The charge transport mechanism in organic semiconductor devices is dependent on the charge carrier mobility which is generally much lower than the inorganic semiconductors. It is thus essential to understand the charge transport mechanisms in organic semiconductors. Despite years of research, there is no absolute rationalization available to characterize conduction in one particular fashion due to the complexity and diversity of the materials and devices available. In this chapter efforts were made to introduce various charge transport mechanisms in organic semiconductors. These include the percolation model, polaron model, MTR and VRH. It is generally agreed that thermal assisted tunnelling, also known as hopping, is the basis of transport in most disordered organic semiconductors. Field and temperature dependence of the carrier mobility is explained in terms of hopping in Gaussian DOS in LEDs whereas the hopping in an exponential DOS explains the gate voltage and temperature dependence in FETs. Although this is the case, one description must be used to explain the electrical characteristics of these organic devices as the polymers described in these devices fit in with the same class of disordered π- conjugated systems.
The Gaussian function and the associated exponential approximate at low energies were used to describe the distribution of states (DOS) in disordered organic semiconductors. It is widely believed that the charge transport model in organic materials co-exist within the discrete energy states defined by the Gaussian distribution. In this thesis, we assume the distribution to follow the exponential DOS as the Fermi level lies in the band tail of the Gaussian DOS. For disordered
organic materials, it is hard to explain the mobility to be constant as the charge carriers on different energy levels in the exponential distribution experience different hopping rates. For this reason, the concept of transport energy is used to describe a position in energy over which it is easier to demonstrate mobility of carriers also known as the mobility edge. Hence effective mobility for organic disordered materials can be described using the universal mobility law in inorganic semiconductors. Moreover to understand effective mobility in polycrystalline organic semiconductors, comparison between ordered grains and disordered grain boundaries is made at the edge of the grain where the grain meets the grain boundary.
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