Blue-emitting Materials Research Articles

7,7-bis(N, N-diethylethylenediamino)-8,8-dicyanoquinodimethane: Effect of Ethyl Moiety on the Photophysical Property besides Thermal Stability.

Tetracyanoquinodimethane (TCNQ) on reaction with primary/secondary amines sequels in mono/di-substituted TCNQ adducts known as diaminodicyanoquinodimethanes (DADQ's) possessing astounding optical or non-linear optical characteristics. Crucially, the subtle choice of amine contributes to the outcome of molecular material aspects. Herein, we present a comprehensive investigation of 7,7-bis(N,N-diethylethylenediamino)-8,8-dicyanoquinodimethane (BDEDDQ); manifesting the impact of ethyl group (existing on the di-substituted nitrogen of N,N-diethylethylenediamine (DEED)); on the crystal structure, optical property and thermal stability. Crystallography study revealed supramolecular self-assemblies among molecular dipoles emanating fluorescence enhancement in the solid state compared to solutions. Quantum yields were primarily ~0.2 to 0.4% in solutions and ~56% in the solid. Stokes shift was noticed to be more in solutions (~90nm) than solid (~67nm), suggesting excess vibrational relaxations in solutions. Differential scanning calorimetry revealed ~182°C as the melting temperature. The heat capacity of solid was found to be 5.03 mJs-1. Thermogravimetric analysis conveyed single stage decomposition process initiated by the two amine side chains. Scanning electron microscopy of films prepared by drop casting solutions imparted divergent morphological features, due to different rates of evaporation accompanied by varied growth kinetics. Accordingly, in this paper we have demonstrated the utilization of simple N,N-diethylethylenediamine (DEED) to successfully generate a noteworthy blue emissive molecular material exhibiting semiconducting feature besides reasonable thermal stability.

Effect of positional isomerism on the functional properties of carbazole-phenanthroimidazole-triphenylamine triads

A series of bipolar 2, 6 or 2, 7 substituted carbazole-based isomeric hybrids featuring phenanthroimidazole as acceptor and N-phenylcarbazole or triphenylamine as donor are synthesized. Structure-property relationship of these blue-emitting materials is established by detailed investigation of physiochemical, thermal and electroluminescence characteristics. The materials showed tunable absorption and emission spectra depending upon nature and position of chromophores attached to carbazole core. The triphenylamine substituted isomers exhibited red-shifted absorption and emission spectra when compared to their respective N-phenylcarbazole-based analogs. It is attributed to the increased intramolecular charge transfer (ICT) in the electron-rich triphenylamine derivatives as further confirmed in solvatochromism studies. However, the N-phenylcarbazole derivatives showed less solvent dependence in spectra attesting less polar ground and excited state due to comparatively poor donor strength of N-phenylcarbazole. Similarly, dyes containing electron-rich chromophores showed facile removal of electron with low oxidation potentials. The thermal robustness of the compounds was attested by high thermal decomposition temperatures (Td) which varied from 438 to 481 οC. The EL performance of 3 wt% doped device fabricated with emitter derived from 2,7-disubstituted carbazole featuring N-phenylcarbazole and PI chromophores showed deep-blue CIE coordinates of (0.16, 0.06) and maximum external quantum efficiency of 5.3%.

Excitonic characteristics of blue-emitting quantum dot materials in group II-VI using hybrid time-dependent density functional theory

Colloidal quantum dots (QDs) of group II-VI are key ingredients of next-generation QD light-emitting diodes technology for display and lighting, yet the understanding of their luminescent characteristics are far from being mature. Using a hybrid time-dependent density functional theory, we have studied the electronic and excitonic properties of blue-emitting colloidal QDs within group II-VI containing a thousand atoms or more, including CdSe, CdS, ZnSe, and ZnS QDs, considering both quantum confinement and surface ligand effects. It is found that the calculated optical gaps are in excellent quantitative agreement with experiment, irrespective of the QD nature. Scaling laws of size-dependent energy gaps governed solely by quantum confinement effects have further been explored at both single-particle level and correlated excitonic level for all QDs. With concurrently stoichiometric control and enhancing quantum confinement effects, we have predicted an unusual switching of symmetry character of the highest occupied molecular orbital state from a ${\mathrm{\ensuremath{\Gamma}}}_{3}$ to a ${\mathrm{\ensuremath{\Gamma}}}_{1}$ symmetry at ultrasmall size ($\ensuremath{\sim}1$ nm) for all QDs. After the switching, pronounced linearly polarized band-edge excitonic emission is activated. The radiative exciton decay lifetime is found to increase monotonically with increasing the QD size and tends to saturate at larger sizes. Finally, we have explored the surface passivation mechanism of inorganic chloride ligand, and identified various favorable Cd-Cl bonding configurations which enable an effective surface passivation resembling the commonly applied pseudohydrogen passivation scheme. We find that chloride ligand serves as a hole delocalization ligand and tends to redshift the absorption spectra, reduce the absorption intensity, and significantly enhance the exciton decay lifetime. Our results provide a guideline for spectroscopic studies of excitonic characteristics of colloidal QDs within group II-VI.

Bismuth activated blue phosphor with high absorption efficiency for white LEDs

Trivalent bismuth (Bi3+) activated blue-emitting materials have attracted considerable attention because they effectively avoid the reabsorption demerits compared to traditional rare earth activators. Unfortunately, these materials are often plagued by mismatch to the near-ultraviolet (n-UV) chip and short blue emission peak as well as low n-UV absorption efficiency. In this work, we successfully designed a blue-emitting phosphor SrZnSO:Bi3+ with a wide n-UV excitation band ranging from 310 to 400 nm and an ideal blue emission band centered at 460 nm. Furthermore, SrZnSO:0.03Bi3+ phosphor exhibits an extraordinary absorption efficiency of 73.21% ascribed to the intense absorption of the matrix and Bi3+ in the n-UV region. The relationship between the excitation peak position of Bi3+-related phosphors and the size of the polyhedron were also summarized. A white light-emitting diode (WLED) was fabricated using a 370 nm n-UV chip, the composition-optimized sample SrZnSO:0.03Bi3+ and the commercial phosphors. Interestingly, the temperature-sensitive luminescence behavior is beneficial to the performance improvement of the WLED. The WLED demonstrates a warm white light whose Commission Internationale de L′Eclairage (CIE) coordinates are (0.3817, 0.3885), outstanding color rendering index of 93.4 and low correlated color temperature of 4051 K at current of 80 mA. The study suggests that SrZnSO:Bi3+ blue emitting phosphor has great potential in LED-based applications. Also inspires researchers to explore Bi3+-related phosphors with low energy excitation band and high absorption efficiency to develop high-quality n-UV pumped WLEDs.

The structural studies of 2-[(4-tert-butylphenyl)-5–(4-biphenyl)]-1, 3, 4- oxadiazole (PBD) incorporated γ ray irradiated chitosan films for optoelectronic applications

Herein, versatile biopolymer chitosan films doped with 2-[(4-‑tert-butylphenyl)-5–(4-biphenyl)]-1, 3, 4- oxadiazole (PBD) a known OLED material were prepared by intercalation method. These films witnessed crosslinking which in turn exhibited exciting properties due to gamma irradiation with varied dosages. The radical mediated crosslinking of NH2 group with t‑butyl group was envisaged by the increased crystallinity of the films. The XPS analysis showed the formation of the bond between dopant and polymer backbone. The changes in binding energies of unirradiated and irradiated CHPBD films for N1s from 399.08 eV (1° amine) to 400.7 eV (2°amine), supported reaction of NH2 group with t‑butyl group of PBD in crosslinking of dopant and polymer matrix with the formation of CN linkage. In PXRD, peaks centered around 21.24° for pristine chitosan film, but for the unirradiated CHPBD, it was shifted to the lower angle16.09° to form amorphous film. However, for the irradiated CHPBD shifting to the higher angle 22.9°, 23.08°, 27.23° suggesting that dopant had chemically interacted with the polymer matrix. The Young's modulus, tensile strength indicated supportive properties. An interesting change in UV absorption behavior was observed for the unirradiated CHPBD exhibiting λmax at 290 nm with a hypsochromic shift with respected to dopant (λmax,309 nm). This indicated weak head-to-head alignment of dyes in an unirradiated matrix; whereas irradiation brought about changes through chemical crosslinking in the architecture of films with a redshift to 309 nm. Hyperchromism was also the effect of irradiation on films. The unirradiated films exhibited emission maxima around 369 nm whereas irradiated films showed 412 nm. Hence, a simple method was developed for the preparation of blue emissive biopolymeric materials.

One-step preparation of blue-emitting CsPbBr3 quantum dots loaded on natural mineral halloysite nanotube

All-inorganic cesium lead halide perovskite quantum dots (QDs) have attracted widespread attention due to unique photoelectric properties. However, blue-emitting (CsPbClxBr3−x, 0 < x ≤ 3) QDs have poor stability and low photoluminescence quantum yields (PLQY), which limits its application in perovskite based light-emitting diodes (LEDs). The chlorine-free CsPbBr3 QDs have fewer crystal defects and a more stable crystal structure. Here, we prepared CsPbBr3 QDs with adjustable photoluminescence (PL) (489–519 nm) by controlling the reaction temperature and composited it with natural mineral halloysite (Hal) nanotubes. The as-prepared CsPbBr3@Hal (100 °C) have high PL intensity, bright blue emission and enhanced stability. Under an open ambient condition, the CsPbBr3@Hal (100 °C) can still maintain 92.4% of original PL intensity after 60 days of storage. In addition, after 280 h of continuous irradiation with 365 nm UV lamp, its PL intensity still maintained 71%. Finally, a white LED (WLED) fabricated by incorporating the CsPbBr3@Hal (100 °C) and commercial red CaAlSiN3: Eu2+ phosphor on the 395 nm InGaN blue-violet chip showed the CIE chromaticity coordinates of (0.3358, 0.3331). This work provides a new way for the synthesis and application of blue-emitting CsPbBr3 perovskite materials in WLEDs and display devices.

Non-Doped Deep-Blue OLEDs Based on Carbazole-π-Imidazole Derivatives.

In this work, we designed and synthesized four bipolar blue-emitting materials with carbazole, imidazole, and biphenyl as donor, acceptor, and p bridge, respectively. The twisted phenylimidazole acceptor leads to a wider band-gap and hence deeper blue emission than the conjugated phenanthrimidazole acceptor. For the substituents on the carbazole donor, the t-butyl group could prevent the intramolecular charge transfer (ICT) process more effectively than the methoxy group. A non-doped deep-blue organic light-emitting diodes (OLED) is obtained with CIE coordinates of (0.159, 0.080), a maximum luminance of 11,364 cd/m2, and a maximum EQE of 4.43%.

Preparation and luminescence properties of Tm3+-doped SrBi2Ta2O9 blue-emitting phosphor

Various novel SrBi2Ta2O9:Tm3+ blue-emitting materials were achieved via solid-state synthesis. The structure and phase purity of prepared SrBi2Ta2O9:xTm3+ (x = 0.005-0.30 mol) were examined by X-ray powder diffraction. The surface morphology of SrBi2Ta2O9:0.01Tm3+ were studied by scanning electron microscopy. Photoluminescence properties were systematically explored under the monitoring emission (λem = 468 nm) and excitation (λex = 303 nm) spectra. The optimum mole ratio of as-synthesized phosphors was 0.01 mol. The concentration quenching mechanism in the SrBi2Ta2O9 host was due to electric multipole interaction. Particularly, the chromaticity coordinates (0.1334, 0.0474) of SrBi2Ta2O9:0.01Tm3+ are near to those of the commercial BaMgAl10O17:Eu2+. These results validated the SrBi2Ta2O9:Tm3+ phosphor can be utilized good blue-emitting candidate for W-LEDs.

High-performance non-doped blue OLEDs based on 1,2,4-triazole-phenanthroimidazole derivatives with negligible efficiency roll-off

Triazole and phenanthroimidazole moieties are commonly used to construct efficient blue-emitting materials as acceptors and donors, respectively.

Intramolecular hydrogen bond – enhanced electroluminescence performance of hybridized local and charge transfer (HLCT) excited-state blue-emissive materials

OLED devices based on o-hydroxyphenyl phenanthroimidazole (HPI) emitters showed a high maximum external quantum effciency of 8.13% with high brightness of 18 100 cd m−2 and a maximum singlet exciton utilization efficiency of as high as 94%.

2-aryloxybenzo[d]oxazoles as deep blue solid-state emitters: Synthesis, aggregation-induced emission properties and crystal structure

The design and exploration of efficient organic fluorophores have attracted significant scientific interest for their various potential applications. As blue fluorescent materials are essential for the achievement of a full-color display and white emission, the development of new efficient organic blue-emitting materials is of great importance. In this work, we found that simple organic molecules 2-aryloxybenzo[d]oxazoles could act as novel organic fluorophores with special AIE properties and efficient blue fluorescent emission in the solid states. Compounds 3 and 4 possess moderately high quantum yields at 10.3% and 11.4%, respectively. The CIE coordinates indicate that most of these compounds are deep-blue emitters. The single crystal structure of compound 3 shows that it has a twisted V-shape molecular structure with J-type aggregation. Noncovalent interactions widely exist in the crystal structures, while π-π stacking interactions are not observed between adjacent aromatic rings. The special fluorescent properties of these new emitters could ascribe to the unique molecular packing mode in the crystal structure. This work provides a new strategy to access organic solid-state fluorophores with deep-blue emission.

High external quantum efficiency and low efficiency roll-off achieved simultaneously in nondoped pure-blue organic light-emitting diodes based on a hot-exciton fluorescent material

“Hot-exciton” fluorescent materials can efficiently convert triplet excitons into singlet excitons through a path from high-lying triplet excited states (Tn, n > 1) to singlet excited states (Sm, m ≥ 1). The fast reverse intersystem crossing (RISC) process of the hot-exciton channel promotes a high exciton utilization efficiency (EUE) and reduces the efficiency roll-off (ηroll-off) caused by the accumulation of low-lying triplet excitons (T1). Herein, a pure-blue-emitting molecule, PICNAnCz, exhibiting hot-exciton fluorescent emission is proposed. The optimized PICNAnCz-based nondoped organic light-emitting diode (OLED) device achieves a high external quantum efficiency of 9.05% corresponding to a large EUE of 87% and a low ηroll-off of 13%, achieving both high efficiency and a small ηroll-off. The maximum current efficiency and power efficiency of the nondoped device are 9.07 cd A−1 and 5.76 lm W−1, respectively. The nondoped device shows a novel blue electroluminescence (EL) emission with a peak wavelength of 448 nm and Commission Internationale de l’Eclairage coordinates of (0.16, 0.11). These results are among the best reported for hot-exciton blue-emitting materials for nondoped blue fluorescent OLEDs. The excellent EL performance is attributed to the nanosecond-scale RISC process from the high-lying triplet excited state (T2) to the lowest singlet excited state (S1).

Highly Efficient Deep Blue Cd-Free Quantum Dot Light-Emitting Diodes by a p-Type Doped Emissive Layer.

Environmentally friendly ZnSe/ZnS core/shell quantum dots (QDs) as an alternative blue emission material to Cd-based QDs have shown great potential for use in next-generation displays. However, it remains still challenging to realize a high-efficiency quantum dot light-emitting diode (QLED) based on ZnSe/ZnS QDs due to their insufficient electrical characteristics, such as excessively high electron mobility (compared to the hole mobility) and the deep-lying valence band. In this work, the effects of QDs doped with hole transport materials (hybrid QDs) on the electrical characteristics of a QLED are investigated. These hybrid QDs show a p-type doping effect, which leads to a change in the density of the carriers. Specifically, the hybrid QDs can balance electrons and holes by suppressing the overflow of electrons and improving injection of holes, respectively. These electrical characteristics help to improve device performance. In detail, an external quantum efficiency (EQE) of 6.88% is achieved with the hybrid QDs. This is increased by 180% compared to a device with pure ZnSe/ZnS QDs (EQE of 2.46%). This record is the highest among deep-blue Cd-free QLED devices. These findings provide the importance of p-type doping effect in QD layers and guidance for the study of the electrical properties of QDs.

Yb3+-Yb3+ cooperative upconversion in oxyfluoride glass and glass ceramics

Yb3+-doped oxyfluoride glasses and glass ceramics containing KZnF3 nanocrystals were obtained by melt-quenching. Under excitation of a 980 nm laser, a strong blue emission from Yb3+-Yb3+ cooperative upconversion was observed in the glass sample, which increased by five times in glass ceramics after heat treatment for 20 h. The photoluminescence mechanism and sample properties were investigated by X-ray diffraction, transmission electron microscopy, fluorescent lifetime, absorption and emission spectra. The size of KZnF3 nanocrystals is in the range 5 nm–20 nm, making the glass ceramic samples highly transparent. The STEM-EDS measurements indicate that the distribution of Yb is consistent with that of the KZnF3 nanocrystals. Fluroide phase provides a low-phonon-energy environment for Yb3+-Yb3+ ion pairs, which facilitates an intense fluorescent blue emission in the KZnF3 crystal lattice. Both theoretical and experimental results demonstrate that Yb3+-doped oxyfluoride glasses and glass ceramics containing KZnF3 nanocrystals are promising materials for blue upconversion emission.

Machine-Learning-Accelerated Perovskite Crystallization

Summary Perovskites have seen significant research interest in the last decade. As ternary and quaternary compounds, their chemical space is exceptionally large, yet perovskite development has been limited to a restricted set of chemical constituents often discovered through trial and error. Here, we report a high-throughput experimental framework for the discovery of new perovskite single crystals. We use machine learning (ML) to guide the sequence of ever-improved robotic synthetic trials. We perform high-throughput syntheses of perovskite single crystals with a protein crystallization robot and characterize the outcomes with the aid of convolutional neural network-based image recognition. We then use an ML model to predict the optimal conditions for the synthesis of a new perovskite single crystal, enabling us to report the first synthesis of (3-PLA)2PbCl4.This material exhibits strong blue emission, illustrating the applicability of the method in identifying new optoelectronic materials.

Molecular‐Structure and Device‐Configuration Optimizations toward Highly Efficient Green Electroluminescence with Narrowband Emission and High Color Purity

AbstractThermally activated delayed fluorescence (TADF) materials that are able to realize electroluminescence showing simultaneously narrow emission band, high color purity, and high efficiency are highly demanded for display applications. Up to now, a few blue emissive TADF materials with such characteristics have been reported, while green TADF materials with such features are extremely scarce. Herein, two TADF compounds with highly efficient narrowband bluish green/green emission (full‐width at half‐maximum (FWHM): 22 and 21 nm in toluene) induced by multiple resonance effect are reported. An optimized organic light‐emitting diode based on the green emissive compound shows a fairly narrow electroluminescent spectrum (FWHM: 33 nm), as well as an excellent color purity with the Commission Internationale de L'Eclairage (CIE) coordinates of (0.20, 0.65), which is a close resemblance to the standard green‐light CIE coordinates of (0.21, 0.71) defined by the National Television System Committee. Meanwhile, the maximum external quantum efficiency (EQE) up to 25.5% is achieved and a high EQE of 20.1% is maintained at a practical high luminance of 1000 cd m−2. The outstanding device performance of the compound makes it attractive for potential practical applications.

Efficient non-doped blue fluorescent OLEDs based on bipolar phenanthroimidazole-triphenylamine derivatives

Blue organic emissive materials are still the most important bottlenecks for the development of organic light-emitting diodes (OLEDs). To enrich the material library, herin, three bipolar phenanthroimidazole derivatives, namely MePPIM-TPA, ClPPIM- TPA and BuPPIM-TPA, are synthesized using triphenylamine as electron-donor and phenanthroimidazole as the electron-acceptor. The photophysical, thermal and electrochemical properties of three compounds are investigated with high decomposition temperature up to 350 °C, and strong blue emission. Single-carrier devices are fabricated to show that three compounds have good bipolar carrier transport characteristic. The non-doped fluorescent OLEDs devices using three compounds as emitting layers are fabricated among which the devices based on MePPIM-TPA achieved the maximum luminance of 1743 cd/m2, the maximum external quantum efficiency (EQE) of 2.99% which is relatively comparable to commonly used blue emitters.

A Cocktail of Multiple Cations in Inorganic Halide Perovskite toward Efficient and Highly Stable Blue Light-Emitting Diodes

Blue-emitting materials and their light-emitting diodes (LEDs) are always challenging in lighting and display applications. Recently, perovskite LEDs (PeLEDs) with efficient and stable green/red em...

Preparation and spectroscopic properties of Ca2MgTeO6:Tm3+ blue-emitting tellurate phosphors

Various novel Ca2MgTeO6:Tm3+ blue-emitting tellurate materials were synthesized via solid-state reaction. The structure and phase purity of prepared Ca2MgTeO6:xTm3+ (x = 0.0025-0.10 mol) were examined by X-ray powder diffraction. The Ca2MgTeO6:Tm3+ phosphors emit blue emission at 359 nm excitation. The optimum doping concentration was 0.02 mol. The concentration quenching mechanism in the Ca2MgTeO6 host was due to the electric dipole-dipole interaction. The The CIE chromaticity coordinates of Ca2MgTeO6:Tm3+ phosphors located in the blue region. These results validated the Ca2MgTeO6:Tm3+ tellurate phosphor can be used as good blue-emitting candidate for W-LEDs.

Synthesis and photoluminescence properties of Ba2Y3(SiO4)3F:Tm3+ blue-emitting fluorosilicate phosphors

Various novel Ba2Y3(SiO4)3F:Tm3+ blue-emitting fluorosilicate materials were achieved via solid-state synthesis. The structure and phase purity of prepared Ba2Y3(SiO4)3F:xTm3+ (x = 0.001-0.10 mol) were examined by X-ray powder diffraction. The surface morphology of Ba2Y3(SiO4)3F:0.01Tm3+ was studied by scanning electron microscopy. Photoluminescence properties were systematically explored under the monitoring emission (λem = 468 nm) and excitation (λex = 302 nm) spectra. The optimum mole ratio of as-synthesized phosphors was 0.01 mol. The concentration quenching mechanism in the Ba2Y3(SiO4)3F host was due to electric multipole interaction. Particularly, the chromaticity coordinates (0.1334, 0.0474) of Ba2Y3(SiO4)3F:0.01Tm3+ are near to those of the commercial BaMgAl10O17:Eu2+. These results validated the Ba2Y3(SiO4)3F:Tm3+ fluorosilicate phosphor can be used as a good blue-emitting candidate for W-LEDs.