4.1 Introduction

Despite decades of research progress, some rather ubiquitous features of the charge transport in organic semiconductors are still far from being well understood. One such example is the relation between conductivity and doping [81,82]. The doping of organic semiconductors is just beginning to be quantitatively studied [83,84,85,86,87]. Early studies have shown that the doping of organic semiconductors (partially oxidizing or reducing them) can increase their conductivity by many orders of magnitude. There are also early studies of the effect of adding molecular dopant to thin films of organic semiconductors in an attempt to improve their photovoltaic behavior [88,89]. Although the doping process of organic semiconductors can largely be depicted by a standard model used for crystalline inorganic semiconductors [90], a general doping model for organic semiconductors still remains a challenge. Because of the weak intermolecular forces, doping of organic semiconductors is quite difficult compared to the doping of common semiconductors. In common semiconductors, the strong covalent or covalent-ionic bonds ease doping [91]. Bending or breaking the high energy interatomic bonds at crystal defects and grain boundaries, or incorporating impurities of a valence different than the valence of the host, often produce electronic states near enough to the band edge to generate free carriers. For these reasons, it is difficult to produce truly intrinsic common semiconductors. On the other hand, organic semiconductors are van der Waals solids. Bending or breaking these low energy intermolecular bonds, or adding different molecular (PPEEB or F4-TCNQ) into the lattice, only inefficiently produce free carriers.

At the same time, the mobile charge in organic semiconductors can be trapped by some states. These charge traps are known as deep traps, and they are not well understood.

In this chapter, we present an analytical model for hopping transport in doped, disordered organic semiconductors based on the VRH and the percolation theory. This model can successfully explain the superliner increase of conductivity with doping observed in several experimental data sets. It can also be used to describe the trapping characteristics of organic semiconductors.