Transport Properties of Diodes

The model suggests that at higher voltages the electron and hole currents in PPV-based devices with low contact barriers (as expected for selective Ohmic contacts) are determined by the bulk conduction properties of the polymer and not by the injection properties of the contacts. The conduction of holes is governed by space-charge effects and field dependent mobility, while electron transport is limited by traps. 

One has to be aware that the I/V response of a diode is always a superposition of its transport properties and metal/semiconductor properties. However, in many cases it is possible to separate these two responses due to their 

For the high voltage regime it is reasonable to suppose that a semiconductor of thickness L is contacted with two electrodes which, by virtue of a low energy barrier at the interface, are able to support the transport of an infinite number of one type of mobile carrier. The current will then become limited by its own space charge, and this can in the extreme case reduce the electric field at the injecting contact to zero. This is realized when the number of carriers per unit area inside the sample approaches the capacitor charge of the diode, i.e., eeo/e. This number of carriers can be transported per unit transit time ttT = L/fj,. 

The charge carrier density decreases with increasing distance from the injection contact according to 

The average space charge density (per unit volume) becomes 

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