Yoshinori Murazaki

Red-Enhanced White-Light-Emitting Diode Using a New Red Phosphor

We fabricated a high-color-rendering, red-enhanced white-light-emitting diode (LED) using a new red phosphor and a short-wavelength YAG phosphor. When the new white-LED was operated at a forward-bias current of 20 mA at room temperature (RT), color temperature (Tcp), the general color rendering index (Ra) and luminous efficiency were 4670 K, 87.7 and 25.5 lm/W, respectively. Most of the color-rendering indexes (CRIs) of the new white-LED were larger than those of current white-LEDs, in which only YAG is used. In particular, the CRI-No.9 value, which shows the color reproduction in the red region, is improved from -2.5 to 62.6.

Mechanism of the persistent phosphorescence in Sr4Al14O25:Eu and SrAl2O4:Eu codoped with rare earth ions

Luminescence and phosphorescence properties are investigated in two series of strontium aluminates, Sr4Al14O25:Eu2+ and SrAl2O4:Eu2+, codoped with rare earth (lanthanoid, Ln) ions. Persistent phosphorescence is observed for the phosphors codoped with Pr, Nd, Dy, Ho, and Er. In the phosphorescence process of these phosphors, the excited electron in the Eu2+ ion is transferred between the Eu and the codopant Ln ions through the 5d states of these ions. When the trivalent codopant ion Ln3+ captures the excited electron in the 4f shell, the ion is reduced to the divalent state and, therefore, the energy necessary to thermally relieve the electron from the ion is determined by the energy difference between the 4fn and the 4fn−15d configurations of the divalent Ln ion. The trapping depths (activation energies) of the Ln ions, estimated theoretically on the basis of this mechanism, coincide with the observed depths (0.8±0.2eV) for the persistently phosphorescent phosphors, and those of the nonpersistent Ln ions ...

A Methodological Study of the Best Solution for Generating White Light Using Nitride-Based Light-Emitting Diodes

In search of suitable white-LED for general illumination, we fabricated various types of white-LEDs using different methods. As the first method, we used the multichip method in which multiple emitters were mounted in one package. This type showed a good general color-rendering index (Ra) = 90 by the optimizing the emission wavelength of each LED chip. However, the electric driving circuitry was too complex for use in general illumination. Secondly, we used a monolithic white-LED by using the multicolor emitting multiple-quantum well (MQW) for the active layers, which consisted of quantum wells (QWs) with different In compositions. A high Ra = 80.1 was obtained in the three-color-emitting white-LED but the luminous efficacy (η L ) was only 8.11 1m/W. As the third method, we used the color conversion method using phosphors. We fabricated a white-LED which consisted of a near-UV-LED chip and blue/yellow phosphors in order to improve the luminous efficacy of the white-LED under high forward-bias current. At 100 mA, the luminous flux (I L ) was estimated to be 7.61m. However, this white-LED degraded quickly, because the epoxy resin used for package was the general purpose one and deteriorated under the UV-light from the n-UV-LED. Next, we improved the Ra and η L of a traditional white-LED which consisted of blue-LED chip and yellow phosphor. In order to improve the Ra, we added a newly developed red phosphor. We obtained a Ra = 87.7 at low-color-temperature. Then, in order to improve the efficiency of the white-LED, we improved the extraction efficiency (η EX ) of the blue-LED by using a patterned sapphire substrate and a high reflection Rh-mesh-patterned p-electrode. Then, we obtained a 62.0 1m/W at 20 mA. As a result, we concluded that the color conversion method of using a blue-LED for general illumination has advantages in efficiency, color-rendering, cost and lifetime. It also has simpler electric driving circuitry.