Organic (opto)electronic materials have received considerable attention due to their applications in thin-film-transistors, light-emitting diodes, solar cells, sensors, photorefractive devices, and many others. The technological promises include low cost of these materials and the possibility of their room-temperature deposition from solution on large-area and/or flexible substrates. The article reviews the current understanding of the physical mechanisms that determine the (opto)electronic properties of high-performance organic materials. The focus of the review is on photoinduced processes and on electronic properties important for optoelectronic applications relying on charge carrier photogeneration. Additionally, it highlights the capabilities of various experimental techniques for characterization of these materials, summarizes top-of-the-line device performance, and outlines recent trends in the further development of the field. The properties of materials based both on small molecules and on conjug...

Organic Optoelectronic Materials: Mechanisms and Applications

Conjugated polymers and molecular semiconductors are emerging as a viable semiconductor technology in industries such as displays, electronics, renewable energy, sensing and healthcare. A key enabling factor has been significant scientific progress in improving their charge transport properties and carrier mobilities, which has been made possible by a better understanding of the molecular structure–property relationships and the underpinning charge transport physics. Here we aim to present a coherent review of how we understand charge transport in these high-mobility van der Waals bonded semiconductors. Specific questions of interest include estimates for intrinsic limits to the carrier mobilities that might ultimately be achievable; a discussion of the coupling between charge and structural dynamics; the importance of molecular conformations and mesoscale structural features; how the transport physics of conjugated polymers and small molecule semiconductors are related; and how the incorporation of counterions in doped films—as used, for example, in bioelectronics and thermoelectric devices—affects the electronic structure and charge transport properties. Organic semiconductors are making their way into applications ranging from display technology to flexible electronics and biomedical applications. This Review discusses current understanding of charge carrier transport in these materials and strategies to improve their performance.

Charge transport in high-mobility conjugated polymers and molecular semiconductors

The nanosegregated structures of columnar, smectic and bicontinuous cubic liquid crystals can provide well-organized, nano- and sub-nanosized 1D, 2D and 3D channels capable of ion and electron transport. The molecular shape, intermolecular interactions and nanosegregation of the molecular structures can influence their self-assembly into a range of functional liquid-crystalline nanostructures. The formation of stable and soft liquid-crystalline materials leads to their application as electrolytes for batteries and photovoltaics, semiconductors, electroluminescence and electrochemical devices. In addition, electrochemical devices are obtained by using redox-active liquid crystals. In this Review, we focus on the design of liquid-crystalline phases, the resultant self-assembled structures, the transport mechanisms, and the fabrication, function and future development of devices incorporating nanostructured liquid crystals. Liquid-crystalline nanostructures can form well-organized 1D, 2D and 3D channels capable of transporting ions or electrons. In this Review, the design of liquid-crystalline phases, their self-assembled structures, and the fabrication and function of devices incorporating them are described.

Transport of ions and electrons in nanostructured liquid crystals

Crystalline thin films of organic semiconductors are a good candidate for field effect transistor (FET) materials in printed electronics. However, there are currently two main problems, which are associated with inhomogeneity and poor thermal durability of these films. Here we report that liquid crystalline materials exhibiting a highly ordered liquid crystal phase of smectic E (SmE) can solve both these problems. We design a SmE liquid crystalline material, 2-decyl-7-phenyl-[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-10), for FETs and synthesize it. This material provides uniform and molecularly flat polycrystalline thin films reproducibly when SmE precursor thin films are crystallized, and also exhibits high durability of films up to 200 °C. In addition, the mobility of FETs is dramatically enhanced by about one order of magnitude (over 10 cm2 V−1 s−1) after thermal annealing at 120 °C in bottom-gate-bottom-contact FETs. We anticipate the use of SmE liquid crystals in solution-processed FETs may help overcome upcoming difficulties with novel technologies for printed electronics.

液晶物質の有機トランジスタへの展開 ; Liquid Crystals for Organic Thin Film Transistors

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

2D Organic Materials for Optoelectronic Applications

Polycrystalline thin films of small molecules hold promise for organic thin-film transistors because of their large charge mobility, but are currently limited by poor film homogeneity and thermal durability. Here, Iino et al. design an ordered liquid crystal phase to overcome these two problems.

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Liquid crystals for organic thin-film transistors

A Windmill-Shaped Discotic Columnar Liquid Crystal with Fast Ambipolar Charge Carrier Transport

A new organic semiconductor material with both a self-organized molecular alignment in smectic liquid crystals and a solid nature in amorphous materials, i.e., a glassy liquid crystalline organic semiconductor has been proposed, and its model compound, 1,3,5-benzenetricarboxylic acid, tris{12-[6′-(4′′-octylphenyl)-2′-naphthyloxy]-1-dodecyl ester} (LCG-triester 8-PNP-O12), was synthesized, and we characterized its phase transition behaviors in slow cooling and rapid cooling. It exhibited smectic A (SmA) and smectic B (SmBhex) phases in slow cooling, and a SmBhex glassy phase with a high Tg close to 60 °C when cooled rapidly from the SmA and SmBhex phases. Its charge carrier transport properties in the smectic mesophases, including the liquid crystal glassy phase, were first studied by the time-of-flight (TOF) technique, which revealed that the hole mobility in the SmBhex glassy phase was about 3.5 × 10−4 cm2 v−1s−1, comparable to that in the SmBhex liquid crystal phase. We concluded that the high charge transport properties of the smectic mesophase are successfully maintained in the solid glassy state.

Synthesis of a novel smectic liquid crystalline glass and characterization of its charge carrier transport properties

Self-organizing n-type hexaazatrinaphthylenes (HATNAs) with various bay-located side chains have been synthesized. The HATNA derivatives are able to form long-range molecular columns with self-directed growth directions. In particular, alkyl-substituted HATNAs showed in-plane molecular columns with axes parallel to substrates, whereas the columnar orientation of the HATNAs with alkylethynyl or alkylthio groups strongly depended on the length of the introduced side chains. Interestingly, the derivative with octylthio chains exhibited out-of-plane molecular columns, in which electron mobility of up to 10(-3)  cm(2)  V(-1)  s(-1) was determined through the time-of-flight technique, highlighting the fact that such molecular columns based on bay-substituted HATNAs are promising n-type semiconductors for device applications.

Takayuki Usui

Papers

Liquid crystals for organic thin-film transistors

液晶物質の有機トランジスタへの展開 ; Liquid Crystals for Organic Thin Film Transistors

A Windmill-Shaped Discotic Columnar Liquid Crystal with Fast Ambipolar Charge Carrier Transport

Synthesis of a novel smectic liquid crystalline glass and characterization of its charge carrier transport properties

Self‐Directed Orientation of Molecular Columns Based on n‐Type Hexaazatrinaphthylenes (HATNAs) for Electron Transport