Effective charge collection area during conductive and photoconductive atomic force microscopy

Abstract

Conductive atomic force microscopy (C-AFM) has been widely used to map the nanoscale electrical properties of conducting polymers, nanomaterials, and organic electronic devices. While these measurements provide valuable insight into the spatial dependence of electrical performance, reported current densities and electrical conductivities measured by C-AFM are consistently much higher than those measured at the macroscopic scale. Here, we demonstrate that these anomalously high current densities and conductivities arise from ignoring current spreading and hence underestimating the current-carrying area. We present a simple experimental means of estimating the effective charge collection area during C-AFM measurements. Using semiconducting polymer poly(3-hexylthiophene) films as a test case, we find that the effective charge collection area can be as much as three orders of magnitude larger than the mechanical contact area between the probe and the film. Calibrated conductivity maps are obtained, with a quantitative correspondence with accepted values, and C-AFM photocurrent measurements of a nanostructured hybrid organic-inorganic solar cell active layer yield short-circuit current densities that match those reported for macroscopic devices. Finally, we address the observation that current spreading increases the effective charge collection area beyond the size of the probe-sample contact but does not preclude an imaging resolution below 10 nm.