White light emitting diodes with enhanced CCT
uniformity and luminous
flux using ZrO
2
nanoparticles
Kuo-Ju Chen,aHau-Vei Han,aHsin-Chu Chen,aChien-Chung Lin,*b
Shih-Hsuan Chien,aChung-Ching Huang,aTeng-Ming Chen,cMin-Hsiung Shihad
and Hao-Chung Kuo*a
To enhance the uniformity of correlated color temperature (CCT) and luminousflux, we integrated ZrO2
nanoparticles into white light-emitting diodes. This novel packaging scheme led to a more than 12% increase in luminousflux as compared to that in conventional dispensing structures. This was attributed to the scattering effect of ZrO2nanoparticles, which enhanced the utilization of blue light. Moreover, the
CCT deviation was reduced from 522 to 7 K in a range of70 to +70, and essentially eliminated the yellow ring phenomenon. The haze measurement indicated strong scattering across the visible spectrum in the presence of ZrO2in the silicone layer, and thisfinding also substantiates our claim. In addition, the
chromaticity coordinate shift was steady in the ZrO2dispensing package structure as the drive current
increased, which is crucial for indoor lighting. Combined with its low cost, easy fabrication, and superior optical characteristics, ZrO2 nanoparticles can be an effective performance enhancer for the future
generation of white light-emitting devices.
Introduction
Recently, light-emitting diodes (LEDs) have been widely used as a solid-state lighting source due to their low cost, longer life, higher efficiency and environmental sustainability.1–4 One of
the very major applications of LEDs is their use as indoor lighting replacement for light bulbs. The most basic and common method for producing white light is combining blue LEDs with yellow phosphor (Y3Al5O12:Ce3+) in the package.
Although this strategy is widely used in the industry, the primary disadvantage is a poor color rendering index (CRI).5 Therefore, several methods have been pursued in the eld addressing white LED technology. To enhance the CRI value of white LEDs, some advanced red phosphor technologies have been reported.6,7Moreover, the use of multiple lateral quantum wells (QWs) and various facets have resulted in multiple emis-sion spectra and obtained white LEDs.8,9 Furthermore, recent
studies have also used various methods such as large overlap
QWs10–13and the surface plasmon approach14,15for suppressing
the charge separation problem in InGaN QWs to improve the IQE in green/yellow/red spectral regimes, which are imperative for tricolor white LEDs with only InGaN QWs.
In many cases, a freely dispensed method has been adapted for easy implementation and low cost, but its luminous effi-ciency and uniformity of correlated color temperature (CCT) still require improvement.16Increasing the amount of received photons passing through these layers of packages is crucial for the luminous efficiency of an LED. In the past, there have been numerous methods for enhancing light extraction. The dual-layer graded-refractive index (RI) encapsulant was used to enhance light extraction.17Luo et al. employed the
phosphor-on-top packaging conguration to enhance the phosphor effi-ciency.18In another attempt to use a remote phosphor design,19
the great separation between the blue LEDs and phosphor was used to prevent backscattering of the phosphor.
In addition to lumen efficiency, color uniformity is one of the major problems in white LED fabrication. According to a previous study, the problem is associated with the different ratios of blue and yellow emissions, which results in different CCTs at various angles.19 In white LEDs, one of the direct consequences of nonuniform CCTs is called a yellow ring phenomenon, and it becomes critical when the device package used is large or sophisticated; thus, this problem must be solved. Kuo et al. used the remote phosphor structure pattern to ameliorate CCT deviation.20 A conformal-phosphor structure
was also proposed to reduce angular CCT deviations.21Other
aDepartment of Photonics & Institute of Electro-Optical Engineering, National Chiao
Tung University, Hsinchu 30010, Taiwan. E-mail: [email protected]; Fax: +886-3-5735601; Tel: +886-3-5712121#56304
bInstitute of Photonic System, National Chiao Tung University, Tainan 711, Taiwan.
E-mail: [email protected]; Fax: 3032535; Tel: +886-6-3032121#57754
cPhosphors Research Laboratory, Department of Applied Chemistry, National Chiao
Tung University, Hsinchu 30010, Taiwan
dResearch Center for Applied Sciences, Academia Sinica, 128 Academia Rd., Sec. 2
Nankang, Taipei 115, Taiwan Cite this:Nanoscale, 2014, 6, 5378
Received 30th December 2013 Accepted 4th February 2014 DOI: 10.1039/c3nr06894c www.rsc.org/nanoscale
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Published on 17 February 2014. Downloaded by National Chiao Tung University on 25/12/2014 01:49:21.
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methods, such as improved silicon lens design22 and modi-cation of the shape of the surface phosphor layer23 were proposed and demonstrated. Moreover, some researches demonstrate the uniform angular CCT for white LED by the optimized design package and using with patterned sapphire substrate.24,25 Furthermore, the graded-refractive-index
multi-layer encapsulation structure was also demonstrated by incor-porating nanoparticles into the packaging materials.26 The
scattering effect of nanoparticles could strongly inuence the optical path and change the CCT deviation in white LEDs.27
However, the luminous ux of this structure must still be enhanced. Therefore, simultaneously achieving high luminous efficiency and excellent light quality is a critical factor that could help white LEDs become the primary solid-state lighting source in the market.
In this study, ZrO2nanoparticles were employed to enhance
the luminous efficiency and light quality of LEDs. By codoping the ZrO2nanoparticles with the phosphors, the enhancement of
the light scattering effect enabled the improved utilization of blue light, resulting in an increased luminous ux. Simulta-neously, the presence of ZrO2 nanoparticles also provided a
scattering capability that reduced angle-dependent CCT deviations.
Experiment
The dispensing method was modied to incorporate ZrO2
nanoparticles (Moretech Precision Technology) into white LEDs. Fig. 1(a) and (b) illustrates the schematic diagrams of a ZrO2-doped device and a conventional device. The experimental
ow was as follows: First, a GaN-based blue chip with an emission wavelength of 450 nm was bonded in the lead-frame package. Second, the ZrO2nanoparticles were uniformly mixed
with the YAG phosphor (Intematix) and the silicone encapsu-lant was dispensed in the package. In the reference samples, only the phosphors were uniformly mixed with the silicone
encapsulant. The YAG phosphor powder shows that the full width at half maximum of emission was approximately 100 nm. The output power of the selected blue LED chips was 120 mW at a driving current of 120 mA. To investigate the inuence of the ZrO2nanoparticles on the improvement of the CCT and
lumi-nousux of a package, different weight percentages of the ZrO2
were added to the phosphor and silicone encapsulant. Fig. 1(c) shows the cross-sectional view of the scanning electron microscopy (SEM) image of the ZrO2nanoparticles in the
sili-cone encapsulant. The particle sizes of the YAG and ZrO2
nanoparticles were approximately 10 mm and 300 nm, respectively.
Results and discussion
Shown in Fig. 2(a) are the luminousuxes of the white LEDs in the packages with different contents of ZrO2 nanoparticles
measured at 120 mA. The phosphor concentrations of the conventional structure and ZrO2 nanoparticle (1 wt%)
dispensing structure are the same, but the lumen output of the ZrO2 nanoparticle dispensing structure was 12% higher than
that of the conventional structure. The nanoparticle-embedded device had a higher yellow ray intensity than the conventional device because of the improved conversion ratio from blue photons, resulting in a higher luminous efficiency. The scat-tering effect of the ZrO2nanoparticles enhanced the efficiency
because the prolonged optical path of the blue light caused by scattering led to the higher possibility of exciting the yellow phosphor; thus increasing yellow photon generation. However,
Fig. 1 Schematic cross-sectional view of (a) nanoparticle dispense (b) conventional dispense phosphor structure (c) SEM images of cross section of nanoparticle dispense structure and the inset shows the SEM images of ZrO2nanoparticle.
Fig. 2 (a) The lumenflux and (b) correlated color temperature with the different concentration of ZrO2nanoparticle.
as the concentration of the ZrO2nanoparticles becomes larger,
the transmittance of the ZrO2layer would be lower, as shown in
Fig. 1. Therefore, LED device with high concentration ZrO2
nanoparticle leads to eventual lower lumen efficiency due to the light trapping and absorption phenomenon between the phosphor materials.19 Fig. 2(b) shows the CCTs with
the different concentrations of ZrO2nanoparticles. The CCT of
the no-ZrO2reference sample was 5319 K, and it dropped from
this value as the concentration of ZrO2increased because of the
higher yellow conversion ratio.
The luminous ux and the luminous efficiency measured using a calibrated integrating sphere are plotted in Fig. 3(a) as a function of injection currents ranging from 50 to 500 mA. Regarding luminousux, the optimized concentration of ZrO2
nanoparticles is 1 wt% and its luminousux exceeded that of conventional devices over the entire current range. Measured at a 120 mA current injection, the emission spectra of the refer-ence and ZrO2-doped devices are shown in Fig. 3(b). The
scat-tering effect of the ZrO2nanoparticles in the encapsulant resin
prevented the original Lambertian blue ray from escaping the resin directly, which increased the possibility to excite the yellow phosphor. Therefore, the increased utilization rate of the blue ray increased the output of the yellow light, resulting in the enhancement of lumen efficiency.
To understand the inuence of the scattering effect on the variations of the CCT and luminousux, the angle-dependent CCTs of LED packages containing different amounts of ZrO2
nanoparticles were investigated and shown in Fig. 4(a). The uniformity of the angle-dependent CCTs was greatly improved when the devices were doped with ZrO2 nanoparticles. This
observation indicates that increasing the ZrO2 nanoparticle
concentration of the dopant yielded a stronger scattering effect. In general, the uniformity of CCTs is dened as the maximum CCT minus the minimum CCT. Without doping with ZrO2
nanoparticles, the reference CCT was located at a high level (approximately 5319 K), and a higher CCT implied a higher extraction of blue light, which caused higher CCT deviation. When the devices were doped with ZrO2nanoparticles, the CCT
difference observed at 0 and 70 was essentially eliminated.
The inset picture in Fig. 4(a) shows the far-eld images of uniform white light generated from a ZrO2nanoparticle-doped
LED. Fig. 4(b) shows the gure of merit (FOM), which was dened as
FOM ¼LumenZrO2 LumenNo ZrO2
DCCT (1)
It was discovered that the CCT deviation dropped rapidly from 522 K (reference) to 57 K (3 wt% ZrO2doping), and then to
7 K (10 wt% ZrO2 doping). Using high ZrO2-doping rates
resulted in uniform CCT angular distributions; however, it also
Fig. 3 (a) Luminousflux and the luminous efficiency of nano-particle dispense and the conventional dispense phosphor structure driven at the current from 50 to 500 mA (b) the emission spectra of nano-particle dispense and the conventional remote phosphor structure at 120 mA.
Fig. 4 (a) The angular-dependent correlated color temperature of ZrO2 nano-particles dispense phosphor structure and (b) the CCT
deviation and thefigure of merit of different concentrations of ZrO2
nano-particles in dispensed phosphor structure.
resulted in lumen reduction, as shown in Fig. 2(a). According to our denition of FOM, the optimal ZrO2-doping concentration
was discovered to be at 3%.
The effect of ZrO2doping on the phosphor and silicone was
strong for the performance of the packaged device. The optical properties of ZrO2-dopedlms, however, remained unclear. To
probe further, a series of thin-lm experiments, including transmission–absorption and haze, were performed to charac-terize this ZrO2–phosphor–silicone mixture. Compared with
that of the conventional dispensing structure, the absorption percentage in the ZrO2nanoparticle dispensing structure was
discovered to increase from approximately 32% to nearly 42% at the wavelength of 460 nm. This improvement led to the generation of a higher portion of yellow light in the ZrO2-doped
samples; thus, luminous efficiency was increased.
In addition, haze measurement was employed to investigate the scattering effect of ZrO2nanoparticles with the phosphor
layer, and the haze intensity was dened as28
Haze intensity ¼ Tdiffraction/Ttotal 100% (2)
where Tdiffractionwas the diffractive transmittance (excluding the 0-order diffraction), and Ttotal was the total transmittance.
Fig. 5(b) shows the haze intensities at various ZrO2-doping
concentrations in the phosphor layer. The measured haze intensity was observed to increase from 45% to 94% at a wavelength of 460 nm aer doping with ZrO2 nanoparticles.
When more ZrO2nanoparticles were used in the dopants, the
haze intensity became stronger; it increased to 100% when the dopant used had 10 wt% ZrO2 nanoparticles. From the
perspective of haze measurement, we could certainly see higher scattering effect by larger ZrO2 particles. However, there is a
limitation on the actual size of ZrO2 nano-particles when the
output lumen and CCT are both considered as important characteristics of a LED. From previous research,29,30 a rising
extinction coefficient can be seen in the 0–1mm range. A higher extinction coefficient means stronger re-absorption of photons due to back-scattering, and thus not very favorable for enhancement of LED performance. From our own experiment, as shown in Fig. 2(a), when the concentration of ZrO2is up, the
output lumen is not always increasing but dropping aer 10%. The effective volume of ZrO2 particles increases as we mixed
more, so thisgure can be treated as an indirect proof of the limitation on the particle size.
In this study, aer doping the ZrO2 nanoparticle in the
phosphor layer, the effective index will be changed with the different ZrO2 nanoparticle concentration. Moreover, the
refractive indices (RI) of silicone, phosphor, ZrO2nanoparticle
are 1.4, 1.8 and 2.23, which are obtained from the ref. 31 Thus, the RI of the phosphor layer with ZrO2nanoparticle is calculated
using the following equation32
RI ¼ V1RI1+ V2RI2+ V3RI3,
where V1, V2 and V3 are the concentrations of the materials,
which is calculated in the weight ratio of the materials. For the ZrO2nanoparticle dispensing structure, the mixing ratio of the
ZrO2 nanoparticles to phosphor layer in the dispensing
struc-ture were 1 wt% and 3 wt%, respectively. Therefore, the RIs of the phosphor layer in each layer were 1.428 and 1.445. To discuss the inuence of the different refractive index layers, a TFCalc32 simulation was used.33Compared with the conven-tional dispense structure, the light extraction for ZrO2
nano-particle dispensing structure is almost the same due to the nearly identical refractive index. Thus, the enhancement of lumenux for ZrO2nanoparticle dispensing structure might be
attributed solely to the scattering effect of the ZrO2
nanoparticle.
To numerically evaluate the scattering effect of the ZrO2
nanoparticles, a Mie-scattering simulation was performed to analyze the scattering effect of the different ZrO2 dopant
concentrations.34–36In our model, there were no phosphors and only ZrO2nanoparticles were present in the medium to reduce
the complexity of the model. The RI of the ZrO2nanoparticle
with silicone was 2.23 at the wavelength of 460 nm. The particle size of ZrO2was approximately 300 nm and the dopant content
of ZrO2 nanoparticles was approximately 1% and 3%,
respec-tively, as represented in Fig. 6(a) and (b). The haze intensity of the simulated device structure with lower ZrO2dopant content
showed almost 100% prior to reaching 500 nm and decreased slowly when the wavelength was longer than 500 nm. According to the simulated results, the scattering effect of ZrO2
corre-sponds with our experimental results. When doping with a higher content of ZrO2, the haze intensity was nearly the same
as that for the wavelength ranging from 300 to 700 nm.
Fig. 5 (a) Absorption and (b) the haze intensity of the different concentration of ZrO2nano-particle.
Finally, the color quality of ZrO2-doped devices must be
evaluated. One of the widely adapted standards is to measure its chromaticity coordinates under normal operations. The chro-maticity coordinates of the dispensing structure with different contents of ZrO2dopant at 120 mA are shown in Fig. 7(a). As the
ZrO2-nanoparticle content increased, the chromaticity
coordi-nates gradually shied to the yellow region, and this observa-tion indicated that the intensity of yellow light became stronger andnally led to lower CCTs. Fig. 7(b) illustrates the detailed shiing of chromaticity coordinates with different contents of ZrO2-nanoparticle dopants at current injections of 50 to 500 mA.
Although it is not obvious, the chromaticity coordinates show slighter shiing in the ZrO2 dispensing package structure
compared with the conventional structure with increasing driving current, which can be attributed to the scattering effect of ZrO2nanoparticles. The maximum color deviation value of
the structure with ZrO2 nanoparticles was 0.006, indicating
superior CCT stability with the increasing driving current. The angular dependence of the emission intensity was also exam-ined to investigate the performance of package structures with different contents of ZrO2-nanoparticle dopants and that of the
conventional dispensing structure, as shown in Fig. 7(c). The far-eld emission pattern of our ZrO2-doped LEDs also showed
the characteristics of a Lambertian source, as shown in Fig. 7(d). With these properties combined, this type of ZrO2
nanoparticle-modied LED can exhibit superior performance under various driving current densities and generate high-quality white light.
Conclusion
In conclusion, the effect of ZrO2-nanoparticle doping in the
package was investigated for improving white LEDs. It was
Fig. 6 The simulated results of haze intensity in the concentration of (a) 1% and (b)3%.
Fig. 7 Chromaticity coordinate of LED (a) with different concentration of ZrO2nano particle (b) with different current from 50 to 500 mA (c)
the relative lumen with different concentration of ZrO2nano-particle
(d) 1% andfitting line at different incident from 90to 90.
revealed that this novel packaging method leads to at least a 12% higher lumen than that of conventional structures at 1 wt% ZrO2concentration. This improvement is due to the scattering
effect of ZrO2 nanoparticles and the enhanced utilization of
blue light. Moreover, the deviation of the CCT is also reduced from 522 K (reference) to 7 K (10 wt% ZrO2doping), and this
result is comparable to that of a conventional diffuser plate; however, the ZrO2-doped design does not sacrice the output
luminous efficiency. Based on the haze measurements, the haze intensity becomes stronger and increases to 100% as the amount of ZrO2 nanoparticles used increases, which
corre-sponds with the simulation results. The chromaticity coordi-nate shi is stable in the ZrO2-dispensed package structure with
the increasing drive current and the emission pattern is close to Lambertian. The doping of ZrO2nanoparticles to achieve highly
uniform CCTs and high luminous efficiencies provides an appropriate solution for applications in solid-state lighting.
Funding sources
This work was funded by the National Science Council in Tai-wan under grant numbers NSC101-3113-E-009-002-CC2 and NSC-99-2120-M-009-007.
Acknowledgements
The authors express their gratitude to EPISTAR Corporation and Helio Opto. Corporation for their technical support. C. C. Lin would like to thank the support of Ministry of Science and Technology via contract: NSC101-2221-E-009-046-MY3.
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