25th Anniversary Article: Isoindigo‐Based Polymers and Small Molecules for Bulk Heterojunction Solar Cells and Field Effect Transistors

Driven by the potential advantages and promising applications of organic solar cells, donor‐acceptor (D‐A) polymers have been intensively investigated in the past years. One of the strong electron‐withdrawing groups that were widely used as acceptors for the construction of D‐A polymers for applications in polymer solar cells and FETs is isoindigo. The isoindigo‐based polymer solar cells have reached efficiencies up to ～7% and hole mobilities as high as 3.62 cm² V^?1 s^?1 have been realized by FETs based on isoindigo polymers. Over one hundred isoindigo‐based small molecules and polymers have been developed in only three years. This review is an attempt to summarize the structures and properties of the isoindigo‐based polymers and small molecules that have been reported in the literature since their inception in 2010. Focus has been given only to the syntheses and device performances of those polymers and small molecules that were designed for use in solar cells and FETs. Attempt has been made to deduce structure‐property relationships that would guide the design of isoindigo‐based materials. It is expected that this review will present useful guidelines for the design of efficient isoindigo‐based materials for applications in solar cells and FETs.     

Organic Semiconductors based on Dyes and Color Pigments

Organic dyes and pigments constitute a large class of industrial products. The utilization of these compounds in the field of organic electronics is reviewed with particular emphasis on organic field‐effect transistors. It is shown that for most major classes of industrial dyes and pigments, i.e., phthalocyanines, perylene and naphthalene diimides, diketopyrrolopyrroles, indigos and isoindigos, squaraines, and merocyanines, charge‐carrier mobilities exceeding 1 cm² V^?1 s^?1 have been achieved. The most widely investigated molecules due to their n‐channel operation are perylene and naphthalene diimides, for which even values close to 10 cm² V^?1 s^?1 have been demonstrated. The fact that all of these π‐conjugated colorants contain polar substituents leading to strongly quadrupolar or even dipolar molecules suggests that indeed a much larger structural space shows promise for the design of organic semiconductor molecules than was considered in this field traditionally. In particular, because many of these dye and pigment chromophores demonstrate excellent thermal and (photo‐)chemical stability in their original applications in dyeing and printing, and are accessible by straightforward synthetic protocols, they bear a particularly high potential for commercial applications in the area of organic electronics.     

Charge Injection in Solution‐Processed Organic Field‐Effect Transistors: Physics,Models and Characterization Methods

A high‐mobility organic semiconductor employed as the active material in a field‐effect transistor does not guarantee per se that expectations of high performance are fulfilled. This is even truer if a downscaled, short channel is adopted. Only if contacts are able to provide the device with as much charge as it needs, with a negligible voltage drop across them, then high expectations can turn into high performances. It is a fact that this is not always the case in the field of organic electronics. In this review, we aim to offer a comprehensive overview on the subject of current injection in organic thin film transistors: physical principles concerning energy level (mis)alignment at interfaces, models describing charge injection, technologies for interface tuning, and techniques for characterizing devices. Finally, a survey of the most recent accomplishments in the field is given. Principles are described in general, but the technologies and survey emphasis is on solution processed transistors, because it is our opinion that scalable, roll‐to‐roll printing processing is one, if not the brightest, possible scenario for the future of organic electronics. With the exception of electrolyte‐gated organic transistors, where impressively low width normalized resistances were reported (in the range of 10 Ω·cm), to date the lowest values reported for devices where the semiconductor is solution‐processed and where the most common architectures are adopted, are ～10 kΩ·cm for transistors with a field effect mobility in the 0.1–1 cm²/Vs range. Although these values represent the best case, they still pose a severe limitation for downscaling the channel lengths below a few micrometers, necessary for increasing the device switching speed. Moreover, techniques to lower contact resistances have been often developed on a case‐by‐case basis, depending on the materials, architecture and processing techniques. The lack of a standard strategy has hampered the progress of the field for a long time. Only recently, as the understanding of the rather complex physical processes at the metal/semiconductor interfaces has improved, more general approaches, with a validity that extends to several materials, are being proposed and successfully tested in the literature. Only a combined scientific and technological effort, on the one side to fully understand contact phenomena and on the other to completely master the tailoring of interfaces, will enable the development of advanced organic electronics applications and their widespread adoption in low‐cost, large‐area printed circuits.     

Organic High Electron Mobility Transistors Realized by 2D Electron Gas

A key breakthrough in inorganic modern electronics is the energy‐band engineering that plays important role to improve device performance or develop novel functional devices. A typical application is high electron mobility transistors (HEMTs), which utilizes 2D electron gas (2DEG) as transport channel and exhibits very high electron mobility over traditional field‐effect transistors (FETs). Recently, organic electronics have made very rapid progress and the band transport model is demonstrated to be more suitable for explaining carrier behavior in high‐mobility crystalline organic materials. Therefore, there emerges a chance for applying energy‐band engineering in organic semiconductors to tailor their optoelectronic properties. Here, the idea of energy‐band engineering is introduced and a novel device configuration is constructed, i.e., using quantum well structures as active layers in organic FETs, to realize organic 2DEG. Under the control of gate voltage, electron carriers are accumulated and confined at quantized energy levels, and show efficient 2D transport. The electron mobility is up to 10 cm² V^?1 s^?1, and the operation mechanisms of organic HEMTs are also argued. Our results demonstrate the validity of tailoring optoelectronic properties of organic semiconductors by energy‐band engineering, offering a promising way for the step forward of organic electronics.     

Photochemical Molecular Tailoring for Efficient Diffusion and Reorganization of Organic Nanocrystals for Ultra‐Flexible Organic Semiconductor Arrays

Solution‐processed organic single crystals with high carrier mobility have been actively investigated for diverse applications such as displays, sensors, and next generation electronics on a flexible platform. However, the lack of precise alignment and growth control of organic single crystals impedes the widespread adoption of organic materials in an industrial perspective. Here, a photochemical modification approach is reported tailoring the solubility and molecular diffusivity of polymeric sacrificial layer and sequential batch‐type vapor annealing to implement high‐performance (average saturation mobility: 8.01 cm² V^?1 s^?1) organic single‐crystal thin film transistors with large channel width including multiple aligned single crystals. Additionally, the mechanical properties of the organic single crystals are systematically investigated with extreme strain conditions such as bending radius of 150 μm.     

Molecular Semiconductors for Logic Operations: Dead-End or Bright Future?

The field of organic electronics has been prolific in the last couple of years, leading to the design and synthesis of several molecular semiconductors presenting a mobility in excess of 10 cm² V⁻¹ s⁻¹. However, it is also started to recently falter, as a result of doubtful mobility extractions and reduced industrial interest. This critical review addresses the community of chemists and materials scientists to share with it a critical analysis of the best performing molecular semiconductors and of the inherent charge transport physics that takes place in them. The goal is to inspire chemists and materials scientists and to give them hope that the field of molecular semiconductors for logic operations is not engaged into a dead end. To the contrary, it offers plenty of research opportunities in materials chemistry.     

From Single Molecules to Thin Film Electronics,Nanofibers, e‐Textiles and Power Cables: Bridging Length Scales with Organic Semiconductors

Organic semiconductors are the centerpiece of several vibrant research fields from single‐molecule to organic electronics, and they are finding increasing use in bioelectronics and even classical polymer technology. The versatile chemistry and broad range of electronic functionalities of conjugated materials enable the bridging of length scales 15 orders of magnitude apart, ranging from a single nanometer (10^?9 m) to the size of continents (10⁶ m). This work provides a taste of the diverse applications that can be realized with organic semiconductors. The reader will embark on a journey from single molecular junctions to thin film organic electronics, supramolecular assemblies, biomaterials such as amyloid fibrils and nanofibrillated cellulose, conducting fibers and yarns for e‐textiles, and finally to power cables that shuffle power across thousands of kilometers.     

MoS2/Rubrene van der Waals Heterostructure: Toward Ambipolar Field‐Effect Transistors and Inverter Circuits

2D transition metal dichalcogenides are promising channel materials for the next‐generation electronic device. Here, vertically 2D heterostructures, so called van der Waals solids, are constructed using inorganic molybdenum sulfide (MoS₂) few layers and organic crystal – 5,6,11,12‐tetraphenylnaphthacene (rubrene). In this work, ambipolar field‐effect transistors are successfully achieved based on MoS₂ and rubrene crystals with the well balanced electron and hole mobilities of 1.27 and 0.36 cm² V^?1 s^?1, respectively. The ambipolar behavior is explained based on the band alignment of MoS₂ and rubrene. Furthermore, being a building block, the MoS₂/rubrene ambipolar transistors are used to fabricate CMOS (complementary metal oxide semiconductor) inverters that show good performance with a gain of 2.3 at a switching threshold voltage of ?26 V. This work paves a way to the novel organic/inorganic ultrathin heterostructure based flexible electronics and optoelectronic devices.     

Metal‐Layer Assisted Growth of Ultralong Quasi‐2D MOF Nanoarrays on Arbitrary Substrates for Accelerated Oxygen Evolution

Controlled growth of metal–organic frameworks (MOFs) nanocrystals on requisite surfaces is highly desired for myriad applications related to catalysis, energy, and electronics. Here, this challenge is addressed by overlaying arbitrary surfaces with a thermally evaporated metal layer to enable the well‐aligned growth of ultralong quasi‐2D MOF nanoarrays comprising cobalt ions and thiophenedicarboxylate acids. This interfacial engineering approach allows preferred chelation of carboxyl groups in the ligands with the metal interlayers, thereby making possible the fabrication and patterning of MOF nanoarrays on substrates of any materials or morphologies. The MOF nanoarrays grown on porous metal scaffolds demonstrate high electrocatalytic capability for water oxidation, exhibiting a small overpotential of 270 mV at 10 mA cm^?2, or 317 mV at 50 mA cm^?2 as well as negligible decay of performance within 30 h. The enhanced performance stems from the improved electron and ion transport in the hierarchical porous nanoarrays consisting of in situ formed oxyhydroxide nanosheets in the electrochemical processes. This approach for mediating the growth of MOF nanoarrays can serve as a promising platform for diverse applications.     

25th Anniversary Article: Organic Field‐Effect Transistors: The Path Beyond Amorphous Silicon

Over the past 25 years, organic field‐effect transistors (OFETs) have witnessed impressive improvements in materials performance by 3–4 orders of magnitude, and many of the key materials discoveries have been published in Advanced Materials. This includes some of the most recent demonstrations of organic field‐effect transistors with performance that clearly exceeds that of benchmark amorphous silicon‐based devices. In this article, state‐of‐the‐art in OFETs are reviewed in light of requirements for demanding future applications, in particular active‐matrix addressing for flexible organic light‐emitting diode (OLED) displays. An overview is provided over both small molecule and conjugated polymer materials for which field‐effect mobilities exceeding > 1 cm² V^–1 s^–1 have been reported. Current understanding is also reviewed of their charge transport physics that allows reaching such unexpectedly high mobilities in these weakly van der Waals bonded and structurally comparatively disordered materials with a view towards understanding the potential for further improvement in performance in the future.     

Organic Photorefractive Materials and Applications

This review describes recent advances and applications in the field of organic photorefractive materials, an interesting area in the field of organic electronics and promising candidate for various aspects of photonic applications. We describe the current state of knowledge about the processes involved in the formation of photorefractive gratings in organic materials and focus on the chemical and photo‐physical aspects of the material structures employed in low glass‐transition temperature amorphous composites and organic photorefractive glasses. State‐of‐the art materials are highlighted and recent demonstrations of photonic applications relying on the reversible holographic nature of the photorefractive materials are discussed.     

A General Approach for Lab‐to‐Manufacturing Translation on Flexible Organic Solar Cells

The blossoming of organic solar cells (OSCs) has triggered enormous commercial applications, due to their high‐efficiency, light weight, and flexibility. However, the lab‐to‐manufacturing translation of the praisable performance from lab‐scale devices to industrial‐scale modules is still the Achilles' heel of OSCs. In fact, it is urgent to explore the mechanism of morphological evolution in the bulk heterojunction (BHJ) with different coating/printing methods. Here, a general approach to upscale flexible organic photovoltaics to module scale without obvious efficiency loss is demonstrated. The shear impulse during the coating/printing process is first applied to control the morphology evolution of the BHJ layer for both fullerene and nonfullerene acceptor systems. A quantitative transformation factor of shear impulse between slot‐die printing and spin‐coating is detected. Compelling results of morphological evolution, molecular stacking, and coarse‐grained molecular simulation verify the validity of the impulse translation. Accordingly, the efficiency of flexible devices via slot‐die printing achieves 9.10% for PTB7‐Th:PC₇₁BM and 9.77% for PBDB‐T:ITIC based on 1.04 cm² . Furthermore, 15 cm² flexible modules with effective efficiency up to 7.58% (PTB7‐Th:PC₇₁BM) and 8.90% (PBDB‐T:ITIC) are demonstrated with satisfying mechanical flexibility and operating stability. More importantly, this work outlines the shear impulse translation for organic printing electronics.     

Toward Environmentally Robust Organic Electronics: Approaches and Applications

Recent interest in flexible electronics has led to a paradigm shift in consumer electronics, and the emergent development of stretchable and wearable electronics is opening a new spectrum of ubiquitous applications for electronics. Organic electronic materials, such as π‐conjugated small molecules and polymers, are highly suitable for use in low‐cost wearable electronic devices, and their charge‐carrier mobilities have now exceeded that of amorphous silicon. However, their commercialization is minimal, mainly because of weaknesses in terms of operational stability, long‐term stability under ambient conditions, and chemical stability related to fabrication processes. Recently, however, many attempts have been made to overcome such instabilities of organic electronic materials. Here, an overview is provided of the strategies developed for environmentally robust organic electronics to overcome the detrimental effects of various critical factors such as oxygen, water, chemicals, heat, and light. Additionally, molecular design approaches to π‐conjugated small molecules and polymers that are highly stable under ambient and harsh conditions are explored; such materials will circumvent the need for encapsulation and provide a greater degree of freedom using simple solution‐based device‐fabrication techniques. Applications that are made possible through these strategies are highlighted.     

Wearable Textile‐Based Co−Zn Alkaline Microbattery with High Energy Density and Excellent Reliability

Wearable in‐plane Zn‐based microbatteries are considered as promising micropower sources for wearable electronics due to their high capacity, low cost, high safety, and easy integration. However, their applications are severely impeded by inadequate energy density arising from unsatisfactory capacity of cathode and poor cycling stability caused by degradation of electrode materials and Zn dendrite. Additionally, the short‐circuit induced safety issue caused by Zn dendrite is still a roadblock for Zn‐based microbatteries. Herein, a textile‐based Co?Zn microbattery with ultrahigh energy density and excellent cycling stability is demonstrated. Benefiting from the fast electron transport of three‐dimensional (3D) porous Ni‐coated textile and synergistic effect from the hierarchical Co(OH)₂@NiCo layered double hydroxide (LDH) core?shell electrode, the fabricated Co?Zn microbattery with high flexibility delivers superior energy/power densities of 0.17 mWh cm^?2/14.4 mW cm^?2, outperforming most reported micro energy storage devices. Besides, the trench‐type configuration as well as the 3D porous Zn@carbon clothes can avoid the short‐circuit‐induced safety issues, resulting in excellent cycling stability (71% after 800 cycles). The unique core?shell structure and novel configuration provide a brand‐new design strategy for high‐performance wearable in‐plane microdevices.     

Large‐Area Organic Solar Cells: Material Requirements,Modular Designs,and Printing Methods

The printing of large‐area organic solar cells (OSCs) has become a frontier for organic electronics and is also regarded as a critical step in their industrial applications. With the rapid progress in the field of OSCs, the highest power conversion efficiency (PCE) for small‐area devices is approaching 15%, whereas the PCE for large‐area devices has also surpassed 10% in a single cell with an area of ≈1 cm². Here, the progress of this fast developing area is reviewed, mainly focusing on: 1) material requirements (materials that are able to form efficient thick active layer films for large‐area printing); 2) modular designs (effective designs that can suppress electrical, geometric, optical, and additional losses, leading to a reduction in the PCE of the devices, as a consequence of substrate area expansion); and 3) printing methods (various scalable fabrication techniques that are employed for large‐area fabrication, including knife coating, slot‐die coating, screen printing, inkjet printing, gravure printing, flexographic printing, pad printing, and brush coating). By combining thick‐film material systems with efficient modular designs exhibiting low‐efficiency losses and employing the right printing methods, the fabrication of large‐area OSCs will be successfully realized in the near future.     

Isoindigo‐Based Polymers with Small Effective Masses for High‐Mobility Ambipolar Field‐Effect Transistors

So far, most of the reported high‐mobility conjugated polymers are p‐type semiconductors. By contrast, the advances in high‐mobility ambipolar polymers fall greatly behind those of p‐type counterparts. Instead of unipolar p‐type and n‐type materials, ambipolar polymers, especially balanced ambipolar polymers, are potentially serviceable for easy‐fabrication and low‐cost complementary metal‐oxide‐semiconductor circuits. Therefore, it is a critical issue to develop high‐mobility ambipolar polymers. Here, three isoindigo‐based polymers, PIID‐2FBT , P1FIID‐2FBT , and P2FIID‐2FBT are developed for high‐performance ambipolar organic field‐effect transistors. After the incorporation of fluorine atoms, the polymers exhibit enhanced coplanarity, lower energy levels, higher crystallinity, and thus increased µ _e. P2FIID‐2FBT exhibits n‐type dominant performance with a µ _e of 9.70 cm² V^?1 s^?1. Moreover, P1FIID‐2FBT exhibits a highly balanced µ _h and µ _e of 6.41 and 6.76 cm² V^?1 s^?1, respectively, which are among the highest values for balanced ambipolar polymers. Moreover, a concept “effective mass” is introduced to further study the reasons for the high performance of the polymers. All the polymers have small effective masses, indicating good intramolecular charge transport. The results demonstrate that high‐mobility ambipolar semiconductors can be obtained by designing polymers with fine‐tuned energy levels, small effective masses, and high crystallinity.     

2D Organic Materials for Optoelectronic Applications

The remarkable merits of 2D materials with atomically thin structures and optoelectronic attributes have inspired great interest in integrating 2D materials into electronics and optoelectronics. Moreover, as an emerging field in the 2D‐materials family, assembly of organic nanostructures into 2D forms offers the advantages of molecular diversity, intrinsic flexibility, ease of processing, light weight, and so on, providing an exciting prospect for optoelectronic applications. Herein, the applications of organic 2D materials for optoelectronic devices are a main focus. Material examples include 2D, organic, crystalline, small molecules, polymers, self‐assembly monolayers, and covalent organic frameworks. The protocols for 2D‐organic‐crystal‐fabrication and ‐patterning techniques are briefly discussed, then applications in optoelectronic devices are introduced in detail. Overall, an introduction to what is known and suggestions for the potential of many exciting developments are presented.     

Infrared Quantum Dots

Colloidal nanocrystals are quantum‐size‐effect tunable; offer an abundance of available surface area for electronic and chemical interactions; and are processible from organic or aqueous solution onto substrates rigid or flexible, smooth or rough, flat or curved, inorganic or organic (including biological), crystalline or amorphous, conducting, semiconducting, or insulating. With the benefit of over a decade's progress in visible‐light‐emitting colloidal‐quantum‐dot synthesis, physical chemistry, and devices, significant progress has recently been made in infrared‐active colloidal quantum dots and devices. This progress report summarizes the state‐of‐the‐art in infrared colloidal quantum dots, with an emphasis on applications and devices. The applications of interest surveyed include monolithic integration of fiber‐optic and free‐space‐communications photonic components with electronic substrates such as silicon and glass; in‐vivo biological tagging in infrared spectral bands in which living tissue is optically penetrable to a depth of 5–10 cm; solar and thermal photovoltaics for energy conversion; and infrared sensing and imaging based on non‐visible, including thermal, signatures. The synthesis and properties of quantum dots are first reviewed: photoluminescence quantum efficiencies greater than 50 % are achievable in solution, and stable luminescent dots are available in organic and aqueous solvents. Electroluminescent devices based on solution processing have been reported with external quantum efficiencies approaching 1 %. Photoconductive devices have been realized with 3 % internal quantum efficiencies, and a photovoltaic effect was recently observed. Electro‐optic modulation achieved by either field‐ or charge‐induced modification of the rate of optical absorption has been demonstrated based both on interband and intersubband (intraband) transitions. Optical gain from these processible materials with a threshold of 1 mJ cm^–2 and an optical net modal gain coefficient of 260 ± 20 cm^–1 have been reported.     

Direct Electron‐Beam Writing of Highly Conductive Wires in Functionalized Fullerene Films

This work demonstrates the patterning of thin films (≈25 nm) of a newly synthesized fullerene derivative by direct‐write electron‐beam lithography to produce highly conducting carbon microstructures. Scanning electron microscopy and atomic force microscopy are used to characterize the resulting microstructure morphology, whilst the resistivities of the structures are probed using four‐point probe electrodes deposited on the microstructures by lift‐off. The microstructures have a resistivity of ≈9.5 × 10^?3 Ω cm after exposure to an electron dose of 0.1 C cm^?2. The method may have applications in the generation and electrical contacting of organic electronics, organic photovoltaics, and lab‐on‐a‐chip devices.     

A 2D Conductive Organic–Inorganic Hybrid with Extraordinary Volumetric Capacitance at Minimal Swelling

Rational design and synthesis of 2D organic–inorganic hybrid materials is important for transformative technological advances for energy storage. Here, a 2D conductive hybrid lamella and its intercalation properties for thin‐film supercapacitors are reported. The 2D organic–inorganic hybrid lamella comprises periodically stacked 2D nanosheets with 11.81 Å basal spacing, and is electronically conductive (605 S m^?1). In contrast to the pre‐existing organic‐based 2D materials, this material has extremely low gas‐permeable porosity (16.5 m² g^?1) in contrast to the high ionic accessibility. All these structural features collectively contribute to the high capacitances up to 732 F cm^?3, combined with small structural swelling at as low as 4.8% and good stability. At a discharge time of 6 s, the thin‐film intercalation electrode delivers an energy density of 24 mWh cm^?3, which universally outperforms the surface‐dominant capacitive processes in porous carbons.