Anisotropic organic glasses

While the last decades have seen considerable efforts to control molecular packing in organic crystals, the idea of controlling packing in organic glasses is relatively unexplored. Glasses have many advantageous properties that crystals lack, such as macroscopic homogeneity and compositional flexibility, but packing in organic glasses is generally considered to be isotropic and highly disordered. Here we review and compare four areas of recent research activity showing control over anisotropic packing in organic glasses: (1) anisotropic glasses of low molecular weight organic semiconductors prepared by physical vapor deposition, (2) the use of mesogens to produce anisotropic glasses by cooling equilibrium liquid crystal phases, (3) the preparation of highly anisotropic glassy solids by vapor-depositing low molecular weight mesogens, and (4) anisotropic films of polymeric semiconductors prepared by spin-coating or solution casting. We delineate the connections between these areas with the hope of cross-fertilizing progress in the development of anisotropic glassy materials.     

Liquid-crystalline semiconducting polymers with high charge-carrier mobility

Organic semiconductors that can be fabricated by simple processing techniques and possess excellent electrical performance, are key requirements in the progress of organic electronics. Both high semiconductor charge-carrier mobility, optimized through understanding and control of the semiconductor microstructure, and stability of the semiconductor to ambient electrochemical oxidative processes are required. We report on new semiconducting liquid-crystalline thieno[3,2-b ]thiophene polymers, the enhancement in charge-carrier mobility achieved through highly organized morphology from processing in the mesophase, and the effects of exposure to both ambient and low-humidity air on the performance of transistor devices. Relatively large crystalline domain sizes on the length scale of lithographically accessible channel lengths ( approximately 200 nm) were exhibited in thin films, thus offering the potential for fabrication of single-crystal polymer transistors. Good transistor stability under static storage and operation in a low-humidity air environment was demonstrated, with charge-carrier field-effect mobilities of 0.2-0.6 cm(2) V(-1) s(-1) achieved under nitrogen.     

25th Anniversary Article: Recent Advances in n‐Type and Ambipolar Organic Field‐Effect Transistors

The advantages of organic field‐effect transistors, such as low cost, mechanical flexibility and large‐area fabrication, make them potentially useful for electronic applications such as flexible switching backplanes for video displays, radio frequency identifications and so on. A large amount of molecules were designed and synthesized for electron transporting (n‐type) and ambipolar organic semiconductors with improved performance and stability. In this review, we focus on the advances in performance and molecular design of n‐type and ambipolar semiconductors reported in the past few years.     

N-type doping and thermoelectric properties of co-sublimed cesium-carbonate-doped fullerene

Organic semiconductors exhibit a large Seebeck coefficient and a poor thermal conductivity allowing them to become strong candidates for thermoelectric applications. These materials have been widely used in organic electronics with the fabrication of organic light-emitting diodes, organic solar cells, and transistors. However, few studies have reported on thermoelectric properties of organic materials even though they offer specific advantages such as cost-effectiveness and flexibility. In this article, we discuss the fabrication and characterization of fullerene C60 doped with cesium carbonate (Cs₂CO₃). The evolution of the morphology, electrical conductivity, and Seebeck coefficient was analyzed as a function of the dopant concentration. An optimal power factor of 28.8 μWm^?1 K^?2 was obtained at room temperature for a molar ratio of 15.2 %. Thus far, this power factor value constitutes the best thermoelectric performance achieved with N-type organic materials.     

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%.     

Poly(α-methyl styrene) polymer additive for organic thin film transistors

While great progress has been achieved in the research of various solution-processed organic semiconductors, the randomized crystal orientations and charge carrier mobility variations have posed tremendous challenges to implement the organic semiconductors for organic electronic device applications. Among the miscellaneous polymer additives reported to tune the crystal growth and modulate charge transport, poly(α-methyl styrene) (PαMS) has been extensively studied for its capability to improve semiconductor crystallization, reduce bulk crystal misorientation, induce phase segregation, enhance morphological uniformity and boost electrical performance of organic thin film transistors and organic electronic devices. In the first section of this article, we review the recent progress of organic electronics and highlight the crystal misorientation and mobility variation as the challenges that need to be overcome. Then, the various merits from mixing polymeric additives with organic semiconductors are discussed. In the second section, we provide an overview of the previous works that employ PαMS for regulating the crystal orientation alignment and modulating charge transport of miscellaneous solution-processed small-molecular organic semiconductors including 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) and 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene (diF-TESADT). By discussing these important examples, we intend to demonstrate that PαMS can be versatilely implemented to improve other new organic semiconductor crystallization and mobility for high-performance organic electronic applications.

     

Ordered materials for organic electronics and photonics

We present a critical review of semiconducting/light emitting, liquid crystalline materials and their use in electronic and photonic devices such as transistors, photovoltaics, OLEDs and lasers. We report that annealing from the mesophase improves the order and packing of organic semiconductors to produce state-of-the-art transistors. We discuss theoretical models which predict how charge transport and light emission is affected by the liquid crystalline phase. Organic photovoltaics and OLEDs require optimization of both charge transport and optical properties and we identify the various trade-offs involved for ordered materials. We report the crosslinking of reactive mesogens to give pixellated full-colour OLEDs and distributed bi-layer photovoltaics. We show how the molecular organization inherent to the mesophase can control the polarization of light-emitting devices and the gain in organic, thin-film lasers and can also provide distributed feedback in chiral nematic mirrorless lasers. We update progress on the surface alignment of liquid crystalline semiconductors to obtain monodomain devices without defects or devices with spatially varying properties. Finally the significance of all of these developments is assessed.     

Perspectives and challenges for organic thin film transistors: materials, devices, processes and applications

This paper reviews recent advancements in the field of organic electronics. Performance of p- and n-type conducting polymers and small molecule organic semiconductors is presented primarily in terms of mobility and current on/off ratio. Moreover, it presents a deep insight into different organic/inorganic materials used for the dielectric layer, electrodes and substrate for thin film transistors (TFTs). The electrical characteristics and performance parameters of single and dual gate structures are compared. In addition, performance dependence of organic TFT (OTFT) is discussed on the basis of contact resistance, channel length and thickness of the active layer. The paper thoroughly discusses several important applications of OTFTs including inverter, organic static random access memory, radio frequency identification tag and DNA sensors. It also includes several limitations and future prospects of organic electronics technology.     

Multifunctional Organic‐Semiconductor Interfacial Layers for Solution‐Processed Oxide‐Semiconductor Thin‐Film Transistor

The stabilization and control of the electrical properties in solution‐processed amorphous‐oxide semiconductors (AOSs) is crucial for the realization of cost‐effective, high‐performance, large‐area electronics. In particular, impurity diffusion, electrical instability, and the lack of a general substitutional doping strategy for the active layer hinder the industrial implementation of copper electrodes and the fine tuning of the electrical parameters of AOS‐based thin‐film transistors (TFTs). In this study, the authors employ a multifunctional organic‐semiconductor (OSC) interlayer as a solution‐processed thin‐film passivation layer and a charge‐transfer dopant. As an electrically active impurity blocking layer, the OSC interlayer enhances the electrical stability of AOS TFTs by suppressing the adsorption of environmental gas species and copper‐ion diffusion. Moreover, charge transfer between the organic interlayer and the AOS allows the fine tuning of the electrical properties and the passivation of the electrical defects in the AOS TFTs. The development of a multifunctional solution‐processed organic interlayer enables the production of low‐cost, high‐performance oxide semiconductor‐based circuits.     

Selectivity of Threefold Symmetry in Epitaxial Alignment of Liquid Crystal Molecules on Macroscale Single‐Crystal Graphene

Epitaxial alignment of organic liquid crystal (LC) molecules on single‐crystal graphene (SCG), an effective epitaxial molecular assembly template, can be used in alignment‐layer‐free liquid crystal displays. However, selectivity among the threefold symmetric easy axes of LCs on graphene is not well understood, which limits its application. Here, sixfold symmetric radial LC domains are demonstrated by dropping an LC droplet on clean SCG, which reveals that the graphene surface does not have an intrinsic preferential direction. Instead, the first contact geometry of the LC molecules determines the direction. Despite its strong anchoring energy on graphene, the LC alignment direction is readily erasable and rewritable, contrary to previous understanding. In addition, the quality of the threefold symmetric alignment is sensitive to alien residue and graphene imperfections, which can be used to detect infinitesimal impurities or structural defects on the graphene. Based on this unique epitaxial behavior of LCs on SCG, an alignment‐layer‐free electro‐optical LC device and LC alignment duplication, which can result in practical graphene‐based flexible LC devices, are realized.     

Stability of perovskite materials and devices

Due to the excellent optoelectronic properties, organic–inorganic perovskites have drawn much attention and have been applied in different electronics with remarkable performance. However, the poor stability creates a massive barrier for the commercialization of perovskite electronic devices. In this review, we discuss intrinsic and extrinsic factors causing instabilities of perovskites and perovskite devices such as solar cells, liquid crystal displays (LCDs), light emitting diodes (LEDs), ionizing radiation detectors, transistors, memristors and sensors. We further review the stabilization approaches, including composition engineering, adoption of lower dimensional compositions, quantum dots, interface engineering, defects engineering and so on.     

Organic materials for printed electronics

Organic materials can offer a low-cost alternative for printed electronics and flexible displays. However, research in these systems must exploit the differences - via molecular-level control of functionality - compared with inorganic electronics if they are to become commercially viable.     

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.     

Flexible Electronics Based on Organic Semiconductors: from Patterned Assembly to Integrated Applications

Organic flexible electronic devices are at the forefront of the electronics as they possess the potential to bring about a major lifestyle revolution owing to outstanding properties of organic semiconductors, including solution processability, lightweight and flexibility. For the integration of organic flexible electronics, the precise patterning and ordered assembly of organic semiconductors have attracted wide attention and gained rapid developments, which not only reduces the charge crosstalk between adjacent devices, but also enhances device uniformity and reproducibility. This review focuses on recent advances in the design, patterned assembly of organic semiconductors, and flexible electronic devices, especially for flexible organic field-effect transistors (FOFETs) and their multifunctional applications. First, typical organic semiconductor materials and material design methods are introduced. Based on these organic materials with not only superior mechanical properties but also high carrier mobility, patterned assembly strategies on flexible substrates, including one-step and two-step approaches are discussed. Advanced applications of flexible electronic devices based on organic semiconductor patterns are then highlighted. Finally, future challenges and possible directions in the field to motivate the development of the next generation of flexible electronics are proposed.     

Highly Crystalline C8‐BTBT Thin‐Film Transistors by Lateral Homo‐Epitaxial Growth on Printed Templates

Highly crystalline thin films of organic semiconductors offer great potential for fundamental material studies as well as for realizing high‐performance, low‐cost flexible electronics. The fabrication of these films directly on inert substrates is typically done by meniscus‐guided coating techniques. The resulting layers show morphological defects that hinder charge transport and induce large device‐to‐device variability. Here, a double‐step method for organic semiconductor layers combining a solution‐processed templating layer and a lateral homo‐epitaxial growth by a thermal evaporation step is reported. The epitaxial regrowth repairs most of the morphological defects inherent to meniscus‐guided coatings. The resulting film is highly crystalline and features a mobility increased by a factor of three and a relative spread in device characteristics improved by almost half an order of magnitude. This method is easily adaptable to other coating techniques and offers a route toward the fabrication of high‐performance, large‐area electronics based on highly crystalline thin films of organic semiconductors.     

Planar Anchoring of C70 Liquid Crystals Using a Covalent Organic Framework Template

The surface‐induced anchoring effect is a well‐developed technique to control the growth of liquid crystals (LCs). Nevertheless, a defined nanometer‐scale template has never been used to induce the anchored growth of LCs with molecular building units. Scanning tunneling microscopy results at the solid/liquid interface reveal that a 2D covalent organic framework (COF‐1) can offer an anchoring effect to template C₇₀ molecules into forming several LC mesophases, which cannot be obtained under other conditions. Through comparison with the C₆₀ system, a stepwise breakdown in ordering of C₇₀ LC is observed. The process is described in terms of the effects of molecular anisotropy on the epitaxial growth of molecular crystals. The results suggest that using a surface‐confined template to anchor the initial layer of LC molecules can be a modular and potentially broadly applicable approach for organizing molecular mesogens into LCs.     

Present status of amorphous In–Ga–Zn–O thin-film transistors

Abstract

The present status and recent research results on amorphous oxide semiconductors (AOSs) and their thin-film transistors (TFTs) are reviewed. AOSs represented by amorphous In–Ga–Zn–O (a-IGZO) are expected to be the channel material of TFTs in next-generation flat-panel displays because a-IGZO TFTs satisfy almost all the requirements for organic light-emitting-diode displays, large and fast liquid crystal and three-dimensional (3D) displays, which cannot be satisfied using conventional silicon and organic TFTs. The major insights of this review are summarized as follows. (i) Most device issues, such as uniformity, long-term stability against bias stress and TFT performance, are solved for a-IGZO TFTs. (ii) A sixth-generation (6G) process is demonstrated for 32″ and 37″ displays. (iii) An 8G sputtering apparatus and a sputtering target have been developed. (iv) The important effect of deep subgap states on illumination instability is revealed. (v) Illumination instability under negative bias has been intensively studied, and some mechanisms are proposed. (vi) Degradation mechanisms are classified into back-channel effects, the creation of traps at an interface and in the gate insulator, and the creation of donor states in annealed a-IGZO TFTs by the Joule heating; the creation of bulk defects should also be considered in the case of unannealed a-IGZO TFTs. (vii) Dense passivation layers improve the stability and photoresponse and are necessary for practical applications. (viii) Sufficient knowledge of electronic structures and electron transport in a-IGZO has been accumulated to construct device simulation models.     

Wearable Electronics Based on Stretchable Organic Semiconductors

Wearable electronics are attracting increasing interest due to the emerging Internet of Things (IoT). Compared to their inorganic counterparts, stretchable organic semiconductors (SOSs) are promising candidates for wearable electronics due to their excellent properties, including light weight, stretchability, dissolubility, compatibility with flexible substrates, easy tuning of electrical properties, low cost, and low temperature solution processability for large-area printing. Considerable efforts have been dedicated to the fabrication of SOS-based wearable electronics and their potential applications in various areas, including chemical sensors, organic light emitting diodes (OLEDs), organic photodiodes (OPDs), and organic photovoltaics (OPVs), have been demonstrated. In this review, some recent advances of SOS-based wearable electronics based on the classification by device functionality and potential applications are presented. In addition, a conclusion and potential challenges for further development of SOS-based wearable electronics are also discussed.     

Carbon-supported growth of cross-linked platinum nanowires by surfactant templating and their elecrochemical characterization

Platinum/carbon (Pt/C) composite materials were prepared by the hydrazine reduction of H2PtCl6 confined to a mixed surfactant lytropic liquid crystal (LC)/C mixture with varying amounts of water. The reaction at relatively low water contents successfully yielded cross-linked Pt nanowires with wire-widths of 2-5 nm. The novel Pt nanostructure is believed to be from poorly hydrated hexagonal domains formed together with layered domains by the phase separation of the precursory LC mixture in the presence of carbon. Electrochemical measurements using cyclic volutammetry and membrane electrode assemblies revealed that the cross-linked nanowired Pt/C composite exhibits fairly high electrocatalytic activity for oxygen reduction reaction, as well as a high performance as the cathode material for polymer electrolyte fuel cells.