Shudong Yu

Color uniformity enhancement for COB WLEDs using a remote phosphor film with two freeform surfaces

The color uniformity (CU) of chip-on-board (COB) white light emitting diodes (WLEDs) has been improved by using remote phosphor films with two freeform surfaces (TFS-RPFs). The finite-difference time-domain (FDTD), Monte Carlo ray-tracing, and color-thickness feedback (CTFB) methods were used to design the TFS-RPFs: the blue light distribution of COB WLEDs is greatly affected by the angular thickness distribution of TFS-RPFs, and a high CU can be achieved iteratively. The directional inconsistency of incident and emergent blue light, scattering effect of TFS-RPFs, and illumination characteristics of the COB source were also investigated. COB WLEDs containing optimized TFS-RPFs achieved high CU with a decrease of 26.2% in maximum CCT deviation; thus, TFS-RPFs can improve the CU of COB WLEDs.

Highly reflective nanofiber films based on electrospinning and their application on color uniformity and luminous efficacy improvement of white light-emitting diodes

Based on electrospinning technology, in this study, we fabricated poly(lactic-co-glycolic acid) (PLGA) nanofiber films with high reflectivity and scattering properties. Various films with different thicknesses and fiber diameters were fabricated by changing the electrospinning time and solution concentration, respectively. Detailed optical measurements demonstrate that the film reflectance and scattering ability increase with the thickness, whereas fiber diameter contributes little to both properties. With optimized film thickness and fiber diameter, nanofiber films feature whiteness with a reflectance of 98.8% compared to the BaSO4 white plate. Furthermore, when deposited on the reflector surface of a remote phosphor-converted light-emitting diode lamp, nanofiber films witness a correlated color temperature deviation decrease from 8880 K to 1407 K and a luminous efficiency improvement of 11.66% at 350 mA. Therefore, the nanofiber films can be applied in lighting systems as a highly reflective coating to improve their light efficacy and quality.

ACU Optimization of pcLEDs by Combining the Pulsed Spray and Feedback Method

In this paper, the pulse-sprayed (PS) phosphor coating technique and feedback method were combined to optimize the angular color uniformity of remote phosphor-converted light-emitting diodes (pcLEDs). The geometry of the phosphor-converted element (PCE) was controlled by the PS technique and the curvature of the phosphor bearing surface. Meanwhile, the yellow and blue light irradiance distributions are as the feedback function to optimize the mass distribution of the PS PCE during the feedback iteration process. With the method proposed herein, the correlative color temperature of the optimized remote pcLED ranges from 5192 to 5263 K with a maximum deviation of only 71 K.

Freeform illumination lens design combining energy and intensity mapping

A composite method combining energy and intensity mapping is proposed to address the issue of surface error caused by the irregular sampling phenomenon in freeform illumination lens design. In the combined method, the central region of the freeform lens is designed by the intensity mapping method, whereas the peripheral region is designed by the energy-mapping method. Furthermore, an iterative feedback optimization is added to scale out the application in extended light sources. As an evaluating example, a freeform lens with a 120-deg viewing angle, fitted with an appropriate number of points is designed by the proposed combined method. Compared with that designed by the energy method, the lens forms a more uniform illumination on the target surface without the appearance of a hot spot in the central region. The proposed method also exhibits superiority in extended-source design, where only a one-time optimization is needed to achieve the preset uniformity with the proper choice of coefficients.

Energy feedback freeform lenses for uniform illumination of extended light source LEDs

Using freeform lenses to construct uniform illumination systems is important in light-emitting diode (LED) devices. In this paper, the energy feedback design is used for freeform lens (EFFL) constructions by solving a set of partial differential equations that describe the mapping relationships between the source and the illumination pattern. The simulation results show that the method can overcome the illumination deviation caused by the extended light source (ELS) problem. Furthermore, a uniformity of 95.6% is obtained for chip-on-board (COB) compact LED devices. As such, prototype LEDs manufactured with the proposed freeform lenses demonstrate significant improvements in luminous efficiency and emission uniformity.

Improvement in Color-Conversion Efficiency and Stability for Quantum-Dot-Based Light-Emitting Diodes Using a Blue Anti-Transmission Film

In this report, a blue anti-transmission film (BATF) has been introduced to improve the color-conversion efficiency (CCE) and the stability of quantum dot (QD) films. The results indicate that the CCE can be increased by as much as 93% using 15 layers of BATFs under the same QD concentration. Therefore, the same CCE can be achieved using BATF-QD hybrid films with a lower QD concentration when compared with standard QD films. The hybrid and QD films with the same CCE of 60% were aged at an environmental temperature of 25°C and with a 10 mA injection current light-emitting diode source. The CCE and luminous efficacy that are gained by the hybrid film increased by 42.8% and 24.5%, respectively, when compared with that gained by the QD film after aging for the same time period of approximately 65 h. In addition, the hybrid film can effectively suppress the red-shift phenomenon of the QD light spectra, as well as an expansion of the full-width at half maximum. Consequently, these BATF-QD hybrid films with excellent optical performance and stability show great potential for illumination and display applications.

Effect of ZnO nanostructures on the optical properties of white light-emitting diodes

White light produced by blue LEDs with yellow phosphor is the most widely used methods, but it results in poor quality in angular CCT uniformity. In this work, a novel technique was introduced to solve this problem by integrating different ZnO nanostructures into white light-emitting diodes. The experiment of ZnO doped films and the simulation of Finite-Difference Time-Domain (FDTD) were carried out. The result indicated scattering effect of ZnO nanoparticles could improve uniformity of scattering energy effectively. Moreover, the effect of ZnO nanostructures on white light-emitting diodes (wLEDs) devices was also investigated. The CCT deviation of wLEDs devices would decrease from 3455.49 K to 96.30 K, 40.03 K and 60.09 K when the node-like (N-ZnO), sheet-like (S-ZnO) and rod-like ZnO (R-ZnO) respectively applied. The higher CCT uniformity and little luminous flux dropping were achieved when the optimal concentrations of N-ZnO, S-ZnO, and R-ZnO nanostructures were 0.25%, 0.75%, and 0.25%. This low-cost and green manufacturing method has a great impact on development of white light-emitting diodes.

Enhanced color conversion efficiency of remote phosphor-converted light-emitting diodes using micro-concavity arrays

Remote phosphor-converted LEDs (rpc-LEDs), which rely on a phosphor layer located away from the LED chip, are a particularly attractive technology benefitting from a higher luminous efficiency and from an improved stability compared with on-chip LEDs. However, systems based on thin-film remote phosphor layers still face a low color conversion efficiency (CCE). This mostly originates from an insufficient interaction of the exciting blue light with the phosphors. To overcome this limitation, we propose to couple the thin-film converting layers to a micro-concavity array (MCA) designed to enhance the optical pathlength of the exciting light, resulting in an improved CCE. This is achieved by exploiting the excellent light scattering and retro-reflection properties of MCA. We experimentally verify that the MCA transmit 95% of the incoming blue light into the converting layer, whereby 84% of this share corresponds to scattered light. Moreover, the measured retro-reflection amounts to 21% for normally incident light. The potential of the fabricated MCA films is tested by integrating them on the illuminated side of remote light converting thin-film layers with sub-millimeter thickness. Two examples, including quantum dots (QDs)- and rare-earth phosphor- based LEDs, are investigated. Our results show that the CCE of both rpc-LEDs are improved due to the enhanced excitation of the downconverted materials and to the effective extraction of the backscattered light. Thus, the CCE values of QDs-based and phosphor-based and rpc-LEDs are increased by 8.1% and by 12.7%, respectively, compared to devices without MCA films. In the latter case, the angular color uniformity is additionally improved under the effect of light scattering.

Influence of lens structure on the mechanical strength of high-power light emitting diodes

The paper presents the failure mode of high-power light emitting diodes (LEDs) under mechanical pressure and verifies the bonding wire as the weakest structure of LEDs. Different lens radiuses and heights are studied to demonstrate the influence of lens parameters on the mechanical strength of LEDs. The experimental results have demonstrated that a LED device has a higher failure force when the lens radius or height is larger. The loading force increases with the compressing displacement until an open circuit happens, which is caused by the bonding wire fracture detected by X-ray imaging. The fracture position lies in the part over the first bond, addressed as heat affected zone, which corresponds with the stress concentration position in the simulation.