Next-generation organic blend semiconductors for high performance solution-processable field Effect Transistors

Abstract

Ambitions for transparent, lightweight, flexible and inexpensive electronic technologies that can be printed over large area substrates have driven substantial advances in the field of organic/printed electronics in recent years. Amongst the various technologies investigated, solution-processed, organic thin-film transistors (OTFTs) have received extraordinary attention, primarily due to the enormous potential for simple, cost-effective manufacturing. Two exciting research areas relevant to OTFT development that offer tremendous potential are those of the small molecule/polymer organic semiconducting blends and the science and engineering of molecular doping. However, the lack of organic semiconducting blends that surpass the benchmark charge carrier mobility of 10 cm2/Vs, and the numerous challenges associated with the practical utilisation of molecular doping, have prevented adaptation of OTFTs as a viable technology for application in the emerging sector of plastic electronics. The work in this thesis focuses on an organic semiconducting system for OTFTs that addresses these two points. The first part of this thesis describes the development of advanced organic semiconducting blends, the so-called 3rd generation (3G) blend systems. Specifically, a new blend based on the small-molecule C8-BTBT and the conjugated polymer C16DT-BT is introduced. A third component, the molecular p-dopant, C60F48, is then added to the blend system and it is found to have remarkably positive effects on OTFT performance. The ternary blend system is then combined with a solvent-mixing approach, resulting in devices with an exceptional hole mobility value exceeding 13 cm2/Vs. Through the use of complementary characterisation techniques, it is shown that key to this achievement is the unusual three-component material distribution, hinting at the existence of an unconventional doping mechanism. Furthermore, by considering alternative processing techniques, the maximum mobility of the resulting OTFTs is improved further to a value in excess of 23 cm2/Vs. The second part of the thesis focuses on the impact of p-doping in the ternary C8 BTBT:C16IDT BT:C60F48 blend on other important operating characteristics of the OTFTs. The intentional and simple to implement doping process is shown to improve key device parameters such as bias-stress stability, parasitic contact resistance, threshold voltage and the overall device-to-device parameter variation (i.e. narrowing of the parameter spread). Importantly, the inclusion of the dopant is not found to adversely affect the nature of the C8 BTBT crystal packing at the OTFT channel. The final part of this thesis describes the incorporation of 3G blend-based OTFTs into fully functional logic electronic circuits. Hybrid inverter circuits (i.e. NOT gates) are fabricated at low temperatures from solution-phase by combining the high hole mobility C8-BTBT:C16IDT-BT:C60F48 blend OTFTs as the p-channel device and a novel In2O3/ZnO heterojunction metal oxide semiconducting system as the n-channel transistor. The resulting complementary inverters exhibit excellent signal gain and high noise margins, making this hybrid circuitry a promising contender for application in the emerging field of printed microelectronics.