Interchain Interactions in Organic π‐Conjugated Materials: Impact on Electronic Structure,Optical Response,and Charge Transport

The pioneering work of Heeger, MacDiarmid, and Shirakawa, rewarded by the 2000 Nobel Prize in Chemistry, has paved the way for the development of the fields of plastic electronics and photonics. Functional organic molecular materials and conjugated oligomers or polymers now allow the low‐cost fabrication of thin films for insertion into new generations of electronic and optoelectronic devices. The performance of these devices relies on the understanding and optimization of several complementary processes (see sketch). Our goal is to discuss, from a theoretical standpoint, the electronic structure characteristics and interfacial properties that are of importance in all these areas. The concept of interface should be taken here in the microscopic sense, i.e., molecular interactions among two or several chains/molecules (of the same or of a different nature). Specifically, we will address the impact of interchain interactions within an organic layer on the transport and optical properties. These issues will therefore be more directly related to transistor and light‐emitting diode applications; however, in all instances, the aspects related to interfacial charge or energy transfer processes will dictate the ultimate performance of a material in a given device.     

Patterning π‐Conjugated Polymers

π‐Conjugated polymers show promise as active materials in application areas such as microelectronics, electro‐optics, opto‐electronics, and photonics. A critical feature in this emerging technology is device fabrication and the reproducible deposition of active material. This review focuses on current trends in the spatial deposition of conjugated polymers.     

Stable Molecular Diodes Based on π–π Interactions of the Molecular Frontier Orbitals with Graphene Electrodes

In molecular electronics, it is important to control the strength of the molecule–electrode interaction to balance the trade‐off between electronic coupling strength and broadening of the molecular frontier orbitals: too strong coupling results in severe broadening of the molecular orbitals while the molecular orbitals cannot follow the changes in the Fermi levels under applied bias when the coupling is too weak. Here, a platform based on graphene bottom electrodes to which molecules can bind via π–π interactions is reported. These interactions are strong enough to induce electronic function (rectification) while minimizing broadening of the molecular frontier orbitals. Molecular tunnel junctions are fabricated based on self‐assembled monolayers (SAMs) of Fc(CH₂)₁₁X (Fc = ferrocenyl, X = NH₂, Br, or H) on graphene bottom electrodes contacted to eutectic alloy of gallium and indium top electrodes. The Fc units interact more strongly with graphene than the X units resulting in SAMs with the Fc at the bottom of the SAM. The molecular diodes perform well with rectification ratios of 30–40, and they are stable against bias stressing under ambient conditions. Thus, tunnel junctions based on graphene with π–π molecule–electrode coupling are promising platforms to fabricate stable and well‐performing molecular diodes.     

High‐Performance Long‐Term‐Stable Dopant‐Free Perovskite Solar Cells and Additive‐Free Organic Solar Cells by Employing Newly Designed Multirole π‐Conjugated Polymers

Perovskite solar cells (PSCs) and organic solar cells (OSCs) are promising renewable light‐harvesting technologies with high performance, but the utilization of hazardous dopants and high boiling additives is harmful to all forms of life and the environment. Herein, new multirole π‐conjugated polymers (P1–P3) are developed via a rational design approach through theoretical hindsight, further successfully subjecting them into dopant‐free PSCs as hole‐transporting materials and additive‐free OSCs as photoactive donors, respectively. Especially, P3‐based PSCs and OSCs not only show high power conversion efficiencies of 17.28% and 8.26%, but also display an excellent ambient stability up to 30 d (for PSCs only), owing to their inherent superior optoelectronic properties in their pristine form. Overall, the rational approach promises to support the development of environmentally and economically sustainable PSCs and OSCs.     

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.     

Strong,Conductive, Foldable Graphene Sheets by Sequential Ionic and π Bridging

The goal of this work is to develop an inexpensive low‐temperature process that provides polymer‐free, high‐strength, high‐toughness, electrically conducting sheets of reduced graphene oxide (rGO). To develop this process, we have evaluated the mechanical and electrical properties resulting from the application of an ionic bonding agent (Cr³⁺), a π–π bonding agent comprising pyrene end groups, and their combinations for enhancing the performance of rGO sheets. When only one bonding agent was used, the π–π bonding agent is much more effective than the ionic bonding agent for improving both the mechanical and electrical properties of rGO sheets. However, the successive application of ionic bonding and π–π bonding agents maximizes tensile strength, toughness, long‐term electrical stability in various corrosive solutions, and resistance to mechanical abuse and ultrasonic dissolution. Using a combination of ionic bonding and π–π bonding agents, high tensile strength (821 MPa), high toughness (20 MJ m^?3), and electrical conductivity (416 S cm^?1) were obtained, as well as remarkable retention of mechanical and electrical properties during ultrasonication and mechanical cycling by both sheet stretch and sheet folding, suggesting high potential for applications in aerospace and flexible electronics.     

Wafer‐Scale Fabrication of High‐Performance n‐Type Polymer Monolayer Transistors Using a Multi‐Level Self‐Assembly Strategy

Wafer‐scale fabrication of high‐performance uniform organic electronic materials is of great challenge and has rarely been realized before. Previous large‐scale fabrication methods always lead to different layer thickness and thereby poor film and device uniformity. Herein, the first demonstration of 4 in. wafer‐scale, uniform, and high‐performance n‐type polymer monolayer films is reported, enabled by controlling the multi‐level self‐assembly process of conjugated polymers in solution. Since the self‐assembly process happened in solution, the uniform 2D polymer monolayers can be facilely deposited on various substrates, and theoretically without size limitations. Polymer monolayer transistors exhibit high electron mobilities of up to 1.88 cm² V^?1 s^?1, which is among the highest in n‐type monolayer organic transistors. This method allows to easily fabricate n‐type conjugated polymers with wafer‐scale, high uniformity, low contact resistance, and excellent transistor performance (better than the traditional spin‐coating method). This work provides an effective strategy to prepare large‐scale and uniform 2D polymer monolayers, which could enable the application of conjugated polymers for wafer‐scale sophisticated electronics.     

Highly Selective Dispersion of Carbon Nanotubes by Using Poly(phenyleneethynylene)‐Guided Supermolecular Assembly

Isolation of single‐walled carbon nanotubes (SWNTs) with specific chirality and diameters is critical for achieving optimum performance of SWNTs in various applications. A water‐soluble π‐conjugated polymer, poly[(m‐phenyleneethynylene)‐alt‐(p‐phenyleneethynylene)], 3 , is found to exhibit high selectivity in dispersing SWNT (6,5). The polymer's ability to sort out SWNT (6,5) appears to be related to the carbon–carbon triple bond, whose free rotation allows a unique assembly of chromophores in a helical conformation. The observation is consistently supported by fluorescence, Raman, and UV‐vis‐NIR absorption spectra. The intriguing selectivity of 3 to SWNT (6,5), however, is not observed for the vinylene analogue polymer 1 , showing that the carbon–carbon triple bond could play a unique role in sorting out a specific SWNT. The observed selectivity from 3 could be attributed to a combination of the helical cavity size restrain and electronic interaction associated with the local chromophore arrangement. This strategy could be expanded for efficient SWNT sorting when the helical conformation is further finely tuned.     

Emergence of an Antiferromagnetic Mott Insulating Phase in Hexagonal π‐Conjugated Covalent Organic Frameworks

While the search for 2D organic semimetallic Dirac materials displaying, like graphene, a Dirac cone at the Fermi level remains active, attention is also being paid to the quantum phase transition from semimetal to antiferromagnet. Such a transition in graphene‐like materials is predicted based on theoretical investigations of the 2D honeycomb lattice; it occurs (within a Hubbard model) when the on‐site electron–electron Coulomb repulsion (U) is much larger than the nearest‐neighbor inter‐site electronic coupling (t). Here, monomers carrying long‐lived radicals are considered and used as building blocks to design 2D hexagonal π‐conjugated covalent organic frameworks (COFs). Both the nonmagnetic semimetallic phase and magnetically ordered phases are evaluated. It is found that the electronic coupling between adjacent radical centers in these COFs is more than an order of magnitude smaller than in graphene while the on‐site Coulomb repulsion is reduced to a lesser extent. The resulting large U/t ratio drives these COFs into the antiferromagnetic side of the phase diagram. This work provides a first theoretical evidence of the realization of an antiferromagnetic Mott insulating phase in 2D π‐conjugated COFs and allows a strategy to achieve quantum phase transitions from antiferromagnet to spin liquid and to semimetal to be outlined.     

Self‐Suspended Nanomesh Scaffold for Ultrafast Flexible Photodetectors Based on Organic Semiconducting Crystals

Self‐standing nanostructures are of fundamental interest in materials science and nanoscience and are widely used in (opto‐)electronic and photonic devices as well as in micro‐electromechanical systems. To date, large‐area and self‐standing nanoelectrode arrays assembled on flexible substrates have not been reported. Here the fabrication of a hollow nanomesh scaffold on glass and plastic substrates with a large surface area over 1 mm² and ultralow leakage current density (≈1–10 pA mm^?2 @ 2 V) across the empty scaffold is demonstrated. Thanks to the continuous sub‐micrometer space formed in between the nanomesh and the bottom electrode, highly crystalline and dendritic domains of 6,13‐bis(triisopropylsilylethinyl)pentacene growing within the hollow cavity can be observed. The high degree of order at the supramolecular level leads to efficient charge and exciton transport; the photovoltaic detector supported on flexible polyethylene terephthalate substrates exhibits an ultrafast photoresponse time as short as 8 ns and a signal‐to‐noise ratio approaching 10⁵. Such a hollow scaffold holds great potential as a novel device architecture toward flexible (opto‐)electronic applications based on self‐assembled micro/nanocrystals.     

From One‐ to Three‐Dimensional Organic Semiconductors: In Search of the Organic Silicon?

Organic semiconductors based on π‐conjugated systems are the focus of considerable interest in the emerging area of soft or flexible photonics and electronics. Whereas in recent years the performances of devices such as organic light‐emitting diodes (OLEDs), organic field‐effect transistors (OFETs), or solar cells have undergone considerable progress, a number of technical and fundamental problems related to the low dimensionality of organic semiconductors based on linear π‐conjugated systems remain unsatisfactorily resolved. This low dimensionality results in an anisotropy of the optical and charge‐transport properties, which in turn implies a control of the material organization/molecular orientation during or after device fabrication. Such a constraint evidently represents a problem when device fabrication by solution‐based processes, such as printing techniques, is envisioned. The aim of this short Review is to illustrate possible alternative strategies based on the development of organic semiconductors with higher dimensionality, capable to exhibit isotropic electronic properties.     

Synergistic Effect of Aligned Graphene Nanosheets in Graphene Foam for High‐Performance Thermally Conductive Composites

Graphene shows a great potential for high‐performance thermally conductive composite applications because of its extremely high thermal conductivity. However, the graphene‐based polymer composites reported so far only have a limited thermal conductivity, with the highest thermal conductivity enhancement (TCE) per 1 vol% graphene less than 900%. Here, a continuous network of graphene foam (GF), filled with aligned graphene nanosheets (GNs), is shown to be an ideal filler structure for thermally conductive composite materials. Compared to previous reports, a clear thermal percolation is observed at a low graphene loading fraction. The GNs/GF/natural rubber composite shows the highest TCE of 8100% (6.2 vol% graphene loading) ever reported at room temperature, which gives a record‐high TCE per 1 vol% graphene of 1300%. Further analyses reveal a significant synergistic effect between the aligned GNs and 3D interconnected GF, which plays a key role in the formation of a thermal percolation network to remarkably improve the thermal conductivity of the composites. Additionally, the use of this composite for efficient heat dissipation of light‐emitting diode (LED) lamps is demonstrated. These findings provide valuable guidance to design high‐performance graphene‐based thermally conductive materials, and open up the possibility for the use of graphene in high‐power electronic devices.     

Programmable Negative Differential Resistance Effects Based on Self‐Assembled Au@PPy Core–Shell Nanoparticle Arrays

The negative differential resistance (NDR) effect observed in conducting polymer/Au nanoparticle composite devices is not yet fully clarified due to the random and disordered incorporation of Au nanoparticles into conducting polymers. It remains a formidable challenge to achieve the sequential arrangement of various components in an optimal manner during the fabrication of Au nanoparticle/conducting polymer composite devices. Here, a novel strategy for fabricating Au nanoparticle/conducting polymer composite devices based on self‐assembled Au@PPy core–shell nanoparticle arrays is demonstrated. The interval between the two Au nanoparticles can be precisely programmed by modulating the thickness of the shell and the size of the core. Programmable NDR is achieved by regulating the spacer between two Au nanoparticles. In addition, the Au/conducting polymer composite device exhibits a reproducible memory effect with read–write–erase characteristics. The sequentially controllable assembly of Au@PPy core–shell nanoparticle arrays between two microelectrodes will simplify nanodevice fabrication and will provide a profound impact on the development of new approaches for Au/conducting polymer composite devices.     

Creating Graphitic Carbon Nitride Based Donor‐π–Acceptor‐π–Donor Structured Catalysts for Highly Photocatalytic Hydrogen Evolution

Conjugated polymers with tailored donor–acceptor units have recently attracted considerable attention in organic photovoltaic devices due to the controlled optical bandgap and retained favorable separation of charge carriers. Inspired by these advantages, an effective strategy is presented to solve the main obstructions of graphitic carbon nitride (g‐C₃N₄) photocatalyst for solar energy conversion, that is, inefficient visible light response and insufficient separation of photogenerated electrons and holes. Donor‐π–acceptor‐π–donor polymers are prepared by incorporating 4,4′‐(benzoc 1,2,5 thiadiazole‐4,7‐diyl) dianiline (BD) into the g‐C₃N₄ framework (UCN‐BD). Benefiting from the visible light band tail caused by the extended π conjugation, UCN‐BD possesses expanded visible light absorption range. More importantly, the BD monomer also acts as an electron acceptor, which endows UCN‐BD with a high degree of intramolecular charge transfer. With this unique molecular structure, the optimized UCN‐BD sample exhibits a superior performance for photocatalytic hydrogen evolution upon visible light illumination (3428 µmol h^?1 g^?1), which is nearly six times of that of the pristine g‐C₃N₄. In addition, the photocatalytic property remains stable for six cycles in 3 d. This work provides an insight into the synthesis of g‐C₃N₄‐based D‐π–A‐π–D systems with highly visible light response and long lifetime of intramolecular charge carriers for solar fuel production.     

Organische Funktionsschichten in Polymerelektronik und Polymersolarzellen

Organic functional layers in polymer electronics and polymer solar cells Thin layers of organic functional polymers play the predominant role in polymer electronics like organic field effect transistors (OFET's) and in organic photovoltaic devices. The well‐known advantages of these solution‐processable materials opened the way for their welcoming now in application fields, which were fully occupied by inorganic semiconductors in the past. However, the polymer semiconductors show also some disadvantages, like a relatively low charge carrier mobility and a not yet sufficient long‐term stability. However, fore the aim of R&D for polymer electronics is not the replace of well‐tried electronic materials and technologies but the opening of new application fields for the new kind of low‐cost /low‐performance electronics. The paper presents recent results of OFET's with thin layers from conjugated polymers like poly(3‐alkylthiophenes), as active semiconducting material, and poly(4‐vinylphenol) as gate dielectricum. Experiments concerning generation of source‐drain electrodes based on polyaniline or Baytron P by laser ablation are represented. Additionally, printing techniques or laser modification are used for patterning of conducting polymers. The described polymer solar cells use for the photoactive layer a composite from polyalkylthiophenes, as light absorbing and charge generating polymer, and fullerene derivatives, responsible for fast electron transfer. Donator‐acceptor cells containing substituted fullerenes give also internationally the best efficiency with η ≈ 3%.     

High Conductivity and Electron‐Transfer Validation in an n‐Type Fluoride‐Anion‐Doped Polymer for Thermoelectrics in Air

Air‐stable and soluble tetrabutylammonium fluoride (TBAF) is demonstrated as an efficient n‐type dopant for the conjugated polymer ClBDPPV. Electron transfer from F^? anions to the π‐electron‐deficient ClBDPPV through anion–π electronic interactions is strongly corroborated by the combined results of electron spin resonance, UV–vis–NIR, and ultraviolet photoelectron spectroscopy. Doping of ClBDPPV with 25 mol% TBAF boosts electrical conductivity to up to 0.62 S cm^?1, among the highest conductivities that have been reported for solution‐processed n‐type conjugated polymers, with a thermoelectric power factor of 0.63 µW m^?1 K^?2 in air. Importantly, the Seebeck coefficient agrees with recently published correlations to conductivity. Moreover, the F^?‐doped ClBDPPV shows significant air stability, maintaining the conductivity of over 0.1 S cm^?1 in a thick film after exposure to air for one week, to the best of our knowledge the first report of an air‐stable solution‐processable n‐doped conductive polymer with this level of conductivity. The result shows that using solution‐processable small‐anion salts such as TBAF as an n‐dopant of organic conjugated polymers possessing lower LUMO (lowest unoccupied molecular orbital), less than ?4.2 eV) can open new opportunities toward high‐performance air‐stable solution‐processable n‐type thermoelectric (TE) conjugated polymers.     

Direct Growth of High Mobility and Low‐Noise Lateral MoS2–Graphene Heterostructure Electronics

Reliable fabrication of lateral interfaces between conducting and semiconducting 2D materials is considered a major technological advancement for the next generation of highly packed all‐2D electronic circuitry. This study employs seed‐free consecutive chemical vapor deposition processes to synthesize high‐quality lateral MoS₂–graphene heterostructures and comprehensively investigated their electronic properties through a combination of various experimental techniques and theoretical modeling. These results show that the MoS₂–graphene devices exhibit an order of magnitude higher mobility and lower noise metrics compared to conventional MoS₂–metal devices as a result of energy band rearrangement and smaller Schottky barrier height at the contacts. These findings suggest that MoS₂–graphene in‐plane heterostructures are promising materials for the scale‐up of all‐2D circuitry with superlative electrical performance.     

1D/2D Carbon Nanomaterial‐Polymer Dielectric Composites with High Permittivity for Power Energy Storage Applications

With the development of flexible electronic devices and large‐scale energy storage technologies, functional polymer‐matrix nanocomposites with high permittivity (high‐k) are attracting more attention due to their ease of processing, flexibility, and low cost. The percolation effect is often used to explain the high‐k characteristic of polymer composites when the conducting functional fillers are dispersed into polymers, which gives the polymer composite excellent flexibility due to the very low loading of fillers. Carbon nanotubes (CNTs) and graphene nanosheets (GNs), as one‐dimensional (1D) and two‐dimensional (2D) carbon nanomaterials respectively, have great potential for realizing flexible high‐k dielectric nanocomposites. They are becoming more attractive for many fields, owing to their unique and excellent advantages. The progress in dielectric fields by using 1D/2D carbon nanomaterials as functional fillers in polymer composites is introduced, and the methods and mechanisms for improving dielectric properties, breakdown strength and energy storage density of their dielectric nanocomposites are examined. Achieving a uniform dispersion state of carbon nanomaterials and preventing the development of conductive networks in their polymer composites are the two main issues that still need to be solved in dielectric fields for power energy storage. Recent findings, current problems, and future perspectives are summarized.     

High‐Performance pH‐Switchable Supramolecular Thermosets via Cation–π Interactions

Supramolecular chemistry has provided versatile and affordable solutions for the design of intelligent soft materials, but it cannot be applied in stiff materials. This paper describes a new concept for the design of high‐performance supramolecular thermosets by using the noncovalent cation–π interaction as cross‐linking. These supramolecular thermosets are a class of infusible and insoluble stiff polymers having excellent mechanical properties even at temperatures exceeding 300 °C. The cation–π interaction can be locally and reversibly installed and removed by aqueous treatments at high or low pH, respectively. Local manipulation of cross‐linking confers these thermosets with multiple stimuli‐responsive functions, such as recyclability, healability, adhesion, and nondestructive detection of cross‐linking and mechanical properties.     

Solution Processed Metal Oxide High‐κ Dielectrics for Emerging Transistors and Circuits

The electronic functionalities of metal oxides comprise conductors, semiconductors, and insulators. Metal oxides have attracted great interest for construction of large‐area electronics, particularly thin‐film transistors (TFTs), for their high optical transparency, excellent chemical and thermal stability, and mechanical tolerance. High‐permittivity (κ) oxide dielectrics are a key component for achieving low‐voltage and high‐performance TFTs. With the expanding integration of complementary metal oxide semiconductor transistors, the replacement of SiO₂ with high‐κ oxide dielectrics has become urgently required, because their provided thicker layers suppress quantum mechanical tunneling. Toward low‐cost devices, tremendous efforts have been devoted to vacuum‐free, solution processable fabrication, such as spin coating, spray pyrolysis, and printing techniques. This review focuses on recent progress in solution processed high‐κ oxide dielectrics and their applications to emerging TFTs. First, the history, basics, theories, and leakage current mechanisms of high‐κ oxide dielectrics are presented, and the underlying mechanism for mobility enhancement over conventional SiO₂ is outlined. Recent achievements of solution‐processed high‐κ oxide materials and their applications in TFTs are summarized and traditional coating methods and emerging printing techniques are introduced. Finally, low temperature approaches, e.g., ecofriendly water‐induced, self‐combustion reaction, and energy‐assisted post treatments, for the realization of flexible electronics and circuits are discussed.