For recent several years, white light emitting diodes (white LEDs) have been used as an illumination light source as well as components of display devices because LEDs have advantages such as low power consumption, long life time, and so on. There are several methods to fabricate white LEDs. Among these methods, coating a yellow-emitting phosphor on a blue LED is used widely. And $Y-3Al_5O_{12}:Ce^{3+}$ (YAG:Ce) phosphor has been used as a yellow-emitting phosphor since its excitation band is in blue light region. However white LEDs show poor color rendering property due to the weak emission of the YAG:Ce phosphor in red spectral region.
There are two possible methods to strengthen the emission intensity of YAG:Ce phosphor in red spectral region. One is to add a red emitting activator into the YAG:Ce phosphor, and the other is to shift the emission band to longer wavelength. In this study, we performed experiments using both methods, respectively.
At first, synthesis condition of YAG:Ce phosphor was optimized before increasing emission intensity in red spectral region. We performed experiments on parameters such as firing temperature, activator concentration, and a flux so as to improve emission intensity of YAG:Ce phosphor. Optimized firing temperature and activator Ce concentration were 1500 ℃, and 0.06 mol, respectively. When $BaF_2$ was used as a flux, YAG:Ce phosphor has high crystallinity and round particle shape and PL intensity increased by 31 % in comparison with YAG:Ce phosphor without flux.
To make a emission peak in red spectral region, a red emitting activator was added into YAG:Ce phosphor. Eu and Pr were used as a red emitting activator, respectively and Pr showed higher excitation intensity in blue light region. In the case of Pr addition, the emission peak appeared at around 608 nm and color coordinates of YAG:Ce phosphor changes from x = 0.445, y = 0.535 to x
= 0.457, y = 0.525. To shift the emission band to longer wavelength, Y site in YAG:Ce phosphor was substituted by another element. Gd and Tb were chosen as substitutional elements and $(Y_{1 x}Gd_x)_3Al_5O_{12}:Ce^{3+}$ and $(Y_{1 x}Tb_x)_3Al_5O_{12}:Ce^{3+}$ phosphors were synthesized. PL emission and excitation spectra showed that red shift occurred in emission and excitation bands. From the PL excitation analysis, we could verify that excitation energy of 4f electron in $Ce^{3+}$ ion became low.
In the consequence of addition of third elements such as Pr, Gd, and Tb, CIE color coordinates of YAG:Ce phosphors shifted from the greenish yellow region to the yellow region. And experiments revealed that Tb was most effective among three elements because $(Y_{1 x}Tb_x)_3Al_5O_{12}:Ce^{3+}$ phosphors showed good color coordinates (x = 0.492, y = 0.500) and high luminescent intensity as much as 83 % that of YAG:Ce phosphor.
Finally the white LEDs were fabricated by using $Y_3Al_5O_{12}:Ce^{3+}$, $Pr^{3+}$, $(Y_{1 x}Gd_x)_3Al_5O_{12}:Ce^{3+}$ and $(Y_{1 x}Tb_x)_3Al_5O_{12}:Ce^{3+}$ phosphors, respectively. The white LED using $(Y_{1 x}Tb_x)_3Al_5O_{12}:Ce^{3+}$
phosphors showed high luminous flux of 2.354 lm at 20 mA, good CIE color coordinates (x = 0.315, y = 0.330) and high color rendering index (Ra ~ 80). In this case, we can expect that color rendering property of the white LED composed of both a blue LED and YAG:Ce phosphor is excellent.
최근 백색 LED에 대한 응용이 증가하고 있다. 백색 LED를 구현하는 방법 중 청색 LED에 황색 형광체를 도포하는 방법이 많이 사용되고 있는데 청색 LED와 황색 형광체를 이용해 제작된 백색 LED는 휘도가 우수한 반면 연색 특성이 좋지 않은 단점이 있다. 본 연구에서는 연색 특성을 개선하고자 형광체의 발광 특성을 개선하였다. 황색 형광체로 이용되는 $Y_3Al_5O_{12}:Ce^{3+}$ 형광체는 주 발광 파장이 550 nm 근처로 greenish yellow 발광을 나타낸다. 따라서 적색 스펙트럼 영역의 발광 강도가 약한 단점이 있다. 이를 개선하고자 먼저 형광체의 합성 최적화를 통해 발광 강도를 증가시킨 후 적색 발광 활성제인 Pr을 co-activator로써 도핑하여 적색 스펙트럼 영역에 발광 peak을 첨가하였다. 또한 Gd, Tb를 첨가하여 발광 밴드를 장파장 쪽으로 이동시킴으로써 적색 영역의 발광 강도를 증가시켰다. $(Y_{1-x}Tb_x)_3Al_5O_{12}:Ce^{3+}$ 형광체의 경우 형광체의 발광이 우수할 뿐 아니라 적색 스펙트럼 영역에서의 발광 강도도 강한 것으로 나타났다. 이 형광체를 청색 LED 위에 도포하여 백색 LED를 제조한 결과 2.3 lm 의 luminous flux와 약 80의 연색 지수를 나타내었다.
