Contact Resistance at Interfaces Buried Within Organic Transistors Imaged Using Scanning Microwave Impedance Microscopy

Contact resistance has been a consistent challenge in the 40-year history of the organic thin-film transistor (OTFT). The problem arises because contact resistance at the metal electrode/organic interface is an amalgamation of physical phenomena: From ubiquitous defect states and Fermi level pinning, to the energy difference between the electrode work function and semiconductor transport level. Here, an original approach is taken to address the established OTFT contact resistance challenge. First, a new characterization technique, scanning microwave impedance microscopy (sMIM), is introduced for imaging buried metal electrode/organic interfaces. sMIM indicates contact resistance is arguably intrinsic in certain metal electrode/organic systems; namely, contact resistance exists when charge injection is theoretically Ohmic. The latter is explained by Fermi level pinning far from the organic semiconductor ionization energy, which is confirmed using ultraviolet photoelectron spectroscopy (UPS). Second, organic source-gated transistors (OSGTs) are developed by exploiting contact resistance as a design principle. OSGTs made from small-molecule/polymer blends, using 2,8-difluoro-5,11-triethylsilylethynyl anthradithiophene (diF-TES ADT) and poly(triarylamine) (PTAA), show distinct, low-voltage saturation, while temperature-dependent measurements indicate localized carriers govern charge transport. Overall, combining a new measurement technique with novel transistor operating mechanisms leads to a new approach for addressing the complex, and established, contact resistance challenge.