White LED has become the next generation of lighting due to its energy saving, environmental protection and long life. Currently, commercial white LED mainly uses blue light chips to excite YAG:Ce3+ yellow phosphor. The blue light emitted by the chip mixes with the yellow light emitted by the phosphor to form white light. However, the emission spectrum of the YAG:Ce3+ phosphor contains insufficient red light components, resulting in the inability to obtain warm white light with low correlated color temperature (CCT <4500 K) and high color rendering index (CRI >80) using a single YAG:Ce3+ phosphor, thus limiting its application in indoor general lighting.
To solve this problem, appropriate red phosphor should be added to the component to supplement the red light component, thereby preparing a warm white LED with low color temperature and high color rendering index. Currently, commercial red phosphors with better performance are mainly rare earth-doped nitrogen (oxide) materials. However, this type of phosphor has limitations such as a wide emission bandwidth and high pressure required for preparation, resulting in low lumen efficiency and high price. Therefore, the development of low-cost, narrow-band red-emitting phosphors that can be effectively excited by blue-light wafers, especially to replace rare earth luminescent materials, has become the focus of attention. This is also the key to improving the lumen efficiency of warm white LEDs.
K2TiF4:Mn4+ red phosphor synthesized by wet chemical method and high-efficiency warm white light emitting diode
The research team led by Professor Chen Xueyuan of the Fujian Institute of Physics, Chinese Academy of Sciences, Professor Liu Ruxi and postdoctoral fellow Lin Qunzhe of the Department of Chemistry, National Taiwan University, successfully prepared Mn4+-doped K2TiF6, K2SiF6, NaYF4 and NaGdF4 red phosphors for the first time using an efficient ion exchange method. This type of phosphor has a strong absorption band at ~460 nm (bandwidth ~50 nm), it is very suitable for the excitation of blue light wafers. At the same time, it emits sharp spectral line red light emission of ~630 nm, which has higher lumen efficiency than nitrile (oxy) compound red light phosphors.
The absolute quantum efficiency of K2TiF6:Mn4+ phosphor at room temperature reaches 98%, which is better than most existing red phosphors. At the same time, the phosphor has good fluorescence thermal stability, and its luminous intensity at 150 degrees reaches 98% of that at room temperature. The warm white LED encapsulated using the combination of this red phosphor and YAG:Ce3+ yellow phosphor has a lumen efficiency of up to 116 lm/W under the conditions of 60 mA driving current, color temperature 3556 K, and color rendering index (Ra) 81. The ion exchange preparation method developed by the research team is simple, can be prepared at room temperature and normal pressure, and the raw materials are cheap, so it has good market application prospects.
In addition, the research team also conducted in-depth research on the spectral characteristics of Mn4+ ions in the fluoride matrix. Through low-temperature high-resolution laser spectroscopy and other methods, they revealed its electronic energy level structure and explained its abnormal luminous intensity-temperature dependence. These provide a reliable theoretical basis for further research and development of this type of non-rare earth red luminescent materials. The above-mentioned research results were published online in full-text form on July 8, 2014 in Nature. Commun. 2014, DOI: 10.1038/ncomms5312.
In recent years, the research team led by Professor Liu Ruxi has taken into account both basic and practical applications in the research of phosphorescent materials. In addition to publishing many important works in important international journals such as Angew.
ANNA