Stationary Dark Currents

Stationary dark currents in disordered thin films which contain no intrinsic charge carriers require the injection of charge carriers. If Ohmic contacts have been prepared, then the stationary dark current is not limited by the contact resistance, but instead by the mobility and the injected space charge. Since about 1985, stationary dark currents through disordered thin films have attracted great interest, especially since at that time, suitable contacting methods were developed to permit injection of both electrons from the one electrode as well as holes from the other. This made 

In order to understand the dark current through disordered films, primarily experiments with unipolar currents must be carried out and analysed. In Sect. 8.6.5.1, we will therefore initially present a few typical and important experimental results on dark currents in selected disordered films of molecules or polymers. In Sects. 8.6.5.2 and 8.6.5.3, we then give the analysis of these results. Beyond a simple measurement of the temperature and voltage dependence of the current, /(V, T), this analysis is reasonable only if the experiments were carried out on carefully and reproducibly prepared films and the parameters film thickness d, temperature T and applied voltage Vgxt were varied over the widest range possible. 

The following experimental results were obtained from two groups of carefully prepared samples, PPV polymer films and molecular Alqs vapour-deposited films. They were chosen from the large number of molecular and polymeric films which have been studied, because the fundamental empirical results in these two groups of samples can be especially clearly presented. 

Finally, as last and most important experimental result, Fig. 8.28 (which we have already seen in another context) shows the temperature dependence of the j(V) characteristic for the electron current in Alq3 with a layer thickness of d = 294 nm. Clearly, the temperature, along with the layer thickness, is a decisive parameter in determining the behaviour of the extremely nonlinear characteristics in this system. For example, the current increases by a factor of about 40 on doubling the voltage V at 320 K, and at 100 K, by a factor of about 10. And for V = 8 V, the ratio of the current at 160 K to that at 320 K is roughly 10.  

In order to analyse these very notable experimental results on the dark current of injected charge carriers in disordered films, we first describe in the next Sect. 8.6.5.2 the model of space-charge-limited currents for the case that the semiconductor is not an ultrapure single crystal, but rather a disordered molecular film. 

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A fairly linear dependence of the photocurrent with pH is observed in the range of 4.5-9.5. The dark current is in the range of lO" " A. Upon illumination, the current increases sharply and becomes stationary after some 10 to 100 s. The photocurrent is in the range of nA. The results are reversible and reproducible. The dependence of the current on pH is shown in Figure 3.14. 

FIGURE 6.24. Current-potential curves of stationary n-type (a) and p-type (b) crystalline silicon electrodes in 2mM MVCh, 0.1 M KCl aqueous solution. Measurements with the n-type electrode were made in the dark, whereas the p-type electrode was in the dark (dashed line) and under illumination (solid line). After Kooij. (Reprinted with permission.)... 

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