Recent Progress in Emerging Organic Semiconductors.

Abstract

We live in a materials’ world, and their development is decisive for new technologies. As one type of material, organic semiconductors (OSCs) are receiving wide research interest due to their attractive properties, such as low-cost preparation, light weight, mechanical flexibility, easy processing, tuning of functions by molecular design, and rich availability compared to inorganic materials. In fact, OSCs are becoming key elements in the preparation of flexible, printable, and scalable electronics such as light-emitting diodes, solar cells, transistors, thermoelectrics, sensors, bioelectronics, and biomimetics, because diverse synthetic/processing technologies can provide versatile platforms to approach various functional materials. Also, the surge of new organic semiconductors will provide unknown physical properties and novel applications. Thus, it is timely to present a special issue collecting the most recent progress in emerging OSCs and helping to shape the future development of this field. We would like to dedicate this special issue to Prof. Daoben Zhu, one of the pioneers of OSCs in China, on the occasion of his upcoming 80th birthday (August 20, 2022). Currently, OSCs have become one of the core research topics in the multidisciplinary areas of chemistry, physics, materials science, medicine, and biology from fundamental studies to potential applications. In fact, these materials can represent one of the most dynamic fields, and have emerged as new “hot spots” of organic materials science. As presented in this special issue of Advanced Materials (8 Reviews, 2 Perspectives, and 15 Research Articles), recent progress in emerging OSCs has led to the study of many interesting properties, where some have demonstrated potential practical applications and some may address key challenges at the heart of physical sciences. This special issue focuses on current developments, milestones achieved, and future research topics regarding OSCs, where the leading scientists have contributed to this issue, covering the following seven key aspects of emerging OSCs. External-stimulus-responsive covalent organic frameworks. The application of smart materials in our life has dramatically changed our lifestyles. Thus, developing novel smart materials (or external-stimulus-responsive materials) is very important and highly desirable. Prof Qichun Zhang and co-workers (article number 2101175) highlight the recent progress in external-stimulus-responsive 2D covalent organic frameworks, including their response mechanisms, design strategies, and applications, offering some guidelines for advanced photoelectronics. De novo synthesis, structure modulation, self-assembly, and morphology control. Precise modulation of electrical/optical proprieties of OSCs through synthesis or self-assembly is an efficient strategy. Prof. Fraser Stoddart and co-workers (article number 2101487) present a temperature-inspired supramolecular assembly strategy by utilizing heat to heal defects and disorder, which can precisely control the assembled structures to afford novel supramolecular electronics. Prof. Yunqi Liu and co-workers (article number 2104325) review acceptor modulation strategies for improving electron transport in semiconducting polymers for high-performance organic field-effect transistors. Prof. Yuliang Li and co-workers (article number 2102811) summarize the up-to-date design strategies for realizing multiscale OSCs, for which chemical structure design and self-assembly process control can be synergistically adopted. Prof. Frank Würthner and co-workers (article number 2104678) focus on the underlying relationship of optical properties based on the molecular-packing arrangement and the resultant electronic coupling, and provide fundamental insights into the rational design of thin-film optoelectronic materials for organic solar cells and organic photodiodes. Prof. Wenping Hu and co-workers (article number 2104166) demonstrate a two-step strategy to fabricate high-resolution layer-controlled 2D OSC crystal arrays, where the organic field-effect transistors based on C6-DPA 2D OSC crystal arrays exhibit high performance with a uniform mobility. Prof. Eiichi Nakamura and co-workers (article number 2106465) perform de novo synthesis of a 3 nm-thick nanofilm by inserting a hydrogen-bonded network between two layers of fullerene molecules, forming a free-standing flexible 2D intercalated nanofilm over tens of cm2, which can be transferred to flat or perforated substrates. Prof. Zuoquan Jiang and co-workers (article number 2104125) review representative molecular scaffolds and newly developed π-stacked systems based on through-space charge transfer, which provides new insights into the theory, materials, and devices for OSCs. New physics. Understanding the optical/electrical physical processes of OSCs in various devices is extremely important in the construction of devices as well as the design and selection of materials. Prof. Frank Ortmann, Prof. Natalie Banerji and co-workers (article number 2101784) adopt femtosecond transient absorption and electro-absorption spectroscopy to quantify charge transfer and recombination dynamics as well as transport at early timescales, and conclude that a long excited-state lifetime, high local charge mobility, and fine-tuned interfacial/bulk disorder are key parameters for avoiding recombination losses in low-offset fullerene- or nonfullerene acceptor (NFA)-based organic photovoltaic systems. Prof. Jenny Nelson and co-workers (article number 2104654) report that charge generation in copper thiocyanate (CuSCN)-based hybrid inorganic–organic heterojunctions can proceed via emissive charge-transfer (CT) states similar to those observed at all-organic heterojunctions, where the dissociation of the CT-exciton and subsequent charge separation facilitates the fabrication of planar photovoltaic devices nearly without nonradiative voltage losses. Prof. Thomas Anthopoulos and co-workers (article number 2108524) overcome the inherent limitations in conventional organic Schottky diodes including the low carrier mobility of organic semiconductors and the high parasitic resistance and capacitance, and address the importance of the planar nanogap architecture in emerging radio frequency (RF) electronics, paving the way to further research toward cost-effective large-area RF electronics. Energy-related devices. OSCs have been widely witnessed to find applications in energy-related devices. Prof. Jun Chen and co-workers (article number 2104150) shed light on the redox processes and electrochemical performance of organic carbonyl electrode materials, providing a deep understanding of the detailed redox processes and their correlations with performance of rechargeable batteries. Prof. Yongfang Li and co-workers (article number 2104161) highlight the molecular design strategies and structure–properties relationship of quinoxaline (Qx)-based D–A copolymers, which can serve as a polymer donor and hole-transport material in polymer/perovskite solar cells. Prof. Peter Bäuerle and co-workers (article number 2103573) report solution-processed single-material (oligothiophene–fullerene dyad) organic solar cells with a power conversion efficiency of 5.34% in an inverted cell architecture. Although vacuum thermal evaporation (VTE) has proven to be an efficient route to acquire organic solar cells, the systematic investigation of the properties of donor:acceptor blends relative to their photovoltaic performance is rare. Prof. Moritz Riede and co-workers (article number 2107584) employ VTE to fabricate organic thin films and solar cells, and use ellipsometry and external quantum efficiency measurements with high dynamic range to quantify absorption, voltage losses, and charge-carrier mobility. Their research provides a summary of what current VTE organic solar cells can achieve in the stipulated categories, and they outline what improvements will be needed for VTE organic solar cells to rival their solution-processed counterparts. Prof. Guillermo Carlos Bazan and co-workers (article number 2104206) lay a foundation for assessing the figures of merit for pseudocapacitors based on conjugated polyelectrolytes for pseudocapacitive energy storage and the techniques adopted for their evaluation. Doping. Borrowing the idea from the silicon industry that the inherent properties can be enhanced through doping, the optical and electrical properties of OSCs can also be tuned through “doping”. Molecular doping—the use of redox-active small molecules as dopants for organic semiconductors—has seen a surge in research interest driven by emerging applications in sensing, bioelectronics, and thermoelectrics. However, molecular doping carries several intrinsic problems stemming directly from the redox-active character of these materials. In this special issue, Prof. Dichtel and co-workers (article number 2101932) demonstrate that the use of cobaltocene (CoCp2) can control the n-doping of naphthalene diimide (NDI) units, which improves the bulk conductivity of two NDI-based 2D polymers. Prof. Henning Sirringhaus and co-workers (article number 2102988) investigate the equilibrium and kinetics of ion-exchange doping in a model system, namely, PBTTT doped with FeCl3 and BMP TFSI, which achieves conductivities over 1000 S cm−1 and ion-exchange efficiencies beyond 99%. Stretchable electronics. Flexibility and stretchability are important targets in modern electronics. Prof. Zhenan Bao and co-workers (article number 2104747) demonstrate a strategy to modulate polymer semiconductor packing-structure through a molecular additive of dioctyl phthalate, which can act as a molecular spacer to insert between the amorphous chain networks and disrupt the crystalline packing, affording an enhanced mechanical stretchability without affecting charge transport. There still remain challenges in fabricating intrinsically stretchable electroluminescent devices with high and robust performance, including stretchable conductors, emissive materials, and compatible processes. Prof. Qibing Pei and co-workers (article number 2106184) review the recent progress in this field and offer a basis of the materials satisfying both the mechanical and electronic requirements and significant techniques to obtain the stretchable EL devices. In addition to stretchable electronics, creating curved-surface electronics from non-stretchable devices is highly desirable and practicable. Prof. Takao Someya and co-workers (article number 2106683) report the wrinkling of parylene-based devices and the effects of shrinkage on electrical components, acquiring shrinkable touch sensors and organic photovoltaics that are laminated to various non-developable surfaces without affecting their performance. Photo-related applications. It is highly desirable for OSCs to display diverse photo-related behaviors, including light-emission, photodetection, photocatalysis, and so on. Prof. Fraser Stoddart and co-workers (article number 2105405) present a review on color-tunable materials and focus on host–guest complexes, supramolecular assemblies, and crystalline materials, including the noncovalent synthesis of room-temperature phosphorescent materials and the adjustment of their luminescent behaviors. Prof. Lian Duan and co-workers (article number 2103102) demonstrate a double-emissive-layer device of white organic light-emitting diodes, which displays a high external quantum efficiency over 30% and an extended device lifetime. In addition, Prof. Huanli Dong and co-workers (article number 2105665) creatively construct a high-performance polarized photodetector based on the intrinsic linear dichroism of 2,6-diphenyl anthracene single crystals, which demonstrate a high linear dichroic ratio up to ≈1.9, and suggest wide-range applications in target detection, remote sensing, security surveillance, as well as machine vision. Since OSCs have also been employed as effective photocatalysts, Prof. Iain McCulloch and co-workers (article number 2105007) describe a promising strategy in the engineering of OSC nanomaterial-based photocatalysts through modification with (oligo)ethylene glycol sidechains, which markedly improves the photocatalytic H2 evolution efficiency. To close, we greatly appreciate the kind support from the editorial team of Advanced Materials, in particular Dr. Aron Urbatsch, Dr. Duoduo Liang, and Dr. Jos Lenders. We are also very grateful to the colleagues and friends, who share their research and great insights to this exciting special issue of emerging organic semiconducting materials: Preparation, Physics, and Applications. Finally, we would like to use this chance to say “Happy Birthday” to Prof. Daoben Zhu. The authors declare no conflict of interest. Qichun Zhang received his B.S. at Nanjing University in China in 1992, his M.S. in physical organic chemistry at Institute of Chemistry, Chinese Academy of Sciences in 1998, and his M.S. in organic chemistry at University of California, Los Angeles (USA, 2003), and completed his Ph.D. in chemistry at University of California Riverside in 2007. Then, he joined Prof. Kanatzidis's group at Northwestern University as a Postdoctoral Fellow (October 2007 to December 2008). In January 2009, he joined the School of Materials Science and Engineering at Nanyang Technological University (NTU, Singapore) as an Assistant Professor. On March 1, 2014, he was promoted to Associate Professor with tenure. On September 1, 2020, he moved to the Department of Materials Science and Engineering at City University of Hong Kong as a full professor. From 2018 to 2021, he has been recognized as one of highly cited researchers (top 1%) in cross-field in Clarivate Analytics. He is a fellow of the Royal Society of Chemistry. Currently, his research focuses on carbon-rich conjugated materials and their applications. Up to now, he has published more than 435 papers and 5 patents (h-index: 92). Wenping Hu is a Professor of Tianjin University. He received his Ph.D. from Institute of Chemistry, Chinese Academy of Sciences (ICCAS) in 1999 (Supervisor: Prof. Daoben Zhu and Prof. Yunqi Liu). Then he joined Osaka University and Stuttgart University as a research fellow of the Japan Society for the Promotion of Sciences and Alexander von Humboldt, respectively. In 2003 he worked at Nippon Telephone and Telegraph (NTT), and then rejoined ICCAS and was promoted to full professor. He was appointed as assistant president of Tianjin University in 2013 and was promoted to vice president and executive vice president in 2016 and 2021. He served as a Visiting Scholar at the Department of Chemistry, Stanford University in 2007 and a Visiting Professor at the Department of Chemistry, National University of Singapore in 2013. He focuses on organic optoelectronics and has published more than 650 peer reviewed papers (IF > 10, over 260 papers) with 36 000 citations (h-index = 93). He served as an Associate Editor of Polymer Chemistry from 2013 to 2016, and is now Co-Editor-in-Chief of SmartMat (since 2020) and an editorial board member of Advanced Energy Materials, Advanced Electron Materials, Nano Research, Science China Materials, Chemistry—An Asian Journal, and Applied Physics Letters. Henning Sirringhaus, FRS, is the Hitachi Professor of Electron Device Physics at the Cavendish Laboratory of the University of Cambridge and currently holds a Royal Society Research Professorship. He works on the charge transport, and photo- and device physics of polymer and molecular semiconductors and other functional materials. He received his Ph.D. from the Institute of Solid-State Physics at ETH Zürich, Switzerland, in 1995 and worked as a post-doctoral research scientist at Princeton University from 1995–1997. He was a visiting professor at the Institute of Chemistry, Chinese Academy of Sciences, in 2011. He is co-founder of the spin-off company, Plastic Logic/FlexEnable, commercializing organic transistor technology. He has published more than 400 peer-reviewed scientific papers with an h-index of 105. Klaus Müllen was director at the Max Planck Institute for Polymer Research and is continuing research at the universities of Heidelberg and Cologne. His broad research interests range from new polymer-forming reactions, to the chemistry and physics of single molecules as well as graphene, dendrimers, and biosynthetic hybrids. He has published about 2000 papers. He has received many awards, honorary doctorates and honorary professorships and he is member of national and international academies. From 2008 to 2009 he served as president of the German Chemical Society (GDCh). In 2013–2014 he was president of the German Association for the Advancement of Science and Medicine. In 2010 he won an ERC Advanced Grant for his work on nanographene. He was associate editor of the Journal of the American Chemical Society.