Largely Enhancing Luminous Efficacy, Color-Conversion

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 diodes (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

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. (a) HRTEM images of CdSe/ZnS QDs. The morphology of (b) SBA-15 MPs measured by SEM. The inset shows the SBA-15 powder with (right) and without (left) UV-light illumination; the HRTEM images of pores of SBA-15 observed from (c) the top view and (d) the lateral view, respectively. (e, f) The HRTEM images of SBA-15 MP/QD hybrid particles; the inset in (e) shows the MP/QD hybrid powder with (right) and without (left) UV-light illumination, respectively.

Figure 3. Cross-sectional views of the electromagnetic field of the propagating light wave: (a−d) the MP/QD hybrid particle and (f−i) the QD particle cloud without SBA-15 MP. (e) Electromagnetic field of the propagating light wave at the top surface of the MP/QD hybrid particle, and (j) electromagnetic field of the QD particle cloud. injection current of the LEDs was kept at 200 mA (injection electrical The SBA-15 MPs with 2D-HPS and pore sizes comparable power of 0.64 W) using a power source from Keithley. The with QDs have been selected due to their potential waveguide morphology of the particles was observed using a scanning electron effect for light emitting from QDs. The TEM images of top- microscope (SEM), a high-resolution transmission electron micro- view and lateral-view SBA-15 MPs are given in Figure 2c,d, scope (HRTEM), and a scanning transmission electron microscope respectively; the 2D-HPS without adsorbing QDs can be (STEM). The pore size of MPs is measured by an automatic surface and porosity analyzer from Micromeritics (ASAP2020HD88). clearly observed. To suppress the reabsorption of QD light through the 2D-HPS, it is necessary to disperse the QDs inside SBA-15 MPs. In this paper, a facile postadsorption process was 3. RESULTS AND DISCUSSION used to realize this by incorporation of the MPs in the QD 3.1. Principle. 3.1.1. Realization of QDs inside 2D-HPS. composites. According to the BET measurement shown in The TEM image of CdSe/ZnS QDs is given in Figure 2a, and Figure S3, the SBA-15 MPs have a pore size distribution the QDs have a particle size distribution ranging from 6 to 13 ranging from 9 to 13 nm, mostly concentrated at 11.2 nm. nm, mostly concentrated at 9.4 nm according to Figure S2. Therefore, the pore size of SBA-15 MPs is large enough for Figure 2b shows a SEM image of SBA-15 particles. It is seen adsorbing QDs by a simple wet mixing process.41 It is worth that they have a rod-shaped geometry with a large aspect ratio mentioning that this method is entirely compatible with the with a white color under both sunlight and UV light (inset). current white LED packaging process. Figure 2e,f show the

C DOI: 10.1021/acsami.8b22298 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX ACS Applied Materials & Interfaces Research Article

TEM images of MP/QD hybrid particles, where spherical QDs Figure 4a shows a fabricated LED using MP/QD hybrid are located at the surface 2D-HPS of SBA-15 MPs. The inset in composites under UV light illumination. Figure 4b shows the Figure 2e shows the MP/QD hybrid particles with a uniform yellow color and green color under sunlight and UV light, respectively, obtained by removing the chloroform solution after stirring for 15 min. To support that QDs have been adsorbed inside the internal 2D-HPS of SBA-15 MPs, the top- view STEM image of the SBA-15 MP/QD hybrid particles is presented in Figure S4a, and the elemental Si, Se, Cd, S, and Zn mapping images at the same location are given in Figure S4b−f. It is evident that the elements of QDs, such as S and Zn, are observed even though QDs are hardly seen on the surface of SBA-15 MPs, indicating that these QDs have been adsorbed inside the 2D-HPS. The large amount of elements from QDs shown in the EDS spectra (Figure S5) can further support these results. Therefore, the SBA-15 MPs can serve as a good matrix to disperse QDs. Most importantly, the pore size of the MPs is similar to the diameter of the QDs, and this can also help to separate QDs by their hexagonal wall and prevent Figure 4. (a) Prototype LED using MP/QD hybrid composites under an injection current of 200 mA. (b) Prototype LEDs with SBA-15 the aggregation of QDs at the same pore location. concentrations of 0, 0.3, 0.7, 1.5, and 3.0% (from left to right) using 3.1.2. Effect of 2D-HPS on the Reabsorption of QDs. To ff MP/QD hybrid composites with a QD concentration of 0.3 wt % (top investigate the e ect of 2D-HPS of SBA-15 MPs on the samples) and MP-only (bottom samples). (c) MP/QD hybrid film reabsorption properties of adsorbed QDs, a three-dimensional with (inset) and without UV-light illumination. (d) Optical images of (FDTD) simulation was performed (simulation details in various films with different SBA-15 concentrations (the inset table Figure S6). In this simulation, the dipole source with isotropic shows the concentrations of SBA-15 and QD for each film). emission is assumed as the QD light source, and the down- conversion processes are not considered in the FDTD optical photos of various prototype LEDs with different SBA- 32,45 model. Figure 3a−d,f−i show the electromagnetic field 15 concentrations of 0, 0.3, 0.7, 1.5, and 3.0% (from left to of the escaping light waves at the cross-sectional view of the right), respectively, with a QD concentration of 0.3 wt % (top MP/QD hybrid particle and the QD particle cloud without samples) and MP-only (bottom samples). Figure 4c shows the SBA-15 MP, respectively. The dipole source with isotropic fabricated MP/QD hybrid films with (inset) and without UV- emission is located at the center of the hybrid particle and the light illumination, and Figure 4d shows the optical photos of QD particle cloud. Figure 3b shows that a portion of light wave the MP/QD hybrid film, QD-only film, and MP-only film with is blocked by the 2D-HPS wall of SBA-15 MP and trapped different SBA-15 concentrations. From these figures, it can be inside. Furthermore, the light wave is cut off near the wall of observed that prototype LEDs using MP-only composites and the SBA-15 MP at the top region and then it is transferred to MP-only films exhibit a slight decrease in transparency, which the top surface of the wall as shown in Figure 3d. This means is probably due to the scattering effects by the MP materials. that most of the photon energy propagating into the wall is However, it is important to note that both LEDs using MP/ constrained without uniform emission in the free region, QD hybrid composites and MP/QD hybrid films have better leading to a concentrated photon energy at the top surface of transparency than LEDs using QD-only composites and QD- the wall. The electromagnetic field at the top surface of the only films, respectively. This implies that the scattering effect hybrid particle is given in Figure 3e. It is clear that most of the of the QDs can be suppressed by the incorporation of the SBA- photon energy is concentrated at the top surface of each wall. 15 MPs, which may be attributed to the reduction in QD When the light wave is far away from the top surface of the aggregation and will be discussed in the next part. SBA-15 MP, their photon energy is mixed together to become Figure 5a−d show the wavelength-dependent transmittance, continuous due to the lack of constraint, as shown in Figure reflection, haze, and absorption of MP-only, QD-only, and S7. However, the light wave of the QD particle cloud at the MP/QD hybrid films, respectively. As the MP concentration same location is continuous with a spherical subwave surface as increases, the transmittance of the MP-only film decreases, shown in Figure 3f−j. This is because Si-based SBA-15 has a which can be explained by the increment in reflection and refractive index higher than silicone for visible light. In other absorption. In addition, the haze of the MP-only film increases words, the refractive index of the wall is larger than that of the dramatically as the MP concentration increases, and this hole inside SBA-15 MP to establish a waveguide structure that implies that MP has a strong scattering ability to increase the can constrict the light wave propagating within the wall of reflection of incident light in the MP-only film. Most SBA-15 MP. The transmittance of the source after propagating importantly, this strong scattering behavior can minimize the through these two structures is summarized in the inset of amount of light captured by the QDs. The comparison Figure 3j. The hybrid particle can significantly increase the between the QD-only film and the MP/QD hybrid film is transmittance by 62.4% compared with that of the QD particle made to further demonstrate this point since the QD cloud without a SBA-15 MP. Therefore, the waveguide effect concentration is the same in each case. It should be noted of the 2D-HPS is helpful to limit the reabsorption losses of that these spectra are affected by the conversion processes of QDs. QDs. The excitation of QD light can increase the trans- 3.2. Optical Performances of QD Films. Experiments mittance, reflection, and haze spectra at the emission have been conducted to validate the photon management of wavelength of QDs. It can be observed that the MP/QD SBA-15 MPs by fabricating QD white LEDs and QD films. hybrid film has a higher transmittance than QD-only films due

D DOI: 10.1021/acsami.8b22298 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX ACS Applied Materials & Interfaces Research Article

Figure 5. (a) Transmittance, (b) reflection, (c) haze, and (d) absorption spectra of QD-only films, MP-only films, and MP/QD hybrid films.

Figure 6. (a) Luminous efficacy and (b) total radiant power of LEDs using MP/QD hybrid composites of various concentrations. (c) The correlated color temperature (CCT) of white LEDs with a QD concentration of 1 wt %. (d) The color-conversion efficiency (CCE) of LEDs using MP/QD hybrid composites of various concentrations. to the reduction in its reflection and absorption. In particular, in the simulation part. Although the MP/QD hybrid film can MP can decrease the absorption spectra for the emission generate more QD light due to the reduced reabsorption wavelength of the QD light (approximate 525 nm), as shown events, their reflection is significantly smaller than those from in the inset of Figure 5d. These results clearly demonstrate that the QD-only film. A reasonable explanation is that the MPs MP can decrease the reabsorption probability of QD light, facilitate the improved dispersion of QDs. Moreover, QDs on thereby decreasing the conversion losses. Notably, previous the order of several nanometers appear to be weakly studies have not found that mesoporous silica with randomly scattered.32,46,47 In the case of the QD-only film, the size of spherical pore structures can decrease the reabsorption loss for aggregated QDs is large and comparable to the wavelength of QDs,41 further supporting that the 2D-HPS of SBA-15 plays an visible light to cause enhanced scattering. These explanations important role in reducing the reabsorption loss, as discussed are supported by the haze spectra as the haze of the MP/QD

E DOI: 10.1021/acsami.8b22298 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX ACS Applied Materials & Interfaces Research Article hybrid film is similar to that of the MP-only film with the same concentration is too high (over the optimal point), the radiant MP concentration of 1.5 wt %. However, the QD-only film power of QD decreases as observed, while the radiant power of with less QD light generation appears to have a larger haze. LED light shows little changes. This suggests that the These results clearly support that the QD-only film has a reduction in the radiant power of QD light comes from the stronger scattering ability than the MP/QD hybrid film, and reabsorption losses instead of backscattered losses due to the this is reflected in the experimental results of the better reabsorption events between these hybrid particles, further transparency of the MP/QD hybrid films as shown in Figure 4. supporting that the reabsorption is mainly suppressed for those In summary, SBA-15 MPs with 2D-HPS can decrease the QD orderings in the 2D-HPS as discussed in the simulation reabsorption losses between QDs and promote a better part. dispersion of QDs to decrease the backscattered losses for Figure 6c shows the correlated color temperature (CCT) for high transmittance. these QD white LEDs, which is a critical parameter for their 3.3. Optical Performances of QD White LEDs. applications. Please note that the CCTs of QD white LEDs 3.3.1. Light-Extraction Performances. The testing results of with QD concentrations of 0.1, 0.3, and 0.5 wt % are almost ffi luminous e cacy and total radiant power of LEDs using MP/ over 9000 K and even as high as infinite, which are hard to be QD hybrid composites are shown in Figure 6a,b, respectively. used as white LEDs; therefore, we only give the CCT values ffi It is observed that the luminous e cacy increases as the QD when using a QD concentration of 1 wt % (the color concentration increases from 0.1 to 0.5 wt %. For QD coordinates are presented in Figure S11), and the large ffi concentration of 1 wt %, the luminous e cacy decreases as enhancement in luminous efficacy for white LEDs with a high fi high QD concentration can signi cantly absorb the emission QD concentration over 50% is extremely meaningful to light (see the QD absorption spectra in Figure S1).39 ffi practical applications. Such a great enhancement is larger Furthermore, the luminous e cacy increases as the MP than previous results reported in QD white LEDs without concentration increases and eventually saturates or reduces. ffi changing the packaging materials (such as phosphor and The maximum e cacy increments are 9.6, 8.5, 28.1, and 56.1% encapsulant) and even comparable to those using the liquid- for QD concentrations of 0.1, 0.3, 0.5, and 1.0 wt %, 30 22,29,48−50 type packaging method. Please note that the enhancement respectively. Previous studies have shown that micro- percentage instead of absolute performances is an essential or nanoparticles embedded in the composite are helpful in ffi 29 parameter for white LEDs when making comparisons, this is improving the light-extraction e ciency but only up to 10% because the optical properties (such as reflection and and the total radiant power at high phosphor concentration is transmittance) of packaging elements (such as lead-frame, reduced. In our study, lower total radiant power is also chips, and phosphor) and packaging structures can greatly observed for LEDs with high QD concentrations in Figure 6b affect the performances of white LEDs. In addition, the CCT is mainly due to high reabsorption. However, the total radiant slightly increased with increasing MP concentration due to the power of QD white LEDs increases as the MP concentration much more blue light emission as discussed above, while it is increases. Please note that the blue LEDs without QDs show still within the applicable ranges. In addition, the CCE is given no increase in total radiant power as the MP concentration in Figure 6d, which is the ratio of QD light radiant power to increases (Figure S8). As a result, such an increment in the the LED light absorption power, it is generally used to evaluate total radiant power is not due to the reduction in TIR by the ffi scattering effect of MPs. Besides, the luminous flux of LEDs the conversion e ciency of QDs in white LEDs. It is clear that using the other similar MPs of MCM-41 is given in Figure S9, the CCE of white LEDs with 1 wt % QD concentration can increase by as high as 56.3%, which is similar to the luminous which also have the same hexagonal pore structure as that of ffi SBA-15 MPs except for their small pore size ranging from 2 to e cacy. A higher CCE is essential to provide a better thermal 4 nm. However, the enhancement in luminous flux is not performance for QD white LEDs by reducing the heat power observed when using the MCM-41 MPs. These results are generating from QDs, which will be discussed in the next attributed to their small pores that cannot adsorb the large section. The absolute values of CCE can be further improved QDs and they just operate like scattering particles. Therefore, by optimizing the packaging structures of QD white LEDs. the improvement in optical performance is mainly due to the The spectra of LEDs based on the QD/MP hybrid ff adsorption of QDs inside the 2D-HPS of SBA-15 MPs as composites with di erent concentrations are investigated in discussed in the simulation part. Figure S12. In general, the radiant power increases as the The reabsorption issue is further investigated by analyzing concentration of the MPs increases, especially at high QD the total radiant power from the chip light (380−495 nm) and concentrations, similar to those LEDs discussed in the previous QD light (495−730 nm), respectively, by integrating the sections. Moreover, when the QD concentration decreases emission spectra as shown in Figure S10a,b. In general, the from 1.0 to 0.5 wt %, the peak wavelength of the QD light has scattering effectinducedbyparticlescanincreasethe a blue shift of 5 nm as shown in Figure S7c,d mainly due to the 51,52 absorption events to decrease the radiant power of the chip reabsorption phenomena between QDs. The normalized light and increase that of the QD light.22,29,50 Our results show spectra of QD light ranged from 495 to 580 nm as shown in both the radiant power of the chip light and QD light increases the insets of Figure S12. It is interesting that a blue shift as the MP concentration increases as the ordering of QDs in phenomenon can also be observed as the MP concentration 2D-HPS of MPs can decrease the absorption probability of increases and this phenomenon is more significant for LEDs QDs. Therefore, the ratio of the radiant power of QD light to with higher QD concentrations. At a QD concentration of 1.0 that of the whole system slightly decreases as the MP wt %, a blue shift of 5 nm can be observed using 3 wt % MPs. concentration increases, as shown in Figure S10c. It should Therefore, in addition to the improvement in optical be further noted that MPs can help in increasing the radiant performance, MPs can also effectively suppress the red shift power of QD light as high as 48.9% at a QD concentration of phenomenon for display applications that require accurate 1.0 wt % as shown in Figure S10b. However, when the MP color output.

F DOI: 10.1021/acsami.8b22298 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX ACS Applied Materials & Interfaces Research Article

3.3.2. Stability Performances. The aging tests are photon management of QD light using the 2D-HPS of SBA-15 investigated using MP/QD hybrid composites with QD-only MPs; future studies on this issue are still required. In addition, LEDs as the reference and the concentrations of MP and QD this method is based on a facile adsorbing process by are selected as 1.5 and 0.75 wt %, respectively. These devices incorporating both MPs and QDs inside the encapsulant were continuously injected with a current of 200 mA (an (silicone), which is easily manufactured and entirely electrical power of 0.64 W) at room temperature (25 °C) compatible with the current packaging technique of white without heat sinks as a typical harsh condition when studying LEDs. Consequently, the proposed method is simple and the stability for QD white LEDs.13 The radiant power effective to comprehensively improve the optical performances maintenance (RPM) and luminous flux maintenance (LFM) of QD white LEDs with high stability, which can greatly are given in Figure 7a,b, respectively. Evidently, the LED using accelerate the commercial applications of QDs in white LEDs. ■ ASSOCIATED CONTENT *S Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b22298. Absorption and emission spectra of QDs; particle size distribution of QDs; adsorption−desorption isotherm and pore size distribution of SBA-15 MPs; top-view STEM image of the SBA-15 MP/QD hybrid particles and its elemental mapping images; EDS spectra of SBA- 15 MP/QD hybrid particles; three-dimensional FDTD model of a MP/QD hybrid particle, the HRTEM image of lateral-view hybrid particles, showing the distribution Figure 7. (a) Radiant power maintenance (RPM) and (b) luminous fi fl of QDs in SBA-15; electromagnetic eld of escaping ux maintenance (LFM) of LEDs using MP/QD hybrid composites light at locations with different distances from the top and QD-only composites at different aging times. The concentrations of MP and QD are 1.5 and 0.75 wt %, respectively. Aging conditions: surface of a MP/QD hybrid particle; radiant power of ° LEDs using MP-only composites with various MP environmental temperature of 25 C, injection electrical power of 0.64 fl W, without heat sink for thermal management. concentrations; luminous ux of LEDs using MP/QD hybrid composites of various MCM-41 concentrations; MP/QD hybrid composites has a higher RPM and LFM than radiant power of chip light, radiant power of QD light, those of LEDs using QD-only composites. The LFM of 70% is and QD light proportion for LEDs using MP/QD hybrid used to define the lifetime of LEDs as recommended by the composites of various concentrations; CIE 1931 standard of Energy Star.53 The LED using MP/QD hybrid coordinates of white LEDs with a QD concentration composites achieves a lifetime of 16.1 h, which is 1.9 times of 1 wt % and SBA-15 MPs of various concentrations; longer than that of the LED using QD-only composites. The and spectra of LEDs using MP/QD hybrid composites working temperature of QDs is the major factor for the with various concentrations (PDF) degradation of QD white LEDs.13,34,54 These results indicate that less thermal power is generated from QDs in MP/QD ■ AUTHOR INFORMATION hybrid composites as SBA-15 MPs with 2D-HPS can reduce Corresponding Authors the conversion losses of QDs. *E-mail: [email protected] (Z.L.). *E-mail: [email protected] (X.D.). 4. CONCLUSIONS ORCID In this report, SBA-15 MPs with 2D-HPS are investigated to Zongtao Li: 0000-0001-7745-2783 improve the optical performances of QD white LEDs with Notes ffi better luminous e cacy and CCE over 50% for LEDs with The authors declare no competing financial interest. approximately 7000 K. The red shift phenomenon associated with the QD emission spectra can be suppressed by adding ■ ACKNOWLEDGMENTS SBA-15 MPs for more accurate color output. Moreover, LEDs This work was supported by the National Natural Science using MP/QD hybrid composites can achieve a lifetime of 1.9 Foundation of China (51775199, 51735004); Natural Science times longer than those of LEDs using QD-only composites Foundation of Guangdong Province (2014A030312017); and due to the lower conversion loss. FDTD simulation reveals that the Project of Science and Technology New Star in Zhujiang 2D-HPS can significantly reduce the reabsorption losses for Guangzhou City (201806010102). QD orderings inside due to the waveguide effect, which constrains the QD light inside the wall of 2D-HPS and reduces ■ REFERENCES the probability of QD light being captured by QDs inside the hole of 2D-HPS. These are responsible for the excellent (1) Dai, X.; Zhang, Z.; Jin, Y.; Niu, Y.; Cao, H.; Liang, X.; Chen, L.; Wang, J.; Peng, X. Solution-Processed, High-Performance Light- performances of QD white LEDs after incorporating with SBA- − fi Emitting Diodes Based on Quantum Dots. Nature 2014, 515,96 99. 15 MPs. The optical measurement of QD lms also supports (2) Zeng, Q.; Zhang, X.; Feng, X.; Lu, S.; Chen, Z.; Yong, X.; that SBA-15 MPs with 2D-HPS can facilitate improved Redfern, S.; Wei, A. H.; Wang, H.; Shen, H.; et al. Polymer-Passivated dispersions of QDs for the suppression of backscattered losses Inorganic Cesium Lead Mixed-Halide Perovskites for Stable and and, most importantly, reduce the reabsorption of QD light. 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G DOI: 10.1021/acsami.8b22298 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX ACS Applied Materials & Interfaces Research Article

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I DOI: 10.1021/acsami.8b22298 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX