Phosphor conversion for WLEDs: YBO₃: Ce³⁺, Tb³⁺ and its effects on the luminous intensity and chromatic properties of dual-layer WLED model

The use of phosphor to convert the light from emitted photons for white light-emitting diodes has been considered as a simpler fabricating technique as it probably reduces the electronic complexity, compared to the WLED combing three red-, blue-, and green-emitting LEDs [1, 2]. Thus, the urge in finding suitable phosphor materials to combine or replace the commercially used YAG:Ce³⁺ phosphor has been immersed. The yellow phosphor YAG:Ce³⁺ though can present good luminous flux yet the shortage of red spectral energy is problematic for WLEDs require high accuracy in color reproduction [3,4,5]. The phosphor materials that could be most suitable for high conversion efficiency in the blue wavelength of the blue-emitting LED chip should possess good excitability and absorption strength. Research articles reported the ability of sulfide phosphors (ZnS:Cu⁺,Al³⁺ or SrGa₂S₄:Eu²⁺) for the conversion efficiency from near UV wavelength to green wavelength, yet the problem is their unsuitable chemical stability for practical applications [6,7,8,9].

Yttrium borate (YBO₃) phosphors are recognized for their flexible framework arrangement that greatly influences the luminescence characteristics, while YBO₃ with rare-earth dopants, i.e. YBO₃:Tb³⁺, is extensively studied due to its strong intensity of photoluminescence, high transparency in the wavelength bands of from near UV to visible ones, and good thermal and chemical stability [10, 11]. However, the rare-earth dopants with 4f – 4f narrow line emitter (i.e. Tb³⁺ and Eu³⁺) seem to be inappropriate as their excitation band is not broad enough to match the wavelength of the pumping LED (near UV and blue spectrum regions). The solution is co-doping these 4f – 4f emitters with a suitable and efficient sensitizer to extend their excitation spectral region. Cerium ion (Ce³⁺) is reported to be a good sensitizer for the luminescence of Tb³⁺ rare earth dopant [12, 13]. Besides, the emission peak of Tb³⁺ is compatible with the green light that is required to generate white light with intense luminous flux and moderate color rendering level. Therefore, co-doping the ions Ce³⁺ and Tb³⁺ into the YBO₃ host could take advantage of the broad emission of Ce³⁺ and essential green emission peak of Tb³⁺ to achieve the desirable luminescence intensity and color creation of WLEDs. For this reason, in this study, we use YBO₃: Ce³⁺, Tb³⁺ for WLED models to provide a broad and sharp green emission for the improvement in white light luminous efficiency and color quality. Besides, mentioned reports barely analyzed and discussed the influences of YBO₃: Ce³⁺, Tb³⁺ phosphor on the luminosity and color rendition of a dual-layer remote phosphor configuration. Thus, our study uses the WLEDs constructed with two-layer remote phosphor geometry in the analysis of YBO₃: Ce³⁺, Tb³⁺ phosphor influences on the white light performances of WLEDs.

The green-emitting phosphor YBO₃: Ce³⁺, Tb³⁺ photoluminescence properties are investigated utilizing the computations of energy transfer between the two ions Ce³⁺ and Tb³⁺ [14]. The results demonstrate that the phosphor shows nonradiative Ce³⁺ → Tb³⁺ energy transfer. The broad excitation spectrum following the strong absorption band that overlaps the wavelength of near-UV excitation source is observed with concentrations of Ce³⁺ and Tb³⁺ of 3% and 15%, respectively. Then, the optical influences of YBO₃: Ce³⁺, Tb³⁺ phosphor on the WLED's lumen output and chromaticity are monitored with the adjustment of the green phosphor concentration. The findings reveal the potential of YBO₃: Ce³⁺, Tb³⁺ phosphor in acquiring greater-efficiency WLED packages.

The concentration of yellow phosphor YAG:Ce³⁺ reduces as a function of increasing concentration of the green phosphor YBO₃:Ce³⁺,Tb³⁺ layer. The lower YAG:Ce³⁺ concentration is more noticeable in the case of higher CCTs (4000 K – 5000 K) of the WLED with dual-layer phosphor remote configuration, as shown in Figure 1. The decreasing concentration of YAG:Ce³⁺ phosphor is essential for the consistency of WLED's preset CCT values when doping higher amount of green phosphor YBO₃:Ce³⁺,Tb³⁺ in the phosphor package. Moreover, the lower yellow phosphor concentration could stimulate scattering ability of the phosphor layers and result in significant changes in the optical output.

Fig. 1

Consequently, the change in YBO₃: Ce³⁺, Tb³⁺ concentration influences the total emitted power of the WLED package, which is illustrated in Figure 2. The difference in the sum power of two YBO₃: Ce³⁺, Tb³⁺ concentrations (5% and 10%) is obvious at 3000 K, while that figure in the case of 4000 K and 5000 K can barely see. Besides, at 5000 K, the increasing intensities in both blue and green regions are easy to observe. This means the luminous flux at each correlated color temperature (CCT) is distinct. Generally, the green emission of the phosphor package is improved with the addition of YBO₃: Ce³⁺, Tb³⁺, at all CCTs. However, 5% YBO₃: Ce³⁺, Tb³⁺ shows a more remarkable enhancement of the spectra intensity than 10% YBO₃: Ce³⁺, Tb³⁺, at 3000 K. Meanwhile, the greater blue emission in the case of 5000 K indicates the enhancement in blue-light conversion and scattering efficiency, besides the green light one. That the blue scattering is enhanced, the higher color consistency can be attained since the larger proportion of the blue light escaping from the LED could be blended with the yellow one to form white light, or to reduce the yellow ring on the illuminated surfaces.

Fig. 2

Regarding the total power in Figure 2(a), the luminous flux of the WLED with CCT of 3000 K could be predicted. Figure 3(a) shows that the luminous intensity of 3000-K WLED is higher with 5% concentration of YBO₃: Ce³⁺, Tb³⁺, with a luminescence peak at about 90 lm. At 4000 K, the difference is not really considerable between the two concentration values but the higher concentration of the green phosphor generally results in greater luminous output. Meanwhile, the luminous enhancement at higher concentration of YBO₃: Ce³⁺, Tb³⁺ is apparently demonstrated in the case of 5000 K CCT, especially with larger particle sizes (≥ 5 μm), see Figure 3(c). With 10% YBO₃: Ce³⁺, Tb³⁺, the highest luminous flux can be obtained at approximate 175 lm.

Fig. 3

The consistency of chromaticity can be assessed with the color difference at a specific CCT. The lower the CCT difference, the better the color uniformity of the WLED source. Similar to the trends of the sum power and the luminous flux, the CCT variation at 3000 K with 5% YBO₃: Ce³⁺, Tb³⁺ is lower than with 10% YBO₃: Ce³⁺, Tb³⁺, as shown in Figure 4(a). At 4000 K and 5000 K, the higher concentration of the green phosphor (10%) exhibits smaller variation of CCT, see Figure 4(b) and (c).

Fig. 4

The similar changes in both luminous flux and CCT variation at three different CCTs can be attributed to the reduction of yellow phosphor and performance of sum power. Referring to Figure 1 and 2, at 3000 K, the difference in the reduction of yellow phosphor concentration is negligible between two concentrations of the green phosphor. Meanwhile, the total power is stronger with 5% of green phosphor concentration. The more intense the total power, the higher the luminous output and the conversion efficiency. These enhancements probably resulted from the lower proportion of light loss and higher proportion of the light straightly extracted from the LED because of enhanced scattering. According to this, the better luminous flux and CCT consistency with 10% concentration of YBO₃: Ce³⁺, Tb³⁺ in the cases of 4000 K and 5000 K are understandable.

The chromatic consistency is important factor in evaluating the color quality of the WLED structure. Besides, the accuracy of color reproduction is another critical factor of color assessment for white light of LEDs. The color reproduction accuracy could be performed using color rendering index CRI and color quality scale CQS. Recently, the CRI has been unfavorable for evaluating the fidelity of a light source since it is reported to give high value even in the case of bluish or reddish lights, and is not sufficient to assess the color reproduction of narrowband light sources. Thus, the CQS is introduced as an alternative to the CRI one. The CQS not only accesses the CRI aspects, but also examines the color coordinate and human factor – the comfort of eyes when interacting with the light source [25, 26]. Moreover, the color sample used for accuracy evaluation of CQS is increased to 15, not 8 samples as of the CRI. This means the chromatic reproduction is performed with smaller color gamut, leading to the higher accuracy.

Figure 5 and 6 illustrate the CRI and CQS values in relation with concentration of YBO₃: Ce³⁺, Tb³⁺ green phosphor, respectively. The increase in green phosphor concentration is obviously disadvantageous to both CRI and CQS, regardless the CCT values. In general, the concentration of 5% results in the moderate CRI and CQS values, about 60 – 63, while that of 10% causes the CRI and CQS to decrease. This might be the downside of scattering enhancement. Particularly, the more the green phosphor scatters lights, the more the green-light components are generated. Thus, the green light is over the blue and yellow lights, which could significantly affect the accuracy of chromatic reproduction.

Fig. 5

Fig. 6

This work focuses on using YBO₃: Ce³⁺, Tb³⁺ green-emitting phosphor to enhance the lighting properties of dual-layer WLED structure. Through analyzing the impacts of YBO₃: Ce³⁺, Tb³⁺ phosphor on critical optical properties of WLEDs at three different CCTs, the phosphor is realized as a potential phosphor-conversion material for achieving the wavelength conversion efficiency in the near-UV or blue region. The great absorption strength followed by the broad emission spectrum from the 4f – 5d and 5d – 4f transitions of Ce³⁺ and the 4f – 4f transition of Tb³⁺, respectively. The presence of the phosphor results in the enhancement of the luminescence and color consistency, owing to the improved scattering and light conversion effectiveness. The influences of CCT values and green phosphor concentration on the spectra power and color consistency are also observed. at 3000 K, 5% YBO₃: Ce³⁺, Tb³⁺ resulted in higher values of these two factors, and vice versa at the other CCTs. The CRI and CQS, on the other hand, show decreased values with increasing concentration of YBO₃: Ce³⁺, Tb³⁺ due to the excessive proportion of green lights, a drawback of too much scattering enhancement. The phosphor is potential to the application of high-luminescence WLED with moderate CRI and CQS of around 60 – 63, yet the recommendation is modifying the concentration of YBO₃: Ce³⁺, Tb³⁺ green phosphor based on the determined CCT of the WLED. Though the CRI and CQS are not equal to the adequate level of 70 in this work, the optical enhancement made by YBO₃: Ce³⁺, Tb³⁺ addition is clarified. Moreover, the photoluminescence properties of YBO₃: Ce³⁺, Tb³⁺ are suitable with n-UV and blue-pumping LED chips. Thus, further research on YBO₃: Ce³⁺, Tb³⁺ utilization for specific WLED devices should be carried out in the future.