Flexible and Stretchable Light-Emitting Diodes and Photodetectors for Human-Centric Optoelectronics.

Optoelectronic devices with unconventional form factors, such as flexible and stretchable light-emitting or photoresponsive devices, are core elements for the next-generation human-centric optoelectronics. For instance, these deformable devices can be utilized as closely fitted wearable sensors to acquire precise biosignals that are subsequently uploaded to the cloud for immediate examination and diagnosis, and also can be used for vision systems for human-interactive robotics. Their inception was propelled by breakthroughs in novel optoelectronic material technologies and device blueprinting methodologies, endowing flexibility and mechanical resilience to conventional rigid optoelectronic devices. This paper reviews the advancements in such soft optoelectronic device technologies, honing in on various materials, manufacturing techniques, and device design strategies. We will first highlight the general approaches for flexible and stretchable device fabrication, including the appropriate material selection for the substrate, electrodes, and insulation layers. We will then focus on the materials for flexible and stretchable light-emitting diodes, their device integration strategies, and representative application examples. Next, we will move on to the materials for flexible and stretchable photodetectors, highlighting the state-of-the-art materials and device fabrication methods, followed by their representative application examples. At the end, a brief summary will be given, and the potential challenges for further development of functional devices will be discussed as a conclusion.

Toward Full-Color Electroluminescent Quantum Dot Displays.

Colloidal quantum dots (QDs) exhibit unique characteristics such as facile color tunability, pure color emission with extremely narrow bandwidths, high luminescence efficiency, and high photostability. In addition, quantum dot light-emitting diodes (QLEDs) feature bright electroluminescence, low turn-on voltage, and ultrathin form factor, making them a promising candidate for next-generation displays. To achieve the overarching goal of the full-color display based on the electroluminescence of QDs, however it is essential to enhance the performance of QLEDs further for each color (e.g., red, green, and blue; RGB) and develop novel techniques for patterning RGB QD pixels without cross-contamination. Here, we present state-of-the-art material, process, and device technologies for full-color QLED-based displays. First, we highlight recent advances in the development of efficient red-, green-, and blue-monochromatic QLEDs. In particular, we focus on the progress of heavy-metal-free QLEDs. Then, we describe patterning techniques for individual RGB QDs to fabricate pixelated displays. Finally, we briefly summarize applications of such QLEDs, presenting the possibility of full-color QLED-based displays.

3D Antidrying Antifreezing Artificial Skin Device with Self-Healing and Touch Sensing Capability.

Hydrogels are attractive, active materials for various e-skin devices based on their unique functionalities such as flexibility and biocompatibility. Still, e-skin devices are generally limited to simple structures, and the realization of optimal-shaped 3D e-skin devices for target applications is an intriguing issue of interest. Furthermore, hydrogels intrinsically suffer from drying and freezing issues in operational capability for practical applications. Herein, 3D artificial skin devices are demonstrated with highly improved device stability. The devices are fabricated in a target-oriented 3D structure by extrusion-based 3D printing, spontaneously heal mechanical damage, and enable stable device operation over time and under freezing conditions. Based on the material design to improve drying and freezing resistance, an organohydrogel, prepared by solvent displacement of hydrogel with ethylene glycol for 3 h, exhibits excellent drying resistance over 1000 h and improved freezing resistance by showing no phase transition down to -60 °C while maintaining its self-healing functionality. Based on the improved drying and freezing resistance, artificial skin devices in target-oriented optimal 3D structures are presented, which enable accurate positioning of touchpoints even on a complicated 3D structure stably over time and excellent operation at temperatures below 0 °C without losing their flexibility.

MoS2 Liquid Cell Electron Microscopy Through Clean and Fast Polymer-Free MoS2 Transfer.

Two dimensional (2D) materials have found various applications because of their unique physical properties. For example, graphene has been used as the electron transparent membrane for liquid cell transmission electron microscopy (TEM) due to its high mechanical strength and flexibility, single-atom thickness, chemical inertness, etc. Here, we report using 2D MoS2 as a functional substrate as well as the membrane window for liquid cell TEM, which is enabled by our facile and polymer-free MoS2 transfer process. This provides the opportunity to investigate the growth of Pt nanocrystals on MoS2 substrates, which elucidates the formation mechanisms of such heterostructured 2D materials. We find that Pt nanocrystals formed in MoS2 liquid cells have a strong tendency to align their crystal lattice with that of MoS2, suggesting a van der Waals epitaxial relationship. Importantly, we can study its impact on the kinetics of the nanocrystal formation. The development of MoS2 liquid cells will allow further study of various liquid phenomena on MoS2, and the polymer-free MoS2 transfer process will be implemented in a wide range of applications.

Route to the Smallest Doped Semiconductor: Mn(2+)-Doped (CdSe)13 Clusters.

Doping semiconductor nanocrystals with magnetic transition-metal ions has attracted fundamental interest to obtain a nanoscale dilute magnetic semiconductor, which has unique spin exchange interaction between magnetic spin and exciton. So far, the study on the doped semiconductor NCs has usually been conducted with NCs with larger than 2 nm because of synthetic challenges. Herein, we report the synthesis and characterization of Mn(2+)-doped (CdSe)13 clusters, the smallest doped semiconductors. In this study, single-sized doped clusters are produced in large scale. Despite their small size, these clusters have semiconductor band structure instead of that of molecules. Surprisingly, the clusters show multiple excitonic transitions with different magneto-optical activities, which can be attributed to the fine structure splitting. Magneto-optically active states exhibit giant Zeeman splittings up to elevated temperatures (128 K) with large g-factors of 81(±8) at 4 K. Our results present a new synthetic method for doped clusters and facilitate the understanding of doped semiconductor at the boundary of molecules and quantum nanostructure.

n-Type nanostructured thermoelectric materials prepared from chemically synthesized ultrathin Bi2Te3 nanoplates.

We herein report on the large-scale synthesis of ultrathin Bi(2)Te(3) nanoplates and subsequent spark plasma sintering to fabricate n-type nanostructured bulk thermoelectric materials. Bi(2)Te(3) nanoplates were synthesized by the reaction between bismuth thiolate and tri-n-octylphosphine telluride in oleylamine. The thickness of the nanoplates was ~1 nm, which corresponds to a single layer in Bi(2)Te(3) crystals. Bi(2)Te(3) nanostructured bulk materials were prepared by sintering of surfactant-removed Bi(2)Te(3) nanoplates using spark plasma sintering. We found that the grain size and density were strongly dependent on the sintering temperature, and we investigated the effect of the sintering temperature on the thermoelectric properties of the Bi(2)Te(3) nanostructured bulk materials. The electrical conductivities increased with an increase in the sintering temperature, owing to the decreased interface density arising from the grain growth and densification. The Seebeck coefficients roughly decreased with an increase in the sintering temperature. Interestingly, the electron concentrations and mobilities strongly depended on the sintering temperature, suggesting the potential barrier scattering at interfaces and the doping effect of defects and organic residues. The thermal conductivities also increased with an increase in the sintering temperature because of grain growth and densification. The maximum thermoelectric figure-of-merit, ZT, is 0.62 at 400 K, which is one of the highest among the reported values of n-type nanostructured materials based on chemically synthesized nanoparticles. This increase in ZT shows the possibility of the preparation of highly efficient thermoelectric materials by chemical synthesis.

Recent Advances in Patterning Strategies for Full-Color Perovskite Light-Emitting Diodes.

Metal halide perovskites have emerged as promising light-emitting materials for next-generation displays owing to their remarkable material characteristics including broad color tunability, pure color emission with remarkably narrow bandwidths, high quantum yield, and solution processability. Despite recent advances have pushed the luminance efficiency of monochromic perovskite light-emitting diodes (PeLEDs) to their theoretical limits, their current fabrication using the spin-coating process poses limitations for fabrication of full-color displays. To integrate PeLEDs into full-color display panels, it is crucial to pattern red-green-blue (RGB) perovskite pixels, while mitigating issues such as cross-contamination and reductions in luminous efficiency. Herein, we present state-of-the-art patterning technologies for the development of full-color PeLEDs. First, we highlight recent advances in the development of efficient PeLEDs. Second, we discuss various patterning techniques of MPHs (i.e., photolithography, inkjet printing, electron beam lithography and laser-assisted lithography, electrohydrodynamic jet printing, thermal evaporation, and transfer printing) for fabrication of RGB pixelated displays. These patterning techniques can be classified into two distinct approaches: in situ crystallization patterning using perovskite precursors and patterning of colloidal perovskite nanocrystals. This review highlights advancements and limitations in patterning techniques for PeLEDs, paving the way for integrating PeLEDs into full-color panels.

Nanomaterial-Based Synaptic Optoelectronic Devices for In-Sensor Preprocessing of Image Data.

With the advance in information technologies involving machine vision applications, the demand for energy- and time-efficient acquisition, transfer, and processing of a large amount of image data has rapidly increased. However, current architectures of the machine vision system have inherent limitations in terms of power consumption and data latency owing to the physical isolation of image sensors and processors. Meanwhile, synaptic optoelectronic devices that exhibit photoresponse similar to the behaviors of the human synapse enable in-sensor preprocessing, which makes the front-end part of the image recognition process more efficient. Herein, we review recent progress in the development of synaptic optoelectronic devices using functional nanomaterials and their unique interfacial characteristics. First, we provide an overview of representative functional nanomaterials and device configurations for the synaptic optoelectronic devices. Then, we discuss the underlying physics of each nanomaterial in the synaptic optoelectronic device and explain related device characteristics that allow for the in-sensor preprocessing. We also discuss advantages achieved by the application of the synaptic optoelectronic devices to image preprocessing, such as contrast enhancement and image filtering. Finally, we conclude this review and present a short prospect.

Ultrathin Self-Powered Heavy-Metal-Free Cu-In-Se Quantum Dot Photodetectors for Wearable Health Monitoring.

Mechanically deformable photodetectors (PDs) are key device components for wearable health monitoring systems based on photoplethysmography (PPG). Achieving high detectivity, fast response time, and an ultrathin form factor in the PD is highly needed for next-generation wearable PPG systems. Self-powered operation without a bulky power-supply unit is also beneficial for point-of-care application. Here, we propose ultrathin self-powered PDs using heavy-metal-free Cu-In-Se quantum dots (QDs), which enable high-performance wearable PPG systems. Although the light-absorbing QD layer is extremely thin (∼40 nm), the developed PD exhibits excellent performance (specific detectivity: 2.10 × 1012 Jones, linear dynamic range: 102 dB, and spectral range: 250-1050 nm at zero bias), which is comparable to that of conventional rigid QD-PDs employing thick Pb-chalcogenide QD layers. This is attributed to material and device strategies─materials that include Cu-In-Se QDs, a MoS2-nanosheet-blended poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) hole transport layer, a ZnO nanoparticle electron transport layer, Ag and ITO electrodes, and an ultrathin form factor (∼120 nm except the electrodes) that enable excellent mechanical deformability. These allow the successful application of QD-PDs to a wearable system for real-time PPG monitoring, expanding their potential in the field of mobile bioelectronics.

Dimension-controlled synthesis of CdS nanocrystals: from 0D quantum dots to 2D nanoplates.

The dimension-controlled synthesis of CdS nanocrystals in the strong quantum confinement regime is reported. Zero-, one-, and two-dimensional CdS nanocrystals are selectively synthesized via low-temperature reactions using alkylamines as surface-capping ligands. The shape of the nanocrystals is controlled systematically by using different amines and reaction conditions. The 2D nanoplates have a uniform thickness as low as 1.2 nm. Furthermore, their optical absorption and emission spectra show very narrow peaks indicating extremely uniform thickness. It is demonstrated that 2D nanoplates are generated by 2D assembly of CdS magic-sized clusters formed at the nucleation stage, and subsequent attachment of the clusters. The stability of magic-sized clusters in amine solvent strongly influences the final shapes of the nanocrystals. The thickness of the nanoplates increases in a stepwise manner while retaining their uniformity, similar to the growth behavior of inorganic clusters. The 2D CdS nanoplates are a new type of quantum well with novel nanoscale properties in the strong quantum confinement regime.

Efficiency improvement of organic solar cells by tuning hole transport layer with germanium oxide.

Improving optical property is critical for optimizing the power conversion efficiency of organic solar cells. In the present research, we show that modification of poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) layer with GeO2 leads to 15% improvement of power conversion efficiency in a polymer solar cells through enhancement of short circuit currents. Modified PEDOT:PSS layer with optimized concentration of GeO2 assists active layer absorbing much light by playing a role of optical spacer. Using AFM and grazing incidence X-ray diffraction (GIXD) data, we also present the evidence that an addition of GeO2 does not affect crystallinity of active layer.

Materials and design strategies for stretchable electroluminescent devices.

Stretchable displays have recently received increasing attention as input and/or output interfaces for next-generation human-friendly electronic systems. Stretchable electroluminescent (EL) devices are a core component of stretchable displays, and they can be classified into two types, structurally stretchable EL devices and intrinsically stretchable EL devices, according to the mechanism for achieving their stretchability. We herein present recent advances in materials and design strategies for stretchable EL devices. First, stretchable devices based on ultrathin EL devices are introduced. Ultrathin EL devices are mechanically flexible like thin paper, and they can become stretchable through various structural engineering methods, such as inducing a buckled structure, employing interconnects with stretchable geometries, and applying origami/kirigami techniques. Secondly, intrinsically stretchable EL devices can be fabricated by using inherently stretchable electronic materials. For example, light-emitting electrochemical cells and EL devices with a simpler structure using alternating current have been developed. Furthermore, novel stretchable semiconductor materials have been presented for the development of intrinsically stretchable light-emitting diodes. After discussing these two types of stretchable EL devices, we briefly discuss applications of deformable EL devices and conclude the review.

Integration of synaptic phototransistors and quantum dot light-emitting diodes for visualization and recognition of UV patterns.

Synaptic photodetectors exhibit photon-triggered synaptic plasticity, which thus can improve the image recognition rate by enhancing the image contrast. However, still, the visualization and recognition of invisible ultraviolet (UV) patterns are challenging, owing to intense background noise. Here, inspired by all-or-none potentiation of synapse, we develop an integrated device of synaptic phototransistors (SPTrs) and quantum dot light-emitting diodes (QLEDs), facilitating noise reduction and visualization of UV patterns through on-device preprocessing. The SPTrs convert noisy UV inputs into a weighted photocurrent, which is applied to the QLEDs as a voltage input through an external current-voltage-converting circuit. The threshold switching characteristics of the QLEDs result in amplified current and visible illumination by the suprathreshold input voltage or nearly zero current and no visible illumination by the input voltage below the threshold. The preprocessing of image data with the SPTr-QLED can amplify the image contrast, which is helpful for high-accuracy image recognition.

Generalised optical printing of photocurable metal chalcogenides.

Optical three-dimensional (3D) printing techniques have attracted tremendous attention owing to their applicability to mask-less additive manufacturing, which enables the cost-effective and straightforward creation of patterned architectures. However, despite their potential use as alternatives to traditional lithography, the printable materials obtained from these methods are strictly limited to photocurable resins, thereby restricting the functionality of the printed objects and their application areas. Herein, we report a generalised direct optical printing technique to obtain functional metal chalcogenides via digital light processing. We developed universally applicable photocurable chalcogenidometallate inks that could be directly used to create 2D patterns or micrometre-thick 2.5D architectures of various sizes and shapes. Our process is applicable to a diverse range of functional metal chalcogenides for compound semiconductors and 2D transition-metal dichalcogenides. We then demonstrated the feasibility of our technique by fabricating and evaluating a micro-scale thermoelectric generator bearing tens of patterned semiconductors. Our approach shows potential for simple and cost-effective architecturing of functional inorganic materials.

Ultrahigh-resolution full-color perovskite nanocrystal patterning for ultrathin skin-attachable displays.

High-definition red/green/blue (RGB) pixels and deformable form factors are essential for the next-generation advanced displays. Here, we present ultrahigh-resolution full-color perovskite nanocrystal (PeNC) patterning for ultrathin wearable displays. Double-layer transfer printing of the PeNC and organic charge transport layers is developed, which prevents internal cracking of the PeNC film during the transfer printing process. This results in RGB pixelated PeNC patterns of 2550 pixels per inch (PPI) and monochromic patterns of 33,000 line pairs per inch with 100% transfer yield. The perovskite light-emitting diodes (PeLEDs) with transfer-printed active layers exhibit outstanding electroluminescence characteristics with remarkable external quantum efficiencies (15.3, 14.8, and 2.5% for red, green, and blue, respectively), which are high compared to the printed PeLEDs reported to date. Furthermore, double-layer transfer printing enables the fabrication of ultrathin multicolor PeLEDs that can operate on curvilinear surfaces, including human skin, under various mechanical deformations. These results highlight that PeLEDs are promising for high-definition full-color wearable displays.

Simple fabrication of asymmetric high-aspect-ratio polymer nanopillars by reusable AAO templates.

We present a simple method of utilizing anodized aluminum oxide (AAO) as a reproducible template for fabricating high-aspect-ratio uniformly bent polymeric nanopillars that can be used as a physical adhesive. It is shown how to achieve straight high-aspect-ratio nanopillars with concepts of the work of adhesion and lateral collapse between polymer pillars without serious damage to the master template. With the support of manufacturing polymeric nanopillars from the reusable AAO, a simple route to asymmetric dry adhesive nanopillars bent by residual stresses was demonstrated.

Solution-Processed Stretchable Ag2 S Semiconductor Thin Films for Wearable Self-Powered Nonvolatile Memory.

Compared with the large plastic deformation observed in ductile metals and organic materials, inorganic semiconductors have limited plasticity (<0.2%) due to their intrinsic bonding characters, restricting their widespread applications in stretchable electronics. Herein, the solution-processed synthesis of ductile α-Ag2 S thin films and fabrication of all-inorganic, self-powered, and stretchable memory devices, is reported. Molecular Ag2 S complex solution is synthesized by chemical reduction of Ag2 S powder, fabricating wafer-scale highly crystalline Ag2 S thin films. The thin films show stretchability due to the intrinsic ductility, sustaining the structural integrity at a tensile strain of 14.9%. Moreover, the fabricated Ag2 S-based resistive random access memory presents outstanding bipolar switching characteristics (Ion /Ioff ratio of ≈105 , operational endurance of 100 cycles, and retention time >106 s) as well as excellent mechanical stretchability (no degradation of properties up to stretchability of 52%). Meanwhile, the device is highly durable under diverse chemical environments and temperatures from -196 to 300 °C, especially maintaining the properties for 168 h in 85% relative humidity and 85 °C. A self-powered memory combined with motion sensors for use as a wearable healthcare monitoring system is demonstrated, offering the potential for designing high-performance wearable electronics that are usable in daily life in a real-world setting.

Face selection in one-step bending of Janus nanopillars.

We introduce a one-step procedure of bending nanopillars, which simply involves oblique metal deposition at a tilted angle of 45 degrees on the pillars by thermal evaporation. The face selection in the bending procedure was determined by the nature of residual stress generated in the metal film during evaporation. If the stress was tensile as with many metals (sigma(f) > 0), the Janus nanopillars were bent toward the metal face; if the residual stress was compressive as in the case of Al (sigma(f) < 0), they were bent toward the polymer face. It has also been demonstrated that groups of Janus nanopillars could be bent in different directions on the same substrate with the aid of a shadow-mask deposition. The degree of bending increased with the decrease in pillar diameter in the range of 360-800 nm for a fixed height of 1 microm.

Materials engineering, processing, and device application of hydrogel nanocomposites.

Hydrogels are widely implemented as key materials in various biomedical applications owing to their soft, flexible, hydrophilic, and quasi-solid nature. Recently, however, new material properties over those of bare hydrogels have been sought for novel applications. Accordingly, hydrogel nanocomposites, i.e., hydrogels converged with nanomaterials, have been proposed for the functional transformation of conventional hydrogels. The incorporation of suitable nanomaterials into the hydrogel matrix allows the hydrogel nanocomposite to exhibit multi-functionality in addition to the biocompatible feature of the original hydrogel. Therefore, various hydrogel composites with nanomaterials, including nanoparticles, nanowires, and nanosheets, have been developed for diverse purposes, such as catalysis, environmental purification, bio-imaging, sensing, and controlled drug delivery. Furthermore, novel technologies for the patterning of such hydrogel nanocomposites into desired shapes have been developed. The combination of such material engineering and processing technologies has enabled the hydrogel nanocomposite to become a key soft component of electronic, electrochemical, and biomedical devices. We herein review the recent research trend in the field of hydrogel nanocomposites, particularly focusing on materials engineering, processing, and device applications. Furthermore, the conclusions are presented with the scope of future research outlook, which also includes the current technical limitations.

Extremely Vivid, Highly Transparent, and Ultrathin Quantum Dot Light-Emitting Diodes.

Displaying information on transparent screens offers new opportunities in next-generation electronics, such as augmented reality devices, smart surgical glasses, and smart windows. Outstanding luminance and transparency are essential for such "see-through" displays to show vivid images over clear background view. Here transparent quantum dot light-emitting diodes (Tr-QLEDs) are reported with high brightness (bottom: ≈43 000 cd m-2 , top: ≈30 000 cd m-2 , total: ≈73 000 cd m-2 at 9 V), excellent transmittance (90% at 550 nm, 84% over visible range), and an ultrathin form factor (≈2.7 µm thickness). These superb characteristics are accomplished by novel electron transport layers (ETLs) and engineered quantum dots (QDs). The ETLs, ZnO nanoparticle assemblies with ultrathin alumina overlayers, dramatically enhance durability of active layers, and balance electron/hole injection into QDs, which prevents nonradiative recombination processes. In addition, the QD structure is further optimized to fully exploit the device architecture. The ultrathin nature of Tr-QLEDs allows their conformal integration on various shaped objects. Finally, the high resolution patterning of red, green, and blue Tr-QLEDs (513 pixels in.-1 ) shows the potential of the full-color transparent display.