Unambiguous Measurement of Local Hole Current in Organic Semiconductors Using Conductive Atomic Force Microscopy

Conductive atomic force microscopy (C-AFM) is a widely used tool for studying the charge transport properties of organic semiconductor films with nanoscale resolution. Local hole current is commonly measured by electrically contacting the film with a high work function C-AFM probe on top and an underlying electrode coated with a hole transport layer. The two voltage polarities, corresponding to the probe injection and substrate injection of holes, are both found in the C-AFM literature; nevertheless, there has been a lack of consideration about the possible influence of voltage polarity on image contrast and charge transport mechanisms. By analyzing local hole current maps and current-voltage curves for three organic semiconductors (a small molecule and two polymers), we find that probe and substrate injection leads to drastically different hole current maps and charge transport mechanisms. Specifically, the substrate injection of holes exhibits ohmic characteristics at low voltages and space-charge-limited current behavior at elevated voltages. Conversely, the probe injection of holes leads to injection-limited current that is sensitive to the state of the probe- sample interface. These measurements provide a blueprint for ensuring that C-AFM measurements are unambiguously probing the bulk properties of organic semiconductor films.