Commercial White Light-emitting Diodes Research Articles

Flexible Triboelectric Nanogenerators Based on Cadmium Metal-Organic Framework/Eethyl Cellulose Composites as Energy Harvesters for Selective Photoinduced Bromination Reaction.

The fabrication of self-driven systems with flexibility and tunable output for organic photoinduction is highly desirable but challenging. In this study, a 3D cadmium metal-organic framework (Cd-MOF) is synthesized and used as a filler for ethyl cellulose (EC) to create mechanically durable and flexible Cd-MOF@EC composite films. Due to its well-established platform with periodically precise structure nature, the outputs of Cd-MOF-based TENG are much higher than those of ligand-based TENGs. Furthermore, composite films with different doping ratios of Cd-MOF are employed to assemble Cd-MOF@EC-based triboelectric nanogenerators (TENGs). The results reveal that a doping ratio of 10 wt.% Cd-MOF in Cd-MOF@EC provides the highest TENG output. Subsequently, a flexible 10 wt.% Cd-MOF@EC-based TENG (FCEC-TENG), working in the contact-separation model, is constructed to harvest mechanical energy from the human body, demonstrating excellent output performance and stability. The energy harvested from FCEC-TENG can directly illuminate 14 commercial white light-emitting diodes (LEDs), providing visible light for the photoinduction of the bromination reaction, and generating bromide with good yield and tolerance. This study presents an effective method for constructing flexible MOF-based TENG for self-powered photoinduced organic transformation systems.

Improving the performance of triboelectric nanogenerators using flexible polyurethane nanocomposites foam filled with montmorillonite

Flexible polyurethane (FPU) foam is flexible and adaptable, making it a promising candidate for utilization as a triboelectric layer in triboelectric nanogenerator (TENG) devices. Nevertheless, the mechanical properties and output voltages of FPU foam are limited, prompting the need to explore modifications that can enhance its performance. In this study, FPU foam was modified through the incorporation of natural montmorillonite (Na-MMT) nanoclay at different weight ratios (0.1, 0.3, 0.5, and 0.7 wt%), resulting in the development of FPU/Na-MMT nanocomposites. The exfoliation of Na-MMT within the FPU matrix was established using XRD and TEM techniques. Morphology analysis by SEM confirmed that adding Na-MMT reduced the cavity and pore diameters of the FPU foam. The compressive strength of the FPU/Na-MMT0.3 composite showed the highest increase of 27.75% when compared to the pristine FPU foam. Moreover, adding Na-MMT to FPU showed an improvement in the output voltage of the TENG for all nanocomposites, with the FPU/Na-MMT0.3 exhibiting the highest improvement of + 62.37% at a force of 4.8 N and a frequency of 5 Hz, compared to pristine FPU. The fabricated TENG device achieved a maximum power density of 0.0328 mW/cm2 at a load resistance of 20 MΩ with a contact area of 9×5 cm2. This device could power three commercial white light-emitting diodes (LEDs) connected in series. Additionally, the fabricated TENG device was employed to charge a 10 µF capacitor, reaching an energy storage of 1185.8 µJ. This demonstrates the device’s effectiveness in energy harvesting. Consequently, the prepared flexible FPU/Na-MMT nanocomposite foams are a promising material to use in TENG devices to power flexible electronic devices.

New green garnet-type phosphor LuMgAl5SiO12:Ce3+ for filling cyan gap to achieve full-spectrum and high colour-rendering-index white light-emitting diodes

Phosphors with simple preparation, high luminous efficiency and excellent thermal stability are highly desirable for commercial white light-emitting diodes (W-LED). In the paper, a new garnet-type green phosphor LuMgAl5SiO12:Ce3+ (LMASO:Ce3+) has been successfully prepared by a high-temperature solid-state reaction. The crystal structure and photoluminescence properties were investigated in detail. X-ray diffraction and the Rietveld refinement demonstrated the crystal structure of LMASO belongs to cubic systems with an Ia 3¯ d (230) space group. The unit cell parameters of LMASO:0.08Ce3+ phosphor were found to be a = 11.90418 Å and V = 1686.935 Å3. LMASO:Ce3+ phosphors have two excitation bands with peaks at 340 and 436 nm, attributed to the 4f → 5d transition of Ce3+ ions. Under excitation at 436 nm, LMASO:0.08Ce3+ phosphor exhibits a broad and asymmetric emission band in the 450–700 nm range. Notably, a W-LED device was created by combining a 455 nm blue LED chip with the obtained green sample and the commercially available red CaAlSiN3:Eu3+ phosphor. The device emitted a bright warm white light with a low correlated color temperature of 3584 K and a high color rendering index of 96.0 at a drive current of 40 mA. The article indicates that LMASO:Ce3+ samples can serve as a potential green phosphor for W-LEDs, and offers a new strategy for designing efficient phosphors.

Synthesis and photoluminescence properties of single-phase Sr3B14O24:RE3+ (RE = Ce, Tb, Sm, Eu) phosphors and glass for WLEDs, UV and NIR shielding

Commercial white light-emitting diodes (WLEDs) usually face problems such as lack of red emission, color difference caused by different degradation rates of multiphase phosphors and the poor thermal stability of the organic encapsulants. The development of new single-phase phosphors and glass can solve these problems well. Sr3B14O24:RE3+ (RE = Ce, Tb, Sm, Eu) phosphors and glass were synthesized by solid-phase method. The obtained Sr3B14O24:RE3+ (RE = Ce, Tb, Sm, Eu) samples showed multicolor emission including blue, green, orange and red, respectively. The optimal excitation wavelength of these samples was located in the blue or ultraviolet (UV) light regions, which were well matched with the blue-violet light chips to manufacture WLEDs. More interestingly, UV absorption was found in Sr3B14O24:Ce3+/4+ glass samples. The doping of dual-valence cerium ions could enhance the shielding performance in the light of inferences drawn from the UV-Vis absorption and transmission spectra of glass samples. By co-doping different concentrations of Cs+ ions in Sr3B14O24:Ce3+/4+ glass samples, the function of near infrared (NIR) shielding was also realized. As a new optical multifunctional material, rare earth doped Sr3B14O24 have great potential for application in WLEDs, UV and NIR shielding.

Study on the structure and luminescence properties of a novel Li3Sc2(PO4)3: Eu3+ orange-red emission phosphor for wLEDs

The manufacture of high-efficiency and stable red phosphor is the key to solve the matter of high color temperature and poor color rendering of commercial white light-emitting diodes (wLEDs). In this work, a series of Li3Sc2-x(PO4)3: xEu3+ (x = 0–0.4) phosphor samples were prepared by conventional solid-phase reaction. The structure and optical properties of Li3Sc2(PO4)3: Eu3+ are investigated. When the doping amount of Eu3+ is 0.3 mol, phosphors have the highest luminescence intensity. The temperature-dependent characteristics of the prepared phosphors are studied. The combined emission intensity at 423K is 77.4% of that at 298K, showing significant thermal stability. Finally, the prepared Li3Sc2(PO4)3 phosphor package is tested in the LED device. The results of the color temperature of 5423K, as well as color rendering index (CRI) (Ra = 81.6) and color coordinate CIE (0.3343, 0.3471), are satisfactory, suggesting that this red phosphor has potential for wLEDs applications.

A novel reddish-orange-emitting phosphor Ca3Gd7(PO4)(SiO4)5O2: Sm3+ for white light-emitting diodes

Phosphor is the decisive material in white light-emitting diodes (WLED), affecting light quality. Silica phosphate has good stability, low cost and environmentally friendly features, so it has great application potential. In this paper, a self-activated reddish-orange phosphor Ca3Gd7(PO4)(SiO4)5O2: xSm3+ was prepared by the high-temperature solid-phase method. The phase purity of the sample was verified by X-ray diffraction (XRD). The Rietveld refinement method was used to resolve further the sample structure and the location of the substitution of rare earth ions. The photoluminescence properties were studied in detail using fluorescence spectra, temperature-dependent emission spectra and other characterization methods. The electron transition and energy transfer processes of Gd3+ in Ca3Gd7(PO4)(SiO4)5O2 were analyzed. The fluorescence decay curve of the sample was obtained, which belongs to the double exponential curve, and the decay lifetime depending on the Sm3+ doping concentration was calculated. Furthermore, a WLED device was assembled, and warm white light was obtained, with a spectrum showing characteristic red emission, proving to complement the missing red component of commercial WLEDs. These results indicate that Ca3Gd7(PO4)(SiO4)5O2: xSm3+ phosphor has great potential for application.

One-pot Solvothermal Synthesis of Full-color Carbon Quantum Dots for Application in Light Emitting Diodes

Full-color Carbon Quantum Dots (FCQDs) have intrinsic characteristics as components for White Light Emitting Diode (WLED) technology, but are not yet applied in the current lighting industry. In this work, FCQDs were prepared by optimizing the synthetic route in order to obtain a simple and one-step solvothermal carbonization of citric acid and urea in dimethylformamide. The synthesis at 166 °C for 11 h and 24 min (FCQD2) was selected from spectroscopy in the ultraviolet–visible (UV–Vis) region and its fluorescence indicated emission over almost the entire visible region of the electromagnetic spectrum with a 380 nm excitation source and a concentration of 0.22 mg mL −1. The morphological, structural and optical properties were evaluated and they showed that nanoparticles with dimensions between 2 and 10 nm consisted mainly of carbon atoms with functional groups on the surface. FCQD2 was incorporated in a polymeric matrix (PVA) and the coordinates of the Color Rendering Index (CRI) showed better color quality than a commercial WLED. It appears that FCQD2, with an easy synthesis and being ecofriendly, has potential for incorporation in a solid matrix, as well as application in WLEDs for the improvement of optoelectronic devices, replacing inorganic quantum dots.

Rational Amphiphilic Ligand Engineering Enables Enhanced Stability and Efficiency of CsPbBr3 Nanocrystals Based Light Emitting Diodes

AbstractMetal halide perovskites have shown great potential for lighting. However, their low stability under irradiation/thermal stress and/or ambient storage conditions are critical for light‐emitting diodes (LEDs). Among the stabilization strategies, ligand surface modification is effective toward stable perovskites, but the dynamic ligand adsorption/desorption process on the surface is a limiting factor. Herein, a new family of biogenic and amphiphilic capping agents, phosphatidyl‐L‐serine (Ptd‐L‐Ser), combining stronger multibinding motifs compared to conventional capping agents has led to superior CsPbBr3 (CsPbBr3‐Ptd‐L‐Ser) with significantly enhanced stability upon storage/heating/water, keeping excellent photoluminescence quantum yields of ≈80% over half year. Spectroscopic/theoretical studies reveal that the origin of this behavior is the increased exciton binding energy associated to the versatility of multiple bindings. This results in CsPbBr3‐Ptd‐L‐Ser nanocrystals‐based green‐LEDs featuring excellent stabilities of >700 h (20 mA) and >200 h (100 mA) that strongly contrast with the reference devices with pristine CsPbBr3 nanocrystals (120 h (20 mA) and 27 h (100 mA)). White LEDs (WLEDs) with chromaticity coordinates of (0.34, 0.33) and high luminous efficiency of 76 lm W–1, keeping stable over weeks, are further demonstrated under continuous operational conditions, thereby suggesting CsPbBr3‐Ptd‐L‐Ser nanocrystals can be a potential candidate for commercial WLED technology.

Multicolor Phosphate Glasses for Potential White LED Lighting and X‐Ray Detections

AbstractThough widely applied in light‐emitting diode (LED) lighting and X‐ray detection, respectively, rare‐earth ions doped phosphor and scintillating single crystals still face many challenges in practicalities, like aging of package material and difficulty to construct tiny array structure. Using phosphate glass as the host is quite beneficial for solving these problems while enabling distinguished luminescence. Therefore, Eu3+ and Tb3+ doped phosphate glasses which, respectively, show strong red/green emissions are researched. Superior thermal stabilities (78.69% and 89.35% emission intensities remain at 150 °C) and high internal quantum efficiencies (87.3% and 68.6%) are found to guarantee the excellent photoluminescence performances. The brilliant X‐ray excited luminescence properties are demonstrated by general 19.49% and 34.74% integrated emission intensities of that of commercial Bi4Ge3O12 single crystal scintillator with the change of X‐ray power. Meanwhile, Tb3+ doped scintillating fiber is drawn. Finally, excited by 380 nm LED, color manipulation and white light generation are achieved in LEDs with a structure of stacked emitting layers (mainly Eu3+ and Tb3+ doped phosphate glasses). Many of the as‐constructed white LED's performance parameters are much better than the commercial white LED. These results show that Eu3+ and Tb3+ doped phosphate glasses possess promising potential in LED lighting and X‐ray detections.

Spectra dependent photonic structure design for energy harvesting by indoor photovoltaic devices

In this work, we report spectra-dependent energy harvesting by optimizing the photon management of an indoor photovoltaic device while taking into consideration the degradation of electrical transport characteristics caused by the nano-photonic structures. For the test case of a CdTe-based photovoltaic device, it has been shown that although the incorporation of dielectric-filled nanoholes in the absorber layer can enhance light absorption by about 40%, the optical-to-electrical conversion efficiency of the device is significantly diminished because of the degradation of the electrical transport characteristics. Instead, the best performance metrics are obtained when the nanostructures are incorporated in the window layer of the device alone. A finite difference time domain based numerical analysis, coupled with Poisson’s equation and continuity equation, shows that by controlling the areal density of the optimized structure in direct correlation with spectral characteristics of the illuminating light source, it is possible to maximize the overall power conversion efficiency of the indoor photovoltaic device. In the case of commercial white light-emitting diodes (LEDs), large arealdensities of holes are found to be more conducive for harvesting energy from cool-white LEDs, whereas smaller areal densities of holes provide better performances for illumination under warm-glow white LEDs.

Improvement of the luminescent thermal stability and water resistance of K2SiF6:Mn4+ by surface passivation

Mn4+-doped fluoride phosphors can solve the problem for lack of red emitting component in commercial white light-emitting diodes (WLEDs). However, its application is seriously hindered by its easy hydrolysis. Here, we propose to use sodium sulfite as a passivator to treat K2SiF6:Mn4+. After passivation, a Mn4+-rare K2SiF6 protective layer can be formed in situ on the surface of the phosphor, and lead to improved emission intensity, luminescent thermal stability and moisture resistance. When soaking in water for 6 h, the integrated fluorescent intensity of the passivated sample maintained 90.8% of the initial value, while the intensity of the un-passivated sample sharply decreased to 10.2% of the initial value. Mechanisms to improve the emission, water resistance and thermal stability of the luminescence are proposed and discussed. WLED was assembled with the passivated sample, and good performance of warm white light (CCT = 2963 K, Ra = 90.4) was obtained.

THE INFLUENCE OF TEMPERATURE ON THE SPECTRAL CHARACTERISTICS OF LEDS WITH A LUMINESCENT COATING BASED ON PHOSPHATE-TUNGSTANE GLASS-CERAMICS

Nowadays, the light-emitting diode (LED) lamps are the most energy-efficient light-emitting devices, the emission spectrum of which can be adjusted in accordance with a specific practical task. One of the important characteristics is the stability of the emission intensity during the operation of such devices. An urgent task is the development of luminescent coatings for white LEDs based on glasses and glass ceramics, because, in comparison with polymers, they are more resistant to high temperatures and powerful light fluxes. The aim of the study was to determine the effect of temperature on the intensity and spectral composition of radiation of the combined system "UV LED + Fluorescent Glass Ceramics" (SD + LCS) and to compare the obtained data with temperature dependences of such characteristics for commercial white LEDs. In this work, the change in the intensity and spectrum of the light emission of the model combined system "UV LED + glass ceramics" when heated up to 105 ° C was studied. The results obtained were analyzed together with the dependences of the luminescence intensity on temperature for a commercial white LED. The emission spectra of the combined system contain both narrow luminescence peaks associated with transitions in europium ions and broad bands elated with the emission from the LED chip and the luminescence of the glass host. When heated, the position of the emission band of the chip is slightly shifted to the long-wavelength side, both for commercial LEDs and for model combined systems. It was also found that the temperature stability of model "white" light sources based on phosphate-tungstate glass ceramics is higher than the corresponding characteristic of commercial white LEDs in the temperature range of 50 - 105 ° C.

Blue-light-excitable broadband yellow-emitting CaGd2HfSc(AlO4)3:Ce3+ garnet phosphors for white light-emitting diode devices with improved color rendering index

The current commercial white light-emitting diodes (LEDs) are generally based on the combination of blue LED chips and Y3Al5O12:Ce3+ yellow phosphors. However, because of the lack of red component, such white LED devices exhibit cool white-light emissions with low color rendering index (Ra < 75, R9 < 0). Therefore, it is urgent to discover new blue-light-excitable yellow-emitting phosphors with enhanced red emissions for fabricating high color-quality white LEDs. In the present work, we demonstrate a novel broadband yellow-emitting CaGd2HfScAl3O12:Ce3+ garnet phosphor for blue-light-excited white LEDs with improved color rendering index. The as-prepared CaGd2HfScAl3O12:Ce3+ garnet phosphor possesses a cubic structure with Ia3¯d space group, and the unit cell parameters of the representative CaGd2HfScAl3O12:2%Ce3+ phosphor are a = b = c = 12.450 Å, α = β = γ = 90°, and V = 1,929.59(4) Å3. Impressively, we find that the CaGd2HfScAl3O12:Ce3+ garnet phosphor shows an intense absorption band in the 300–500 nm wavelength range with a maximum at 452 nm owing to the 4f→5d transition of Ce3+ ions. On 452 nm excitation, the optimal CaGd2HfScAl3O12:2%Ce3+ sample exhibits a broad asymmetric yellow emission band in the wavelength range of 470–750 nm with peak at 564 nm and full width at half maximum of 151 nm. The Commission Internationale de l’Eclairage chromaticity coordinates and internal quantum efficiency of the CaGd2HfScAl3O12:2%Ce3+ sample are (0.4485, 0.5157) and 30.4%, respectively. Finally, a white LED device is fabricated by combing a 450 nm blue LED chip with commercial Y3Al5O12:Ce3+ yellow-emitting phosphor, which generates white light with low color rendering index (CRI; Ra = 74.7, R9 = −12.7) and high correlated color temperature (CCT = 6,554 K) under the 60 mA driving current. In sharp contrast, another white LED device, which is made by coating our as-prepared CaGd2HfScAl3O12:2%Ce3+ yellow-emitting phosphors onto the surface of a 450 nm blue LED chip, produces white-light emission with high CRI value (Ra = 84.5, R9 = 26.3) and relatively low CCT of 5,649 K. This work reveals that the newly discovered broadband yellow-emitting CaGd2HfScAl3O12:Ce3+ phosphors can serve as a potential color converter in high-color-quality phosphor-converted white LEDs.

Broadband Short-Wave Infrared Light-Emitting Diodes Based on Cr3+-Doped LiScGeO4 Phosphor

Short-wave infrared (SWIR) spectroscopy has recently emerged as an important technology across a wide range of areas, whether industrial, biomedical, or environmental. Nevertheless, it is still a long-standing challenge to develop robust SWIR light sources. The SWIR phosphor-convert light emitting diodes (LEDs) by coating blue LED chips with desirable SWIR-emitting phosphors are becoming an ideal alternative for solid-state SWIR light sources due to its compactness, low-cost, and long operating lifetime, as does the commercial white LEDs. Herein, we report a blue-pumped Cr3+-doped LiScGeO4 SWIR phosphor as a luminescent converter for phosphor-convert SWIR LEDs. This phosphor shows an intense SWIR emission band with a peak wavelength at ∼1120 nm owing to the 4T2 → 4A2 electron transition of Cr3+ when exciting with blue light. The full width at half-maximum (fwhm) of the phosphor is ∼300 nm and the absolute quantum efficiency is determined to be ∼26%. SWIR LED prototypes are constructed by combining the optimized phosphor materials with commercial blue InGaN LED chips, which can generate a commendable emission band in the SWIR region over 800-1600 nm and achieve a maximum output power of ∼4.78 mW at 60 mA with the photoconversion efficiency of 4.4%. The current exploration of Cr3+-doped SWIR-emitting phosphors will lay the foundation to engineer phosphor-convert SWIR LEDs for applications in night-vision surveillance and SWIR spectroscopy technology. These blue-light-excitable SWIR-emitting phosphors can serve as an important complement to the spectral gap of the current Cr3+-doped phosphors in the SWIR region and will pave the way toward cost-effective phosphor-converted solid-state SWIR light sources.

Robust thermal performance of red-emitting phosphor composites for white light-emitting diodes: Energy transfer and oxygen-vacancy induced electronic localization

Ce3+ ion can effectively sensitize Sm3+ ion via energy transfer, and this phenomenon can led to the development of white light-emitting diodes (WLED). However, interestingly, high correlated color temperature (CCT), poor color-rending index (CRI), poor thermal stability, and low efficacy of available red phosphor still pose immense challenges. Herein, we undertook a combined analysis: X-ray diffraction (XRD), crystal refinement, electron spin resonance (ESR), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and diffuse reflection spectroscopy (DRS). We also observed the optical properties of the resulting samples. The Ce3+ and Sm3+ dopants on the Sr2+ and La3+ sites in the mixed cation borate Sr3LaAl3B4O15 (SLAB) phosphors were quantitatively evaluated. A cerium ion merged as a sensitizer, improving the red emission intensity by enhancing it 3.9 times. The energy transfer (ET) between Ce3+ and Sm3+ was examined experimentally and with theoretical models as a function of Ce3+ concentrations at ambient temperatures. Several theoretical models were employed to simulate the luminescence decays of Ce3+ and Sm3+ doped samples at different doping levels and their transfer mechanisms were studied depending on forced electric dipole at each ion. Notably, the electronic sites created by the oxygen vacancies around the Ln3+ ions can effectively justify the highly efficient bluish-red phosphor. Additionally, the SL0.95AB:0.02Ce3+,0.03Sm3+ exhibited outstanding thermal-quenching (TQ) resistance and has > 94.8% intensity at 425 K. WLEDs made with the use of SL0.95AB:0.02Ce3+,0.03Sm3+ furnished an exceptional CRI exceeding 88 and low at CCT 4503 K. These results are superior to the parameters of commercial WLED containing Y3Al5O12:Ce3+ phosphor and blue LED chip (CCT≈7746 K, CRI≈75), and they could be a cornerstone for the fabrication of warm WLEDs.

High-Performance Perovskite Dual-Band Photodetectors for Potential Applications in Visible Light Communication

Due to the rapid development of smart technology infusion, visible light communication (VLC) has been promising as a connection belt among real estates due to the appealing features including fast speed of data transmission and high bandwidth. Unfortunately, the issues of crosstalk, interference, or data leakage in the VLC impose rigorous requests for the receiver terminal, photodetector, including fast and accurate signal recognition, rapid decoding, etc. In pursuit of distinctive merits, a dual-band photodetector is proposed as an efficient receiver terminal for VLC in this work. With MAPbBr3 and MAPbI3 as photoactive layers, a device by stacking two photodiodes in opposite polarity is constructed to sense the signals at two different wavelengths from a commercial white light-emitting diode (LED) transmitter. By manipulating the applied bias direction, the response of a single device can be controllably switched between 300-570 and 630-800 nm with an optical crosstalk of less than -30 dB. The performance with an Ion/Ioff ratio of about 108, a response bandwidth (f-3dB) of ∼33 kHz, a response switching rate approaching 1000 Hz, and a detectivity of 1.75 × 1010 Jones ensures its application as an efficient data receiver. We believe that this work will provide the motivation to explore novel functional perovskite optoelectronic devices and put them to practical applications in the special field.

A Novel Transceiver Model for Polar Transform Optical (PTO --) OFDM for V isible L ight C ommunication (VLC) Based on Peak to Average Power Ratio Reduction u sing Precoding

Indoor visible light communication (VLC) has the potential of providing high data rates for short-range wireless communication with a relative spatial elevated security in contrast to a radiofrequency wireless one. To support that high data stream, Orthogonal Frequency Division Multiplexing (OFDM) is used; however, due to the limited operational bandwidth of the commercial white light-emitting diode (LED), signal processing techniques are used to increase the efficiency of the OFDM and to adapt OFDM to VLC systems. As a major concern, the intensity modulation direct detection necessary for VLC requires positive real signal, this is dealt with by imposing Hermitian pre-possessing or Cartesian to polar conversion post-processing to the OFDM. The use of the Cartesian to polar converter allows the transmission of complex OFDM symbols through the intensity modulation channel. A polar transform optical (PTO-) OFDM presented here as an improvement and simplification of previous polar optical OFDM schemes gives an efficient transceiver architecture. Nevertheless, both OFDM transmission techniques for Visible optical links, similar to radiofrequency (RF), suffer greatly from irregular excessive Peak-to-Average power ratio (PAPR). Higher PAPR reduces the power efficiency of the On-Off Keying (OOK) based on pulse amplitude modulation (PAM). Furthermore, it also is recommendable to reduce the PAPR for conformity with eye safety. A precoding technique is proposed to reduce the PAPR of intensity-modulated for direct detectability of the OFDM signal destined for the wireless optical link using Cartesian-to-Polar conversion. Based on the enhanced processing at the front ends and using MATLAB simulation, it is proven that the presented model can improve the link parameters including the bit error rate (BER) and signal to noise ratio (SNR) and bandwidth efficient compared to Hermitian modified ones.

Tunable cool to warm white emission in Ca1-xLa2x/3TiO3 coped with Dy3+ and Sm3+ for white LED application

The luminescence properties of Ca1-xLa2x/3TiO3:Dy3+, Sm3+ phosphors were investigated in this paper. They were characterized by their photoluminescence, thermal quenching, and electroluminescence. At an excitation wavelength of 388 nm, the photoluminescence spectra of the Ca1-xLa2x/3TiO3:Dy3+ phosphors exhibited two emission peaks at 482 and 573 nm which were attributed to 4F9/2 → 6H15/2 and 4F9/2 → 6H13/2 transitions of Dy3+, respectively. Orange-red Ca1-xLa2x/3TiO3:Sm3+ phosphors had three emission peaks at 565, 601 and 648 nm at an excitation wavelength of 404 nm due to the 4G5/2 → 6HJ/2 (J = 5, 7 and 9) transitions of Sm3+. When Dy3+ and Sm3+ were co-doped into Ca1-xLa2x/3TiO3, the phosphors achieved conversion of cool white and warm white light by varying the concentrations of Dy3+ and Sm3+. The temperature-dependent emission spectra indicated that the Ca1-xLa2x/3TiO3:Dy3+, Sm3+ phosphors exhibited good thermal stability. White light-emitting diodes (WLEDs) demonstrated that the fabricated Ca0.7La0.2TiO3:Dy3+, Sm3+ phosphors had correlated color temperature (CCT) and color rendering index (Ra) values of 6064 K and 71, respectively. They had better performances than commercial WLEDs with a combination of blue LEDs coated with yellow emitting Y3Al5O12:Ce3+ phosphors (CCT =7760 K, and Ra < 65). This study demonstrates that Ca1-xLa2x/3TiO3:Dy3+, Sm3+ phosphors can be promising candidates for NUV chip-based WLEDs.

Eu(II)‐Activated Silicates for UV Light‐Emitting Diodes Tuning into Warm White Light

White‐light‐emitting diodes (WLEDs) are the main white light source for general lighting due to high‐luminous efficacy (&gt;150 lm W−1), long lifespan (5 × 104 h), and environmentally friendly properties. Despite the current maturity and reliability of the solid state‐lighting field, the control of the correlated color temperature (CCT) tuning is still a challenge, arising from the lack of red emission color in the current design of commercial WLEDs. Moreover, as this aspect is often associated with dysregulations of the human circadian rhythm, the need for novel WLEDs is of enlarged societal relevance. In this scope, herein, warm WLED prototypes are fabricated by coating near‐UV‐emitting LEDs with a mixture of the yellow‐emitting polyphasic Sr2SiO4:Eu(II) (S2S) and the blue‐emitting BaMgAl10O17:Eu(II) (BAM) phosphors dispersed in poly(methyl methacrylate). As the amount of S2S increases in the phosphor mixture, the CCT values of the WLEDs decrease from 6500 K (ideal for daylight application) to 4000 K (ideal for night light application), with a luminous efficacy of radiation of 404 lm W−1, among the best figures of merit for WLEDs. Furthermore, all the WLED devices also display a desirable color rendering index (CRI &gt; 75), comparable with the commercial WLED, independent on the operating voltage.

Self‐Quenching‐Resistant Red Emissive Carbon Dots with High Stability for Warm White Light‐Emitting Diodes with a High Color Rendering Index

AbstractDue to the excellent characteristics of energy‐saving, environmentally friendly and long‐life, the solid‐state lighting based on white light‐emitting diodes (WLEDs) is an emerging technology to replace traditional lighting solutions. However, the lack of red component of the most established method for fabricating commercial WLEDs results in high correlated color temperature (CCT) and low color rendering index (CRI). Herein, red‐emitting carbon dots (R‐CDs) with self‐quenching‐resistant property are synthesized using melamine and dithiosalicylic acid as precursors. In addition, R‐CDs exhibit high thermal and photostability under high temperature and strong blue light irradiation. On the basis of the unique features of R‐CDs, a WLED is fabricated by combining a blue‐LED chip with commercial Ce3+:Y3Al5O12 and R‐CDs phosphors, which exhibits warm white light with a color coordinate of (0.41, 0.38), a high CRI of 92.7 and a CCT of 3827 K. Besides, the WLED exhibits good optical stability upon the increase of drive current from 20 to 100 mA. The results suggest R‐CDs can be used as a color converter to fabricate high‐quality warm WLEDs.