Emerging Thin‐Film Transistor Technologies and Applications

Thin-film transistors (TFTs) have radically transformed the way we think and interact with electronics and they are now a mainstay of almost every aspect our lives from personal use to big industries, healthcare, security and beyond. It is hard to overstate the importance of TFT technologies in modern day electronics and their broader societal impact. TFTs have single handedly created a new electronics industry that deviates significantly from the traditional form of small chip-size, monolithic silicon electronics. This rapidly expanding technology strand, often referred to as large-area electronics, has traditionally relied on established semiconductor technologies, but the past decade has witnessed the emergence of a library of new materials for an increasing range of applications in different technology sectors. The mass deployment of TFT-based electronics in our daily lives kick-started with the commercial proliferation of flat panel liquid-crystal displays (LCDs), by enabling the realization of large-area driving backplanes. We are now witnessing similar devices being applied to the latest display technologies, such as organic light-emitting diodes (OLEDs), for applications in TVs, computers, and smart phones, to name but a few. These ubiquitous displays, in tandem with touch sensing capabilities, have given rise to a unique human-machine interface that most likely will keep dominating our lives for years to come and re-defining the way we work, shop, and interact with each other. As large-area electronics continue to advance, the research on TFTs becomes more elaborate and divergent, extending to new materials, device concepts, manufacturing processes, and integrated systems that offer improved performance and more functionalities, ultimately leading to new markets. This Special Issue entitled “Emerging Thin-Film Transistor Technologies and Applications” attempts to consolidate key recent developments in the broader area of thin-film transistors with a focus on the fundamental properties of emerging materials, from organic semiconductors to metal oxides, from quantum dots to hybrid perovskites, and their integration in innovative new devices that interact with light or biological systems. The issue collates reviews and progress reports that discuss the past, present and future of TFT technologies along with several original contributions. The charge transport mechanism in the semiconductor channel plays a crucial role in transistor performance, ultimately determining the success of the technology. Park and co-workers (article number 1904545) review important developments in high mobility organic polymer semiconductors, while Podzorov et al., report (article number 1903617) original research on the influence of grain boundaries on the elusive Hall Effect observed in a handful of polycrystalline organic semiconductors. The role of contact resistance and other nonidealities together with possible mitigation strategies, are reviewed by Jurchescu et al., (article number 1904576) and the device physics of ideal and non-ideal TFTs is described by Liu et al., (article number 1903889). Xu et al., (article number 1904508) discuss the methods for precise extraction of the charge carrier mobility in organic transistors, while Cho et al., (article number 1904590) focus on summarizing recent progress in understanding the bias stability of organic TFTs. Several review articles focus on the design and performance aspects of various TFT technologies. Klauk et al., (article number 1903812) discuss the importance of material characteristics and device design for gigahertz (GHz) organic transistors, while Caironi et al., (article number 1907641) present the recent progress towards solution-processed GHz organic TFTs. Leo et al., review the promising branch of vertical organic transistors (article number 1907113), while Wakayama et al., (article number 1903724) discuss the operating principles and recent progress of organic based antiambipolar transistors and their applications. Importantly, a substantial part of the issue is dedicated to multifunctional TFTs. Device concepts covered include, hybrid perovskite phototransistors (Yan et al., article number 1903907), quantum dot light-emitting transistors (Loi et al., article number 1904174), organic light-emitting transistors (Namdas et al., article number 1905282), thin-film light-emitting transistors (Zaumseil et al., article number 1905269), optically switchable organic polymer transistors (Samori et al., article number 1907507), and biosensor transistors (Torsi et al,. article number 1904513). Fabrication and integration of TFTs on conventional rigid substrates is well established, but transferring the technology to mechanically flexible, bendable, or even stretchable substrates, presents additional challenges. Gelinck et al., review recent advances in image sensors (article number 1904205), and Ko et al., discuss recent attempts to mimic human skin with flexible electronics (article number 1904523). Ng et al., report on the use of flexible pressure sensors for physiological signal monitoring (article number 1905241), while Kim et al., describe the use of ionic tactile sensors for human-interactive technologies (manuscript number 1904532). Finally, different approaches for tackling the economic and technological challenges associated with emerging TFTs technologies, are also covered. Anthopoulos et al., review the use of photonic processing as a viable route for scalable, cost-efficient manufacturing of metal oxide electronics (article number 1906022), while Frisbie et al., report original work on low-voltage ZnO transistors and circuits (article number 1902028). Lastly, Noh et al., (article number 1904588) and Kim et al., (article number 1904632) review the advances on printable semiconductors for TFT backplanes for OLED displays, and low-temperature solution processed metal oxides TFTs for flexible electronics, respectively. By highlighting many of these exciting recent developments in the field of TFTs, as well as the challenges currently faced, we hope to provide researchers with a broader picture of the current state-of-the-art while at the same time inspire further interactions between scientists from relevant, and not only, disciplines. We are grateful to Dr. Jos Lenders, deputy editor of Advanced Functional Materials, for proposing the idea of a special issue on this topic as well as all authors and editorial staff for their sincere contributions and help, respectively. Thomas D. Anthopoulos is a Professor of Material Science and Engineering at King Abdullah University of Science and Technology (KAUST) in Saudi Arabia. He received his B.Eng. and D.Phil. degrees from Staffordshire University in the UK followed by postdoctoral appointments at the University of St. Andrews (UK) and Philips Research Laboratories (The Netherlands), before joining Imperial College London (UK) from 2006 to 2017. His research interests are diverse and cover the development and application of innovative processing paradigms and the physics, chemistry, and application of functional materials. Yong-Young Noh is a Professor of Chemical Engineering at Pohang University of Science and Technology (POSTECH) in Republic of Korea. He received his Ph.D. degrees from GIST in Republic of Korea and worked as a postdoctoral associate at the Cavendish Laboratory at the University of Cambridge (UK). Since then he was a senior researcher at ETRI, an assistant professor at Hanbat National University and an associate professor at Dongguk University. His research interests are the development and application of semiconducting materials and it manufacturing processes for various functional devices. Oana D. Jurchescu is an Associate Professor of Physics at Wake Forest University (WFU) in USA. She received her BS and MS degrees from The West University of Timișoara (Romania) a Ph.D. from University of Groningen (The Netherlands), and was a postdoctoral researcher at the National Institute of Standards and Technology (NIST, USA) from 2007 to 2009. Her expertise is in charge transport in organic semiconductors, device physics, and structure-property relationships in organic and hybrid functional materials.