1. Introduction

Since high-power gallium nitride (GaN)-based blue LEDs were developed, phosphor-converted LEDs have become one of the most popular options for obtaining high efficiency commercial white LEDs [1–3]. It consists of a blue LED chip with high efficiency yellow phosphors, which are typically based on yttrium aluminum garnets and rare earth-doped nitrides/silicates [4,5]. Recently, the remote phosphor approach has been extensively investigated as a solution to prevent thermal degradation of the phosphor-embedded resin for white LEDs by separating the phosphor from the surface of high power emitter [6–8]. However, trapped loss of light in the air-gap from the emitter and total internal reflection (TIR) occurring within the remote structure are the primary problems to be overcome for effective light extraction [9,10]. As the demand of LED industry has been ever raised for chip-on-board multi-packaging module, endeavers to minimize total reflection loss have been reported in theoretical and practical aspects for the large area modules [11,12]. Surface roughening or texturing has been recognized as a desirable approach to minimize the TIR in the emitter and phosphor components [13–15]. As an example, our recent work introduced a random texturing process of the printed phosphor layer with a significant improvement of luminous efficacy by ~22.5% [16]. These approaches require additional chemical or physical processes for the surface modification of substrate although those are effective in improving the luminous efficiency.

Here, we introduce an effective method of improving luminous efficacy of the remote yellow phosphor layer-LED structure by applying a prismatic pattern for better light extraction. The prismatic pattern on a PET film was separately prepared using a simple roll-to-roll process utilizing the subsequent UV-curing step of acrylate-based polymer. The prepared prismatic film was applied to the printed remote phosphor player on a glass substrate. This method does not require additional chemical treatments of substrate for rougher surfaces. The remote phosphor layer was fabricated via screen-printing of a viscous paste consisting of silicate yellow phosphor and UV-curable polymer. Two different pitch-patterns are used here to observe the effect of each prismatic pattern on the luminous efficacy and color distribution of the resultant remote phosphor-LED structure. A substantial improvement of luminous efficacy by ~29% was obtained owing to the use of the prismatic pattern. The origin of the luminescence improvement is discussed in terms of light extraction and reflection at each interface with experimental evidences.

2. Experimental

For the remote phosphor layer, a commercially available yellow silicate phosphor based on (Ba,Sr,Ca)₂SiO₄:Eu²⁺ (PA556, Force4 CO., Korea: average particle size ~11.9 μm) was used. The phosphor powder was mixed with a UV-curable organic binder (Loctite 3321, Henkel Co., USA) using a paste mixer in a weight ratio of 70/30 of the phosphor/organic vehicle. The resultant paste was screen-printed on a soda-lime silicate glass substrate and subsequently UV-cured at room temperature for 10 min under a 500 W UV light in a UV furnace, resulting in ~50 μm thickness of the remote layer with double-printings.

Prismatic patterns with two pitch sizes were separately prepared using a roll-to-roll imprinting process as demonstrated in the schematic of Fig. 1(a). A commercial UV-curable acrylate polymer is supplied onto the PET film using a nozzle from one side of a conveyer belt and subsequently passes through a serrated wheel to provide a fixed prismatic pattern. The prismatic pattern became rigid after subsequent UV-curing process. Two prismatic patterns with different pitch sizes of ~30 μm and ~9 μm in height (with the length of each pitch being ~50 and 15 μm, respectively) were evaluated here for better light extraction. The final structure of the planar remote layer/glass substrate/prismatic film is illustrated in Fig. 1(b). The refractive index of UV-curable polymer, PET and phosphor layer was assumed as 1.51, 1.58 and 1.51, respectively. The light was illuminated from the remote phosphor side for luminescence measurement.

 

Fig. 1 (a) Schematic of the roll-to-roll preparation process of the UV-cured prismatic pattern on a PET substrate and (b) schematic of the remote phosphor structure of prismatic pattern/PET/glass substrate/phosphor layer.

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The different pitch patterns of ~30 μm and ~9 μm in height were designated as pattern 1 and pattern 2, respectively. The apex angle of prism was designed to have a fixed angle of 90°. Figure 2 shows the top and cross-sectional SEM images of the patterns 1 and 2, which were cured under the UV exposure. The prismatic patterns were attached properly to the PET substrate through the roll-to-roll curing process as observed in the cross-sectional images of both the cases. The prepared prismatic film was attached onto the other side of bare glass substrate surface.

 

Fig. 2 Surface and cross-sectional SEM image of the prismatic patterns having (a) 30 μm and (b) 9 μm-height pitch sizes,

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Surface and cross-sectional microstructures of the remote phosphor layer and the prismatic patterns were observed using a field-emission scanning electron microscope (FESEM: JSM-5410, JEOL, Japan). Emission spectra, luminous efficacy and commission Internationale de l'Eclairage (CIE) chromaticity coordinates of the resultant LED device were measured via an integrating sphere system (ISP 1000, Instrument Systems, Germany) using a 450 nm blue diode as an excitation source under a current of 200 mA.

3. Result and discussion

Figure 3(a, b) shows the cross-sectional SEM images of the prismatic patterns 1 and 2, which are located on the top of PET/glass substrate. Satisfactory adhesion can be observed without detectable macro-gaps at the interfaces between the PET and glass substrate. The other side of the glass substrate corresponds to the remote phosphor layer consisting of the yellow phosphor particles dispersed in the UV-cured polymer matrix as observed in the cross-sectional SEM image of Fig. 3(c). Properly dispersed phosphor particles are readily discernible across the phosphor layer of ~50 μm thickness. Our previous study demonstrated that this UV-curable phosphor system can be applied with exceptional ease for planar printable devices with high performance and long-term reliability [17]. The additional surface SEM image of Fig. 3(d) confirms the uniform distribution of the phosphor particles in the cured polymer matrix. It is especially interesting to note that this solid matrix forms within an exceedingly short time of ~10 min.

 

Fig. 3 Cross-sectional SEM images of prismatic PET films with (a) 30 μm and (b) 9 μm-height pitches, (c) cross-sectional SEM image of the remote silicate phosphor-dispersed layer on glass substrate, and (d) surface images of the dispersed phosphor particles in cured polymer matrix.

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Figure 4(a) demonstrates the emission spectra of the pattern 1 and pattern 2-applied phosphor samples, compared to the flat case, with a real image of white LED module. The yellow emission peak at ~580 nm was prominently improved with the prismatic patterns, suggesting that the prism structure is beneficial in producing stronger emissions. The relative radiant power P was calculated based on the curves of Fig. 4(a) by using the equation of P = ${\displaystyle {\int }_{380}^{780}S(\lambda )d\lambda }$, where P is the radiant power and S(λ) is the spectral power intensity at the wavelength λ. The calculated P values indicated the enhancements of radiant power by ~13 and ~16% by applying the patterns 1 and 2, respectively, which confirms the positive effect of the prismatic pattern.

 

Fig. 4 (a) Emission spectra of prismatic patterned phosphor samples on a blue LED chip, with an image of actual LED module, and (b) luminous efficacy of the remote phosphor-prismatic pattern samples with demonstrations of real white illuminations.

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Figure 4(b) shows the change in luminous efficacy values for the three different phosphor structures, relative to the reference case, with the real white-illuminating LED images. Compared with the reference luminous efficacy of ~71.0 lm/W, the adaptation of the prismatic pattern exhibited higher efficacy values of ~88.1 and ~91.6 lm/W for the patterns 1 and 2, respectively. It indicates that the prismatic patterns aid in producing better light extractions. The best efficiency value for the pattern 2 corresponds to the ~29% enhancement in the luminescent property with respect to the reference case with the prismatic pattern. It is evident that the light extraction efficiency can be enhanced via deducing the total internalreflection (TIR) with a relatively high possibility for light to escape. TIR is associated with the critical angle and the distribution of angles depending on surface condition, which determines the levels of light reflection and extraction in this phosphor system [18].

According to Snell’s law, θ_c = sin⁻¹ (n_air / n_glass) [19], the critical angle was 41° for the white light to escape from the flat surface of glass substrate, assuming that the refractive indices n_glass and n_air are 1.52 and 1.0, respectively [20]. Only those lights with angles of incidence less than 41° could escape from the surface. The light passing through the interface between the glass substrate and PET film is not much affected by TIR, because the refractive index of PET is slightly higher than that of the glass substrate [21]. The light is refracted towards the upper portion of the PET film. Figure 5 illustrates the schematic of the potential light paths over the prismatic pattern structure when the light comes from the bottom. At the prism structure with the apex angle of 90°, internally refracted light at the prism-air interface can escape from the other side of the prism. Except for the light that TIR toward the PET, all light rays can pass through prism structure and reach air with upward directions. With the array of the prism on top, a higher possibility for the light to escape with the effect of broadening the critical angle for TIR should exist [22]. Their escape probability is presumably enhanced by increasing the possibilities of light extraction with the direct escape of light through the vertical straight path [23]. Light may escape along the vertical paths without refractions at vertex of prism structure. Although a difference exists in luminous efficacy between the 9 and 30 μm-pitch patterns, it is likely that there is a relatively high possibility of light extraction with respect to the smaller pitch pattern having a larger number of prisms per unit area resulting in an enhancement of the luminescence. The smaller pitch pattern may provide more vertical escape paths for emissive lights [12].

 

Fig. 5 Illustration of potential light paths and enhanced light extractions over the prismatic pattern on the PET film.

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Figure 6 shows the CIE chromaticity coordinates of the LED samples depending on the surface condition. Regardless of the pattern of prismatic film, the CIE chromaticity coordinates appeared in the nearly identical white-light region close to x = 0.32 and y = 0.32. In addition, the correlated color temperature (CCT) was estimated to determine the suitability of the obtained color range. The calculated CCT value was ~6103 K, which is approximately equal to the true daylight CCT value of ~6500 K [24,25]. The color rendering index Ra of ~83.1 for the LED samples was reasonable when compared with the reported values of 80 to 85 for yellow phosphor-based devices [26].

 

Fig. 6 CIE color coordination diagrams of the remote phosphor-prismatic pattern white LED samples.

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We additionally did experiments to demonstrate the effects of layer thickness and relative content of phosphor power in the layer only for the cases of 9 μm pitch-pattern structure. Figure 7(a,b) shows emission spectra of the 9 μm pitch samples with the different number of printing up to 3 times and with the different content of phosphor up to 70 wt%. As expected, the intensities of the yellow emission increased with the thicker layer and the higher content of phosphor, while the intensities of the blue emission decreased. The dependence of the parameters on the luminous efficacy of the patterned structures is shown in Fig. 7(c). It suggests that the higher density of phosphors in the layer boosted the conversion of emitted blue light to the yellow emission with the overall reduction of blue emission. The luminous efficacy became the maximum value of 98.1 lm/W with 3 times-printing of 70 wt% phosphor paste. However, the maximum value accompanied a lowered color rendering index as seen in the inset of the Ra plot due to the decreased intensity of blue emission.

 

Fig. 7 Emission spectra of prismatic patterned phosphor structures with the variations in (a) the number of coatings (for the case of 70 wt% phosphor powder) and (b) the relative content of phosphor powder (for the case of twice printings), and (c) the variations of luminous efficacy as functions of the number of coating and phosphor content. The inset figure is the variation of Ra according to the number of coating for the 70 wt% phosphor sample. All the values were obtained for the 9 mm pitch-pattern structure.

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4. Conclusion

A polymer film having a prismatic pattern was successfully applied to enhance the luminous efficacy of the printed remote phosphor layer. The degree of enhancement depended on the pitch size of the prism structure. The prismatic pattern on the PET film was prepared using a serrated wheel that the UV-curable polymer passes through. Another UV-polymer paste was also used to fabricate a solid remote phosphor layer of ~50 μm thickness consisting of yellow phosphor particles dispersed in cured UV-polymer matrix. The resulting LED structure with the smaller pitch pattern integrated on the rear side of the yellow silicate phosphor exhibited an improved luminous efficacy by 29% compared with the reference flat surface. The smaller pitch pattern was assumed to have a more significant possibility for light to escape, resulting in a noteworthy improvement of the luminous efficacy. Although the 9 μm pitch-pattern demonstrated better performance than the 30 μm pattern here, there might be the optimal pitch size for the best performance.