The Emergence of Organic Single-Crystal Electronics

Flexible Organic Molecular Single Crystal-Based Triboelectric Device as a Self-Powered Tactile Sensor

Triboelectric nanogenerators (TENGs) have proven to be effective at converting mechanical energy into electrical power, making them a viable technology for operating self-powered electronic devices used in medical diagnostics and environmental monitoring. In the present study, we demonstrate the utility of the flexible single crystals of an organic compound for the fabrication of a TENG as a self-powered tactile sensor. Triboelectrification was attained in single crystals as a result of surface functionalization with positively and negatively charged moieties, viz. Zn2+ and F-, respectively, which resulted in a variable surface potential and reversible adhesion through electrostatic interaction and induction phenomena. TENG incorporating the single crystals showed an output voltage of 2.4 V, a current density of ∼2.2 μA/m2, and a power density of ∼850 mW/m2 and was capable of charging commercial capacitors thereby ensuring its ability to be used as a self-powered touch sensor. Capitalizing on these features, a self-powered tactile sensor was fabricated to demonstrate limb movements. The excellent mechano-electric sensitivity (∼102 mV/kPa until 6 kPa range) and response time (∼38 ms) establish the viability of flexible organic single crystals for mechanical energy harvesting and biosensing applications that could pave the way for their utilization as biomedical wearable devices.

Enhancing 2D Photonics and Optoelectronics with Artificial Microstructures

By modulating subwavelength structures and integrating functional materials, 2D artificial microstructures (2D AMs), including heterostructures, superlattices, metasurfaces and microcavities, offer a powerful platform for significant manipulation of light fields and functions. These structures hold great promise in high-performance and highly integrated optoelectronic devices. However, a comprehensive summary of 2D AMs remains elusive for photonics and optoelectronics. This review focuses on the latest breakthroughs in 2D AM devices, categorized into electronic devices, photonic devices, and optoelectronic devices. The control of electronic and optical properties through tuning twisted angles is discussed. Some typical strategies that enhance light-matter interactions are introduced, covering the integration of 2D materials with external photonic structures and intrinsic polaritonic resonances. Additionally, the influences of external stimuli, such as vertical electric fields, enhanced optical fields and plasmonic confinements, on optoelectronic properties is analysed. The integrations of these devices are also thoroughly addressed. Challenges and future perspectives are summarized to stimulate research and development of 2D AMs for future photonics and optoelectronics.

Organic-Inorganic Rubrene/WS2 Heterostructure for Broadband Detection and Polarization Imaging

Organic single crystals exhibit improved carrier mobility, longer exciton diffusion length, anisotropic charge transport, and unique linear dichroism, while its high exciton binding energy seriously limits the free-carrier generation and photoelectric conversion efficiency. Layered van der Waals heterostructures, which integrate organic crystals with high mobility two-dimensional (2D) inorganic semiconductors, are promising for promoting exciton dissociation and boosting sensitivity by utilizing the interfacial potential and photogating effect. In this work, organic single-crystal rubrene is integrated with a few-layer WS2 to design the high-performance photodetector. The device exhibits an excellent responsivity of 1000 A W-1, and a fast speed of 180 μs, which is far superior to the individual WS2 device. Equally importantly, this device provides excellent polarization detection performance by virtue of the anisotropic properties of rubrene, and the dichroic ratios are 1.56, 1.5, and 1.7 for 375, 405, and 658 nm irradiation, respectively. Finally, several high-resolution single-pixel broadband polarization imaging was demonstrated. Our work shows that organic-inorganic heterostructure is an essential candidate for improving optoelectronics performance and has potential for polarization imaging.

A Naphthalenetetracarboxdiimide-Containing Covalent Organic Polymer: Preparation, Single Crystal Structure and Battery Application

Inspired by dative boron-nitrogen (B←N) bonds proven to be the promising dynamic linkage for the construction of crystalline covalent organic polymers/frameworks (COPs/COFs), we employed 1,4-bis(benzodioxaborole) benzene (BACT) and N,N'-Di(4-pyridyl)-1,4,5,8-naphthalenetetracarboxdiimide (DPNTCDI) as the corresponding building blocks to construct a functional COP (named as CityU-25), which had been employed as an anode in rechargeable lithium ion batteries. CityU-25 displayed an excellent reversible lithium storage capability of 455 mAh/g after 170 cycles at 0.1 A/g, and an impressive one of 673 mAh/g after 720 cycles at 0.5 A/g. These findings suggest that CityU-25 is a standout candidate for advanced battery technologies, highlighting the potential application of this type of materials.

Surface coating induced room-temperature phosphorescence in flexible organic single crystals

Materials exhibiting room temperature phosphorescence (RTP) are in high demand for signage, information encryption, sensing, and biological imaging. Due to weak spin-orbit coupling and other non-radiative processes that effectively quench the triplet excited states, RTP is sparsely observed in organic materials. Although the incorporation of a heavy atom through covalent or non-covalent modification circumvents these drawbacks, heavy-atom-containing materials are undesirable because of their deleterious side effects. Here, we designed and synthesized a new naphthalidenimine-boron complex as a coating material for the single crystals of 4,4'-dimethoxybenzophenone. The coated surface was observed to exhibit yellowish-green phosphorescence with ms lifetimes at ambient conditions through Förster resonance energy transfer (FRET). Importantly, the mechanical flexibility of the single crystals was observed to be retained after coating. The fluorescence-phosphorescence dual emission was utilised for colour-tunable optical waveguiding and anti-counterfeiting applications. As organic single crystals that can sustain mechanical deformations are emerging as the next-generation materials for electronic device fabrication, the flexible RTP organic crystals showing colour-tuneable optical waveguiding could be omnipotent in electronics.

Flexible molecular crystals for optoelectronic applications

The cornerstones of the advancement of flexible optoelectronics are the design, preparation, and utilization of novel materials with favorable mechanical and advanced optoelectronic properties. Molecular crystalline materials have emerged as a class of underexplored yet promising materials due to the reduced grain boundaries and defects anticipated to provide enhanced photoelectric characteristics. An inherent drawback that has precluded wider implementation of molecular crystals thus far, however, has been their brittleness, which renders them incapable of ensuring mechanical compliance required for even simple elastic or plastic deformation of the device. It is perplexing that despite a plethora of reports that have in the meantime become available underpinning the flexibility of molecular crystals, the "discovery" of elastically or plastically deformable crystals remains limited to cases of serendipitous and laborious trial-and-error approaches, a situation that calls for a systematic and thorough assessment of these properties and their correlation with the structure. This review provides a comprehensive and concise overview of the current understanding of the origins of crystal flexibility, the working mechanisms of deformations such as plastic and elastic bending behaviors, and insights into the examples of flexible molecular crystals, specifically concerning photoelectronic changes that occur in deformed crystals. We hope this summary will provide a reference for future experimental and computational efforts with flexible molecular crystals aimed towards improving their mechanical behavior and optoelectronic properties, ultimately intending to advance the flexible optoelectronic technology.

Ultraflexible Wireless Imager Integrated with Organic Circuits for Broadband Infrared Thermal Analysis

Flexible imagers are currently under intensive development as versatile optical sensor arrays, designed to capture images of surfaces and internals, irrespective of their shape. A significant challenge in developing flexible imagers is extending their detection capabilities to encompass a broad spectrum of infrared light, particularly terahertz (THz) light at room temperature. This advancement is crucial for thermal and biochemical applications. In this study, a flexible infrared imager is designed using uncooled carbon nanotube (CNT) sensors and organic circuits. The CNT sensors, fabricated on ultrathin 2.4 µm substrates, demonstrate enhanced sensitivity across a wide infrared range, spanning from near-infrared to THz wavelengths. Moreover, they retain their characteristics under bending and crumpling. The design incorporates light-shielded organic transistors and circuits, functioning reliably under light irradiation, and amplifies THz detection signals by a factor of 10. The integration of both CNT sensors and shielded organic transistors into an 8 × 8 active-sensor matrix within the imager enables sequential infrared imaging and nondestructive assessment for heat sources and in-liquid chemicals through wireless communication systems. The proposed imager, offering unique functionality, shows promise for applications in biochemical analysis and soft robotics.

Eclipsed and Twisted Excimers of Pyrene and 2-Azapyrene: How Nitrogen Substitution Impacts Excimer Emission

Due to their unique photophysical and electronic properties, pyrene and its analogues have been the subject of extensive research in recent decades. The propensity of pyrene and its derivatives to form excimers has found wide application in various fields. Nitrogen-substituted pyrene derivatives display similar photophysical properties, but for them, excimer emission has not been reported to date. Here, we use time-dependent density functional theory (TD-DFT) calculations to investigate the low-lying exciton states of dimers of pyrene and 2-azapyrene. The excimer equilibrium structures are determined and the contribution of charge transfer (CT) excitations and intermolecular interactions to the exciton states is disclosed using a diabatization procedure. The study reveals that the dimers formed by the two molecules have quite similar exciton-state patterns, in which the relevant CT contributions govern the formation of excimer states, along with the La/Lb state inversion. In contrast with pyrene, the dipole-dipole interactions in 2-azapyrene stabilize the dark eclipsed excimer structure and increase the barrier for conversion into a bright twisted excimer. It is suggested that these differences in the nitrogen-substituted derivative might influence the excimer emission properties.

Coplanar Conformational Structure of π-Conjugated Polymers for Optoelectronic Applications

Hierarchical structure of conjugated polymers is critical to dominating their optoelectronic properties and applications. Compared to nonplanar conformational segments, coplanar conformational segments of conjugated polymers (CPs) demonstrate favorable properties for applications as a semiconductor. Herein, recent developments in the coplanar conformational structure of CPs for optoelectronic devices are summarized. First, this review comprehensively summarizes the unique properties of planar conformational structures. Second, the characteristics of the coplanar conformation in terms of optoelectrical properties and other polymer physics characteristics are emphasized. Five primary characterization methods for investigating the complanate backbone structures are illustrated, providing a systematical toolbox for studying this specific conformation. Third, internal and external conditions for inducing the coplanar conformational structure are presented, offering guidelines for designing this conformation. Fourth, the optoelectronic applications of this segment, such as light-emitting diodes, solar cells, and field-effect transistors, are briefly summarized. Finally, a conclusion and outlook for the coplanar conformational segment regarding molecular design and applications are provided.

Benchmark Data Set of Crystalline Organic Semiconductors

This work reports a Benchmark Data set of Crystalline Organic Semiconductors to test calculations of the structural and electronic properties of these materials in the solid state. The data set contains 67 crystals consisting of mostly rigid molecules with a single dominant conformer, covering the majority of known structural types. The experimental crystal structure is available for the entire data set, whereas zero-temperature unit cell volume can be reliably estimated for a subset of 28 crystals. Using this subset, we benchmark r2SCAN-D3 and PBE-D3 density functionals. Then, for the entire data set, we benchmark approximate density functional theory (DFT) methods, including GFN1-xTB and DFTB3(3ob-3-1), with various dispersion corrections against r2SCAN-D3. Our results show that r2SCAN-D3 geometries are accurate within a few percent, which is comparable to the statistical uncertainty of experimental data at a fixed temperature, but the unit cell volume is systematically underestimated by 2% on average. The several times faster PBE-D3 provides an unbiased estimate of the volume for all systems except for molecules with highly polar bonds, for which the volume is substantially overestimated in correlation with the underestimation of atomic charges. Considered approximate DFT methods are orders of magnitude faster and provide qualitatively correct but overcompressed crystal structures unless the dispersion corrections are fitted by unit cell volume.

Preparation of Novel Organic Polymer Semiconductor and Its Properties in Transistors through Collaborative Theoretical and Experimental Approaches

Conjugated polymer semiconductors based on donor-acceptor structures are commonly employed as core materials for optoelectronic devices in the field of organic electronics. In this study, we designed and synthesized a novel acceptor unit thiophene-vinyl-diketopyrrolopyrrole, named TVDPP, based on a four-step organic synthesis procedure. Stille coupling reactions were applied with high yields of polymerization of TVDPP with fluorinated thiophene (FT) monomer. The molecular weight and thermal stability of the polymers were tested and showed high molecular weight and good thermal stability. Theoretical simulation calculations and 2D grazing-incidence wide-angle X-ray scattering (GIWAXS) tests verified the planarity of the material and excellent stacking properties, which are favorable for achieving high carrier mobility. Measurements based on the polymer as an organic thin film transistor (OTFT) device were carried out, and the mobility and on/off current ratio reached 0.383 cm2 V-1 s-1 and 104, respectively, showing its great potential in organic optoelectronics.

Autonomous self-healing organic crystals for nonlinear optics

Non-centrosymmetric molecular crystals have a plethora of applications, such as piezoelectric transducers, energy storage and nonlinear optical materials owing to their unique structural order which is absent in other synthetic materials. As most crystals are brittle, their efficiency declines upon prolonged usage due to fatigue or catastrophic failure, limiting their utilities. Some natural substances, like bone, enamel, leaf and skin, function efficiently, last a life-time, thanks to their inherent self-healing nature. Therefore, incorporating self-healing ability in crystalline materials will greatly broaden their scope. Here, we report single crystals of a dibenzoate derivative, capable of self-healing within milliseconds via autonomous actuation. Systematic quantitative experiments reveal the limit of mechanical forces that the self-healing crystals can withstand. As a proof-of-concept, we also demonstrate that our self-healed crystals can retain their second harmonic generation (SHG) with high efficiency. Kinematic analysis of the actuation in our system also revealed its impressive performance parameters, and shows actuation response times in the millisecond range.

Fully (Re)configurable Interactive Material through a Switchable Photothermal Charge Transfer Complex Gated by a Supramolecular Liquid Crystal Elastomer Actuator

Charge transfer complexes (CTCs) based on self-assembled donor and acceptor molecules allow light absorption of significantly redshifted wavelengths to either the donor or acceptor. In this work, we demonstrate a CTC embedded in a hydrogen-bonded liquid crystal elastomer (LCE), which in itself is fully reformable and reprocessable. The LCE host acts as a gate, directing the self-assembly of the CTC. When hydrogen bonding is present, the CTC behaves as a near-infrared (NIR) dye allowing photothermal actuation of the LCE. The CTC can be disassembled in specific regions of the LCE film by disrupting the hydrogen bond interactions, allowing selective NIR heating and localized actuation of the films. The metastable non-CTC state may persist for weeks or can be recovered on demand by heat treatment. Besides the CTC variability, the capability of completely reforming the shape, color, and actuation mode of the LCE provides an interactive material with unprecedented application versatility.

Wafer-Scale Epitaxial Growth of Two-dimensional Organic Semiconductor Single Crystals toward High-Performance Transistors

The success of state-of-the-art electronics and optoelectronics relies heavily on the capability to fabricate semiconductor single-crystal wafers. However, the conventional epitaxial growth strategy for inorganic wafers is invalid for growing organic semiconductor single crystals due to the lack of lattice-matched epitaxial substrates and intricate nucleation behaviors, severely impeding the advancement of organic single-crystal electronics. Here, an anchored crystal-seed epitaxial growth method for wafer-scale growth of 2D organic semiconductor single crystals is developed for the first time. The crystal seed is firmly anchored on the viscous liquid surface, ensuring the steady epitaxial growth of organic single crystals from the crystal seed. The atomically flat liquid surface effectively eliminates the disturbance from substrate defects and greatly enhances the 2D growth of organic crystals. Using this approach, a wafer-scale few-layer bis(triethylsilythynyl)-anthradithphene (Dif-TES-ADT) single crystal is formed, yielding a breakthrough for organic field-effect transistors with a high reliable mobility up to 8.6 cm2 V-1 s-1 and an ultralow mobility variable coefficient of 8.9%. This work opens a new avenue to fabricate organic single-crystal wafers for high-performance organic electronics.

Ambipolar to Unipolar Conversion in C70/Ferrocene Nanosheet Field-Effect Transistors

Organic cocrystals, which are assembled by noncovalent intermolecular interactions, have garnered intense interest due to their remarkable chemicophysical properties and practical applications. One notable feature, namely, the charge transfer (CT) interactions within the cocrystals, not only facilitates the formation of an ordered supramolecular network but also endows them with desirable semiconductor characteristics. Here, we present the intriguing ambipolar CT properties exhibited by nanosheets composed of single cocrystals of C70/ferrocene (C70/Fc). When heated to 150 °C, the initially ambipolar monoclinic C70/Fc nanosheet-based field-effect transistors (FETs) were transformed into n-type face-centered cubic (fcc) C70 nanosheet-based FETs owing to the elimination of Fc. This thermally induced alteration in the crystal structure was accompanied by an irreversible switching of the semiconducting behavior of the device; thus, the device transitions from ambipolar to unipolar. Importantly, the C70/Fc nanosheet-based FETs were also found to be much more thermally stable than the previously reported C60/Fc nanosheet-based FETs. Furthermore, we conducted visible/near-infrared diffuse reflectance and photoemission yield spectroscopies to investigate the crucial role played by Fc in modulating the CT characteristics. This study provides valuable insights into the overall functionality of these nanosheet structures.

Organic Semiconductor Single Crystal Arrays: Preparation and Applications

The study of organic semiconductor single crystal (OSSC) arrays has recently attracted considerable interest given their potential applications in flexible displays, smart wearable devices, biochemical sensors, etc. Patterning of OSSCs is the prerequisite for the realization of organic integrated circuits. Patterned OSSCs can not only decrease the crosstalk between adjacent organic field-effect transistors (OFETs), but also can be conveniently integrated with other device elements which facilitate circuits application. Tremendous efforts have been devoted in the controllable preparation of OSSC arrays, and great progress has been achieved. In this review, the general strategies for patterning OSSCs are summarized, along with the discussion of the advantages and limitations of different patterning methods. Given the identical thickness of monolayer molecular crystals (MMCs) which is beneficial to achieve super uniformity of OSSC arrays and devices, patterning of MMCs is also emphasized. Then, OFET performance is summarized with comparison of the mobility and coefficient of variation based on the OSSC arrays prepared by different methods. Furthermore, advances of OSSC array-based circuits and flexible devices of different functions are highlighted. Finally, the challenges that need to be tackled in the future are presented.

Reducing Contact Resistance in Organic Field-Effect Transistors: A Comprehensive Comparison between 2D and Microrod Single Crystals

A comprehensive comparison of organic single crystals based on a single material but with different dimensions provides a unique approach to probe their carrier injection mechanism. In this report, both two-dimensional (2D) and microrod single crystals with the same crystalline structure of an identical thiopyran derivative, 7,14-dioctylnaphtho[2,1-f:6,5-f']bis(cyclopentane[b]thiopyran) (C₈-SS), are grown on a glycerol surface with the space-confined method. Organic field-effect transistors (OFETs) based on the 2D C₈-SS single crystal exhibit superior performance compared with those based on the microrod single crystal, particularly in their contact resistance (R_C). It is demonstrated that the resistance of the crystal bulk in the contact region plays a key role in R_C of the OFETs. Thus, among the 30 devices tested, the microrod OFETs typically appear contact-limited, whereas the 2D OFETs possess significantly reduced R_C arising from the tiny thickness of the 2D single crystal. The 2D OFETs show high operational stability and high channel mobility up to 5.7 cm²/V·s. The elucidation of the contact behavior highlights the merits and great potential of 2D molecular single crystals in organic electronics.

Recent Research Progress of Organic Small-Molecule Semiconductors with High Electron Mobilities

Organic electronics has made great progress in the past decades, which is inseparable from the innovative development of organic electronic devices and the diversity of organic semiconductor materials. It is worth mentioning that both of these great advances are inextricably linked to the development of organic high-performance semiconductor materials, especially the representative n-type organic small-molecule semiconductor materials with high electron mobilities. The n-type organic small molecules have the advantages of simple synthesis process, strong intermolecular stacking, tunable molecular structure, and easy to functionalize structures. Furthermore, the n-type semiconductor is a remarkable and important component for constructing complementary logic circuits and p-n heterojunction structures. Therefore, n-type organic semiconductors play an extremely important role in the field of organic electronic materials and are the basis for the industrialization of organic electronic functional devices. This review focuses on the modification strategies of organic small molecules with high electron mobility at molecular level, and discusses in detail the applications of n-type small-molecule semiconductor materials with high mobility in organic field-effect transistors, organic light-emitting transistors, organic photodetectors, and gas sensors.

Limited Stability of 6,13-Bis(tri(isopropyl)silylethynyl)pentacene upon One-Electron Oxidation: Electrochemically Induced (4 + 2) Cycloaddition between an Alkynyl-Substituted Acene and Its Radical Cation

6,13-Bis(tri(isopropyl)silylethynyl)pentacene, a particularly stable acene derivative important for (opto)electronic materials, turns reactive upon electrochemical one-electron oxidation. One of the typically stabilizing tri(isopropyl)silylethynyl substituents becomes involved in a (4 + 2) cycloaddition after redox umpolung. The electrosynthetic dimerization of the title compound provides easy access under mild conditions to a complex scaffold, which includes an intact pentacene, an anthracene, and a phenylene unit, all electronically separated. The product's electrochemical redox properties are explained by superimposed cyclic voltammetric features of the pentacene and the anthracene moieties. The reaction path is analyzed on the basis of electroanalytical and ESR data, and an oxidation-cycloaddition-reduction sequence is elaborated. The contribution of homogeneous electron transfers (electron transfer chain reaction) is negligible, in accordance with the relative formal redox potentials of the starting compound and the product. Quantum chemical calculations indicate that the central cycloaddition should be described as a two-step process with a distonic radical cation intermediate. We suggest an extended notation to define the contribution of the components with respect to electron count in the two-step cycloaddition, [3 + 1, 1 + 1].

Solution-Processed Isoindigo- and Thienoisoindigo-Based Donor-Acceptor-Donor π-Conjugated Small Molecules: Synthesis, Morphology, Molecular Packing, and Field-Effect Transistor Characterization

Molecular design and precise control of thin-film morphology and crystallinity of solution-processed small molecules are important for enhancing charge transport mobility of organic field-effect transistors and gaining more insight into the structure-property relationship. Here, two donor-acceptor-donor (D-A-D) architecture small molecules TRA-IID-TRA and TRA-TIID-TRA comprising an electron-donating triarylamine (TRA) and two different electron-withdrawing cores, isoindigo (IID) and thienoisoindigo (TIID), respectively, were synthesized and characterized. Replacing the phenylene rings of central IID A with thiophene gives a TIID core, which reduces the optical band gap and upshifts the energy levels of frontier molecular orbitals. The single-crystal structures and grazing-incidence wide-angle X-ray scattering (GIWAXS) analysis revealed that TRA-TIID-TRA exhibits the relatively tighter π-π stacking packing with preferential edge-on orientation, larger coherence length, and higher crystallinity due to the noncovalent S···O/S···π intermolecular interactions. The distinctly oriented and connected ribbon-like TRA-TIID-TRA crystalline film by the solution-shearing process achieved a superior hole mobility of 0.89 cm² V^-1 s^-1 in the organic field-effect transistor (OFET) device, which is at least five times higher than that (0.17 cm² V^-1 s^-1) of TRA-IID-TRA with clear cracks. Eventually, rational modulation of fused core in the π-conjugated D-A-D small molecule provides a new understanding of structural design for enhancing the performance of solution-processed organic semiconductors.

A Strategy for Polar Crystals with Dipolar Heterohelicenes

Polar crystals have attracted interest for the applications to polar materials with piezo- and pyroelectricity, and second harmonic generation. Despite their potential utility for flexible polar materials, a strategy for ordering polar helicenes have remained elusive. Here, we demonstrate creation of polar crystal with unsymmetrically substituted aza[5]helicenes tuned by substituents. The usymmetric aza[5]helicenes have been prepared through regioselective monoprotiodesilylations. We disclosed triisopropylsilyl-substituted derivatives show 1D chain columnar packings. In particular, enantiopure crystals showed spontaneous polarization. Optical and single-crystal X-ray diffraction experiments with other derivatives, as well as theoretical calculations, revealed that the presence of triisopropylsilyl or electron-withdrawing aryl substituents is essential for forming the 1D chain columnar structure. Hirshfeld surface analyses further showed that CH-π interactions between 1D chain columns regulate the polar assembly. Finally, we determined the polarizability of the nitro derivative by ab initio calculation to be 4.53 µC/cm 2 . This value corroborates the first example of a spontaneously polar crystal of helicenes. We believe that this study will contribute to the development of polar materials from organic molecules.

Challenging Bendable Organic Single Crystal and Transistor Arrays with High Mobility and Durability toward Flexible Electronics

Bendable organic single crystals are promising candidates for flexible electronics owing to their superior charge-transport properties. However, large-area high-quality organic single crystals are rarely available on the polymer substrates generally used in flexible electronics. Here, a surface-assisted assembly strategy based on a polymer modification, poly(amic acid) (PAA), is developed to grow large-area organic singe crystals on polymer substrates using a simple drop-casting method. The unique surface properties of PAA that enable molecular solution superwetting and promote molecular ordered assembly produce an extraordinary self-driven "meniscus-guided coating" behavior, enabling the fabrication of millimeter-sized, highly aligned organic single crystals for a variety of organic semiconductors. Organic field-effect transistors based on a mode molecule of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene demonstrate the highest (average) mobility of 18.6 cm² V^-1 s^-1 (15.9 cm² V^-1 s^-1 ), attractively low operating voltage of -3 V, and high flexible durability. The results shed light on the large-area fabrication of organic single crystals on polymer dielectrics toward high-performance and integrated plastic electronics.

Harnessing Polymer-Matrix-Mediated Manipulation of Intermolecular Charge-Transfer for Near-Infrared Security Applications

Precise recognition of near-infrared (NIR) signals holds great prospects in optical communication, remote sensing, information security, and anti-counterfeiting. For these applications, filters with good NIR transparency are typically essential components. Currently, such NIR transparent filters are dominated by inorganic materials such as chalcogenide glasses. There are, so far, only a handful of organic molecules with suitable optical properties due to the rarity of organic materials with good NIR transparency and relatively flat absorption over the UV-visible region. Here, it is found that the library of NIR-transparent organic materials can be expanded by forming a charge-transfer complex (CTC) between a donor (D) and an acceptor (A) molecule that are commercially available. Via regulating the DA interaction, the CTC filter shows tunable absorption from the visible to NIR region with a relatively high penetration of NIR radiation (≈80%). The CTC filter can successfully highlight NIR information hidden in a complex environment and allow reading of NIR security images for advanced anti-counterfeiting. Moreover, the CTC filter can be used for viewing protected NIR information with good resolution, and thus provide a convenient tool for different security applications using NIR-encoded information.

Growth of Pentacene Crystals by Naphthalene Flux Method

We report the crystal growth of pentacene from a solution of naphthalene. The solubility of pentacene in naphthalene was evaluated by optical absorption at elevated temperature. The crystal growth was performed in an H-shaped sealed glass tube or metal vessels sealed with ultrahigh-vacuum compatible flanges placed in heated two-zone aluminum blocks. The obtained crystals had a single-crystal-like appearance and flat surface. They were made of aligned microtwins of the "bulk type" (interlayer spacing 14.5 Å) polymorph.

Flexible organic crystals. Understanding the tractable co-existence of elastic and plastic bending

As an emerging class of flexible materials, mechanically bendable molecular crystals are broadly classified as elastic or plastic. Nevertheless, flexible organic crystals with mutually exclusive elastic and plastic traits, with contrasting structural requirements, co-existing under different stress settings are exceptional; hence, it is imperative to establish the concurring factors that beget this rare occurrence. We report a series of halogen-substituted benzil crystals showing elastic bending (within ∼2.45% strain), followed by elastoplastic deformation under ambient conditions. Under higher stress settings, they display exceptional plastic flexibility that one could bend, twist, or even coil around a capillary tube. X-ray diffraction, microscopy, and computational data reveal the microscopic and macroscopic basis for the exciting co-existence of elastic, elastoplastic, and plastic properties in the crystals. The layered molecular arrangement and the weak dispersive interactions sustaining the interlayer region provide considerable tolerance towards breaking and making upon engaging or releasing the external stress; it enables restoring the original state within the elastic strain. Comparative studies with oxalate compounds, wherein the twisted diketo moiety in benzil was replaced with a rigid and coplanar central oxalate moiety, enabled us to understand the effect of the anisotropy factor on the crystal packing induced by the C[double bond, length as m-dash]O⋯C tetral interactions. The enhanced anisotropy depreciated the elastic domain, making the oxalate crystals more prone to plastic deformation. Three-point bending experiments and the determined Young's moduli further corroborate the co-existence of the elastic and plastic realm and highlight the critical role of the underlying structural elements that determine the elastic to plastic transformation. The work highlights the possible co-existence of orthogonal mechanical characteristics in molecular crystals and further construed the concurrent role of microscopic and macroscopic elements in attaining this exceptional mechanical trait.

Multifunctional Heteropentalenes: From Synthesis to Optoelectronic Applications

In the broad family of heteropentalenes, the combination of two five-membered heterocyclic rings fused in the [3,2-b] mode has attracted the most significant attention. The relatively straightforward access to these structures, being a consequence of the advances in the last two decades, combined with their physicochemical properties which match the requirements associated with many applications has led to an explosion of applied research. In this Perspective, we will discuss the recent progress of heteropentalenes' usefulness as an active element of organic light-emitting diodes and organic field-effect transistors. Among the myriad of possible combinations for the different heteroatoms, thieno[3,2-b]thiophenes and 1,4-dihydropyrrolo[3,2-b]pyrroles are subject to the most intense studies. Together they comprise a potent optoelectronics tool resulting from the combination of appreciable photophysical properties, chemical reactivity, and straightforward synthesis.

Temperature-Triggered Supramolecular Assembly of Organic Semiconductors

Supramolecular assembly is a promising bottom-up approach for producing materials that behave as charge transporting components in electronic devices. Although extensive advances have been made during the past two decades, formidable challenges exist in controlling the local randomness present in supramolecular assemblies. Here, a temperature-triggered supramolecular assembly strategy using heat to heal defects and disorders is reported. The central concept of the molecular design-named the "Tetris strategy" in this research-is to: i) increase the rotational freedom of the molecules through thermal perturbation, ii) induce conformation-fitting of adjacent molecules through two different kinds of intermolecular [π···π] interactions, and finally iii) lock the nearby molecules in inactive co-conformations. Experimentally, upon heating to 57 °C, amorphous solid-state films undergo spontaneous assembly, leading to the growth of uniform and highly ordered microwire arrays. Temperature-triggered supramolecular assembly provides an approach closer to the precision control of assembled structures and presents with a broad canvas to work on in approaching a new generation of supramolecular electronics. Tetris is a registered trademark of Tetris Holding, LLC, used with permission.

High-performance five-ring-fused organic semiconductors for field-effect transistors

Organic molecular semiconductors have been paid great attention due to their advantages of low-temperature processability, low fabrication cost, good flexibility, and excellent electronic properties. As a typical example of five-ring-fused organic semiconductors, a single crystal of pentacene shows a high mobility of up to 40 cm² V^-1 s^-1, indicating its potential application in organic electronics. However, the photo- and optical instabilities of pentacene make it unsuitable for commercial applications. But, molecular engineering, for both the five-ring-fused building block and side chains, has been performed to improve the stability of materials as well as maintain high mobility. Here, several groups (thiophenes, pyrroles, furans, etc.) are introduced to design and replace one or more benzene rings of pentacene and construct novel five-ring-fused organic semiconductors. In this review article, ∼500 five-ring-fused organic prototype molecules and their derivatives are summarized to provide a general understanding of this catalogue material for application in organic field-effect transistors. The results indicate that many five-ring-fused organic semiconductors can achieve high mobilities of more than 1 cm² V^-1 s^-1, and a hole mobility of up to 18.9 cm² V^-1 s^-1 can be obtained, while an electron mobility of 27.8 cm² V^-1 s^-1 can be achieved in five-ring-fused organic semiconductors. The HOMO-LUMO levels, the synthesis process, the molecular packing, and the side-chain engineering of five-ring-fused organic semiconductors are analyzed. The current problems, conclusions, and perspectives are also provided.

Growth direction dependent separate-channel charge transport in the organic weak charge-transfer co-crystal of anthracene-DTTCNQ

Co-crystallization is an efficient way of molecular crystal engineering to tune the electronic properties of organic semiconductors. In this work, we synthesized anthracene-4,8-bis(dicyanomethylene)4,8-dihydrobenzo[1,2-b:4,5-b']-dithiophene (DTTCNQ) single crystals as a template to study the crystal growth direction dependent charge transport properties and attempted to elucidate the mechanism by proposing a separate-channel charge transport model. Single-crystal anthracene-DTTCNQ field-effect transistors showed that ambipolar transport properties could be observed in all crystal growth directions. Furthermore, upon changing the measured crystal directions, the electronic properties experienced a weak change from n-type dominated ambipolar, balanced ambipolar, to p-type dominated ambipolar properties. The theoretical calculations at density functional theory (DFT) and higher theory levels suggested that the anthracene-DTTCNQ co-crystal motif was a weak charge-transfer complex, in line with the experiment. Furthermore, the detailed theoretical analysis also indicated that electron or hole transport properties originated from separated channels formed by DTTCNQ or anthracene molecules. We thus proposed a novel separate-channel transport mechanism to support additional theoretical analysis and calculations. The joint experimental and theoretical efforts in this work suggest that the engineering of co-crystallization of weak charge-transfer complexes can be a practical approach for achieving tuneable ambipolar charge transport properties by the rational choice of co-crystal formers.

Organoplatinum(II) Cruciform: A Versatile Building Block to Fabricate 2D Microcrystals with Full-Color and White Phosphorescence and Anisotropic Photon Transport

Conventional square-planar platinum complexes typically form one-dimensional assemblies as a result of unidirectional metallophilic and/or π⋅⋅⋅π intermolecular interactions. Organoplatinum(II) complexes with a cruciform shape are presented herein to construct two-dimensional (2D) microcrystals with full-color and white phosphorescence. These 2D crystals show unique monocomponent π⋅⋅⋅π stacking, from either the cyclometalating or noncyclometalating ligand, and the bicomponent alternate π⋅⋅⋅π stacking from both ligands along different facet directions. Anisotropic tri-directional waveguiding is further implemented on a single hexagonal microcrystal. These results demonstrate the great capability of the organoplatinum(II) cruciform as a general platform to fabricate 2D phosphorescent micro-/nanocrystals for advanced photonic applications.

Nitrogen-Containing Perylene Diimides: Molecular Design, Robust Aggregated Structures, and Advances in n-Type Organic Semiconductors

ConspectusOrganic semiconductors (OSCs) have attracted much attention because of their potential applications for flexible and printed electronic devices and thus have been extensively investigated in a variety of research fields, such as organic chemistry, solid-state physics, and device physics and engineering. Organic thin-film transistors (OTFTs), a class of OSC-based devices, have been expected to be an alternative of silicon-based metal oxide semiconductor field-effect transistors (MOSFETs), which is the indispensable element for most of the current electronic devices. However, the noncovalently aggregated, van der Waals solid nature of the OSCs, by contrast to covalently bound silicon, conventionally exhibits lower carrier mobilities, limiting the practical applications of OTFTs. In particular, electron-transporting (i.e., n-type) OSCs lag behind their hole-transporting (p-type) counterparts in carrier mobility and ambient stability as OTFTs. This is primarily because of the difficulty in achieving compatibility between the aggregated structure exhibiting excellent carrier mobility and that with enough electron affinity. Recent understandings of carrier transport in OSCs explain that large and two-dimensionally isotropic transfer integrals coupled with small fluctuations are crucial for high carrier mobilities. In addition, from a practical point of view, the compatibility with practical device processes is highly required. Rational molecular design principles, therefore, are still demanded for developing OSCs and OTFTs toward high-end device applications.Herein, we will show our recent progress in the development of n-type OSCs with the key π-electron core (π-core) of benzo[de]isoquinolino[1,8-gh]quinolinetetracarboxylic diimide (BQQDI) on the basis of single-crystal OTFT technologies and the band-transport model enabled by two-dimensional molecular packing arrangements. The critical point is the introduction of electronegative nitrogen atoms into the π-core: the nitrogen atoms in BQQDI not only deepen the molecular orbital energies but also allow hydrogen-bonding-like attractive intermolecular interactions to control the aggregated structures, unlike the conventional role of the nitrogen introduced into OSCs only for the former role. Hence, the BQQDI analogues exhibit air-stable OTFT behavior and two-dimensional brickwork packing structures. Specifically, phenethyl-substituted analogue (PhC₂-BQQDI) has been shown as the first principal BQQDI-based material, demonstrating solution-processable thin-film single crystals, fewer anisotropic transfer integrals, and an effective suppression of molecular motions, leading to band-like electron-transport properties and stress-durable n-channel OTFT performances, in conjunction with the support of computational calculations. Insights into more fundamental points of view have been found by side-chain derivatization and OTFT studies on polycrystalline and single-crystal films. We hope that this Account provides readers with new strategies for designing high-performance OSCs by two-dimensional control of the aggregated structures.

Capillary-Confinement Crystallization for Monolayer Molecular Crystal Arrays

Organic single-crystalline semiconductors are highly desired for the fabrication of integrated electronic circuits, yet their uniform growth and efficient patterning is a huge challenge. Here, a general solution procedure named the "soft-template-assisted-assembly method" is developed to prepare centimeter-scale monolayer molecular crystal (MMC) arrays with precise regulation over their size and location via a capillary-confinement crystallization process. It is remarkable that the field-effect mobility of the array is highly uniform, with variation less than 4.4%, which demonstrates the most uniform organic single-crystal arrays ever reported so far. Simulations based on fluid dynamics are carried out to understand the function mechanism of this method. Thanks to the ultrasmooth crystalline orientation surface of MMCs, high-quality p-n heterojunction arrays can be prepared by weak epitaxy growth of n-type material atop the MMC. The p-n heterojunction field-effect transistors show ambipolar characteristics and the corresponding inverters constructed by these heterojunctions exhibit a competitive gain of 155. This work provides a general strategy to realize the preparation and application of logic complementary circuits based on patterned organic single crystals.

An asymmetric 2,3-fluoranthene imide building block for regioregular semiconductors with aggregation-induced emission properties

For organic semiconductors, the development of electron-deficient building blocks has lagged far behind that of the electron-rich ones. Moreover, it remains a significant challenge to design organic molecules with efficient charge transport and strong solid-state emission simultaneously. Herein, we describe a facile synthetic route toward a new π-acceptor imide building block, namely 2,3-fluoranthene imide, based on which four regioregular small molecules (F1–F4) are synthesized by tuning the imide orientations and the central linkage bridges. All molecules exhibit attractive aggregation-induced emission (AIE) characteristics with strong far-red emission in the powder state, and F3 shows the highest photoluminescence quantum yield of 5.9%. F1 and F3 with a thiophene bridge present an obvious p-type characteristic, while for F3 with an outward imide orientation, the maximum hole mobility from a solution-processed field-effect transistor (FET) device reaches 0.026 cm² V⁻¹ s⁻¹, being ∼10⁴ times higher than the value of F1 with an inward imide orientation. By using a fluorinated thiophene bridge, the resulting F2 and F4 can be turned into n-type semiconductors, showing an electron mobility of ∼1.43 × 10⁻⁴ and ∼3.34 × 10⁻⁵ cm² V⁻¹ s⁻¹, respectively. Our work not only demonstrates that asymmetric 2,3-fluoranthene imide is a promising building block for constructing organic materials with high carrier mobility and strong solid-state emission, but also highlights the importance of regioregular structures in the materials' properties.

A new electron-deficient 2,3-fluoranthene imide unit was easily synthesized through a one-pot reaction for constructing small molecule regioregular semiconductors with good carrier transport ability and strong solid-state emission.

A-Site Substitute for Fabricating All-Inorganic Perovskite CsPbCl3 with Application in Self-Powered Ultraviolet Photodetectors

Because of its stable chemical properties and wide band gap, CsPbCl₃ perovskite has shown great application prospects in ultraviolet photodetectors (UPDs). However, the poor solubility of CsCl in organic solvents impedes the fabrication of high-quality CsPbCl₃ films. Herein, we introduced an A-site substitute route for fabricating a high-quality CsPbCl₃ microcrystalline (MC) film by spin-coating cesium acetate on a MAPbCl₃ MC film followed by a high-temperature annealing process. To enhance the device performance of the FTO/SnO₂/CsPbCl₃ MCs/carbon structure UPD, a pressure-assisted annealing strategy was carried out, which reduced the void density and surface roughness of the microcrystal film. Finally, our optimized PDs showed high device performances with an on/off ratio of 6 × 10⁴, a responsivity of 0.13 A W^-1, a detectivity of as high as 1.07 × 10¹² Jones, and a rise/fall time of 10/24 μs. Moreover, our unpacked PDs showed good storage and light stability. Our results lay a foundation for the application of all inorganic perovskite in the ultraviolet region.

Tuning p-type to n-type Semiconductor Nature by Charge Transfer Cocrystallization: Effect of Transfer Integral vs. Reorganization Energy

In this contribution, 1:2 mixed stack (··DADA·· arrangement) donor acceptor cocrystal comprised of hole transport material CBP (4,4ʹ-bis(9H-carbazole-9-yl)biphenyl) as the donor (D), and TCNQ (7,7ʹ,8,8ʹ-tetracyano-1,4-quinodimethane) as the acceptor (A) was...

Organic Single Crystals with High Photoluminescence Quantum Yields Close to 100% and High Mobility for Optoelectronic Devices

Organic single crystals with excellent optical and electrical properties are critical for the development of organic optoelectronics. Herein, two compounds 9,10-bis([N,N-diphenyl]-4'-phenylethynyl)anthracene (TPA-An) and 9,10-bis([1',3'-diphenyl]-5'-phenylethynyl)anthracene (TBA-An) are synthesized by introducing two different luminescent groups, triphenylamine and 1,3-diphenylbenzene, at the 9,10 positions of anthracene via triple bond connection. Single crystals based on TPA-An and TBA-An with a ribbon morphology obtained through the slow solvent-evaporation method exhibit high photoluminescence quantum yields (PLQYs) of 98% and 99% at room temperature, and remarkable hole mobilities of 0.45 and 0.15 cm² V^-1 s^-1 in single-crystal organic field-effect transistors (SC-OFETs). Furthermore, UV phototransistors based on the two single crystals obtain photosensitivities of 1.03 × 10³ and 3.45 × 10⁴ , ultrahigh photoresponsivities of 7.19 × 10⁵ and 1.50 × 10⁵ A W^-1 , and the detectivities exceeding 1.40 × 10¹⁶ and 1.60 × 10¹⁷ Jones.

Quasi-Homoepitaxial Junction of Organic Semiconductors: A Structurally Seamless but Electronically Abrupt Interface between Rubrene and Bis(trifluoromethyl)dimethylrubrene

Single-crystalline organic semiconductors exhibiting band transport have opened new possibilities for the utilization of efficient charge carrier conduction in organic electronic devices. The epitaxial growth of molecular materials is a promising route for the realization of well-crystallized organic semiconductor p-n junctions for optoelectronic applications enhanced by the improved charge carrier mobility. In this study, the formation of a high-quality crystalline interface upon "quasi-homoepitaxial" growth of bis(trifluoromethyl)dimethylrubrene (fmRub) on the single-crystal surface of rubrene was revealed by using out-of-plane and grazing-incidence X-ray diffraction techniques. Ultraviolet photoelectron spectroscopy results indicated abrupt electronic energy levels and the occurrence of band bending across this quasi-homoepitaxial interface. This study verifies that the minimization of the lattice mismatch enhances the crystalline qualities at the heterojunctions even for van der Waals molecular condensates, potentially opening an untested route for the realization of high-mobility organic semiconductor optoelectronics.

Controlled Assembly of Conjugated Ladder Molecules with Different Bridging Structures toward Optoelectronic Application

Structural design of organic π-conjugated small molecules allows the energy band structure and electronic properties of the molecules to be tuned as needed, which provides a feasible strategy for enhancing the performance of optoelectronic devices. The introduction of bridging structures is a common structural modification method to adjust the rigidity and coplanarity of the molecular backbone, thus affecting the molecular packing. However, patterning of organic single-crystalline microstructures based on conjugated ladder molecules with different bridging structures still remains challenging for large-area integration of optoelectronic devices. In this paper, a controlled dewetting process is applied to obtain organic single-crystalline arrays with precise positioning and a regular morphology based on two isomers with silicon-oxygen bridging and their two carbon-oxygen-bridged analogues. Molecules with different bridging structures show disparate packing models due to the difference of dihedral angles and ring tensions. A microwire-array ultraviolet photodetector based on the oxygen-silicon-bridging ladder molecule exhibits a high light on/off ratio of 24 and a responsivity of 0.63 mA W^-1 owing to the effective π-π stacking governed by the molecular planarity. This work not only provides a universal method for the integration of organic optoelectronic devices but also explains the effect of bridging structure engineering on molecular assembly and optoelectronic performance.