Organic Thin-Film Transistors: an Investigation of Device Properties, Applications and Market Perspectives

The possibility for printing circuits completely made of plastics and polymers is paving the way for exploiting both innovative and challenging applications that one could never have thought about only a few years ago. In addition, the low fabrication costs of the devices make them appealing for the market place. Thus, both scientific community and investors focus on the synthesis of new materials able to achieve better and better performances so that they can be marketable very soon. Unfortunately, organic materials are still too sensitive to ambient conditions such as oxygen, moisture and light that degrade and consequently change their pristine properties. Though many steps forward have been done, still a better control on the performances is needed for aiming to realistic applications of organic electronics. However, this marked environmental sensitivity can be exploited in those applications where it can represent a plus rather than a minus such as gas, humidity or temperature sensing or in low speed switching circuits. The first part of this thesis deals with organic thin-film transistors (OTFTs) and successively proposes their application as chemical sensors. It finally ends with an overview of the real market perspectives for organic-based sensors. In particular, •Chapter 1 mainly deals with organic materials properties and the related transport mechanisms. Then the OTFT equation and working principles as well as the threshold voltage and the parasitic resistances role are discussed. Finally, the main device fabrication techniques used throughout this work are presented. •Chapter 2 describes several techniques used to characterize both device and interfaces. •Chapter 3 is focused on the non-idealities in OTFTs such as bias stress, aging and finally short-channel effects. •Chapter 4 examines non-planar geometry OTFTs, more specifically the cylindrical as well as the double-gate case. •First in Chapter 5 an overview of the main organic dielectrics employed as the gate insulator in OTFTs fabrication will be presented; then the dielectrics we have been testing throughout this thesis work as well as the role of the interfaces will be discussed together with the trapping mechanisms occurring when dealing with organic materials, and finally how semiconductor morphology and chemical-physical nature of the interface can influence the OTFTs electrical performances. •In Chapter 6, a brief overview of what is in literature on organic-based chemical sensors will be given. Starting from a deep insight into the Ion-Sensitive FET (ISFET) and its organic version, namely the Ion-Sensitive Organic FET (ISOFET). We will then examine the Charge-Modulated FET working principles useful to understand the Charge-Modulated Organic FETs which have been part of this thesis work and whose experimental results will be eventually shown and discussed. •Finally, Chapter 7 is dedicated to the analysis of the real organic electronics market perspectives focusing then on OTFTs used as sensors. This study was possible thanks to a J. W. Fulbright I was awarded in 2007. In fact the American Embassy in Italy created a pilot program called BEST (Business Exchange for Student Training) that is intended to foster entrepreneurship skills in young researchers having a smart scientific-based entrepreneurial idea to bring to the market place.