Since the discovery of highly conducting polyacetylene by Shirakawa, MacDiarmid, and Heeger in 1977, π-conjugated systems have attracted much attention as futuristic materials for the development and production of the next generation of electronics, that is, organic electronics. Conceptually, organic electronics are quite different from conventional inorganic solid state electronics because the structural versatility of organic semiconductors allows for the incorporation of functionality by molecular design. This versatility leads to a new era in the design of electronic devices. To date, the great number of π-conjugated semiconducting materials that have either been discovered or synthesized generate an exciting library of π-conjugated systems for use in organic electronics. 11 However, some key challenges for further advancement remain: the low mobility and stability of organic semiconductors, the lack of knowledge regarding structure property relationships for understanding the fundamental chemical aspects behind the structural design, and realization of desired properties. Organic field-effect transistors (OFETs) are a kind of device consisting of an organic semiconducting layer, a gate insulator layer, and three terminals (drain, source, and gate electrodes). OFETs are not only essential building blocks for the next generation of cheap and flexible organic circuits, but they also provide an important insight into the charge transport of πconjugated systems. Therefore, they act as strong tools for the exploration of the structure property relationships of πconjugated systems, such as parameters of field-effect mobility (μ, the drift velocity of carriers under unit electric field), current on/off ratio (the ratio of the maximum on-state current to the minimum off-state current), and threshold voltage (the minimum gate voltage that is required to turn on the transistor). 17 Since the discovery of OFETs in the 1980s, they have attracted much attention. Research onOFETs includes the discovery, design, and synthesis of π-conjugated systems for OFETs, device optimization, development of applications in radio frequency identification (RFID) tags, flexible displays, electronic papers, sensors, and so forth. It is beyond the scope of this review to cover all aspects of π-conjugated systems; hence, our focus will be on the performance analysis of π-conjugated systems in OFETs. This should make it possible to extract information regarding the fundamental merit of semiconducting π-conjugated materials and capture what is needed for newmaterials and what is the synthesis orientation of newπ-conjugated systems. In fact, for a new science with many practical applications, the field of organic electronics is progressing extremely rapidly. For example, using “organic field effect transistor” or “organic field effect transistors” as the query keywords to search the Web of Science citation database, it is possible to show the distribution of papers over recent years as shown in Figure 1A. It is very clear

Semiconducting π-conjugated systems in field-effect transistors: a material odyssey of organic electronics.

A review was presented to demonstrate a historical description of the synthesis of light-emitting conjugated polymers for applications in electroluminescent devices. Electroluminescence (EL) was first reported in poly(para-phenylene vinylene) (PPV) in 1990 and researchers continued to make significant efforts to develop conjugated materials as the active units in light-emitting devices (LED) to be used in display applications. Conjugated oligomers were used as luminescent materials and as models for conjugated polymers in the review. Oligomers were used to demonstrate a structure and property relationship to determine a key polymer property or to demonstrate a technique that was to be applied to polymers. The review focused on demonstrating the way polymer structures were made and the way their properties were controlled by intelligent and rational and synthetic design.

Synthesis of Light-Emitting Conjugated Polymers for Applications in Electroluminescent Devices

Organic materials that exhibit thermally activated delayed fluorescence (TADF) are an attractive class of functional materials that have witnessed a booming development in recent years. Since Adachi et al. reported high-performance TADF-OLED devices in 2012, there have been many reports regarding the design and synthesis of new TADF luminogens, which have various molecular structures and are used for different applications. In this review, we summarize and discuss the latest progress concerning this rapidly developing research field, in which the majority of the reported TADF systems are discussed, along with their derived structure–property relationships, TADF mechanisms and applications. We hope that such a review provides a clear outlook of these novel functional materials for a broad range of scientists within different disciplinary areas and attracts more researchers to devote themselves to this interesting research field.

Recent advances in organic thermally activated delayed fluorescence materials.

In the past few years, spiropyran has emerged as the molecule-of-choice for the construction of novel dynamic materials. This unique molecular switch undergoes structural isomerisation in response to a variety of orthogonal stimuli, e.g. light, temperature, metal ions, redox potential, and mechanical stress. Incorporation of this switch onto macromolecular supports or inorganic scaffolds allows for the creation of robust dynamic materials. This review discusses the synthesis, switching conditions, and use of dynamic materials in which spiropyran has been attached to the surfaces of polymers, biomacromolecules, inorganic nanoparticles, as well as solid surfaces. The resulting materials show fascinating properties whereby the state of the switch intimately affects a multitude of useful properties of the support. The utility of the spiropyran switch will undoubtedly endow these materials with far-reaching applications in the near future.

/pdf/spiropyran-based-dynamic-materials-4c1onvrhup.pdf

Spiropyran-based dynamic materials

The bulk-heterojunction blend of an electron donor and an electron acceptor material is the key component in a solution-processed organic photovoltaic device. In the past decades, a p-type conjugated polymer and an n-type fullerene derivative have been the most commonly used electron donor and electron acceptor, respectively. While most advances of the device performance come from the design of new polymer donors, fullerene derivatives have almost been exclusively used as electron acceptors in organic photovoltaics. Recently, nonfullerene acceptor materials, particularly small molecules and oligomers, have emerged as a promising alternative to replace fullerene derivatives. Compared to fullerenes, these new acceptors are generally synthesized from diversified, low-cost routes based on building block materials with extraordinary chemical, thermal, and photostability. The facile functionalization of these molecules affords excellent tunability to their optoelectronic and electrochemical properties. Within t...

Nonfullerene Acceptor Molecules for Bulk Heterojunction Organic Solar Cells

Recent progress in second-order nonlinear optical polymers and dendrimers

Hybrid polymer–inorganic nanocrystal (NC) solar cells consisting of a hole-conducting conjugated polymer and inorganic semiconducting quantum dots, like cadmium selenide (CdSe), lead sulfi de (PbS) and lead selenide (PbSe), have attracted considerable research attention, since the advantages of two classes of materials can be effectively combined. Some of the potential advantages offered by this hybrid approach include: a) low fabrication costs due to easy solution-based processing; b) high optical absorbance and fl exible substrate manufacturing of the conjugated polymer matrix; and c) high electron conductance, tunable optical bandgap, and carrier multiplication due to the semiconducting NCs. [ 1–14 ] The performance of these hybrid devices depends on energy band matching, blend fi lm morphology, composition ratio, and NC shape, etc . [ 15–20 ] In addition, it is crucial to control interfacial contact at nanometer length scales in hybrid blend fi lms to ensure effi cient exciton generation and charge transfer. Generally, it is desirable to perform ligand exchange on the as-prepared NCs since they typically are shielded by long alkyl chain ligands such as trioctylphosphine oxide (TOPO) and oleic acid (OA) that act as insulating layers. For example, ligand exchange with shorter ligands like pyridine and butylamine have been shown to result in better charge photogeneration and effi cient charge transport. [ 16 , 19 , 21 , 22 ] Such pre-fabrication ligand exchange processing requires that good dispersibility of the NCs is maintained for fi lm fabrication. This method has been successfully applied to CdSe/poly(3-hexyl thiophene) (P3HT) hybrid devices to show signifi cantly improved performance. [ 15 ] Recently, the use of a low-bandgap polymer with CdSe NCs resulted in power conversion effi ciency of 3.2% with a broad absorption spectrum from UV to ∼ 800 nm. [ 23 ] To achieve better light harvesting including nearand mid-infrared (IR) wavelengths, PbS and PbSe NCs have been investigated in hybrid polymer solar cells. [ 24–28 ] While these materials will require appropriate encapsulation and proper disposal to avoid environmental contamination, they represent the best choice

Efficient Heterojunction Photovoltaic Cell Utilizing Nanocomposites of Lead Sulfide Nanocrystals and a Low‐Bandgap Polymer

Thermally activated delayed fluorescence (TADF) materials have attracted much attention in the field of organic light-emitting diodes (OLEDs), with their state-of-the-art performance in terms of external quantum efficiencies (EQEs), turn-on voltages, and color coordinates. TADF materials exhibited EQEs above 25% due to harvesting both singlet and triplet excitons via reverse intersystem crossing (RISC). A small singlet–triplet energy gap (ΔEST) is essential for TADF materials to exhibit efficient upconversion from the lowest triplet excited state (T1) to the lowest singlet excited state (S1, T1 → S1). Moreover, these materials are purely organic and thus not costly. Therefore, the TADF approach provides the best alternative to conventional fluorescent and phosphorescent OLEDs, regarding device efficiency and cost. On the other hand, blue light-emitting devices are facing several issues related to their stability and efficiency, making their development quite challenging for researchers. Herein, we review the recent advances in the use of blue TADF dopants and hosts in OLEDs.

https://pubs.rsc.org/en/content/articlepdf/2017/tc/c6tc90213h

Thermally activated delayed fluorescence blue dopants and hosts: from the design strategy to organic light-emitting diode applications

Thermally activated delayed fluorescence (TADF) materials have attracted significant attention owing to their state-of-the-art performance in the field of organic light-emitting diodes (OLEDs). TADF materials have the capability of harvesting both singlet and triplet excitons through reverse intersystem crossing, thus, the maximum internal quantum efficiency can be expected to be 100%. Thanks to TADF materials that are developed from pure organic entities so that they are inexpensive and external quantum efficiencies as high as 38% have been achieved to date. Despite the high performance of doped OLEDs, precise control of the host–guest ratio and phase separation at high driving voltages are the foremost challenges. In order to circumvent these demerits, the use of numerous small molecules, dendritic molecules, and polymers as non-doped emitters has been reported. Non-doped OLEDs consist of only a single molecule emitter in the emissive layer, thus simplifying the device fabrication process. Furthermore, non-doped OLEDs exhibit better device stability compared to doped devices, because the former do not exhibit phase separation or crystallinity and maintain a homogeneous surface film morphology during device operation. As a result, non-doped OLEDs are capable of overcoming the most pivotal issues of device complexity and stability, which makes them effective in rendering OLEDs commercially viable. Although maximum external quantum efficiencies of nearly 23.0% in blue, 22.0% in green, 22.0% in yellow, 17.0% in orange, and 10.0% in red emitting materials have been realized with non-doped OLEDs, still further progress is essential to reach the doped devices. This review systematically describes the molecular design, photophysical properties, and electroluminescence performance data of non-doped TADF emitters. We strongly believe that this review will be beneficial in terms of presenting a specific direction for the design of a variety of non-doped TADF emitters in the near future.

Recent breakthroughs in thermally activated delayed fluorescence organic light emitting diodes containing non-doped emitting layers

Charge-transport phenomena and photoinduced charge generation of organic semiconductors have potential applications in many electronic and optoelectronic devices. Organic fi eld-effect transistors (OFETs) are the most promising electronic devices fabricated using either well-defi ned single crystals (SCs) or thin fi lms as the charge-transporting layers. [ 1–3 ] In particular, soluble organic semiconductors are attracting attention for use in the fabrication of low-cost, large-scale, and practical devices. [ 4 , 5 ]

Min Ju Cho

Papers

Recent progress in second-order nonlinear optical polymers and dendrimers

Efficient Heterojunction Photovoltaic Cell Utilizing Nanocomposites of Lead Sulfide Nanocrystals and a Low‐Bandgap Polymer

Thermally activated delayed fluorescence blue dopants and hosts: from the design strategy to organic light-emitting diode applications

Recent breakthroughs in thermally activated delayed fluorescence organic light emitting diodes containing non-doped emitting layers

Highly Photosensitive J‐Aggregated Single‐Crystalline Organic Transistors