Probing Out-Of-Plane Charge Transport in Organic Semiconductors Using Conductive Atomic Force Microscopy.

High contact resistance remains the primary obstacle that hinders further advancements of organic semiconductors (OSCs) in electronic circuits. While significant effort has been directed toward lowering the energy barrier at OSC/metal contact interfaces, approaches toward reducing another major contributor to overall contact resistance - the bulk resistance - have been limited to minimizing the thickness of OSC films. However, the out-of-plane conductivity of OSCs, a critical aspect of bulk resistance, has largely remained unaddressed. In this study, multi-layered 2D crystalline, solution-processed films of the high-mobility molecular semiconductor 2,9-dioctylnaphtho[2,3-b] naphtha[2',3':4,5]thieno[2,3-d]thiophene (C8-DNTT-C8) are investigated using conductive-probe atomic force microscopy (C-AFM) to evaluate out-of-plane charge transport. The findings reveal a linear increase in out-of-plane resistance with the number of molecular layers in the film, which is modeled using an equivalent circuit model with multiple tunneling barriers connected in series. Building upon these results, a vertical transfer length method (V-TLM) is developed, allowing one to determine the out-of-plane resistivity of OSC and providing insights into charge transport properties at a single molecule length scale. The V-TLM approach highlights the potential of C-AFM for investigating out-of-plane charge transport in OSC thin films and holds promise for accelerating the screening of molecules for high-performance electronic devices.