Effect of ZnO nanostructures on the optical properties of white light-emitting diodes

White light produced by blue LEDs with yellow phosphor is the most widely used methods, but it results in poor quality in angular CCT uniformity. In this work, a novel technique was introduced to solve this problem by integrating different ZnO nanostructures into white light-emitting diodes. The experiment of ZnO doped films and the simulation of Finite-Difference Time-Domain (FDTD) were carried out. The result indicated scattering effect of ZnO nanoparticles could improve uniformity of scattering energy effectively. Moreover, the effect of ZnO nanostructures on white light-emitting diodes (wLEDs) devices was also investigated. The CCT deviation of wLEDs devices would decrease from 3455.49 K to 96.30 K, 40.03 K and 60.09 K when the node-like (N-ZnO), sheet-like (S-ZnO) and rod-like ZnO (R-ZnO) respectively applied. The higher CCT uniformity and little luminous flux dropping were achieved when the optimal concentrations of N-ZnO, S-ZnO, and R-ZnO nanostructures were 0.25%, 0.75%, and 0.25%. This low-cost and green manufacturing method has a great impact on development of white light-emitting diodes. © 2017 Optical Society of America OCIS codes: (230.1980) Diffusers; (230.3670) Light-emitting diodes; (290.0290) Scattering. References and links 1. H. C. Chen, K. J. Chen, C. H. Wang, C. C. Lin, C. C. Yeh, H. H. Tsai, M. H. Shih, H. C. Kuo, and T. C. Lu, “A novel randomly textured phosphor structure for highly efficient white light-emitting diodes,” Nanoscale Res. Lett. 7(1), 188 (2012). 2. H. Jeong, D. J. Park, H. S. Lee, Y. H. Ko, J. S. Yu, S. B. Choi, D. S. Lee, E. K. Suh, and M. S. Jeong, “Light-extraction enhancement of a GaN-based LED covered with ZnO nanorod arrays,” Nanoscale 6(8), 4371– 4378 (2014). 3. B. R. Lee, E. D. Jung, J. S. Park, Y. S. Nam, S. H. Min, B. S. Kim, K. M. Lee, J. R. 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Introduction
Light-emitting diodes (LEDs) are regarded as the next generation solid-state lighting source because of their long life, high efficiency, low cost and environmental advantages in comparison to conventional incandescent and fluorescent light sources [1][2][3][4].Currently, combining a blue LED with yttrium aluminum garnet (YAG:Ce) phosphor is the most common method of producing a white light-emitting diodes (wLEDs) [5,6].However, this phosphor-free dispensing method results in poor quality in angular correlated color temperature (CCT) and the unwanted phenomenon such as "yellow ring" [7][8][9].To solve the problem, several methods such as conformal-phosphor structure design [10], surface phosphor layer modification [11], lens design [12], and package structure optimization [13,14], were reported.Although these methods were confirmed to achieve high CCT uniformity, they also brought large fabrication difficulties and high-cost for the mass production, which would limit their applications in LED packaging industry.Therefore, it is necessary to find a method which not only realizes CCT uniformity but also is with low cost and simple fabrication.
In present, a promising strategy, adopted for improving the CCT uniformity is the diffuser-load encapsulation method, owing to advantages of easy fabrication and low cost.Accordingly, some of diffuser-based LEDs have been designed to enhance angular CCT uniformity by using metal oxide diffusers materials, such as TiO 2 , ZrO 2 and SiO 2 [15][16][17][18].Chen et al. [15] embedded the ZrO 2 particles (300 nm) into a remote phosphor structure and reduced angular-dependent CCT deviations from 1000 K to 420 K in the range of −70° to 70°. Lee et al. [18] adopted TiO 2 powders with 320 nm particles size as diffusers dispersed on the phosphor layer or encapsulation layer, and the angular CCT deviations of wLEDs declined as the diffuser concentration increased.However, among reports on the use of diffusers in the wLEDs devices, they are almost focus on study the effect of single particles or different particles concentration, yet there have been few reports on the effect of different morphologies of these nanoparticles in wLEDs [5,19,20].Meanwhile, due to the lack of intensive theories studies and sophisticated technical guidance, choosing proper particles which can improve CCT uniformity efficiently has been a tedious and time-consuming work.
Recently, ZnO nanostructures with scattering effect have been used in LEDs owing to the advantages of controllable morphology, low cost and a simple synthetic process [21][22][23][24].By controlling the reaction conditions such as temperature, time, precursor concentration and so on, various structures would easily achieved.ZnO nanostructures can be used as gradient refractive index layer because the refractive index of ZnO (n = 2.0) is between GaN (n = 2.5) and air (n = 1).Until now, researchers found that doping ZnO, especially with special nanostructures, into GaN-based blue LEDs was able to improve the light extraction [25][26][27][28][29]. Yin et al. [26] applied a hybrid patterned ZnO micro-cylinders and nanorods array on the surface of LED chip to improve the light extraction efficiency for GaN-LEDs.Yin et al. [28] demonstrated a remarkable enhancement of light extraction efficiency in GaN based blue LEDs with rough beveled ZnO nanocone arrays grown on the planar ITO layer.Lee et al. [29] reported a controllable way of tuning the light emission pattern of the GaN-based blue LEDs using the shape of ZnO nanorods.All of these researches exhibit a huge potential that ZnO nanostructures can be used to enhance the light extraction enhancement of LEDs.However, to the best of our knowledge, there are few reports on the different ZnO nanostructures were used as diffusers in LED packaging to enhance the optical properties of wLEDs, especially the ZnO with different morphologies effect on the CCT and light intensity uniformity of wLEDs.Therefore, there remains scope for research into enhancing the CCT and light intensity uniformity of wLEDs by using ZnO nanostructures.
In this paper, the node-like ZnO (N-ZnO), sheet-like (S-ZnO), and rod-like ZnO (R-ZnO) nanostructures were synthesized by a shape-selective manner using simple hydrothermal method and first time employed to enhance CCT uniformity of wLEDs.A scattering energy distribution and Finite-Difference Time-Domain (FDTD) methods were carried out to study the scattering effect of ZnO nanostructures.The result indicated that ZnO nanoparticles have a great scattering effect and could improve uniformity of scattering energy effectively.Moreover, we co-doped different concentrations and morphologies of ZnO nanoparticles with silicone as an encapsulant of wLEDs, greatly enhancing CCT uniformity of the wLEDs devices.

Synthesis of ZnO nanostructures
Three different highly crystalline and single phase ZnO structures (N-ZnO, S-ZnO and R-ZnO) were synthesized by a shape-selective manner using simple hydrothermal method [30].Typically, the zinc acetate dihydrate [Zn(CH 3 COO) 2 •2H 2 O] and sodium hydroxide (NaOH) were used as the precursors, and deionized water as the solvent.For the synthesis of N-ZnO nanostructures, a colorless transparent solution was prepared by dissolving zinc acetate dehydrate (2.19 g, 0.01 mol) in 60 ml of deionized water, followed by few min of continuous stirring at room temperature.Then an aqueous solution (60 ml) of sodium hydroxide (0.8 g, 0.02 mol) was slowly dripped into zinc acetate solution with 30 min of vigorous stirring and ultrasonic treatment for homogeneous dispersion.The initial molar ratio of Zn 2+ and OH -ions was 1:2.After that, the mixing solution was transferred to a 200 ml stainless steel teflon-lined autoclave.Subsequently, the autoclave was placed in an electric oven and heated at 150 °C for a period of 24 h.When the reaction was complete, the autoclave was allowed to cool naturally to room temperature.The precipitate was centrifuged and washed 4~5 times by using deionized water and ethanol to remove impurities.Finally, the purified sample was dried at 80 °C for 6 h before further analysis.Similar procedures were used for the synthesis of S-ZnO and R-ZnO nanostructures, by using 2 g or 8 g of sodium hydroxide, respectively, leading to the initial molar ratio of Zn 2+ and OH -ion being 1:5 or 1:20, correspondingly.

Preparation of ZnO-doped films
A series of ZnO-doped thin films of around 300 µm thickness were fabricated by the template method.The various of ZnO nanoparticles were mixed into polydimethylsiloxane (PMDS), and then vigorously tirred using a vacuum homogenizer at 1360 rpm and 0.2 MPa vacuum for 6 min to degas.After that, the mixture was injected into a cleaned and dried steel die.Thereafter, the steel die was put into an electric oven and preheated at 80 °C for a period of 30 min.Subsequently, the temperature was increased to 120 °C for 30 min to ensure that the ZnO-doped films solidified.Finally, the ZnO-doped films were cooled to room temperature (25 °C) and stored in a drying cabinet.

Fabrication of ZnO-based wLED devices
A schematic of the ZnO-based wLED devices and images of samples with different concentrations and morphologies of ZnO nanostructure are respectively shown in Figs.1(a)  -(c).First, a GaN-based blue LED chip with emission wavelength of 450 nm and rated power of 1 W at a driving current of 350 mA was placed in the commercial lead-frame package.Then, YAG:Ce yellow phosphor (D 50%, 13 ± 2 µm) was mixed with silicone in a weight ratio of 3:8 by a vacuum homogenizer at 1360 rpm and 0.2 MPa vacuum for 12 min.The mixture was dropped on the surface of the blue LED chip and solidified at 120 °C for 3 h under ambient atmosphere to form a phosphor layer.A hemispherical lens was used for encapsulation to prevent contamination and damage.Finally, the ZnO nanoparticles were uniformly dispensed in silicon via vacuum stirring, degassed, and then injected into the into the gap between the lens and blue LED chip by a high pressure injector, and dried at room temperature (25 °C).

Physical and optoelectronic characterization
The surface morphology of ZnO was characterized by a field emission scanning electron microscope (FE-SEM, Merlin).The transmittance of ZnO-doped films was measured using a UV-Vis spectrometer (UV-Vis, Agilent Cary 5000).Optoelectronic measurements of ZnO-doped films and ZnO-based wLED devices were performed using an integrating sphere system (Multi Spectrums T-950/930).All measurements were performed in air and at room temperature (25 °C).

The properties of ZnO-doped films
In order to explore the effect of different concentrations and morphologies of ZnO nanoparticles on the optical properties, a series of ZnO-doped films were investigated.The morphology and size of the ZnO nanostructures doped in the films were investigated using the field emission scanning electron microscope (FE-SEM).Figures 2(a)-2(c) show the FE-SEM images of N-ZnO, S-ZnO and R-ZnO nanostructures, respectively.The typical N-ZnO nanostructures consisted of two short column-like particles of 170~200 nm in width and 280 nm in length, and the S-ZnO with a sheet-like polygon section of an average thickness of 40 nm, while the regular R-ZnO of 200~230 nm in width and several micrometers in length.The size of ZnO surface nanostructure just has several nanometers and it can be ignored the impact on scattering effect of ZnO.These nanoparticles with obvious morphology character were selected in order to distinguish their scattering ability.suggests that the uniformity of scattering energy distribution was greatly improved when the films were doped with ZnO nanoparticles.This means that increasing the ZnO nanoparticles concentration of the dopant produces a remarkable scattering effect.The scattering effect of ZnO nanoparticles could strongly influence the path of the incident light, leading to the higher possibility of energy dispersion.In the reference sample without ZnO nanoparticles, the reference scattering energy distribution zone is narrow because most of the energy transit is by specular transmission.Even doping with ZnO at a relatively low concentration [Fig.3(a)], the scattering effect of ZnO nanoparticles is still noteworthy.When the films were doped with 2.5% ZnO nanoparticles [Fig.3(e)], the distribution of scattering energy is similar to the Lambertian distribution.
Meanwhile, the different surface morphologies of the ZnO nanostructures also have a great effect on the scattering energy distribution.As shown in Figs.3(a) and 3(b), where the concentration of ZnO is less than or equal to 0.25%, the difference of scattering energy distribution among the three kinds of ZnO-doped films is small, but still different from the reference.When the concentration of scattering particles are at a low level, the number of particles are very rare per unit volume and are not sufficient to strongly affect light paths; thus, the effect of morphology on light scattering is not obvious.As the concentration of ZnO nanoparticles increases, the scattering energy distribution of ZnO-doped films becomes more even, as shown in Figs.3(c)-3(e).According to general scattering theory, when the concentration of particles is less than a critical value, then scattering is according to particle cloud definitions, and the total scattered power per unit volume of space is proportional to the single particles' scattering power, which results in the aforementioned consequence [31].Meanwhile, the effect of morphology becomes stronger as the particle concentration increases.When the concentration of ZnO-doped films ranges from 0.75% to 1.25% [Figs.3(c) and 3(d)], it can be seen that the scattering effect of the S-ZnO nanostructure is superior to the others.This can be attributed to the specific surface area of S-ZnO being much larger and thus can more strongly influence optical paths.However, as the content of ZnO in films was increased from 0.75% to 1.25%, the scattering effect of N-ZnO nanostructure excelled over the R-ZnO nanostructure.This change may be related to grain size, particle quantity, and particle orientation.The average grain size of N-ZnO is less than R-ZnO (from Fig. 2); thus, the quantity of particles of the former is larger than the latter, such that particle orientation becomes more random in an equal weight, which will increase possibility of changing the exit light path.When the concentration of ZnO doping of the films reaches 2.5% [Fig.3(e)], complications arise because of these high concentrations, and multiple scattering and particle interaction effects become significant, while the effect of morphology cannot be shown.The above explanation clearly suggests that the morphology of ZnO nanostructures could obviously influence light path.The reflection measurements demonstrate that the morphology of the ZnO nanostructure has an enormous effect on the scattering energy distribution, as shown in Figs.4(a)-4(e).When the concentration of ZnO in the films ranged from 0.75% to 2.5%, the scattering effect of S-ZnO nanostructure was also superior to the others, in support of the transmission results.High ZnO doping concentrations resulted in uniform scattering energy distribution; however, this also produced more back scattering.Even with ZnO doping at a low concentration (0.25%), the normalized back scattering energy was located at a high level (approximately 0.5), and a higher back scattering energy implied extraction of light.
In addition, the scattering properties of ZnO nanoparticles doped in the film can be characterized by another evaluation index, the scattering energy ratio, which is defined as where η is the scattering energy ratio of transmission or reflection, E Total is the total extraction energy, and E specular is the specular transmission or reflection energy.The scattering energy ratios of transmission and reflection measured using optical testing systems are plotted in Figs.5(a) and 5(b) as a function of ZnO doping concentration ranging from 0.125% to 2.5%.It is obvious that the scattering energy ratio increases with the concentration of ZnO, as opposed to the transmission or reflection measurements; this also supports the normalized scattering energy distribution results.Among the different ZnO nanostructures, the scattering energy ratio of transmission or reflection for S-ZnO doped films was higher than others.Besides, when the doping concentration was increased from 0.75% to 1.25%, the scattering energy ratio of N-ZnO was larger than R-ZnO; this result further confirmed the consequences of the normalized scattering energy distribution.When the concentration of ZnO reaches a certain value, there is a difference among the various ZnO nanostructures of the scattering energy distribution; nevertheless, the transmittance of the ZnO-doped films would be lower, as shown in Fig. 6.For a concentration of 0.125%, the transmittance of the films decreased by 4.13%, 6.31% and 9.12% for N-ZnO, S-ZnO and R-ZnO, respectively.It can be seen that the transmittance of R-ZnO nanostructure dropped more than the others, because the R-ZnO nanostructure has a larger grain and aspect ratio that will weaken light transit.As doping concentration increases, the transmittance of S-ZnO nanostructure decreases faster than the others, but little difference is observed between S-ZnO and R-ZnO nanostructures; this may be related to specific surface area and aspect ratio.With 2.5% concentration, the transmittance of films decreased by up to 49.3%, 59.8%, and 57.4%.for N-ZnO, S-ZnO and R-ZnO, respectively.This would not be suitable for practical use, in spite of the high scattering ability.Therefore, the scattering effect and transmittance of ZnO-doped films should both be considered when choosing an optimum solution for use in different environments.
To numerically evaluate the scattering ability of the three different morphology ZnO nanoparticles, a Finite-Difference Time-Domain (FDTD) method was carried out to calculate their scattering phase function according to our previous study [32,33].Figures 7 (a)-(d) shows their scattering phase distributions (SPDs) at three typical light incident angle of 0°, 45°, 90° and their average scattering phase distributions of three light incident angle, respectively.In our model, there was only one ZnO nanoparticle to reduce simulation complexity.The size of ZnO is the same as the SEM measured (as shown in Fig. 2).The wavelength of incident light is 455 nm.When the light incident angle is 0° [Fig.7

The properties of ZnO-based wLED devices
The scattering effect of different concentrations and morphology of ZnO doping strongly affected the performance of the films.However, the optical properties of ZnO-based LED devices remained unclear.To investigate this, experiments were performed on ZnO-based wLED devices, including measuring CCT spatial distribution, light intensity spatial distribution, and luminous flux, to evaluate the scattering effects of the ZnO-loaded encapsulation.All the devices were measured with a driving current of 350 mA.The yellow phosphor concentrations of the traditional structure and ZnO doping structure were identical.According to previous research, the uniformity of the angle-dependent CCT and light intensity can be defined as the maximum of the CCT or light intensity minus the minimum of the CCT or light intensity, respectively.In general, the range of spatial angles from −70° or 70° was used to evaluate uniformity of the angle-dependent CCT and light intensity, because the luminous intensity outside this range is weak and sometimes cannot be used because of the hollow effect of the lens or package frame [34].
Figures 8(a)-8(e) show the angle-dependent CCT distribution of wLEDs devices doped with different ZnO nanostructures at different concentrations.It can be seen that the uniformity of the angle-dependent CCT was greatly improved when ZnO nanoparticles were introduced into the devices.Moreover, as the amount of ZnO increased, the angle-dependent CCT became more uniform.This indicates that increasing the concentration of ZnO nanoparticles in the wLED devices improves the scattering effect.From the pictures demonstrated in Figs.8(a)-8(e), it can be seen that the key factors that influence the optical properties of the wLED devices are the concentration and morphology of the ZnO nanoparticles.Compared to the reference sample (bare LED) without ZnO nanoparticles, the angular-dependent CCT distribution is similar to the angular CCT distribution of the light emitted from the bare LED chip, which can be considered to be Lambertian type.When the concentration of ZnO nanoparticles doped in the wLED devices was increased from 0 to 2.5%, the CCT deviation was reduced from 3455.49K to 96.30 K, 40.03 K, and 60.09 K for N-ZnO, S-ZnO, and R-ZnO, respectively.These changes are ascribed to the scattering effect of the ZnO nanoparticles.This result further verified the previous analysis that the scattering effect increases with the concentration of ZnO nanoparticles.Besides, the morphology of the ZnO nanostructure also influences the distribution of the angular CCT, as shown in Fig. 8(a).Even with a relatively low doping concentration, the CCT deviation was reduced from 3455.49K to 1097.90 K, 592.11K, and 945.66K for N-ZnO, S-ZnO and R-ZnO, respectively.The wLED devices doped with S-ZnO have better CCT uniformity.However, when the doping concentration is 0.25% [Fig.8(b)], among three kinds of wLED devices, the wLED devices with R-ZnO showed better CCT uniformity than others.This phenomenon is maybe related to the aspect ratio and surface morphology properties of R-ZnO.Continuing increase the concentration of ZnO nanoparticles, the variation of the CCT of wLED devices with N-ZnO and R-ZnO are similar to that with S-ZnO [Figs.8(c)-(e)].However, the curve of CCT variation amplitude of these devices was smaller than the others [Figs.8(a) and 8(b)]; this result is similar to the scattering energy analysis.Meanwhile, the differences between different ZnO morphologies decrease for increasing concentration, especially for S-ZnO and R-ZnO.Doping at higher concentrations results in increased overlapping opportunities between particles and weakens the surface morphology effect.
The effect of different concentrations and morphologies of ZnO nanostructures on light intensity were investigated in a similar manner to before, as shown in Figs.9(a)-9(e).The light intensity from the undoped reference sample was quite high (about 6.32 mcd), but had poor uniformity.Meanwhile, the wLED devices doped with N-ZnO nanoparticles have stonger light intensity than others due to it has better transmissivity (Fig. 6), while the wLED devices doped with S-ZnO and R-ZnO nanoparticles have better light intensity uniformity.When the wLED devices were loaded with ZnO nanoparticles, the light intensity uniformity was dramatically improved.The stronger scattering ability of the S-ZnO nanostructure resulted in better intensity uniformity.All these result real that the morphology of the ZnO nanostructure has a great effect on light intensity.Besides, as the concentration of ZnO nanoparticles increased, the light intensity would quickly decrease because of lower transmission, especially for the devices with S-ZnO nanostructure.

Fig. 5 .
Fig. 5. Scattering energy ratio of different concentrations of the different ZnO-doped films by using (a) transmission and (b) reflection measurement methods.
(a)], both the R-ZnO and S-ZnO nanoparticles have almost the same forward scattering ability, while the N-ZnO has smallest scattering ability among three different morphology ZnO nanoparticles.Besides, the S-ZnO nanoparticles have a large backward scattering.This result indicates that S-ZnO nanoparticles have better scattering ability than others, owing to S-ZnO nanoparticles with larger specific surface area.When the light incident angle is 45°or 90° [Fig.7(b) or Fig. 7(c)], the S-ZnO nanoparticles demonstrate a remarkable scattering ability than the R-ZnO and N-ZnO nanoparticles.In addition, different light incident angle with remarkable difference scattering power distribution among three different morphology ZnOs, which shows ZnO morphology have a great effect on scattering effect.Figure 7(d) represents a average of three light incident angle scattering power distributions, which obviously reveal the S-ZnO nanoparticles with best scattering ability among them.All these results confirm scattering energy distribution analysis that the S-ZnO nanoparticles with best scattering ability.

Fig. 10 .
Fig. 10.(a) Luminous flux and (b) CCT deviation and luminous flux reduction at different concentrations of ZnO.Although the wLEDs with high concentrations of ZnO nanoparticles have better uniformity of the angle-dependent CCT and light intensity, this resulted in luminous flux reduction.The luminous flux measured as shown in Fig. 10(a) at a driving current of 350 mA.As the concentration of ZnO increased, the luminous flux rapidly decreased, which may be caused by the light trapping and absorption phenomenon between the phosphor materials.The luminous