Largely Enhancing Luminous Efficacy, Color-Conversion Efficiency

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Surfaces, Interfaces, and Applications

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 ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22298 • Publication Date (Web): 18 Apr 2019 Downloaded from http://pubs.acs.org on April 18, 2019

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ACS Applied Materials & Interfaces

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,1,2 YONG TANG,1 ZONGTAO LI,1,2,* XINRUI DING,1,* BINHAI YU,1 AND LIWEI LIN3 1Engineering

Research Center of Green Manufacturing for Energy-Saving and New-Energy

Technology, South China University of Technology, Guangdong, 510640, China 2Foshan

Nationstar Optoelectronics Company Ltd., Foshan 528000, China

3Department

of Mechanical Engineering, University of California, Berkeley, CA 94720-5800, USA

* [email protected] * [email protected]

Abstract: Quantum dots (QDs) white light-emitting diodes (LEDs) are promising in illumination and display applications due to the excellent color quality. Although they have a high quantum yield closed 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 colorconversion efficiency of QD white LEDs in excess of 50%. The reduction in conversion loss also help QD white LEDs to achieve a lifetime 1.9 times longer than that of LEDs using QD-only composites at a harsh aging condition. Simulation and testing results suggest the waveguide effect of the 2D-HPS helps 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 SBA15 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

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1. Introduction Quantum dots (QDs) have attracted great attention in research and practical applications owing to their desirable properties in high quantum yield (QY), narrow emission spectra, and ease of manufacturing1 for high-volume, photo-electric applications including solar cells2, LEDs3, and detectors4. Significant efforts have been focused on improving the QY by optimizing surface functional groups5, 6, band structures7, 8, as well as the surface passivation9, 10. For example, the QY has been increased to 90% using a core/shell structure by CdSe/ZnS QDs1, 11 for possible replacement of LEDs made of traditional rare-earthbased phosphor materials12, greatly improving the lighting quality of white LEDs. Generally, QDs are dispersed into a transparent matrix in order to form QD composites and to prevent oxidation13, 14. This has been used to convert blue LED light (light emission from LED chips) to QD light (light emission from QDs) to facilitate the control of the chromatic properties of LEDs15-17. However, there are some challenges in achieving high luminous efficacy, particularly in the development of color-convertors for LEDs12,

18.

One such

challenge is the host matrix effect, including the ligand destruction and aggregation induced quenching (AIQ)19, 20 during matrix exchanging. Previously, the in-situ syntheses of QDs in cross-linked polymers such as PVA21, PDMS22, and gel glass23 have been proposed to solve this issue and a similar approach has been utilized in the in-situ synthesis of QDs in nano and micro particles20, 24-29. The recently proposed liquid packaging method has also been considered as a promising method to solve this issue30. Another is that QDs can strongly absorb their emitting light, leading to high reabsorption losses31, 32. In particular, high QD concentration is preferable to increase the ratio of the light radiant power from the QD for illumination and display applications. However, this can simultaneously cause to a heavy reabsorption events to result in high conversion loss. This becomes the bottleneck that limits the luminous efficacy of QD white LEDs, causing their efficiency far lower than that of LEDs based on traditional phosphor composites33. Therefore, the luminous efficacy of white LEDs using QDs as the only color converter is generally lower than 80 lm/W30, only the red QDs can realize commercialization for white LEDs when combining with traditional yellow phosphor materials34-36. Since QDs are


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extremely sensitive to heat, the high conversion loss can also result in the increase of working temperature to accelerate the thermal quenching of QDs, resulting in the extremely low stability of QD white LEDs37. Transition metals, such as Mn2+, are generally used in the doping processes to introduce large Stokes shifts and minimize the reabsorption loss of QDs38, 39. However, their quantum yield is lower than that of CdSe/ZnS QDs for practical applications in white LEDs. By far, effective approaches to prevent the reabsorption of QD light are bared seen from the prospective of photon management by optical structures40. Mesoporous particles (MPs) have been widely used to adsorb QDs inside themselves for excellent environmental stability and dispersity41, while most of these MPs haven’t been explored in the photon management of QDs yet. Especially, SBA-15 MPs42-44 are with the unique two-dimensional hexagonal pore structure (2D-HPS), which shows great potential in reducing the reabsorption of QDs. Herein, we introduce SBA-15 MPs with 2DHPS to solve the reabsorption issue for QD white LEDs by a facial incorporating process. The morphology characterization and a three-dimensional finite-difference time-domain (FDTD) simulation were performed to study the effect of 2D-HPS on the reabsorption of QDs. Finally, the QD films and QD white LEDs were fabricated and tested to investigate the influence of SBA-15 MPs on their optical performances. Results indicate that SBA-15 MPs can largely enhance the luminous efficacy and color-conversion efficiency (CCE) of QD white LEDs in exceed of 50% using their 2D-HPS with waveguide effect for QD light, such great enhancement is higher than previous reports in QD white LEDs without changing the packaging materials (such as phosphor and encapsulant), and the higher CCE is also beneficial to enlarge the device’s lifetime by 1.9 times compared with the traditional QD white LEDs. 2. Experiments 2.1 Materials Green core/shell structure QDs of CdSe/ZnS were purchased from China Beijing Beida Jubang Science & Technology Co., Ltd (QY of 90%, emission peak of 520 nm). The SBA15 MPs (XFF01) and MCM-41 MPs (XFF02) were purchased from Nanjing XFNANO Co. Ltd. The silicone was purchased from Dow Corning. Chloroform solution was purchased from Aladdin Reagents. Blue LED devices were purchased from Foshan



NationStar Optoelectronics Co. Ltd (emission wavelength centered at 455 nm). All chemicals were used directly without any further purification. 2.2 Fabrication Methods

Fig. 1. Diagram outlining the fabrication method for the MP/QD hybrid composites.

A diagram outlining the fabrication method for the MP/QD hybrid composite is shown in Fig. 1. Firstly, CdSe/ZnS QD powder was added to 1.5 mL chloroform solution. The mass of the QDs was adjusted to control their mass ratio in the silicone matrix which was 2.0 mg, 6.0 mg, 10.0 mg, and 20.0 mg. The QD-chloroform solution was stirred for several seconds until the QDs were uniformly dispersed in the solution. SBA-15 MPs were then added to the QD-chloroform solution. Similarly, the mass of the MPs was adjusted to control their mass ratio in the silicone matrix as 1.2 mg, 2.4 mg, 6.0 mg, 24.0 mg, 30.0 mg, 45.0 mg, and 60.0 mg. The MP-QD-chloroform solution was then sealed with a cover (to avoid the evaporation of chloroform) and moved to a planetary type stirring machine. The solution was stirred for 15 min to allow MPs to physical adsorb QDs. Subsequently, 2 g of silicone was added to the solution for matrix exchange, and the solution was stirred for an additional 45 min. The goal of this process is to uniformly disperse the MPs and QDs in the silicone matrix by completely evaporating the chloroform solution. Finally, the prepared MP-QD-silicone composite was injected into LED devices and cured at a temperature of 150 °C for 1.5 h to make MP/QD hybrid LEDs. Similarly, MP/QD hybrid films can be fabricated by injecting the MP-QD silicone composite into molds for curing under the same conditions. In addition, the MP LEDs (QD LEDs) and MP films (QD films)


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were fabricated based on the aforementioned procedure, but without the addition of QDs (MPs). 2.3 Characterization Methods. The emission/absorption spectra of the fabricated films were measured using a dual-beam UV-Vis spectrophotometer TU-1901. Other optical properties, such as the transmittance, reflection, haze, and absorption properties were also characterized. The optical performance of the LEDs, including their radiant power, luminous flux, and spectra, were measured using an integrating sphere system from Instrument Systems GmbH. The injection current of the LEDs was kept at 200 mA (injection electrical power of 0.64 W) using a power source from Keithley. The morphology of the particles was observed using a scanning electron microscope (SEM), a high-resolution transmission electron microscope (HRTEM), and a scanning transmission electron microscope (STEM). The pore size of MPs is measured by an automatic surface and porosity analyzer from Micromeritics (ASAP2020HD88). 3. Results And Discussion 3.1 Principle 3.1.1 Realization of QDs inside 2D-HPS The TEM image of CdSe/ZnS QDs is given in Fig. 2(a), and the QDs have a particle size distribution ranged from 6 to 13 nm, mostly concentrated at 9.4 nm according to Fig. S2. Fig. 2(b) shows an SEM image of SBA-15 particles. It is seen that they have a rod-shaped geometry with a large aspect ratio with a white color under both sunlight and UV-light (inset). The SBA-15 MPs with 2D-HPS and pore sizes comparable with QDs have been selected due to their potential waveguide effect for light emitting from QDs, the TEM images of the top-viewed and the lateral-viewed SBA-15 MPs are given in Fig. 2(c) and (d), respectively; the 2D-HPS without adsorbing QDs can be 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 post adsorption process was used to realize this by incorporation the MPs in the QD composites. According to the BET measurement shown in Figs. S3, the SBA-15 MPs have a pore size distribution ranged from 9 to 13 nm, mostly



concentrated at 11.2 nm. Therefore, the pore size of SBA-15 MPs is large enough for adsorbing QDs by a simple wet mixing process41. It is worthy to mention that this method is entirely compatible with current white LED packaging process. Figs. 2(e)-(f) show the TEM images of MP/QD hybrid particles where spherical QDs are located in the surface 2D-HPS of SBA-15 MPs. The inset in Fig. 2(e) 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-viewed STEM image of the SBA-15 MP/QD hybrid particles is presented in Fig. S4(a), and the elemental Si, Se, Cd, S, and Zn mapping images at the same location are given in Figs. S4(b)-(f). It is evident that the elements of QDs, such as S and Zn, are observed even though QDs are hardly seen in 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 (Fig. 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, this can also help to separate QDs by their hexagonal wall and prevent the aggregation of QDs at the same pore location.

Fig. 2. (a) The HRTEM images of CdSe/ZnS QDs. The morphology of (b) SBA-15 MPs measured by SEM. The inset is 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) is the MP/QD hybrid powder with (right) and without (left) UV-light illumination, respectively.


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3.1.2 Effect of 2D-HPS on the Reabsorption of QDs To investigate the effect of 2D-HPS of SBA-15 MPs on the reabsorption properties of adsorbed QDs, a three-dimensional (FDTD) simulation was performed (simulation details in Fig. S6). In this simulation, the dipole source with isotropic emission is assumed as the QD light source, and the down-conversion processes are not considered in FDTD model32, 45.

Figs. 3(a)-(d) and Figs. 3(f)-(i) show the electromagnetic field of the escaping light

waves at the cross-section view of the MP/QD hybrid particle and the QD particle cloud without SBA-15 MP, respectively. The dipole source with isotropic emission is located at the center of the hybrid particle and the QD particle cloud. Fig. 3(b) shows that a portion of light wave is blocked by the 2D-HPS wall of SBA-15 MP and trapped inside. Furthermore, the light wave is cut off near the wall of the SBA-15 MP at the top region, and then it is transferred to the top surface of the wall as shown in Fig. 3(d). This means that most of photon energy propagating into the wall is constrained without uniformly emission to the free region, leading to a concentrated photon energy at the top surface of the wall. The electromagnetic field at the top surface of the hybrid particle is given in Fig. 3(e). It is clear that most of the photon energy is concentrated at the top surface of each wall. When the light wave is far away from the top surface of the SBA-15 MP, their photon energy is mixed together to become continuous due to the lack of constrain, as shown in Figs. S7. However, the light wave of QD particle cloud at the same location is continuous with spherical subwave surface as shown in Figs. 3(f)-(j). This is because that Si-based SBA-15 has a refractive index higher than that of the silicone for visible light. In other words, the refractive index of the wall is larger than that of the hole inside SBA-15 MP to establish a waveguide structure that can constrict the light wave propagating within the wall of the SBA-15 MP. The transmittance of the source after propagating through these two structures is summarized at the insert Fig. of Fig. 3(j). The hybrid particle can significantly increase the transmittance by 62.4% compared with that of the QD particle cloud without a SBA-15 MP. Therefore, the waveguide effect of the 2D-HPS is helpful to limit the reabsorption losses of QDs.



Fig. 3. The 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.

3.2 Optical Performances of QD films Experiments have been conducted to validate the photon management of SBA-15 MPs by fabricating QD white LEDs and QD films. Fig. 4(a) shows a fabricated LED using MP/QD hybrid composites under the UV light illumination. Fig. 4(b) shows the optical photos of various prototype LEDs with different SBA-15 concentrations of 0, 0.3, 0.7, 1.5 and 3.0% (from left to right), respectively, with QD concentration of 0.3 wt% (top samples) and MPonly (bottom samples). Fig. 4(c) shows fabricated MP/QD hybrid films with (inset) and without UV-light illumination and Fig. 4(d) shows optical photos of the MP/QD hybrid film, QD-only film, and MP-only film with different SBA-15 concentrations. From these Figs., it can be observed that prototype LEDs using MP-only composites and MP-only films exhibit a slight decrease in transparency, which is probably due to the scattering effects by the MP materials. However, it is important to note that both LEDs using MP/QD hybrid composites and MP/QD hybrid films have better transparency than those of LEDs using QD-only composites and QD-only films, respectively. This implies that the scattering


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effect of the QDs can be suppressed by the incorporation of the SBA-15 MPs, this may be attributed to the reduction in QD aggregations and will be discussed at the next part.

Fig. 4. (a) A prototype LED using MP/QD hybrid composites under an injection current of 200 mA. (b) Prototype LEDs with SBA-15 concentrations of 0, 0.3, 0.7, 1.5 and 3.0% (from left to right) using MP/QD hybrid composites with QD concentration of 0.3 wt% (top samples) and MP-only (bottom samples). (c) MP/QD hybrid film with (inset) and without UV-light illumination. (d) Optical images of various films with different SBA-15 concentrations (the inset table shows the concentrations of SBA15 and QD for each film).

Figs. 5(a), (b), (c), and (d) show the wavelength-dependent transmittance, reflection, haze, and absorption of MP-only, QD-only and MP/QD hybrid films, respectively. As the MP concentration increases, the transmittance of the MP-only film decreases, which can be explained by the increment in reflection and absorption. In addition, the haze of the MPonly film increases dramatically as the MP concentration increases, and this implies that MP has a strong scattering ability to increase the reflection of incident light in the MP-only film. Most importantly, this strong scattering behavior can minimize the amount of light captured by the QDs. The comparison between the QD-only film and the MP/QD hybrid film is made to further demonstrate this point since the QD concentration is the same in each case. It should be noted that these spectra are affected by the conversion processes of QDs. The excitation of QD light can increase the transmittance, reflection, and haze spectra at the emission wavelength of QDs. It can be observed that the MP/QD hybrid film has a higher transmittance compared with that of QD-only films due to the reduction in its reflection and absorption. In particular, MP can decrease the absorption spectra for the emission wavelength of the QD light (approximate 525 nm), as shown in the inset Fig. of



Fig. 5(d). These results clearly demonstrate that MP can decrease the reabsorption probability of QD light, thereby decreasing the conversion losses. Notably, previous studies have not found that the mesoporous silica with randomly spherical pore structures can decrease the reabsorption loss for QDs41, further supporting that the 2D-HPS of SBA15 plays an important role in reducing the reabsorption loss, as discussed in the simulation part. Although the MP/QD hybrid film can generate more QD light due to the reduced reabsorption events, their reflection is significantly smaller as compared with those from the QD-only film. A reasonable explanation is that the MPs facilitate the improved dispersion of QDs. Moreover, QDs on the order of several nanometers appear to be weakly scattered32, 46, 47. In the case of the QD-only film, the size of aggregated QDs is large and comparable to the wavelength of visible light to cause enhanced scattering. These explanations are supported by the haze spectra as the haze of the MP/QD hybrid film is similar to that of the MP-only film with the same MP concentration of 1.5 wt%. However, the QD-only film with less QD light generation appears to have a larger haze. These results clearly support that the QD-only film has a stronger scattering ability as compared with that of the MP/QD hybrid film and this is reflected in the experimental results of the better transparency of the MP/QD hybrid films as shown in Fig. 4. In summary, SBA-15 MPs with 2D-HPS can decrease the reabsorption losses between QDs and promote a better dispersion of QDs to decrease the back-scattered losses for high transmittance.


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Fig. 5. The (a) transmittance, (b) reflection, (c) haze, and (d) absorption spectra of QD-only films, MPonly films, and MP/QD hybrid films.

3.3 Optical Performances of QD white LEDs 3.3.1 Light-extraction Performances The testing results of luminous efficacy and total radiant power of LEDs using MP/QD hybrid composites are shown in Figs. 6(a) and (b), respectively. It is observed that the luminous efficacy increases as the QD concentration increases from 0.1 to 0.5 wt%. For QD concentration of 1 wt%, the luminous efficacy decreases as high QD concentration can significantly absorb the emission light (see the QD absorption spectra in Fig. S1)39. Furthermore, the luminous efficacy increases as the MP concentration increases and eventually saturates or reduces. The maximum efficacy increments are 9.6%, 8.5%, 28.1%, and 56.1% for QD concentrations of 0.1 wt%, 0.3 wt%, 0.5 wt%, and 1.0 wt%, respectively. Previous studies22,

29, 48-50

have shown that micro or nanoparticles embedded in the

composite are helpful in improving light extraction efficiency but only up to 10%29 and the total radiant power at high phosphor concentration is reduced. In our study, lower total radiant power is also observed for LEDs with high QD concentrations in Fig. 6(b) mainly due to the high reabsorption. However, the total radiant power of QD white LEDs increases as the MP concentration increases. Please note that the blue LEDs without QDs show no increase in total radiant power as the MP concentration increases (Fig. S8). As a result, such increment in the total radiant power is not due to the reduction in TIR by the scattering effect of MPs. Besides, the luminous flux of LEDs using the other similar MPs of MCM41 is given in Fig. S9, which also have the same hexagonal pore structure as that of SBA15 MPs except for their small pore size ranged from 2 to 4 nm. However, the enhancement in luminous flux is not observed when using the MCM-41 MPs, these results are attributed to their small pore that cannot adsorb the large QDs and they just operate like scattering particles. Therefore, the improvement in optical performance is mainly due to the adsorption of QDs inside the 2D-HPS of SBA-15 MPs as discussed in the simulation part.



Fig. 6. The (a) luminous efficacy and (b) total radiant power for LEDs using MP/QD hybrid composites of various concentrations. (c) The correlated color temperature (CCT) of white LEDs with QD concentration of 1 wt%. (d) The color-conversion efficiency (CCE) of LEDs using MP/QD hybrid composites of various concentrations.

The reabsorption issue is further investigated by analyzing the total radiant power from the chip light (380 nm to 495 nm) and QD light (495 nm to 730 nm), respectively, by integrating the emission spectra as shown in Figs. S10 (a) and (b). In general, the scattering effect induced by particles can increase the absorption events to decrease the radiant power of the chip light and increase that of the QD light22, 29, 50. Our results show both the radiant power of the chip light and QD light increases as the MP concentration increases as the ordering of QDs in 2D-HPS of MPs can decrease the absorption probability of QDs. Therefore, the ratio of the radiant power of QD light to that of the whole system slightly decreases as the MP concentration increases, as shown in Fig. S10 (c). It should be further noted that MPs can help increasing the radiant power of QD light as high as 48.9% at a QD concentration of 1.0 wt% as shown in Fig. S10 (b). However, when the MP concentration is too high (over the optimal point), the radiant power of QD decreases as observed, while the radiant power of LED light shows little changes. This suggests that the reduction in the


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radiant power of QD light comes from the reabsorption losses instead of back-scattered losses due to the reabsorption events between these hybrid particles, further supporting that the reabsorption is mainly suppressed for those QDs ordering in the 2D-HPS as discussed in the simulation part. Fig. 6(c) shows the correlated color temperature (CCT) for these QD white LEDs, which is a critical parameter for their applications. Please note that the CCT of QD white LEDs with QD concentration of 0.1 wt%, 0.3 wt%, and 0.5 wt% are almost over 9000 K and even as high as infinite, which are hard to be used as white LEDs, therefore, we only give the CCT values when using QD concentration of 1 wt% (the color coordinates are presented in Fig. S11), and the large enhancement in luminous efficacy for white LEDs with high QD concentration over 50% is extremely meaningful to practical applications. Such great enhancement is larger than previous results reported in QD white LEDs without changing the packaging materials (such as phosphor and encapsulant), and even comparable to those using liquid-type packaging method30. Please note that the enhancement percentage instead of absolute performances is an essential parameter for white LEDs when making comparisons, this is because that the optical properties (such as reflection and transmittance) of packaging elements (such as lead-frame, chips, and phosphor) and packaging structures can greatly affect the performances of white LEDs. In addition, the CCT is slightly increased with increasing MP concentration due to the much more blue light emission as discussed above, while it is still within the applicable ranges. In addition, the CCE is given in Fig. 6(d), which is the ratio of QD light radiant power to the LED light absorption power, it is generally used to evaluate the conversion efficiency of QDs in white LEDs. It is clear that the CCE of white LEDs with 1 wt% QD concentration can increase by as high as 56.3%, which is similar to that of luminous efficacy. The higher CCE is essential to provide a better thermal performance for QDs white LEDs by reducing heat power generating from QDs, which will be discussed at the next section. And the absolute values of CCE can be further improved by optimizing the packaging structures of QD white LEDs. The spectra of LEDs based on the QD/MP hybrid composites with different concentration are investigated in Fig. S12. In general, the radiant power increases as the concentration of the MPs increases, especially at high QD concentrations, similar to those



LEDs discussed in the previous sections. Moreover, when the QD concentration decreases from 1.0 wt% to 0.5 wt%, the peak wavelength of the QD light has a blue-shift of 5 nm as shown in Figs. S7(c) and (d) mainly due to the reabsorption phenomena between QDs51, 52. The normalized spectra of QD light ranged from 495 nm to 580 nm as shown in the insets of Fig. S12. It is interesting that a blue-shift phenomenon can also be observed as the MP concentration increases and this phenomenon is more significant for LEDs with higher QD concentrations. At a QD concentration of 1.0 wt%, a blue-shift of 5 nm can be observed using 3 wt% MPs. Therefore, in addition to the improvement in optical performance, MPs can also effectively suppress the red-shift phenomenon for display applications that require accurate color output. 3.3.2 Stability Performances The aging tests are investigated using MP/QD hybrid composites with QD-only LEDs as the reference and the concentration of MP and QD are selected as 1.5 wt% and 0.75 wt%, respectively. These devices were continuously injected with a current of 200 mA (an electrical power of 0.64W) at room temperature (25°C) without heat sinks as a typical harsh condition when studying the stability for QD white LEDs13. The radiant power maintenance (RPM) and luminous flux maintenance (LFM) are given in Figs. 7(a) and (b), respectively. Evidently, the LED using MP/QD hybrid composites has a higher RPM and LFM compared with those of LEDs using QD-only composites. The LFM of 70% is used to define the lifetime of LEDs as recommended by the standard of Energy Star53. The LED using MP/QD hybrid composites achieves a lifetime of 16.1 h, which is 1.9 times longer than that of the LED using QD-only composites. The working temperature of QDs is the major factor for the degradation of QD white LEDs13, 34, 54. These results indicate that less thermal power is generated from QDs in MP/QD hybrid composites as SBA-15 MPs with 2D-HPS can reduce the conversion losses of QDs.


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Fig. 7. (a) Radiant power maintenance (RPM) and (b) luminous flux maintenance (LFM) of LEDs using MP/QD hybrid composites and QD-only composites at different aging time. The concentration of MP and QD are 1.5 wt% and 0.75 wt%, respectively. Aging conditions: environmental temperature of 25°C, injection electrical power of 0.64W, without heat sink for thermal management.

4

Conclusions

In this report, SBA-15 MPs with 2D-HPS are investigated to improve the optical performances of QD white LEDs with better luminous efficacy and CCE over 50% for LEDs with approximately 7000 K. The red-shift phenomenon associate with the QD emission spectra can be suppressed by adding SBA-15 MPs for outputting more accuracy color. Moreover, LEDs using MP/QD hybrid composites can achieve a lifetime of 1.9 times longer than those of LEDs using QD-only composites due to the lower conversion loss. FDTD simulation reveals that the 2D-HPS can significantly reduce the reabsorption losses for QDs ordering inside due to the waveguide effect, which constrains the QD light inside the wall of 2D-HPS and reduces the probability of QD light being captured by QDs inside the hole of 2D-HPS. These are responsible to the excellent performances of QD white LEDs after incorporating with SBA-15 MPs. The optical measurement of QD films also support that SBA-15 MPs with 2D-HPS can facilitate improved dispersions of QDs for the suppression of back-scattered losses and, most importantly, reduce the reabsorption of QD light. Therefore, we indicate a new path to effectively improve the photon management of QD light using the 2D-HPS of SBA-15 MPs, future studies on this issue are still required. In addition, this method is based on a facile adsorbing process by incorporating both MPs and QDs inside the encapsulant (silicone), which is of easy manufacture and entirely compatible with the current packaging technique of white LEDs. Consequently, the proposed method is simple and effective to comprehensively improve the optical



performances of QD white LEDs with high stability, which can greatly accelerate the commercial applications of QDs in white LEDs. Supporting Information The absorption and emission spectra of QDs; the particle size distribution of QDs; the adsorptiondesorption isotherm and pore size distribution of SBA-15 MPs; the top-viewed STEM image of the SBA-15 MP/QD hybrid particles and its elemental mapping images; EDS spectra of SBA-15 MP/QD hybrid particles; the three-dimensional FDTD model of a MP/QD hybrid particle, the HRTEM image of the lateral-viewed hybrid particle showing the distribution of QDs in SBA-15; the electromagnetic field of escaping light at locations with different distance from the top surface of a MP/QD hybrid particle; the radiant power of LEDs using MP-only composites with various MP concentrations; the luminous flux of LEDs using MP/QD hybrid composites of various MCM-41 concentrations; the radiant power of chip light, radiant power of QD light, and QD light proportion for LEDs using MP/QD hybrid composites of various concentrations; the CIE 1931 coordinates of white LEDs with QD concentration of 1 wt% and SBA-15 MPs of various concentration; and the spectra of LEDs using MP/QD hybrid composites with various concentrations.

Acknowledgments This work is supported by the National Natural Science Foundation of China (51775199, 51735004); Natural Science Foundation of Guangdong Province (2014A030312017); and the Project of Science and Technology New Star in Zhujiang Guangzhou City (201806010102).

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TOC


Largely Enhancing Luminous Efficacy, Color-Conversion Efficiency

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