Chao-Hsun Wang

Patterned structure of remote phosphor for phosphor-converted white LEDs.

High efficiency white light-emitting diodes with superior color-mixing have been investigated. It is suggested that the patterned remote phosphor structure could improve the uniformity of angular-dependent correlated color temperature (CCT) and achieve high chromatic stability in wider operating current range, as compared to the conventional remote phosphor coating structure. In this experiment, we employed a pulse spray coating method to place the patterned phosphor on the package and to leave a window region. The window area, a clear space without coating of the phosphor not only increases the extraction efficiency of blue rays at large angle, but also improves the stability of angular-dependent CCT. Moreover, the CCT deviation could be reduced from 1320 K to 266 K by this patterned remote phosphor method, and the stray blue/yellow light within the package can be effectively reduced and controlled. The design was verified both experimentally and theoretically.

Improvement in uniformity of emission by ZrO2 nano-particles for white LEDs

The high luminous efficiency and superior uniformity of angular-dependent correlated color temperature (CCT) white light-emitting diodes have been investigated by ZrO₂ nano-particles in a remote phosphor structure. By adding ZrO₂ nano-particles with silicone onto the surface of the phosphor layer, the capability of light scattering could be enhanced. In particular, the intensity of blue light at large angles was increased and the CCT deviations could be reduced. Besides, the luminous flux was improved due to the ZrO₂ nano-particles with silicone providing a suitable refractive index between air and phosphor layers. This novel structure reduces angular-dependent CCT deviations from 1000 to 420 K in the range of -70° to 70°. Moreover, the enhancement of lumen flux was increased by 2.25% at a driving current 120 mA, compared to a conventional remote phosphor structure without ZrO₂ nano-particles. Consequently, the ZrO₂ nano-particles in a remote phosphor structure could not only improve the uniformity of lighting but also increase the light output.

The Influence of the Thermal Effect on CdSe/ZnS Quantum Dots in Light-Emitting Diodes

This study investigates the effect of temperature on CdSe/ZnS quantum dots (QDs) in GaN-based light-emitting diodes (LEDs) using the phosphor conversion efficiency (PCE) and LED junction temperature. In our simulation, the blue chip and CdSe/ZnS QDs temperature are similar because of their minimal thickness. Furthermore, to verify the effect of temperature on CdSe/ZnS QDs, we use continuous wave and pulsed current sources to measure the relationship between the temperature and relative PCE. Higher junction temperatures are observed with greater CdSe/ZnS QD volume in LEDs. This is attributed to the thermal conduction and nonradiative energy between CdSe/ZnS QDs and blue chip. Therefore, if thermal management is improved, CdSe/ZnS QDs are expected to be used as color converting material in LEDs.

Hole injection and electron overflow improvement in InGaN/GaN light-emitting diodes by a tapered AlGaN electron blocking layer.

A tapered AlGaN electron blocking layer with step-graded aluminum composition is analyzed in nitride-based blue light-emitting diode (LED) numerically and experimentally. The energy band diagrams, electrostatic fields, carrier concentration, electron current density profiles, and hole transmitting probability are investigated. The simulation results demonstrated that such tapered structure can effectively enhance the hole injection efficiency as well as the electron confinement. Consequently, the LED with a tapered EBL grown by metal-organic chemical vapor deposition exhibits reduced efficiency droop behavior of 29% as compared with 44% for original LED, which reflects the improvement in hole injection and electron overflow in our design.

Enhanced Luminous Efficiency of WLEDs Using a Dual-Layer Structure of the Remote Phosphor Package

This study demonstrates how the insertion of a thin silicone layer into a dual-layer remote phosphor structure enhances light extraction in white light-emitting diodes (WLEDs). In the experiment, a dual-layer phosphor structure yielded a higher intensity of blue and yellow components than a conventional structure. Moreover, the lumen flux was 5% higher than a conventional remote phosphor package at the same correlated color temperature (CCT). Using a TFCalc32 simulation, the electric field intensity was calculated for different thicknesses of the dual-layer remote phosphor structures, and the enhanced use of blue rays was verified. Additionally, the dual-layer structure reduces chromaticity deviations as the driving current increases.

An Investigation of the Optical Analysis in White Light-Emitting Diodes With Conformal and Remote Phosphor Structure

An effective emission model of phosphor film is proposed by using bidirectional scattering distribution function system (BSDF), and the model is verified by white light-emitting diodes (LEDs) with conformal and remote phosphor structure. The emission model is built to clarify the optical characteristics by analyzing the angular-dependent distribution of emission and excitation behaviors in phosphor film. The white LEDs with conformal and remote phosphor structure are also fabricated for experimental comparison. The uniformity of angular correlated color temperature (CCT) in white LEDs can be determined by the angular distribution of blue and yellow light, which is in turns decided by the refractive index variation between chip a©nd phosphor layers. Finally, the experimental results are found to have good agreement with the simulation results performing by the Monte Carlo method.

Efficiency and Droop Improvement in Hybrid Warm White LEDs Using InGaN and AlGaInP High-Voltage LEDs

This study investigates the optical and electrical characteristics in hybrid warm white high-voltage light-emitting diodes (HV-LEDs). The luminous efficiency of the hybrid warm white LED in this study improved by 11% and 51%, compared to conventional cool and warm LEDs, respectively, solving the warm white gap in white LEDs. The efficiency droop of the hybrid warm white LED was reduced to 21.8% from 26.8% for the conventional cool white LED, and from 26.3% in the conventional warm white LED at 40 mA (35 A/cm2) the operated current. Furthermore, the color rendering index (CRI) and angular correlated color temperature (CCT) were analyzed, indicating a significant improvement in hybrid warm white HV-LEDs.

Efficiency and Droop Improvement in InGaN/GaN Light-Emitting Diodes by Selective Carrier Distribution Manipulation

Efficiency and droop behavior in InGaN/GaN light-emitting diodes are both improved by selectively-graded-composition multi-quantum barriers (SGQB). Simulation results show that SGQB could moderately improve the hole transport in active region. In the meantime, spatial distribution overlap between electrons and holes in active region could be also well-considered. Therefore, the radiative recombination of SGQB LED is more efficient than that of conventional LED. The overall efficiency and droop behavior are simultaneously improved in SGQB LED, at both low and high current density.

Improvement of angular-dependent CCT uniformity by ZrO 2 nano-particles in remote phosphor white LEDs

The superior uniformity of angular-dependent CCT white LEDs have been investigated by ZrO2-type remote phosphor structure. This novel structure reduces angular-dependent CCT deviations from 1000K to 420K in range of -70 to 70 degrees.

Improving the efficiency of gallium nitride-based LEDs for high-power applications

Over the past decade, solid-state lighting has been a rising star for next-generation illumination sources. It has the advantages of saving energy and providing high-quality lighting, as well as offering a range of flexible design features.1 In particular, there has been much interest in the potential of solid-state lighting using gallium nitride (GaN)-based LEDs. However, for highpower applications, LEDs need to be operated at a very high current density, which can diminish device efficiency.2 In LEDs, the device structure gives rise to a separation of charge carriers, that is, electrons and holes. When a bias voltage is applied, the electrons and holes recombine, emitting light. Carrier overflow—electron overflow out of the radiativerecombination region—has been identified as the main cause of the drop in efficiency at high current densities.3, 4 An aluminum gallium nitride (AlxGa1 xN) ‘electron-blocking layer’ (EBL) has been widely adopted in LED structures to to suppress carrier overflow. However, it has been reported that the large polarization field in the AlxGa1 xN EBL reduces the effective energy barrier height for electrons,5 weakening the stopping power of this barrier layer. To make the situation worse, the polar nature of the group III nitride material and lattice mismatch give rise to a polarization field at the interfaces of the GaN and EBL. The polarization field bends the electronic energy band structure. In addition, there is a difference in the energy of the outermost electrons: a valance band offset. The band bending and offset are thought to retard the injection of holes, preventing recombination and further reducing efficiency.2, 5 By adapting the concept of band engineering, we designed a graded-composition EBL (GEBL) for GaN-based LEDs. The GEBL not only suppressed the electron overflow from the active region but also enhanced hole injection. Our concept of Figure 1. Schematic diagram of the concept of band engineering. EBL is the electron blocking layer and MQWs are multiple quantum wells. The dotted blue circle indicates the triangular valence band structure. Electrons are injected from the n-side and holes from the p-side. n, p: Semiconductor types.