Features of Capacitance and Mobility of Injected Carriers in Organic Layers Measured by Impedance Spectroscopy

Transport of charge carriers in organic layers plays a relevant role in the performance of electronic devices such as light-emitting diodes, solar cells, and photodetectors. The presence of energetically distributed traps severely affects the measured transport coefficient and the charge-storage features in the film. We summarize recent theoretical work on impedance spectroscopy models for space–charge limited current of a single carrier in a multiple trapping model, which describes the experimental behavior usually observed in organic layers with injected charge carriers well. Two main physical effects are obtained from the numerical and analytical treatment. First, carriers in slow traps that do not follow alternating current modulation provide an increase in the capacitance at low frequency, and second, those in fast traps remain in equilibrium with the transport state increasing the transit time. Analysis also provides a unified interpretation of models with field- or carrier-density dependence on mobility.