Organic semiconductor conductive doping

In the context of this chapter, we focus on the undoped or lightly doped 7i-conjugated systems that are commonly referred to as organic semiconductors. Conducting polymers, such as PEDOT PSS, plexcore, polyaniline, polypyrrole, and others are not addressed here as their charge transfer mechanisms are rather different and would warrant an article in its own right. 

More specifically, there is a transition in the increase of the conductivity of organic semiconductor upon doping, which is manifested by a change in the slope of the curve, as shown in Eig. 10. The parameters are the same as in Fig. 9. There we can see that the conductivity increases linearly for low, and superlinearly for high doping levels. This transition has been interpreted in [25] in terms of the broadening of the transport manifold due to the enhanced disorder from the dopant. 

Attempts to dope organic semiconductors have been made very early in the field, motivated by the prospect of possibly reaching metallic conductivities [108, 109]. These synthetic metals, however, have not been realized. While p-type doping could be obtained, for example, with iodine gases for poly-p-phenylene vinylene (PPV) derivatives, and n-type doping was demonstrated with sodium for a cyano-derivative of PPV, the doping levels obtained were not stable with time. The dopant molecules readily diffused into the organic semiconductor, yet also out of it. Due to the lack of stability, these approaches were not suitable for commercial applications. 

The signal travels through a thick, or even molecularly thin, semiconductor that connects these electrodes it could be an inorganic semiconductor (doped Si, doped Ge), an organic conducting polymer (polyaniline, polythiophene, polyacetylene), a carbon nanotube, or an organic semiconductor (sexithiophene). 

Several polymer conductors are commercially available, and have been used in the demonstration of printed transistors. These include PEDOT PSS, which is a commercially available polymer conductor, as well as various versions of polyaniline. The latter is typically doped with an acid or salt to increase conductivity. Both of these material systems are water soluble and easily printable. They also typically form good interfaces to organic semiconductors, making them attractive for use in printed transistors. As with polymer dielectrics, however, it is important to note that their usability with inorganic semiconductors is questionable, of course. 

Alternatively, undesirable side reactions may lead to persistent cation radicals. Due to these side reactions doping (p-type) of the organic semiconductor may occur, leading to a higher conductivity, but lower luminescence efficiency (photo and electroluminescence). However, by chemical or thermal (post-bake step at 180 °C) treatment, complete dedoping is possible and the luminescence efficiency is fully recovered. Additionally, in some cases the cation radical is able to attack the oxetane through nucleophilic reaction and ultimately start the same chain reaction as above [33]. 

One possibility to realize low-threshold organic solid-state lasers is to utilize guest-host systems. In the following this is demonstrated for the example of DCM- and DCM2-doped organic semiconductors. As host materials the electron-transport material Alq3 and the hole-conducting materials NPD and CBP are used. 

Many unsubstituted polymeric organic semiconductors are not easily dissolved in solvents. After mechanical processing, doping with acids, and/or processing with surfactants many of these materials can be formed into conductive dispersions and applied from solution to substrates. 

When an organic semiconductor is weakly n- or p-doped, thus when dopants lead to the charge-transfer reactions mentioned above, it becomes an Ohmic conductor. A measurement of the specific conductivity a can then in optimal cases permit the concentration of the dopants to be computed, if the mobility /x is known. 

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