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In this study, we demonstrated a reliable technique combination a little phosphor and composite silica photonic crystals (c-SPhCs) for developing the candlelight light-emitting diodes (LEDs). We used a UV adhesive curing method for improving the adhesion properties of SPhCs. The warm-white LEDs with c-SPhCs exhibit a correlated color temperature of 2089 K, a color-rendering index of 80, and a luminous flux of 34.5 lm (5.4 times that of a candle). The LEDs were subjected to a reliability analysis (RA) test, applying a high temperature and high relative humidity (85 °C/85 RH) during operation current at 120 mA. During a RA test of 3300 h, no visible degradation in optical performance has been observed. We implemented a reliable and inexpensive technique for producing the residential lighting source.
© 2015 Optical Society of America
1. Introduction
Recently, white light-emitting diode (WLED) lighting is booming, and LED industry has turned into maturity stage. WLEDs have been rapidly developed and are currently rivalling traditional light sources as the predominant choice of lighting for new lighting installations [1]. Because of their multiple advantages of the high efficiency, high reliability, long lifetime; WLEDs consequently offer the lighting industry a wide range of benefits and opportunities [2]. To date, commercially available WLEDs generally involve using the phosphor-converted WLEDs (PC-WLEDs), which is the most efficient method, because PC-WLEDs provide high luminous efficiency at low cost [3,4]. However, to achieve the correlated color temperature (CCT; Tcp) of candlelight LED at approximately 2000 K, a large amount of red-emitting phosphor, such as nitride-based phosphor, is adopted in package process to achieve the high color-rendering index (CRI; Ra) [5,6]. Currently, all nitride-based red-emitting phosphors are typically 10 times more expensive than are YAG-based yellow-emitting phosphors, a phenomenon that substantially contribute to the high cost of manufacturing candlelight LEDs. Reducing the cost of candlelight LEDs in the worldwide lighting market, we try hard to develop the technology going to high value and creative future.
Previously, the luminescence emission spectrum of warm WLEDs (w-WLEDs) has been modified using polystyrene (PS) colloidal photonic crystals (PhCs) to resemble candlelight LEDs [5,6]. Unfortunately, the PS material has a glass transition temperature (Tg) of approximately 90 °C, causing the WLEDs with composite PS PhC to be unreliable in high temperature environment [7]. To resolve the aforementioned problems, a silica nanosphere was used to solve the Tg and a UV adhesive curing method was applied to enhance the adhesion properties of the SPhCs. In this study, we deposited w-WLEDs with composite silica PhC (c-SPhC) thin films to fabricate low-cost candlelight LEDs. The luminescence spectrum of w-WLEDs containing c-SPhCs was modified to resemble candlelight by using photonic bandgap (PBG) of c-SPhCs. Thus, the w-WLED CCT of approximately 3000 K decreased to the candlelight CCT of approximately 2000 K without increasing the weight percentage (wt%) of the phosphor layer. In addition, a method to test the reliability of the candlelight LEDs was conducted and applied in this study.
2. Experimental
Commercial w-WLEDs that produced a CCT of approximately 3000 K and a CRI of approximately 83 were fabricated by Lextar Electronics Corp. in Taiwan. In the current study, the w-WLEDs fabricated as follows: First, a 35 × 22 mil2 chip of GaN-based patterned sapphire substrate (PSS) LEDs (emission wavelength, 455 nm) was bonded to a commercial 5630-type plastic-leaded chip carrier (PLCC) and packaged using silver paste and gold wire. The GaN PSS LED structures used in this study were identical to those described previously [5]. Subsequently, a green phosphor (Lu3Al5O12:Ce3+, LuAG) and a red phosphor (CaAlSiN3:Eu2+, CASN) of various wt% concentrations were used to obtain the w-WLEDs. The w-WLEDs had a LuAG and CASN phosphor concentration of 23.1 wt% and 4.5 wt%, respectively. They were uniformly mixed with silicone and then poured into the lead frame by using traditional phosphor-dispensing techniques.
Then, we prepared the latex (100.0 mg/mL) by using a silica nanosphere with a diameter (D) approximately 220 nm. Silica nanospheres were fabricated by modified Stober method [10]. Monodispersed silica nanospheres were synthesized using ammonium hydroxide (NH4OH, 28%; SHOWA), the deionized (DI) water, and the tetraethoxysilane (TEOS, 98%; Aldrich) condensation was controlled in anhydrous ethanol solution [8,9]. In this synthesis process, 230 mL of anhydrous ethanol, 12 mL of NH4OH, and 10 mL of DI water were added to a 1000 mL three-necked flask, which was placed in a water bath at 35 °C. Subsequently, 10 mL of TEOS was dropped to the mixture while stirring at 200 rpm. After stirring for 6 h, monodispersed silica nanospheres with an average diameter of 220 nm were obtained. By varying the amount of TEOS, silica colloidal nanospheres with different sizes were synthesized with the same method. Next, the SPhCs (10.0 μL) were deposited onto the entire emission region of the w-WLEDs by employing a micro-pipette and a vertical deposition method. Furthermore, the interstitial structure of the SPhCs caused the colloidal nanospheres to adhere ineffectively. Therefore, to enable the SPhCs to be reliable in high relative humidity (RH) conditions, a photocurable resin of a UV adhesive solution was used as a dispersion medium, enhancing the linkage among the colloidal silica nanospheres [11]. A simple method was developed using the UV adhesive solution to prepare c-SPhCs that exhibited high optical transparency and physical rigidity. The crosslinked UV adhesive enhanced the interactions among the silica nanospheres in the SPhC thin film, thereby improving the adhesion. We used an optimal volume of 2.0 μL for the UV adhesive solution, which infiltrated the interstitial spaces in the SPhC thin films; the UV adhesive solution with a concentration of 16.7 wt% was prepared by mixing 1.0 mg of commercial UV adhesive (KA-700L; Hitichi Chemical co.) and 5.0 mL of an acetone (ACE) solution. After the UV adhesive solution had infiltrated the SPhC thin film and entered the gaps under the capillary forces (see Media 1), and then we exposed the sample to UV light for 10 min to ensure that the photopolymerization of the UV adhesive was complete. Figures 1(a) and 1(b) show the optical microscopy image, respectively, without and with the c-SPhC thin film deposited onto the 5630-type PLCC-packaged w-WLED. They were exhibited under natural lighting conditions, revealing the blue reflection light of c-SPhCs (also shown in Figs. 2(a) and 2(b)), and pink color of w-WLED containing c-SPhCs. We used the field-emission scanning electron microscopy (FESEM) to investigate the crystalline structure of the c-SPhCs on the w-WLEDs. Figures 1(c) show images of the outer (111) face of the c-SPhCs with face-centered cubic (fcc) structures, and illustrates that the UV adhesive solution infiltrated the interstices of the silica nanospheres. Due to the interstices of SPhCs were not fill with the UV adhesive solution, that caused the low refractive indices of the total interstices. This simple approach improved the adhesion of c-SPhCs, thereby enhancing their practical application. In addition, the average 4.5-μm-thick of c-SPhC thin film was prepared on the w-WLED surface, which obtained the CRI of approximately 80 and CCT of approximately 2000 K.
Fig. 1 (a) and (b) Optical microscopy image without and with the c-SPhCs deposited onto the 5630-type PLCC-packaged w-WLEDs, respectively. (c) FESEM images of the c-SPhCs prepared using silica nanospheres with a diameter of 220 nm and display a well-organized (111) plane of fcc structures. Inset: the fcc Brillouin zone with symmetry points.
Fig. 2 (a) and (b) Pictures of before and after photocrosslinking was performed using a UV adhesive solution, respectively. (c) The peak positions of the reflection spectra measured from the SPhCs and c-SPhCs are 484 and 497 nm, respectively.
The c-SPhCs feature the PBG properties of the 3D PhCs [12]. The Bragg reflection wavelength (λR) of c-SPhCs can be calculated according to the Bragg’s law (1) [13]:
where λR is the wavelength of the reflection peak, d111 is the interplanar spacing between the (111) planes, neff is the effective refractive index of the crystalline lattice, and θ111 is the incident angle into the (111) planes. Regarding the c-SPhC structure, in this study, neff was obtained using (2):where nSilica = 1.45, nUV = 1.49, and nIS = 1.17 are the refractive indices of the silica nanospheres, the UV adhesive solution, and the interstices of the silica nanospheres, respectively; fSilica = 0.74 is the volume fraction of silica nanosphere. The reflectance measurement system was equipped with a Y-type optical fiber, a Xenon lamp, and a monchromator, that was used to measure the reflection wavelength of the c-SPhCs. The normal reflectance spectrum (θ111 = 0°) for the SPhCs (Fig. 2(a)) and c-SPhCs (Fig. 2(b)) exhibited a reflection peak at 484 nm and 497 nm, respectively, that adhered to the Bragg’s law, as shown in Fig. 2(c). The PBG position was slightly red shifted compared with that of the original SPhCs when the UV adhesive solution had infiltrated the thin film, which was monitored based on the nIS higher to nair [6].3. Results and discussion
The measured luminescence spectra of the w-WLEDs without and with c-SPhC thin films (candlelight LEDs) were measured using a 20-in integration sphere with a current setting of 120 mA, as displayed in Fig. 3(a).The luminescence spectrum of candlelight LEDs modified when a high-quality c-SPhC was self-assembled on the w-WLEDs. Due to the optical properties of c-SPhCs exhibit a lowest-order PBG at the L symmetry point of the fcc Brillouin zone in the [111] direction [8–10]. Figure 3(a) shows the blue light emission wavelengths of the candlelight LEDs, which were suppressed by the c-SPhC thin films, because the PBGs caused the blue light to diffract back into the phosphor layer, which excites the red-emitting phosphor again [5,6], thereby affecting the luminescence spectrum of candlelight LEDs. The reflected light was incidental to the package surfaces, indicating a conversion of reflected light into the phosphor layer of the w-WLEDs. Candlelight LEDs constantly require adequate red light; therefore, recycling as much red light as possible before it is absorbed is essential to improving the luminous efficacy.
Fig. 3 The w-WLED and candlelight LED driven at currents from 10 to 150 mA. (a) Luminescence spectra (at 120 mA), (b) Luminous flux.
In this experiment, we measured the luminous flux, luminous efficacy, CRI, CCT, Duv in relation to current characteristics by using the 20-in integration sphere (Labsphere) that was equipped with a radiometer and photometer (SC-610). Figure 3(b) and Table 1 show that, at an input power of 0.37 W, the luminous flux of the w-WLEDs and candlelight LEDs were 43.3 and 34.5 lm, respectively. The luminous flux of the candlelight LEDs was decreased by 25.8% more than the w-WLEDs at current 120 mA, because the blue light emission was decreased by c-SPhCs. The luminous flux of candlelight LEDs was approximately 5.4 times than a candle. The luminous efficacy of these w-WLEDs was 117 and 93 lm/W, respectively. Additionally, the Tcp of w-WLEDs containing c-SPhCs decreased from approximately 3000 K to 2089 K without an increase in the wt% of the LuAG and CASN phosphors. The 30.7 wt% of a LuAG phosphor and the 7.5 wt% of a CASN phosphor was used in the w(Oblique-font)-WLEDs, that optical properties nearly showed the candlelight LEDs. Table 1 lists the optical properties of all light sources and the International Commission on Illumination (CIE) color chromaticity coordinates (x, y). Duv is defined the closest distance from the Planckian locus on the CIE diagram. In this case, the CIE (x, y) coordinates of the candlelight LEDs was located above the Planckian locus of the CIE chromaticity diagram, where Duv is 0.004. Accordingly, the American National Standards Institute (ANSI) C78.377-2008 standard defines a series of chromaticity quadrangles that fall along the Planckian locus in CIE chromaticity space [14,15]. ANSI C78.377-2008 standard defines tolerance Duv is ± 0.006 and the tolerance Tcp (ΔTcp) that was obtained using (3):
In this study, our purposed the Tcp is 2000 K that the ΔTcp is ± 104 K. Therefore, the candlelight LEDs and candle have the same relative chromaticity tolerance. In addition, candlelight LEDs have a CRI of approximately 80, a standard that is required for use in general illumination applications.
Table 1. Optical characteristics of all light sources.
The angular emission distribution of candlelight LEDs was measured using the angular-resolved spectrum technique [6]. A continuous current setting of 120 mA was applied to the candlelight LEDs at room temperature. The angular-resolved spectra as a function of detection angle θout were obtained using a fiber probe coupled to an optical spectrometer (Horiba CP-140). The optical spectrometer was detected from θout = −90° (horizontal direction) to + 90° in increments of 0.5° and the fiber-to-device distance was maintained at 25 cm. The measured plane of the candlelight LEDs was fixed along the K-L-U direction of the fcc Brillouin zone. The luminescence spectra were then displayed on a wavelength versus detection angle plot, with color intensity according to a log scale bar. Figures 4(a) and 4(b) shows the unpolarized luminescence spectra measured w-WLEDs and along the K-L-U direction of candlelight LEDs, respectively. The angular-resolved measurement results show that the PBGs for the guided modes shifted toward a shorter wavelengths when the θout detection angles increased. This indicates that light wave propagation is impossible within this region because of the photonic band structure of the c-SPhCs. The candlelight LEDs exhibited the first photonic pseudogap in the K-L-U direction, as shown in Fig. 4(b), which were measured and analyzed based on the photonic band structures [5,6]. The azimuthal of the angular emission distribution has measured as a function of the azimuthal angle φ by using the angular-resolved luminescence-measuring apparatus. In azimuthal measurement results also showed that the PBGs which attributed to the photonic band structure of the c-SPhCs. In addition, we demonstrated angular-dependent Tcp of these w-WLEDs, which is critical for the use of these w-WLEDs in illuminating spaces. The 3D-emitted photometric distribution images were taken using imaging spheres (Radiant imaging IS-LI) with a current setting of 120 mA as the Tcp comparison, as shown in Fig. 4(c). The candlelight LEDs exhibited the orange-red emission. The w-WLEDs featuring the c-SPhC structure stabilized the angular-dependent Tcp. Figure 4(c) presents the angular-dependent Tcp values measured from −70° to 70°; the Tcp deviations of the w-WLEDs and the candlelight LEDs were 75 K and 111 K, respectively. The measured data indicated that the angular-dependent Tcp of the candlelight LEDs was not affected uniformly by angle distribution.
Fig. 4 (a) and (b) Unpolarized angular-resolved luminescence measurements, (c) angular-dependent Tcp values of the w-WLEDs and candlelight LEDs, respectively. Inset (b): the dashed red lines represent the PBG of the s-CPhCs of the fcc structure.
Finally, the reliability analysis (RA) test [16,17], which was conducted by Lextar Electronics Corp. in Taiwan, is crucial in the application of w-WLED devices. The RA test is used to determine the operation time during which the output luminous flux of WLED devices decreases to 70% of the initial value of the WLED devices. We used a wet, high temperature operating life test to evaluate the reliability performance of the candlelight LEDs. We performed the RA test at a high temperature and high RH of 85 °C and 85 RH, respectively, with 120 mA on candlelight LED samples for 3300 h, comparing the results with those from commercial w-WLEDs. A summary of the RA test, shown in Fig. 5, the w-WLEDs show a weak drift of lumen maintenance due to the LuAG phosphor of the oxidation of Ce3+ to Ce4+ and the CASN phosphor of the oxidation of Eu2+ to Eu3+, respectively [18–20]. This failure analyses have been demonstrated by Lextar Electronics Corp. in Taiwan. Candlelight LED samples were nearly the same as commercial w-WLEDs, indicating that the c-SPhCs deposited on w-WLEDs were undamaged. Therefore, the practical c-SPhCs process was not suffer adverse effects according to the experimental results. This novel technique can be used to reduce phosphor use, thereby producing low-cost candlelight LEDs.
4. Conclusions
In this study, we developed a reliable reduced-phosphor technique for fabricating low-cost candlelight LEDs. The candlelight LEDs operating in 0.37 W produced a CCT of 2089 K, and exhibited a CRI of approximately 80 and a luminous flux of 34.5 lm (5.4 times that of a candle) without an increase in the concentration of the LuAG and CASN phosphors. In the candlelight LEDs, the c-SPhCs thin films substantially modified the luminescence spectrum because of the PBG influence and multiple phosphor reemissions. In addition, the RA test showed no visible degradation in optical performance for candlelight LEDs. This technique applied to w-WLEDs producing candlelight LEDs that reduces fabrication costs and provides new options for lighting.
Acknowledgments
This work is supported by the National Science Council, and Ministry of Science and Technology in Taiwan, under contract numbers NSC102-2221-E-035-046, NSC102-2622-E-035-030-CC2, MOST102-2632-E-035-001-MY3, MOST103-2221-E-035-029, and MOST103-2622-E-035-007-CC2. The authors appreciate the Precision Instrument Support Center of Feng Chia University in providing the fabrication and measurement facilities.
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