Applications In Light-emitting Devices Research Articles

Blue and green double band luminescent carbon quantum dots: Synthesis, origin of photoluminescence, and application in white light-emitting devices

Carbon quantum dots (CQDs) have become an excellent candidate for light-converting phosphors. However, it is a challenge to develop white emission CQDs and overcome the agglomeration in the process of solid-state CQDs. Here, white fluorescence CQDs with graphite core structure were synthesized by a one-step hydrothermal method from 1,3-dihydroxynaphthalene and hydrochloric acid. The photoluminescence spectra and surface structure characterization along with UV-Vis absorption spectrum, photoluminescence excitation, and time-resolved photoluminescence spectra together reveal the luminescence of CQDs stems from carbon-oxygen-related surface fluorophores. The principle of solvent similarity compatibility is an effective thought to choose isolating matrix material for the sake of overcoming the agglomeration of CQDs in the process of forming solid film. Solid-state white light emitting device (WLED) was fabricated using the CQDs embedded in polymethyl methacrylate matrix as a single white-light converter excited by a 365 nm UV-LED chip. The fabricated WLED based on CQDs is cold white light source with CIE coordinates (0.3122, 0.3429) and with CCT of 6428 K.

SiC siligraphene: a novel SiC allotrope with wide tunable direct band gap and strong anisotropy

By employing an ab initio evolutionary algorithm, we predict two novel metastable structures whose energies are just higher than g-SiC but lower than other two-dimensional (2D) SiC siligraphenes reported so far. One is composed of 4-6-8 C–Si rings named tho-SiC siligraphene and the other one is composed of 5-6-8 rings named pho-SiC siligraphene. They are almost equal in energy, but interestingly, our calculations demonstrate that tho-SiC siligraphene is a semiconductor like g-SiC, while pho-SiC siligraphene is a 2D metal. Additionally, unlike g-SiC which is very easy to switch from a direct band gap to indirect band gap semiconductor and exhibits homogenous isotropy characteristic under biaxial strains, tho-SiC siligraphene keeps a consistently direct band gap from 1.02 eV to 1.98 eV as the uniaxial strain increases from −9% to 9% and it shows strong anisotropic electronic structures. The obtained calculation results indicate that tho-SiC siligraphene may be a better candidate than g-SiC in the application of light-emitting devices in the future.

Stable and color tunable MEH-PPV/PMMA polymer blends for light-emitting applications

Light Emitting Polymers (LEPs) have gained attention in optoelectronics. However, stability and regulation of emission color remain a challenge. This study aims to overcome the challenge by formulating a Polymer Blend (PB) which consists of a LEP – poly [2-methoxy,5-(2-ethylhexyloxy)1,4-phynylenevinylene (MEH-PPV) and a non-conjugated polymer – poly(methyl methacrylate) (PMMA). A simple approach of in situ polymerization of the non-conjugated polymer in the MEH-PPV solution leads to alteration of size of the MEH-PPV polymer chain and modified the emission color from red-orange to bluish-green. The barrier properties of PMMA improve the stability of PB in an ambient environment with unnoticeable luminescence loss. This type of PB shall find application in Organic Light-Emitting Devices.

Nanosecond-time-scale delayed fluorescence towards fast triplet-singlet spin conversion for efficient orange-red OLEDs with negligible efficiency roll-off

Efficient thermally activated delayed fluorescence (TADF) relies on fast reverse intersystem crossing (RISC) that promotes triplet-singlet spin conversion to shorter excited state lifetime, which is a decisive factor to achieve high electroluminescence performance. In this study, two new TADF emitters BenCN-PXZ and BDCN-PXZ employing an asymmetric donor-acceptor framework are designed and synthesized by combining different CN-modified acceptor units with phenoxazine (PXZ) as a strong electron donor unit. The influence of CN-group with different substituting sites is systematically compared to highlight the structure-property relationship. BenCN-PXZ and BDCN-PXZ exhibit red emission with emission peak over 600 nm and short delayed lifetime within 0.6 µs in the neat film. Surprisingly, the delayed lifetime of BDCN-PXZ is still shorter than 1 µs even being doped into CBP host, implying their promising application in organic light-emitting devices (OLEDs). Both Non-doped and doped OLEDs are fabricated for comparison, among which, doped device based on BDCN-PXZ: CBP presented superior device performances and negligible efficiency roll-off (2.8%) at 1000 cd m−2, which is almost the best results for orange-red TADF-based OLEDs.

Recent research on the luminous mechanism, synthetic strategies, and applications of CuInS2 quantum dots

We discuss the synthesis and luminescence mechanisms of CuInS2 QDs, the strategies to improve their luminous performance and their potential application in light-emitting devices, solar energy conversion, and the biomedical field.

Synthesis of Triphenylmethyl Modified Oil-soluble Carbon Quantum Dots and Their Applications in Light-emitting Devices

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Novel ultrabright luminescent copper nanoclusters and application in light-emitting devices.

Two newly synthesized ultra-small copper nanoclusters, [Cu3(μ3-H)(μ2-dppy)4](ClO4)2 (1) and [Cu4(μ4-H)(μ2-dppy)4(μ2-Cl)2](ClO4) (2) (dppy = diphenyl-2-pyridylphosphine), have been shown to exhibit ultrabright yellow and yellow-green room-temperature phosphorescence (RTP) emission, with high quantum yields of 71.8% and 63.5%, respectively. Therefore, nanocluster 1 has been applied for the first time as a single component phosphor for yellow and white light-emitting diodes (LEDs) with favourable characteristics.

Highly efficient carbazolylgold(iii) dendrimers based on thermally activated delayed fluorescence and their application in solution-processed organic light-emitting devices.

A new class of C^C^N ligand-containing carbazolylgold(iii) dendrimers has been designed and synthesized. High photoluminescence quantum yields of up to 82% in solid-state thin films and large radiative decay rate constants in the order of 105 s−1 are observed. These gold(iii) dendrimers are found to exhibit thermally activated delayed fluorescence (TADF), as supported by variable-temperature emission spectroscopy, time-resolved photoluminescence decay and computational studies. Solution-processed organic light-emitting diodes (OLEDs) based on these gold(iii) dendrimers have been fabricated, which exhibit a maximum current efficiency of 52.6 cd A−1, maximum external quantum efficiency of 15.8% and high power efficiency of 41.3 lm W−1. The operational stability of these OLEDs has also been recorded, with the devices based on zero- and second-generation dendrimers showing maximum half-lifetimes of 1305 and 322 h at 100 cd m−2, respectively, representing the first demonstration of operationally stable solution-processed OLEDs based on gold(iii) dendrimers.

Singly Occupied Molecular Orbital−Highest Occupied Molecular Orbital (SOMO−HOMO) Conversion

Singly occupied molecular orbital−highest occupied molecular orbital (SOMO−HOMO) conversion (inversion), SHC, is a phenomenon in which the SOMO is lower in energy than the doubly occupied molecular orbitals (DOMO, HOMO). A non-Aufbau electronic structure leads to unique properties such as a switch in bond dissociation energy and the generation of high-spin species on one-electron oxidation. In addition, the pronounced photostability of these species has been reported recently for application in organic light-emitting devices. In this review article, we summarise the chemistry of SOMO−HOMO converted (inverted) species reported to date.

Molecular design of efficient yellow- to red-emissive alkynylgold(iii) complexes for the realization of thermally activated delayed fluorescence (TADF) and their applications in solution-processed organic light-emitting devices.

Here, we report the design and synthesis of a new class of fused heterocyclic alkynyl ligand-containing gold(iii) complexes, which show tunable emission colors spanning from the yellow to red region in the solid state and exhibit thermally activated delayed fluorescence (TADF) properties. These complexes display high photoluminescence quantum yields of up to 0.87 and short excited-state lifetimes in sub-microsecond timescales, yielding high radiative decay rate constants on the order of up to 106 s−1. The observation of the drastic enhancement in the emission intensity of the complexes with insignificant change in the excited-state lifetime upon increasing the temperature from 200 to 360 K indicates an increasing radiative decay rate. The experimentally estimated energy splitting between the lowest-lying singlet excited state (S1) and the lowest-lying triplet excited state (T1), ΔES1–T1, is found to be as small as ∼0.03 eV (250 cm−1), comparable to the value of ∼0.05 eV (435 cm−1) obtained from computational studies. The delicate choice of the cyclometalating ligand and the fused heterocyclic ligand is deemed the key to induce TADF through the control of the energy levels of the intraligand and the ligand-to-ligand charge transfer excited states. This work represents the realization of highly emissive yellow- to red-emitting gold(iii) TADF complexes incorporated with fused heterocyclic alkynyl ligands and their applications in organic light-emitting devices.

Monodisperse Long-Chain Sulfobetaine-Capped CsPbBr3 Nanocrystals and Their Superfluorescent Assemblies.

Ligand-capped nanocrystals (NCs) of lead halide perovskites, foremost fully inorganic CsPbX3 NCs, are the latest generation of colloidal semiconductor quantum dots. They offer a set of compelling characteristics—large absorption cross section, as well as narrow, fast, and efficient photoluminescence with long exciton coherence times—rendering them attractive for applications in light-emitting devices and quantum optics. Monodisperse and shape-uniform, broadly size-tunable, scalable, and robust NC samples are paramount for unveiling their basic photophysics, as well as for putting them into use. Thus far, no synthesis method fulfilling all these requirements has been reported. For instance, long-chain zwitterionic ligands impart the most durable surface coating, but at the expense of reduced size uniformity of the as-synthesized colloid. In this work, we demonstrate that size-selective precipitation of CsPbBr3 NCs coated with a long-chain sulfobetaine ligand, namely, 3-(N,N-dimethyloctadecylammonio)-propanesulfonate, yields monodisperse and sizable fractions (>100 mg inorganic mass) with the mean NC size adjustable in the range between 3.5 and 16 nm and emission peak wavelength between 479 and 518 nm. We find that all NCs exhibit an oblate cuboidal shape with the aspect ratio of 1.2 × 1.2 × 1. We present a theoretical model (effective mass/k·p) that accounts for the anisotropic NC shape and describes the size dependence of the first and second excitonic transition in absorption spectra and explains room-temperature exciton lifetimes. We also show that uniform zwitterion-capped NCs readily form long-range ordered superlattices upon solvent evaporation. In comparison to more conventional ligand systems (oleic acid and oleylamine), supercrystals of zwitterion-capped NCs exhibit larger domain sizes and lower mosaicity. Both kinds of supercrystals exhibit superfluorescence at cryogenic temperatures—accelerated collective emission arising from the coherent coupling of the emitting dipoles.

Morphology Controlled Synthesis of Composition Related Plasmonic CuCdS Alloy Nanocrystals.

Cu-based ternary alloy nanocrystals have emerged for extensive applications in solar cells, light-emitting devices (LEDs), and photoelectric detectors because of their low-toxicity, tunable band gaps, and large absorption coefficients. It is still an enormous challenge that regulating optical and electrical properties through changing their compositions and shapes in alloy nanocrystals. Herein, we present a facile method to synthesize CuCdS alloy nanocrystals (NCs) with tunable compositions and shapes at relatively low temperature. Different morphologies of monodisperse CuCdS nanocrystals are tailored successfully by simply adjusting the reaction temperature and Cu:Cd precursor molar ratio. The as-synthesized nanocrystals are of homogeneous alloy structures with uniform obvious lattice fringes throughout the whole particles rather than heterojunction structures. The localized surface plasmon resonance (LSPR) absorption peaks of CuCdS NCs are clearly observed and can be precisely tuned by varying the Cu:Cd molar ratio. Moreover, current–voltage (I–V) behaviors of different shaped CuCdS nanocrystals show certain rectification characteristics. The alloy CuCdS NCs with tunable shape, band gap, and compositionpossess a potential application in optoelectronic devices.

Kill Three Birds with One Stone: Poly(3,4-ethylenedioxythiophene)-Hosted Ag Nanoclusters with Boosted Cathodic Electrochemiluminescence for Biosensing Application.

Metal nanoclusters (NCs) have attracted extensive interest in electrochemiluminescence (ECL) field, but it is still a significant challenge to prepare high ECL efficiency NCs, which tremendously precludes their application in sensing and imaging. Herein, we report poly(3,4-ethylenedioxythiophene) (PEDOT) as a functional ligand for NCs with a "kill three birds with one stone" role, acting as a stabilizer like existing templates, excitingly, excellent electrical conductivity to accelerate the injection of interfacial electrons, and outstanding electrocatalytic activity toward coreactants (S2O82-), which breaks the convention that traditional ligands act as a double-edged sword in ECL field. As an illustration, PEDOT-hosted Ag NCs were prepared with an unprecedented ECL intensity with S2O82- as a cathodic coreactant, which indicates that this novel ligand strategy will bring exciting opportunities, not only in opening up new horizons for rational development of high ECL efficiency metal NCs but also in advancing their potential applications in light-emitting devices and clinical biosensing. As a proof of concept, the PEDOT-hosted Ag NCs were applied as neoteric ECL emitters to achieve sensitive detection of dopamine (DA), which showcased a wide linear response from 1 nM to 10 mM and a low detection limit of 0.17 nM.

FRET-Based Analysis of AgInS2/ZnAgInS/ZnS Quantum Dot Recombination Dynamics.

Ternary quantum dots (QDs) are very promising nanomaterials with a range of potential applications in photovoltaics, light-emitting devices, and biomedicine. Despite quite intensive studies of ternary QDs over the last years, the specific relaxation channels involved in their emission mechanisms are still poorly understood, particularly in the corresponding core-shell nanostructures. In the present work, we have studied the recombination pathways of AgInS2 QDs stabilized with the ZnAgInS alloy layer and the ZnS shell (AIS/ZAIS/ZnS QDs) using time-resolved fluorescence spectroscopy. We have also investigated FRET in complexes of AIS/ZAIS/ZnS QDs and cyanine dyes with the absorption bands overlapping in the different regions of the QD emission spectrum, which allowed us to selectively quench the radiative transitions of the QDs. Our studies have demonstrated that FRET from QDs to dyes results in decreasing of all QD PL decay components with the shortest lifetime decreasing the most and the longest one decreasing the least. This research presents important approaches for the investigation of ternary QD luminescence mechanisms by the selective quenching of recombination pathways. These studies are also essential for potential applications of ternary QDs in photodynamic therapy, multiplex analysis, and time-resolved FRET sensing.

Ultraviolet Light-Emitting Diodes Using Aluminium Indium Nitride Nanowire Structures

Ultraviolet (UV) light-emitting diodes (LEDs) have various applications in the field of biochemistry, medicine, water/air purification, etc. To achieve efficient UV LEDs, researchers have essentially focused on the use of AlGaN alloys in the active region. However, the practical applications of AlGaN based UV LEDs are severely limited due to poor external quantum efficiency (EQE). Some reasons behind the low EQE in AlGaN based UV LEDs are efficiency droop, mostly due to electron leakage into the p-region, poor light extraction efficiency (LEE) which is mainly due to the total internal reflection and the absorption of UV light in the p-GaN contact layer. Identifying and developing the potential of alternative UV materials will be critical to make further progress in the development of deep UV emitters. In this regard, AlxIn1-xN alloy has not been widely studied even though it holds great potential applications in UV and visible light-emitting devices [1]. In this study, we have performed the epitaxial growth, electrical and optical properties of spontaneously grown AlInN nanowires on Si (111) substrate by molecular beam epitaxy (MBE) and demonstrated a high crystalline quality of AlInN nanowire LED structures with stable and strong EL emission at ~299 nm peak wavelength.The AlInN nanowire LED structure, shown in Fig. 1(a), was spontaneously grown on n-Si (111) substrates under nitrogen rich conditions by a Veeco GEN II MBE system equipped with a RF plasma-assisted nitrogen source. The growth conditions for GaN nanowire segments include a growth temperature of ~ 770 °C, nitrogen flow rate of 1 sccm, and forward plasma power of ~ 400 W. The device active region consists of ~ 100 nm n-Al0.78In0.22N, 40 nm undoped Al0.75In0.25N well and 100 nm p-Al0.78In0.22N layers. Growth temperature of the active region was at ~ 670 ̶ 700 °C to increase the In incorporation in AlInN segments and the nitrogen flow rate was kept at 2.5 sccm, while the plasma power was fixed at 400 W. After being grown by MBE, the nanowire LED samples were fabricated using standard lithography method as reported elsewhere [2]. The device characterization was performed on LED with an area size of 500×500 µm2. Photoluminescence (PL) spectrum of the grown nanowire LEDs exhibits strong emission in the UV wavelength range as shown in Fig. 1(b). The peak emission at ~ 299 nm and ~ 280 nm is responsible for the emission from the AlInN quantum well and barriers, respectively. The AlInN nanowire UV LEDs have strong EL intensity with the peak wavelength at ~ 299 nm, as shown in Fig. 1(c). This work is being developed toward the achievement of high power AlInN nanowire UV LEDs, and the result will be presented. Reference R. T. Velpula, B. Jain, M. R. Philip et al., Sci. Rep., 10(1) 1-10 (2020).R. T. Velpula, B. Jain, H. Q. T. Bui et al., Optical Materials Express, 10(2) 472-483 (2020) Figure 1

Triptycene-imbedded thermally activated delayed fluorescence emitters with excellent film morphologies for applications in efficient nondoped and doped organic light-emitting devices

Thermally activated delayed fluorescence (TADF) materials with high luminescence efficiencies, short lifetimes, excellent thermal stability and high-quality film formation are highly desired to realize efficient organic light emitting diodes (OLEDs) with low efficiency roll-off. Herein, two asymmetric TADF molecules consisting of triptycene-fused benzophenone serving as an electron acceptor, and dimethylacridin and phenoxazine as electron donors are reported. The comparative study reveals that the introduction of triptycene scaffold strengthens the molecular rigidity and the intermolecular interactions in aggregated states, which endow the titled materials with significantly improved thermal stability, film formation quality and mechanofluorochromism behaviors. The triptycene-imbedded compound TCO-DMAC emits fast efficient delay fluorescence with photoluminescence quantum yield (PLQY) of 0.92 and short lifetime of 4.94 μs in doped evaporated film, based on which the bluish-green doped OLED achieved an external quantum efficiency (EQE) of 21.2%, a current efficiency (CE) of 57.3 cd/A, a power efficiency (PE) of 62.3 lm/W, and a maximum brightness of 15780 cd/m2. It is worth mentioning that the evaporated neat film of TCO-DMAC exhibit high PLQY of 0.69 and short delay fluorescence lifetime of 4.37 μs. The non-doped OLED achieved an external quantum efficiency (EQE) of 15.6%, a turn-on voltage of 2.8 V and a maximum brightness of 13830 cd/m2 with only minor efficiency roll-off of 3.8% at a high luminance of 1000 cd/m2.

Influence of micro-patterning of the growth template on defect reduction and optical properties of non-polar GaN

We investigate the influence of different types of template micro-patterning on defect reduction and optical properties of non-polar GaN using detailed luminescence studies. Non-polar (or a-plane) GaN exhibits a range of different extended defects compared with its more commonly used c-plane counterpart. In order to reduce the number of defects and investigate their impact on luminescence uniformity, non-polar GaN was overgrown on four different GaN microstructures. The micro-patterned structures consist of a regular microrod array; a microrod array where the −c-side of the microrods has been etched to suppress defect generation; etched periodic stripes and finally a subsequent combination of etched stripes and etched microrods (double overgrowth). Overall the presence of extended defects, namely threading dislocations and stacking faults (SFs) is greatly reduced for the two samples containing stripes compared with the two microrod samples. This is evidenced by more uniform emission and reduction in dark regions of non-radiative recombination in room temperature cathodoluminescence imaging as well as a reduction of the SF emission line in low temperature photoluminescence. The observed energy shifts of the GaN near band edge emission are related to anisotropic strain relaxation occurring during the overgrowth on these microstructures. A combination of stripes and microrods is a promising approach for defect reduction and emission uniformity in non-polar GaN for applications in light-emitting devices as well as power electronics.

High quantum efficiency and stability of biohybrid quantum dots nanojunctions in bacteriophage-constructed perovskite

Forming heterojunctions is a traditional approach for improving the performance of semiconducting nanomaterials. However, using a biological nanostructure to achieve such a goal has rarely been studied. Here we showcase a novel biohybrid junction comprising a functional nanobiomaterial (bacteriophage) and a semiconductor nanomaterial (perovskite quantum dots). We found that M13 bacteriophage, genetically modified to bear increased negative charges, could assist the growth of cubic cesium lead bromide (CsPbBr3) perovskite quantum dots with a size of 13.25 ± 2.69 nm. The M13 bacteriophage further functioned as a scaffold for the assembly of the formed CsPbBr3 quantum dots into a bacteriophage-perovskite biohybrid with a photoluminescence quantum yield of 40.1%, 2.99-fold higher than that of conventional CsPbBr3 quantum dots (13.4%). In addition, compared to the conventional perovskite quantum dots, the perovskite quantum dots in the bacteriophage-based biohybrids exhibited a decreased full width at half maximum (FWHM) of the photoluminescence, indicating that the biohybrids are a better color gamut for light-emitting device applications. Such high optical performance arose from the ordered arrangement of the quantum dots by the bacteriophage, which further affected the charge density due to the interaction between the bacteriophage and CsPbBr3 surface. The materials lifetime of the M13 bacteriophage-assisted perovskite quantum dots was also 1.76 times greater than that of the conventional counterparts without the assistance of bacteriophages due to the reduced valence energy level in the biohybrids. The experiment using bacteriophage with artificially increased surface charge density through genetic manipulation proved our assumption that the surface charge density of the bacteriophage contributes to the stabilization of perovskite quantum dots. The light-emitting diode (LED) with perovskite quantum dots assembled and functionalized by bacteriophage exhibited a 17-fold higher maximum luminance than that of conventional CsPbBr3 quantum dots. This work demonstrates a novel nanobiotechnological approach to the production and assembly of perovskite quantum dots for light-emitting applications.

Exciton Coupling and Conformational Changes Impacting the Excited State Properties of Metal Organic Frameworks.

In recent years, the photophysical properties of crystalline metal-organic frameworks (MOFs) have become increasingly relevant for their potential application in light-emitting devices, photovoltaics, nonlinear optics and sensing. The availability of high-quality experimental data for such systems makes them ideally suited for a validation of quantum mechanical simulations, aiming at an in-depth atomistic understanding of photophysical phenomena. Here we present a computational DFT study of the absorption and emission characteristics of a Zn-based surface-anchored metal-organic framework (Zn-SURMOF-2) containing anthracenedibenzoic acid (ADB) as linker. Combining band-structure and cluster-based simulations on ADB chromophores in various conformations and aggregation states, we are able to provide a detailed explanation of the experimentally observed photophysical properties of Zn-ADB SURMOF-2: The unexpected (weak) red-shift of the absorption maxima upon incorporating ADB chromophores into SURMOF-2 can be explained by a combination of excitonic coupling effects with conformational changes of the chromophores already in their ground state. As far as the unusually large red-shift of the emission of Zn-ADB SURMOF-2 is concerned, based on our simulations, we attribute it to a modification of the exciton coupling compared to conventional H-aggregates, which results from a relative slip of the centers of neighboring chromophores upon incorporation in Zn-ADB SURMOF-2.

Influence of SO42− anionic group substitution on the enhanced photoluminescence behaviour of red emitting CaMoO4:Eu3+ phosphor

CaMoO4:xEu3+(x = 0.5, 1.0, 1.5, 2.0, and 2.5) powder phosphors incorporating SO42− anions were synthesized at high temperature using the solid-state reaction technique. The structural, morphological and optical properties of these phosphors were analysed using X-ray diffraction (XRD), field emission scanning electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and optical spectroscopy. The XRD results indicate that the incorporation of SO42− anions and Eu3+ dopant ions did not affect the crystal structure of the CaMoO4, but largely influenced the luminescence properties of the CaMoO4–SO4:Eu3+ phosphors.The optical properties of our materials were examined using the UV–vis absorption spectroscopy. The absorption edges of the phosphors with different concentrations of Eu3+ were less than the band gap energy of the CaMoO4 and their values ranged from 3.30 to 4.75 eV. The intensity of the red photoluminescence (PL) from CaMoO4:Eu3+ phosphors was enhanced considerably upon incorporation of SO42− anions, suggesting that SO42− acted to capture primary excitation energy and transfer it non-radiatively to Eu3+ ions. In addition, the incorporation of SO42− ions also improved the fluorescence decay life-time values of the CaMoO4:xEu3+ phosphors significantly. Tunable emission was observed when the Eu3+ concentration was varied. Our PL results indicated that the CaMoO4–SO4:Eu3+ phosphor exhibited the highest red emission intensity compared to CaMoO4: Eu3+ phosphors, suggesting that CaMoO4–SO4:Eu3+ could be a promising red component material for potential application in white light-emitting diode devices.