Highly Efficient Blue Fluorescent Organic Light-Emitting Devices Based on λ5-Phosphinine Derivatives.

Although λ5-phosphinine derivatives are known as a promising class of blue fluorescent emitters, those photoluminescent quantum yield (PLQY) values have been reached up to 92 %, however, only a few examples have been explored as an emitter for blue organic light-emitting device (OLED), and the external quantum efficiency (EQE) has been below 2.4 % so far. In this study, we newly developed two types of blue λ5-phosphinine derivatives namely CN-COCF3 and CO2Me-CHO, and investigated the photophysical properties in the solid states. The photophysical analyses in solid state films suggested that the strong electron-accepting nature of these λ5-phosphinine derivatives caused the inferior PLQY values, and the exciplex formation with the host and neighboring materials should be avoided to improve the device efficiency. By choosing suitable host and neighboring materials with deep ionization potentials, we successfully realized efficient blue fluorescent OLEDs with EQE of over 4 % and CIE (0.14, 0.18). This is among the best in λ5-phosphinine-based blue OLEDs so far.

Synergistic Effect of Cation Composition Engineering of Hybrid Cs1-x FAx PbBr3 Nanocrystals for Self-Healing Electronics Application.

Mixed-cation hybrid perovskite nanocrystal (HPNC) with high crystallinity, color purity, and tunable optical bandgap offers a practical pathway toward next-generation displays. Herein, a two-step modified hot-injection combined with cation compositional engineering and surface treatment to synthesize high-purity cesium/formamidinium lead bromide HPNCs(Cs1-x FAx PbBr3 ) is presented. The optimized Cs0.5 FA0.5 PbBr3 light-emitting devices (LEDs) exhibit uniform luminescence of 3500 cd m-2 and a prominent current efficiency of 21.5 cd A-1 . As a proof of concept, a self-healing polymer (SHP) integrated with white LED backlight and laser prototypes exhibited 4 h autonomous self-healing through the synergistic effect of weak reversible imine bonds and stronger H-bonds. First, the SHP-HPNCs-initial and SHP-HPNCs-cut possess high long-term stability and dramatically suppressed lead leakage as low as 0.6 ppm along with a low leakage rate of 1.11 × 10-5 cm2 and 3.36 × 10-5 cm2 even over 6 months in water. Second, the Cs0.5 FA0.5 PbBr3 HPNCs and SHP-induced shattered-repaired perovskite glass substrate show the lowest lasing threshold values of 1.24 and 8.58 µJ cm-2 , respectively. This work provides an integrative and in-depth approach to exploiting SHP with intrinsic and entropic self-healing capabilities combined with HPNCs to develop robust and reliable soft-electronic backlight and laser applications.

Robust Spirobifluorene Core Based Hole Transporters with High Mobility for Long-Life Green Phosphorescent Organic Light-Emitting Devices.

Using a tailored high triplet energy hole transport layer (HTL) is a suitable way to improve the efficiency and extend the lifetime of organic light-emitting devices (OLEDs), which can use all molecular excitons of singlets and triplets. In this study, dibenzofuran (DBF)-end-capped and spirobifluorene (SBF) core-based HTLs referred as TDBFSBF1 and TDBFSBF2 were effectively developed. TDBFSBF1 exhibited a high glass transition temperature of 178 °C and triplet energy of 2.5 eV. Moreover, a high external quantum efficiency of 22.0 %, long operational lifetime at 50 % of the initial luminance of 89,000 h, and low driving voltage at 1000 cd m-2 of 2.95 V were achieved in green phosphorescent OLEDs using TDBFSBF1. Further, a high-hole mobility µh value of 1.9×10-3  cm2 V-1 s-1 was recorded in TDBFSBF2. A multiscale simulation successfully reproduced the experimental µh values and indicated that the reorganization energy was the primary factor in determining the mobility differences among these SBF core based HTLs.

Precision Synthesis and Atomistic Analysis of Deep-Blue Cubic Quantum Dots Made via Self-Organization.

As a crystal approaches a few nanometers in size, atoms become nonequivalent, bonds vibrate, and quantum effects emerge. To study quantum dots (QDs) with structural control common in molecular science, we need atomic precision synthesis and analysis. We describe here the synthesis of lead bromide perovskite magic-sized nanoclusters via self-organization of a lead malate chelate complex and PbBr3- under ambient conditions. Millisecond and angstrom resolution electron microscopic analysis revealed the structure and the dynamic behavior of individual QDs─structurally uniform cubes made of 64 lead atoms, where eight malate molecules are located on the eight corners of the cubes, and oleylammonium cations lipophilize and stabilize the edges and faces. Lacking translational symmetry, the cube is to be viewed as a molecule rather than a nanocrystal. The QD exhibits quantitative photoluminescence and stable electroluminescence at ≈460 nm with a narrow half-maximum linewidth below 15 nm, reflecting minimum structural defects. This controlled synthesis and precise analysis demonstrate the potential of cinematic chemistry for the characterization of nanomaterials beyond the conventional limit.

Nonbonding/Bonding Molecular Orbital Regulation of Nitrogen-Boron-Oxygen-embedded Blue/Green Multiresonant TADF Emitters with High Efficiency and Color Purity.

Invited for the cover of this issue is the group of H. Sasabe and J. Kido at Yamagata University in Japan. The image depicts the molecular structures of N-B-O embedded multi-resonant thermally activated delayed fluorescent (MR-TADF) emitters, which achieved ultra-pure deep-blue/green emission with high efficiency in OLEDs. Read the full text of the article at 10.1002/chem.202201605.

Nonbonding/Bonding Molecular Orbital Regulation of Nitrogen-Boron-Oxygen-embedded Blue/Green Multiresonant TADF Emitters with High Efficiency and Color Purity.

In this study, we synthesized and characterized multiresonant thermally activated delayed fluorescent (TADF) materials embedded with nitrogen-boron-oxygen (N-B-O), exhibiting color-tunability between blue and green, namely NBO, m-DiNBO, and p-DiNBO. The three emitter materials showed a high photoluminescence quantum yield (PLQY) and a state-of-the-art narrow full width at half maximum (FWHM) of 96 %/25 nm, 87 %/17 nm, and 99 %/19 nm, respectively. For m-DiNBO and p-DiNBO, the emission color could be tuned from blue to green by regulating the nonbonding/bonding molecular orbital characters. Owing to the expanded planar molecular structure, m-DiNBO and p-DiNBO showed high horizontal dipole ratio (Θ) of 88 % and 92 %, respectively. OLEDs were prepared with NBO, m-DiNBO, and p-DiNBO, exhibiting high external quantum efficiencies of 16.8 %, 24.2 %, and 21.6 %, respectively. NBO and m-DiNBO exhibited pure-blue emission with CIE coordinates of (0.137, 0.142) and (0.126, 0.098), respectively. p-DiNBO showed pure-green emission with a CIE coordinate of (0.258, 0.665).

Energy Transfer between Size-Controlled CsPbI3 Quantum Dots for Light-Emitting Diode Application.

Perovskite quantum dots (PQDs) are applicable in light-emitting diodes (LEDs) owing to their color tunability, high color purity, and excellent photoluminescence quantum yield (PLQY) in the solution state. However, a PQD film obtained through nonradiative recombination by concentration quenching and the formation of surface defects exhibited a low PLQY. In this study, we focused on the energy transfer between PQDs with different energy gaps (Eg) to reduce nonradiative recombination in the film state and consequently achieve high device performance. We prepared size-controlled PQDs measuring 10.7 nm (large-size QD; LQD) and 7.9 nm (small-size QD; SQD) with different Eg values and observed a spectral overlap between SQD emission and LQD absorption. To investigate the Förster resonance energy transfer (FRET) from SQDs to LQDs, we prepared SQD-LQD mixed QDs (MQDs). The MQD film enhanced LQD emission and exhibited a higher PLQY (52%) with a longer PL decay time (7.4 ns) than those exhibited by the neat LQD film (38% and 6.2 ns). This energy transfer was determined to be FRET by photoluminescence excitation and PL decay times. Moreover, the external quantum efficiency of an MQD-based LED increased to 15%, indicating that the FRET process can enhance the PLQY of the film and LED efficiency.

Gel permeation chromatography process for highly oriented Cs3Cu2I5 nanocrystal film.

The emergence of green materials has attracted considerable attention in the field of optoelectronics. Copper-based lead-free metal halide (with a near-unity quantum yield) obtained from Cs3Cu2I5 nanocrystals (NCs) can exhibit blue emission with a wavelength of 440 nm and provide outstanding stability for various applications. However, in practical applications, colloidal dispersion purity and film quality are inadequate toward a high-performance device. In this study, antisolvent-free gel permeation chromatography is used to purify Cs3Cu2I5 NCs. The purified Cs3Cu2I5 NCs exhibit a high photoluminescent quantum yield and provide a highly oriented single-crystal film. Density functional theory calculation results indicate that the iodide-rich surface in the NCs makes them highly stable. In addition, it has been demonstrated for the first time that the mixture of polymethyl methacrylate (PMMA) and Cs3Cu2I5 NCs has waterproofing capabilities. The composite film consisting of Cs3Cu2I5 NCs and PMMA can survive in water for several days. This result opens up more possibilities for the application of these green material.

Constructing Soluble Anthracene-Based Blue Emitters Free of Electrically Inert Alkyl Chains for Efficient Evaporation- and Solution-Based OLEDs.

Anthracene derivatives are one of the most promising blue emitters employed in organic light-emitting devices (OLEDs) because of their electrochemical and thermal stabilities. However, their high crystallinity owing to their large π-planar structures severely impedes the progress in the development of solution-based systems. In this work, we developed two types of highly soluble multifunctional anthracene derivatives terminated with ortho-biphenyl and triphenylamine moieties and showed high solubility in general organic solvents such as toluene, tetrahydrofuran, and cyclohexanone at high concentrations (>10 mg mL-1 ), and showed blue emission with a peak wavelength of ∼465 nm and a high photoluminescence quantum yield that ranges up to 81 %. Notably, these emitters are suitable for fabricating both evaporation- and solution-based systems. The evaporation-based system OLED achieved a high external quantum efficiency (EQE) of 5.4 %. While the solution-processed system realized 4.8 %, exhibiting the best performance among the anthracene-based solution-processed OLEDs so far.

Two-Step Crystallization for Low-Oxidation Tin-Based Perovskite Light-Emitting Diodes.

Metal halide perovskites attract significant attention because of their excellent optoelectronic and semiconducting properties. However, there are environmental concerns related to the toxicity of the lead metal that is mainly used in these perovskites. PEA2SnI4 perovskite is a potential candidate for lead-free perovskites because of its pure red emission. Although, undesired Sn4+ oxidation results in the deterioration of PEA2SnI4 perovskite. We demonstrate the two-step crystallization of PEA2SnI4 through the (i) reprecipitation and (ii) recrystallization processes. A film prepared using this method exhibits narrowed emission, with a full width at half-maximum from 30.0 to 26.1 nm, because of its homogeneous emission. Moreover, the Sn4+ content of two-step-crystallized PEA2SnI4 films is five times lower than that of a control film. Diffusion-ordered spectroscopy analysis indicates that the two-step precursor exhibits a smaller hydrodynamic radius crystal seed, which enhances crystallization during spin coating. The resulting two-step crystallized PEA2SnI4-based light-emitting diode (LED) exhibits a maximum external quantum efficiency (EQE) of 0.4% with an average of 0.2%, which is two times greater than that of the control device. This two-step approach may be generalized to synthesize other lead-free materials.

Four Dibenzofuran-Terminated High-Triplet-Energy Hole Transporters for High-Efficiency and Long-Life Organic Light-Emitting Devices.

The weak stability of a hole-transporter upon approaching the anion state is one of the major bottlenecks for developing long-life organic light-emitting devices (OLEDs). Therefore, in this study, we developed a series of thermally and electrically stable hole-transporters that are end-capped with four dibenzofuran units. These materials exhibit i) high bond dissociation energy (BDE) toward the anion state, ii) a high glass transition temperature (Tg >130 °C), and iii) high triplet energy (ET >2.7 eV), thereby enabling approximately 20 % high external quantum efficiency (EQE) and significantly prolonging the stability of both thermally activated delayed fluorescent (TADF) and phosphorescent OLEDs with an operation lifetime at 50 % (LT50 ) of 20 000-30 000 h at 1000 cd m-2 . In addition, investigating their structure-property relationship revealed that ionization potential (IP ), BDE, and Tg are critical prerequisites for the hole-transporter to prolong lifetime in OLEDs.

Bis(Triphenylamine)Benzodifuran Chromophores: Synthesis, Electronic Properties and Application in Organic Light-Emitting Diodes.

A series of bis(triphenylamine)benzodifuran chromophores have been synthesized and fully characterised. Starting from suitably functionalized benzodifuran (BDF) precursors, two triphenylamine (TPA) moieties are symmetrically coupled to a central BDF unit either at 4,8-positions through double bonds (1) and single bonds (2) respectively, or at 2,6-positions through double bonds (3). Their electronic absorption and photoluminescence properties as well as redox behaviour have been investigated in detail, indicating that the π-extended conjugation via vinyl linkers in 1 and 3 leads to comparatively strong electronic interactions between the relevant redox moieties TPA and BDF. Due to intriguing electronic properties and structural planarity, 3a has been applied as a dopant emitter in organic light-emitting diodes. A yellowish-green OLED exhibits a high external quantum efficiency (EQE) of 6.2%, thus exceeding the theoretical upper limit most likely due to energy transfer from an interface exciplex to an emissive layer and/or favorable horizontal orientation.

Asymmetric Spirobiacridine-based Delayed Fluorescence Emitters for High-performance Organic Light-Emitting Devices.

Invited for the cover of this issue is Hisahiro Sasabe, Junji Kido and co-workers at Yamagata University in Japan. This image depicts that the chemical structure of the acceptor is one of the most important keys to maximize the potential of triazine/acridine-based thermally activated delayed fluorescence (TADF) emitters realizing high external quantum efficiency (EQE) of over 30%. Read the full text of the article at 10.1002/chem.202101188.

Metabolic responses to polychromatic LED and OLED light at night.

Light exposure at night has various implications for human health, but little is known about its effects on energy metabolism during subsequent sleep. We investigated the effects of polychromatic white light using conventional light-emitting diodes (LED) and an alternative light source, organic light-emitting diodes (OLED), producing reduced spectral content in the short wavelength of blue light (455 nm). Ten male participants were exposed to either LED, OLED (1000 lx), or dim (< 10 lx) light for 4 h before sleep in a metabolic chamber. Following OLED exposure, energy expenditure and core body temperature during sleep were significantly decreased (p < 0.001). Fat oxidation during sleep was significantly reduced (p = 0.001) after the exposure to LED compared with OLED. Following exposure to OLED, fat oxidation positively correlated with the 6-sulfatoxymelatonin levels, suggesting that the role of melatonin in lipolysis differs depending on the light. These findings advance our knowledge regarding the role of light in energy metabolism during sleep and provide a potential alternative to mitigate the negative consequences of light exposure at night.

Asymmetric Spirobiacridine-based Delayed Fluorescence Emitters for High-performance Organic Light-Emitting Devices.

Recently, researchers have focused on thermally activated delayed fluorescence (TADF) for efficient future lighting and displays. Among TADF emitters, a combination of triazine and acridine is a promising candidate for realizing high-efficiency organic light-emitting devices (OLEDs). However, simultaneous development of perfect horizontal orientation (Θ=100 %) and an external quantum efficiency (EQE) of over 40 % is still challenging. Here, to obtain insights for further improvements of a triazine/acridine combination, various asymmetric spirobiacridine (SBA)-based TADF emitters with a unity photoluminescence quantum yield and high Θ ratio of over 80 % were developed. Furthermore, the substitution effects of the triazine acceptor unit on the photophysical properties were studied, including molecular orientations and OLED performance. The corresponding OLED exhibited sky-blue emission with a high EQE of over 30 %.

π-Extended Carbazole Derivatives as Host Materials for Highly Efficient and Long-Life Green Phosphorescent Organic Light-Emitting Diodes.

High-performance organic light-emitting diodes (OLEDs) that use phosphorescent and/or thermally activated delayed fluorescence emitters are capable of realizing 100 % electron-to-photon conversion. The host materials in these OLEDs play crucial roles in determining OLED performance. Carbazole derivatives are frequently used as host materials, among which 3,3-bis(9H-carbazol-9-yl)biphenyl (mCBP) is often used for lifetime testing in scientific studies. In this study, the π conjugation of the carbazole unit was expanded to enhance OLED lifetime by designing and developing two benzothienocarbazole (BTCz)-based host materials, namely m1BTCBP and m4BTCBP. Among these host materials, m1BTCBP formed a highly efficient [Ir(ppy)3 ]-based OLED with an operational luminescence half-life (LT50 ) of over 300 h at an initial luminance of approximately 12000 cd m-2 (current density: 25 mA cm-2 ). The LT50 value at 1000 cd cm-2 was estimated to be about 23 000 h. This performance is clearly higher than that of mCBP-based OLEDs (LT50 ≈8500 h).

Novel Series of Mononuclear Aluminum Complexes for High-Performance Solution-Processed Organic Light-Emitting Devices.

Light metal complexes, such as lithium (Li), sodium (Na), magnesium (Mg), and aluminum (Al) complexes, are attractive candidates for the fabrication of thermally activated delayed fluorescent (TADF) materials. Nevertheless, mononuclear Al complexes with delayed fluorescence have not been developed so far. In this study, we successfully developed a novel series of highly luminescent Al complexes with two phenylacridine-modified asymmetric acetylacetonate-type ligands. These complexes exhibit high photoluminescence quantum yields (PLQYs) of up to 79 % in the solid state with a short delayed fluorescence lifetime of approximately 4 µs. Solution-processed organic light-emitting devices (OLEDs) using these Al complexes exhibit excellent performance with an external quantum efficiency of 17.5 % at 100 cd m-2 . This is the best performance in light metal-based TADF OLEDs reported so far. The results are expected to guide the advancement of the next-generation solid-state lighting technology.

Neodymium Chloride-Doped Perovskite Nanocrystals for Efficient Blue Light-Emitting Devices.

Metal halides doping of perovskite nanocrystals (NCs) has been shown to precisely control nonradiative pathways and to improve photoluminescence quantum yield (PLQY). Here, we report a trivalent lanthanide halide neodymium (III) chloride (NdCl3)-doped perovskite NCs prepared with a post-synthetic room temperature treatment for efficient blue light-emitting devices (LEDs). The Nd 3d and Cl 2p core peaks were observed in the NdCl3-doped NCs, which allowed for simultaneous doping of Nd3+ and Cl- into the pristine CsPbBr3 NCs. The NdCl3-doped NCs exhibited blue emission at a peak wavelength of 478 nm with a high PLQY of 97% in solution. We found that the Nd3+ cation incorporated into the NCs more effectively suppressed nonradiative recombination compared with common halide anion exchange from temperature dependence of optical properties. Blue LEDs based on NdCl3-doped NCs had an external quantum efficiency of 2.7%, which represents a considerable performance improvement compared with LEDs based on organic chloride salt-doped NCs.

Surface Crystal Growth of Perovskite Nanocrystals via Postsynthetic Lead(II) Bromide Treatment to Increase the Colloidal Stability and Efficiency of Light-Emitting Devices.

The surface modification of metal halide perovskite nanocrystals (NCs) significantly impacts their optical properties and colloidal stability. This subsequently affects the performance of light-emitting devices (LEDs). Therefore, numerous surface passivation techniques like ligand exchange and metal halide doping have been explored to passivate the surface defects of perovskite NCs and obtain highly efficient LEDs. In this study, we demonstrated the postsynthetic metal halide doping treatment using lead(II) bromide (PbBr2) to passivate the surface defects of the CsPbBr3 NCs at a moderate reaction temperature of 80 °C. The alkyl quaternary ammonium salt, didodecyldimethylammonium bromide (DC12AB), enabled the complete dissolution of PbBr2 in a nonpolar solvent, toluene. Because of surface crystal growth, the particle sizes of the PbBr2-doped CsPbBr3 NCs were higher than those of the as-synthesized CsPbBr3 NCs. The photoluminescence quantum yield of the CsPbBr3 NCs drastically increased from 26.8 to 83.9% after the PbBr2 doping treatment. Moreover, the PbBr2-doped CsPbBr3 NCs possessed long-term colloidal stability of more than 2 months that indicates the strong bonding between the NCs and ligands. We observed that the alkyl chain length of the quaternary alkyl ammonium salts affected the luminance and device stability during operations. In this study, a promising strategy was devised to achieve highly luminescent perovskite NCs with excellent colloidal stability that can enhance the performance of LEDs.

Molecular Orientations of Delayed Fluorescent Emitters in a Series of Carbazole-Based Host Materials.

Molecular orientation is one of the most crucial factors to boost the efficiency of organic light-emitting devices. However, active control of molecular orientation of the emitter molecule by the host molecule is rarely realized so far, and the underlying mechanism is under discussion. Here, we systematically investigated the molecular orientations of thermally activated delayed fluorescence (TADF) emitters in a series of carbazole-based host materials. Enhanced horizontal orientation of the TADF emitters was achieved. The degree of enhancement observed was dependent on the host material used. Consequently, our results indicate that π-π stacking, CH/n (n = O, N) weak hydrogen bonds, and multiple CH/π contacts greatly induce horizontal orientation of the TADF emitters in addition to the molecular shape anisotropy. Finally, we fabricated TADF-based organic light-emitting devices with an external quantum efficiency (ηext) of 26% using an emission layer with horizontal orientation ratio (Θ) of 79%, which is higher than that of an almost randomly oriented emission layer with Θ of 62% (ηext = 22%).