Phosphor Converted Three-Band White LED

2004 Bulletin of the Korean Chemical Society (Print)  
The development of wide band gap III-V nitride compound semiconductors has led to the commercial production of high-efficiency LEDs. 1,2 The recent advent of blue InGaN technology has made it possible to produce a conventional white LED in which white light is obtained by coating a Y3Al5O12:Ce (or Sr3SiO5:Eu) phosphor onto a blue LED chip. [3] [4] [5] [6] In this device, known as a two-band white LED, white light is generated by additive color mixing of the blue light emitted by the blue LED
more » ... by the blue LED and the yellow light emitted by the Y3Al5O12:Ce phosphor. The development of a white LED is important because it opens the way for LED applications such as light bulbs and fluorescent lamps with high durability and low energy consumption. However, the spectral composition of the light produced by the conventional two-band white LED differs from that of natural white light, particularly in the red region. The color properties of conventional two-band white LEDs can potentially be improved by adding another component to create a white LED based on three emission bands (a three-band white LED). Full-color fluorescent display devices have been developed by using a combination of ZnS:Ag (blue), ZnS:Cu,Al (green), and ZnCdS:Ag (red) phosphors excited by a near-UV LED. 7 In addition, a white light source has been obtained by intergrating ZnS:Ag (blue), ZnS:Cu,Al (green), and Y2O2S:Eu (red) phosphors, and a UV-LED (350 nm). 8 White light has also been achieved by combining a blue LED (460 nm) with SrGa2S4:Eu (green) and SrS:Eu (red) phosphors. 9 Previously, we constructed a three-band white LED by combining a blue LED (465 nm) with SrGa2S4:Eu (green) and ZnCdS:Ag,Cl (red) phosphors. 10 In the present work, we investigated the optical properties of a white LED which was obtained by using a blue LED (465 nm) in conjunction with SrGa2S4:Eu (green) and SrY2S4:Eu (red) phosphors. The SrY2S4:Eu phosphor was chosen over the ZnCdS:Ag,Cl phosphor used in our previous work in order to improve the red color characteristics of the white LED. Experimental Section Green-emitting SrGa2S4:Eu phosphor was synthesized using a decomposition method that does not involve the use of the toxic compound hydrogen sulfide. SrGa2S4:Eu was synthesized by heating 0.95 mmol of strontium sulfide (SrS), 2.0 mmol of gallium dimethyldithiocarbamate (Ga[(CH3)2-NCS2]3), 0.05 mmol of the europium complex {[(CH3)4N] Eu[(CH3)2NCS2]4}, and excess sulfur for 2 hours at 850 o C. 10,11 However, the red-emitting SrY2S4:Eu phosphor could not be synthesized using this decomposition method; instead, it was prepared by solid-state reaction of 0.98 mmol SrCO3, 2.0 mmol Y2CO3, 0.02 mmol of Eu2CO3, and the appropriate amount of Na2CO3 as a flux, at 1300 o C for 8 hours in H2S steam. 12 Photoluminescence (PL) excitation and emission measurements were carried out using a 0.275 m monochromator, a photomultiplier tube, and a 500 W Xe lamp as an excitation source. The incident beam was perpendicular to the surface of the sample, and the observation angle was 45 o to the excitation source. The PL and chromaticity of the three-band white LED were measured using a 15 cm diameter integration sphere. A blue LED (Nichia, λmax = 465 nm) was used. One gram of SrGa2S4:Eu or SrY2S4:Eu phosphor was dispersed in an aqueous solution containing 4.0 g of polyvinyl-alcohol (polymerization degree: 200). This solution was applied onto a poly(ethylene terephthalate) (PET) film. Although, the thickness and concentration of the film were not measured, the relative amounts of phosphor could be varied by adjusting the number of phosphor flims. Figure 1 shows the PL excitation and emission spectra of the SrGa2S4:Eu and SrY2S4:Eu phosphors. The excitation spectra show that both phosphors absorb strongly absorptions at 465 nm, which is the emission wavelength of the blue LED used. The emission spectra show that, under 465 nm excitation, the SrGa2S4:Eu and SrY2S4:Eu phosphors emit green (535 nm) and red (640 nm), respectively. Therefore, SrGa2S4:Eu and SrY2S4:Eu are suitable as greenand red-emitting phosphors when excited by a blue LED. Figure 2 (A) shows the PL spectra of blue-and greenemitting LEDs prepared by coating the blue LED with SrGa2S4:Eu. Two distinct emission peaks are observed at 465 nm and 535 nm, which correspond to the wavelengths of the blue LED and green emission from the SrGa2S4:Eu phosphor, respectively. As the amount of SrGa2S4:Eu phosphor is increased by increasing the number of * Corresponding Author. Results and Discussion
doi:10.5012/bkcs.2004.25.10.1585 fatcat:3ykqm53kpfcafijnpmqs2y5vki