Optical Performance Enhancement of Quantum Dot ... - IEEE Xplore

IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 63, NO. 2, FEBRUARY 2016

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Optical Performance Enhancement of Quantum Dot-Based Light-Emitting Diodes Through an Optimized Remote Structure Xiang Lei, Huai Zheng, Xing Guo, Jingcao Chu, Sheng Liu, Fellow, IEEE, and Peizhao Liu

Abstract— Quantum dot (QD)-based light-emitting diodes (LEDs) always show low efficiency and unstable performance at high driving currents due to the saturation effect of QDs. In this paper, a novel remote packaging structure was proposed to reduce the saturation effect and enhance optical performances of QD-based LEDs. In the proposed packaging structure, a crater lens and an air gap between the QD-polymer film and the lens were introduced. The comparison experiments between the proposed structure and the current structures, a fully filling structure, and an air-gap structure with a spherical lens were conducted. The luminous flux increasing rates of current structures were far lower than that of the proposed packaging structure at high driving currents. Consequently, compared with the fully filling structure and the air-gap structure with the spherical lens, the proposed packaging structure has increased the luminous flux by 70.5% and 50.1% at the driving current of 800 mA. In terms of the correlated color temperature stability with the driving current, the proposed packaging structure shows the best performance. In addition, the angular color uniformity has been greatly improved by the proposed packaging structure. Index Terms— Lens, light-emitting diodes (LEDs), luminous flux, quantum dots (QDs), saturation effect.

I. I NTRODUCTION

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UE TO extraordinary characteristics, such as energy saving, high luminous efficiency, and long life and high reliability, high-power white light-emitting diodes (LEDs) have been widely used in our daily lives [1]–[5]. Currently, most of the white LEDs are generated through the combination of the yellow phosphor and blue chip. However, because of the shortage of long-wavelength light, the light quality, particularly the color-rendering index presents poor performance.

Manuscript received September 10, 2015; revised November 3, 2015 and December 3, 2015; accepted December 9, 2015. Date of publication December 25, 2015; date of current version January 20, 2016. This work was supported in part by the National Natural Science Foundation of China under Grant U1201254 and Grant U1501241, and in part by the Fundamental Research Funds through the Huazhong University of Science and Technology under Grant CX15-044. (Corresponding author: Huai Zheng.) X. Lei, X. Guo, and J. Chu are with the School of Mechanical Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China. H. Zheng and S. Liu are with the School of Power and Mechanical Engineering, Cross-Disciplinary Institute of Engineering Science, Wuhan University, Wuhan 430072, China (e-mail: [email protected]). P. Liu is with the Faculty of Materials Science & Engineering, Hubei University, Wuhan 430062, China. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TED.2015.2508026

Recently, quantum dots (QDs), solution-processed nanoscale crystals of semiconducting materials, have attracted considerable scientific attention [6]–[9]. QDs have many advantages over the conventional light-emitting materials, such as inorganic phosphors, and fluorescent and phosphorescent organic polymers. QDs show tunable photoluminescence (PL) properties by changing the size or composition, narrow band emission, saturated color, and high quantum efficiency. Moreover, the colloidal synthesis process is more simplified, more easily scalable, and less expensive. Furthermore, QDs show almost nonscattering or very weak-scattering property, since the nanocrystal size is much smaller than the optical wavelength [9]–[11]. Initially, QD-based LEDs had been fabricated through the same processes with the phosphor-converted LEDs. QDs were physically blended into a commercial resin encapsulant. In addition, the mixture was coated around LED chips. However, such a process usually accompanied the undesired results, such as QD agglomeration, which leads to nonradiative energy loss, the luminous efficacy reduction, and incomplete resin hardening due to the so-called catalyst poisoning effect [12]–[14]. A QD-polymer film, where QDs are homogeneously distributed in the transparent polymeric matrix, is a good way to overcome above problems of the direct coating process [15]. The utilization of the QD-polymer film is through the remote packaging structure. The QD-polymer film is mounted on the top surface of the lead-frame substrate. The gap between the film and the LED chip is filled fully with an encapsulant. However, LEDs based on the QD-polymer film always show low optical power output and luminous efficiency. Besides, they also display the saturation effect at high driving currents, which results from the absorption saturation of QDs, when the received light energy is beyond the QDs’ absorption threshold. Results of saturation effect present dropping in light conversion efficiency (CE), increasing in correlated color temperature (CCT) and changing in color coordinates with increasing driving current [11], [16], [17]. However, few works have been reported to solve the above critical packaging issues of LEDs based on the QD-polymer film. Shin et al. [18] proposed an improved structure by introducing the spherical lens and the air gap between the lens and the film. With the air-gap structure, the optical power and CE were enhanced compared with those of the conventional fully filling structure. Nevertheless, their experiments were

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conducted at the low driving current of 6 mA. The optical power and stability of the air-gap structure at high driving currents should be examined further. The issue of saturation effect seems not to be solved, yet. In this paper, a new and optimized remote packaging structure, an air-gap structure with a patterned lens, such as a crater, was proposed to reduce the saturation effect and make full use of QDs. Two kinds of current remote packaging structures, the fully filling structure and the air-gap structure with the spherical lens presented in [18], were adopted to evaluate the proposed packaging structure. Their optical performances with the driving current ranging from 20 to 800 mA were measured. Experimental results were presented and discussed. II. P RINCIPLE Besides the inherent low saturation threshold of QDs, the improper packaging structure is also a factor resulting in the significant saturation effect of QD-based LED devices. Since the Lambert distribution of the blue light emitted from the LED chip and the very weak-scattering property of QDs, the absorbed light distribution of the QD-polymer film is obviously nonuniform. It decreases from the film center to sides sharply. As a result, the saturation effect first appears at the film center with the driving current increasing. At high driving currents, the QD emission at the film center is saturated. However, QD emission in sides is still far lower than its saturation threshold, and QDs are not fully utilized. Based on the above saturation reason analysis of QD-based LEDs, it can be obtained that reducing the difference of the light absorption between the center and the sides will benefit reducing the saturation effect and making full use of the QDs. Fig. 1(c) indicates the proposed novel remote packaging structure. In such a structure, a patterned lens, such as a crater, is applied, and an air gap exists between the lens and the QD-polymer film. The lens with the crater can modulate the blue light illumination to be more uniform [19], [20]. Thus, in principle, it is thought to be able to reduce the saturation effect. Fig. 1(a) and (b) shows the other two remote packaging structures, the traditional fully filling structure and the air-gap structure with the spherical lens, which are used for performance comparison in this paper. For convenience, as shown in Fig. 1, the fully filling structure and the air-gap structure with the spherical lens are named structures A and B, respectively. The air-gap structure with the patterned lens is named structure C. III. E XPERIMENTS A green and modified route was used to synthesize highquality luminescent CdSe QDs with the inorganic passivation of ZnS [21]. The prepared QDs were first centrifugally washed three times by chloroform. After drying and weighting, QDs were dispersed into chloroform. Next, some polystyrene particles were added and homogeneously dissolved by ultrasonic oscillating. The solution was poured into flat-bottomed disk molds for film preparation. By curing with the protection of N2 , chloroform solvent was removed. QD-polymer films

Fig. 1. Schematics of three kinds of remote packaging structures of QD-based LEDs.

were obtained. The weight of polystyrene particles was controlled to make sure that the QD concentration in the final composite film was 0.5 wt%. In addition, the thickness of the QD-polymer film was kept ∼550 µm. LED devices with three kinds of remote structures, as shown in Fig. 1, were fabricated. Their fabricating processes had a little difference. For structure A, the silicone was first filled into the cavity of a lead-frame substrate, and the QD-polymer film was covered on the top surface of a silicone resin. After that the whole module was cured at 100 °C. The lens fabrication processes need to be accomplished for two air-gap structures. For structure B, a half-sphere lens was fabricated on the chip in the package by dispensing silicone on a circle bump [22]. For structure C, the patterned silicone lens, such as a crater, was prepared in the LED package by the polymer dispensing and embossing technology [19]. And then, the QD-polymer film was attached on the packages, followed by thermal curing at 100 °C. The vertical injection LED chip is used with the peak wavelength of 454 nm and the chip size is 45 × 45 mil. The diameter and height of the cavity of lead-frame substrate are 6 and 4 mm, respectively. And the cavity reflectivity

LEI et al.: OPTICAL PERFORMANCE ENHANCEMENT OF QD-BASED LEDs

Fig. 2. (a) High-resolution TEM images of CdSe/ZnS QDs. (b) Normalized absorption and PL spectra of CdSe/ZnS QDs. Inset: photograph of the QD dispersion under UV (365 nm) illumination.

is ∼85%. During experiments, the optical performances of chips were measured before fabricating lenses. In addition, chips with almost the same optical performances, such as the peak wavelength of emission light and light output power, were chosen in order to fairly assess the optical performance of presented three packaging structures. UV-visible absorption and PL spectra were obtained by the Lampda spectrometer (VERTEX 70, Bruker, Germany) and the fluorescence spectrometer (FP-6500, Jasco, Japan). The QDs’ dispersion and size distribution were observed by a high-resolution transmission electron microscope (TEM) (TecnaiG2 20, FEI, Netherlands). Light intensity distribution (LID) was obtained by LID curve tester (GO1900L, Everfine, China). Luminous flux, CCT, and electroluminescent (EL) spectra were measured by integrating sphere (HAAS-2000, Everfine, China). The angular color uniformity (ACU) was measured by a setup with a digital colorimeter, which was built in our lab [23]. IV. R ESULTS AND D ISCUSSION High-resolution TEM images of inorganic passivated CdSe/ZnS QDs are shown in Fig. 2(a). The prepared QDs present good monodispersion and their average size is ∼5.2 nm. The UV-visible absorption and PL spectra of CdSe/ZnS QDs are shown in Fig. 2(b). It can be seen that the CdSe/ZnS QDs have a high absorbance for the LED blue light range (420–480 nm). The wavelength of emission peak of QDs is located at 573 nm with a full width at half maximum of 28 nm. The sharp and narrow emission peak results from highly monodispersed QDs. The PL quantum yield of the QDs is measured to be 68%. The cross sections of LED modules with the spherical silicone lens and the patterned silicone lens are shown in Fig. 3. In these modules, the QD-polymer films are not integrated yet. For structure B, the spherical lens is almost a half sphere with the radius of 2 mm. For structure C, the patterned lens is with a crater at its top and center. The crater diameter (d) is 1.7 mm, and the crater depth (h) is 0.7 mm. For structure A, the silicone top surface is flat. In addition, it is not shown, herein, due to its simplicity.

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Fig. 3. Cross sections of LED modules with (a) spherical silicone lens and (b) patterned silicone lens.

Fig. 4. LID curves for packaged monochromatic blue light LEDs with three kinds of lenses, and without QD-polymer films.

Fig. 4 shows the LID curves of the monochromatic blue light LEDs with three kinds of lenses, a flat lens, a spherical lens, and a patterned lens, and without covering QD-polymer films. From Fig. 4, it can be seen that the LID of the LED module with the flat lens is close to the Lambert distribution. The LID gets a little modification by the spherical lens, and the center light intensity turns to be a little evener. However, its type still presents the Lambert distribution. The LID is significantly changed by the patterned lens. In addition, the strongest light intensity varies from the center to the sides. It means that more light was redirected toward large view angles. The relative light extraction efficiency (LEE) of the lens is defined as the ratio of the light output power of the LED module with a lens to that of the LED module without lens [24]. The relative LEEs of three kinds of lenses are measured at the driving current of 20 mA. For the spherical lens, it shows the largest relative LEE, 122.7%. For the patterned lens, it shows slightly lower relative LEE, 106.5%. For the flat lens, it is the lowest value, only 78.5%. To test the performance against saturation effect, the evolutions of EL spectra of QD-based LEDs with three kinds of remote structures with increasing driving current from 20 to 800 mA are measured. Fig. 5 shows the measurement results. It can be seen that the QD emission shows the saturation effect at high driving currents in QD-based LEDs with structures A and B. The increasing rate of QD emission peak

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Fig. 6. Luminous flux variations of LEDs with three kinds of remote structures when the driving current increases from 20 to 800 mA.

Fig. 5. Evolutions of EL spectra of QD-based LEDs with three kinds of remote structures when the driving current increases from 20 to 800 mA. (a) Structure A. (b) Structure B. (c) Structure C.

becomes lower with the driving current rising. In addition, it can be roughly found that the saturation phenomenon starts at the driving current of 300 mA. However, the QD emission peak of QD-based LEDs with structure C keeps almost the same increasing rate and it hardly presents the saturation effect with increasing current. The CE can be obtained based on the data shown in Fig. 5. The CE is defined as the ratio of converted yellow light intensity to the original blue light intensity emitted from bare LED chips. The CE of structure A drops from 29.5% to 19.9%, when the driving current increases from 20 to 800 mA, and

that of structure B also drops from 37.8% to 26.7%. However, the PE of structure C slightly decreases from 34.0% to 33.3%. Hence, compared with structures A and B, the CE of structure C presents higher value and higher stability at high driving currents. The packaging efficiency (PE) is defined as the ratio of light output power of the packaging LED modules to that of the original blue light emitted from bare LED chips [25]. These three structures show the same PE variation trend with that of the CE. The PE of structure A drops from 59.9% to 43.3%, when the driving current increases from 20 to 800 mA, and that of structure B also drops from 70.9% to 50.4%. However, the PE of structure C only decreases from 68.6% to 61.3%. Hence, compared with structures A and B, structure C improves the PE by 41.6% and 21.6% at the driving current of 800 mA. Fig. 6 indicates the variations of the luminous flux with the driving current increasing from 20 to 800 mA for QD-based LEDs with three kinds of remote structures. At the driving current of 20 mA, the luminous flux of structure B has the highest value. The lowest luminous flux occurs in structure A. And the luminous flux of structure B is 29.5% larger than that of structure A. Structure C shows the same-level value with structure B. Its value is ∼95% of the luminous flux of structure B. With the driving current increasing, the luminous flux increments of structures A and B gradually slow down. However, the luminous flux of structure C increases with the same trend. Consequently, the maximal luminous flux difference between structure C and other packaging structures occurs at the driving current of 800 mA. Compared with structures A and B, structure C increases the luminous flux by 70.5% and 50.1%. Fig. 7 presents the luminous flux variations of LEDs with three kinds of lenses, and without the QD-polymer film when the driving current increases from 20 to 800 mA. It can be observed that all the luminous fluxes of LEDs with three kinds of lenses increase in almost the same trend. The luminous flux of LED with the spherical lens is a bit larger than that of LED with the patterned lens all the time. In addition, they are far larger than that of LED with the flat lens. It can be used to explain that why structure B have the highest value


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Fig. 7. Luminous flux variations of LEDs with three kinds of lenses, and without QD-polymer films when the driving current increases from 20 to 800 mA.

Fig. 9. Angular color coordinates. (a) International Commission on Illumination (CIE) x and (b) CIE y of QD-based LEDs with three kinds of remote structures. Fig. 8. CCT variations of QD-based LEDs with three kinds of remote structures when the driving current increases from 20 to 800 mA.

at the driving current of 20 mA, which is shown in Fig. 6. At the driving current of 800 mA, the maximum luminous flux in Fig. 7 occurs at the LED with the spherical lens. It is different with the phenomenon in Fig. 6 that structure C shows the largest luminous flux. These differences demonstrate that the saturation effect of QDs is the main reason, which reduces the luminous flux at high driving currents. In addition, structure C is beneficial to reduce the saturation effect. Fig. 8 shows the CCT variations of QD-based LEDs with three kinds of remote structures, when the driving current increases from 20 to 800 mA. It can be seen that QD-based LEDs with structures A and B both show the saturation effect at high driving currents. With the driving current increasing, their CCT increases. In addition, the CCT increasing rate of structure A is larger at high driving currents. The CCT of structure A increases by 3200 K, from 6782 to 9982 K. The CCT of structure B increases by 2600 K, from 6679 to 9279 K. However, the CCT of structure C almost keeps the same. The CCT variation of structure C is only 153 K, from 6741 to 6588 K. Therefore, compared with structures A and B, structure C reduces the CCT variation by 95.2% and 94.1%. It should be mentioned that the trend of CCT variation of structure C is different with those of other structures, decreas-

ing with the driving current increasing. It may be due to the thermal effect on LED chip emission. With the driving current increasing, the LED chip gets higher junction temperature and the peak wavelength of the LED chip emission gets slightly red shift [26], [27]. Besides, structure C suffers from a little saturation effect. The comprehensive effect results in the CCT slightly reducing with the driving current increasing. ACU is also measured for QD-based LEDs with three kinds of remote structures at the driving current of 100 mA. The ACU is presented by the variation of angular color coordinates in degrees from −70° to 70°, and the results are shown in Fig. 9. For three kinds of remote structures, all the color coordinates (CIE x and CIE y, created by the International Commission on Illumination) show the lowest value at 0° and the largest value at 70°. The angular CIE x variation and angular CIE y variation of structure A are 0.0257 and 0.0103, respectively. Shown as the same-level value, those values of structure B are 0.0236 and 0.0115, respectively. However, the angular color coordinate variations of structure C are much smaller. The angular CIE x variation is 0.0143 and angular CIE y variation is 0.0055. Therefore, compared with structures A and B, structure C reduces the angular CIE x variation by 44.4% and 39.4%. Meanwhile, it reduces the angular CIE y variation by 46.6% and 52.2%. According to the CIE coordinates, the CCT values are calcu-

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V. C ONCLUSION In order to reduce the saturation effect and make full use of QDs, a novel remote structure, the air-gap structure with the patterned lens, such as a crater, was proposed. The measurement results of optical performances demonstrate that the proposed new packaging structure has a significant effect on reducing the saturation effect and reducing the bad influence of the saturation effect on QD-based LEDs. Consequently, compared with the fully filling structure and the air-gap structure with the spherical lens, the proposed packaging structure increases the luminous flux by 70.5% and 50.1% at 800 mA. When the driving current increases from 20 to 800 mA, the proposed packaging structure reduces the CCT variation by 95.2% and 94.1%. Furthermore, compared with the other structures, it reduces the angular variation of CIE x by 44.4% and 39.4% and angular variation of CIE y by 46.6% and 52.2% at the same time. R EFERENCES

Fig. 10. Schematics of light progressing paths in three kinds of remote structures.

lated. The angular CCT deviation (ACCTD) is also applied to assess the color uniformity performance [28]. The ACCTD of structure A is 1496 K, from 6997 to 5501 K, that of structure B is 1368 K, from 6882 to 5514 K, and that of structure C is only 828 K, from 6617 to 5789 K. Consequently, compared with structures A and B, structure C improves the CCT angular uniformity by 44.7% and 39.5%. All above measurement results of optical performances can be explained as follows. Fig. 10 schematically illustrates the light progressing paths in three kinds of remote structures. The initial blue light emitted from the LED chip presents as the Lambert distribution in which the light intensity decreases from the center to the sides sharply. In structures A and B, the blue light distribution at the QD-polymer film is still close to the Lambert type due to very weak refraction effect. However, in structure C, because of the refraction and the total internal reflection phenomenon at an inverted cone structure, more blue light is largely redirected from the center to large view angles. In addition, it results in the uniform light distribution at the QD-polymer film. All above light progressing analyses can be verified by results, as shown in Fig. 4. Because the light distribution of structure C is more uniform than those of other two packaging structures, its saturation effect could be effectively reduced at high driving currents, as shown in Fig. 5. As a result, structure C shows a higher luminous flux, as shown in Fig. 6. Meanwhile, less saturation effect leads to more color stability against the increasing driving current, as shown in Fig. 8. Conventional QD-based LEDs have a bad ACU mostly because of the weak intensity of blue light at large view angles. In structure C, blue light enhancement is achieved at large viewing angles. So the ACU is improved in structure C, as shown in Fig. 9.

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Huai Zheng received the Ph.D. degree from the Huazhong University of Science and Technology, Wuhan, China, in 2014. He is currently an Assistant Professor with the School of Power and Mechanical Engineering, Wuhan University, Wuhan. His current research interests include LED packaging, thermal management of electronics, electrohydrodynamics, and flow dynamics of 3-D printing. He has authored over 20 articles and holds seven issued patents.

Xiang Lei received the B.S. degree from the School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, China, in 2012, where he is currently pursuing the Ph.D. degree. His current research interests include LED packaging and thermal management of optoelectronic devices.

Peizhao Liu received the B.S. degree from the School of Materials Science and Engineering, Hubei University, Wuhan, China, in 2014, where he is currently pursuing the master’s degree. His current research interests include quantum dots (QDs) synthesis and QD-LED.

Xing Guo received the B.S. degree from the School of Mechanical and Electronic Engineering, Wuhan University of Technology, Wuhan, China, in 2010. He is currently pursuing the Ph.D. degree with the School of Mechanical Science and Engineering, Huazhong University of Science and Technology, Wuhan. His current research interests include LED packaging, microfluidics, and electrohydrodynamics technology.

Jingcao Chu received the B.S. degree from the School of Logistics Engineering, Wuhan University of Technology, Wuhan, China, in 2013. He is currently pursuing the M.S. degree with the School of Mechanical Science and Engineering, Huazhong University of Science and Technology, Wuhan. His current research interests include LED packaging, thermal management, and mechanical structure design.

Sheng Liu (F’14) received the B.S. and M.S. degrees in flight vehicle design from the Nanjing University of Aeronautics and Astronautics, Nanjing, China, in 1983 and 1986, respectively, and the Ph.D. degree in mechanical engineering from Stanford University, Stanford, CA, USA, in 1992. He has over 22 years of experience in integrated circuit packaging and 12 years in optoelectronic/LED packaging. His current research interests include LED/MEMS/IC packaging, mechanics, and sensors.