Semiconductors, doped, transport organic

Arkhipov VI, Fishchuk II, Kadashchuk A, Bassler H (2007) Charge transport in neat and doped random organic semiconductors. In Hadziioannou G, Malliaras GG (eds) Semiconducting polymers. Chemistry, physics and engineering, 2nd, completely revised and enlarged edn. Wiley, Weinheim, pp 275-383... 

Charge transport in disordered organic semiconductors generally occurs by hopping between adjacent conjugated segments. This process has been investigated in detail in molecularly doped polymers and even in doped crystalline materials... 

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. 

Fig. 5.1. A schematic diagram of the energy levels and fiUed/empty states for (a) a band-transport semiconductor, (b) a metal, and (c) an organic semiconductor in the absence of thermal excitation or doping. In crystalline and organic semiconductors, an energy gap exists, whereas for a metal, energy levels immediately above the filled states are permitted. Note that carriers ejected beyond the vacuum level can be at any energy.

In conducting polymers, the extra carriers added upon doping are able to drift under an applied electrical field. In semiconducting polymers, no carriers are available except those thermally excited across the gap. However, negative (positive) carriers can be injected into the material by metallic contacts when the barrier between the metal work function and the LUMO (HOMO) molecular levels is overcome. Then, the injected carriers can move inside the semiconductor if a bias field is applied. Injection of carriers and their transport is a fundamental issue for all electronic devices and transistors in particular. In the following, main transport properties of organic semiconductors (both small molecules and polymers-based) used as active materials in transistors will be reviewed. 

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