Poole-Frenkel emission

The results of several studies were interpreted by the Poole-Frenkel mechanism of field-assisted release of electrons from traps in the bulk of the oxide. In other studies, the Schottky mechanism of electron flow controlled by a thermionic emission over a field-lowered barrier at the counter electrode oxide interface was used to explain the conduction process. Some results suggested a space charge-limited conduction mechanism operates. The general lack of agreement between the results of various studies has been summarized (57). 

Figure 50 Quenching efficiency (<5) as a function of dc electric field applied to the electrophosphorescent (EPH) and phosphorescent system. The curves are fits to the Poole—Frenkel (see lower inset) and Onsager (see upper inset) models for charge pair dissociation in external electric fields. The quenching efficiency is defined as a relative difference between the emission efficiency at a given field F[0(F)] and at a field F0[4>(F0)] where a decrease in the EPH efficiency becomes observed (<) (F0) (7 1)]/

In order to predict absolute dielectric strengths we need to have more detailed information than is yet available about electronic states and mobilities in polymers. For the present we can only conclude that there is satisfactory agreement between the form of the theoretical results, based on a rather general electronic model, and the best experimental results. To the extent that the model is a very reasonable one, we can say that we can understand intrinsic breakdown behaviour. Measurement of pre-breakdown currents, especially with pointed electrodes which impose regions of very high field strength at their tips when embedded in the material, suggests that electronic carrier production either by injection from the electrodes (Schottky emission) or from impurities (Poole-Frenkel effect) may play a part in the breakdown process. More work is required, however, before this can be fully understood. 

Tunneling in multilayered LB films is defect-mediated via trap sites within the conduction band of the molecules (Poole conduction), or by Schottky emission between widely spaced trap sites (Poole Frenkel conduction) in thicker samples [13]. With good molecular conductors the current from molecular conduction should dominate the small contribution from tunneling. However, the conduction mechanism between adjacent layers is not always obvious, due to the complexity of the interface structure. 

A more searching analysis of the Poole-Frenkel mechanism performed for polysiloxane on the basis of the Hill model ( ) showed that charge carrier emission should proceed from the isolated Coulomb centre and should take place in the hemisphere related to that centre. The depth of the centres, determined form the activation dependence of the temperature, was =... 

Equation (8.15) may well cause one to wonder whether Poole-Frenkel (P-F) emission [268] or Schottky emission [208] of the charges underlies the charge transport. For the latter the charge emission is assumed to take place across the Schottky barrier between the electrodes and the semiconductive materials. By testing the linearity of log(7/7 ) versus F IkT plots [208] where J is the current density, it was found that the linearity is worse than that displayed in Figure 8.66, implying that Schottky emission is irrelevant and that P-F emission is more likely. The P-F-like feature... 

The emission of the Poole-Frenkel type assumes the formation of coulombian centers in the grain-intergranular layer interface region. The relationship that describes this type of emission is on Equation 9, where the external electric field variations are more relevant than for issue... 

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