Organic semiconductor carrier-injection barriers

Explanation of the experimental data required taking into account disorder-induced broadening of the density of states of the organic semiconductor, which provides carriers with injection pathways through deep states in the DOS, leading to a reduced effective barrier at low temperatures. Similar conclusions have recently been drawn on the basis of channel length scaling experiments [111]. 

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. 

Besides the transport properties, the contact formation between the source/drain electrodes and the organic semiconductor is crucial for the OFET performance. To illustrate this, balanced charge carrier transport properties ii = i2 in the above evaluated model are assumed and the injection of the complementary charge carrier species is hindered, e.g., by a pronounced injection barrier at the drain/semiconductor contact. The source contact is supposed to inject freely charge carriers of type 1 and out of simplicity they are assumed to be ejected freely at the drain electrode as well. For Vd —> IV g] the channel depletes more and more in the vicinity of the drain electrode and for Vg < Vd the charge carriers of type 1... 

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