Largely Enhancing Luminous Efficacy, Color-Conversion
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Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX www.acsami.org Largely Enhancing Luminous Efficacy, Color-Conversion Efficiency, and Stability for Quantum-Dot White LEDs Using the Two- Dimensional Hexagonal Pore Structure of SBA-15 Mesoporous Particles † ‡ † † ‡ † † § Jiasheng Li, , Yong Tang, Zongtao Li,*, , Xinrui Ding,*, Binhai Yu, and Liwei Lin † Engineering Research Center of Green Manufacturing for Energy-Saving and New-Energy Technology, South China University of Technology, Guangdong 510640, China ‡ Foshan Nationstar Optoelectronics Company Ltd., Foshan 528000, China § Department of Mechanical Engineering, University of California, Berkeley, California 94720-5800, United States *S Supporting Information ABSTRACT: Quantum-dot (QD) white light-emitting di- odes (LEDs) are promising for illumination and display applications due to their excellent color quality. Although they have a high quantum yield close to unity, the reabsorption of QD light leads to high conversion loss, significantly reducing the luminous efficacy and stability of QD white LEDs. In this report, SBA-15 mesoporous particles (MPs) with two- dimensional hexagonal pore structures (2D-HPS) are utilized to largely enhance the luminous efficacy and color-conversion efficiency of QD white LEDs in excess of 50%. The reduction in conversion loss also helps QD white LEDs to achieve a lifetime 1.9 times longer than that of LEDs using QD-only composites at harsh aging conditions. Simulation and testing results suggest that the waveguide effect of 2D-HPS helps in reducing the reabsorption loss by constraining the QD light inside the wall of 2D-HPS, decreasing the probability of being captured by QDs inside the hole of 2D-HPS. As such, materials and mechanisms like SBA-15 MPs with 2D-HPS could provide a new path to improve the photon management of QD light, comprehensively enhancing the performances of QD white LEDs. KEYWORDS: quantum-dot white light-emitting diode, SBA-15 mesoporous particle, two-dimensional hexagonal pore structure, luminous efficacy, stability, reabsorption 1. INTRODUCTION ing19,20 during matrix exchanging. Previously, the in situ syntheses of QDs in cross-linked polymers such as poly(vinyl Quantum dots (QDs) have attracted great attention in 21 22 23 research and practical applications owing to their desirable alcohol), poly(dimethylsiloxane), and gel glass have been properties in high quantum yield (QY), narrow emission proposed to solve this issue and a similar approach has been 1 utilized in the in situ syntheses of QDs in nano- and spectra, and ease of manufacturing for high-volume, photo- 20,24−29 Downloaded via SOUTH CHINA UNIV OF TECHNOLOGY on May 22, 2019 at 09:09:24 (UTC). 2 See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. electric applications including solar cells, light-emitting diodes microparticles. The recently proposed liquid packaging 3 4 fi ff method has also been considered as a promising method to (LEDs), and detectors. Signi cant e orts have been focused 30 on improving the QY by optimizing surface functional solve this issue. 5,6 7,8 9,10 Another is that QDs can strongly absorb their emitting light, groups, band structures, and surface passivation. For 31,32 example, the QY has been increased to 90% using a core/shell leading to high reabsorption losses. In particular, high QD structure by CdSe/ZnS QDs1,11 for possible replacement of concentration is preferable to increase the ratio of the light LEDs made of traditional rare-earth-based phosphor materi- radiant power from the QD for illumination and display als,12 greatly improving the lighting quality of white LEDs. applications. However, this can simultaneously cause a heavy Generally, QDs are dispersed into a transparent matrix to form reabsorption event, which results in high conversion loss. This ffi QD composites and to prevent oxidation.13,14 This has been becomes the bottleneck that limits the luminous e cacy of QD white LEDs, making their efficiency far lower than that of used to convert blue LED light (light emission from LED 33 chips) to QD light (light emission from QDs) to facilitate the LEDs based on traditional phosphor composites. Therefore, 15−17 ffi control of the chromatic properties of LEDs. However, the luminous e cacy of white LEDs using QDs as the only there are some challenges in achieving high luminous efficacy, particularly in the development of color convertors for Received: December 21, 2018 LEDs.12,18 One such challenge is the host matrix effect, Accepted: April 18, 2019 including ligand destruction and aggregation-induced quench- Published: April 18, 2019 © XXXX American Chemical Society A DOI: 10.1021/acsami.8b22298 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX ACS Applied Materials & Interfaces Research Article Figure 1. Diagram outlining the fabrication method for the MP/QD hybrid composites. color converter is generally lower than 80 lm/W;30 only the 2. EXPERIMENTS red QDs can realize commercialization for white LEDs when 2.1. Materials. Green core/shell structure QDs of CdSe/ZnS 34−36 combining with traditional yellow phosphor materials. were purchased from China Beijing Beida Jubang Science & Since QDs are extremely sensitive to heat, the high conversion Technology Co., Ltd (QY of 90%, emission peak of 520 nm). The loss can also result in the increase of working temperature to SBA-15 MPs (XFF01) and MCM-41 MPs (XFF02) were purchased accelerate the thermal quenching of QDs, resulting in the from Nanjing XFNANO Co. Ltd. Silicone was purchased from Dow extremely low stability of QD white LEDs.37 Transition metals, Corning. Chloroform solution was purchased from Aladdin Reagents. such as Mn2+, are generally used in doping processes to Blue LED devices were purchased from Foshan NationStar Optoelectronics Co. Ltd (emission wavelength centered at 455 introduce large Stokes shifts and minimize the reabsorption fi 38,39 nm). All chemicals were used directly without any further puri cation. loss of QDs. However, their quantum yield is lower than 2.2. Fabrication Methods. A diagram outlining the fabrication that of CdSe/ZnS QDs for practical applications in white method for the MP/QD hybrid composite is shown in Figure 1. First, LEDs. By far, effective approaches to prevent the reabsorption CdSe/ZnS QD powder was added to 1.5 mL of chloroform solution. of QD light are barely seen from the prospective of photon The mass of the QDs was adjusted to control their mass ratio in the management by optical structures.40 silicone matrix, which were 2.0, 6.0, 10.0, and 20.0 mg. The QD- Mesoporous particles (MPs) have been widely used to chloroform solution was stirred for several seconds until the QDs adsorb QDs inside themselves for excellent environmental were uniformly dispersed in the solution. SBA-15 MPs were then 41 added to the QD-chloroform solution. Similarly, the mass of the MPs stability and dispersity, while most of these MPs have not was adjusted to control their mass ratio in the silicone matrix as 1.2, been explored in the photon management of QDs yet. In − 42−44 2.4, 6.0, 24.0, 30.0, 45.0, and 60.0 mg. The MP QD-chloroform particular, SBA-15 MPs are with unique two-dimensional solution was then sealed with a cover (to avoid the evaporation of hexagonal pore structures (2D-HPS), which show great chloroform) and moved to a planetary-type stirring machine. The potential in reducing the reabsorption of QDs. Herein, we solution was stirred for 15 min to allow MPs to physically adsorb introduce SBA-15 MPs with 2D-HPS to solve the reabsorption QDs. Subsequently, 2 g of silicone was added to the solution for issue for QD white LEDs by a facile incorporating process. The matrix exchange and the solution was stirred for an additional 45 min. morphology characterization and a three-dimensional finite- The goal of this process is to uniformly disperse the MPs and QDs in ff the silicone matrix by completely evaporating the chloroform solution. di erence time-domain (FDTD) simulation were performed to Finally, the prepared MP−QD-silicone composite was injected into study the effect of 2D-HPS on the reabsorption of QDs. ° fi LED devices and cured at a temperature of 150 C for 1.5 h to make Finally, QD lms and QD white LEDs were fabricated and MP/QD hybrid LEDs. Similarly, MP/QD hybrid films can be tested to investigate the influence of SBA-15 MPs on their fabricated by injecting the MP−QD silicone composite into molds for optical performances. The results indicate that SBA-15 MPs curing under the same conditions. In addition, the MP LEDs (QD can largely enhance the luminous efficacy and color-conversion LEDs) and MP films (QD films) were fabricated based on the efficiency (CCE) of QD white LEDs in excess of 50% using aforementioned procedure but without the addition of QDs (MPs). ff 2.3. Characterization Methods. The emission/absorption their 2D-HPS with a waveguide e ect for QD light; such a fi great enhancement is higher than previous reports on QD spectra of the fabricated lms were measured using a TU-1901 dual-beam UV−vis spectrophotometer. Other optical properties, such white LEDs without changing the packaging materials (such as as transmittance, reflection, haze, and absorption properties, were also phosphor and encapsulant), and the higher CCE is also characterized. The optical performances of the LEDs, including their beneficial for enlarging the device’s lifetime by 1.9 times radiant power, luminous flux, and spectra, were measured using an compared with the traditional QD white LEDs. integrating sphere system from Instrument Systems GmbH. The B DOI: 10.1021/acsami.8b22298 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX ACS Applied Materials & Interfaces Research Article Figure 2.