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Abstract
In this paper, a centrifugation-based quasi-horizontal separation (c-HS) structure is proposed to enhance the QD light extraction of QD–phosphor hybrid white LEDs (WLEDs), effectively suppressing the backscattered loss from phosphor at the top region of the QD layer. Results indicate that a large centrifugation speed and dispensing mass of the QD layer is more beneficial to reducing the local density of phosphor at the top region, realizing quasi-horizontal separation between phosphor and QDs. Moreover, WLEDs with c-HS structure and conventional vertically layered packaging reference structure were compared at different correlated color temperatures (CCT). The radiant power and luminous flux achieved by the c-HS structure were 13.6% and 10.8%, respectively, higher than the reference structure at a typical warm white color of ∼4000 K. Consequently, this study can provide a new perspective on designing the separation structure for QD–phosphor hybrid WLEDs considering the backscattering loss of QD light.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Light-emitting diodes (LEDs) are gradually replacing traditional torches, candles, and incandescent lamps owing to their high efficiency and long lifetimes [1,2], and become one of the most promising solid-state light sources in our daily life, with applications such as illumination, display, medical equipment, etc. [3]. Commercial white LEDs (WLEDs) generally use GaN-based blue LED chips and yellow Y3Al5O12:Ce3+ (YAG) phosphor to achieve white light [4–6], while the deficient red-light component leads to a poor color rendering index (CRI) [7,8]. To overcome this limit, a hybrid phosphor structure is proposed for WLEDs combining red and yellow/green phosphor [9,10]. However, the inter-reabsorption of red and green phosphor is serious owing to the overlapping of absorption and emission spectra, and thus causes serious conversion loss. Recently, quantum dots (QDs), which have wide absorption spectra, narrow emission spectra, and high photoluminescence quantum yield (PLQY), have attracted great attention regarding replacing the red phosphor in WLEDs [11–14]. A new type of WLED is produced by mixing the red CdSe-based QDs and yellow phosphor, demonstrating a higher luminous efficacy compared with that using all phosphor materials [15]. However, such a QD–phosphor hybrid structure still faces significant challenges owing to the high reabsorption loss of QDs [16], leading to high thermal power and reducing the operating stability [17,18]. It is still of great necessity to reduce the optical energy loss in QD–phosphor hybrid WLEDs (Q–P hybrid WLEDs).
Vertically layered packaging (VLP) structures have been widely studied to improve the optical performance of Q–P hybrid LEDs [19–28]. It has been proven that QDs should be placed near the bottom to comprehensively improve the performances of Q–P hybrid WLEDs. Firstly, QD near the bottom is beneficial to increase the absorption probability of blue light by QDs, leading to highly cost-effective devices [20]. Secondly, this solution can effective avoid QDs absorbing the yellow light from phosphor and thus improve overall efficiency [22]. Thirdly, it is beneficial to improve the heat dissipate of QDs by moving them closer to the lead-frame with high thermal conductivity, enhancing the operating stability [29]. Unfortunately, backscattering of phosphor is not solved in such VLP structures. This means that a portion of red light from the QDs is inevitably backscattered by the upper phosphor layer [26] and then propagates into the QD layer, probably leading to serious reabsorption of red light between the QDs [30].
In this paper, a centrifugation-based quasi-horizontal separation (c-HS) structure was proposed to enhance the QD light extraction in Q–P hybrid WLEDs. A convex QD layer is preformed to control the settlement distributions of phosphor particles, leading to a quasi-horizontal separation between the QDs and phosphor. The centrifugation speed, dispensing mass, and phosphor concentration were studied in detail to investigate the effect of c-HS structure on optical performance, demonstrating an effective solution for enhancing the efficiency of Q–P hybrid WLEDs by suppressing the backscattering of phosphor.
2. Methods
The QDs was purchased from Beijing Beida Jubang Science & Technology Co., Ltd, emission peak of 625 nm. The YAG phosphor was purchased from Shenzhen Jianlong Technology Co., Ltd, emission peak of 558 nm. The absorption and emission spectra of CdSe-based red QDs used in our cases are given in Fig. 1; the photoluminescence (PL) emission peak is located at 625 nm and the full width at half maximum (FWHM) of the emission spectrum is 32.8 nm. The emission spectrum of YAG phosphor is also given in Fig. 1 to show the spectra overlapping between the emission spectrum of phosphor and the absorption spectrum of the QDs, confirming that the yellow light from phosphor can be absorbed by QDs. The red QDs and yellow phosphor used in our cases have mean particle sizes of 10.3 nm and 15 µm, respectively. The fabrication process of Q–P hybrid WLEDs is shown in Fig. 2. Firstly, QD slurry (QDs mixed with silicone) was dispensed onto the blue chip with a peak emission wavelength of 455 nm [31], and then cured in an oven at a temperature of 150 °C. The QD concentration was kept at 0.05% in our cases. A hollow lens with semi-spherical shape was packaged to the LED devices and then phosphor slurry (phosphor mixed with silicone) was injected to fill the chamber. As for reference devices with conventional VLP structure (the reference structure in our cases), these devices after phosphor injection were immediately moved to an oven at a temperature of 100 °C for 3 h. Regarding achieving c-HS structure, it is most important to control the phosphor distribution in the device. Generally, the centrifugal technology is widely used in LED packaging industry to increase the manufacture consistency by controlling the phosphor distributions [32], which has been also adopted in recent reports to improve the color uniformity of phosphor-converted LEDs by adjusting the phosphor distributions [33]. Accordingly, we also introduce the centrifugation technology to Q-P hybrid WLEDs. After phosphor injection, the devices were moved to the centrifugation machine for phosphor settlement, changing the phosphor distributions. In our cases, the centrifugation radius was 4.0 cm and the centrifugation time was kept at 1 min for high productivity. Once the centrifugation parameters and the phosphor slurry are specific, the phosphor distributions can be well repeated according to the phosphor settlement model [32]. After centrifugation, the phosphor layer was cured at the same conditions given above to finish the packaging process. The optical performances of these devices were measured by an integrating sphere system, and the injection current was kept at 100 mA. And the luminous flux deviation of c-HS structure devices is 2.55%, calculated from 24 samples.
Fig. 2. Diagram of the fabrication process of WLEDs with reference (VLP) structure and c-HS structure.
3. Results and discussion
The centrifugation speed is used to control the settlement distributions of phosphor. A series of Q-P hybrid WLED with different centrifugation speed were fabricated to investigate this issue. In this case, the dispensing mass and concentration of the QD layer are kept at 6 mg and 0.05 wt%, respectively, while the phosphor concentration is kept at 16.7 wt%. Generally, much more phosphor settles to the bottom with increasing centrifugation by overcoming the viscous force. The sedimentation of phosphor can be supported by the X–ray images and cross-sectional views of Q–P hybrid WLEDs with different centrifugation speed, as shown in Fig. 3. As for the same phosphor layer, the darker place means that it is harder for X-ray to pass through, demonstrating a denser phosphor particles (higher phosphor concentration) in this region. It can be seen that YAG phosphor particles are uniform distributed in the device before centrifugation (centrifugation speed of 0 rpm) as shown in Fig. 3(a). However, the darker region in phosphor layer is an arched shape along with QD layer when centrifugation is 3500 rpm, as shown in Fig. 3(b). It means that much more phosphor particles are concentrated around the QD layer as the centrifugation speed increases, while there is less in the top region of the QD layer. In particular, phosphor particles are barely observed near the QD layer and most of them are settled to the bottom of the lead-frame, when the centrifugation speed is 6000 rpm as shown in Fig. 3(c). It can be further supported by the cross-sectional view of the device as shown in Fig. 3(d). Notably, although the accurate distributions for each phosphor particle are difficulty to obtain, the current results are sufficient to indicate that the centrifugation helps to reduce the local density of phosphor on the top region of QD layer and distribute phosphor particles around the QD layer to obtain the c-HS structure proposed in this paper.
Fig. 3. Phosphor distributions of Q-P hybrid WLEDs with different centrifugation speeds. (a)-(b) The centrifugation speeds are 0 and 3500 rpm, respectively, showing the X-ray images of upper right side of WLEDs. (c)-(d) The centrifugation speed is 6000 rpm, showing the X-ray image of lower right side of WLEDs and the photograph of cross-sectional view of WLEDs. Scale bar: 0.2 mm.
To clarify the influence of centrifugation on the optical performances of Q-P hybrid WLEDs, the electroluminescence (EL) spectra achieved by different centrifugation speed are given in Fig. 4(a). For convenience, the EL spectra are divided into blue, yellow, and red components as shown in Figs. 4(b)–4(d), respectively. In Fig. 4(b), the EL intensity of blue light increases as the centrifugation speed increases, while the EL intensity of yellow and red light decrease as shown in Figs. 4(c) and 4(d), respectively. As discussed above, centrifugation is beneficial to changing VLP reference structure to c-HS structure by reducing the number of phosphor particles above the QD layer. It is equivalent to reducing the local concentration of YAG phosphor, resulting that the EL intensity of blue chip is increased and the EL intensity of YAG phosphor is decreased, which are consistent with the earlier studies on phosphor concentration [34]. However, the main concern of this article is to study the effect of phosphor sedimentation on QD light extraction of Q-P hybrid LEDs, which has been barely studied before. In addition, it should be mentioned that the reduction in red light is attributed to overlapping between the PL spectra of yellow phosphor and red QDs (Fig. 1). Therefore, fewer phosphor conversion events also lead to a reduction in the EL intensity of the red component, which cannot be regarded as a weakened EL intensity of QDs; more detailed discussion is given in subsequence to figure out this issue.
Fig. 4. EL spectra of Q–P hybrid WLEDs with different centrifugation speeds. (a)–(d) are wavelengths from 400 to 700 nm, 420 to 480 nm, 510 to 600 nm, and 610 to 650 nm, to clearly show the changes in all components combined and in the blue, yellow, and red components, respectively.
To have a better understanding of the changes in EL intensity that originated from QDs, we introduce normalized parameters including chip intensity (CI), phosphor intensity (PI), and QD intensity (QI), respectively. In the EL spectra, the blue and yellow component are treated as only from the LED chip and the phosphor, respectively. Herein, we use the peak intensity of 446 nm and 530 nm to represent the CI and PI, respectively. However, the peak intensity in the red region cannot represent the QI owing to the overlapping of conversion light from phosphor in the red component. To ignore the influence of spectra overlapping in the red-light region, the QI is defined as the ratio of peak intensity in the red component (645 nm) to that in the yellow component (530 nm). In our cases, we are mainly concerned about the EL intensity changes after centrifugation, and therefore the CI, PI, and QI are all normalized to those before centrifugation (centrifugation speed of 0 rpm), as shown in Fig. 5(a). Similar to the discussion in EL spectra, the CI significantly increases while the PI decreases as the centrifugal speed increases, which is attributed to the less conversion events caused by phosphor at the top region.
Fig. 5. (a) Normalized radiant power originating from the LED chip (CI), phosphor (PI), and QDs (QI), respectively, at different centrifugation speeds. (b) EL intensity of WLEDs with and without packaging an upper phosphor layer. The insert is their integrated radiant power from QDs (RPQ).
Most importantly, it is interesting that the QI increases with increasing the centrifugation speed. Generally, phosphor has a strong scattering effect and the light emitted by phosphor is isotropic [35]. This means that the backscattered blue light and backward-emission yellow light can propagate into the QD layer again as shown in the diagram of Fig. 6(a), which is beneficial to increase the absorption probability of QDs and thus generating much more red light. Actually, as the centrifugation speed increases, the backscattered blue light and backward-emission yellow light decrease owing to lower local phosphor concentration on the top region, while the QI keeps increasing. These results demonstrate that much more red light of QDs is extracted from devices as the centrifugation speed increases. One reasonable explanation is that much more phosphor settles towards the lateral region of the QD layer owing to the convex surface, as shown in Fig. 3. Less phosphor in the top region of the QD layer is more beneficial to reducing the backscattering of red light, suppressing a portion of red light propagating into the QD layer and thereby reducing the reabsorption loss, as compared between the diagrams of Figs. 6(b) and 6(c). The spectra of WLEDs with and without the upper phosphor layer are given in Fig. 5(b) to further support this issue. As for WLEDs with the upper phosphor layer, the radiant power from QDs (RPQ) is the difference between the integration EL intensity of the blue line and that of the green line (600 to 700 nm), whereas for WLEDs without the upper phosphor layer, the RPQ is the integration EL intensity of the red line (600 to 700 nm). The RPQs of both devices are summarized in the insert; it is interesting that the RPQ of WLEDs after packaging with the upper phosphor layer is even decreased by 8%. These results suggest that optical loss of red light backscattered by phosphor, rather than the enhanced absorption of backscattered blue light or backward-emission yellow light, is the major factor influencing the QI. Therefore, the c-HS structure is beneficial for less backscattered red light and better light-extraction performances of QD layer.
Fig. 6. Diagram of the light-extraction mechanism in Q–P hybrid WLEDs. (a) Backscattered blue light and backward-emission yellow light in the top region of the VLS reference structure; (b) backscattered red light in the top region of the VLS reference structure; (c) extraction red light with less backscattering in the top region of the c-HS structure.
In the c-HS structure, phosphor slurry is injected onto the QD layer and then undergoes centrifugation, the dispensing mass of the QD layer affects the curvature of the convex surface where the phosphor settles, which probably influences the QD light extraction. Obviously, the shape of QD layer is more convex with a larger dispensing mass as shown in Fig. 7(a), providing a resultant force much more towards the lateral region of the QD layer and avoiding phosphor settlement in the top region of the QD layer. Therefore, the more convex shape of the QD layer is more beneficial to settling phosphor when using the c-HS structure, leading to a better QD light extraction.
Fig. 7. (a) Photographs of the QD layer with large (6 mg) and small (3 mg) dispensing masses and their corresponding force analytical diagrams for a phosphor particle. (b) Normalized radiant power originated from the LED chip (CI), phosphor (PI), and QDs (QI), respectively, at different dispensing masses of the QD layer.
To support these explanations, the effect of dispensing mass on the optical performances of Q-P hybrid WLEDs is also investigated at a centrifugation speed of 3500 rpm according to Fig. 3. Similarly, the corresponding CI, PI, and QI are given to ignore the influence of spectra overlapping, as shown in Fig. 7(b). Herein, the CI, PI, and QI values are also normalized to that before centrifugation (VLS reference structure); for example, the c-HS structure and VLS reference structure have the same QD light extraction when the QI value equals to unity, and a lager QI value indicates that the c-HS structure leads to much more QD light extraction compared with the VLS reference structure at the same dispensing mass. It is evident that the QI increases as the dispensing mass increases, which is increased from 1.04 to 1.64 when the dispensing mass increases from 3 to 6 mg. These results demonstrate that much more QD light of c-HS structure is extracted compared with that of VLS reference structure at a larger dispensing mass. However, the CI is decreased by 0.6 times when the dispensing mass increases from 3 to 6 mg, demonstrating that there are much more phosphor settling as the dispensing mass increases. Therefore, we can safely indicate that a convex geometry of QD layer is essential to increase the QD light extraction by suppressing the backscattering loss from phosphor. In our cases, the dispensing mass is 6.0 mg for subsequent comparisons limited by the surface tension of QD slurry, a more convex shape can be obtained by further increasing the surface tension or optimizing the geometry of lead frame according to previous studies [36,37].
Although the c-HS structure leads to better light extraction of the QD layer, it also decreases the conversion probability for phosphor particles owing to the settlement, which leads to a lower EL intensity of yellow component at the same phosphor concentration as discussed above. To ensure a fair comparison in device efficiency with similar yellow component, Q–P hybrid WLEDs with different correlated color temperature (CCT) values are prepared by adjusting the phosphor concentration. It should be noticed that this is a widely used approach to compare device performance ignoring the influence of device costs by changing the phosphor concentration [38,39]. The EL spectra of Q–P hybrid WLEDs with reference structure and c-HS structure are given in Figs. 8(a) and 8(b), respectively. Notably, the EL spectra of these two devices are normalized to themselves, respectively. The EL intensities of the yellow component increases as the phosphor concentration increases for both structures. These results demonstrate that the yellow component of WLED can be increased simply by increasing the phosphor concentration though using the c-HS structure. In addition, the phosphor concentration of c-HS structure is approximately 1.5 times higher than that of VLS reference structure to achieve the similar yellow component, as shown in Fig. 8(c), demonstrating that the c-HS structure has disadvantage of increasing phosphor usage. This is also the reason that we use a moderate centrifugation speed (3500 rpm in our cases) to avoid all phosphor particles settling at the bottom as shown in Fig. 3. Previously, there have been lots of studies focused on reducing the phosphor usage by enhancing its absorption probability, including scattering particles [40], nanofiber reflectors [41], etc., which can be further combined with our c-HS structure to solve this issue in future, while it is out of the scope of our current topic.
Fig. 8. Efficiency comparisons between the VLS reference structure and c-HS structure at different CCT values. (a)–(b) EL spectra obtained by the VLS reference structure and c-HS structure, respectively, with different phosphor concentrations.(c) Phosphor centrifugation between the VLS reference structure and c-HS structure at different CCT values.
The CI, PI, and QI of these devices are used to confirm the better QD light extraction of the c-HS structure at various CCT values, as shown in Figs. 9(a)–9(c), respectively; the CCT values are inversely proportional to the phosphor concentration according to Fig. 8(c). In Figs. 9(a) and 9(b), Q-P hybrid WLEDs using the c-HS structure have the same CI and PI as that of the reference structure, demonstrating that the blue and yellow component can be simply adjusted to be the same for both structures by tailoring the phosphor concentration. Most importantly, although both of these two structures have the same QD concentration, their QI values show great differences as shown in Fig. 9(c). As discussed above, the light-extraction of the QD layer is suppressed by the backscattering of phosphor, the red light from QDs can be backscattered and then reabsorbed by the QD layer and other packaging elements. Therefore, the QI of both structures is increased as the CCT values increase with decreasing phosphor concentration (weaker backscattered ability). It is obvious that the c-HS structure significantly contributes to increasing the QI value, confirming that the backscattered loss of QD light is effectively suppressed in our devices compared with that in the reference even compared at similar CCT values.
Fig. 9. Normalized radiant power originating from the (a) LED chip (CI), (b) phosphor (PI), and (c) QDs (QI), respectively, at different CCT values.
The radiant power and luminous flux of both devices with various CCT values are compared as shown in Figs. 10(a) and 10(b), respectively. It is unsurprising that the radiant power for both structures is increased as the CCT values increase owing to the fewer conversion events and lower backscattering caused by the phosphor, and the luminous flux is decreased mainly owing to the fewer conversion events. Most importantly, the radiant power and luminous flux of Q–P hybrid WLEDs using c-HS structure are obviously higher than those using the VLS reference structure at various CCT values, showing increases of 13.6% and 10.8%, respectively, at a typical warm white color of ∼4000 K. As discussed above, the efficiency enhancement is attributed to the better QD light extraction (increased QI values) of the c-HS structure. In addition, the red light component contributes less to the luminous function compared with the yellow light component, therefore, it is reasonable that the enhancement in luminous flux is slightly lower than that of radiant power. Nevertheless, the c-HS structure is promising to enhance the QD light extraction of Q–P hybrid WLEDs at a wide range of output colors.
Fig. 10. (a) Radiant power and (b) luminous flux of the VLS reference structure and c-HS structure at different CCT values.
4. Conclusion
In this study, we have proposed a c-HS structure to enhance the optical efficiency of Q–P hybrid WLEDs with a better QD light extraction. The QD layer is prepared on chips to provide a convex surface for controlling the phosphor settlement distribution, reducing the local density of phosphor at the top region of the QD layer; therefore, quasi-horizontal separation between QDs and phosphor is obtained. Results indicate that the increased centrifugation speed and dispensing mass of the QD layer lead to much more phosphor concentrated on the lateral region of the QD layer, demonstrating a less backscattered loss from phosphor in the top region, which is helpful for increasing the extraction of light emission from the QD layer. Furthermore, the optical performances of c-HS structure are compared with the reference VLS structure under different CCT values by simply increasing its phosphor concentration. Spectral analysis confirms that the c-HS structure effectively increases the QI value at different CCT values compared with the reference structure (while their CI and PI values are almost the same, respectively), demonstrating a better light extraction of QD layer in the c-HS structure. Therefore, the c-HS structure contributes to a higher radiant power and luminous flux at a wide range of CCT values compared with the reference structure, showing increases of 13.6% and 10.8% at a typical CCT of ∼4000 K, respectively. The proposed c-HS structure is beneficial to increasing the efficiency of Q–P hybrid WLEDs, providing a new perspective to design the separation structure for Q–P hybrid WLEDs considering the backscattering loss of QD light.
Funding
Natural Science Foundation of Guangdong Province (2018B030306008); National Natural Science Foundation of China (51775199); National Natural Science Foundation of China (51735004); Project of Science and Technology New Star in Zhu jiang Guangzhou City (201806010102).
Disclosures
The authors declare no conflicts of interest.
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