Cite this article as:
Thi, M.H.N.,Thai, N.L., Bui, T.M., Ho, S.D., 2023. The Light Features and Bredigite Layout for Orthosilicate Phosphor in WLED Devices. International Journal of Technology. Volume 14(4), pp. 911-920
We created a green phosphor Ca14-xEuxMg2[SiO4]8
orCMS: Eu2+ to be utilized in WLED devices. The phosphor
offers a wide spectrum achieving the highest value of 505 nm when excited at
400 nm, as a result of a shift between the excited state of 4f 65d
and the ground state of 4f 7 in an ion of Eu2+.
The interactivity of dipole-dipole appeared to be a primary power shift for the
electrical multipolar nature of the phosphor. We acquired a critical distance
measured at 12.9as well as 14.9 via a critical Eu2+ concentration as well
as the Dexter hypothesis on power shift. Via an encapsulant, we combined CMS:Eu2+
as well as a phosphor in red with a LED device having a value of 395 nm and managed to acquire white
illumination having a CRI value measured at 91 at a 20-milliampere forward bias
current. In addition, we also examined the layout as well as the light features
in CMS:Eu2+.
Color homogeneity; Double-layer phosphor; Luminous flux; Monte Carlo theory; WLEDs
Introduction
WLED devices have become increasingly common due to
their many advantages which include small power drain, high durability, high
performance, etc (Alexeev et al., 2021; Chang et al., 2021; Bindai, Annapurna, and Tarafder,2019). On the other hand, these devices still have
certain limitations that prevent them from becoming standard optical devices.
It is possible to create white illumination via daubing yellow phosphor on blue
chips of LED. Such a procedure may yield a significant lumen exceeding 30 lm/W.
On the other hand, it results in low CRI below 65, which is caused by a faint
red discharge (Huu and Thi, 2022; Hakim et al., 2021). It is worth noting that
the majority of phosphors do not possess an absorption line between 440 nm and
460 nm. This downside is not present in the yellow phosphor (Chen et al., 2020). Therefore, the industry has managed to a novel
WLED form that offers desirable features of hue generation. Many novel
phosphors have been created for WLED, such as oxide, oxyfluoride, nitride,
oxynitride host latticework, etc (Hsin et al., 2021;
Jain et al., 2020) subjected to non-ultraviolet light. While these
phosphors have proved to be promising, we would still need further research for
the task of eliminating certain problems to achieve strong performance,
superior heat consistency, as well as improvement in hue features.
When it comes to WLED devices, The best options would be alkaline-earth
silicate phosphors as they offer great performance as well as heat and chemical
reliability. Some mixtures were acknowledged and proposed, which have a
universal structure of Sr2SiO4 that crystallizes within
the tetragonal space group Pmnb (S. G. No. 62). With this taken into account,
our investigation focused on Ca14Mg2[SiO4]8
(CMS, S. G. No. 34, Pnn2) - a bregidite mineral. The earliest examination
concerning the powder X-ray data for CMS on the basis of a novel pinwheel
setting was done by Araki and his colleagues. According to Sun et al. (2022), along with Bredigite appeared to be distinctive
phases. Li et al. (2020) claimed that appeared to be based on K2[SO4].
The space group pedigree of bredigite results in Pmnn Pnn2, caused by the tetrahedral’s leaning. This would
decrease the symmetrical mechanic to Pnn2 (Ma et al., 2021; Qin, Shi, and Leon,2020).
Our investigation focuses on
the layout as well as light features of the phosphor CMS:Eu2+.
Specifically, the investigation concerns the power shift of Eu2+
with critical concentration as well as Dexter's hypothesis on power shift. We
constructed a WLED device by creating a merger between an InGaN chip of LED
with value of 395 nm and CMS:Eu2+.
Furthermore, we also examined the heat abatement for the luminescence and
compared the mechanism in the phosphor Sr2SiO4.
Experimental Methods
Computational
Simulation
2.1. Preparation
of green-emitting Ca14-xEuxMg2[SiO4]8(CMS:Eu2+) phosphor
Table 1 The constituents and
preparation of CMS:Eu2+
Table 2 The characteristics of
CMS:Eu2+
We created the LED device
via daubing an InGaN LED with a compound containing CMS:Eu2+, a
phosphor in red and translucent resin. The electroluminescence was determined
by putting a separate LED constructed above m-plane GaN on silver
headers linked with golden wires to work under electricity. Afterward, we
covered the LED with a compound of phosphor and silicone. The compound was put
above the headers and cured. Once the preparation was finished, we set the LED
for assessment within a sphere with DC bias forward states. The unit cell for CMS appears to be a structured mixture of Ca:Mg that
possesses a desirable polyhedral layout of X[12]X2[9]Y[10]M[6][TO4]4
with X and Y being the huge polyhedral, M being the
octahedron with T being tetrahedron. The desirable upper
restriction for Mg presence in bredigite would be Ca14Mg2[SiO4]8.
For the layout of bredigite, the MgO6 octahedra bond through the
tetrahedra to create bindings traveling parallel to the axis of For the layout,
Ca would settle in the locations of 2a and 4c, Mg would settle
in the location of 2b, while Si and O would settle in the location
of 4c. The eight locations of Ca would be related to polyhedral having 9-,
10-, and 12- coordination. These locations would be replaced with the ions of
Eu. If the ions merge with the CMS crystal layout, they could replace every
cation location which includes Ca2+, Mg2+, and Si4+.
On the other hand, with the ion radius and the permitted value of oxygen
coordination (n) taken into account, Ca2+ (1.12 n
value of 8), Mg2+ (0.72n
value of 6) and Si4+ (0.26n value of 4), replacing
the ions of Mg2+ as well as Si4+ with those of Eu2+
(1.25 n value of 8) would be a challenging task (Shadalou,
Cassarly, and Suleski, 2021; Shi et al., 2021).
Figure 1 Photograph of WLEDs
2.2. Spectra
optimization of the CCT
Figures 2(b) and 2(c)
demonstrate the correlation between the apex location, intensity of discharge,
and replacement of Eu2+. As the concentration of Eu2+
rises, the discharge line with value of 400 nm will move
towards a greater wavelength, which is the result of the repeating absorbing
activity among the ions of Eu2+ instead of the alterations within
the crystal field surrounding Eu2+ which can be expressed via the following equation (Shih
et al., 2020):
where Dq represents the crystal
field of the octahedral symmetrical mechanism. R represents the range
separating the center ion from its ligands. Z represents the anion’s
charge or valence. e
represents the electron’s charge. r
represents the d
wavefunction’s radius. Therefore, the cleavage of the crystal field would not
be the primary cause of the red shift because of the ion radius for Ca2+
with 8- coordinate being inferior to Eu2+ (1.12 and 1.25). It is clear in Figure 2(c) that the optimal replacement for Eu2+from CMS:Eu2+ would be an x value of 0.3. If this value rises, the relative intensity of discharge
will occur, caused by the abatement of concentration.It is possible to assess the critical range
of power shift or Rc for CMS:Eu2+ via the layout data
containing the unit cell’s volume (V), the quantity of locations of Eu2+ for
each cell (N), as well as the critical concentration (Xc).
where Rc
represents the mean splitting among the closest ions of Eu2+under Xc.
With a V value of 1356 3 , an N
value of 8, and an Xc value of 0.3, the critical shift range
for Eu2+ would be roughly 12.9.
Figure 2 (a) Discharge and emission spectra in CMS:Eu2+ when excited
at 400 nm under different concentrations of Eu2+ (x). (b) The location for
the maximal discharge. (c) The relative intensity of discharge along with
replacement of Eu2+ (x)
The
Dexter expression was utilized to determine the shift for the electrical
dipole-dipole interactivity as the symmetrical permitted shifts of Eu2+
are involved (Singh
et al., 2021). The critical range is calculated based on Blasse
expression:
where P
represents oscillator potency for the ion of Eu2+. E
represents the energy for the maximal spectrum overlay. represents
the outcome of normalized spectrum forms for discharge as well as excitation.
From Figure 2, it is possible to acquire E as well as via
the spectrum information. In the case of P related to the wide absorbing line
of Based on the
hypothesis of Dexter, we ought to manage the power shift activity via electric
multipole-multipole interactivity (Tian et al., 2020). The following
formula will determine the intensity of discharge or I for each trigger
ion (Tuyet
et al., 2020):
where C
represents trigger concentration that abates itself. k and are constants corresponding to
interactivity within an identical excitation state of a host’s latticework. If
C >> Xc, the non-radioactive depletions could be caused
by a multipolar shift. It is possible to alter the formula (5) (Wang et al.,
2020):
where k1
represents a constant. Thevalues
of 6, 8 and 10 corresponds to dipole-dipole, dipole-quadrupole and
quadruple-quadruple. It is possible to assess via
the slopfrom
the linear dash demonstrated by Figure 3. The Figure puts log(I/C) and log(C)
on a logarithmic scale. The result forwould
reach -2.16, and as such, assumes a value of roughly 6. Such an
outcome points out the dipole-dipole interactivity would be the concentration
abatement activity for the discharge of Eu2+. From Figure 4, we can
see the diffuse reflectance absorption spectra of CMS:Eu2+. It is
possible to determine the Kubelka-Munk absorption coefficient (K/S) via
a correlation: the absorbance (A) for CMS as well as CMS:Eu2+
can be assessed with the reflectances (R) via an expression (Yu et al.,
2021):
Using the shown
expression, the line gap power or Eg for CMS:Eu2+
assumed a value of 2.56 eV. From this result, it might be impossible for the
excitation power for n-UV (400 nm, 45000 cm-1) to shift between the
valence line and conduction band. CMS does not display any absorption at below
380 nm and became white as a result. Hence, it is possible to excite the
electrons of the 4f7 states in Eu2+ via n-UV
illumination within the 4f65d
state. We determined the quantum performance via a 400-nm excitation cause. The
performance reached roughly 23% under room temperature, lower than the 85%
result of Sr2SiO4:Eu2+.
Figure
3 The
logarithm for the intensity of discharge for each trigger ion (log I/CEu) along
with the logarithm for the concentration of Eu2+ (log CEu) from
CMS:Eu2+ with value
of 400 nm
Figure
4 The
diffuse reflectance spectra of (a) CMS and (b) CMS:Eu2+ when excited
at 400 nm
Between room temperature and 150oC, the intensities of
photoluminescence in CMS:Eu2+ and Sr2SiO4:Eu2+
suffered a penalty of 11% and 18%, which is demonstrated by Figure 5(a). If the
task is to examine the heat abatement features, we need to determine the
trigger power via Arrhenius expression (Zhang et al., 2020; Yuce
et al., 2019):
represents the first
intensity. I(T)
indicates the intensity under a specific (T) temperature. A represents a constant. E represents the trigger
power of heat abatement. k represents
the constant of Boltzmann. In Figure 5(b), we can see ln and 1/(kT). Through utilizing the Arrhenius
expression, the values of trigger power (E) for CMS:Eu2+ and Sr2SiO4:Eu2+
appeared to be 0.348 eV and 0.158 eV.
Figure 5 (a) The intensities of
discharge reliant on temperature. (b) The heat abatement in Sr2SiO4:Eu2+
as well as CMS:Eu2+ when Arrhenius expression is utilized
Figure 6 Sustaining the
median CCT level via the modification of concentration: (a) 3000 K-5000 K; (b)
6000 K-8000 K
With
the use of the software LightTools 9.0 and the Monte Carlo approach, the flat
phosphor sheet for a multi-chip WLED (MCW-LED) was reproduced (Zhang et al.,
2021). The recreation procedure would have two main phases. First, assessing
as well as constructing the layout samples along with optical characteristics
in MCW-LED lights. Second, managing the phosphor mixture and the light
influences through many CMS:Eu2+
doping amount percentages. For the assessment of YAG:Ce3+ and CMS:Eu2+ compound’s
effects on WLED lights’ performance, generating variants would be compulsory.
Examining two kinds of compounds under the median CCT values of 3000 K, 4000 K,
and 5000 K, a remote phosphor layout with two sheets would be necessary. Via
Figure 1, the modeling illustration of the MCW-LED with a conformal phosphor
structure and a great 8500 K CCT value is presented. It also reveals that the
initial MCW-LEDs’ recreation does not involve CMS:Eu2+. The WLED components’ dimensions can be
listed as follows. A reflector with 2.07×8×9.85 mm for height and bottom- and
top-surface length, respectively; a 0.08 mm thick film of conformal phosphor
compounding would be daubed on the chips; a set of nine chips with a size of
1.14×0.15 mm (length × height) and radiant-flux value of 1.16 W, and 453 nm
peak wavelength for each is attached to LED reflector’s gap.
Results and Discussion
Figure 6 depicts the relation between the utilizing concentration of CMS:Eu2+ and YAG:Ce3+.
In particular, it shows the inverse data trend between these two phosphor
amounts: the increase (2-20%) of CMS:Eu2+
content leads to the decrease ofYAG:Ce3+
content, at both 4000 K and 8000 K. This opposite serves as a median to sustain
the CCT value and cause changes to either absorptivity or dispersion of light
in the WLED model. As a result, we can manage to achieve the desired results
for WLED illumination by modifying the CMS:Eu2+
concentration.
Figure 7 The emission spectra in WLED devices and the CMS:Eu2+ concentrations: (a) 4000 K; (b) 8000 K
Figure 7 illustrates how the
concentration of green phosphor CMS:Eu2+influence the emission power of the WLED at 4000 K and 8000 K, from which the producer can determine suitable CMS:Eu2+ concentration to
apply to their LED products. If they want to get high and strong luminescence
for backlight application, for example, they can allow a minor deduction in the
chromatic features. As depicted in both Figures 7(a) and 7(b), two critical
spectral regions for white light generation, 420-480 nm, and 500-640 nm are
improved with increasing CMS:Eu2+
concentration, suggesting the enhancement in the luminescent output of the WLED
model. Additionally, the addition of green phosphor
seems to stimulate the emission and dispersion of blue light, which will
contribute to increasing the possibility of blue-light conversion and
extraction. Thus, the active dispersion and color consistency could be
improved. This result is crucial and essential for the application of CMS:Eu2+ phosphor to serve
the enhancement of WLED-lighting performance, especially at high CCT like 8000
K.
The discussed states can be further
demonstrated with the data in Figures 8 and 9. The luminous intensity of the
white light from the WLED in connection with CMS:Eu2+ concentration is shown in Figure 8. When the
concentration of CMS:Eu2+
grows in the range of 2-25%, a notable rise is recorded at 5000 K – 8000 K. In
Figure 9, the decreases in deviating CCT proportions decline as CMS:Eu2+ content becomes
higher, suggesting the enhancement in CCT consistency. The green phosphor
scattering and absorbing activities could be applied to explain this
phenomenon. The green phosphor particles will absorb the blue illumination more
significantly than the yellow illumination, as the blue emission wavelength is
shorter than the yellow one. This allows greener illuminations to be generated
by converting these absorbed illuminations. In other words, the green spectral
component is increased as the concentration ofCMS:Eu2+ is higher,
contributing to compensating the green-light energy shortage to result in
better light color consistency and luminosity.
Figure 8 The lumen in the WLED devices with respective CMS:Eu2+ concentrations:
(a) 3000 K-5000 K; (b) 6000 K-8000 K
Figure 9 The hue aberration in the WLED device along with CMS:Eu2+ concentrations: (a) 3000 K-5000 K; (b) 6000
K-8000 K
Though
color uniformity is a vital and essential factor for high white-light color
quality, the color rendering intent and reproduction efficiency should not be
ignored. Thus, the CRI (color rendering index) and CQS (color quality scale)
are subsequently examined and discussed. Figures 10 and 11 present the data of
CRI and CQS, respectively. As shown, CRI values decline gradually with the
surge of CMS:Eu2+content
percentage. Meanwhile, the CQS displays an increase with the rising
concentration of CMS:Eu2+within 2-10%. More than 10% CMS:Eu2+
obviously causes the CQS to degrade. Such events can be attributed to
green-energy domination, which means the green light is generated excessively,
causing the lack of other critical emission colors (blue and yellow).
Therefore, the balance and homogeneity of white-light emission colors are
damaged, leading to the mentioned degradation in CRI and CQS. However, the rise
and stability in CQS with <10% CMS:Eu2+ is an important result
because the CQS is more complex and difficult to control than the CRI.
Specifically, managing CQS means managing to satisfy the three critical
factors: color coordinates, CRI, and the observer’s visual inclination. So, we
should consider the optical goals to determine the suitable concentration of
CMS:Eu2+.