Improving optical properties of remote phosphor LED using green Y2O3:Ho3+ and red Mg4(F)(Ge, Sn)O6:Mn2+ layers

TELKOMNIKA Telecommunication, Computing, Electronics and Control
Vol. 18, No. 6, December 2020, pp. 3228~3233
ISSN: 1693-6930, accredited First Grade by Kemenristekdikti, Decree No: 21/E/KPT/2018
DOI: 10.12928/TELKOMNIKA.v18i6.13797  3228
Journal homepage: http://journal.uad.ac.id/index.php/TELKOMNIKA
Improving optical properties of remote phosphor LED using
green Y2O3:Ho3+
and red Mg4(F)(Ge, Sn)O6:Mn2+
layers
My Hanh Nguyen Thi1
, Nguyen Thi Phuong Loan2
, Nguyen Doan Quoc Anh3
1
Faculty of Mechanical Engineering, Industrial University of Ho Chi Minh City, Vietnam
2
Faculty of Fundamental 2, Posts and Telecommunications Institute of Technology, Vietnam
3
Power System Optimization Research Group, Faculty of Electrical and Electronics Engineering,
Ton Duc Thang University, Vietnam
Article Info ABSTRACT
Article history:
Received Aug 4, 2019
Revised Apr 6, 2020
Accepted Jul 9, 2002
The lighting device that employs diodes to create white light (WLEDs) with
quantum dots (QDs) and phosphor layers is a promising lighting method that
is increasingly used in many fields on account of the remarkable color
expressing ability. The QDs film is usually placed apart from the phosphor
layer according to the packaging configuration to prevent light loss due to
backscattering as well as preserve the consistency of the ligands on the QDs
surface. The article also conducted experiments to compare the lighting
properties and thermal output of the two packaging orders of QDs and
phosphor. The heat discharing ranges were simulated with thermography
technology, moreover, other parameters such as light energy emission and PL
spectra are acquired to evaluate the efficiency of the packaging order. The
results from the practical experiment show that while under 10% wt., the
luminous output (LO) of green QDs-on-phosphor structure reaches 1130 lm,
higher than the red QDs-on-phosphor structure with 878 lm, and the color
rendering value in the configuration with red QDs on phosphor is Ra = 74 are
higher than Ra = 68 index of the green QDs-on-phosphor structure. As a result,
the QDs-on-phosphor is determined as the better packaging configuration to
choose to achieve an overall improvement in lighting efficiency, color
rendering index
Keywords:
Color rendering index
Lumen output
Mg4(F)(Ge,Sn)O6:Mn2+
WLEDs
Y2O3:Ho3+
This is an open access article under the CC BY-SA license.
Corresponding Author:
Nguyen Doan Quoc Anh,
Power System Optimization Research Group,
Faculty of Electrical and Electronics Engineering,
Ton Duc Thang University,
Ho Chi Minh City, Vietnam.
Email: nguyendoanquocanh@tdtu.edu.vn
1. INTRODUCTION
White light-emitting diodes (WLEDs) with their phenomenon lighting properties such as high lighting
capacity, power saving and excellent endurance are being increasingly utilized in modern lighting applications
and flat monitor display [1-3]. Fabricating white light by merging discharged light from the blue chip and
the phosphor material Y3Al5O12:Ce3+
(YAG:Ce) is the most commonly used method. The idea of this method
is based on the color-integrating principle, which is combining red and yellow to create white light. While
this LEDs model has high lighting efficacy (LE), the red deficiency within the radiation spectrum, which is
a much-needed chromatic light to improve the color expression ability, prevents it from obtaining high quality
status [4]. Many solutions were suggested in previous researches to address this issue including adding red

TELKOMNIKA Telecommun Comput El Control 
Improving optical properties of remote phospor LED using green Y2O3:Ho3+
... (My Hanh Nguyen Thi)
3229
phosphor materials to enhance the color rendering ability of pc-LEDs [5-8]. The only problem with this method
is the inability of keeping the luminous efficacy index at an acceptable level due to part of their extensive red
emitting spectrum exceeds the human perception [9]. The solid-state lighting field in recent time is focusing
on the semiconductor quantum dots (QDs), an innovative material that can bring about immense changes to
the lighting performance of LEDs, with exceptional optical parameters, namely, the adjustable bandwidth,
small emitting range and high quantum productivity [10-13]. The positive effects of QDs on the color rendering
index (CRI), color gamut as well as the lighting efficacy of pc-LEDs have been verified both in theory and
through practices [14-16]. The most frequently used packaging configurations for pc-LEDs with QDs are applying
a compound of QDs and the phosphorous silicone resin or coating the QDs and the phosphor component separately.
The QDs and the phosphor are blended together and put on the LED chip in the first configuration,
the combined configuration [17, 18], while in the latter configuration the QDs and phosphor layer are applied
individually onto the chip, as separated configuration [19, 20]. The issue with the combined configuration is
the QDs in this structure is part of the reflector and, therefore, too close to the LED chip and results in high
optical density. The remote structure, on the other hand, with a gap between the QDs sheet and the chip
experiences low optical power density. However, the remote structure can stabilize the chemical discrepancy
among QDs molecules at the surface and phosphor silicone expoxy by alternating the polymeric condition that
the QDs sheet is in [21-23]. With this characteristic, the remote packaging configuration appears to be
the optimal solution and was normally applied to create WLEDs with QDs and phosphor.
2. PREPARATION AND SIMULATION
Figure 1 (a) illustrates the scenario where the green QDs particles are placed upon the phosphor layer.
Meanwhile, Figure 1 (b) the red QDs lay upon the phosphor layer. The package configuration is a deciding
factor to the emitted light and heat radiation, thus deciding the lighting performance of WLEDs. While
the relation between the package configuration and the QDs-WLEDs optical performance was thoroughly
expressed in previous studies, there was no practical experiment on the transition of lighting power between
QDs and phosphor. The findings from said research are an important contribution to the QDs-WLEDs
construction guideline, however, the amount of light energy loss during transition in both structures as well as
the effect of QDs on the optical performance are the important aspects that have not been discussed.
(a) (b)
Figure 1. Simulation diagram of two remote WLEDs types with different packaging sequences:
(a) green QDs-on-phosphor type, (b) red QDs-on-phosphor type
Therefore, the goal of this article is to gather the information needed for a quantitative research that
relates to the aspects mentioned above. The primary method is to analyze the decline of lighting power between
QDs and the phosphor layer in both structures, and then compare their operating efficiency to determine which
one is the optimal configuration. To serve this experiment, a red emitting QDs CdSe/ZnS and a WLEDs based
on yellow emitting phosphor and a band of QDs were created. The emission of lighting energy and PL range
indexes needed for the comparison process are computed using an integrating sphere system while
the temperature fields on the devices during operation are imitated by putting the optical indexes in thermal
simulation and then use an infrared thermal imager to analyze the result. The outcomes from the experiments
show that with better lighting efficiency and color rendering index in addition to lower device generated heat,
the QDs-on-phosphor is the ideal packaging order to choose for better performance.

 ISSN: 1693-6930
TELKOMNIKA Telecommun Comput El Control, Vol. 18, No. 6, December 2020: 3228 - 3233
3230
3. RESULTS AND DISCUSSION
The results in Figure 2 expressed the different changes occur in terms of concentration between green
phosphor Y2O3:Ho3+
, red phosphor Mg4(F)(Ge, Sn)O6:Mn2+
and yellow phosphor YAG:Ce3+
. These changes
in concentration affect the ability to absorb or scatter of the phosphor layers in WLEDs, which determine
the color quality and the amount of light emitted. Therefore, the concentration setting of the two phosphor
layers Y2O3:Ho3+
and Mg4(F)(Ge, Sn)O6:Mn2+
are extremely important to the optical performance of WLEDs.
Moreover, the decreases in the concentration of yellow phosphor YAG:Ce3+
corresponding to the increases in
concentrations of the Y2O3:Ho3+
is to maintain the average correlated color
temperatures. More specifically, YAG:Ce3+
concentration decline to preserve the ACCTs when
the concentrations of Y2O3:Ho3+
and Mg4(F)(Ge, Sn)O6: Mn2+
are rising in the range from 2 to 20% wt.
The influences of red phosphor Mg4(F)(Ge, Sn)O6:Mn2+
and green phosphor Y2O3:Ho3+
are expressed in Figure
3. According to Figure 3, the intensity in the range from 420-480 nm and 500 to 640 nm increase with
Y2O3:Ho3+
concentration which confirmed that the luminous flux in these ranges is also increase. Moreover,
the scattered blue light also increases, which means the scattering property is improved leading to better
correlated color. The advantages that occur when applying the Y2O3:Ho3+
are notable results that can be utilized
to validate the benefit of this green phosphor on the optical properties of WLEDs. In the case of
Mg4(F)(Ge,Sn)O6:Mn2+
, the presence of this phosphor cause the intensity from 648 to 738 nm to rise, however,
this effect is meaningless without the increase in the two remaining spectrum ranges, which are
420-480 nm and 500-640 nm. Similar to the green phosphor, if the intensity in the remaining spectrums of red
phosphor rise that would result in better blue light scattering effect. The emission spectrum that ties to
the emitted temperature is crucial in improving the optical performance, particularly, when the emission
spectrum increase, the color quality and luminous flux become better. These are important findings when
utilizing the red phosphor Mg4(F(Ge, Sn)O6:Mn2+
. The research results show that Mg4(F)(Ge, Sn)O6:Mn2+
guarantee improvement in color quality despite the color temperature, this is a valuable characteristic,
especially when maintaining high color quality at high temperature is considered hard to obtain. The final
decision relies on the manufacturers to choose the suitable option to reach their goal. For WLEDs that focus
on the high chromatic performance, it is advisable to give up a bit of emitted light to achieve highest color
quality possible.
Figure 2. The alteration in phosphor concentration
of remote WLEDs to maintain the average CCT
Figure 3. Emission spectra intensity in dual-layer
phosphor structures
The CRI expresses the chromatic outcome of the irradiated object. This index denotes the ability to
express the color of an object and is based on the balance of blue, yellow, and green. If the proportions of
the constituent colors are uneven, the color balance will loss and results in reduced color quality. According to
results of CRI presented in Figure 4, the green remote phosphor layer Y2O3:Ho3+
is detrimental to the CRI.
Although the color rendering index decreases slightly whenever there is a green phosphor layer in the structure,
this is not a serious defect due to CRI is only part of color quality scale (CQS). The CQS, as can be seen in
Figure 5 remain static when the concentration of Y2O3:Ho3+
is below 8%. In comparison to CRI, the CQS is
a priority because it covers more optical properties and is harder to obtain. As a result, it is acceptable to have
the concentration of Y2O3:Ho3+
under 8% if the luminous flux is also appropriate.
The following section is the calculating formula for the unequal SPD of single color LED with
Gaussian function [24, 25]:

3231
𝑃𝜆 = 𝑃𝑜𝑝𝑡
1
𝜎√2𝜋
𝑒𝑥𝑝 [−0.5 ∗
(𝜆−𝜆 𝑝𝑒𝑎𝑘)2
𝜎2 ] (1)
In this formula, 𝑃𝜆 is the spectral power distribution (SPD) (mW/nm) while λ indicates the wavelength (nm)
and 𝜆 𝑝𝑒𝑎𝑘 describes the peak wavelength (nm). 𝑃𝑜𝑝𝑡 are used to describe the optical power (W) of the WLED.
When 𝜎 relies on the peak wavelength 𝜆 𝑝𝑒𝑎𝑘, it is possible to define the full-width at half-maximum (FWHM)
Δ𝜆 (nm) as following with 𝜆1, 𝜆2 being the wavelengths at half of the peak intensity,ℎ represents Planck’s
constant (J.s) and c is the speed of light (m.s−1
).
𝜎 =
𝜆 𝑝𝑒𝑎𝑘
2
𝛥𝐸
2ℎ𝑐√2 𝑙𝑛 2
=
2
(
ℎ𝑐
𝜆1
−
ℎ𝑐
𝜆2
)
=
2
(
ℎ𝑐𝛥𝜆
𝜆1 𝜆2
)
(2)
Figure 4. The color rendering index corresponding
to the concentration of Y2O3:Ho3+
and
Mg4(F)(Ge, Sn)O6:Mn2+
phosphors
Figure 5. The color quality scale corresponding to
the concentration of Y2O3:Ho3+
and
phosphors
In theory, WLED with YAG phosphor and blue light emitting chip has the combination between blue
and yellow spectra as SPD. In practical experiment, yellow phosphor actually active in both yellow and green
emission ranges. With that being said, it is possible to use the green spectrum to depict the discrepancy between
the SPD in practice and blue-yellow mixed spectrum in cases with blue and yellow are the chosen spectra.
As a result, the green spectrum is added to the model for practical circumstances and the analytical tri-spectrum
(B-G-Y) is expressed as in (3) then later on modified to (4):
𝑃𝜆 = 𝑃𝑜𝑝𝑡_𝑏
1
𝜎 𝑏√2𝜋
𝑒𝑥𝑝 [−0.5 ∗
(𝜆 − 𝜆 𝑝𝑒𝑎𝑘_𝑏)2
𝜎 𝑏
2 ]
𝑃𝑜𝑝𝑡 𝑔
1
𝜎 𝑔√2𝜋
𝑒𝑥𝑝 [−0.5 ∗
(𝜆−𝜆 𝑝𝑒𝑎𝑘 𝑔)2
𝜎 𝑔
2 ] (3)
𝑃𝑜𝑝𝑡_𝑦
1
𝜎 𝑦√2𝜋
𝑒𝑥𝑝 [−0.5 ∗
(𝜆 − 𝜆 𝑝𝑒𝑎𝑘_𝑦)2
𝜎 𝑦
2 ]
𝑃𝜆 = 𝜂 𝑏 𝑃𝑜𝑝𝑡 𝑡𝑜𝑡𝑎𝑙
1
𝜎 𝑏√2𝜋
𝑒𝑥𝑝 [−0.5 ∗
(𝜆 − 𝜆 𝑝𝑒𝑎𝑘 𝑏
)2
𝜎 𝑏
2 ]
𝜂 𝑔 𝑃𝑜𝑝𝑡 𝑡𝑜𝑡𝑎𝑙
1
𝜎𝑔√2𝜋
𝑒𝑥𝑝 [−0.5 ∗
(𝜆 − 𝜆 𝑝𝑒𝑎𝑘 𝑔
)2
𝜎𝑔
2 ]
𝜂 𝑦 𝑃𝑜𝑝𝑡_𝑡𝑜𝑡𝑎𝑙
1
𝜎 𝑦√2𝜋
𝑒𝑥𝑝 [−0.5 ∗
(𝜆−𝜆 𝑝𝑒𝑎𝑘_𝑦)2
𝜎 𝑦
2 ] (4)
where 𝑃𝑜𝑝𝑡_𝑏, 𝑃𝑜𝑝𝑡_𝑔, 𝑃𝑜𝑝𝑡_𝑦and 𝑃𝑜𝑝𝑡_𝑡𝑜𝑡𝑎𝑙 stand for the optical power (W) in each separate spectrum of blue,
green, yellow and white. 𝜆 𝑝𝑒𝑎𝑘_𝑏, 𝜆 𝑝𝑒𝑎𝑘_𝑔 and 𝜆 𝑝𝑒𝑎𝑘_𝑦in order are the peak wavelengths (nm) of the spectra

 ISSN: 1693-6930
TELKOMNIKA Telecommun Comput El Control, Vol. 18, No. 6, December 2020: 3228 - 3233
3232
emitting the chromatic lights. The non-dimensional ratios of chromatic lights spectra to the range of white
light, are expressed respectively as 𝜂 𝑏, 𝜂 𝑔 and 𝜂 𝑦. 𝜎𝑏, 𝜎𝑔 and 𝜎 𝑦 in turn are the coefficient of FWHM (nm) for
the wavelength ranges of chromatic lights. This mathematical models for SPC in WLEDs with phosphor layer
can be perceived as a tricolor spectrum and an extended Gaussian model. Next, we study the luminous flux of
QDs WLED with the QDs concentration as in Figure 6.
The content of Figure 6 demonstrates that when the concentration range of Y2O3:Ho3+
is from 2% wt
to 20% wt the luminous will sharply increase. However, the luminous flux in remote phosphor structure with
dual layers is a result of both phosphor layers including the red phosphor Mg4(F)(Ge, Sn)O6:Mn2+
.
The extinction coefficient increase with Mg4(F)(Ge, Sn)O6:Mn2+
concentration while opposing to light
transmission energy as the application of Beer’s Law suggests. Therefore, when choosing the concentration for
the phosphor layers it may occur the lumen output decreases when the concentration of Mg4(F)(Ge, Sn)O6:Mn2+
increases, and drastically decline especially when the concentration reaches 20%. Despite the drawback in
luminous flux, using red phosphor Mg4(F)(Ge,Sn)O6:Mn2+
is still beneficial because of the enhancements it
brings to the CRI and CQS, not to mention the luminous flux in these dual-layer phosphor structures is better
than the single-layer ones that do not have a red phosphor layer. These references help the manufacturers
choose the exact concentration of the phosphor to serve their purpose in mass production.
Figure 6. The luminous flux corresponding to the concentration of Y2O3:Ho3+
phosphors
4. CONCLUSION
The results of this research paper confirmed the benefits of green phosphor Y2O3:Ho3+
and red
phosphor Mg4(F)(Ge, Sn)O6:Mn2+
on the optical properites of remote structure with two phosphor layers.
Through the study of scattering characteristic with scattering theory and the Beer’s law, it is confirmed that
the red phosphor Mg4(F)(Ge, Sn)O6: Mn2+
is capable of boosting the chromatic quality. On the other hand,
the green phosphor Y2O3:Ho3+
is the excellent substance to improve the luminous flux, the effect is not only
applicable to low color temperature WLEDs but also with the ones at high color temperature. With
the aforementioned results, this research has succeeded in a very difficult task that is enhancing the color quality
in remote phosphor structure. On another note, the concentration of both phosphor layers Y2O3:Ho3+
or
must be kept at an appropriate level as the excessive amount of phosphor can damage
the color quality as well as the luminous flux. Managing the concentration of phosphor is crucial in producing
WLEDs, however, this article has presented all the much-needed information about this topic to
the manufacturers so they can apply it to create WLEDs with the desired quality.
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Improving optical properties of remote phosphor LED using green Y2O3:Ho3+ and red Mg4(F)(Ge, Sn)O6:Mn2+ layers

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