High-Density Integration of Ultrabright OLEDs on a Miniaturized Needle-Shaped CMOS Backplane.

Direct deposition of organic light-emitting diodes (OLEDs) on silicon-based complementary metal-oxide-semiconductor (CMOS) chips has enabled self-emissive microdisplays with high resolution and fill-factor. Emerging applications of OLEDs in augmented and virtual reality (AR/VR) displays and in biomedical applications, e.g., as brain implants for cell-specific light delivery in optogenetics, require light intensities orders of magnitude above those found in traditional displays. Further requirements often include a microscopic device footprint, a specific shape and ultrastable passivation, e.g., to ensure biocompatibility and minimal invasiveness of OLED-based implants. In this work, up to 1024 ultrabright, microscopic OLEDs are deposited directly on needle-shaped CMOS chips. Transmission electron microscopy and energy-dispersive X-ray spectroscopy are performed on the foundry-provided aluminum contact pads of the CMOS chips to guide a systematic optimization of the contacts. Plasma treatment and implementation of silver interlayers lead to ohmic contact conditions and thus facilitate direct vacuum deposition of orange- and blue-emitting OLED stacks leading to micrometer-sized pixels on the chips. The electronics in each needle allow each pixel to switch individually. The OLED pixels generate a mean optical power density of 0.25 mW mm-2 , corresponding to >40 000 cd m-2 , well above the requirement for daylight AR applications and optogenetic single-unit activation in the brain.

An Exciplex-Based Light-Emission Pathway for Solution-State Electrochemiluminescent Devices.

Electrochemiluminescence (ECL) allows the design of unique light-emitting devices that use organic semiconductors in a liquid or gel state, which allows for simpler and more sustainable device fabrication and facilitates unconventional device form-factors. Compared to solid-state organic LEDs, ECL devices (ECLDs) have attracted less attention due to their currently much lower performance. ECLD operation is typically based on an annihilation pathway that involves electron transfer between reduced and oxidized luminophore species; the intermediate radical ions produced during annihilation dramatically reduce device stability. Here, the effects of radical ions are mitigated by an exciplex formation pathway and a remarkable improvement in luminance, luminous efficacy, and operational lifetime is demonstrated. Electron donor and acceptor molecules are dissolved at high concentrations and recombined as an exciplex upon their oxidization/reduction. The exciplex then transfers its energy to a nearby dye, allowing the dye to emit light without undergoing oxidation/reduction. Furthermore, the application of a mesoporous TiO2 electrode increases the contact area and hence the number of molecules participating in ECL , thereby obtaining devices with a very high luminance of 3790 cd m-2 and a 30-fold improved operational lifetime. This study paves the way for the development of ECLDs into highly versatile light sources.

Crystal Facet Engineering of TiO2 Nanostructures for Enhancing Photoelectrochemical Water Splitting with BiVO4 Nanodots.

Although bismuth vanadate (BiVO4) has been promising as photoanode material for photoelectrochemical water splitting, its charge recombination issue by short charge diffusion length has led to various studies about heterostructure photoanodes. As a hole blocking layer of BiVO4, titanium dioxide (TiO2) has been considered unsuitable because of its relatively positive valence band edge and low electrical conductivity. Herein, a crystal facet engineering of TiO2 nanostructures is proposed to control band structures for the hole blocking layer of BiVO4 nanodots. We design two types of TiO2 nanostructures, which are nanorods (NRs) and nanoflowers (NFs) with different (001) and (110) crystal facets, respectively, and fabricate BiVO4/TiO2 heterostructure photoanodes. The BiVO4/TiO2 NFs showed 4.8 times higher photocurrent density than the BiVO4/TiO2 NRs. Transient decay time analysis and time-resolved photoluminescence reveal the enhancement is attributed to the reduced charge recombination, which is originated from the formation of type II band alignment between BiVO4 nanodots and TiO2 NFs. This work provides not only new insights into the interplay between crystal facets and band structures but also important steps for the design of highly efficient photoelectrodes.

External Quantum Efficiency Exceeding 24% with CIEy Value of 0.08 using a Novel Carbene-Based Iridium Complex in Deep-Blue Phosphorescent Organic Light-Emitting Diodes.

Deep-blue triplet emitters remain far inferior to standard red and green triplet emitters in terms of exhibiting high-color-purity Commission International de l'Éclairage (CIE) y values of ≤0.1, external quantum efficiencies (EQEs), and high electroluminescent brightnesses in phosphorescent organic light-emitting diodes. In fact, no deep-blue triplet emitter with color purity and high device performance has previously been reported. In this study, a deep-blue triplet emitter, mer-tris(N-phenyl, N-benzyl-pyridoimidazol-2-yl)iridium(III) (mer-Ir1) is developed, which meets the requirements of the National Television System Committee (NTSC) CIE(x, y) coordinates of (0.149, 0.085) with an extremely high EQE of 24.8% and maximum brightness (Lmax ) of 6453 cd m-2 , by a device with a 40 vol% doping ratio. Moreover, another device demonstrates an EQEmax of 21.3%, an Lmax of 5247 cd m-2 , and CIE(x, y) coordinates of (0.151, 0.086) at a 30 vol% doping ratio. This is the first report of a high-performance, deep-blue phosphor, carbene-based Ir(III) complex device with outstanding CIE(x, y) color coordinates and a high EQE. The results of this study indicate that the novel dopant mer-Ir1 is a promising candidate for reducing power consumption in display applications.

A Broadband Multiplex Living Solar Cell.

Developing renewable and sustainable energy sources is a compelling goal in materials science and engineering. In particular, natural photosynthesis with its infinite energy reservoir provides profound inspiration for energy conversion and storage systems. Here, we report a multiplex living solar cell that offers a drastic power enhancement by harnessing the broadband spectra of the visible wavelength range for photosynthesis. Cyanobacteria are embedded into a nanostructural complex composed of Au nanoparticles (NPs) and ZnO nanorods (NRs). This nanocomposite system is capable of not only generating excitons but also amplifying the photosynthetic performance of the cell via a far-field scattering effect in the broadband region of the light, resulting in multiplex energy harvesting with a peak power density of 6.15 mW/m2. We envision that this study will provide a strategic way to enhance the performance of biophotovoltaics, enabling efficient and durable energy generation.

Lensfree OLEDs with over 50% external quantum efficiency via external scattering and horizontally oriented emitters.

High efficiency is important for successful deployment of any light sources. Continued efforts have recently made it possible to demonstrate organic light-emitting diodes with efficiency comparable to that of inorganic light-emitting diodes. However, such achievements were possible only with the help of a macroscopic lens or complex internal nanostructures, both of which undermine the key benefits of organic light-emitting diodes as an affordable planar light source. Here we present a systematic way to achieve organic light-emitting diodes with ultrahigh efficiency even only with an external scattering film, one of the simplest low-cost outcoupling structures. Through a global, multivariable analysis, we show that scattering with a high degree of forwardness has a potential to play a critical role in realizing ultimate efficiency. Combined with horizontally oriented emitters, organic light-emitting diodes equipped with particle-embedded films tailored for forward-intensive scattering achieve a maximum external quantum efficiency of 56%.

Unraveling the orientation of phosphors doped in organic semiconducting layers.

Emitting dipole orientation is an important issue of emitting materials in organic light-emitting diodes for an increase of outcoupling efficiency of light. The origin of preferred orientation of emitting dipole of iridium-based heteroleptic phosphorescent dyes doped in organic layers is revealed by simulation of vacuum deposition using molecular dynamics along with quantum mechanical characterization of the phosphors. Consideration of both the electronic transitions in a molecular frame and the orientation of the molecules at the vacuum/molecular film interface allows quantitative analyses of the emitting dipole orientation depending on host molecules and dopant structures. Interactions between the phosphor and nearest host molecules on the surface, minimizing the non-bonded van der Waals and electrostatic interaction energies determines the molecular alignment during the vacuum deposition. Parallel alignment of the main cyclometalating ligands in the molecular complex due to host interactions rather than the ancillary ligand orienting to vacuum leads to the horizontal emitting dipole orientation.Iridium-based phosphors show high photoluminescence quantum yield in organic light-emitting diodes. Here, Moon et al. reveal the mechanism responsible for the preferred orientation of iridium complexes in an organic host and highlight the interaction between phosphor and host molecules at play.

Combined Inter- and Intramolecular Charge-Transfer Processes for Highly Efficient Fluorescent Organic Light-Emitting Diodes with Reduced Triplet Exciton Quenching.

Inter- and intramolecular charge-transfer processes are combined using an exciplex-forming host and a thermally activated delayed fluorescent dopant, for fabricating efficient fluorescent organic light-emitting diodes along with the reduced efficiency roll-off at high current densities. Extra conversion on the host from triplet exciplexes to singlet exciplexes followed by energy transfer to the dopant reduces population of triplet excitons on dopant molecules, thereby reducing the triplet exciton annihilations at high current densities.

Quantitative Analysis of the Efficiency of OLEDs.

We present a comprehensive model for the quantitative analysis of factors influencing the efficiency of organic light-emitting diodes (OLEDs) as a function of the current density. The model takes into account the contribution made by the charge carrier imbalance, quenching processes, and optical design loss of the device arising from various optical effects including the cavity structure, location and profile of the excitons, effective radiative quantum efficiency, and out-coupling efficiency. Quantitative analysis of the efficiency can be performed with an optical simulation using material parameters and experimental measurements of the exciton profile in the emission layer and the lifetime of the exciton as a function of the current density. This method was applied to three phosphorescent OLEDs based on a single host, mixed host, and exciplex-forming cohost. The three factors (charge carrier imbalance, quenching processes, and optical design loss) were influential in different ways, depending on the device. The proposed model can potentially be used to optimize OLED configurations on the basis of an analysis of the underlying physical processes.

Highly efficient non-doped deep blue fluorescent emitters with horizontal emitting dipoles using interconnecting units between chromophores.

New deep blue fluorescent emitters composed of anthracene as an electron rich unit, a diphenyltriazine as a strong electron acceptor unit, and phenyl or xylene as interconnecting units were synthesised. The interconnecting unit between chromophores increased the singlet transition energy and the ratio of horizontal emitting dipoles. As a result, a non-doped blue fluorescent organic light-emitting diode (OLED) using a new emitter was demonstrated, with an external quantum efficiency (EQE) of 6.6% and Commision Internationale de l'Eclairage (CIE) colour coordinates of (0.145, 0.068). This device performance has been the highest EQE observed in deep blue non-doped OLEDs with CIE coordinates less than (0.145, 0.068) to date.

Na3 SbS4 : A Solution Processable Sodium Superionic Conductor for All-Solid-State Sodium-Ion Batteries.

All-solid-state sodium-ion batteries that operate at room temperature are attractive candidates for use in large-scale energy storage systems. However, materials innovation in solid electrolytes is imperative to fulfill multiple requirements, including high conductivity, functional synthesis protocols for achieving intimate ionic contact with active materials, and air stability. A new, highly conductive (1.1 mS cm(-1) at 25 °C, Ea =0.20 eV) and dry air stable sodium superionic conductor, tetragonal Na3 SbS4 , is described. Importantly, Na3 SbS4 can be prepared by scalable solution processes using methanol or water, and it exhibits high conductivities of 0.1-0.3 mS cm(-1) . The solution-processed, highly conductive solidified Na3 SbS4 electrolyte coated on an active material (NaCrO2 ) demonstrates dramatically improved electrochemical performance in all-solid-state batteries.

Phosphorescent OLEDs: Sky-Blue Phosphorescent OLEDs with 34.1% External Quantum Efficiency Using a Low Refractive Index Electron Transporting Layer (Adv. Mater. 24/2016).

J.-J. Kim and co-workers achieve highly efficient blue organic light-emitting diodes (OLEDs) using a low-refractive-index layer. As described on page 4920, an external quantum efficiency over 34% is achieved, owing to the low refractive index of the materials. A milepost and a shining entrance of the castle are the metaphor indicating the way to highly efficient blue OLEDs. On the way to the castle, the depicted chemical structures serve as the light-emitting layer.

Sky-Blue Phosphorescent OLEDs with 34.1% External Quantum Efficiency Using a Low Refractive Index Electron Transporting Layer.

Blue-phosphorescent organic light-emitting diodes (OLEDs) with 34.1% external quantum efficiency (EQE) and 79.6 lm W(-1) are demonstrated using a hole-transporting layer and electron-transporting layer with low refractive index values. Using optical simulations, it is predicted that outcoupling efficiencies with EQEs > 60% can be achieved if organic layers with a refractive index of 1.5 are used for OLEDs.

Highly Efficient Sky-Blue Fluorescent Organic Light Emitting Diode Based on Mixed Cohost System for Thermally Activated Delayed Fluorescence Emitter (2CzPN).

The mixed cohosts of 1,3-bis(N-carbazolyl)benzene and 2,8-bis(diphenylphosphoryl)dibenzothiophene have been developed for a highly efficient blue fluorescent oragnic light emitting diode (OLED) doped with a thermally activated delayed fluorescence (TADF) emitter [4,5-di (9H-carbazol-9-yl) phthalonitrile (2CzPN)]. We have demonstrated one of the highest external quantum efficiency of 21.8% in blue fluorescent OLEDs, which is identical to the theoretically achievable maximum electroluminescence efficiency using the emitter. Interestingly, the efficiency roll-off is large even under the excellent charge balance in the device and almost the same as the single host based devices, indicating that the efficiency roll-off in 2CzPN based TADF host is related to the material characteristics, such as low reverse intesystem crossing rate rather than charge imbalance.

Crystal Organic Light-Emitting Diodes with Perfectly Oriented Non-Doped Pt-Based Emitting Layer.

Organic light-emitting diodes with external quantum efficiency of 38.8% are realized using a Pt-based thin-film emitting layer with photoluminescence quantum yield of 96% and transition dipole ratio of 93%. The emitting dipole orientation of the thin films fabricated using Pt complexes is investigated and the structural relationship between X-ray structural analysis and the structures in thin films are discussed based on quantum chemical calculations.

Efficient Vacuum-Deposited Ternary Organic Solar Cells with Broad Absorption, Energy Transfer, and Enhanced Hole Mobility.

The use of multiple donors in an active layer is an effective way to boost the efficiency of organic solar cells by broadening their absorption window. Here, we report an efficient vacuum-deposited ternary organic photovoltaic (OPV) using two donors, 2-((2-(5-(4-(diphenylamino)phenyl)thieno[3,2-b]thiophen-2-yl)thiazol-5-yl)methylene)malononitrile (DTTz) for visible absorption and 2-((7-(5-(dip-tolylamino)thiophen-2-yl)benzo[c]-[1,2,5]thiadiazol-4-yl)methylene)malononitrile (DTDCTB) for near-infrared absorption, codeposited with C70 in the ternary layer. The ternary device achieved a power conversion efficiency of 8.02%, which is 23% higher than that of binary OPVs. This enhancement is the result of incorporating two donors with complementary absorption covering wavelengths of 350 to 900 nm with higher hole mobility in the ternary layer than that of binary layers consisting of one donor and C70, combined with energy transfer from the donor with lower hole mobility (DTTz) to that with higher mobility (DTDCTB). This structure fulfills all the requirements for efficient ternary OPVs.

Luminescence from oriented emitting dipoles in a birefringent medium.

We present an optical model to describe the luminescence from oriented emitting dipoles in a birefringent medium and validate the theoretical model through its applications to a dye doped organic thin film and organic light emitting diodes (OLEDs). We demonstrate that the optical birefringence affects not only far-field radiation characteristics such as the angle-dependent emission spectrum and intensity from the thin film and OLEDs, but also the outcoupling efficiency of OLEDs. The orientation of emitting dipoles in a birefringent medium is successfully analyzed from the far-field radiation pattern of a thin film using the model. In addition, the birefringent model presented here provides a precise analysis of the angle-dependent EL spectra and efficiencies of OLEDs with the determined emitting dipole orientation.

Vacuum nanohole array embedded phosphorescent organic light emitting diodes.

Light extraction from organic light-emitting diodes that utilize phosphorescent materials has an internal efficiency of 100% but is limited by an external quantum efficiency (EQE) of 30%. In this study, extremely high-efficiency organic light emitting diodes (OLEDs) with an EQE of greater than 50% and low roll-off were produced by inserting a vacuum nanohole array (VNHA) into phosphorescent OLEDs (PhOLEDs). The resultant extraction enhancement was quantified in terms of EQE by comparing experimentally measured results with those produced from optical modeling analysis, which assumes the near-perfect electric characteristics of the device. A comparison of the experimental data and optical modeling results indicated that the VNHA extracts the entire waveguide loss into the air. The EQE obtained in this study is the highest value obtained to date for bottom-emitting OLEDs.

Phosphorescent dye-based supramolecules for high-efficiency organic light-emitting diodes.

Organic light-emitting diodes (OLEDs) are among the most promising organic semiconductor devices. The recently reported external quantum efficiencies (EQEs) of 29-30% for green and blue phosphorescent OLEDs are considered to be near the limit for isotropically oriented iridium complexes. The preferred orientation of transition dipole moments has not been thoroughly considered for phosphorescent OLEDs because of the lack of an apparent driving force for a molecular arrangement in all but a few cases, even though horizontally oriented transition dipoles can result in efficiencies of over 30%. Here we use quantum chemical calculations to show that the preferred orientation of the transition dipole moments of heteroleptic iridium complexes (HICs) in OLEDs originates from the preferred direction of the HIC triplet transition dipole moments and the strong supramolecular arrangement within the co-host environment. We also demonstrate an unprecedentedly high EQE of 35.6% when using HICs with phosphorescent transition dipole moments oriented in the horizontal direction.

A fluorescent organic light-emitting diode with 30% external quantum efficiency.

Almost 100% internal quantum efficiency (IQE) is achieved with a green fluorescent organic light-emitting diode (OLED) exhibiting 30% external quantum efficiency (EQE). The OLED comprises an exciplex-forming co-host system doped with a fluorescent dye that has a strong delayed fluorescence as a result of reverse intersystem crossing (RISC); the exciplex-forming co-hosts stimulate energy transfer and charge balance in the system. The orientation of the transition dipole moment of the fluorescent dye is shown to have an influence on the EQE of the device.