Solid-state Light-emitting Devices Research Articles

Vermiculite-derived cationic molecular-intercalated fluorescent MgAl-LDHs for assembling ultrathin solid-state light-emitting films

Solid-state light-emitting devices are of special interest in optoelectronic applications, owing to inherent advantages in terms of low cost, high efficiency and desirable durability. However, the accumulation of solid fluorescent chromophores causes serious energy transfer, resulting in fluorescence quenching. Herein, exfoliated vermiculite (VMT) was structurally disassembled into Mg-O octahedral and Si-O tetrahedral layers to produce hydrotalcite (LDHs) and SiO2, respectively, realizing efficient utilization of each component and VMT surface. The fluorescent intercalated LDHs was intercalated with organic fluorescent dyes, including rhodamine 6G (R6G), pyronine G (PG) and sodium fluorescein (Fl-Na), to prepare high-performance fluorescent LDHs, which was further assembled with VMT nanosheets to form ultrathin solid-state light-emitting films. Based on the electrostatic self-assembly strategy, the localization of chromophores and light-emitting intensity of solid films can be precisely controlled by adjusting ordered multilayers. In fact, the exfoliated VMT could cause the quenching of organic fluorescent dyes in liquid, therefore VMT can be used as fluorescent quencher, which could be regenerated by heating. The designed assembly strategy avoids the fluorescence quenching of VMT. Without the addition of any surfactant, both anionic and cationic organic fluorescent dye can be intercalated into hydrotalcite, overcoming the disadvantage of LDHs that can only intercalate anionic molecules. In particular, the VMT encapsulation overcomes the quenching problem of fluorescent dye and improves the stability of the light-emitting films. This work provides a potential strategy for assembling solid-state light-emitting films and opens new applications in various optoelectronics, such as surface-emitting light source and full-color flat-panel displays.

Aluminum Nitride Ultraviolet Light-Emitting Device Excited via Carbon Nanotube Field-Emission Electron Beam

With the progress of wide bandgap semiconductors, compact solid-state light-emitting devices for the ultraviolet wavelength region are of considerable technological interest as alternatives to conventional ultraviolet lamps in recent years. Here, the potential of aluminum nitride (AlN) as an ultraviolet luminescent material was studied. An ultraviolet light-emitting device, equipped with a carbon nanotube (CNT) array as the field-emission excitation source and AlN thin film as cathodoluminescent material, was fabricated. In operation, square high-voltage pulses with a 100 Hz repetition frequency and a 10% duty ratio were applied to the anode. The output spectra reveal a dominant ultraviolet emission at 330 nm with a short-wavelength shoulder at 285 nm, which increases with the anode driving voltage. This work has explored the potential of AlN thin film as a cathodoluminescent material and provides a platform for investigating other ultrawide bandgap (UWBG) semiconductors. Furthermore, while using AlN thin film and a carbon nanotube array as electrodes, this ultraviolet cathodoluminescent device can be more compact and versatile than conventional lamps. It is anticipated to be useful in a variety of applications such as photochemistry, biotechnology and optoelectronics devices.

Mechanical Stabilities and Properties of Graphene-like 2D III-Nitrides: A Review

Mechanical stabilities and properties are critical in real applications of materials, as well as material and machine design. With the success of graphene, graphene-like materials arose tremendous interest in the past few years. Different from bulk materials, two-dimensional (2D) materials have prominent non-linear elastic behaviors. Here, we briefly review the mechanical stabilities and properties of graphene-like 2D III-nitrides, including boron nitride (BN), aluminum nitride (AlN), gallium nitride (GaN), indium nitride (InN), and thallium nitride (TlN). These nitrides are excellent wide band gap semiconductors very suitable for modern electronic and optoelectronic applications. As a result, they play a central role in solid-state light-emitting devices. Their Young’s modulus, Poisson’s ratio, ultimate tensile strength, and elastic limits under various strains are extensively studied, as well as their high-order elastic constants and non-linear behaviors. These studies provide a guide for their practical applications and designs.

Versatile Electroluminescence Color-Tuning Strategy of an Efficient Light-Emitting Electrochemical Cell (LEC) by an Ionic Additive.

The color-tuning strategies of solid-state light-emitting devices (ss-LEDs) are mainly focused on engineering molecular structures. In this paper, for the first time, we developed a facile strategy for tuning the electroluminescence (EL) color from orange to green through the addition of the ionic additive TBAP (tetrabutylammonium perchlorate). To achieve the active ionic emissive compound for use in a light-emitting electrochemical cell (LEC), the neutral biscyclometalated bromo tetrazole iridium(III) [Ir(ppy)2(BrTz)] was exchanged to its cationic complex, [Ir(ppy)2(BrTz-Me)]ClO4 (ppy = 2-phenyl pyridine, BrTz = 4-bromo-2-pyridine tetrazole, BrTz-Me = 4-bromo-2-pyridine methyl tetrazole) with a new synthetic strategy. This method allows employing neutral Ir-cyclometalated complexes, which are ruled out for use in LECs because of their non-ionic behaviors. In the following, an LEC based on the new cationic [Ir(ppy)2(BrTz-Me)]ClO4 as the emissive layer was fabricated between the FTO (fluorine-doped tin oxide) anode and Ga:In alloy cathode without using any additive or polymers, which makes this configuration the simplest ss-LED so far. By adding the ionic additives, the electroluminescence characteristics of [Ir(ppy)2(BrTz-Me)]ClO4 were dramatically increased, including luminance (L) from 162.8 cd/m2 for the device with an additive to 212.9 and 355.9 cd/m2 for devices containing LiTFSI (bis(trifluoromethane)sulfonamide lithium salt) and TBAP, respectively. In particular, when TBAP was added to the [Ir(ppy)2(BrTz-Me)]ClO4 complex, the irradiance was significantly increased from 166.4 to 220.8 μW/cm2 with an efficacy of 1.78 cd/A and external quantum efficiency (EQE) value of 2.14%. The obtained EL results clearly showed that adding TBAP and LiTFSI significantly improved the electroluminescence characteristics and tuned the electroluminescence color.

One-step large-scale fabricating aggregation-induced emission carbon dots with strong solid-state fluorescence emission

Carbon dots (CDs) possessing favorable stability and optical properties have attracted extensive attention for their potential application in solid-state light-emitting devices. However, due to the stacking of π–π conjugated systems, solid-state CDs often suffer aggregation-caused quenching effect, resulting in fluorescence quenching. There are few reports on solid-state fluorescent CDs fabricated without using other solid matrices. In this work, we report a facile approach for one-step synthesis of yellow-green solid-state CDs without using matrix. The synthesized CDs show an independent excitation and emission in various organic solvents. Importantly, the prepared CDs show solvatochromism and aggregation-induced emission redshift effect. By simply drying the colloidal solution of CDs, yellow-green solid-state fluorescent CD powders with a photoluminescence quantum yield of 25.5% can be obtained. High-performance white light-emitting diodes with Commission Internationale de L'Eclairage coordinates at (0.35, 0.37) are prepared by employing the yellow-green solid-state CD powders. The corresponding color temperature and color rendering index are 4691 K and 91.5 under ultraviolet excitation of 365 nm, respectively. The facile synthesis method of solid-state fluorescent CDs with unique optical features offers promising future for CDs-based optoelectronic devices.

Mechanism and quantum efficiency of yellow emission at the organic-inorganic F8T2/p-GaN interface

An organic/inorganic F8T2/gallium nitride (GaN) hybrid light-emitting device is fabricated and found to emit white light. The white light emission is determined to be the combination of the blue emission of the GaN light-emitting diode and the yellow emission of the F8T2 layer. Analysis of the results of this work reveals the following: (1) the yellow emission of the F8T2 layer in the F8T2/GaN light-emitting device is caused by both electroluminescence and photoluminescence and (2) the origin of the majority of the yellow emission is the electroluminescence mechanism.A potential barrier in the valence band at the F8T2/p-GaN interface has been observed and determined to be 1.05 eV by ultraviolet photoelectron spectroscopy analysis. The number of hole carriers flowing over the potential barrier (1.05 eV) into the valence band of GaN; multiple quantum wells (MQWs) governs the quantum efficiency of the yellow emission. Because the percentage of the overflowed hole carriers in the valence band increases with the current input, the quantum efficiency of the active region in the F8T2 layer increases with the current input; this observed behavior is opposite to that of existing solid-state light-emitting devices.

One-pot synthesis of double silane-functionalized carbon dots with tunable emission and excellent coating properties for WLEDs application

Silane-functionalized carbon dots (SiCDs) can be exploited as effective color converting materials for the solid-state light-emitting devices. However, most of SiCDs reported thus far have shown photoluminescence emissions in the blue and green spectral range, which limit them to construct an efficient white light-emitting diodes (WLEDs) due to the lack of long-wavelength emission. Herein, a series of double silane-functionalized carbon dots (DSiCDs) were prepared via a one-step solvothermal method. The results show that the organic functional group of the silane has great influence on the optical properties of DSiCDs and the number of alkoxy group in the silane has great influence on coating properties of DSiCDs. In addition, the DSiCDs prepared by (3-aminopropyl)triethoxysilane and N-[3-(trimethoxysilyl)propyl]ethylenediamine with molar ratio of 7:3 show excellent optical properties with the maximum emission at 608 nm under 570 nm excitation. Furthermore, they can be completely cured within 1 h at room temperature to form fluorescent coating with high stability and strong adhesion to the substrate. Together with their excellent optical and coating properties, they can be directly coated on LED chips to prepare WLEDs, with a CIE coordinate of (0.33,0.31), color rendering index of 81.6, and color temperature of 5774 K.

Solid-state fluorescent nitrogen doped graphene quantum dots with yellow emission for white light-emitting diodes

Light-emitting graphene quantum dots (GQDs) are widely investigated due to their distinct merits. However, GQDs generally suffer from aggregation-induced luminescence quenching, which means they are highly emissive in a solution state with uniform dispersion but dramatically quenched in a solid or aggregated state. This problem significantly limits the application of GQDs, partially in the solid-state light-emitting devices. In this report, we successfully developed a simple and efficient hydrothermal method for the production of nitrogen doped graphene quantum dots (N-GQDs) with strong solid-state fluorescence (SSF) by using citric acid and o-Phenylenediamine as precursors. Under the 365 nm UV light illumination, the produced N-GQDs in an aqueous state exhibit blue color, with a quantum yield of 58%. As the concentration of N-GQDs increases, the photoluminescence exhibits an obvious red-shift from blue to yellow. For the N-GQDs in solid state, yellow luminescence with a high photoluminescence quantum yield (PLQY) of 28% is achieved under the 405 nm excitation. Finally, via the simple adjustment of thickness and the concentration of N-GQDs in blue emitting InGaN chips, color converter is enabled by constructing white light-emitting diode (WLED) device with improved color rendering index (CRI) and correlated color temperature (CCT).

Fluorine-defects induced solid-state red emission of carbon dots with an excellent thermosensitivity

Up to date, solid-state carbon dots (CDs) with bright red fluorescence have scarcely achieved due to aggregation-caused quenching (ACQ) effect and extremely low quantum yield in deep-red to near infrared region. Here, we report a novel fluorine-defects induced solid-state red fluorescence (λem = 676 nm, the absolute fluorescence quantum yields is 4.17%) in fluorine, nitrogen and sulfur co-doped CDs (F,N,S-CDs), which is the first report of such a long wavelength emission of solid-state CDs. As a control, CDs without fluorine-doping (N,S-CDs) show no fluorescence in solid-state, and the fluorescence quantum yield/emission wavelength of N,S-CDs in solution-state are also lower/shorter than that of F,N,S-CDs, which is mainly due to the F-induced defect traps on the surface/edge of F,N,S-CDs. Moreover, the solid-state F,N,S-CDs exhibit an interesting temperature-sensitive behavior in the range of 80–420 K, with the maximum fluorescence intensity at 120 K, unveiling its potential as the temperature-dependent fluorescent sensor and the solid-state light-emitting device adapted to multiple temperatures.

Dimensions of fluorescence kinetic concentration of doped morphology homologs synthesized by TCPP and UiO-66 MOF

Metal-organic frameworks (MOFs) have attracted significant attention thanks to their excellent physical and chemical properties. Metal-organic frames are widely used in optics. For example, metal-organic frameworks can be used in fluorescent probes to detect and treat diseases, and can also be used to make solid-state light-emitting devices. Considering the high stability and easy synthesis of Zr-based UiO-66 MOF as well as the strong fluorescent emission of porphyrin molecules, the TCPP-incorporated UiO frameworks would inherit these advantages and exhibit intriguing optical performance. Therefore, in this paper, TCPP was doped into UiO-66, and the optical properties of the resulting MOF structure were observed. To obtain the optimal doping condition, three doping concentrations of TCPP were used. Besides the structural and morphological characterization by X-ray diffraction and scanning electron microscopy, more attentions have paid on the optical testing based on absorption and fluorescence spectroscopy, two-photon fluorescence spectroscopy and time-resolved spectroscopy. As a result, the TCPP-doped MOFs exhibited good fluorescence properties and satisfied our previous expectations of nonlinear optical properties and two-photon absorption. This work would pave a new way in constructing various MOFs-based solid-state devices with widespread optical applications.

Tunable photoluminescence of silver molecular clusters formed in Na+-Ag+ ion-exchanged antimony-doped photo-thermo-refractive glass matrix

Influence of Na+-Ag+ low-temperature ion exchange and subsequent heat treatment durations on the spectral-luminescent properties of silver molecular clusters formed in antimony-doped photo-thermo-refractive (PTR) glass matrix was studied. The ion exchange of PTR glass leads to long-wavelength shift of the ultra-violet (UV) edge of strong absorption. The shift magnitude is shown to linearly depend on the square root of the ion exchange duration. Increase in the ion exchange duration is shown to result in changing photoluminescent properties of silver molecular clusters, which allows the tuning of the emission color from cool and warm white to yellow and orange. Model for growth of silver molecular clusters in Na+-Ag+ ion-exchanged layers of the antimony-doped PTR glass matrix was proposed based on the observed changes in spectral-luminescent properties of the samples. The studied PTR glass samples doped with silver molecular clusters are characterized by photoluminescence quantum yield up to 57%, which paves the wave for the development of solid-state light-emitting devices and spectral converters with tunable emission spectrum.

Effect of a conformal layer on the photonic crystal for light extraction of scintillator

Abstract In this paper, we have investigated the light extraction of scintillator using by the photonic crystal consisting of an array of monolayer polystyrene nanospheres and a conformal layer. The effect of the conformal layer on the extraction efficiency was systematically analyzed. It is found that the conformal layer will strongly affect the optical properties of the photonic crystal. The coupling efficiency can be controlled by increasing mode number through varying the thickness and refractive index of the conformal layer. When the mode number increases, the multi-extraction process plays an important role in the ultimate extraction efficiency. Thus, at this situation, the extraction efficiency is more favorable for a thin scintillator. This conclusion is also applicable to other solid-state light-emitting devices.

Temperature-Dependent Electroabsorption and Electrophotoluminescence and Exciton Binding Energy in MAPbBr3 Perovskite Quantum Dots

Organic–inorganic lead halide perovskite nanocrystals have attracted much attention as promising materials for the development of solid-state light-emitting devices, but the existence of free or bo...

Polypyridyl ligands as a versatile platform for solid-state light-emitting devices.

The replacement of inorganic semiconductors with molecule-based compounds for applications in current-to-light conversion has led to a significant increase in interdisciplinary collaborations worldwide, affording new improved organic-light emitting diodes (OLEDs) ripe for commercial applications, as well as light-emitting electrochemical cells (LECs) that have recently started to head to the market. This review highlights the role that transition metal coordination complexes (TMCs) have played in advancing the field of molecular electronics, from early conception to the advanced development of several polypyridyl complexes currently pursued for both OLED and LEC concepts. In this context, the design and synthesis of Ir(iii), Pt(ii), Cu(i) and Ag(i) complexes as the emissive components of OLEDs and LECs are thoughtfully presented. We discuss how molecular design is pivotal for fine-tuning color and optimizing power efficiencies, highlighting the key roles of the metal, cyclometalate, and ancillary polypyridyl ligands. We provide insight into the strategies exploited for the development of new, improved emitters and their fabrication into OLEDs/LECs with high external quantum efficiencies and stabilities. In addition, we have surveyed the remarkable photophysical properties of third generation TMCs capable of undergoing thermally activated delayed fluorescence (TADF). Since previous reviews of TADF materials are strongly biased towards organic-based systems, this overview compliments other synopses of light emitting TADF materials. Finally, we shed light onto the conceptual challenges that still need to be overcome to advance the rational design of TMC-based TADF emitters with tunable ligands and the subsequent fabrication of OLEDs/LECs, which are tailor-made for each specific application.

Effect of a Phonon Bottleneck on Exciton and Spin Generation in Self-Assembled In1−xGaxAs Quantum Dots

Semiconductor quantum dots (QDs) hold great potential for solid-state light-emitting devices and optical and spin-based quantum information processing, but the efficiency of QD-based spin LEDs remains limited, and their physics not fully understood. Using tunable laser spectroscopy, the authors discover that here an intrinsic obstacle to spin generation is $p\phantom{\rule{0}{0ex}}h\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}n$ $b\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}k$, a lack of phonon-mediated relaxation pathways that can arise in a zero-dimensional system. This insight provides design rules for energy-level engineering in spin-optoelectronic applications of QDs, yielding about a threefold increase in spin generation efficiency, even at elevated temperatures.

White-Light Emission from Sputter Deposited TiOx Thin Film on Si Wafer

Solid state white light emitting devices (LEDs) are needed for the lighting and many other applications [1]. The semiconductor based LED can only emit the narrow band light due to the band gap energy restriction. The white light has to be generated by combining 3 different types of LEDs or using a LED and a phosphor material [2,3]. The single-chip white-light LED is desirable. Kuo and Lin reported a new type solid state LED that emits white light from an amorphous metal oxide film [4-7]. Light is emitted from conductive paths self-aligned to the gate electrode after the dielectric breakdown [4-7]. The TiOx thin film, which has a near zero band offset with Si, has been used as the gate dielectric in DRAMs [8,9]. It may be a good light emission material in the new type of LED. Authors investigated the TiOxbased LED for the electrical and optical properties. The new LED was made of the MOS capacitor on the p-type (100) Si wafer. The TiOx layer was sputter deposited from the Ti target at 5 mTorr, 60 W, and Ar/O2 (1:1) atmosphere for 2 minutes. The post deposition annealing (PDA) was done in the N2 atmosphere at 1,000°C for 3 min. The 80 nm ITO layer was sputter deposited on the sample and wet etched into the 300 µm diameter electrodes. The final post metal annealing (PMA) was done at 400°C for 5 min under the H2/N2(1:9) atmosphere. The current-voltage curves were measured with the Agilent 4155C semiconductor parameter analyzer. The light emission spectra were recorded by the optical emission spectrometer (StellarNet BLK-C-SR-TEC). Figure 1 shows the high-magnification photos of the LED stressed at Vg = -10V, -15V, -20V, and -25V, separately. The light is emitted from many discrete bright dots each of which is from the thermal excitation of a conductive path in the TiOx layer. The uniform light emission from the conventional semiconductor based LED is due to the recombination of the e-h pair or excitons. Both the light intensity and the number of bright dots in this LED increased with the increase of the magnitude of Vg (|Vg |). Figure 2 shows the current density-voltage (J-V) curve of a LED. The inset shows the enlarged view. The J jumped abruptly at the breakdown voltage (VBD ) of -5.9V above which the light emission was observed. Then, the leakage current increased monotonically with the increase of the |Vg | similar to the resistor behavior. The light emission mechanism of this device is the same as that of the incandescent light bulb, i.e., thermal excitation of the conductive filament from the passing of a large current. Figure 3 shows the emission spectra of the same LED at Vg = -10V, -15V, -20V, and -25V, separately. The same broad band light is emitted at different Vg ’s. The light intensity increases with the increase of the |Vg |, which is consistent with the Figs. 1 and 2 results. The device has a CIE 1931 color coordinate of (0.462, 0.426), which is located at the warm white light region. It has a CCT of 2,788K and a CRI Raof 95, which are close to those of the W incandescent light and better than those of many fluorescent or semiconductor-based LED lights. The new LED could be driven at various frequencies and duty cycles (DCs). The power consumption can be lowered with the reduction of the DC. Figure 4 shows the emission spectra of the LED driven from 1 Hz to 100 kHz but with the same DC of 75%. The light emission intensity, CIE color coordinates, CCT, and CRI remain the same, which is contributed by the fast excitation of the short conductive path, e.g., < 5 nm. This is desirable for many industry and biomedical applications. [1] J. M. Phillips et.al., Laser and Photon. Rev. 1, 307 (2007).[2] J. Kundu, et al., Nano Lett.12, 3031 (2012).[3] E. F. Schubert and J. K. Kim, Science 308, 1274 (2005).[4] Y. Kuo et al. , Appl. Phys. Lett. 102, 031117 (2013).[5] Y. Kuo et al., Electrochem. Solid-State Lett. 2, Q59 (2013).[6] Y. Kuo et al., Solid-State Electron. 89, 120 (2013).[7] C. C. Lin et al., J. Vac. Sci. Technol. B 31, 030605 (2013).[8] D. Wei, J. Solid State Sci. and Technol. 2,N110 (2013).[9] Q. Xie et al. Appl. Phys. Lett. 97, 112905 (2010).

Bright light-emitting diodes based on organometal halide perovskite.

Solid-state light-emitting devices based on direct-bandgap semiconductors have, over the past two decades, been utilized as energy-efficient sources of lighting. However, fabrication of these devices typically relies on expensive high-temperature and high-vacuum processes, rendering them uneconomical for use in large-area displays. Here, we report high-brightness light-emitting diodes based on solution-processed organometal halide perovskites. We demonstrate electroluminescence in the near-infrared, green and red by tuning the halide compositions in the perovskite. In our infrared device, a thin 15 nm layer of CH3NH3PbI(3-x)Cl(x) perovskite emitter is sandwiched between larger-bandgap titanium dioxide (TiO2) and poly(9,9'-dioctylfluorene) (F8) layers, effectively confining electrons and holes in the perovskite layer for radiative recombination. We report an infrared radiance of 13.2 W sr(-1) m(-2) at a current density of 363 mA cm(-2), with highest external and internal quantum efficiencies of 0.76% and 3.4%, respectively. In our green light-emitting device with an ITO/PEDOT:PSS/CH3NH3PbBr3/F8/Ca/Ag structure, we achieved a luminance of 364 cd m(-2) at a current density of 123 mA cm(-2), giving external and internal quantum efficiencies of 0.1% and 0.4%, respectively. We show, using photoluminescence studies, that radiative bimolecular recombination is dominant at higher excitation densities. Hence, the quantum efficiencies of the perovskite light-emitting diodes increase at higher current densities. This demonstration of effective perovskite electroluminescence offers scope for developing this unique class of materials into efficient and colour-tunable light emitters for low-cost display, lighting and optical communication applications.

White-Light Emission from Amorphous ZrHfO Thin Film Dielectrics with and without Embedded Nanocrystalline CdSe Dots

White-Light Emission from Amorphous ZrHfO Thin Film Dielectrics with and without Embedded Nanocrystalline CdSe Dots

ChemInform Abstract: Recent Progress in Phosphorescent Organic Light‐Emitting Devices

AbstractReview: 98 refs.

Metal-Lined Semiconductor Nanotubes for Surface Plasmon-Mediated Luminescence Enhancement

Highly efficient solid-state light-emitting devices require semiconductor architectures equipped with high quantum efficiency and integratability on conductive substrates. Surface plasmon (SP)-mediated luminescence enhancement has been considered as one of the most promising solutions, because SP resonance can greatly improve the radiative recombination rate and be achieved using metal entities compatible with the electrode fabrication process. Nevertheless, metal/semiconductor heterostructures have had several fabrication-compatible issues due to metal as a potential contaminant of the semiconductor. We present here a simple fabrication scheme for a metal-lined semiconductor nanotube heterostructure, in which a metal layer is selectively formed on the inner wall of the semiconductor nanotube. The Ag-lining process in a ZnO nanotube resulted in 7.5-fold enhancement of the photoluminescence intensity at 11 K. This SP fabrication technique looks promising for highly efficient solid-state lighting based on semiconductor nanostructures without detrimental effects.