Browsing by Author "Wu, Tingzhu"

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A Novel Liquid Packaging Structure of Deep-Ultraviolet Light-Emitting Diodes to Enhance the Light-Extraction Efficiency
(2019-04-01)
Kang, Chieh-Yu
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Lin, Chih-Hao
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Wu, Tingzhu
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Lee, Po-Tsung
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Chen, Zhong
To realize high-efficiency, AlGaN-based, deep-ultraviolet light-emitting diodes (DUV-LEDs), enhancing their light-extraction efficiency and reducing thermal resistance is very crucial. We proposed a liquid packaging structure that could enhance optical power by 27.2% and 70.7% for flat type and lens type 281-nm DUV-LEDs, respectively. A significant improvement effect at different wavelengths, such as 268 nm and 310 nm, was also observed. Furthermore, using the liquid packaging structure, the thermal resistance was reduced by 30.3% compared to the conventional structure. Finally, the reliability of liquid packaging DUV-LEDs was tested. The light output maintenance of liquid packaging DUV-LEDs was compared to the conventional structure.
Advances in Quantum-Dot-Based Displays
(2020-07-01)
Huang, Yu-Ming
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Singh, Konthoujam James
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Liu, An-Chen
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Lin, Chien-Chung
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Chen, Zhong
In terms of their use in displays, quantum dots (QDs) exhibit several advantages, including high illumination efficiency and color rendering, low-cost, and capacity for mass production. Furthermore, they are environmentally friendly. Excellent luminescence and charge transport properties of QDs led to their application in QD-based light-emitting diodes (LEDs), which have attracted considerable attention in display and solid-state lighting applications. In this review, we discuss the applications of QDs which are used on color conversion filter that exhibit high efficiency in white LEDs, full-color micro-LED devices, and liquid-type structure devices, among others. Furthermore, we discuss different QD printing processes and coating methods to achieve the full-color micro-LED. With the rise in popularity of wearable and see-through red, green, and blue (RGB) full-color displays, the flexible substrate is considered as a good potential candidate. The anisotropic conductive film method provides a small controllable linewidth of electrically conductive particles. Finally, we discuss the advanced application for flexible full-color and highly efficient QD micro-LEDs. The general conclusion of this study also involves the demand for a more straightforward QD deposition technique, whose breakthrough is expected.
Assessment and Optimization of the Circadian Performance of Smartphone-Based Virtual Reality Displays
(2019-01-01)
Wu, Tingzhu
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Liang, Shijie
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Zheng, Lili
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Lin, Yue
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Guo, Ziquan
The non-visual effects of blue light in displays mean that excessive use of smartphones can disturb the human circadian rhythm. Thus, the impact of virtual reality (VR) headsets, which are worn closer to the human eye, may be even more serious. In this paper, based on non-visual parameters, such as the circadian action factor (CAF) and circadian illuminance, the circadian performance of smartphone-based VR displays is quantitatively evaluated by an evaluation system we designed. Moreover, we investigate the improvements in the circadian performance of VR headsets resulting from three practical methods for reducing the blue light content. Finally, a theoretical method of shifting the green-light wavelength of the screen close to 555 nm to optimize the CAF of VR headsets is proposed. Overall, the results of this paper are of significant value in quantifying the effects of VR displays on circadian rhythms and improving the safety of VR headsets with regard to human health.
Design and Fabrication of the Reliable GaN Based Vertical-Cavity Surface-Emitting Laser via Tunnel Junction
(2019-04-01)
Shen, Chih-Chiang
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Lu, Yun-Ting
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Yeh, Yen-Wei
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Chen, Cheng-Yuan
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Chen, Yu-Tzu
In this study, we theoretically designed and experimentally fabricated an InGaN vertical-cavity surface-emitting laser (VCSEL) with a tunnel junction (TJ) structure. From numerical simulation results, the optical loss of the device can be reduced by a TJ structure. Additionally, the leakage current of the VCSEL with TJ structure was much smaller than that of the VCSEL with an Indium-Tin-Oxide (ITO) layer. We have been demonstrated that laser output performance is improved by using the TJ structure when compared to the typical VCSEL structure of the ITO layer. The output power obtained at 2.1 mW was enhanced by a factor of 3.5 by the successful reduction of threshold current density (J(th)) from 12 to 8.5 kA/cm(2), and the enlarged slope efficiency was due to less absorption in VCSEL with a TJ structure. Finally, the samples passed the high temperature (70 degrees C) and high operation current (1.5 x J(th)) test for over 500 h.
Full-color micro-LED display with high color stability using semipolar (20-21) InGaN LEDs and quantum-dot photoresist
(2020-05-01)
Chen, Sung-Wen Huang
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Huang, Yu-Ming
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Singh, Konthoujam James
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Hsu, Yu-Chien
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Liou, Fang-Jyun
Red-green-blue (RGB) full-color micro light-emitting diodes (mu-LEDs) fabricated from semipolar (20-21) wafers, with a quantum-dot photoresist color-conversion layer, were demonstrated. The semipolar (20-21) InGaN/GaN mu-LEDs were fabricated on large (4 in.) patterned sapphire substrates by orientation-controlled epitaxy. The semipolar mu-LEDs showed a 3.2 nm peak wavelength shift and a 14.7% efficiency droop under 200 A/cm(2 )injected current density, indicating significant amelioration of the quantum-confined Stark effect. Because of the semipolar mu-LEDs' emission-wavelength stability, the RGB pixel showed little color shift with current density and achieved a wide color gamut (114.4% NTSC space and 85.4% Rec. 2020). (C) 2020 Chinese Laser Press
Full-color monolithic hybrid quantum dot nanoring micro light-emitting diodes with improved efficiency using atomic layer deposition and nonradiative resonant energy transfer
(2019-04-01)
Chen, Sung-Wen Huang
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Shen, Chih-Chiang
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Wu, Tingzhu
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Liao, Zhen-You
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Chen, Lee-Feng
Full-color displays based on micro light-emitting diodes (mu LEDs) can be fabricated on monolithic epitaxial wafers. Nanoring (NR) structures were fabricated on a green LED epitaxial wafer; the color of NR-mu LEDs was tuned from green to blue through strain relaxation. An Al2O3 layer was deposited on the sidewall of NR-mu LEDs, which improved the photoluminescence intensity by 143.7%. Coupling with the exposed multiple quantum wells through nonradiative resonant energy transfer, red quantum dots were printed to NR-mu LEDs for a full-color display. To further improve the color purity of the red light, a distributed Bragg reflector is developed to reuse the excitation light. (C) 2019 Chinese Laser Press
High-Bandwidth Green Semipolar (20-21) InGaN/GaN Micro Light-Emitting Diodes for Visible Light Communication
(2020-08-19)
Chen, Sung-Wen Huang
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Huang, Yu-Ming
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Chang, Yun-Han
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Lin, Yue
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Liou, Fang-Jyun
The light-emitting diode (LED) is among promising candidates of light sources in visible light communication (VLC); however, strong internal polarization fields in common c-plane LEDs, especially green LEDs, result in low frequency and limited transmission performance. This study aims to overcome the limited 3-dB bandwidth of long-wavelength InGaN/GaN LEDs. Thus, semipolar (20-21) micro-LEDs (mu LEDs) were fabricated through several improved approaches on epitaxy and chip processes. The mu LED exhibits a 525 nm peak wavelength and good polarization performance. The highest 3-dB bandwidth up to 756 MHz and 1.5 Gbit/s data rate was achieved under a current density of 2.0 kA/cm(2). These results suggest a good transmission capacity of green semipolar (20-21) mu LEDs in VLC applications.
Hybrid-type white LEDs based on inorganic halide perovskite QDs: candidates for wide color gamut display backlights
(2019-05-01)
Lin, Chih-Hao
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Verma, Akta
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Kang, Chieh-Yu
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Pai, Yung-Min
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Chen, Tzu-Yu
We demonstrate inorganic halide perovskite quantum-dots-based white light-emitting diodes via three different geometries, including liquid, solid, and hybrid types. Problems of fast anion exchange and aggregation in the cases of liquid- and solid-type devices are discussed in detail and push us to move towards the fabrication of a hybrid-type device structure. The experiment results illustrate that a hybrid-type device has the highest luminance efficiency (51 lm/W) and a wide color gamut (122% of NTSC and 91% of Rec. 2020). Therefore, we conclude that a hybrid-type device can provide an outstanding color gamut for high color gamut display applications. (c) 2019 Chinese Laser Press
Interplay of carriers and deep-level recombination centers of 275-nm light-emitting diodes - Analysis on the parasitic peaks over wide ranges of temperature and injection density
(2019-08-05)
Wu, Tingzhu
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Lin, Yue
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Peng, Zhangbao
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Chen, Huashan
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Shangguan, Zhibin
The low luminance efficiency, poor reliability and parasitic peaks have greatly limited the commercialization of deep ultraviolet (DUV) light-emitting diodes. Tasks of identifying the culprits of these deficits are of paramount importance but remains unaccomplished. We employ the full-range temperature (20 K - 300 K) measurement on 275-nm DUV devices that subjected to a 15-hour current-stress aging. The results suggest that the primary culprit of fast luminous decay is the proliferation of non-radiative centers. The origins of two main parasitic peaks are identified. The 310-nm peak is considered to solely come from deep-level radiative centers (DLRCs) that only dwell in the active region. Whereas, the 400-nm peak is proven to be dual-sources. One is related to the DLRCs in the active region, which only can be observed at very low currents; the other emerging at higher currents are associated with similar kinds of DLRCs located in the p-region, which only are excited when electrons overflow. This new discovery also demonstrates that a thorough investigation on the interplay among carriers and various types of defects should be conducted on the basis of the measurement that is taken under a wide temperature range, as well as under a proper forward voltage. This is to let the quasi-Fermi level shift across deep defect levels, the band-edge, and to over-band, whereby these recombination sites are exposed to deficit, moderate and saturated electron environment so that their natures can be well tested. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Mini-LED and Micro-LED: Promising Candidates for the Next Generation Display Technology
(2018-09-01)
Wu, Tingzhu
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Sher, Chin-Wei
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Lin, Yue
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Lee, Chun-Fu
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Liang, Shijie
Displays based on inorganic light-emitting diodes (LED) are considered as the most promising one among the display technologies for the next-generation. The chip for LED display bears similar features to those currently in use for general lighting, but it size is shrunk to below 200 microns. Thus, the advantages of high efficiency and long life span of conventional LED chips are inherited by miniaturized ones. As the size gets smaller, the resolution enhances, but at the expense of elevating the complexity of fabrication. In this review, we introduce two sorts of inorganic LED displays, namely relatively large and small varieties. The mini-LEDs with chip sizes ranging from 100 to 200 m have already been commercialized for backlight sources in consumer electronics applications. The realized local diming can greatly improve the contrast ratio at relatively low energy consumptions. The micro-LEDs with chip size less than 100 m, still remain in the laboratory. The full-color solution, one of the key technologies along with its three main components, red, green, and blue chips, as well color conversion, and optical lens synthesis, are introduced in detail. Moreover, this review provides an account for contemporary technologies as well as a clear view of inorganic and miniaturized LED displays for the display community.
Multi-Azimuth Failure Mechanisms in Phosphor-Coated White LEDs by Current Aging Stresses
(2018-04-01)
Peng, Zhangbao
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Guo, Ziquan
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Wu, Tingzhu
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Zhuang, Peng
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Ye, Zhicheng
We have experimentally analyzed multi-azimuth degradation mechanisms that govern failures of commercially-available high-power (1 Watt) phosphor-coated white (hppc-W) light-emitting diodes (LEDs) covered with peanut-shaped lenses under three current-stress aging (CSA) conditions. Comprehensive analyses focus on photometric, chromatic, electrical, thermal and packaging characteristics. At the packaging level, (a) the decrease of the phosphor-conversion efficiency; (b) the yellow-browning of the optical lens; and (c) the darkening of the silver-coated reflective layer deposited with extraneous chemical elements (e. g., C, O, Si, Mg, and Cu, respectively) contribute collectively to the integral degradation of the optical power. By contrast, Ohmic contacts, thermal properties, and angles of maximum intensity remain unchanged after 3840 h aging in three cases. Particularly at the chip level, the formation of point defects increases the number of non-radiative recombination centers, and thus decreases the optical power during aging stages. Nevertheless, in view of the change of the ideality factor, the Mg dopant activation and the annealing effect facilitate the increase of the optical power in two specific aging stages (192 h similar to 384 h and 768 h similar to 1536 h). This work offers a systematic guidance for the development of reliable LED-based light sources in general-lighting areas.
Research on a Camera-Based Microscopic Imaging System to Inspect the Surface Luminance of the Micro-LED Array
(2018-01-01)
Zheng, Lili
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Guo, Ziquan
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Yan, Wei
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Lin, Yue
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Lu, Yijun
A majority of luminance measurement systems are currently focused on measuring the total luminance of light-emitting diodes (LEDs). It has largely limit its application on micro-LEDs, which require capturing the microscopic surface luminance. In order to study the microscopic characteristics of micro-LED displays, in this paper we propose a practical method that can accurately measure the luminance of a whole micro-LED array and even the surface luminance distribution of any single micro-led chip on the array. The essentials of this method are a high-precision complementary metal oxide semiconductor camera and a self-made data processing algorithm. Results show that the maximum error of the luminance measurement is less than 8%, which can prove the accuracy of the method. The metrical wavelength range and the accuracy of system can be dynamically set according to the camera and microscope. Endowed with ultrahigh dynamic ranges, this system proves its competency for evaluating the spatially resolved distribution of luminance for high brightness micro-LEDs.
The Impact of Luminous Properties of Red, Green, and Blue Mini-LEDs on the Color Gamut
(2019-05-01)
Guo, Weijie
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Chen, Nan
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Lu, Hao
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Su, Changwen
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Lin, Yue
Color gamut is of the paramount importance in the display. Light-current-voltage(L-I-V) characteristics of red, green, and blue flip-chip mini-light-emitting diodes (LEDs) (100 mu m x 200 mu m) are investigated at temperatures similar to the operational temperatures. Both the ideality factor and the temperature dependence of external quantum efficiency (EQE) suggest that the nonradiative loss in red mini-LED is higher. We also illustrate the intensive lateral luminous intensity fluctuation for red mini-LED under the over-driving current by capturing the spatial emission mapping. The influence of temperature and driving current on the chromaticity coordinates of mini-LEDs is determined. Furthermore, we propose a drive-current algorithm to maximize the color gamut of the trichromatic mixed light at different temperatures. The results indicate that, for the application of self-emitting mini-LED displays, the maximum color gamut is more sensitive to green emission, whereas enhancing the radiative recombination in red mini-LED could contribute a lot to the improvement on net performance.
Ultra-High Light Extraction Efficiency and Ultra-Thin Mini-LED Solution by Freeform Surface Chip Scale Package Array
(2019-04-01)
Huang, Che-Hsuan
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Kang, Chieh-Yu
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Chang, Shu-Hsiu
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Lin, Chih-Hao
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Lin, Chun-Yu
In this study, we present a novel type of package, freeform-designed chip scale package (FDCSP), which has ultra-high light extraction efficiency and bat-wing light field. For the backlight application, mainstream solutions are chip-scale package (CSP) and surface-mount device package (SMD). Comparing with these two mainstream types of package, the light extraction efficiency of CSP, SMD, and FDCSP are 88%, 60%, and 96%, respectively. In addition to ultra-high light extraction efficiency, because of the 160-degree bat-wing light field, FDCSP could provide a thinner and low power consumption mini-LED solution with a smaller number of LEDs than CSP and SMD light source array.
Ultrawide Color Gamut Perovskite and CdSe/ZnS Quantum-Dots-Based White Light-Emitting Diode with High Luminous Efficiency
(2019-09-01)
Lin, Chih-Hao
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Kang, Chieh-Yu
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Verma, Akta
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Wu, Tingzhu
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Pai, Yung-Min
We demonstrate excellent color quality of liquid-type white light-emitting diodes (WLEDs) using a combination of green light-emitting CsPbBr3 and red light-emitting CdSe/ZnS quantum dots (QDs). Previously, we reported red (CsPbBr1.2I1.8) and green (CsPbBr3) perovskite QDs (PQDs)-based WLEDs with high color gamut, which manifested fast anion exchange and stability issues. Herein, the replacement of red PQDs with CdSe/ZnS QDs has resolved the aforementioned problems effectively and improved both stability and efficiency. Further, the proposed liquid-type device possesses outstanding color gamut performance (132% of National Television System Committee and 99% of Rec. 2020). It also shows a high efficiency of 66 lm/W and an excellent long-term operation stability for over 1000 h.