aromatic polymers is not yet really clear The electrical conduction of oxidized PP films... 

For route 2, charge transport mechanism is similar to the hopping mechanism of small molecules. According to the charge transport theory developed hy Bredas et al. [22], the charge carrier transport in organic materials can be described by Marcus electron transfer theory (Eq. 1.3). In organic ciystals, the AG is 0 because the electron transfer happens in a same kind of molecules. Thus the Eq. 1.3 formula can be simplified into Eq. 1.4 as follows ... 

In side-chain polymers, where the chromophore is covalently bonded to a polymer main chain, the concentration of transport molecules can be increased as compared to MDPs without causing crystallization. Because the underlying physical mechanism of charge carrier transport (nearest neighbour hopping between weakly coupled, localized states) is the same for both MDPs and side-chain polymers, the transport properties are qualitatively similar. 

In contrast to the two main groups described above the charge carrier transport in conjugated systems occurs via the polymer backbone. Various substituents are used to tailor the mechanical properties and the processibility. For instance, the chain can be a skeleton with quasi-(T-conjugation like in polysilanes [36, 38-42] or polygermylenes [39]. 

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