Finely regulated luminescent Ag-In-Ga-S quantum dots with green-red dual emission toward white light-emitting diodes

Ag-In-Ga-S(AIGS)quantum dots(QDs)have recently attracted great interests due to the outstanding optical properties and eco-friendly components,which are considered as an alternative replacement for toxic Pb-and Cd-based QDs.However,enormous attention has been paid to how to narrow their broadband spectra,ignoring the application advantages of the broadband emission.In this work,the AIGS QDs with controllable broad green-red dual-emission are first reported,which is achieved through adjusting the size distribution of QDs by controlling the nucleation and growth of AIGS crystals.Resultantly,the AIGS QDs exhibit broad dual-emission at green-and red-band evidenced by photoluminescence(PL)spectra,and the PL relative intensity and peak position can be finely adjusted.Furthermore,the dual-emission is the intrinsic characteristics from the difference in confinement effect of large particles and tiny particles confirmed by temperature-dependent PL spectra.Accordingly,the AIGS QDs(the size consists of 17 nm and 3.7 nm)with 530 nm and 630 nm emission could successfully be synthesized at 220°C.By combining the blue light-emitting diode(LED)chips and dual-emission AIGS QDs,the constructed white light-emitting devices(WLEDs)exhibit a continuous and broad spectrum like natural sunlight with the Commission Internationale de l’Eclairage(CIE)chromaticity coordinates of(0.33,0.31),a correlated color temperature(CCT)of 5425 K,color rendering index(CRI)of 90,and luminous efficacy of radiation(LER)of 129 lm/W,which indicates that the AIGS QDs have huge potential for lighting applications.

Temperature effects on photoluminescence of YAG:Ce^(3+) phosphor and performance in white light-emitting diodes

The well crystalline YAG：Ce^3＋ phosphor was synthesized by sold-state method, and the temperature dependence of excitation and emission spectra of YAG：Ce^3＋ phosphor were investigated in the temperature range from room temperature to 573 K. With temperature increasing, it was noted that the emission intensity of as-repared phosphors decreased considerably more rapidly when pumped by 460 nm than by 340 nm. The temperature-intensity curves under different excitation wavelengths were obtained using an Arrhenius function, and the corresponding activation energies were also obtained respectively. Thus, the experimental phenomenon was discussed in terms of nonradiative decay rate. The effects of as-prepared phosphors on the performance of the white LED with changing temperature were also studied.

Highly efficient emission and high-CRI warm white light-emitting diodes from ligand-modified CsPbBr_(3) quantum dots

All-inorganic CsPbBr_(3) perovskite quantum dots(QDs)have received great attention in white light emission because of their outstanding properties.However,their practical application is hindered by poor stability.Herein,we propose a simple strategy to synthesize excellent stability and efficient emission of CsPbBr_(3) QDs by using 2-hexyldecanoic acid(DA)as a ligand to replace the regular oleic acid(OA)ligand.Thanks to the strong binding energy between DA ligand and QDs,the modified QDs not only show a high photoluminescence quantum yield(PLQY)of 96%but also exhibit high stability against ethanol and water.Thereby warm white light-emitting diodes(WLEDs)are constructed by combining lig-and modified CsPbBr_(3) QDs with red AgInZnS QDs on blue emitting InGaN chips,exhibiting a color rendering index of 93,a power efficiency of 64.8 lm/W,a CIE coordinate of(0.44,0.42)and correlated color temperature value of 3018 K.In ad-dition,WLEDs based on ligand modified CsPbBr_(3) QDs also exhibit better thermal performance than that of WLEDs based on the regular CsPbBr_(3) QDs.The combination of improved efficiency and better thermal stability with high color quality indicates that the modified CsPbBr_(3) QDs are ideal for WLEDs application.

Phosphor-free white light-emitting diodes

The multiple color-matching schemes that could improve the color rendering index for phosphor-free white LEDs are discussed. Then we review a few of the recent research directions for phosphor-free white LEDs, which include the development of monolithic GaN-based white LEDs and hybrid integrated GaN-based and A1GalnP-based white LEDs. These development paths will pave the way toward commercial application of phosphor-free white LEDs in the coming years.

Color-converted remote phosphor prototype of a multiwavelength excitable borosilicate glass for white light-emitting diodes

We report a unique red light-emitting Eu-doped borosilicate glass to convert color for warm white light-emitting diodes. This glass can be excited from 394 nm-peaked near ultraviolet light, 466 nm-peaked blue light, to 534 nm- peaked green light to emit the desired red light with an excellent transmission in the wavelength range of 400-700 nm which makes this glass suitable for color conversion without a great cost of luminous power loss. In particular, when assembling this glass for commercial white light-emitting diodes, the tested results show that the color rendering index is improved to 84 with a loss of luminous power by 12 percent at average, making this variety of glass promising for inorganic ＂remote-phosphor＂ color conversion.

Theoretical Modeling of Luminous Efficacy for High-Power White Light-Emitting Diodes

The photometric characteristics of high-power white light-emitting diode （LED） devices are investigated. A theoretical model for the luminous efficacy o[ high-power white LED devices and LED systems is proposed. With the proposed theoretical model, the mechanism of the luminous efficacy decrease is explained. Meanwhile, the model can be used to estimate the luminous efficacy oF LEDs under general operation conditions, such as different operation temperatures and injection currents. The wide validity of the luminous efficacy model is experimentally verified through the measurements of different types of LEDs. The experimental results demonstrate a high estimation accuracy. The proposed models not only can be applied to estimate the LED photometric performance, but also is helpful for reliability research of LEDs.

Structure and luminescence of Ca_2Si_5N_8:Eu^(2+) phosphor for warm white light-emitting diodes

We have synthesized Ca2Si5N8：Eu^2＋ phosphor through a solid-state reaction and investigated its structural and luminescent properties. Our Rietveld refinement of the crystal structure of Ca1.9Eu0.1Si5N8 reveals that Eu atoms substituting for Ca atoms occupy two crystallographic positions. Between 10 K and 300 K, Ca2Si5N8：Eu^2＋ phosphor shows a broad red emission band centred at -1.97 eV-2.01 eV. The gravity centre of the excitation band is located at 3.0 eV 3.31 eV. The centroid shift of the 5d levels of Eu^2＋ is determined to be -1.17 eV, and the red-shift of the lowest absorption band to be - 0.54 eV due to the crystal field splitting. We have analysed the temperature dependence of PL by using a configuration coordinate model. The Huang-Rhys parameter S = 6.0, the phonon energy hv = 52 meV, and the Stokes shift △S = 0.57 eV are obtained. The emission intensity maximum occurring at -200 K can be explained by a trapping effect. Both photoluminescence （PL） emission intensity and decay time decrease with temperature increasing beyond 200 K due to the non-radiative process.

Monolithic semi-polar(1■01) InGaN/GaN near white light-emitting diodes on micro-striped Si(100) substrate

The epitaxial growth of novel GaN-based light-emitting diode(LED) on Si(100) substrate has proved challenging.Here in this work, we investigate a monolithic phosphor-free semi-polar InGaN/GaN near white light-emitting diode, which is formed on a micro-striped Si(100) substrate by metal organic chemical vapor deposition. By controlling the size of micro-stripe, InGaN/GaN multiple quantum wells(MQWs) with different well widths are grown on semi-polar(1■01)planes. Besides, indium-rich quantum dots are observed in InGaN wells by transmission electron microscopy, which is caused by indium phase separation. Due to the different widths of MQWs and indium phase separation, the indium content changes from the center to the side of the micro-stripe. Various indium content provides the wideband emission. This unique property allows the semipolar InGaN/GaN MQWs to emit wideband light, leading to the near white light emission.

Enhanced red luminescence of Ca_(3)Si_(2−x)M_(x)O_(7):Eu^(3+)(M=Al,P)phosphors via partial substitution of Si^(4+) for applications in white light-emitting diodes

A red-emitting phosphor Ca_(2.91)Si_(2)O_(7):0.09Eu^(3+) with partial Al^(3+)/P^(5+) substitution on Si^(4+) was synthesized via a simple solid-state method,and the effects of the introduction of the M^(3+/5+)(M=Al,P)ions on the crystal structure and photoluminescence performance of Ca_(2.91)Si_(2−x)M_(x)O_(7):0.09Eu^(3+) phosphors were investigated.The X-ray diffraction(XRD),energy-dispersive X-ray spectroscopy(EDS),and X-ray photoelectron spectroscopy(XPS)results revealed that the structure of Ca_(3)Si_(2)O_(7) remained the same after the introduction of Al^(3+) and P^(5+) ions.The characteristic emission of Eu^(3+)-doped Ca_(3)Si_(2−x)M_(x)O_(7) phosphors exhibited two main peaks at 617 nm(red)and 593 nm(orange)under excitation at 394 nm,which originated from the^(5)D_(0)→^(7)F_(2)and^(5)D_(0)→^(7)F_(1) electron transitions of Eu^(3+) ions.After the partial substitution of Al^(3+) and P^(5+),the red emission intensities of the Ca_(2.91)Si_(2)O_(7):0.09Eu^(3+) phosphors were significantly enhanced by 1.88-and 1.42-fold,respectively,which is attributed to the crystal-field effect around Eu^(3+).Meanwhile,the luminescence intensities of the Ca_(2.91)Si_(1.96)Al_(0.04)O_(7):0.09Eu^(3+) and Ca_(2.91)Si_(1.94)P_(0.06)O_(7):0.09Eu^(3+) phosphors at 210℃ were 79.36%and 77.53%of those at 30°C,respectively,indicating their excellent thermal stability.Moreover,the as-prepared Ca_(2.91)Si_(1.96)Al_(0.04)O_(7):0.09Eu^(3+)and Ca_(2.91)Si_(1.94)P_(0.06)O_(7):0.09Eu^(3+) red-emitting phosphors were combined with a near-ultraviolet chip of 395 nm to fabricate red-light-emitting diode(LED)and white(w)-LED devices with excellent chromaticity features.In summary,Al^(3+)/P^(5+)-substituted Ca_(2.91)Si_(2)O_(7):0.09Eu^(3+) can serve as red-emitting phosphors for applications in w-LEDs.

Temperature-dependent photoluminescent behavior of millimeter-scale Cs_(4)PbBr_(6)/CsPbBr_(3)bulk crystals and their application to white light-emitting diodes

The zero-dimensional perovskite composite Cs_(4)PbBr_(6)/CsPbBr_(3)has attracted significant attention for its remarkable photoluminescence(PL),which remains highly effective even in solid state.This work presents a detailed analysis of the steady-state and time-resolved PL(TRPL)behavior of millimeter-scale Cs_(4)PbBr_(6)/CsPbBr_(3)crystals over a temperature range of 80 to 360 K,which covers exciton binding en-ergy,phonon energy,and PL peak energy shifting with increasing temperature.According to the results,Cs_(4)PbBr_(6)/CsPbBr_(3)exhibits high exciton binding energy and phonon energy,with calculated values of 358.7 and 94.8 meV,respectively.Specifically,when the temperature is below~235 K,thermal expan-sion dominates to influence the TRPL and peak energy,whereas electron-phonon interaction becomes the dominant factor as temperature rises from 235 to 325 K.It is found that Cs_(4)PbBr_(6)/CsPbBr_(3)has a PL behavior similar to CsPbBr_(3),and characterization and TRPL results demonstrate that nanometer-scale CsPbBr_(3)crystals embed in the Cs_(4)PbBr_(6)bulk matrix.Meanwhile,a white light-emitting diode(WLED)device based on Cs_(4)PbBr_(6)/CsPbBr_(3)with luminous efficiency of 64.56 Im/W is fabricated,and its color coordinate is measured as(0.34,0.31)under 20 mA,which is in close proximity to the standard white color coordinate.Moreover,the color gamut of the device is measured as 128.66%of the National Televi-sion Systems Committee(NTSC).The WLED electroluminescence(EL)spectra show high Correlated Color Temperature(CCT)stability for the working current varying from 5 to 100 mA,and after continuous oper-ation for 12 h,the EL intensity decreases and stabilizes at~70%of the initial EL intensity.These findings suggest that Cs_(4)PbBr_(6)/CsPbBr_(3)crystals are a promising candidate for WLEDs.

Growth,leaf anatomy,and photosynthesis of cotton(Gossypium hirsutum L.)seedlings in response to four light-emitting diodes and high pressure sodium lamp

Background Light is a critical factor in plant growth and development,particularly in controlled environments.Light-emitting diodes(LEDs)have become a reliable alternative to conventional high pressure sodium(HSP)lamps because they are more efficient and versatile in light sources.In contrast to well-known specialized LED light spectra for vegetables,the appropriate LED lights for crops such as cotton remain unknown.Results In this growth chamber study,we selected and compared four LED lights with varying percentages(26.44%–68.68%)of red light(R,600–700 nm),combined with other lights,for their effects on growth,leaf anatomy,and photosynthesis of cotton seedlings,using HSP lamp as a control.The total photosynthetic photon flux density(PPFD)was(215±2)μmol·m-2·s-1 for all LEDs and HSP lamp.The results showed significant differences in all tested parameters among lights,and the percentage of far red(FR,701–780 nm)within the range of 3.03%–11.86%was positively correlated with plant growth(characterized by leaf number and area,plant height,stem diameter,and total biomass),palisade layer thickness,photosynthesis rate(Pn),and stomatal conductance(Gs).The ratio of R/FR(4.445–11.497)negatively influenced the growth of cotton seedlings,and blue light(B)suppressed stem elongation but increased palisade cell length,chlorophyll content,and Pn.Conclusion The LED 2 was superior to other LED lights and HSP lamp.It had the highest ratio of FR within the total PPFD(11.86%)and the lowest ratio of R/FR(4.445).LED 2 may therefore be used to replace HPS lamp under controlled environments for the study of cotton at the seedling stage.

Flexible perovskite light-emitting diodes for display applications and beyond

The flexible perovskite light-emitting diodes(FPeLEDs),which can be expediently integrated to portable and wearable devices,have shown great potential in various applications.The FPeLEDs inherit the unique optical properties of metal halide perovskites,such as tunable bandgap,narrow emission linewidth,high photoluminescence quantum yield,and particularly,the soft nature of lattice.At present,substantial efforts have been made for FPeLEDs with encouraging external quantum efficiency(EQE)of 24.5%.Herein,we summarize the recent progress in FPeLEDs,focusing on the strategy developed for perovskite emission layers and flexible electrodes to facilitate the optoelectrical and mechanical performance.In addition,we present relevant applications of FPeLEDs in displays and beyond.Finally,perspective toward the future development and applications of flexible PeLEDs are also discussed.

Enhancing the Performance of Perovskite Light-Emitting Diodes via Synergistic Effect of Defect Passivation and Dielectric Screening

Metal halide perovskites,particularly the quasi-two-dimensional perovskite subclass,have exhibited considerable potential for next-generation electroluminescent materials for lighting and display.Nevertheless,the presence of defects within these perovskites has a substantial influence on the emission efficiency and durability of the devices.In this study,we revealed a synergistic passivation mechanism on perovskite films by using a dual-functional compound of potassium bromide.The dual functional potassium bromide on the one hand can passivate the defects of halide vacancies with bromine anions and,on the other hand,can screen the charged defects at the grain boundaries with potassium cations.This approach effectively reduces the probability of carriers quenching resulting from charged defects capture and consequently enhances the radiative recombination efficiency of perovskite thin films,leading to a significant enhancement of photoluminescence quantum yield to near-unity values(95%).Meanwhile,the potassium bromide treatment promoted the growth of homogeneous and smooth film,facilitating the charge carrier injection in the devices.Consequently,the perovskite light-emitting diodes based on this strategy achieve a maximum external quantum efficiency of~21%and maximum luminance of~60,000 cd m^(-2).This work provides a deeper insight into the passivation mechanism of ionic compound additives in perovskite with the solution method.

Physico−mathematical model of the voltage−current characteristics of light-emitting diodes with quantum wells based on the Sah−Noyce−Shockley recombination mechanism

Herein,a physical and mathematical model of the voltage−current characteristics of a p−n heterostructure with quantum wells(QWs)is prepared using the Sah−Noyce−Shockley(SNS)recombination mechanism to show the SNS recombination rate of the correction function of the distribution of QWs in the space charge region of diode configuration.A comparison of the model voltage−current characteristics(VCCs)with the experimental ones reveals their adequacy.The technological parameters of the structure of the VCC model are determined experimentally using a nondestructive capacitive approach for determining the impurity distribution profile in the active region of the diode structure with a profile depth resolution of up to 10Å.The correction function in the expression of the recombination rate shows the possibility of determining the derivative of the VCCs of structures with QWs with a nonideality factor of up to 4.

Samarium doping improves luminescence efficiency of Cs_(3)Bi_(2)Br_(g)perovskite quantum dots enabling efficient white light-emitting diodes

Considering the toxicity problem of lead-based perovskite quantum dots(PQDs),the lead-free Cs_(3)Bi_(2)Br_(g)PQDs has been recognized as one of the promising candidates.However,the low photoluminescence quantum yields(PLQYs)hinder its practical application in optoelectronic devices.Here,w e successfully prepared Sm^(3+)ions doped Cs_(3)Bi_(2)Br_(g)PQDs with effective white light-emission by modified ligandassisted recrystallization method.The realization of white light-emission is attributed to the broadband blue emission of excitons and the red emission(^(4)G_(5/2)-^(6)HJ(J=5/2,7/2,9/2))of Sm^(3+)ions for Sm^(3+)ions doped Cs_(3)Bi_(2)Br_(g)PQDs.More importantly,compared with the undoped Cs_(3)Bi_(2)Br_(g)PQDs,the PLQYs of Sm^(3+)ions doped Cs_(3)Bi_(2)Br_(g)PQDs are improved from 10.9%to 20.8%,and the anti-water stability is also obviously improved.Finally,the Sm^(3+)ions doped PQDs based white light-emitting diodes(LEDs)with luminous efficiency of 12.6 lm/W were explored,which indicates that there is a potential prospect of lead-free PQDs in white light lighting application.

Large-scale synthesis of single-source, thermally stable, and dual-emissive Mn-doped Zn-Cu-ln-S nanocrystals for bright white light-emitting diodes

The global demand for resource sustainability is growing. Thus, the development of single-source, environment-friendly colloidal semiconductor nanocrystal （NC） phosphors with broadband emission spectra is highly desirable for use as color converters in white light-emitting diodes （WLEDs）. We report herein the gram-scale synthesis of single-source, cadmium-free, dual-emissive Mn-doped Zn-Cu-In-S NCs （d-dots） by a simple, non-injection, low-cost, one-pot approach. This synthesis method led to the formation of NCs with continuously varying compositions in a radial direction because each precursor had a different reactivity. Consequently, the d-dots exhibited two emission bands, one that could be attributed to Mn emission and a second that could be ascribed to the band edge of the Zn-Cu-In-S NCs. The emission peaks assigned to band edge were tunable by modifying the particle size and composition. The prepared d-dots also exhibited the characteristic zero self-absorption, a quantum yield of 46%, and good thermal stability. Combining a commercial blue light-emitting diode （LED） chip with optimized d-dots as color converters gave a high color rendering index of up to 90, Commission Internationale de l＇eclairage color coordinates of （0.332, 0.321）, and a correlated color temperature of 5,680 K. These results suggest that cadmium-free, thermally stable, single-phase d-dot phosphors have potential applications in WLEDs.

Angular color uniformity enhancement of white light-emitting diodes by remote micro-patterned phosphor film

Angular color uniformity(ACU)is a key factor used to evaluate the light quality of white-light emitting diodes(LEDs).In this study,a novel double remote micro-patterned phosphor film(double RMPP film)was used to enhance the ACU of a remote phosphor(RP)down-light lamp.A conventional RP film and remote phosphor film with single micro-patterned film(single RMPP film)also were examined for comparison.The angular correlated color temperature(CCT)distributions and the optical performance of the films were experimentally measured.The measurement results showed that double RMPP film configuration exhibited better color uniformity with a CCT deviation of only 441 K,compared with 556 K for the single RMPP film configuration and 1390 K for the RP film configuration.A simulation based on FDTD and ray tracing combined method also confirmed the ACU improvement.In addition,compared with the conventional RP film,the luminous efficiency of single and double RMPP film configurations was increased by 6.68% and 4.69%,respectively,at a driving current of 350 mA.The enhancement of the ACU and luminous efficiency are due to the scattering and mixing effect of the micropatterned film.Moreover,the double RMPP film configuration had better CCT stability at different currents than the other two configurations.The results demonstrated the effectiveness and superiority of double RMPP film in white LED applications.

Dy^(3+)-doped LiYF_4 crystals for UV-excited white light-emitting diodes

A Dy3＋-doped LiYF4 single crystal capable of generating white light by simultaneous blue and yellow light emission of phosphorescent centers is produced. Chromaticity coordinates and photoluminescence intensity vary with excitation wavelength. Under 350, 365, and 388 nm excitation, the crystal shows excellent white light emission. The most efficient wavelength for white light is 388 nm. The CIE coordina.tes are x=0.316 and y =0.321, and the color temperature （Tc） is 6 368 K. These results indicate that the studied crystal is a potential candidate for ultraviolet light-excited white light-emitting diodes.

Realization of high-efficiency AlGaN deep ultraviolet light-emitting diodes with polarization-induced doping of the p-AlGaN hole injection layer

We investigate the polarization-induced doping in the gradient variation of Al composition in the pAl_(0.75)Ga_(0.25)N/Al_xGa_(1-x)N hole injection layer(HIL)for deep ultraviolet light-emitting diodes(DUV-LEDs)with an ultrathin p-GaN(4 nm)ohmic contact layer capable of emitting 277 nm.The experimental results show that the external quantum efficiency(EQE)and wall plug efficiency(WPE)of the structure graded from 0.75 to 0.55 in the HIL reach 5.49%and 5.04%,which are improved significantly by 182%and 209%,respectively,compared with the structure graded from 0.75 to 0.45,exhibiting a tremendous improvement.Both theoretical speculations and simulation results support that the larger the difference between 0.75 and x in the HIL,the higher the hole concentration that should be induced;thus,the DUV-LED has a higher internal quantum efficiency(IQE).Meanwhile,as the value of x decreases,the absorption of the DUV light emitted from the active region by the HIL is enhanced,reducing the light extraction efficiency(LEE).The IQE and LEE together affect the EQE performance of DUV-LEDs.To trade off the contradiction between the enhanced IQE and decreased LEE caused by the decrease in Al composition,the Al composition in the HIL was optimized through theoretical calculations and experiments.

Enhancing performance of inverted quantum-dot light-emitting diodes based on a solution-processed hole transport layer via ligand treatment

The performance of inverted quantum-dot light-emitting diodes(QLEDs)based on solution-processed hole transport layers(HTLs)has been limited by the solvent-induced damage to the quantum dot(QD)layer during the spin-coating of the HTL.The lack of compatibility between the HTL’s solvent and the QD layer results in an uneven surface,which negatively impacts the overall device performance.In this work,we develop a novel method to solve this problem by modifying the QD film with 1,8-diaminooctane to improve the resistance of the QD layer for the HTL’s solvent.The uniform QD layer leads the inverted red QLED device to achieve a low turn-on voltage of 1.8 V,a high maximum luminance of 105500 cd/m2,and a remarkable maximum external quantum efficiency of 13.34%.This approach releases the considerable potential of HTL materials selection and offers a promising avenue for the development of high-performance inverted QLEDs.