Organic semiconductors in general are a family of
electronic materials that are based on 
-conjugated carbon
atoms. In the last three decades electronic devices based on this family of materials, such as
field-effect transistors and light emitting diodes, have attracted
much attention as possible inexpensive and flexible alternatives to inorganic
devices. Although we witnessed considerable
progress in the introduction of new commercial applications that are based on
these materials, the nature of charge transport in these organic materials and
devices has not been understood very well. The main goal of this thesis is
to theoretically investigate the charge transport properties of organic semiconductor
materials and devices.
Charge transport properties presented here are investigated in the framework of variable range hopping theory. In a previously published paper by Vissenberg, a percolation model has been developed in order to explain the temperature dependence of hopping mobility in organic semiconductors. One of our main theoretical goals is to develop different models that can explain the dependence of the mobility in organic semiconductors on electric field, temperature, carrier concentration, and doping and trap concentration. A both temperature and electric field dependent mobility model is developed based on a modified Miller-Abrahams rate equation. The carrier concentration dependent mobility is formulated assuming a Gaussian density of states. A unified mobility model is presented which can explain the temperature, electric field and carrier concentration dependence. The doping and trap dependent mobility model is obtained by assuming a superposition of two exponential density of states functions.
The charge injection process between metal and organic semiconductor is examined for organic light-emitting diodes. For this goal we develop both a diffusion-controlled and a master equation based injection model. These two models can explain the dependence of the injection current on the temperature, electric field and barrier height. Good agreement between calculation and experimental data is found.
We examine closely the space charge limited current (SCLC) and the effect of the Fermi-Dirac statistics on the transport energy. It is found that the SCLC due to a Gaussian density of states is similar to SCLC controlled by shallow traps in regular semiconductors. The Fermi-Dirac statistics plays an important role for transport energy, even at low temperature.
Finally, analytical models applicable to organic thin film transistors and to unbipolar organic light-emitting diodes are presented.