Porous Organic Field‐Effect Transistors for Enhanced Chemical Sensing Performances

The thin‐film structures of chemical sensors based on conventional organic field‐effect transistors (OFETs) can limit the sensitivity of the devices toward chemical vapors, because charge carriers in OFETs are usually concentrated within a few molecular layers at the bottom of the organic semiconductor (OSC) film near the dielectric/semiconductor interface. Chemical vapor molecules have to diffuse through the OSC films before they can interact with charge carriers in the OFET conduction channel. It has been demonstrated that OFET ammonia sensors with porous OSC films can be fabricated by a simple vacuum freeze‐drying template method. The resulted devices can have ammonia sensitivity not only much higher than the pristine OFETs with thin‐film structure but also better than any previously reported OFET sensors, to the best of our knowledge. The porous OFETs show a relative sensitivity as high as 340% ppm^?1 upon exposure to 10 parts per billion (ppb) NH₃. In addition, the devices also exhibit decent selectivity and stability. This general and simple strategy can be applied to a wide range of OFET chemical sensors to improve the device sensitivity.     

En Route to Wide Area Emitting Organic Light-Emitting Transistors for Intrinsic Drive-Integrated Display Applications: A Comprehensive Review

Organic light-emitting transistors (OLET) evolved from the fusion of the switching functionality of field-effect transistors (FET) with the light-emitting characteristics of organic light-emitting diode (OLED) that can simplify the active-matrix pixel device architecture and hence offer a promising pathway for future flat panel and flexible display technology. This review systematically analyzes the key device/molecular engineering tactics that assist in improving the electrode edge narrow emission to wide-area emission for display applications via three different topics, that is, narrow to wide-area emission, vertical architecture, and impact of high-κ dielectric on the device performance. Source–drain electrode engineering such as symmetric/asymmetric, planar/non-planar arrangement, semitransparent nature, multilayer approach comprising charge transport, and work function modification layers enable widening the emission zone. Vertical OLET architecture offers short channel lengths with a high aperture ratio, pixel type area emission, and stable light-emitting area. Transistors utilizing high-κ dielectric materials have assisted in lowering the operating voltage, enhancing luminance and air stability. The promising development in achieving wide-area emission provides a solid basis for constructing OLET research toward display applications; however, it relies on developing highly luminescent and fast charge transporting materials, suitable semitransparent source/drain electrodes, high-κ -dielectrics, and device architectural engineering.     

An Organic Electrochemical Transistor Integrated Photodetector for High Quality Photoplethysmogram Signal Acquisition

The organic photodiode (OPD) is a promising building block for solution-processable, flexible, lightweight, and miniaturized photodetectors, ideal for wearable applications. Despite the advances in materials used in OPDs, their photocurrent and light responsivity are limited, and alternative methods are required to boost the signal response. Herein, a miniaturized organic electrochemical transistor (OECT) is integrated with an OPD module to unlock the potential of OPDs to acquire physiological signals. In this integrated photodetector (IPD) system, the light intensity regulates the OPD voltage output that modulates the OECT channel current. The high transconductance of the OECT provides efficient voltage-to-current conversion, enhancing the signal-to-noise ratio on the sensing site. A microscale, p-type enhancement-mode OECT with high g_m and fast switching speed performs better in this application than depletion-mode OECT of the same geometry. The IPD achieves a photocurrent and responsivity 318 and 140 times higher than the standalone OPD, respectively. It is shown that with the IPD, the amplitude of the photoplethysmogram signals detected by the OPD is enhanced by a factor of 2.9 × 10³, highlighting its potential as a wearable biosensor and to detect weak, often uncaptured, light-based signals from living systems.     

有机发光器件的一种失效机制

制备了结构为铟锡氧化物(ITO)/NPB/插入层/Alq/LiF/Al的有机电致发光(EL)器件,测量了器件发光随电压变化的光谱和电压-电流-亮度特性,观察到这种结构器件在电压升高的过程中总是在某电压附近有一个光谱、亮度和效率等性能突变的不可逆过程,这是由于在发光区域附近的纳米薄层材料将导致电荷在该区域的局部聚集,并引起该薄层材料局部破坏。这一失效的机制表明,尽管在器件制备过程中可能需要在器件中使用几纳米厚的有机层,但是应当考虑尽量避免,以使器件内载流子分布合理,避免此类失效过程发生。     

Color-Tunable Organic Light-Emitting Displays for Interactive Multi-Signal Visualization

Optoelectronic devices integrated with multiple functions for interactive signal visualization have become increasingly important in the intelligent era. Here, a color-tunable organic light-emitting diode (CTOLED) is developed to achieve a user-interaction intelligent device that enables multicolor visualization of infrared light and temperature distribution via the modulation with a multi-stimuli responsive silicon (Si) sensor for the first time. The key to the color-tunable functionality of the CTOLED is an ultrathin exciton-blocking layer between different emitting layers, which leads to the quantitative correlation of driving current and emitted visible color. As a result, the user can manipulate the silicon/color-tunable organic light-emitting diode (Si/CTOLED) device through the infrared pen, and the “writing” spatial information and the infrared intensity can influence the display patterns and colors. Besides, the temperature is sensed by the flexible Si/CTOLED device and is displayed in corresponding different colors, which is promising as a wearable temperature visualization monitor and directly interact with a user. The temperature monitor exhibits ultrasensitivity with a low minimum distinguishable temperature difference of only 65 mK. The Si/CTOLED devices open new possibilities for the development of intelligent devices and have promising multi-disciplinary applications.     

有机电致发光器件寿命测试系统

介绍了一种有机发光器件（OLED）寿命自动检测系统，可同时对16路OLED进行寿命自动测试。高灵敏度的数字采集电路能侦测到低至0．3mV的电压变化。通过采用PC自动记录观测结果大大提高了数据采样的密度，可对材料和工艺的评测提供了更为精确的参考数据。系统通过用光电池取代辉度计降低了系统的成本。     

Polyelectrolyte Dielectrics for Flexible Low‐Voltage Organic Thin‐Film Transistors in Highly Sensitive Pressure Sensing

Organic thin‐film transistors (OTFTs) can provide an effective platform to develop flexible pressure sensors in wearable electronics due to their good signal amplification function. However, it is particularly difficult to realize OTFT‐based pressure sensors with both low‐voltage operation and high sensitivity. Here, controllable polyelectrolyte composites based on poly(ethylene glycol) (PEG) and polyacrylic acid (PAA) are developed as a type of high‐capacitance dielectrics for flexible OTFTs and ultrasensitive pressure sensors with sub‐1 V operation. Flexible OTFTs using the PAA:PEG dielectrics show good universality and greatly enhanced electrical performance under a much smaller operating voltage of ?0.7 V than those with a pristine PAA dielectric. The low‐voltage OTFTs also exhibit excellent flexibility and bending stability under various bending radii and long cycles. Flexible OTFT‐based pressure sensors with low‐voltage operation and superhigh sensitivity are demonstrated by using a suspended semiconductor/dielectric/gate structure in combination with the PAA:PEG dielectric. The sensors deliver a record high sensitivity of 452.7 kPa^?1 under a low‐voltage of ?0.7 V, and excellent operating stability over 5000 cycles. The OTFT sensors can be built into a wearable sensor array for spatial pressure mapping, which shows a bright potential in flexible electronics such as wearable devices and smart skins.     

Thermally Stable,Biocompatible, and Flexible Organic Field‐Effect Transistors and Their Application in Temperature Sensing Arrays for Artificial Skin

Application of degradable organic electronics based on biomaterials, such as polylactic‐co‐glycolic acid and polylactide (PLA), is severely limited by their low thermal stability. Here, a highly thermally stable organic transistor is demonstrated by applying a three‐arm stereocomplex PLA (tascPLA) as dielectric and substrate materials. The resulting flexible transistors are stable up to 200 °C, while devices based on traditional PLA are damaged at 100 °C. Furthermore, charge‐ trapping effect induced by polar groups of the dielectric is also utilized to significantly enhance the temperature sensitivity of the electronic devices. Skin‐like temperature sensor array is successfully demonstrated based on such transistors, which also exhibited good biocompatibility in cytotoxicity measurement. By presenting combined advantages of transparency, flexibility, thermal stability, temperature sensitivity, degradability, and biocompatibility, these organic transistors thus possess a broad applicability such as environment friendly electronics, implantable medical devices, and artificial skin.     

蓝光波段顶发射有机发光二极管

针对顶发射有机发光二极管(TEOLEDs)中的微腔效应会增加蓝光波段TEOLEDs的制作难度这一问题,提出利用高透明金属阴极并结合在阴极表面生长增透膜的方法来减小二极管阴极的反光性,从而抑制二极管中的微腔效应(这里主要是指多光束干涉);同时利用宽角干涉对器件结构进行设计来改善二极管的蓝光强度,制备了基于有机蓝光材料4,4'-bis(2,2'-diphenylvinyl)-1,1'-biphenyl的顶发射有机发光二极管;优化了增透膜的厚度,研究了增透膜对于二极管电光性能的影响;得到了性能(亮度、效率、色纯度等)可以与底发射有机发光二极管相比的蓝光波段TEOLEDs.     

Highly Sensitive Glucose Biosensors Based on Organic Electrochemical Transistors Using Platinum Gate Electrodes Modified with Enzyme and Nanomaterials

Organic electrochemical transistors with glucose oxidase‐modified Pt gate electrodes are successfully used as highly sensitive glucose sensors. The gate electrodes are modified with nanomaterials (multi‐wall carbon nanotubes or Pt nanoparticles) for the first time, which results in a dramatic improvement in the sensitivity of the devices. The detection limit of the device modified with Pt nanoparticles on the gate electrode is about 5 nM, which is three orders of magnitude better than a device without the nanoparticles. The improvement of the device performance can be attributed to the excellent electrocatalytic properties of the nanomaterials and more effective immobilization of enzyme on the gate electrodes. Based on the same principle, many other types of enzyme sensors with high sensitivity and low cost are expected to be realized by modifying the gate electrodes of organic electrochemical transistors with specific enzymes and nanomaterials.     

Precise Extraction of Charge Carrier Mobility for Organic Transistors

Unreliable mobility values, and particularly greatly overestimated values and severely distorted temperature dependences, have recently hampered the development of the organic transistor field. Given that organic field‐effect transistors (OFETs) have been routinely used to evaluate mobility, precise parameter extraction using the electrical properties of OFETs is thus of primary importance. This review examines the origins of the various mobilities that must be determined for OFET applications, the relevant extraction methods, and the data selection limitations, which help in avoiding conceptual errors during mobility extraction. For increased precision, the review also discusses device fabrication considerations, calibration of both the specific gate‐dielectric capacitance and the threshold voltage, the contact effects, and the bias and temperature dependences, which must actually be handled with great care but have mostly been overlooked to date. This review serves as a systematic overview of the OFET mobility extraction process to ensure high precision and will also aid in improving future research.     

High Mobility and Luminescent Efficiency in Organic Single‐Crystal Light‐Emitting Transistors

A high‐performance ambipolar light‐emitting transistor (LET) that has high hole and electron mobilities and excellent luminescence characteristics is described. By using this device, a conspicuous light‐confined edge emission and current‐density‐dependent spectral evolution are observed. These findings will result in broader utilization of device potential and they provide a promising route for realizing electrically driven organic lasers.     

双层有机电致发光器件有机层厚度优化的数值研究

在陷阱电荷限制电流传导理论的基础上,提出了双层有机电致发光器件的数值模型,研究了结构为"阳极/空穴输运层(HTL)/发光层(EML)/阴极"的器件中电流密度和量子效率随有机层的特征陷阱能量、陷阱密度和载流子迁移率的依赖关系. 研究发现,对于给定的HTL和EML的特征陷阱能量、陷阱密度和载流子迁移率,存在一个最优的HTL和EML之间的厚度比率,在此最优厚度比下,器件的电流密度和量子效率达到最大.通过有机层厚度的优化,器件的电流密度和量子效率可提高多达两个数量级.另外,还研究了最优厚度比随有机层特征陷阱能量、总陷阱密度和载流子迁移率之间的定量关系.     

Recent Advances in Structural Design of Efficient Near-Infrared Light-Emitting Organic Small Molecules

Organic light-emitting materials in the near-infrared (NIR) region are important to realize next-generation lightweight and wearable applications in bioimaging, photodynamic therapy, and telecommunications. Inorganic and organometallic light-emitting materials are expensive and toxic; thus, the development of purely organic light-emitting materials is essential. However, the development of highly efficient NIR light-emitting materials made of organic materials is still in its infancy. Therefore, this review outlines molecular design strategies for developing organic small-molecule NIR light-emitting materials with high emission efficiency that can overcome the energy-gap law to be applied to next-generation wearable devices. After briefly reviewing the basic knowledge required for the NIR emission of organic molecules, representative high-efficiency molecules reported over the past 5 years are classified according to their core moieties, and their molecular design, physical properties, and luminescence characteristics are analyzed. Further, the perspective and outlook regarding the development of next-generation high-efficiency NIR organic light-emitting materials are provided.     

Solution‐Processed Monolayer Organic Crystals for High‐Performance Field‐Effect Transistors and Ultrasensitive Gas Sensors

This work innovatively develops a dual solution‐shearing method utilizing the semiconductor concentration region close to the solubility limit, which successfully generates large‐area and high‐performance semiconductor monolayer crystals on the millimeter scale. The monolayer crystals with poly(methyl methacrylate) encapsulation show the highest mobility of 10.4 cm² V^?1 s^?1 among the mobility values in the reported solution‐processed semiconductor monolayers. With similar mobility to multilayer crystals, light is shed on the charge accumulation mechanism in organic field‐effect transistors (OFETs), where the first layer on interface bears the most carrier transport task, and the other above layers work as carrier suppliers and encapsulations to the first layer. The monolayer crystals show a very low dependency on channel directions with a small anisotropic ratio of 1.3. The positive mobility–temperature correlation reveals a thermally activated carrier transport mode in the monolayer crystals, which is different from the band‐like transport mode in multilayer crystals. Furthermore, because of the direct exposure of highly conductive channels, the monolayer crystal based OFETs can sense ammonia concentrations as low as 10 ppb. The decent sensitivity indicates the monolayer crystals are potential candidates for sensor applications.     

High Triplet Energy Hosts for Blue Organic Light-Emitting Diodes

Blue organic light-emitting diodes (OLEDs) have been a bottleneck for OLEDs lighting and flexible displays. Improving the operational lifetimes and simultaneously decreasing efficiency roll-off while maintaining high quantum efficiency is currently a challenge in the scientific community and industry. Optimizing the fabrication process of devices and developing stable and efficient luminescence and transporting materials and developing host materials with high triplet energy is an effective way to overcome this obstacle. On the one hand, the host material can disperse the blue emitters to reduce the possibility of exciton annihilation. On the other hand, it can adjust the carrier transport, exciton formation, and energy transfer in the device. In recent years, many efforts have been undertaken for the design, synthesis, and applications of the novel host. A systematic summary and comments on the recent advances of high triplet energy hosts for blue OLEDs are provided here, which specifically include bipolar transport hosts, single thermally activated delayed fluorescence (TADF) hosts, TADF assistant hosts, exciplex hosts, exciplex free type mixed hosts, and electroplex hosts. Moreover, future prospects for host for high performance blue OLEDs are also proposed.     

Biodegradable Materials for Transient Organic Transistors

Electronic devices that can physically disappear in a controlled manner without harmful by-products unveil a wide range of opportunities in medical devices, environmental monitoring, and next-generation consumer electronics. Their property of transience is indispensable for mitigating the global problem of electronic waste accumulation. Additionally, transient technologies that are biocompatible and can be biologically resorbed are of great potential for applications in temporary medical implants, since it eliminates the need for expensive device recovery surgery. Transistors are the key building blocks of modern electronics, and their fabrication using organic materials is beneficial due to their low cost, unprecedented flexibility and facile processing. This contribution reviews the technological application of biodegradable materials in four major classes of organic transistors, namely organic field-effect transistors (OFETs), organic synaptic transistors, electrolyte-gated OFETs, and organic electrochemical transistors. The fundamental biodegradation mechanism is discussed in detail, followed by a perspective of various biodegradable materials utilized as active semiconductors, dielectrics, electrolytes and substrates in the various types of organic transistor devices. This contribution comprehensively discusses the role and application of biodegradable materials in all of the key modern-day organic transistors, highlighting their unique properties that allow the fabrication of biodegradable, eco-friendly, and sustainable devices.     

Contact Resistance in Organic Field‐Effect Transistors: Conquering the Barrier

Organic semiconductors have sparked interest as flexible, solution processable, and chemically tunable electronic materials. Improvements in charge carrier mobility put organic semiconductors in a competitive position for incorporation in a variety of (opto‐)electronic applications. One example is the organic field‐effect transistor (OFET), which is the fundamental building block of many applications based on organic semiconductors. While the semiconductor performance improvements opened up the possibilities for applying organic materials as active components in fast switching electrical devices, the ability to make good electrical contact hinders further development of deployable electronics. Additionally, inefficient contacts represent serious bottlenecks in identifying new electronic materials by inhibiting access to their intrinsic properties or providing misleading information. Recent work focused on the relationships of contact resistance with device architecture, applied voltage, metal and dielectric interfaces, has led to a steady reduction in contact resistance in OFETs. While impressive progress was made, contact resistance is still above the limits necessary to drive devices at the speed required for many active electronic components. Here, the origins of contact resistance and recent improvement in organic transistors are presented, with emphasis on the electric field and geometric considerations of charge injection in OFETs.     

Interface Functionalities in Multilayer Stack Organic Light Emitting Transistors (OLETs)

Herein is described a multidisciplinary approach to understand the performance limitations of small molecule organic light emitting transistors (OLETs) based on a layered architecture, an innovative architecture potentially competitive with the state of the art and more flexible for spectral emission control. The processes of charge injection and field‐effect transport at metal/organic and organic/organic interfaces are analysed using microscopic and spectroscopic techniques in coordination. Atomic force microscopy and ultrasonic force microscopy are employed to characterize the interface morphology and the initial growth stages of organic films where charge transport actually occurs. X‐ray diffraction and near edge X‐ray dichroic absorption with linearly polarised light allow to determine the unit cell packing and the molecular orientation at the active organic interfaces, as well as the amount of non‐ordered domains. Moreover, chemical reactivity at the interfaces is measured by X‐ray photoelectron spectroscopy. It is found that a strong reaction occurs at the metal‐organic interfaces, with molecular fragmentation. Additionally, the transport properties strongly depend on the nature of the materials forming the organic stack. Specifically, amorphous conjugated films as bottom layers can promote an increased molecular disorder in the upper active layer, with a concomitant deterioration of the conduction properties.