Nanostructured Molecular Packing of Polymer Films Formed on Water Surfaces.

In this study, we examined the nanostructured molecular packing and orientations of poly[[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)] (P(NDI2OD-T2)) films formed on water for the application of nanotechnology-based organic electronic devices. First, the nanoscale molecule-substrate interaction between the polymer and water was modulated by controlling the alkyl side chain length in NDI-based copolymers. Increasing alkyl side chain lengths induced a nanomorphological transition from face-on to edge-on orientation, confirmed by molecular dynamics simulations revealing nanostructural behavior. Second, the nanoscale intermolecular interactions of P(NDI2OD-T2) were controlled by varying the volume ratio of the high-boiling-point additive solvent in the binary solvent blends. As the additive solvent ratio increased, the nanostructured molecular orientation of the P(NDI2OD-T2) films on water changed remarkably from edge-on to bimodal with more face-on crystallites, thereby affecting charge transport. Our finding provides essential insights for precise nanoscale morphological control on water substrates, enabling the formation of high-performance polymer films for organic electronic devices.

Stretchable OLEDs based on a hidden active area for high fill factor and resolution compensation.

Stretchable organic light-emitting diodes (OLEDs) have emerged as promising optoelectronic devices with exceptional degree of freedom in form factors. However, stretching OLEDs often results in a reduction in the geometrical fill factor (FF), that is the ratio of an active area to the total area, thereby limiting their potential for a broad range of applications. To overcome these challenges, we propose a three-dimensional (3D) architecture adopting a hidden active area that serves a dual role as both an emitting area and an interconnector. For this purpose, an ultrathin OLED is first attached to a 3D rigid island array structure through quadaxial stretching for precise, deformation-free alignment. A portion of the ultrathin OLED is concealed by letting it 'fold in' between the adjacent islands in the initial, non-stretched condition and gradually surfaces to the top upon stretching. This design enables the proposed stretchable OLEDs to exhibit a relatively high FF not only in the initial state but also after substantial deformation corresponding to a 30% biaxial system strain. Moreover, passive-matrix OLED displays that utilize this architecture are shown to be configurable for compensation of post-stretch resolution loss, demonstrating the efficacy of the proposed approach in realizing the full potential of stretchable OLEDs.

Understanding of complex spin up-conversion processes in charge-transfer-type organic molecules.

Despite significant progress made over the past decade in thermally activated delayed fluorescence (TADF) molecules as a material paradigm for enhancing the performance of organic light-emitting diodes, the underlying spin-flip mechanism in these charge-transfer (CT)-type molecular systems remains an enigma, even since its initial report in 2012. While the initial and final electronic states involved in spin-flip between the lowest singlet and lowest triplet excited states are well understood, the exact dynamic processes and the role of intermediate high-lying triplet (T) states are still not fully comprehended. In this context, we propose a comprehensive model to describe the spin-flip processes applicable for a typical CT-type molecule, revealing the origin of the high-lying T state in a partial molecular framework in CT-type molecules. This work provides experimental and theoretical insights into the understanding of intersystem crossing for CT-type molecules, facilitating more precise control over spin-flip rates and thus advancing toward developing the next-generation platform for purely organic luminescent candidates.

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.

Organic Phototransistor with Light-Induced Contact Modulation and Sensitivity Enhancement Using a C60/C70:TAPC Hybrid Channel.

Organic phototransistors (OPTs) are attracting a significant degree of interest as devices that have the potential to play multiple roles, including light sensing, signal amplification, and switching for addressing when they are used for matrix arrays. However, it has been challenging to realize OPTs that can perform all of these roles simultaneously at a sufficient performance level because the channel materials with high carrier mobility often exhibit relatively low photoabsorption. In this work, we propose OPTs with a hybrid bilayer channel consisting of a neat C60 layer and a bulk-heterojunction layer of C70 and 1,1-bis(4-bis(4-methyl-phenyl)-amino-phenyl)-cyclohexane (TAPC) as a possible solution to this issue. While the C60 layer serves as the main carrier-transporting layer with high mobility, the C70:TAPC layer operates as a photoactive layer wherein the photogenerated carriers provide photoinduced contact modulation that leads to a significant enhancement in photosensitivity. With the optimal design maximizing the absorption, the proposed hybrid-channel OPTs show a responsivity of ca. 180 A/W, which is 4.5 times higher than that of the control OPT with a C70:TAPC single channel. The operation mechanism and the origin for the improvement are verified by an in-depth analysis of the photoinduced modulation of the channel and contact resistances of the OPTs.

OLED catheters for inner-body phototherapy: A case of type 2 diabetes mellitus improved via duodenal photobiomodulation.

Phototherapeutics has shown promise in treating various diseases without surgical or drug interventions. However, it is challenging to use it in inner-body applications due to the limited light penetration depth through the skin. Therefore, we propose an organic light-emitting diode (OLED) catheter as an effective photobiomodulation (PBM) platform useful for tubular organs such as duodenums. A fully encapsulated highly flexible OLED is mounted over a round columnar structure, producing axially uniform illumination without local hotspots. The biocompatible and airtight OLED catheter can operate in aqueous environments for extended periods, meeting the essential requirements for inner-body medical applications. In a diabetic Goto-Kakizaki (GK) rat model, the red OLED catheter delivering 798 mJ of energy is shown to reduce hyperglycemia and insulin resistance compared to the sham group. Results are further supported by the subdued liver fibrosis, illustrating the immense potential of the OLED-catheter-based internal PBM for the treatment of type 2 diabetes and other diseases yet to be identified.

Fast and rigorous optical simulation of periodically corrugated light-emitting diodes based on a diffraction matrix method.

Increasing the light extraction efficiency has been widely studied for highly efficient organic light-emitting diodes (OLEDs). Among many light-extraction approaches proposed so far, adding a corrugation layer has been considered a promising solution for its simplicity and high effectiveness. While the working principle of periodically corrugated OLEDs can be qualitatively explained by the diffraction theory, dipolar emission inside the OLED structure makes its quantitative analysis challenging, making one rely on finite-element electromagnetic simulations that could require huge computing resources. Here, we demonstrate a new simulation method, named the diffraction matrix method (DMM), that can accurately predict the optical characteristics of periodically corrugated OLEDs while achieving calculation speed that is a few orders of magnitude faster. Our method decomposes the light emitted by a dipolar emitter into plane waves with different wavevectors and tracks the diffraction behavior of waves using diffraction matrices. Calculated optical parameters show a quantitative agreement with those predicted by finite-difference time-domain (FDTD) method. Furthermore, the developed method possesses a unique advantage over the conventional approaches that it naturally evaluates the wavevector-dependent power dissipation of a dipole and is thus capable of identifying the loss channels inside OLEDs in a quantitative manner.

Toward highly efficient deep-blue OLEDs: Tailoring the multiresonance-induced TADF molecules for suppressed excimer formation and near-unity horizontal dipole ratio.

Boron-based compounds exhibiting a multiresonance thermally activated delayed fluorescence are regarded promising as a narrowband blue emitter desired for efficient displays with wide color gamut. However, their planar nature makes them prone to concentration-induced excimer formation that broadens the emission spectrum, making it hard to increase the emitter concentration without raising CIE y coordinate. To overcome this bottleneck, we here propose o-Tol-ν-DABNA-Me, wherein sterically hindered peripheral phenyl groups are introduced to reduce intermolecular interactions, leading to excimer formation and thus making the pure narrowband emission character far less sensitive to concentration. With this approach, we demonstrate deep-blue OLEDs with y of 0.12 and full width at half maximum of 18 nm, with maximum external quantum efficiency (EQE) of ca. 33%. Adopting a hyperfluorescent architecture, the OLED performance is further enhanced to EQE of 35.4%, with mitigated efficiency roll-off, illustrating the immense potential of the proposed method for energy-efficient deep-blue OLEDs.

Heteroleptic Ir(III)-based near-infrared organic light-emitting diodes with high radiance capacity.

Near-infrared organic light-emitting diodes (NIR OLEDs) with heavy metals are regularly reported due to the advantages of their various applications in healthcare services, veil authentication, and night vision displays. For commercial applications, it is necessary to look at radiance capacity (RC) instead of radiance because of power consumption. However, recent papers still reported only simple high radiance performance and do not look at device from the point of view of RC. To overcome this hurdle, we designed Ir(III)-based heteroleptic NIR materials with two types of auxiliary ligand. The proposed emitters achieve a highly oriented horizontal dipole ratio (Ir(mCPDTiq)2tmd, complex 1: 80%, Ir(mCPDTiq)2acac, complex 2: 81%) with a short radiative lifetime (1: 386 ns, 2: 323 ns). The device also shows an extremely low turn-on voltage (Von) of 2.2 V and a high RC of 720 mW/sr/m2/V. The results on the Von and RC of the device is demonstrated an outstanding performance among the Ir(III)-based NIR OLEDs with a similar emission peak.

The effect of proto-type wearable light-emitting devices on serum 25-hydroxyvitamin D levels in healthy adults: a 4-week randomized controlled trial.

BACKGROUND: Many people in modern society have insufficient exposure to ultraviolet B (UVB) sunlight, which may lead to vitamin D deficiency. We aimed to investigate the effect of a proto-type wearable light-emitting diode (LED) device emitting UVB light on serum 25-hydroxyvitamin D levels. METHODS: A total of 136 healthy adults were randomly assigned to receive either an active device emitting UVB light with a peak wavelength of 285 nm (n = 64) or a sham device emitting visible light (n = 72). All participants wore the device for a total of two minutes, one minute on each forearm, every day for 4 weeks. Serum 25-hydroxyvitamin D levels were assessed at baseline, 2, and 4 weeks of intervention, and 2 weeks after the end of the intervention. RESULTS: A significant difference was found between the experimental and control groups in changes in serum 25-hydroxyvitamin D levels from baseline after two (0.25 ± 3.10 ng/mL vs. -1.07 ± 2.68 ng/mL, p = 0.009) and 4 weeks of intervention (0.75 ± 3.98 ng/mL vs. -1.75 ± 3.04 ng/mL, p < 0.001). In the experimental group, the dropout rate due to mild, self-limiting adverse skin reactions was 11.8% (9/76). The mean total 25-hydroxyvitamin D production after UVB exposure was estimated at 0.031 ng/mL per 1 cm2 of skin area. CONCLUSIONS: A prototype wearable LED UVB device was effective for improving 25-hydroxyvitamin D status. The development of a safer wearable LED device for phototherapy may provide a novel daily, at-home option for vitamin D supplementation.

Optimal flickering light stimulation for entraining gamma rhythms in older adults.

With aging, optimal parameters of flickering light stimulation (FLS) for gamma entrainment may change in the eyes and brain. We investigated the optimal FLS parameters for gamma entrainment in 35 cognitively normal old adults by comparing event-related synchronization (ERS) and spectral Granger causality (sGC) of entrained gamma rhythms between different luminance intensities, colors, and flickering frequencies of FLSs. ERS entrained by 700 cd/m2 FLS and 32 Hz or 34 Hz FLSs was stronger than that entrained by 400 cd/m2 at Pz (p < 0.01) and 38 Hz or 40 Hz FLSs, respectively, at both Pz (p < 0.05) and Fz (p < 0.01). Parieto-occipital-to-frontotemporal connectivities of gamma rhythm entrained by 700 cd/m2 FLS and 32 Hz or 34 Hz FLSs were also stronger than those entrained by 400 cd/m2 at Pz (p < 0.01) and 38 Hz or 40 Hz FLSs, respectively (p < 0.001). ERS and parieto-occipital-to-frontotemporal connectivities of entrained gamma rhythms did not show significant difference between white and red lights. Adverse effects were comparable between different parameters. In older adults, 700 cd/m2 FLS at 32 Hz or 34 Hz can entrain a strong gamma rhythm in the whole brain with tolerable adverse effects.

Simple and practical methods for utilizing parylene C film based on vertical deposition and laser patterning.

We propose two novel methods to effectively utilize parylene C films. First, we demonstrate a vertical deposition method capable of depositing a parylene C film of the same thickness on both sides of a sample. Through this method, we have formed parylene C films with a thickness of 4 µm on both sides of the sample with a thickness deviation of less than 2.5%. Further optical verification indicates that parylene C films formed by this method have a very uniform thickness distribution on each side of the surfaces. Second, we propose a debris-tolerant laser patterning method as a mask-less means to fabricate self-supporting ultrathin parylene C films. This method does not involve any photolithography and entails a simple and rapid process that can be performed using only a few materials with excellent biocompatibility. It is demonstrated that patterned parylene C films exhibit a high degree of surface uniformity and have various geometrical shapes so that they can be used for substrates of highly flexible and/or stretchable devices. Finally, we use both of the proposed methods to fabricate flexible, stretchable, and waterproof-packaged bifacial blue LED modules to illustrate their potential in emerging applications that would benefit from such versatile form factors.

Ultralong persistent luminescence from carbon dots.

Hour-level persistent luminescence is realized with carbon dots embedded in cyanuric acid, the composition of which is easily obtained by the microwave-assisted heating of carbon dots and urea. By forming donor-acceptor blends, the proposed composition yields intermediate states with long lifetimes, providing a rare-earth-metal-free route to ultralong persistent luminescence.

Role of Electronic States and Their Coupling on Radiative Losses of Open-Circuit Voltage in Organic Photovoltaics.

Voltage losses (ΔVOC) are a crucial limitation for the performance of excitonic organic solar cells (OSCs) and can be estimated by two approaches─the radiative limit and the Marcus charge-transfer (MCT) model. In this work, we show that combining the radiative limit and MCT models for voltage loss calculations provides useful insights into the physics of emerging efficient OSCs. We studied nine different donor-acceptor systems, wherein the power conversion efficiency ranges from 4.4 to 14.1% and ΔVOC varies from 0.55 to 0.95 V. For these state-of-the-art devices, we calculated the ΔVOC using the radiative limit and the MCT model. Furthermore, we combined both models to derive new insights on the origin of radiative voltage losses (ΔVrad) in OSCs. We quantified the contribution in ΔVrad due to the bulk intramolecular (S1) disorder and interfacial intermolecular (CT) disorder by revisiting the spectral regions of interest for OSCs. Our findings are in agreement with the expected relationship of VOC with Urbach energy (EU), which suggests that the low EU is beneficial for reduced losses. However, unprecedentedly, we also identify a universal, almost linear relationship between the interfacial disorder (λ) and ΔVrad. We believe that these results can be exploited by the organic photovoltaic (OPV) community for the design of new molecules and a combination of donor-acceptors to further improve OSCs.

Identification of a multi-stack structure of graphene electrodes doped layer-by-layer with benzimidazole and its implication for the design of optoelectronic devices.

Optical properties of benzimidazole (BI)-doped layer-by-layer graphene differ significantly from those of intrinsic graphene. Our study based on transmission electron microscopy and X-ray photoelectron spectroscopy depth profiling reveals that such a difference stems from its peculiar stratified geometry formed in situ during the doping process. This work presents an effective thickness and optical constants that can treat these multi-stacked BI-doped graphene electrodes as a single equivalent medium. For verification, the efficiency and angular emission spectra of organic light-emitting diodes with the BI-doped graphene electrode are modeled with the proposed method, and we demonstrate that the calculation matches experimental results in a much narrower margin than that based on the optical properties of undoped graphene.

Blue TADF Emitters Based on B-Heterotriangulene Acceptors for Highly Efficient OLEDs with Reduced Efficiency Roll-Off.

The design of robust boron acceptors plays a key role in the development of boron-based thermally activated delayed fluorescence (TADF) emitters for the realization of efficient and stable blue organic light-emitting diodes (OLEDs). Herein, we report a set of donor (D)-acceptor (A)-type blue TADF compounds (1-3) comprising triply bridged triarylboryl acceptors, the so-called B-heterotriangulenes, which differ depending on the identity of one of the bridging groups: methylene (1), dimethylmethylene (2), or oxo (3). The X-ray crystal structures of 2 and 3 reveal a highly twisted D-A connectivity and a completely planar geometry for the B-heterotriangulene rings. All compounds exhibit blue emissions with the unitary photoluminescence quantum yields and small singlet-triplet energy splitting (<0.1 eV) in their doped host films. The compounds exhibit a fast reverse intersystem crossing rate (kRISC ≈ 106 s-1) with short-lived delayed fluorescence (τd ≈ 2 µs), which is found to be promoted by the strong spin-orbit coupling between the local triplet excited state (3LE, T2) and singlet (S1) states. Using compounds 1-3 as the emitters, highly efficient blue TADF-OLEDs are realized. The devices based on the emitters with B-heterotriangulenes exhibit better performances than the device incorporating a singly bridged reference emitter over the whole luminance range. Notably, the device based on the fully dimethylmethylene-bridged emitter (2) achieves the highest maximum external quantum efficiency (EQE) of 28.2% and the lowest efficiency roll-off, maintaining a high EQE value of 21.2% at 1000 cd/m2.

Optimal flickering light stimulation for entraining gamma waves in the human brain.

Although light flickering at 40 Hz reduced Alzheimer's disease (AD) pathologies in mice by entraining gamma waves, it failed to reduce cerebral amyloid burden in a study on six patients with AD or mild cognitive impairment. We investigated the optimal color, intensity, and frequency of the flickering light stimulus for entraining gamma waves in young adults. We compared the event-related synchronization (ERS) values of entrained gamma waves between four different light colors (white, red, green, and blue) in the first experiment and four different luminance intensities in the second experiment. In both experiments, we compared the ERS values of entrained gamma waves between 10 different flickering frequencies from 32 to 50 Hz. We also examined the severity of six adverse effects in both experiments. We compared the propagation of gamma waves in the visual cortex to other brain regions between different luminance intensities and flickering frequencies. We found that red light entrained gamma waves most effectively, followed by white light. Lights of higher luminance intensities (700 and 400 cd/m2) entrained stronger gamma waves than those of lower luminance intensities (100 and 10 cd/m2). Lights flickering at 34-38 Hz entrained stronger and more widely spread beyond the visual cortex than those flickering at 40-50 Hz. Light of 700 cd/m2 resulted in more moderate-to-severe adverse effects than those of other luminance intensities. In humans, 400 cd/m2 white light flickering at 34-38 Hz was most optimal for gamma entrainment.

Highly efficient, heat dissipating, stretchable organic light-emitting diodes based on a MoO3/Au/MoO3 electrode with encapsulation.

Stretchable organic light-emitting diodes are ubiquitous in the rapidly developing wearable display technology. However, low efficiency and poor mechanical stability inhibit their commercial applications owing to the restrictions generated by strain. Here, we demonstrate the exceptional performance of a transparent (molybdenum-trioxide/gold/molybdenum-trioxide) electrode for buckled, twistable, and geometrically stretchable organic light-emitting diodes under 2-dimensional random area strain with invariant color coordinates. The devices are fabricated on a thin optical-adhesive/elastomer with a small mechanical bending strain and water-proofed by optical-adhesive encapsulation in a sandwiched structure. The heat dissipation mechanism of the thin optical-adhesive substrate, thin elastomer-based devices or silicon dioxide nanoparticles reduces triplet-triplet annihilation, providing consistent performance at high exciton density, compared with thick elastomer and a glass substrate. The performance is enhanced by the nanoparticles in the optical-adhesive for light out-coupling and improved heat dissipation. A high current efficiency of ~82.4 cd/A and an external quantum efficiency of ~22.3% are achieved with minimum efficiency roll-off.

Blue emission at atomically sharp 1D heterojunctions between graphene and h-BN.

Atomically sharp heterojunctions in lateral two-dimensional heterostructures can provide the narrowest one-dimensional functionalities driven by unusual interfacial electronic states. For instance, the highly controlled growth of patchworks of graphene and hexagonal boron nitride (h-BN) would be a potential platform to explore unknown electronic, thermal, spin or optoelectronic property. However, to date, the possible emergence of physical properties and functionalities monitored by the interfaces between metallic graphene and insulating h-BN remains largely unexplored. Here, we demonstrate a blue emitting atomic-resolved heterojunction between graphene and h-BN. Such emission is tentatively attributed to localized energy states formed at the disordered boundaries of h-BN and graphene. The weak blue emission at the heterojunctions in simple in-plane heterostructures of h-BN and graphene can be enhanced by increasing the density of the interface in graphene quantum dots array embedded in the h-BN monolayer. This work suggests that the narrowest, atomically resolved heterojunctions of in-plane two-dimensional heterostructures provides a future playground for optoelectronics.