This paper reports the observation of bipolar, space-charge-perturbed transport in colloidal dispersions using an experimental technique that time-resolves electrophoresis in nonpolar colloidal systems. Unlike existing methods for studying electrophoresis, this technique can be applied to dispersions of diverse types and concentrations over a wide range of electric fields, including the space-charge-perturbed conditions often encountered in practical applications. The phenomenon is investigated as a special case of dielectric relaxation in a leaky capacitor connected in series to a perfect one. Using the first principle charge transport theory, such dielectric relaxation, occurring under the non-Ohmic supply and space-charge-perturbed transport conditions, is shown to differ from that expected by the conventional equivalent-circuit treatment. The combined theoretical analysis and the experimental technique provides a means for independently determining the densities and mobilities of charged species in such systems. Using a liquid developer for electrography as a prototype system, results are presented that illustrate the power of the technique as a new tool to provide new insights into the generic transport and generation mechanisms of charged species in colloidal systems.

The xerographic discharge characteristics, i.e., surface potential vs exposure (light intensity × time), of photoreceptors have been studied theoretically. The effects of field dependence and sublinear flux dependence of the carrier supply and that of space charge perturbation are considered. These effects are found to extend the dynamic range of the photoreceptors.

The transport of charge carriers in a multijunction, consisting of alternating thin p- and n-type sublayers of amorphous semiconductor, has been analyzed theoretically. It is shown that the multijunction behaves in the dark like a pn-junction diode, while it has an advantage in reducing range limitation of photogenerated carriers. These properties are realized when the sublayer thickness is of the order of 100 nm for realistic values of doping levels and localized state density.

The effect of space-charge perturbation due to either trapped or free charge on the xerographic discharge of photoreceptors has been studied. In the case of trapped charge, the xerographic gain vs field curves can be analyzed to yield estimates for the carrier range and this is illustrated by measurements on amorphous selenium. The achievement of spacecharge-limited xerographic discharges in Se: poly(N-vinyl carbazole) layered structures will also be described. In this case it is shown that measurements of parameters which in the past have been viewed only from an applications viewpoint, e. g., contrast potential, can be useful in probing charge transport in appropriate solids.

The surface voltage V of undoped amorphous silicon films has been measured as a function of the applied surface charge Q as in electrophotographic photoreceptor applications. It is found that the Q-V characteristic is nearly linear at low charge, but that the voltage saturates and eventually decreases with continued charging. This phenomenon of negative differential capacitance is explained by considering a field-dependent carrier generation and subsequent carrier transport in an amorphous solid. It is shown that the observed results arise from the buildup of space charge when the electron and the hole μτ products are sufficiently different.