The weak intermolecular interactions inherent in organic semiconductors make them susceptible to defect formation, resulting in localized states in the band-gap that can trap charge carriers at different timescales. Charge carrier trapping is thus ubiquitous in organic semiconductors and can have a profound impact on their performance when incorporated into optoelectronic devices. This review provides a comprehensive overview on the phenomenon of charge carrier trapping in organic semiconductors, with emphasis on the underlying physical processes and its impact on device operation. We first define the concept of charge carrier trap, then outline and categorize different origins of traps. Next, we discuss their impact on the mechanism of charge transport and the performance of electronic devices. Progress in the filed in terms of characterization and detection of charge carrier traps is reviewed together with insights on future direction of research. Finally, a discussion on the exploitation of traps in memory and sensing applications is provided.

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Charge carrier traps in organic semiconductors: a review on the underlying physics and impact on electronic devices

Flexible neuromorphic electronics that emulate biological neuronal systems constitute a promising candidate for next-generation wearable computing, soft robotics, and neuroprosthetics. For realization, with the achievement of simple synaptic behaviors in a single device, the construction of artificial synapses with various functions of sensing and responding and integrated systems to mimic complicated computing, sensing, and responding in biological systems is a prerequisite. Artificial synapses that have learning ability can perceive and react to events in the real world; these abilities expand the neuromorphic applications toward health monitoring and cybernetic devices in the future Internet of Things. To demonstrate the flexible neuromorphic systems successfully, it is essential to develop artificial synapses and nerves replicating the functionalities of the biological counterparts and satisfying the requirements for constructing the elements and the integrated systems such as flexibility, low power consumption, high-density integration, and biocompatibility. Here, the progress of flexible neuromorphic electronics is addressed, from basic backgrounds including synaptic characteristics, device structures, and mechanisms of artificial synapses and nerves, to applications for computing, soft robotics, and neuroprosthetics. Finally, future research directions toward wearable artificial neuromorphic systems are suggested for this emerging area.

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Flexible Neuromorphic Electronics for Computing, Soft Robotics, and Neuroprosthetics

Lenses with tunable focal length play important roles in nature by helping species avoid predators and capture prey. Many practical devices mimic lens concept for imaging, sensing, and detection. This review covers fundamental optics of lenses and its extension to lenses made of liquid crystals (LCs). Three main types of LC lenses are described, namely, lenses with curved surfaces, flat gradient-index lenses and composite lenses. The review discusses advantages of LC lenses over their isotropic counterparts, challenges in their fabrication and control, as well as a variety of potential applications. We also discuss the current challenges associated with nematic LC lenses and their solutions. LC lenses are already having significant impacts on optics and optometry, and these impacts will grow with discovering new LC materials and new lens designs.

Liquid crystal lenses with tunable focal length

Photonic synapses combine sensing and processing in a single device, so they are promising candidates to emulate visual perception of a biological retina. However, photonic synapses with wavelength selectivity, which is a key property for visual perception, have not been developed so far. Herein, organic photonic synapses that selectively detect UV rays and process various optical stimuli are presented. The photonic synapses use carbon nitride (C3 N4 ) as an UV-responsive floating-gate layer in transistor geometry. C3 N4 nanodots dominantly absorb UV light; this trait is the basis of UV selectivity in these photonic synapses. The presented devices consume only 18.06 fJ per synaptic event, which is comparable to the energy consumption of biological synapses. Furthermore, in situ modulation of exposure to UV light is demonstrated by integrating the devices with UV transmittance modulators. These smart systems can be further developed to combine detection and dose-calculation to determine how and when to decrease UV transmittance for preventive health care.

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Retina-Inspired Carbon Nitride-Based Photonic Synapses for Selective Detection of UV Light.

Summary Taking advantage of the coupling effect of contact electrification and electrostatic induction, triboelectric nanogenerators can effectively convert various forms of ambient mechanical energy into electricity, and therefore have attracted much attention, with broad applications in energy harvesting, active sensing, and biomedical therapy, which are anticipated to be an indispensable component in the era of the Internet of things. To improve the mechanical-to-electrical conversion, various strategies have been reported to engineer the materials used at the nanoscale with physical, chemical, biological, and hybrid approaches. These strategies to enhance the output performance and extend the applications of triboelectric nanogenerators are comprehensively reviewed and summarized in this article. Furthermore, perspectives are also discussed in depth, with an emphasis on future research directions to further advance developments within the field.

Engineering Materials at the Nanoscale for Triboelectric Nanogenerators

We demonstrate the physical pictures of the localization of the conductive filaments (CFs) growth in flexible electrochemical metallization (ECM) memristors through an interfacial triggering (IT) into the polymer electrolyte. The IT sites (ITSs), capable of controlling the pathways of the CF growth, are formed at the electrode–polymer interfaces via the Ostwald ripening at low temperatures (below 230 °C). The injection and migration of metal ions and the resultant CF growth are found to be effectively controlled through the ITSs with the local electric field enhancement. The reliability, uniformity, and switching voltage of the device are much improved by the presence of the ITSs. Our flexible ECM memristor exhibits a high mechanical flexibility and a stable memory performance under repeated bending deformations.

Interfacial Triggering of Conductive Filament Growth in Organic Flexible Memristor for High Reliability and Uniformity.

Transparent and flexible high power triboelectric nanogenerator with metallic nanowire-embedded tribonegative conducting polymer

This work was supported in part through the BK21 Program funded by Ministry of Education of Korea.

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Organic Flexible Memristor with Reduced Operating Voltage and High Stability by Interfacial Control of Conductive Filament Growth

Liquid crystal (LC)-based lenses are promising for a wide range of applications from optical communications and signal processing to three-dimensional displays. Among a variety of the LC-based lenses, several classes including the Fresnel type and the lenticular type are comprehensively reviewed from the standpoints of the basic concepts and the device architectures. The underlying mechanisms for the focusing effect and the tuning capability inherent to the lens configuration are described. Recent progress on the LC-based lenses for reconfigurable optical processing and autostereoscopic displays is also presented. The lens architecture combined with the intrinsic anisotropy of the LC provides a versatile platform to build up high-performance LC-based lenses with the tuning capability.

Sin-Doo Lee

Papers

Interfacial Triggering of Conductive Filament Growth in Organic Flexible Memristor for High Reliability and Uniformity.

Transparent and flexible high power triboelectric nanogenerator with metallic nanowire-embedded tribonegative conducting polymer

Organic Flexible Memristor with Reduced Operating Voltage and High Stability by Interfacial Control of Conductive Filament Growth

Design and fabrication of liquid crystal-based lenses

Degradation mechanism of blue thermally activated delayed fluorescent organic light-emitting diodes under electrical stress