Space-charge-limited stationary currents

In fact, in their recent work, Mensfoort et al. [90] conclude that in polyfluorene copolymers hole transport is entirely dominated by disorder. This is supported by a strictly linear In p cx dependence covering a dynamic range of 15 decades with a temperature range from 150 to 315 K (Fig. 8). Based upon stationary space-charge-limited current measurement, where the charge carriers are in quasi equilibrium so that dispersion effects are absent, the authors determine a width a of the DOS for holes as large as 130 meV with negligible polaron contribution. 

We saw previously that concentration polarization results in the decrease of solute concentration near the permselective interface (right at the interface in the electro-neutral version) where most of the system s resistance thus concentrates, and where the space charge develops. The system is expected to be sensitive to the minimum concentration value, and because of nonlinearity nontrivial effects, could be anticipated in response to unsteady disturbances of this value (e.g., provided by harmonic modulation superimposed upon a constant voltage applied to the system). Since it is easier to increase the minimal concentration (close to zero at the limiting current) than to decrease it, we might expect a positive rectification effect for the direct current component, counterintuitive ( anomalous ) in the present system with a convex stationary VC curve. Thus the topic of this section is the rectification effects that arise in the stationary concentration polarization in response to a harmonic voltage modulation. 

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 this case only the rate of polarization giving stationary diffusion profiles in the thin solution layer needs to be found. This results in a stationary limiting current instead of a current peak as for semi-infinite conditions. The rate was experimentally found to be 2 mV/s at a space thickness of less than 100 /xm. The slow rate of polarization does not affect the limiting current, but the charging current of the signal decreases which results in a better lower detection limit. The stationary limiting current can be calculated with the equation first derived by Anderson and Reilley ... 

---