Solution processed organic semiconductor thin-film transistors for flexible electronics [electronic resource] : device physics, device modeling, fabrication technology, and interface engineering
Keywords: Electrical Engineering

Organic or carbon electronics has been a fast-growing field in recent years covering a broad range from nanoelectronic devices to macroelectronic systems. Besides the single-graphene or single-carbon nanotube transistor toward extending the scaling limit of traditional silicon metal-oxide-semiconductor field-effect transistor (MOSFET), organic semiconductor based thin-film transistors have been actively investigated due to their promise in large-area electronics fabricated on flexible substrates using low-cost unconventional means, such as low/room-temperature printing and roll-to-roll processing. This dissertation focuses on the study of device physics, device modeling, fabrication technology, and interface engineering for solution-processed organic field-effect transistors (SPOFET) for flexible electronics applications. There are primarily four parts of contributions originated from this dissertation work. The first part introduces the design and demonstration of high-performance, low-voltage flexible SPOFETs fabricated on plastic substrates with a carrier mobility over 0.2 cm2/Vs, a turn-on voltage of near 0 V, and a record low subthreshold slope of ~80 mV/dec in ambient conditions. These exceptional characteristics are achieved by novel device architecture design, 3-D statistical modeling for solution-shearing process optimization, and phenyl-terminated self-assembled monolayer (SAM) based interface engineering. In the second part, SAM relevant physical effects and chemistry effects at the organic semiconductor-dielectric interface are systematically investigated. Through careful selection of a group of phenyl-terminated SAMs, we elucidate how the performance and reliability of organic transistors are controlled by the critical semiconductor-dielectric interfacial SAMs. In addition, we briefly introduce a spin-coating process for depositing high-quality phenyl-terminated SAMs for organic electronics applications. The third part focuses on the device physics and device modeling of organic transistors. In this dissertation work, we have proposed and developed a universal physical model for organic transistors by incorporating both the charge injection effects and charge transport properties, and successfully applied it to resolve many elusive physical phenomena observed so far, such as the peculiar mobility scaling behavior with respect to the channel length, the contact resistance effect, and the mysterious surface potential profiles of organic transistors which have been experimentally probed yet poorly understood. Of particular importance is that we discover an overshoot region in the mobility scaling behavior and identified the existence of a critical channel length for the peak field-effect mobility. In the last part, we investigate novel contact engineering for organic transistors toward lowering charge injection barrier and reducing the interfacial disorder width or localization states. We have explored and demonstrated Fermi-level depinning at the metal-organic interface for low-resistance Ohmic contacts by inserting an ultrathin interfacial Si3N4 insulator in between. The contact behavior is successfully tuned from rectifying to quasi-Ohmic and to tunneling by varying the Si3N4 thickness within 0-6 nm. Detailed physical mechanisms of Fermi-level pinning/depinning responsible for the metal-organic semiconductor contact behavior are clarified based on a proposed lumped-dipole model.