Efficient Deep-Blue Light-Emitting Diodes Through Decoupling of Colloidal Perovskite Quantum Dots.

Metal halide perovskite light-emitting diodes (PeLEDs) have exceptional color purity but designs that emit deep-blue color with high efficiency have not been fully achieved and become more difficult in the thin film of confined perovskite colloidal quantum dots (PeQDs) due to particle interaction. Here it is demonstrated that electronic coupling and energy transfer in PeQDs induce redshift in the emission by PeQD film, and consequently hinder deep-blue emission. To achieve deep-blue emission by avoiding electronic coupling and energy transfer, a QD-in-organic solid solution is introduced to physically separate the QDs in the film. This physical separation of QDs reduces the interaction between them yielding a blueshift of ≈7 nm in the emission spectrum. Moreover, using a hole-transporting organic molecule with a deep-lying highest occupied molecular orbital (≈6.0 eV) as the organic matrix, the formation of exciplex emission is suppressed. As a result, an unprecedently high maximum external quantum efficiency of 6.2% at 462 nm from QD-in-organic solid solution film in PeLEDs is achieved, which satisfies the deep-blue color coordinates of CIEy < 0.06. This work suggests an important material strategy to deepen blue emission without reducing the particle size to <≈4 nm.

Quantum-Dot Light-Emitting Fiber Toward All-In-One Clothing-Type Health Monitoring.

In the Fourth Industrial Revolution, as the connection between objects and people becomes increasingly important, interest in wearable optoelectronic device-based medical diagnosis is on the rise. Pulse oximetry sensors based on a fiber platform, which is the smallest unit of clothing, could be considered an attractive candidate for this application. In this study, red and green quantum-dot light-emitting fibers (QDLEFs) based on a 250 µm-diameter 1-dimensional fiber were successfully implemented, achieving high current efficiencies of approximately 22.46 mW/sr/A and 23.6 mW/sr/A and narrow full-width at half-maximum (FWHM) of about 33 nm, respectively. In addition, its omnidirectional flexibility was confirmed through a vertical and lateral bending test with 0.92% strain. By employing a transparent and flexible elastomer, a wearable pulse oximeter incorporating QDLEFs was successfully demonstrated for oxygen saturation level (SpO2) monitoring on finger and wrist. It was demonstrated to be washable, and could be operated for up to about 18 h. Due to the elastomer and bottom emission, it exhibited excellent wear resistance characteristics in a 50 cycle reciprocating test conducted at about 2180.43 kPa with 220-grit abrasive paper sheet. A theoretical investigation based on modified photon diffusion analysis (MPDA) modeling also determined that using narrow FWHM light sources, such as QDLEFs, improves the resolution and accuracy of SpO2 monitoring. Accordingly, the proposed QDLEF showed distinguished potential as an all-in-one clothing type pulse oximetry.

Surface-binding molecular multipods strengthen the halide perovskite lattice and boost luminescence.

Reducing the size of perovskite crystals to confine excitons and passivating surface defects has fueled a significant advance in the luminescence efficiency of perovskite light-emitting diodes (LEDs). However, the persistent gap between the optical limit of electroluminescence efficiency and the photoluminescence efficiency of colloidal perovskite nanocrystals (PeNCs) suggests that defect passivation alone is not sufficient to achieve highly efficient colloidal PeNC-LEDs. Here, we present a materials approach to controlling the dynamic nature of the perovskite surface. Our experimental and theoretical studies reveal that conjugated molecular multipods (CMMs) adsorb onto the perovskite surface by multipodal hydrogen bonding and van der Waals interactions, strengthening the near-surface perovskite lattice and reducing ionic fluctuations which are related to nonradiative recombination. The CMM treatment strengthens the perovskite lattice and suppresses its dynamic disorder, resulting in a near-unity photoluminescence quantum yield of PeNC films and a high external quantum efficiency (26.1%) of PeNC-LED with pure green emission that matches the Rec.2020 color standard for next-generation vivid displays.

Increasing the stability of electrolyte-gated organic synaptic transistors for neuromorphic implants.

Electrolyte-gated organic synaptic transistors (EGOSTs) can have versatile synaptic plasticity in a single device, so they are promising as components of neuromorphic implants that are intended for use in neuroprosthetic electronic nerves that are energy-efficient and have simple system structure. With the advancement in transistor properties of EGOSTs, the commercialization of neuromorphic implants for practical long-term use requires consistent operation, so they must be stable in vivo. This requirement demands strategies that maintain electronic and ionic transport in the devices while implanted in the human body, and that are mechanically, environmentally, and operationally stable. Here, we cover the structure, working mechanisms, and electrical responses of EGOSTs. We then focus on strategies to ensure their stability to maintain these characteristics and prevent adverse effects on biological tissues. We also highlight state-of-the-art neuromorphic implants that incorporate these strategies. We conclude by presenting a perspective on improvements that are needed in EGOSTs to develop practical, neuromorphic implants that are long-term useable.

Enhanced Photodetection Performance of an In Situ Core/Shell Perovskite-MoS2 Phototransistor.

While two-dimensional transition metal dichalcogenides (TMDCs)-based photodetectors offer prospects for high integration density and flexibility, their thinness poses a challenge regarding low light absorption, impacting photodetection sensitivity. Although the integration of TMDCs with metal halide perovskite nanocrystals (PNCs) has been known to be promising for photodetection with a high absorption coefficient of PNCs, the low charge mobility of PNCs delays efficient photocarrier injection into TMDCs. In this study, we integrated MoS2 with in situ formed core/shell PNCs with short ligands that minimize surface defects and enhance photocarrier injection. The PNCs/MoS2 heterostructure efficiently separates electrons and holes by establishing type II band alignment and consequently inducing a photogating effect. The synergistic interplay between photoconductive and photogating effects yields a high responsivity of 2.2 × 106 A/W and a specific detectivity of 9.0 × 1011 Jones. Our findings offer a promising pathway for developing low-cost, high-performance phototransistors leveraging the advantages of two-dimensional (2D) materials.

Ultrasmall 2D Sn-Doped MAPbBr3 Nanoplatelets Enable Bright Pure-Blue Emission.

Synthesis of perovskites that exhibit pure-blue emission with high photoluminescence quantum yield (PLQY) in both nanocrystal solutions and nanocrystal-only films presents a significant challenge. In this work, a room-temperature method is developed to synthesize ultrasmall, monodispersed, Sn-doped methylammonium lead bromide (MAPb1- xSnxBr3) perovskite nanoplatelets (NPLs) in which the strong quantum confinement effect endows pure blue emission (460 nm) and a high quantum yield (87%). Post-treatment using n-hexylammonium bromide (HABr) repaired surface defects and thus substantially increased the stability and PLQY (80%) of the NPL films. Concurrently, high-precision patterned films (200-µm linewidth) are successfully fabricated by using cost-effective spray-coating technology. This research provides a novel perspective for the preparation of high PLQY, highly stable, and easily processable perovskite nanomaterials.

Perovskite Colloidal Nanocrystal Solar Cells: Current Advances, Challenges, and Future Perspectives.

The power conversion efficiencies (PCEs) of polycrystalline perovskite (PVK) solar cells (SCs) (PC-PeSCs) have rapidly increased. However, PC-PeSCs are intrinsically unstable without encapsulation, and their efficiency drops during large-scale production; these problems hinder the commercial viability of PeSCs. Stability can be increased by using colloidal PVK nanocrystals (c-PeNCs), which have high surface strains, low defect density, and exceptional crystal quality. The use of c-PeNCs separates the crystallization process from the film formation process, which is preponderant in large-scale fabrication. Consequently, the use of c-PeNCs has substantial potential to overcome challenges encountered when fabricating PC-PeSCs. Research on colloidal nanocrystal-based PVK SCs (NC-PeSCs) has increased their PCEs to a level greater than those of other quantum-dot SCs, but has not reached the PCEs of PC-PeSCs; this inferiority significantly impedes widespread application of NC-PeSCs. This review first introduces the distinctive properties of c-PeNCs, then the strategies that have been used to achieve high-efficiency NC-PeSCs. Then it discusses in detail the persisting challenges in this domain. Specifically, the major challenges and solutions for NC-PeSCs related to low short-circuit current density Jsc are covered. Last, the article presents a perspective on future research directions and potential applications in the realm of NC-PeSCs.

Electrically Reconfigurable Phase-Change Transmissive Metasurface.

Programmable and reconfigurable optics hold significant potential for transforming a broad spectrum of applications, spanning space explorations to biomedical imaging, gas sensing, and optical cloaking. The ability to adjust the optical properties of components like filters, lenses, and beam steering devices could result in dramatic reductions in size, weight, and power consumption in future optoelectronic devices. Among the potential candidates for reconfigurable optics, chalcogenide-based phase change materials (PCMs) offer great promise due to their non-volatile and analogue switching characteristics. Although PCM have found widespread use in electronic data storage, these memory devices are deeply sub-micron-sized. To incorporate phase change materials into free-space optical components, it is essential to scale them up to beyond several hundreds of microns while maintaining reliable switching characteristics. This study demonstrated a non-mechanical, non-volatile transmissive filter based on low-loss PCMs with a 200 × 200 µm2 switching area. The device/metafilter can be consistently switched between low- and high-transmission states using electrical pulses with a switching contrast ratio of 5.5 dB. The device was reversibly switched for 1250 cycles before accelerated degradation took place. The work represents an important step toward realizing free-space reconfigurable optics based on PCMs.

Skin-Mountable Functional Electronic Materials for Bio-Integrated Devices.

Skin-mountable electronic materials are being intensively evaluated for use in bio-integrated devices that can mutually interact with the human body. Over the past decade, functional electronic materials inspired by the skin are developed with new functionalities to address the limitations of traditional electronic materials for bio-integrated devices. Herein, the recent progress in skin-mountable functional electronic materials for skin-like electronics is introduced with a focus on five perspectives that entail essential functionalities: stretchability, self-healing ability, biocompatibility, breathability, and biodegradability. All functionalities are advanced with each strategy through rational material designs. The skin-mountable functional materials enable the fabrication of bio-integrated electronic devices, which can lead to new paradigms of electronics combining with the human body.

Electrically Confined Electroluminescence of Neutral Excitons in WSe2 Light-Emitting Transistors.

Monolayer transition metal dichalcogenides (TMDs) have drawn significant attention for their potential in optoelectronic applications due to their direct band gap and exceptional quantum yield. However, TMD-based light-emitting devices have shown low external quantum efficiencies as imbalanced free carrier injection often leads to the formation of non-radiative charged excitons, limiting practical applications. Here, electrically confined electroluminescence (EL) of neutral excitons in tungsten diselenide (WSe2) light-emitting transistors (LETs) based on the van der Waals heterostructure is demonstrated. The WSe2 channel is locally doped to simultaneously inject electrons and holes to the 1D region by a local graphene gate. At balanced concentrations of injected electrons and holes, the WSe2 LETs exhibit strong EL with a high external quantum efficiency (EQE) of ≈8.2 % at room temperature. These experimental and theoretical results consistently show that the enhanced EQE could be attributed to dominant exciton emission confined at the 1D region while expelling charged excitons from the active area by precise control of external electric fields. This work shows a promising approach to enhancing the EQE of 2D light-emitting transistors and modulating the recombination of exciton complexes for excitonic devices.

MAPbBr3 Halide Perovskite-Based Resistive Random-Access Memories Using Electron Transport Layers for Long Endurance Cycles and Retention Time.

Recent studies have focused on exploring the potential of resistive random-access memory (ReRAM) utilizing halide perovskites as novel data storage devices. This interest stems from its notable attributes, including a high ON/OFF ratio, low operating voltages, and exceptional mechanical properties. Nevertheless, there have been reports indicating that memory systems utilizing halide perovskites encounter certain obstacles pertaining to their stability and dependability, mostly assessed through endurance and retention time. Moreover, the presence of these problems can potentially restrict their practical applicability. This study explores a resistive switching memory device utilizing MAPbBr3 perovskite, which demonstrates bipolar switching characteristics. The device fabrication procedure involves a low-temperature, all-solution process. For the purpose of enhancing the device's reliability, the utilization of TPBI(2,2',2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) as an electron transfer material on the MAPbBr3 switching layer was implemented for the first time. The formation and rupture of Ag filaments in the MAPbBr3 perovskite switching layer are attributed to reduction-oxidation reactions. The TPBI is involved in the regulation of filaments during the SET and RESET processes. Hence, it can be shown that the MAPbBr3 device incorporating TPBI exhibited about 1000 endurance cycles when subjected to continuous voltage pulses. Moreover, the device consistently maintained ON/OFF ratios above 107. In contrast, the original MAPbBr3 device without TPBI demonstrated a significantly lower endurance with only 90 cycles observed. In addition, the MAPbBr3 device integrated with TPBI exhibited a retention time exceeding 3 × 103 s. The findings of this research provide compelling evidence to support the notion that electron transfer materials have promise for the development of halide perovskite memory systems owing to their favorable attributes of dependability and stability.

Valley-centre tandem perovskite light-emitting diodes.

Perovskite light-emitting diodes (PeLEDs) have emerged as a promising new light source for displays. The development roadmap for commercializing PeLEDs should include a tandem device structure, specifically by stacking a thin nanocrystal PeLED unit and an organic light-emitting diode unit, which can achieve a vivid and efficient tandem display; however, simply combining light-emitting diodes with different characteristics does not guarantee both narrowband emission and high efficiency, as it may cause a broadened electroluminescence spectra and a charge imbalance. Here, by conducting optical simulations of the hybrid tandem (h-tandem) PeLED, we have discovered a crucial optical microcavity structure known as the h-tandem valley, which enables the h-tandem PeLED to emit light with a narrow bandwidth. Specifically, the centre structure of the h-tandem valley (we call it valley-centre tandem) demonstrates near-perfect charge balance and optimal microcavity effects. As a result, the h-tandem PeLED achieves a high external quantum efficiency of 37.0% and high colour purity with a narrow full-width at half-maximum of 27.3 nm (versus 64.5 nm in organic light-emitting diodes) along with a fast on-off response. These findings offer a new strategy to overcome the limitations of nanocrystal-based PeLEDs, providing valuable optical and electrical guidelines for integrating different types of light-emitting device into practical display applications.

Bright and stable near-infrared perovskite light emitters supported by multifunctional molecule design strategy.

Perovskite light emitters can realize bright, stable and efficient light-emitting diodes through a molecular design strategy that enables strong endurance on high-current operation.

Room-Temperature, Homogeneous, Single-Step, and Large-Scale Synthesis of Perovskite Nanoplatelets for Blue Light-Emitting Diodes.

Inorganic perovskite nanocrystals (IPNCs) have attracted considerable attention due to their excellent optoelectronic properties. However, problems arise from anion migration during the preparation of a blue light-emitting diode (LED), and only small-scale syntheses have been conducted on a laboratory scale. By using only Br as the anion here, CsPbBr3 was synthesized in the form of nanoplatelets to eliminate the effects of anion migration and to prepare an inorganic perovskite nanoplatelet (IPNPL) emitting blue light. In addition, the synthesis was performed under ambient conditions at room temperature, and the synthetic process was shortened to enable large-scale synthesis. We used a 1 L bottle for large-scale synthesis, and a photoluminescence quantum yield (PLQY) of 78% was observed at 460 nm. We fabricated LEDs by using IPNPLs, and we observed an electroluminescence peak at 461 nm. The developed synthetic method is expected to pave the way for commercialization of IPNCs and the next-generation display market.

Overcoming Charge Confinement in Perovskite Nanocrystal Solar Cells.

The small nanoparticle size and long-chain ligands in colloidal metal halide perovskite quantum dots (PeQDs) cause charge confinement, which impedes exciton dissociation and carrier extraction in PeQD solar cells, so they have low short-circuit current density Jsc , which impedes further increases in their power conversion efficiency (PCE). Here, a re-assembling process (RP) is developed for perovskite nanocrystalline (PeNC) films made of colloidal perovskite nanocrystals to increase Jsc in PeNC solar cells. The RP of PeNC films increases their crystallite size and eliminates long-chain ligands, and thereby overcomes the charge confinement in PeNC films. These changes facilitate exciton dissociation and increase carrier extraction in PeNC solar cells. By use of this method, the gradient-bandgap PeNC solar cells achieve a Jsc = 19.30 mA cm-2 without compromising the photovoltage, and yield a high PCE of 16.46% with negligible hysteresis and good stability. This work provides a new strategy to process PeNC films and pave the way for high performance PeNC optoelectronic devices.

Transition-Metal-Free Upscaling of 2,5-Furandicarboxylic Acid Synthesis and Investigation of the Reaction Mechanism.

An environmentally friendly oxidation system has proposed for the practical and scalable production of value-added 2,5-furandicarboxylic acid from 1 kg of 5-hydroxymethylfurfural. The system is composed of a simple base, oxygen, and a green solvent, thereby providing a sustainable and economical approach to organic synthesis. To gain insight into the mechanism of this oxidation process, NMR spectroscopic analysis and kinetic study are used for the mechanistic investigation of this environmentally friendly oxidation process.

Transition-Metal-Free Unusual Oxidative Cleavage of Homoallylic Alcohol and Its Application in the Upcycling of Terpene to Value-Added Chemicals.

A NaOtBu-O2 -mediated oxidative dehomologation of homoallylic alcohols into structurally different carboxylic acids through direct oxidative cleavage of either the C(sp2 )-C(sp2 ) or C(sp3 )-C(sp3 ) bond utilizing enolate chemistry was demonstrated. Furthermore, under transition-metal-free conditions, this protocol has been applied to convert terpene as biomass feedstock into value-added chemicals.

Advances in Solution-Processed OLEDs and their Prospects for Use in Displays.

This review outlines problems and progress in development of solution-processed organic light-emitting diodes (SOLEDs) in industry and academia. Solution processing has several advantages such as low consumption of materials, low-cost processing, and large-area manufacturing. However, use of a solution process entails complications, such as the need for solvent resistivity and solution-processable materials, and yields SOLEDs that have limited luminous efficiency, severe roll-off characteristics, and short lifetime compared to OLEDs fabricated using thermal evaporation. These demerits impede production of practical SOLED displays. This review outlines the industrial demands for commercial SOLEDs and the current status of SOLED development in industries and academia, and presents research guidelines for the development of SOLEDs that have high efficiency, long lifetime, and good processability to achieve commercialization.

A low-power stretchable neuromorphic nerve with proprioceptive feedback.

By relaying neural signals from the motor cortex to muscles, devices for neurorehabilitation can enhance the movement of limbs in which nerves have been damaged as a consequence of injuries affecting the spinal cord or the lower motor neurons. However, conventional neuroprosthetic devices are rigid and power-hungry. Here we report a stretchable neuromorphic implant that restores coordinated and smooth motions in the legs of mice with neurological motor disorders, enabling the animals to kick a ball, walk or run. The neuromorphic implant acts as an artificial efferent nerve by generating electrophysiological signals from excitatory post-synaptic signals and by providing proprioceptive feedback. The device operates at low power (~1/150 that of a typical microprocessor system), and consists of hydrogel electrodes connected to a stretchable transistor incorporating an organic semiconducting nanowire (acting as an artificial synapse), connected via an ion gel to an artificial proprioceptor incorporating a carbon nanotube strain sensor (acting as an artificial muscle spindle). Stretchable electronics with proprioceptive feedback may inspire the further development of advanced neuromorphic devices for neurorehabilitation.