Phosphor-based White Light-emitting Diodes Research Articles

Toward 200 Lumens per Watt of Quantum-Dot White-Light-Emitting Diodes by Reducing Reabsorption Loss.

In this study, we analyze the influence of the pore structure of an SBA-15 particle on the light emission from its inner adsorbed quantum dots (QDs) and outer light-emitting diode (LED) chips. It is found that the particle features of a high refractive index, comparable feature size of pore structure, and lower amount of QD adsorption help with QD light extraction, demonstrating a mechanism to suppress QD light propagating through pores and thus reducing the reabsorption loss. We consequently developed highly efficient QD white LEDs with wet-mixing QD/SBA-15 nanocomposite particles (NPs) by further optimizing the packaging methods and the introduced NP mass ratio. The LEDs demonstrated a record luminous efficacy (the ratio of luminous flux to electrical power) of 206.8 (entrusted test efficiency of 205.8 lm W-1 certificated by China National Accreditation Service) and 137.6 lm W-1 at 20 mA for white LEDs integrating only green QDs and green-red QD color convertors, respectively, with improved operating stability. These results are comparable to conventional phosphor-based white LEDs, which can be a starting point for white LEDs only using QDs as convertors toward commercialization in the near future.

Effects of cationic substitution on the luminescence behavior of Dy3+ doped orthophosphate phosphor

Effects of cationic substitution on the luminescence behavior of Dy3+ doped orthophosphate (A3B(PO4)2; A = K, Na; B = Y, La) phosphors synthesized by the solution combustion method were investigated. The powder X-ray diffraction (PXRD) and photoluminescence (PL) studies were done to examine the structural and luminescence behavior of Dy3+ ions in the phosphate lattices. The PXRD results ascertained that the phosphors have the desired phases corresponding to their standards. Under UV-excitation, the PL emission spectra of the phosphors exhibited three bands at 481 ± 2 nm (blue band), 575 ± 1 nm (yellow band) and 668 ± 2 nm (red band) ascribed to the 4F9/2 → 6H15/2, 4F9/2 → 6H13/2 and 4F9/2 → 6H11/2 characteristic 4f-4f transition of the Dy3+ ions, respectively. The luminescence behavior, the Stark splitting of the emission band and symmetry around the Dy3+ ions in the host lattice were discussed on the grounds of distortion induced in the host lattice upon Dy3+ doping. The photometric results including high quantum efficiency, commission international de l’Eclairage coordinates near to standard white light coordinates, color correlated temperature (>5000 K) and high thermal stability imply that the phosphor could be an appropriate choice for single phosphor-based white light-emitting diodes.

Metal-enhanced fluorescence in polymer composite films with Au@Ag@SiO2 nanoparticles and InP@ZnS quantum dots.

For white light-emitting diode (LED) applications, semiconductor quantum dots (QDs) have been widely utilized as efficient down-converters to change the blue color of the light source into different emission colors. Because QDs offer spectral tunability over the entire visible light range, as well as improved color purity, they have rapidly replaced conventional phosphor-based white LEDs. However, for the sustainable growth of QD-mediated LEDs, the amount of QDs required must be reduced by enhancing the color-conversion efficiency. For this purpose, we prepared poly(lauryl methacrylate) (PLMA) composite films by the photo-crosslinking polymerization of lauryl methacrylate monomers in the presence of Au@Ag@SiO2 nanoparticles (NPs) and InP@ZnS QDs. In the PLMA composites, the Au@Ag NPs not only amplified the blue light source but also modified the relaxation of the excited QDs via localized surface plasmon resonance. This resulted in a maximum 12.9-fold enhancement in the QD fluorescence. Because the blue light source in this study can be easily replaced by blue LEDs, the enhanced efficiency of QD emissions via the plasmonic effect could potentially increase the performance of QDs for display applications.

Excellent luminescence films of excitation-independent carbon quantum dots toward non-rare-earth phosphor-based white light-emitting diodes

Currently, fluorescent carbon quantum dots have been extensively studied for their promising application in solid-state lighting. However, most of the reported carbon quantum dots exhibit short-wavelength blue-green emissions and the obtained carbon solid powders usually show weak luminescence due to aggregation-induced emissive quenching. In this work, carbon quantum dots with excitation wavelength independent yellow emission were synthesized by a facile and green hydrothermal route using water as solvent and o-phenylenediamine as carbon source. Structural and spectroscopic characterizations evidenced that N-related defect state was responsible for yellow luminescence. Importantly, the carbon quantum dots embedded solid films were successfully prepared, which exhibited strong blue light absorption and retained intense yellow emission. As a proof-of-concept experiment, carbon polymer monolith was demonstrated to be an alternative to traditional Ce: Y3Al5O12 phosphors as color converter for constructing white light-emitting diode device with improved color rendering index and correlated color temperature.

A high-speed phosphorescent LED-based visible light communication system utilizing SQGNRC precoding technique

Peak-over-average power ratio (PAPR) control utilizing precoding technique based on square root-generalized raised cosine function is proposed and verified experimentally through a visible light communication system employing phosphor-based white light-emitting diodes for the first time. The simulation results show that the improved precoding matrix can further reduce PAPR by nearly 1 dB compared with existing precoding matrix based on square root raised cosine function and significantly achieve 3.5 dB improvement in contrast to original signals without precoding. The experimental result also proves that the bit error rate performance can be effectively improved by 1.62 dB at the same bandwidth and 0.76 dB at the same raw data rate in terms of quality factor when proposed precoding technique is applied, which clearly demonstrates its benefit and feasibility.

Phosphor-Free InGaN White Light Emitting Diodes Using Flip-Chip Technology.

Monolithic phosphor-free two-color gallium nitride (GaN)-based white light emitting diodes (LED) have the potential to replace current phosphor-based GaN white LEDs due to their low cost and long life cycle. Unfortunately, the growth of high indium content indium gallium nitride (InGaN)/GaN quantum dot and reported LED’s color rendering index (CRI) are still problematic. Here, we use flip-chip technology to fabricate an upside down monolithic two-color phosphor-free LED with four grown layers of high indium quantum dots on top of the three grown layers of lower indium quantum wells separated by a GaN tunneling barrier layer. The photoluminescence (PL) and electroluminescence (EL) spectra of this white LED reveal a broad spectrum ranging from 475 to 675 nm which is close to an ideal white-light source. The corresponding color temperature and color rendering index (CRI) of the fabricated white LED, operated at 350, 500, and 750 mA, are comparable to that of the conventional phosphor-based LEDs. Insights of the epitaxial structure and the transport mechanism were revealed through the TEM and temperature dependent PL and EL measurements. Our results show true potential in the Epi-ready GaN white LEDs for future solid state lighting applications.

Colour tuning in white hybrid inorganic/organic light-emitting diodes

White hybrid inorganic/organic light-emitting diodes (LEDs) were fabricated by combining a novel organic colour converter with a blue inorganic LED. An organic small molecule was specifically synthesised to act as down-converter. The characteristics of the white colour were controlled by changing the concentration of the organic molecule based on the BODIPY unit, which was embedded in a transparent matrix, and volume of the molecule and encapsulant mixture. The concentration has a critical effect on the conversion efficiency, i.e. how much of the absorbed blue light is converted into yellow light. With increasing concentration the conversion efficiency decreases. This quenching effect is due to aggregation of the organic molecule at higher concentrations. Increasing the deposited amount of the converter does not increase the yellow emission despite more blue light being absorbed. Degradation of the organic converter was also observed during a period of 15 months from LED fabrication. Angular-dependent measurements revealed slight deviation from a Lambertian profile for the blue and yellow emission peaks leading to a small change in ‘whiteness’ with emission angle. Warm white and cool white light with correlated colour temperatures of 2770 K and 7680 K, respectively, were achieved using different concentrations of the converter molecule. Although further work is needed to improve the lifetime and poor colour rendering, these hybrid LEDs show promising results as an alternative approach for generating white LEDs compared with phosphor-based white LEDs.

Long-range high-speed visible light communication system over 100-m outdoor transmission utilizing receiver diversity technology

Visible light communication (VLC) has no doubt become a promising candidate for future wireless communications due to the increasing trends in the usage of light-emitting diodes (LEDs). In addition to indoor high-speed wireless access and positioning applications, VLC usage in outdoor scenarios, such as vehicle networks and intelligent transportation systems, are also attracting significant interest. However, the complex outdoor environment and ambient noise are the key challenges for long-range high-speed VLC outdoor applications. To improve system performance and transmission distance, we propose to use receiver diversity technology in an outdoor VLC system. Maximal ratio combining–based receiver diversity technology is utilized in two receivers to achieve the maximal signal-to-noise ratio. A 400-Mb/s VLC transmission using a phosphor-based white LED and a 1-Gb/s wavelength division multiplexing VLC transmission using a red–green–blue LED are both successfully achieved over a 100-m outdoor distance with the bit error rate below the 7% forward error correction limit of 3.8×10−3. To the best of our knowledge, this is the highest data rate at 100-m outdoor VLC transmission ever achieved. The experimental results clearly prove the benefit and feasibility of receiver diversity technology for long-range high-speed outdoor VLC systems.

(Invited) High Efficiency, Color-Tunable InGaN/GaN Nanowire Light Emitting Diode Arrays

The monolithic integration of red, green and blue (RGB) light-emitting diodes (LEDs) directly on a single chip is critically important for smart lighting and full-color display applications. The resulting color-tunable LEDs can exhibit excellent color rendering capability, high efficiency, and low power consumption, compared to conventional phosphor-based white LEDs. In this context, we have investigated the molecular beam epitaxial growth, fabrication and characterization of InGaN nanowire RGB LEDs monolithically integrated on a single chip, including Si and sapphire substrate. We have achieved controllable and tunable full-color light generation from InGaN LEDs. The correlated color temperature can be continuously varied in the range of 1900 K to 6800 K, while maintaining excellent color rendering index capability (CRI > 90). In this work, RGB InGaN/GaN dot-in-a-wire LED arrays were grown on Si using a three-step selective area growth (SAG) technique, wherein blue, green, and red-emitting LEDs were grown in sequence. In this process, a SiOx mask layer was used to protect the previously grown devices and also to precisely define the growth position of each nanowire LED pixel. Tunable PL emission across the entire visible spectral range (~450 nm to 700 nm) can be readily achieved on a single Si wafer by varying the sizes and/or compositions of the dots. By separately biasing such lateral-arranged multi-color LED subpixels, the CCT value can be varied from ~1900 K to 6800 K, while maintaining relatively high CRI values (up to ~ 90, or higher). We have further investigated the molecular epitaxial growth and fabrication of multi-color nanowire LED arrays on GaN template on sapphire substrate. In this process, a thin (~ 10 nm) Ti layer was utilized as the mask, and nanoscale growth patterns were defined by e-beam lithography. By controlling the nanowire diameters, multi-color emission can be achieved in a single epitaxial growth step. We have also demonstrated multi-color nanowire LEDs on sapphire substrate that can exhibit excellent current-voltage characteristics, high power operation, and reduced efficiency droop. In summary, we have developed tunable, full-color InGaN nanowire LED arrays on Si and sapphire substrate by using special selective area growth techniques. Through controlled light mixing at the wafer-level, we have demonstrated color-tunable light emission with an unprecedented tuning range for the CCT, from 1900 K to 6800K, while maintaining excellent CRI values. The realization of electrically injected RGB InGaN nanowire lasers is being studied and will also be reported.

Perovskite Crystals for Tunable White Light Emission

A significant fraction of global electricity demand is for lighting. Enabled by the realization and development of efficient GaN blue light-emitting diodes (LEDs), phosphor-based solid-state white LEDs provide a much higher efficiency alternative to incandescent and fluorescent lighting, which are being broadly implemented. However, a key challenge for this industry is to achieve the right photometric ranges and application-specific emission spectra via cost-effective means. Here, we synthesize organic–inorganic lead halide-based perovskite crystals with broad spectral tuneability. By tailoring the composition of methyl and octlyammonium cations in the colloidal synthesis, meso- to nanoscale 3D crystals (5–50 nm) can be formed with enhanced photoluminescence efficiency. By increasing the octlyammonium cations content, we observe platelet formation of 2D layered perovskite sheets; however, these platelets appear to be less emissive than the 3D crystals. We further manipulate the halide composition of the p...

Optimized pre-equalization for gigabit polarization division multiplexed visible light communication

We experimentally demonstrated a gigabit visible light communication system employing polarization division multiplexing and adaptive modulation optical-discrete multitones by using a phosphor-based white light-emitting diode. An optimized pre-equalization circuit was used to prevent clipping of the baseband signal, which exceeded the linear operating range of the LED, and to control the power distribution of each subcarrier in order to obtain an efficient bit-loading profile. Using this technique, we achieved 1.025 and 1.016 Gbps transmissions for each polarization channel and a total of 2.041 Gbps transmissions were experimentally verified. In each transmission, bit error rate performances were below the forward error correction limit.

Color coded multiple access scheme for bidirectional multiuser visible light communications in smart home technologies

In optical wireless communications, multiple channel transmission is an attractive solution to enhancing capacity and system performance. A new modulation scheme called color coded multiple access (CCMA) for bidirectional multiuser visible light communications (VLC) is presented for smart home applications. The proposed scheme uses red, green and blue (RGB) light emitting diodes (LED) for downlink and phosphor based white LED (P-LED) for uplink to establish a bidirectional VLC and also employs orthogonal codes to support multiple users and devices. The downlink transmission for data user devices and smart home devices is provided using red and green colors from the RGB LEDs, respectively, while uplink transmission from both types of devices is performed using the blue color from P-LEDs. Simulations are conducted to verify the performance of the proposed scheme. It is found that the proposed bidirectional multiuser scheme is efficient in terms of data rate and performance. In addition, since the proposed scheme uses RGB signals for downlink data transmission, it provides flicker-free illumination that would lend itself to multiuser VLC system for smart home applications.

Is blue optical filter necessary in high speed phosphor-based white light LED visible light communications?

Optical blue filter is usually regarded as a critical optical component for high speed phosphor-based white light emitting diode (LED) visible-light-communication (VLC). However, the optical blue filter plays different roles in VLC when using modulations of on-off keying (OOK) or discrete multi-tone (DMT). We show that in the DMT VLC system, the blue optical filter may be unnecessary, and even degrade the transmission performance (by reducing the optical signal-to-noise ratio (SNR)). Analyses and verifications by experiments are performed. To the best of our knowledge, this is the first time the function of blue filters in VLC is explicitly analyzed.

Hybrid YAG/CdSe Quantum Dots Phosphors for White Light-Emitting Diodes

CdSe quantum dots (QDs) with the emission wavelength of 577 nm, full width at half maximum (FWHM) of 28 nm, quantum yield (QY) of 40% and average particle size of 3.7 +/- 0.5 nm are prepared. 1, 3 and 5 wt% of CdSe QDs is blended with commercial YAG:Ce phosphor (called as composite phosphor) and the device performance of YAG-based and composite phosphor-based white light-emitting diode (WLED) is investigated. The color rendering index (CRI) and luminous efficiency could be improved simultaneous for composite phosphor-based WLED. As the applied current increases, the correlated color temperature (CCT), Commission International d'Eclairage (CIE) chromaticity coordinates and CRI of both WLEDs are almost the same when compared with standard condition (20 mA), while the luminous efficiency of both WLEDs decreases with increasing applied current. It is interesting to find that there is a positive effect of QDs on the luminous efficiency and CRI value. This result also reveals that adding a small amount of yellow emitting QDs can not only reduce the scattering of light effectively but increase utilization efficiency of reflection and refraction light, thereby enhancing the efficiency of composite phosphor-based WLED. Moreover, the emission wavelength of 577 nm QDs is longer than that of YAG, it redound to extends emission range resulting in increase the CRI of devices.

Facile plasma-induced fabrication of fluorescent carbon dots toward high-performance white LEDs

In this study, we report a facile plasma-induced method to fabricate photoluminescent carbon dots (CDs) using acrylamide as the precursor in few minutes. The Fourier transform infrared spectra, UV–Vis absorption spectra, photoluminescence, fluorescent lifetime, and transmission electron microscopy of the as-prepared CDs were investigated thoroughly. The CDs have a narrow size distribution of 3–4 nm and exhibit strong blue fluorescence with quantum yield of ~6 %. More importantly, we explored the CDs as color converters along with CdTe quantum dots to generate white light-emitting diodes (LEDs) using a UV-LED chip as the excitation light source. Compared with the conventional YAG:Ce phosphor-based white LEDs, this resulted LED emitted white light with a higher color rendering index up to 87, which may find their potential in optoelectronic device.

Novel quantum dots: Water-based CdTeSe/ZnS and YAG hybrid phosphor for white light-emitting diodes

Novel water-based core/shell CdTeSe/ZnS quantum dots (QDs) were synthesized by aqueous method. The CdTeSe/ZnS QDs were investigated by high resolution transmission electron microscopy, energy dispersive spectrometry, UV–vis absorption spectra, and photoluminescence spectrum. The as-prepared QDs capped with ZnS shell were spherical in shape with an excellent quantum yield of 16% and emitted bright yellow light. In addition, the CdTeSe/ZnS QDs can be excited by blue or near-UV region, which is an advantage over wavelength converters for white light-emitting diodes (LEDs). White LEDs based on CdTeSe/ZnS QDs, commercially known as Y3Al5O12:Ce3+ (YAG:Ce), and hybrid phosphor of CdTeSe/ZnS QDs and YAG:Ce, were fabricated. The luminescent properties of the resultant white LEDs were evaluated. The higher red-component in the emission spectrum from CdTeSe/ZnS QDs increased the color rendering index (CRI) value of the commercial YAG:Ce-based white LEDs, and the hybrid phosphor-based white LED had CIE-1993 color coordinate, color temperature, and CRI values of (0.3125, 0.2806), 7108 K and 83.3, respectively.

1.1-Gb/s White-LED-Based Visible Light Communication Employing Carrier-Less Amplitude and Phase Modulation

We demonstrate 1.1-Gb/s visible light communication (VLC) employing carrier-less amplitude and phase modulation (CAP) and a commercially available phosphorescent white light emitting diode (LED). Optical blue filtering, precompensation, and decision feedback equalization are used to compensate the frequency response of the phosphor-based white LED. Various modulation orders of CAP signals are investigated to maximize the capacity of the VLC system. The record data rate of 1.1 Gb/s with the bit error rate performance below the FEC limit of 10-3 is successfully achieved >; 23-cm air-transmission via a 220-MBaud 32-CAP signal.

The Mixed Blessing of Phosphor-Based White LEDs

The Mixed Blessing of Phosphor-Based White LEDs

Practical issues for two-light-sources model of phosphor-based white light-emitting diode

We report an intuitive approach to simulating the performance of the phosphor-based light emitting diode (LED) by using the two-light-sources model for an optimized LED lens. The optimized secondary lens shape is achieved by approaching a uniform energy distribution on the detecting plane. Both the spectrum and spatial apodization of the LED structure are discussed, and the results indicate that the two-light-sources model can solve the misleading design on the phosphor-based LED due to imperfect color mixing of light source, that one can satisfy the highly uniformity requirement for specific applications, such as display backlight or optogenetic.

Status and prospects for phosphor-based white LED packaging

The status and prospects for high-power, phosphor-based white light-emitting diode (LED) packaging have been presented. A system view for packaging design is proposed to address packaging issues. Four aspects of packaging are reviewed: optical control, thermal management, reliability and cost. Phosphor materials play the most important role in light extraction and color control. The conformal coating method improves the spatial color distribution (SCD) of LEDs. High refractive index (RI) encapsulants with high transmittance and modified surface morphology can enhance light extraction. Multi-phosphor-based packaging can realize the control of correlated color temperature (CCT) with high color rendering index (CRI). Effective thermal management can dissipate heat rapidly and reduce thermal stress caused by the mismatch of the coefficient of thermal expansion (CTE). Chip-on-board (CoB) technology with a multilayer ceramic substrate is the most promising method for high-power LED packaging. Low junction temperature will improve the reliability and provide longer life. Advanced processes, precise fabrication and careful operation are essential for high reliability LEDs. Cost is one of the biggest obstacles for the penetration of white LEDs into the market for general illumination products. Mass production in terms of CoB, system in packaging (SiP), 3D packaging and wafer level packaging (WLP) can reduce the cost significantly, especially when chip cost is lowered by using a large wafer size.