Enhancing Luminous Efficacy of White Led Lamp Using Ca2MgSi2O7:Eu

Along with the rapid development of science and technology in today's society, lighting technology, especially white light-emitting diodes (WLEDs), has become a critical and essential element in almost every eld of our daily life. Therefore, the enhancement of luminous e ciency have become an important objective in the production of white LEDs to serve the needs of customers. This paper presents the in uence of Ca2MgSi2O7:Eu 2+ green luminescent phosphor on the light quality of WLEDs. In this work, the green-emitting Ca2MgSi2O7:Eu 2+ phosphor is added into the in-cup phosphor con guration, which leads to the varying of the scattering property of this compounding. This is proved through the scattering coe cient from Mie-theory. Besides, the color quality scale which is called CQS index of WLEDs is also veri ed. The achieved results indicate that the luminous e cacy increases significantly if the Ca2MgSi2O7:Eu 2+ concentration is varied. The smaller the Ca2MgSi2O7:Eu 2+ size is used, the higher the luminous ux is obtained. However, the CQS can decrease if Ca2MgSi2O7:Eu 2+ concentration is adjusted in another direction. Therefore, it is vital to select a suitable concentration and size of Ca2MgSi2O7:Eu 2+ for enhancing the luminous e cacy of WLEDs, and the bene t of this phosphor in creating white WLED packages is a potential solution for developing LED illumination


Introduction
Recently, white light emitting diodes (WLEDs) have been more and more important and indispensable in our daily life because of their potential exibility as well as dominant functions.With these outstanding advantages, WLEDs can easily be spread in many dierent illumination applications such as general lighting, medicine, consumer products [1]- [4].The combination of numerous monochromatic LED chips or the mixture of blue light emitted from LED chip with YAG:Ce 3+ phosphor is one of the popular methods in white light emitting diode technology to generate white light which is also called phosphor converted LED.Nevertheless, the proccess of light's absorption going back to the package is very harsh, leading to the attained results of lumen output and angular colour uniformity just reach such bad quality.Hence, the purpose of generating WLEDs with high luminous eciency, excellent colour uniformity, and great CRI has been developing in recent years [5]- [8].Shi's team has proposed the greenemitting Ca 7 (PO 4 ) 2 (SiO 4 ) 2 :Eu 2+ phosphor for improving the optical properties of white light emitting diodes [9].In 2016, Wang's group has applied perovskite CsPbBr 3 quantum dots for enhancing the luminous ecacy of green lightemitting diodes [10].In 2017, Ma's team added Cr 3+ to Y 3 Al 5 O 12 :Ce 3+ to enhance the color rendering index from 63.2 to 77.9.This study shows that Ce 3+ and Cr 3+ co-doped YAG phosphors are suitable for white light-emitting diodes [11].In 2018, Lu's group has presented the synthesis and the photoluminescence characteristics of the LiGd 3 (MoO 4 ) 5 :Eu 3+ red phosphor with high color purity and brightness.The results show that LiGd 3 (MoO 4 ) 5 :Eu 3 may be a promising red phosphor for white light-emitting diodes [12].
To serve the purpose of ameliorating and limiting the loss backscattered light inside LED chip, an opinion based on a structure with remote phosphor conguration designed in which the phosphor layer separated from chip is put forward [13].Although the remote phosphor structure can give the better luminous eciency than the dispensing and conformal structure, it is not easy to achieve the concave surface of this structure which leads to a non-uniform phosphor thickness in fabrication technology.Thus, it is really hard for luminous eciency and uniform colour distribution of WLEDs to meet the demand of dierent cases in illumination applications.During the process of development, the optimization of remote phosphor structure is created via designing the concentric green, red phosphor rings to focus on lessening the problems of backscattering.With this propose, the extraction eciency and the colour rendering property of WLEDs are more excellent than these of WLEDs with a mixed phosphor layer [14]- [18].A dierent phosphor conformation with an innovative double remote micropatterned phosphor lm is used to enrich the colour homogeneity of WLEDs.On the other hand, the conguration of LED's lens need to be researched in many aspects to keep balance in optimization of uniform illumination and high light eciency.Therefore, the luminescent material is concerned as a signicant factor that supports the performance of multi-chip WLEDs, and hence its main function is to overwhelm the condition of poor light extraction and colour uniformity.
Although almost above approaches aim to improving the performance of WLEDs, the luminous eciency and the colour quality of white LEDs are not completely gratied at many dissimilar illumination applications as well as extensive demands in the commercial LED lighting market [19]- [25].In addition, there is only single-chip white LED lamps with low colour correlated temperature, which is chiey concentrated.Moreover, in order to satisfy the competitiveness in lighting market and oer the better light quality, advanced studies might have been extensively conducted to nd more optimal conguration of LEDs or a phosphor material that help the luminescent intensity and colour quality increase at such high temperatures.
This paper proposes a method in which the Ca 2 MgSi 2 O 7 :Eu 2+ phosphor compounding is applied to mix with the YAG:Ce 3+ yellow phosphor compounding of the WLEDs.The main purpose of this combination is to optimize its light extraction, colour quality at high colour correlated temperature.In the process of simulation, the weight proportion and the size of phosphor particle need to be varied in order that it is exact to realize its inuence on the optical performance of WLEDs.The optical properties of Ca 2 MgSi 2 O 7 :Eu 2+ and YAG:Ce 3+ phosphors compounding are surveyed via analysing light absorption and scattering as well as light conversion in LEDs at high correlated colour temperature.In other words, simulations and calculations are fully taken advantage so that they can create a chance which nd the positive factors of concentration and size of Ca 2 MgSi 2 O 7 :Eu 2+ green luminescent phosphor on luminous ux, colour quality.According to the obtained results, we recognize that Ca 2 MgSi 2 O 7 :Eu 2+ green phosphor does not only improve the luminescent properties but also can verify colour quality of the incup phosphor conguration.  of green-emitting Ca 2 MgSi 2 O 7 :Eu 2+ phosphor is calculated in Table 2. Besides, the preparation of the material is described as three following steps: Firstly, we start to mix the material together by slurrying them in methanol plus a few cubic centimeters of water.Next, dry it in the air.When the mixture gets dry, we powderize it.Lastly, re this powder in capped quartz tubes with N 2 at 1000 o C for an hour.Then powderize it, and re the material again in capped quartz tubes with CO, at 1150 o C for an hour.phosphor on the performance of the WLEDs at the high correlated temperature of 8500 K.
In order to prepare for the process of the incup phosphor conguration of WLEDs, we blend the Ca 2 MgSi 2 O 7 :Eu 2+ and YAG:Ce 3+ phosphor compounding as expressed in Fig. 1.Consequently, the phosphor layer of WLEDs contains Ca 2 MgSi 2 O 7 :Eu 2+ green phosphor, the yellow phosphor YAG:Ce 3+ , and the silicone glues.The constituents of simulated WLEDs expressed in the model are blue chips, a reector cup, a phosphor layer, and a silicone layer.A reector with a 2.07 mm depth, a bottom length of 8 mm and a length of 9.85 mm at its top surface is bonded with these chips.The radiant power of each nice blue chip was designed with 1.16 W, a peak wavelength of 453 nm, as shown in Table 1.The radius of the green Ca 2 MgSi 2 O 7 :Eu 2+ phosphor particles were changed from 1 µm to 10 µm. to achieve the optimization of color uniformity and lumen output eciency.The Ca 2 MgSi 2 O 7 :Eu 2+ phosphor particle density can be changed from 0 to 1.4% in simulation process.The refractive index of phosphor particle is set to be 1.85 and 1.83 for Ca 2 MgSi 2 O 7 :Eu 2+ and YAG:Ce 3+ respectively.To maintain the average CCT of white LEDs at 8500 K, the YAG:Ce 3+ phosphor concentration need to change appropriately to the concentration of Ca 2 MgSi 2 O 7 :Eu 2+ .The optical simulation is performed along with the change in Ca 2 MgSi 2 O 7 :Eu 2+ particle size and distribution so that the the eect of Ca 2 MgSi 2 O 7 :Eu 2+ green phosphor on WLEDs could be accurately dened.The scattering of Ca 2 MgSi 2 O 7 :Eu 2+ phosphor particle was analyzed by using the Mie-theory [14].In addition, the scattering cross section Csca for spherical particles can be computed by the following expression through ap-Talbe 1.The actual parameters of LED chip.

LED chip V45H
Voltage (V) (2) where k = πD/λ is particle size parameter, r is the particle radius, λ is the wavelength, ψ n (x) and ξ n (x) are the Riccati-Bessel functions, µ sca (λ) is the scattering coecient, and a n , b n are calculated by: (4) According to the received results shown in Fig. 2, the growth of scattering coecient exits parallel with the decrease of the size of Ca 2 MgSi 2 O 7 :Eu 2+ particle, which proves the truth that the smaller the phosphor particles are, the stronger the abilities of scattering of incident light are and the more color uniform they achieve.At the size of 1 µm, the best luminous ecacy can be reached.Generally, the scattering coecient is the same as the wavelengths varied from 380 nm to 780 nm.The scattering coecient slightly increases with the changes of the wavelength at the size of 1 µm.Moreover, the scattering is slightly sensitive to the incident wavelength.The longer wavelength can lead to the stronger scattering.It implies that the scattering of Ca 2 MgSi 2 O 7 :Eu 2+ phosphor benets considerably to color quality.

DISCUSSION
As demonstrated in Fig. 3, the obtained results show that Ca 2 MgSi 2 O 7 :Eu 2+ phosphor has a positive eect on the luminous ecacy of WLEDs and its luminescent properties are completely inuenced by concentration and particle dimension.Ca 2 MgSi 2 O 7 :Eu 2+ green luminescent phosphor is considered as a good element for green light enhancement and thermal condensation.Therefore, we have an idea that adding these particles to the YAG:Ce 3+ compounding will help the LED achieves higher brightness at high temperature of 8500 K.
To obtain accurate results on the optical properties of Ca 2 MgSi 2 O 7 :Eu 2+ , the simulations with concentrations ranging from 0 to 1.4% corresponding to each size within 1 − 10 µm are performed.The lines in the ow chart show that the luminous ecacy increases so fast and it can reach the maximum values in the concentration range of 0% to 0.6% and the size from 1 to 5 µm as depicted in the Fig. 3. Normally, the luminescence ux can be improved at all particle sizes when adding Ca 2 MgSi 2 O 7 :Eu 2+ to the phosphor compound in concentration.
The small particles usually provide less luminous ux than bigger ones due to the unwanted backward scattering inside WLEDs.It means that the light is more likely to be trapped inside a package and there will be less light from the LEDs when the particles are applied.The intensity of light emission decreases and hardly reaches the maximum for small particles at a color temperature of 8500 K. Thus, the luminous ecacy of WLEDs using Ca 2 MgSi 2 O 7 :Eu 2+ phosphor with the size of 1 µm can decrease with further increase of concentration.This phenomenon occurs at 0.4% wt. of Ca 2 MgSi 2 O 7 :Eu 2+ .For the size of 2 − 3 µm, the reduction of luminous ecacy occurs at 0.6% and 0.8% wt. of Ca 2 MgSi 2 O 7 :Eu 2+ , respectively.The increase of luminous ecacy grows with the size of Ca 2 MgSi 2 O 7 :Eu 2+ particles, from 4 µm to 6 µm regardless the Ca 2 MgSi 2 O 7 :Eu 2+ concentration.With the size of 6 − 10 µm, the tendency of light propagates more vigorously in the forward direction and weakens in the opposite direction of the LED chip, so that large particle sizes show advantages for luminous ecacy.The main pur-

Fig. 3 :
Fig. 3: The luminous ecacy of WLEDs as a function of the size and concentration of Ca 2 MgSi 2 O 7 :Eu 2+ phosphor.

Fig. 4 :
Fig. 4: The color quality scale of WLEDs as a function of the size and concentration of Ca 2 MgSi 2 O 7 :Eu 2+ phosphor.pose of this study is not only to enhance luminous ux but also to verify the color quality scale (CQS).CQS index which can evaluate the overall color quality of WLEDs about color delity, chromatic discrimination, and observer preferences shows slight decrement around particle sizes 6 − 10 µm as Fig. 4. The CQS values slightly decrease within the size of 6 to 10 µm.The Ca 2 MgSi 2 O 7 :Eu 2+ green luminescent phosphor can lead to green light supplementation and deciency of red components in the spectrum of WLEDs, causing the reduction of CQS.
MgSi 2 O 7 :Eu 2+ phosphor is frequently employed for very high loading and long life-time uorescent lamps due to the high quantum eciency, chemical properties, and thermal stability of this material.Ca 2 MgSi 2 O 7 :Eu 2+ composition comes from chemical processes among dierent materials such asCaO, MgO, SiO 2 , Eu 2 O 3 , and NH 4 Cl.
[26] phosphor compounding can be considered as an ecient host candidate for good luminous ecacy of WLEDs[26].The detail composition
MgSi 2 O 7 :Eu 2+ green luminescent phosphor on WLEDs is presented and demonstrated, which helps us be aware of the possibility of improvement both luminous ux and color uniformity at high color correlated temperature basing on the optical simulation.We analyzed the impact of phosphor particle size and concentration of Ca 2 MgSi 2 O 7 :Eu 2+ on luminous ux and CQS.The highest luminous ux accompanies with insignicant decrease in CQS in this scope.As a result, by adding Ca 2 MgSi 2 O 7 :Eu 2+ green phosphor with the concentration range from 0 − 1.4% and the particle size from 6 − 8 µm into phosphor compounding, we can obtain better optical performance of white LEDs.Hence, the employment of Ca 2 MgSi 2 O 7 :Eu 2+ phosphor to white WLED packages is a promising solution for developing LED illumination technology in the future.