Contacts, Injection, Ejection, and Dark Currents

In Sections 8.4.1 and 8.4.2, we introduced the basic principles of photoconductivity. Without optical excitation, electric currents can flow in ideal, non-doped organic 

When a contact and not the organic semiconductor limits the current, it is referred to as a contact-limited current. When, in contrast, the organic semiconductor itself and not the contacts limit the current, the contacts are referred to as Ohmic contacts. Both cases can be of interest the first when the current is to be controlled in the circuit independently of the external resistance and the second, when the process of charge transport in the organic semiconductor is to be investigated. 

Stationary space charge, the characteristic becomes nonlinear in a typical manner (see Sect. 8.4.4). 

Using this expression together with the definitions of the specific conductivity ff and the mobility /u (Eqns. (8.1) and (8.5)) as well as the charge density n (n = oyVol = QJAd), and with the equation of continuity, (9j/9x) = 0, we find for the current density j  

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