Phosphor Layer Research Articles

Fabrication and Analysis of Perovskite Solar Cells (PSCs) by Using Phosphor and TiO₂ Photoelectrode.

In this work, to explore the influence of phosphor additives on the conversion efficiency of perovskite solar cells (PSC), we introduce a Y₃Al5O12:Ce3+ (YAG:Ce3+) phosphor layer. The YAG:Ce3+ nanophosphor acts as a light down-converting material to absorb high energy photons and emit lower energy photons that match well with the absorption of the perovskite layer, yielding more excited photo-generated electron-hole pairs. Therefore, the incident solar light can be harvested more effectively. We used 8 mg/ml of TiO₂ mixed with YAG:Ce3+ in the PSCs and realized a light-to-electric energy conversion efficiency of 13.34%, a short circuit current density of 21.23 mA/cm², an open circuit voltage of 0.97 V, and a FF of 55.96%. Higher efficiencies were achieved for PSCs with phosphor-mixed TiO₂ than for cells with pure TiO₂ nanoparticles. The samples were characterized by XRD, SEM, UV-vis, PL, and IV-curves. Photoelectrode DSSC with light-to-electric energy conversion efficiency was achieved under a simulated solar light irradiation of 100 mW/cm² (AM 1.5).

Failure Mechanism of Phosphors in GaN‐Based White LEDs

The degradation and failure mechanisms of nitride phosphor in GaN‐based white LEDs are studied using an in situ multi‐functional accelerated life test and different analytical technologies. The electroluminescence results show that both the intensities of the blue emission (BL) from the LED chips and the yellow emission (YL) from the phosphors decrease with the stress time increasing. However, the YL/BL intensity ratio increases with a yellow shift for the chromaticity coordinates. The reason is not only due to the degradation of the LED chip but the nitride phosphors. The generated microcracks and delamination of phosphor layer lead to the increase of blue light absorption. Except for that, we find that the reason for the decrease of the phosphor conversion efficiency is related to the oxidization of the phosphor's luminescence center Eu2+ ions instead of phosphor thermal quenching. These findings help further understand failure mechanism of phosphors during the operation, and improve the long‐term color quality for the white LEDs.

The Effects of TiO2 Diffuser-Loaded Encapsulation on Corrected Color Temperature Uniformity of Remote Phosphor White LEDs

With the development of high-efficiency and high-power LEDs, they have become the most energy-efficient and environmentally friendly artificial light source. Phosphor-converted white LEDs are currently mainstream in the market. The remote phosphor is an effective way to enhance the conversion efficiency and lifetime of phosphor-converted LEDs. For applications of high-quality lighting and LCD backlights, the uniformity of angular correlated color temperature (CCT) is very important. This report explored a remote phosphor white LED with low angular CCT variance and high luminous efficiency by using TiO2 diffuser-loaded encapsulation. Experimental results revealed that for the TiO2 diffuser-loaded encapsulation remote phosphor white LED, the angular color uniformity could be improved by 31.82% and the luminous flux by 8.65%. Moreover, the mean CCTs of the TiO2 diffuser-loaded encapsulation and non-diffuser remote phosphor white LEDs were similar at a driving current of 350 mA. Finally, we showed that incorporating the TiO2 diffuser into the phosphor layer of the remote phosphor white LEDs, does not influence the reliability of the LED.

A novel graphene-based electroluminescent gas sensor for carbon dioxide detection

In this research, we report a new application of alternating-current electroluminescent (AC-EL) devices for carbon dioxide (CO2) detection at room temperature. The EL structure was designed for gas-sensing applications by replacing a dielectric layer with a graphene gas-sensing layer. The graphene-based EL gas sensor was fabricated by screen-printing of a ZnS:Cu,Cl phosphor layer and drop-coating of graphene synthesized by chemical vapor deposition (CVD) process. Optical characterizations confirmed that the graphene-based EL gas sensor showed similar optical properties to a typical AC-EL display device. From gas-sensing results, the AC-EL gas sensor exhibited high response to CO2 in concentration range of 100–1000 ppm at room temperature and high selectivity against NH3, C2H5OH and C7H8. Furthermore, the EL gas sensors offer good repeatability, reproducibility and measurement accuracy. A sensing mechanism of AC-EL gas sensor was explained based on the resistance change of graphene sensing layer via a direct charge transfer process. These results could pave the way to develop a new type of gas sensor based on an EL principle.

Multi-wavelength phosphor model based on fluorescent radiative transfer equation considering re-absorption effect

The light propagation properties within the phosphor layer in phosphor-converted light-emitting diodes (pc-LEDs) have been extensively described by the two-wavelength model, in which two individual excitation and emission wavelengths are selected. The ignorance of the wavelength-dependence may cause deviation and cannot return the spectrum. In this paper, we extended the two-wavelength model to a multi-wavelength model based on fluorescent radiative transfer equation (FRTE) to characterize the overall light propagation behavior in the phosphor layer. In addition to the light absorption, forward scattering, and fluorescence characteristics, the re-absorption effect was further considered. The spectral radiance at any spatial location and angular direction for each wavelength was iteratively solved using the spectral element method (SEM). The model was validated by comparing the calculated spectrum and angular correlated color temperature (CCT) with experiments and good agreement was achieved with the corresponding maximum deviations of 7.6% and 3.6%, respectively. We also conducted a comparison between the multi-wavelength model with the two-wavelength model. We found that the two-wavelength model may underestimate or overestimate the optical power in the blue region and yellow region, respectively. It was attributed to assuming the peak absorption coefficient and quantum efficiency for all excitation wavelengths in the two-wavelength model.

Development of optical-thermal coupled model for phosphor-converted LEDs

In this review, first, we discussed the effect of phosphor features on optical properties by the software simulation in detail. A combination of these parameters: phosphor material, phosphor particle size and particle distribution, phosphor layer concentration, phosphor layer thickness, geometry, and location of the phosphor layer, will result in the final optical performance of the phosphor layer. Secondly, we introduced how to improve light extraction efficiency with various proposed methods. Thirdly, we summarized the thermal models to predict the phosphor temperature and the junction temperature. To stabilize the optical performance of phosphor-converted light emitting diodes (PC-LEDs), much effort has been made to reduce the junction temperature of the LED chips. The phosphor temperature, a critical reliability concern for PC-LEDs, should be attracted academic interest. Finally, we summed up optical-thermal coupled model for phosphors and summarized future optical- thermal issues exploring the light quality for LEDs. We foresee that optical-thermal coupled model for PC-LEDs should be paid more attention in the future.

Non-doped phosphorescent organic light-emitting devices with an exciplex forming planar structure for efficiency enhancement

Organic light-emitting devices (OLEDs) consisting of a non-doped phosphorescent dye that inserted in an exciplex forming planar structure have been fabricated, and the thickness of ultrathin phosphor layer is optimized to achieve high efficiency. The results showed that OLEDs based on the exciplex interface and a 0.5 nm thick phosphorescent dye have a power efficiency, a current efficiency, and an external quantum efficiency of 37.4 lm/W, 40.5 cd/A, and 14.3%, respectively, which are almost two-folds higher than those with non-exciplex interface devices. Meanwhile, the efficiency roll-off is significantly suppressed. These improved device performances are attributed to the elimination of triplet energy leakage from exciplexes to constituting molecules, efficient energy up conversion of triplet exciplexes and complete host-guest Förster energy transfer. These results will provide an easily-fabricated and time-saving approach for high-performance OLEDs.

Solution of the problem of impact elastoplastic deformation of a thin layer of mechanoluminophor using the methods of the dislocation microdynamic theory of plasticity

The paper presents the results of numerical simulation of the output optical signals of mechanoluminescent shock sensors. Such sensors operate on the principle of direct conversion of mechanical impact energy into optical radiation energy. The sensing element of such a sensor is a thin layer of phosphor enclosed between two transparent flexible polymer films. The mathematical model of the sensor is based on the process of excitation of the glow centers (activator atoms) in a strong electric field of a moving dislocation. The stress-strain state of the film sensing element under quasi-static uniaxial loading under the action of a single pressure pulse is considered. The analysis of constitutive equations for elasto-plastic deformations and the basic equations of the dynamic theory of dislocations. To calculate the deformation of the sensing element, a microscopic model of an isotropic elastic-plastic medium with hardening is used, according to which the plastic deformation is considered as a result of the movement and multiplication of dislocations, and the hardening is as a result of their partial locking due to the increased density.

Flexible nanophosphor films doped with Mie resonators for enhanced out-coupling of the emission.

Herein, we present a general method to prepare self-standing flexible photoluminescent coatings of controlled opacity for integration into light-emitting diodes (LEDs) employing cost-effective solution-processing methods. From colloidal suspensions of nano-sized phosphors, we fabricate light-emitting transparent films that can be doped with spherical scatterers, which act as Mie resonators that trigger a controlled photoluminescence enhancement, evidenced by the reduction of the guided light along the layer. This results in an enhanced emission compared to that extracted from a bare phosphor layer. We show not only that emission is visible under ultraviolet-LED illumination for both rigid and flexible versions of the coatings, but we also prove the feasibility of the integration of these flexible conversion layers into such devices. We believe these results can contribute to develop more efficient and cost-effective illumination sources by providing efficient and easy-to-handle conversion layers susceptible to excitation by LEDs emitting at wavelengths in the near UV region.

Increasing the extraction efficiency of the LED by using a pyramid-shaped phosphor layer

Increasing the extraction efficiency of the LED by using a pyramid-shaped phosphor layer

Analytical and Monte Carlo comparisons on the optical transport mechanisms of powder phosphors

In indirect medical imaging systems, the optical transfer properties of powder phosphor materials are affected by the intrinsic mechanisms of light extinction. Light extinction (absorption and scattering) can be described either through theoretical models or with Monte Carlo simulation techniques. The purpose of the present paper is to compare both methodologies and assess the association of their optical parameters by examining either the structural (e.g., particle size) or the optical (e.g., light wavelength) intrinsic properties of the phosphor.Comparisons were carried out for the widely used host Gd2O2S for granular phosphors. The parameters considered in Monte Carlo simulation were: (i) a phosphor layer composed of grains with diameter 4–12 μm, (ii) the light wavelength (420 nm, 545 nm and 610 nm), (iii) the refractive index of the phosphor (real part: 2.3, imaginary part: 10−6 and 10−5), (iv) the packing density 50%. The results were compared with the Swank analytical model, based in Boltzman diffusion equation, by considering three cases for optical coefficients determination with the help of which the Luminescence Efficiency (LE) and the Modulation Transfer Function (MTF) of the phosphor could be predicted.It was found that Swank's model seems more appropriate for estimating the MTF values, presenting a deviation less than 5% in all the frequency range. for: wavelength 420 nm, considering (i) grain size 4 μm and thicknesses above 250 μm, (ii) grain size 12 μm and thicknesses above 300 μm, (b) wavelength 545 nm, grain size 8 μm and thicknesses above 160 μm, and (c) wavelength 610 nm, considering (i) grain size 4 μm and thicknesses above 200 μm, (ii) grain size 8 μm and thicknesses above 120 μm and (iii) grain size 12 μm and thicknesses above 80 μm.

Plasmonic-Enhanced Luminescence Characteristics of Microscale Phosphor Layers on a ZnO Nanorod-Arrayed Glass Substrate.

We present a planar luminescent layer for glare-free, long-lifespan white light-emitting diodes (LEDs), with attractive light outputs. The novel and facile remote phosphor approach proposed in this work enhances luminescence properties by combining a waveguiding ZnO-based nanostructure with plasmonic Au nanoparticles. The system comprised a microscale yellow phosphor layer that is applied by simple printing onto an Au nanoparticle-dispersed ZnO nanorod array. This architecture resulted in a considerable enhancement in luminous efficacy of approximately 18% because of the combination of waveguide effects from the nanorod structure and plasmonic effects from the Au nanoparticles. Performance was optimized according to the length of the Zn nanorods and the concentration of Au. An optimal efficiency of ∼84.26 lm/W for a silicate phosphor-converted LED was achieved using long ZnO nanorods and an Au concentration of 12.5 ppm. The finite-difference time-domain method was successfully used to verify the luminous efficacy improvements in the Au nanoparticle-intervened nanostructures via the waveguiding and plasmonic effects.

Controlling graphene work function by doping in a MOCVD reactor

Here we demonstrate a new method for doping graphene using Metal Organic Chemical Vapor Deposition (MOCVD) reactor. The original undoped graphene was of a very high quality mounted on Si/SiO2 substrates, they were then doped in the MOCVD's reactor using tertiarybutylphosphine (TBP) and tertiarybutylarsene (TBA). Post process Raman spectroscopy confirmed the presence of a single layer of phosphor doped graphene (G/P) and Arsine doped graphene (G/As) when doped by TBP or by TBA, respectively. Blue shift of the 2D peak assured p-type doping. The work function determined by ultraviolet photoelectron spectroscopy varied from 4.5 eV for Pristine Graphene to 4.7, 4.8 eV for G/As, G/P, respectively. The increase of the work function is attributed to electron transfer from the graphene to the dopant. Our results suggest that doping graphene by MOCVD with TBA or TBP can easily and effectively alternate the work function by few tenths of eV and improve the electronic properties of graphene. The MOCVD technology of doping graphene opens a new route on which other semiconductors can be epitaxially grown on it in a continues process in the same MOCVD reactor.

Formation of Coated Phosphor Layers by Rotation–Revolution for Remote-Phosphor LED Bulbs and Characterization of LED Light Bulb With Super High Color Rendition

We have developed a new system for coating phosphor globes used in remote-phosphor-type light-emitting diode (RP-LED) bulbs. The system has a rotation–revolution coating apparatus and a heating unit. After dispensing a phosphor slurry mixed with phosphor powder and silicone resin into globes, the phosphor slurry is uniformly coated on the entire inner surface of the globes by the coating system. The phosphor slurry is then cured. We obtained phosphor globes having a phosphor layer with a uniform thickness on the entire inner surface of the globes. We prepared RP-LED bulbs combined with an LED chip-on-board (COB) that emitted light with a wavelength of 400 nm, and phosphor globes containing red, green, and blue phosphor powders obtained using the coating system. The bulbs have a high color rendering because their spectrum is almost the same as that of blackbody radiation. The RP-LED bulbs having a phosphor layer with uniform thickness had more uniform chromaticity and luminescence distribution than an RP-LED bulb having a phosphor layer with nonuniform thickness. The total luminous flux of the RP-LED bulbs was 29% higher than that of a bulb mounted on a COB covered with a phosphor layer. Furthermore, the semiangle at half-power of the RP-LED bulbs was much larger than that of the bulbs mounted on a COB with a phosphor layer.

Color temperature tunable phosphor-coated white LEDs with excellent photometric and colorimetric performances.

The optimization model for the luminous efficacy (LE) of a correlated color temperature (CCT) tunable phosphor-coated white LED (pc-WLED) was developed. The optimal pc-WLEDs with green (498.7nm, 70.0nm), yellow (569.2nm, 83.4nm), and red (638.7nm, 70.0nm) phosphors excited by a blue chip (442.4nm, 35.0nm) could achieve a color fidelity index (Rf) of 97 and LEs of 118-127lm/W at CCTs of 2700-6500K for a radiant efficiency of the blue chip of 60% and a quantum efficiency of a phosphor layer of 90%. Compared to quantum dot WLEDs (QD-WLEDs), the LEs of the optimal pc-WLEDs are lower only by 2.7%-4.8% than the optimal QD-WLEDs at CCTs of 2700-6500K, so that QD-WLEDs have only a slight advantage in terms of luminous efficacy for excellent color rendition. Four real pc-WLEDs with Rfs of 96-97 and LEs of 93-106lm/W at CCTs of 3037K, 4081K, 4951K, and 6443K were demonstrated.

Enhancement of luminous efficacy for LED lamps by introducing polyacrylonitrile electrospinning nanofiber film.

Polyacrylonitrile electrospinning nanofiber film was introduced into a light emitting diode (LED) lamp to exploit the strong reflective and scattering effects. The light extraction mechanism was studied systematically for three different electrospinning types in three different types of LED lamps. For the all-electrospinning types, the luminous efficacy increased for the white LED, outwards remote phosphor layer, and inwards remote phosphor layer lamps by 10.98, 16.97, and 18.35%, respectively, compared with the reference lamp. Lamps with stronger backscatter had larger luminous efficacy enhancements. The reflector-electrospinning type helped redirect lights with large emission angles. The substrate-electrospinning type was beneficial for recycling the total interior reflection lights and increasing the yellow to blue ratio. Additionally, the all-electrospinning white LED lamps remains 97.89% luminous flux after a 96-hour aging process. Electrospinning fiber films are favorable luminous efficacy enhancers for the future generation of LED lamps.

Influence of Dual-layer and Triple-layer Remote Phosphor Package on Optical Properties of White LEDs

In this paper, the influence of the distance between phosphor layers in the dual-layer and triple- layer remote package on luminous flux and color rendering property is presented and analyzed. During the simulation, it is recognized that an appropriate distance can produce higher quality of the multi-chip white LED (MCW-LEDs) through adjusting the distance between two and three phosphor layers. According to the research results, 0.1mm is the outstanding distance between two phosphor layers so that the performance of MCW-LEDs can be accomplished the best optimal effect. In addition, the simulation results show that the dual-layer structure yielded higher optical properties than the triple-layer structure in relation to the distance. The highest lumen output of triple layer-structure can be achieved at the distance of 0.6 mm and dual-layer structure is 0.1 mm. Meanwhile, the color rendering index (CRI) changes insignificantly when the distance increases. Triple layer package is not practical for high power white LEDs due to high cost and low conversion efficiency. Dual-layer remote phosphor package with the distance between two phosphor layers of 0.1mm is an optimal structure of LEDs in improving the luminous efficiency and the color quality.

Simple prismatic patterning approach for nearly room-temperature processed planar remote phosphor layers for enhanced white luminescence efficiency

Efforts have been made to enhance light-extraction by controlling phosphor structure for higher efficiency white light-emitting diodes (LEDs). Here, we introduce a technical method for improving luminous efficacy by applying prismatic patterns for a planar remote yellow phosphor structure. The remote phosphor structure is based on a UV-cured prismatic pattern/polyethylene terephthalate (PET)/glass substrate/silicate phosphor layer. The phosphor layer was formed via another UV-curing process of the screen-printed polymer/ phosphor-mixture. As a result, the prismatic layer of the apex angle 90° of each pitch enhanced the luminous efficacy of the remote phosphor-LED by 29% compared with the reference sample when a smaller pitch size was used. As anticipated, it is assumed that the escape probability of the emitted lights from a blue LED chip through the phosphor structure increases by reducing the possibilities of total internal reflection at the top surfaces of the structure.

The Impacts of Red-Emitting SrwFxByOz:Eu2+,Sm2+ Phosphor on Color Quality of Dual-Layer Phosphor Geometry

When the features of remote phosphor structure are compared with these of conformal phosphor or in-cup phosphor, it is recognized that it is more outstanding than the rest about luminous flux but the quality of color tends to be worse. Through that we have grasped these disadvantages and find out many studies in order to improve the color of the remote phosphor structure. In this study, we propose a dual-layer remote phosphor structure that could improve the color rendering index (CRI) and color quality scale (CQS) for WLEDs. In this study, three similar WLEDs structures but having different color temperatures including 5600 K, 6600 K and 7700K are applied. The principal idea is putting a red phosphoric layer SrwFxByOz:Eu2+,Sm2+ on the yellow phosphorus layer YAG:Ce3+. The results show that SrwFxByOz:Eu2+,Sm2+ brings great benefits to increasing CRI and CQS. Specifically, the greater the concentration of SrwFxByOz:Eu2+,Sm2+ has, the higher CRI and CQS get. However, the declining trend of luminous flux occurs when the SrwFxByOz:Eu2+,Sm2+ concentration exceeds the level. This can be demonstrated through the results of the study and be explained by the Mie dispersion theory and the Lambert-Beer law. The results of this article are important in making WLEDs of higher color quality. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Improved color uniformity in white light-emitting diodes using newly developed phosphors.

We report novel white light-emitting diode (WLED) devices that improve emission color uniformity. The WLEDs consist of a violet chip and a mixed-phosphor layer of three phosphors previously developed by us. It is found that each phosphor does not reabsorb the luminescence from the other phosphors; consequently, the emission color of the WLEDs does not get affected by the mounted quantity of phosphors and/or the variation in chip emission wavelength. Furthermore, an encapsulated WLED with a hemispherical dome-shaped mixed-phosphor layer enables an area to be irradiated with uniform color, producing an excellent color rendering index and improved luminous flux because of the reduced inelastic scattering loss in the phosphor layer.