8.1 Current Progress

Our first contribution was the formulation of a charge transport mobility model, which is one of the most important parameters in organic semiconductors. Using three different analytical models, we separately explain the mobility's dependence on carrier concentration, electric field and temperature. We showed that the density of states function is an important factor for the carrier concentration dependence of the mobility and the exponential DOS function is not entirely reliable for the low carrier concentration regime. Furthermore, a physical model was developed to explain the Poole-Frenkel behavior of the electric field dependent mobility.

In order to deal with the effect of Fermi-Dirac statistics on the transport energy, we extended Baranovskii's transport energy model. This model shows that the Fermi-Dirac statistics plays an important role in transport energy when the temperature is low and carrier concentration is higher.

Then, we developed analytical models to describe the doping and trap characteristics of organic semiconductors. This model can successfully explain the superliner increase of conductivity upon trap concentration and the relation between trap concentration and conductivity.

Despite of the successful application of the Fowler-Nordheim and Richardson-Schottky injection models to some experimental data of organic devices, it is very important to discuss the role of diffusion transport and backflow current in the charge injection process for organic devices. For this goal, we have presented two different injection models for organic light-emitting diodes, one is based on multiple trapping theory and the other on a master equation.

In the next step we investigated the SCLC in organic devices within the frame work of variable range hopping transport. It was shown that the SCLC controlled by a Gaussian density of states distribution obeyed the $j\propto V^2$ relation remarkably similar to SCLC controlled by shallow traps in the low current density regime when the mobility was constant, while field-dependent mobility would change this relation slightly in the high current regime.

Up to now, many of the numerical or analytical organic device models available in commercial device simulators use the same expressions as used for the crystalline devices. However, organic devices present several differences with respect to crystalline devices. In the final part of this thesis, we presented two analytical models that describe the DC characteristics of organic thin film transistors and organic light-emitting diodes. Both models are based on hopping transport theory and good agreement between calculation and experimental data was found.