Probing the Out-of-Plane Charge Transport in Organic Semiconductors Using Atomic Force Microscopy

Recent years have seen a remarkable progress in the field of organic electronics with the in-plane charge carrier mobility routinely surpassing 10 cm²/Vs in the best-performing materials. However, the miniaturization of organic semiconductor devices to achieve sub-micrometer active channel lengths and high switching frequencies is still limited by the contact resistance between metal electrodes and the organic semiconductor material. Efficiently controlling the contact and access resistance is, therefore, of crucial importance in pursuit of developing efficient electronic devices based on organic materials.<br/>Conventional approaches for achieving low contact resistance usually focus on interface engineering to achieve better alignment of energy levels. However, another significant contributor to high contact resistance is the out-of-plane mobility. Despite its importance, the out-of-plane mobility remains an often-overlooked parameter mainly due to the lack of available measurement techniques.<br/>Herein, we report a novel method for evaluating the out-of-plane mobility and contact resistance based on conductive-probe atomic force microscopy (C-AFM). In this study, we prepared multi-layered single crystal films of the molecular semiconductor C8-DNTT-C8 by a solution shearing technique on electrically conducting substrates and investigated them using C-AFM. By using scanning probe microscopy to map the local changes in the out-of-plane electrical current we gain a close-up view of the out-of-plane conductivity and access resistance at an unprecedented molecular length scale. This study opens a new pathway for gaining a better understanding of structure-property relationships and enabling a more accurate engineering of materials and interfaces in organic semiconductor devices.