Impact of Materials versus Geometric Parameters on the Contact Resistance in Organic Thin-Film Transistors

The contact resistance is known to severely hamper the performance of organic thin-film transistors, especially when dealing with large injection barriers, high mobility organic semiconductors, or short channel lengths. Here, the relative significance of how it is affected by materials-parameters (mobility and interfacial level-offsets) and geometric factors (bottom-contact vs top-contact geometries) is assessed. This is done using drift-diffusion-based simulations on idealized device structures aiming at a characterization of the intrinsic situation in the absence of traps, differences in the film morphology, or metal-atoms diffusing into the organic semiconductor. It is found that, in contrast to common wisdom, in such a situation the top-contact devices do not always outperform the bottom-contact ones. In fact, the observed ratio between the contact resistances of the two device structures changes by up to two orders of magnitude depending on the assumed materials parameters. The contact resistance is also shown to be strongly dependent on the hole mobility in the organic semiconductor and influenced by the chosen point of operation of the device.