Determination of injection barriers in organic semiconductor devices from capacitance measurements

The low-frequency differential capacitance of single-carrier (metal/organic semiconductor/metal) devices with a sandwich structure is shown to display a distinct peak if the injection barrier of at least one of the electrodes is sufficiently small. The effect is shown to be caused by the diffusion contribution to the current density. Depending on the height of the injection barriers, the peak voltage can be a few tenths of a volt below the built-in voltage, V-bi. We show how the peak voltage and the peak height can be used to accurately determine the injection barriers and V-bi, and we demonstrate the method by applying it to polyfluorene-based devices.