Yellow Light-emitting Diodes Research Articles

All-vacuum fabrication of yellow perovskite light-emitting diodes

Yellow light-emitting diodes (LEDs) are widely utilized in high-quality lighting, light communication, indicator lamps, etc. Owing to their outstanding material properties and device performance, the metal halide perovskites have demonstrated a significant potential for LED applications. However, the performance of the yellow perovskite LEDs (PeLEDs) is inferior to that of their green and red counterparts, with the maximum external quantum efficiency (EQE) limited to ∼3.1%. Further, a majority of the yellow PeLEDs are fabricated using the spin-coating methods. The current study reports the development of the yellow CsPbBr2I PeLEDs based on an all-vacuum deposition approach, which has been widely employed in the commercial organic LEDs (OLEDs). By controlling the co-evaporation rate of CsI and PbBr2, the growth kinetics of the perovskite layer are regulated to achieve a small grain size of ∼31.8 nm. Consequently, an improved radiative recombination rate (8.04 × 10−9 cm3/s) is obtained owing to the spatial confinement effect. The PeLEDs based on the optimal perovskite film demonstrate the yellow electroluminescence (574 nm) with a maximum EQE of ∼3.7% and luminance of ∼16,200 cd/m2, thus, representing one of the most efficient and bright yellow PeLEDs. Overall, this study provides a useful guideline for realizing the efficient PeLEDs based on the thermal evaporation strategy and highlights the potential of PeLED as an efficient and bright yellow light source.

Mobility aware of WDM-based CMO OFDM communication system

Abstract The continued increase in several mobile applications forces to replace existing limited spectrum indoor radio frequency wireless connections with high-speed ones. Visible light communications (VLC) technology has gained prominence in the development of high data rate transmission for fifth-generation networks. In optical wireless communications, light-emitting diode (LED) transmitters are used in applications that desire mobility as LED divergence enables larger coverage. Since each VLC access point covers a small area, handovers of mobile users are inevitable. Wavelength division multiplexing (WDM) can be used in VLC systems to tackle the above issue and to meet the increasing demand for indoor connectivity with high bit rates. In this paper, a new system architecture for WDM with coded modulated optical in orthogonal frequency division multiplexing (OFDM) VLC system in conjunction with red, green, blue, and yellow (RGBY) LEDs is proposed to reduce the impact of random receiver orientation of indoor mobile users over VLC downlink channels and improves the system’s bit-error-rate (BER) performance. Simulation results show that the proposed method is not affected by the user’s mobility and hence it performs better than other approaches, in terms of BER for all scenarios and at all positions. This study reveals that using WDM-OFDM-VLC with RGBY LEDs to construct a VLC system is very promising.

In Situ Fabrication of Mn‐Doped 2D Perovskite‐Polymer Phosphor Films with Green‐Red Dual Emissions for Yellow Lighting

Abstract2D metal halide perovskites have emerged as highly bright and stable light emitters, which can be employed as phosphors for optically pumped luminescence devices. However, the intrinsic volatile property is the main weakness to achieve their long‐term lighting applications. Here, an in situ doctor blade method is developed to fabricate a flexible large‐area (20 × 25 cm2) PEA2PbI4:2Mn‐PVDF film with green‐red dual emissions. Under UV light‐emitting diode (LED) excitation, this yellow phosphor device shows a maximum luminance value of 250 cd m−2, an extremely large full width at half maximum of 189 nm, and a color coordinate of (0.47, 0.50). The device can pass 120 h of water soaking and 85 °C/85% relative humidity of harsh environment tests, which benefit from the strong interaction of 2D perovskite and hydrophobic PVDF polymer. Different from electroluminescence yellow LED devices, Mn‐doped 2D perovskite‐polymer phosphor films excited by UV LED and fluorescent lamp provide an opportunity to achieve low‐cost indoor yellow lighting in some UV‐sensitive places.

One-pot synthesis of metal-free, yellow-emitting phosphor with organic single crystal as a matrix

In recent years, there is a rapid growth on the research of metal-free phosphor. A typical example is blend of photoluminescent carbon nanomaterials with a matrix. Normally, the C dots were prepared first, which were then mixed with the matrix. A drawback of this strategy is that multiple steps have to be involved, which is time consuming. In this work, we show that by simply heating a readily-available small organic molecule (phthalic acid) in a high-boiling solvent with reactive group (formamide), ready-to-use and metal-free phosphor can be produced in high yields (over 70%). The phosphor contains single crystals of phthalic anhydride with photoluminescent species adsorbed on the surfaces, which shows yellow emission in solid state with a fluorescence quantum yield (Φ) of 28.53%. The phosphor can be directly used to prepare light-emitting diodes (LEDs), which results in a yellow LED on the UV chip and a white LED on the blue-emitting chip.

High special surface area and “warm light” responsive ZnO: Synthesis mechanism, application and optimization

In this study, a new series of zinc oxide (ZnO) with high specific surface area and narrow energy band gap are prepared using a facile microwave-induced method. The corresponding formation mechanism is also discussed for the first time. Due to the introduction of C, these ZnO can be excited by long wave temperature light without harmful short wave radiation, and play an efficient photocatalytic activity. This valuable property fundamentally improves the biological safety of its photocatalytic application. Herein, taking teeth whitening as an example, the photocatalytic performance of ZnO is evaluated. The “pure” yellow light-emitting diode (PYLED) with high biological safety is used as the excitation source. It is found that this method could effectively remove pigment on the tooth surface through physical adsorption. In addition, these ZnO could generate active oxygen to degrade the pigment on the tooth surface under the irradiation of yellow light. Some further optimization of these “warm light” responsive ZnO is also discussed in this systematical study, which could open up new opportunities in biomedical field.

Rational construction of staggered InGaN quantum wells for efficient yellow light-emitting diodes

High-efficiency InGaN-based yellow light-emitting diodes (LEDs) with high brightness are desirable for future high-resolution displays and lighting products. Here, we demonstrate efficient InGaN-based yellow (∼570 nm) LEDs with optimized three-layer staggered quantum wells (QWs) that are grown on patterned sapphire substrates. Numerical simulations show that the electron–hole wavefunction overlap of staggered InGaN QWs with high In content exhibits a 1.7-fold improvement over that of square InGaN QWs. At the same injection current, LEDs with staggered QWs exhibit lower forward voltages and narrower full widths at half maximum than LEDs with square QWs. The light output power and external quantum efficiency of a staggered QW LED are 10.2 mW and 30.8%, respectively, at 15 mA. We combine atomic probe tomography (APT), time-resolved photoluminescence (TRPL), and transmission electron microscopy (TEM) with energy-dispersive x-ray (EDX) mapping spectroscopy to shed light on the origin of enhanced device performance. APT results confirm the staggered In profile of our designed staggered QWs structure, and TRPL results reveal decreased defect-state carrier trapping in staggered QWs. Furthermore, TEM with EDX mapping spectroscopy supports the viewpoint that staggered QWs exhibit uniform elemental distribution and improved crystal quality. Together, these factors above contribute to enhanced LED performance. Our study shows that staggered InGaN QWs provide a promising strategy for the development of LEDs that are efficient in the long-wavelength region.

Step-type quantum wells with slightly varied InN composition for GaN-based yellow micro light-emitting diodes.

In this work, we propose adopting step-type quantum wells to improve the external quantum efficiency for GaN-based yellow micro light-emitting diodes. The step-type quantum well is separated into two parts with slightly different InN compositions. The proposed quantum well structure can partially reduce the polarization mismatch between quantum barriers and quantum wells, which increases the overlap for electron and hole wave functions without affecting the emission wavelength. Another advantage is that the slightly decreased InN composition in the quantum well helps to decrease the valence band barrier height for holes. For this reason, the hole injection capability is improved. More importantly, we also find that step-type quantum wells can make holes spread less to the mesa edges, thus suppressing the surface nonradiative recombination and decreasing the leakage current.

Quantum-dot and organic hybrid tandem light-emitting diodes with color-selecting intermediate electrodes for full-color displays.

The immaturity of the color patterning technology and the poor stability of blue (B) quantum-dots are two of the challenges that restrict the development of quantum-dot electroluminescence displays. In this work, we demonstrate a hybrid tandem light-emitting diode (LED), in which a yellow (Y) quantum-dot LED and a B organic LED are vertically connected using an IZO/Ag intermediate electrode. The transparent IZO sets the resonance wavelength of the Y-QLED and thus selectively converts the yellow emission into R or G emission, while the semi-reflective Ag enhances the optical interference of the B LED and hence improves the color saturation of the B emission. Enabled by the multi-functionalities of color-selection and color-enhancement of the intermediate electrode, the tandem LED can emit independently controllable R, G and B colors with a high color gamut of ∼110% NTSC without the need to pattern the light-emitting layers. The demonstrated tandem LED could provide a promising way for the realization of emitting-layer-patterning-free, color-filter-free and wide-color-gamut electroluminescence displays.

Pseudocapacitive Charge Storage in MXene-V2O5 for Asymmetric Flexible Energy Storage Devices.

Pseudocapacitive asymmetric supercapacitors are promising candidates for achieving high energy density in flexible energy storage devices. However, seeking suitable positive electrode materials that are compatible with negative electrode materials remains a considerable challenge. In the current study, a pseudocapacitive Ti3C2Tx MXene used as negative electrodes is rationally compatible with redox-type V2O5 as positive electrodes, resulting in the assembly of an all-pseudocapacitive Ti3C2Tx MXene//V2O5 asymmetric flexible energy storage device. The solid-state asymmetric device can deliver an energy density of 8.33 mW h cm-3 at a current density of 0.5 A g-1. Moreover, it can operate in an expanded voltage window of 1.5 V, with dominant surface-capacitive charge-storage mechanisms. Additionally, the device can power a yellow light-emitting diode for up to 7 s, indicating the potential of the device for use in practical applications. This study demonstrates the possibility of using other two-dimensional transition-metal carbide nanosheets for high-energy density flexible energy storage devices.

Aromatic-imide-based TADF material as emitter for efficient yellow and white organic light-emitting diodes

Abstract An aromatic-imide based TADF emitter (AI-4Cz) was designed and synthesized. The TADF emitter showed high thermal stability, good electrochemical properties, obvious AIE activity, small ΔEST value of 0.02 eV, and intense yellow emissions with PLQY of 76%. AI-4Cz also exhibited obvious microsecond-scale delayed fluorescence lifetime. Yellow OLED based on AI-4Cz was further fabricated, which achieved EQEmax of 16.7%. Moreover, a white OLED was fabricated through the dual emission layer strategy by using AI-4Cz and DMAC-DPS as emitting dopants, and the device obtained CIE coordinates of (0.30, 0.40), CRI value of 66, and an EQEmax of 13.6%.

Pillared nickel-based metal-organic frameworks as electrode material with high electrochemical performance

A pillared nickel-based MOFs, Ni 2 (bdc) 2 P (bdc is the 1,4-benzenedicarboxylic acid and P is dabco(1,4-diazabicyclo[2.2.2]-octane)), and Zn-doped pillared (Zn/Ni) 2 (bdc) 2 P are synthesized and their electrochemical properties as an electrode material are firstly evaluated. The Ni 2 (bdc) 2 P electrode material has a specific capacity of 177.6C g −1 and 48C g −1 at 0.5 A g −1 and 10 A g −1 , respectively. Much higher specific capacities of 298.5C g −1 at 0.5 A g −1 and 104 C g −1 at 10 A g −1 are obtained for the (Zn/Ni) 2 (bdc) 2 P, indicating good rate capability. In addition, the pillared (Zn/Ni) 2 (bdc) 2 P also possesses more stable cycling performance than the pillared Ni 2 (bdc) 2 P. The excellent electrochemical performance of both Ni 2 (bdc) 2 P and (Zn/Ni) 2 (bdc) 2 P can be attributed to their 3D framework structure with open channels and large surface area, hierarchical porous structure and high electrical conductivity. An asymmetric supercapacitor (ASC) (Zn/Ni) 2 (bdc) 2 P//AC is assembled to evaluate the practical application of (Zn/Ni) 2 (bdc) 2 P electrode. The assembled ASC possesses a high energy density of 12.2 W h kg −1 at a power density of 371.5 W kg −1 and excellent cycling performance. Furthermore, two assembled ASCs in series are used to light up a red and a yellow light-emitting diode (LED). Nickel-based pillared metal-organic frameworks as electrode materials with excellent electrochemical performance and have promising potential applications in energy storage. • Pillared Ni 2 (bdc) 2 P and (Zn/Ni) 2 (bdc) 2 P were synthesized. • The specific capacities of Ni 2 (bdc) 2 P and (Zn/Ni) 2 (bdc) 2 P are 177.6 and 298.5 C g −1 at 0.5 A g −1 , respectively. • (Zn/Ni) 2 (bdc) 2 P//AC has an energy density of 12.2 W h kg −1 and a high power density of 6141.6 W kg −1 .

Development of an optical sensor for measuring opacity changes in polyvinyl toluene scintillators

Polyvinyl toluene (PVT) based detectors are used in radiation portal monitors (RPMs) throughout the world to detect the trafficking of illicit nuclear material. PVT scintillators, which are normally optically clear, have been observed to suffer internal fogging throughout their volume due to prolonged exposure to varying environmental conditions. These changes could lead to reduced performance for RPMs that utilize plastic scintillators. In this research, a proof of concept system, consisting of different color light emitting diodes (LEDs) and a single optical sensor (OS), were used to examine the change in light transmission through a PVT scintillator. This Optical Monitoring System (OMS), coupled with an environmentally exposed PVT detector, was tested in an environmental chamber where it was subjected to changes in temperature and humidity ranging from 55 °C at 100% relative humidity to −20 °C at 40% relative humidity. At temperatures below 10 °C, light transmission was reduced by 81% ± 8% for blue LEDs, 84% ± 5% for yellow LEDs, and 49% ± 4% for green LEDs. Similar reductions in detected light were not recorded when the OMS was tested with only air between the LEDs and OS. Therefore, the significant reductions in transmitted light were attributed to changes occurring within the PVT scintillator. These results indicate that a significant reduction in PVT opacity occurs due to wide environmental changes. A device such as the OMS could be used to track these changes and provide users an early indication that a portal monitor is suffering from reduced performance.

Mixed halide CsPb(Br1-xIx)3 nanocrystals for green, orange, and red light-emitting diodes

Cesium lead halide (CsPbX3; X = Cl, Br, and I)-based nanocrystals (NCs) are an attractive optoelectronic material for next-generation lighting and display applications. Except for green-emitting CsPbBr3-based NCs, however, there are few investigations for another visible wavelength spectral-ranged color emitters, such as yellow, orange, and red light-emitting diodes (LEDs). This paper reports a facile synthesis of CsPb(Br1-xIx)3 NCs emitting wavelengths from green to red based on a halide exchange post-treatment using a trimethylsilyl iodide (TMSI) additive. Microstructural and optical investigations showed that the TMSI modulated the iodine content of CsPb(Br1-xIx)3 NCs, resulting in a successive shift of photoluminescence peaks from 510 to 663 nm. Based on CsPb(Br1-xIx)3 NCs, LEDs emitting from green to red wavelengths were demonstrated. Interestingly, CsPb(Br1-xIx)3 NCs exhibited abnormal electroluminescence (EL) spectra, showing a large red-shift and additional peaks corresponding to the pristine CsPbBr3 NCs. These were attributed to the modification of CsPb(Br1-xIx)3 NCs during the LED fabrication process as well as electrical field-induced phase-segregation during operation.

Efficient emission of InGaN-based light-emitting diodes: toward orange and red

Indium gallium nitride (InGaN)-based light-emitting diodes (LEDs) are considered a promising candidate for red-green-blue (RGB) micro displays. Currently, the blue and green LEDs are efficient, while the red ones are inefficient for such applications. This paper reports our work of creating efficient InGaN-based orange and red LEDs on silicon(111) substrates at low current density. Based on the structure of InGaN yellow LEDs, by simply reducing the growth temperature of all the yellow quantum wells (QWs), we obtained 599 nm orange LEDs with peak wall-plug efficiency (WPE) of 18.1% at 2 A / cm 2 . An optimized QW structure was proposed that changed two of the nine yellow QWs to orange ones. Compared with the sample containing nine orange QWs, the sample with two orange QWs and seven yellow QWs showed similar emission spectra but a much higher peak WPE up to 24.0% at 0.8 A / cm 2 with a wavelength of 608 nm. The improvement of peak WPE can be attributed to the improved QW quality and the reduced active recombination volume. Subsequently, a series of efficient InGaN-based orange and red LEDs was demonstrated. With further development, the InGaN-based red LEDs are believed to be attainable and can be used in micro LED displays.

Optimizing light sources for selective growth of purple bacteria and efficient formation of value-added products

The slow growth and inefficient productivity of value-added products are the main obstacles to industrialization of purple non-sulfur bacteria (PNSB)-based technology for clean production from wastewater. To overcome these, a strain of PNSB was obtained through selective cultivation and purification, which was subsequently identified as Rhodobacter sphaeroides. The impacts of different light sources (i.e. halogen lamp, incandescent lamp, infrared light, and light-emitting diodes (LEDs) of white, red, blue, green and yellow) were investigated on the production of biomass, pigments and protein by Rb. sphaeroides. Results suggested that, in addition to infrared light, green and yellow LED, most light sources managed to keep the purity of Rb. sphaeroides above 96%. After 7-day cultivation, halogen lamp and infrared light yielded the highest concentrations of biomass (5.78 g SS L−1) and pigments (22.6 mg L−1), respectively. Highest protein-to-biomass ratio (82%) was observed under blue LED, while maximum protein concentration (4.43 g protein L−1) was obtained under incandescent lamp. The infrared light and incandescent lamp might account for the higher production of pigments and protein, respectively. Considering the culture screening, productivity of biomass, pigments and protein as well as energy efficiency together, incandescent lamp was the optimal light source in lab scale. It also gave rise to the highest net profit of $ 14.43 per ton of wastewater treated in a desktop extrapolation evaluating economic benefit of full-scale PNSB-based wastewater recovery process. The results jointly demonstrated that appropriate light source (i.e. incandescent lamp) would boost the microbial protein production and enhance the protein content, which is of great importance to cost control in practice.

White-light color conversion with red/green/violet laser diodes and yellow light-emitting diode mixing for 34.8 Gbit/s visible lighting communication

The visible wavelength-division multiplexing (VWDM) optical wireless communication beyond 30 Gbit/s with a white-light beam mixed by red/green/violet (R/G/V) laser diodes (LDs) and yellow (Y) LED is demonstrated via quadrature amplitude modulation discrete multitone modulation (QAM DMT). To facilitate both high-quality indoor lighting and high-speed optical wireless communication, the R/G/V-LD white-light module incorporates with a Y-LED to provide a high color rendering index (CRI) and encapsulates with a frosted glass to enlarge its divergent angle. By respectively encoding the R/G/V-LDs with the filtered QAM DMT data in a back-to-back case, the total raw data rate as high as 34.8 Gbit/s is achieved by encoded R/G/V-LDs with respective VWDM data rates of 18/7.2/9.6 Gbit/s. To fulfill the demanded CRI and correlated color temperature (CCT) for indoor white-lighting, the yellow LED contributes the yellowish-orange luminescence with flexible CCT and CRI varying from 3952 K to 3031 K and from 0 to 45.9, respectively. A cold white-light carrier at a CCT of 4852 K, CRI of 71.6, and CIE of (0.3652, 0.4942) is also approached by attenuating the red LD power, and such a cold white-light spot with an illuminance of 6800 lux and a divergent solid angle of 0.89 steradian (sr) can support VWDM data transmission at 28.4 Gbit/s.

Attributes of Yellow Traps Affecting Attraction of Diaphorina citri (Hemiptera: Liviidae).

Laboratory assays were conducted to evaluate responses of Diaphorina citri to various aspects of visual cues associated with traps in an effort to improve trap effectiveness. Addition of white or UV violet but not yellow light-emitting diodes (LEDs) increased attraction to standard yellow adhesive traps moderately (11–17%), with no difference in attraction between white or UV violet LEDs. Addition of a black border on yellow traps enhanced collections. However, there were no differences between attraction to black patterns on traps. Comparisons were made between different commercial paints, some with UV-reflecting properties or fluorescence. A yellow paint with UV reflectance, used for painting bird decoys (decoy yellow), was more attractive than the standard yellow Olson sticky trap. Addition of white or green pigment to increase intensity or enhance green reflectance, respectively, did not increase attraction. Alteration of reflectance of Olson traps with addition of UV-reflecting or fluorescent pigments did not enhance attraction of D. citri. In field comparisons, decoy yellow and fluorescent yellow sticky traps were more attractive to D. citri than Olson yellow.

Structural and optical properties of Dy3+:YAlO3 phosphors for yellow light-emitting diode applications

The spectroscopic properties of a series of Dy3+ single-doped and Dy3+/Nd3+, Dy3+/Tb3+, and Dy3+/Tm3+ co-doped YAlO3 (yttrium aluminum perovskite, YAP) phosphors were investigated and compared through the measurements of optical absorption, emission spectra, and fluorescence decay curves. For the Dy3+ ion single-doped samples, the intensity of each absorption band increases with an increment in Dy3+ ion doping concentration, and the identified strong absorption peak at 447 nm indicates that Dy3+:YAP phosphors are suitable to be pumped by a blue laser diode (LD). For all co-doped samples, absorption peaks of Dy3+ ion along with some of the absorption bands of Nd3+, Tb3+, and Tm3+ ions are observed. Under 351 and 447 nm excitation, a prominent emission peak at 572 nm was obtained in all the samples, corresponding to Dy3+: 4F9/2 → 6H13/2 transition. Here, 2 at% Dy3+:YAP phosphor exhibits the highest yellow emission intensity under 447 nm pumping. Among the three kinds of Dy3+ co-doped phosphors, Dy3+/Tb3+:YAP phosphor possesses the dominant yellow emission. The fluorescence decay curves show exponential behaviour and are fitted well. The Commission International de L'Eclairage (CIE) chromaticity coordinates were calculated following the respective emission spectra, and it is found that all the coordinates locate in the yellow region. The energy transfer (ET) processes were investigated and the concentration quenching mechanism was discussed. The obtained results suggest that Dy3+-activated YAP phosphors are good candidates for yellow LED applications.

Preferential attraction of different colours of light emitting diodes for Culicoides species in West Bengal, India.

The preferential attraction of adult Culicoides (Diptera: Ceratopogonidae) to specific wavelengths was studied in West Bengal, India. A total of 280 collections were made with suction light traps fitted with various colours of light emitting diodes (LEDs) placed at cattle sheds during June, August and November in 2017. In addition, the numbers that have been collected in the evening and morning were compared. Locally manufactured suction light traps fitted with ultra violet (UV), blue, green, yellow, pink, red and white LEDs were compared. UV light attracted the highest number of midges followed by blue and then green LED. Culicoides peregrinus Kieffer and Culicoides oxystoma Kieffer were the most abundant followed by Culicoides fulvus Sen and Das Gupta, Culicoides innoxius Sen and Das Gupta, Culicoides anophelis Edwards and Culicoides huffi Causey. The species composition remained similar across the wavelengths. Although significant variations in midge population have been observed across the months, no significant difference in dusk and dawn abundance was noticed. The females showed a mixed population with less parous individuals.

Effect of hole blocking layer on V-pit hole injection and internal quantum efficiency in GaN-based yellow LED

Five types of GaN-based yellow light-emitting diodes (LEDs) with both a V-pit and a hole blocking layer (HBL) have been investigated numerically. The simulation results show that the GaN hole blocking layer in the p region (HBLP) can not only increase the ratio of the hole current via the V-pit, but also increase the electron leakage into the p layer via the flat region, leading to the lower internal quantum efficiency (IQE). Compared to the GaN HBLP, the Al0.5Ga0.5N HBLP is helpful in suppressing the electron leakage via the flat region to the p layer; however, it increases the electron leakage via the V-pits to the p layer, resulting in an unsatisfactory improvement of IQE. In order to settle out this issue, the AlN hole blocking layer in the n region (HBLN) is designed in the sidewall of the V-pit. It is found that the HBLN can not only alleviate the electron leakage via the V-pits to the p layer, more importantly, but also block the hole leakage via the V-pits to the n layer, leading to the improvement of IQE.