White light-emitting diode with quasisolar spectrum based on organic fluorescent dyes

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White light-emitting diode with

quasisolar spectrum based on organic

fluorescent dyes

Shuang-Chao Chung

Ming-Chia Li

Ching-Cherng Sun

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White light-emitting diode

with quasisolar spectrum

based on organic

fluorescent dyes

Shuang-Chao Chung,a_{Ming-Chia Li,}b_and

Ching-Cherng Suna,_*

a_{National Central University, Department of Optics and}

Photonics, Chung-Li 320, Taiwan

b_{National ChiaoTung University, Department of Biological}

Science and Technology, Hsinchu 300, Taiwan

Abstract. We present a study of light-emitting diodes (LEDs) using organic fluorescent dyes to replace the general phosphor. The blue die with a specific organic fluorescent dye gives the LED a single color appearance. Through a color-mixing cavity, multiple LEDs are used to produce a quasisolar spectrum at a certain band and white light with a color rendering index as high as 97 at around

[DOI:10.1117/1.OE.54.7.070501]

Keywords: light-emitting diode; color; solar simulator; dye.

Paper 150427L received Apr. 7, 2015; accepted for publication Jun. 4, 2015; published online Jul. 7, 2015.

White light-emitting diodes (LED) have become one of the major light sources in modern lighting because of its advantages, such as fast response, long life, wide-color

range, and lack of mercury.1–6 Generally, a white LED is

made by a blue die with a yellow phosphor and the LED attains approximately 70 on the color rendering index

(CRI),7 which is a measure of a light source’s ability to

show an object’s colors “realistically” or “naturally”

com-pared to a familiar reference source, either incandescent light or daylight. The color performance of the typical phosphor-converted white LED is acceptable in outdoor lighting. In indoor lighting, however, a white LED must pro-vide a higher CRI, therefore, two or more kinds of phosphors

are used to supplement the spectrum.6,8–12_{Even the approach}

of multiple phosphors for a wider spectrum still has some issues that need to be solved. The first issue is that it will cause more light absorption by the multiple phosphors, so

the packaging efficiency5 is at a low level. The second

issue is that it is difficult to control the vivid color of each phosphor because of multiple absorptions between the phos-phors. The third issue is that the spectrum is not as flat as that of solar light and causes poor color performance in a certain spectrum, which may cause serious problems in specific applications. The third issue results in the demand for a white LED to provide a useful solar spectrum in a specific

band.13–19 _{In this letter, we present a study to introduce a}

new way for organic fluorescent dyes to produce a quasisolar spectrum with a blue die. The design concept as well as the experimental measurement is demonstrated.

Organic fluorescent dyes are transparent media with down conversion fluorescence. In contrast to commercial phos-phors, the organic dyes in this study are heavy-metal-free and environmentally friendly. Generally, the spectrum band-width of fluorescent light is larger than that of the phosphor. It is useful for the packaging of the LED to be transparent so that the backward scattering of the blue light is reduced, thus

resulting in a higher packaging efficiency.5A larger

band-width enables a wide range of coverage for the composite spectrum and can even produce a specific spectrum.

In order to have a quasisolar spectrum in the visible band, we select six organic fluorescent dyes with a peak wave-length from 476 to 631 nm, which results in a high efficiency in comparison with other dyes. The dyes selected for this

study are listed in Table1. The detailed procedures for

pre-paring the LED phosphor layer are described as follows. A mixture of 5 mg of organic dye plus 20.0 g of epoxy resin was mixed in a glass bottle and then stirred at 25°C for 1 h to form a clear polymer solution. Subsequently, the polymer solution was dip-coated on the LED chip, and was then ther-mally cured at 150°C under a normal atmosphere for 1 h. The fluorescent thin film was prepared by the dip-coating process

at room temperature. The film thickness was0.2 0.01 mm.

The absorption and emission spectra were measured with spectrophotometer HITACH U-3900H and fluorescence spectrometer HITACH F-7000, respectively. The illustration

of the six organic fluorescent dyes is shown in Table1. The

peak wavelengths (bandwidth) of the six dyes are 476 (75 nm), 481 (50 nm), 505 (100 nm), 536 (102 nm), 615 (81 nm), and 631 nm (78 nm), respectively, which cover the full range of visible light. To avoid a complicated optical effect, each organic fluorescent dye is packaged with a blue die in a single package. The peak wavelength of the blue die is selected as 450 nm. The fluorescence spectrum of each

organic fluorescent dye is shown in Fig. 1. The packaged

LEDs are shown in Fig. 2, where the color appearance is

controlled by the particular organic fluorescent dye being used.

To produce white light, we use a light pipe with a diffuser to form a color-mixing cavity for multiple light sources in

different colors, as shown in Fig.3. The cavity with a diffuser

is shown to be capable of providing good color mixing.20–25

A spectrometer is used to measure the emitted spectrum from the color-mixing cavity. To produce a quasisolar spectrum, three or more LEDs with organic fluorescent dye must be used in the color-mixing cavity. The corresponding simula-tions show that different recipes can be used to produce qua-sisolar spectrum at specific correlated color temperatures

(CCT). Figure4 shows a quasisolar spectrum at a CCT of

5200 K, where the spectra mismatch is always lower than 2% from 430 to 650 nm.

The other benefit for the proposed LED is that this approach can provide a way to produce a white light with

a high CRI. Figure5 shows the simulation results for four

white lights with the color coordinates at the black body radiation. The CCTs are at 2800, 3500, 4500, and 6500 K,

*Address all correspondence to: Ching-Cherng Sun, E-mail:[email protected]

Optical Engineering 070501-1 July 2015 •Vol. 54(7)

OE Letters

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and the corresponding CRIs are as large as 97, 96, 94, and 90, respectively.

In summary, in this letter, we propose and demonstrate the capability of organic fluorescent dyes as new wavelength

conversion materials. The LED is made with a blue die with a specific organic fluorescent dye to produce a single color appearance. With the use of a color-mixing cavity, the white light is demonstrated to produce a quasisolar spectrum at a specific band from 430 to 650 nm. In addition, white light with a high CRI can also be produced with different recipes, where the CRI is as high as 97 when the CCT is around 2800 K.

Table 1 The properties of the six organic fluorescent dyes.

No. Name Extinction coefficient (ε_Acm−1M−1) Quantum yield (Φ_A) Emission peak wavelength (nm) FWHM (nm) Color coordinateðx; yÞ 1 3, 4, 9, 10-Perylenetetracarboxylic

acid disodium salt

56,000 0.8 476 75 (0.1528, 0.3206)

2 Tetrachloro-substituted 3, 4, 9,

10-perylenetetracarboxylic acid disodium salt

55,000 0.55 481 50 (0.1344, 0.4185) 3 Tetrachloro-substituted 3, 4, 9, 10-perylenetetracarboxylic dianhydride 49,500 0.65 505 100 (0.3053, 0.5621) 4 Curcumin 54,000 0.3 536 102 (0.3582, 0.5695) 5 4-(Dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran 44,900 0.6 615 81 (0.6105, 0.3875) 6 Sulforhodamine 101 (S101) 139,000 0.9 631 78 (0.6873, 0.3087)

Fig. 1 The emission spectra of the six organic fluorescent dyes.

Fig. 2 The photo of the packaged light emitting diode (LED) with a single organic fluorescent dye and the corresponding color coordinates.

Fig. 3 (a) The geometry of the color-mixing cavity for producing white light. (b) The schematic diagram of the color mixing. (c) The photos of the single color LEDs and the emitted white light.

0 0.25 0.5 0.75 1 400 450 500 550 600 650 700 750 800

Relative intensity (a.i)

Wavelength (nm)

AM1.5 Spectra mismatch

450 nm ~500 nm 1.92% AM1.5 Spectra mismatch

600 nm ~650 nm 0.42%

CRI 82

AM1.5 Spectra mismatch

600 nm ~650 nm 0.42%

CRI 82

Color temperature (K) 5199 K Color coordinate (x, y) 0.3409 0.3875

(a) (b)

Fig. 4 (a) The spectrum of the quasisolar white light at the correlated color temperature (CCT) of 5200 K. (b) The table describing the spec-tra mismatch and color performance.

OE Letters

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Acknowledgments

This study was supported by the Ministry of Science and Technology of the Republic of China with Contract Nos. NSC100-3113-E-008-001, 100-2221-E-008-088-MY3, 101-2221-E-008-085-MY3, 101-2221-E-008-108, and 103-2221-E-008-063-MY3.

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