Fluorescence is a valuable physical process, which finds applications in many fields. The application of fluorescent materials, such as phosphors, to achieve fluorescence is ubiquitous. Phosphor is a kind of luminescent substance with strong fluorescence, and the materials of phosphor are widely used in fluorescent lamps, high definiiton televisions and high efficiency lamps that produce fluorescent light
Phosphor is composed of two parts: host and activator. Host is a luminescence medium that transfers the energy of light to activator. The activator absorbs the light and emits light of a different wavelength from its original wavelength after stimulated. The host may also emit some glow light of different wavelength from that of activator due to the energy transfer. And that is the phosphorescence.
Fluorescence phosphor is an important form of phosphor. The fluorescence phosphor refers to the material which can convert ultraviolet and visible light into visible light after being excited by ultraviolet and visible light. There are mainly two kinds of phosphorescence: direct excitation phosphorescence and the indirect excitation phosphorescence. The phosphorescence of direct excitation type depends on direct excitation of light, while the indirect excitation phosphorescence is caused by energy transfer from an activator to another material.
When visible light or ultraviolet light is incident upon the fluorescent material, the energy is absorbed by an electronic shell of an activator and its electrons will take a short jump from the original energy level to a higher energy level. The electrons can then return to their original energy level by radiating visible light when stimulated. During the short instant, the electrons tend to return to the lower energy level by non-radiative transition or energy transfer process. The non-radiative transition or energy transfer will make the fluorescent material return to higher energy level and emit visible light.
The main properties of fluorescence phosphor are stability and luminescence. The fluorescence phosphor should be stable chemically and thermally to provide stable light emission. The luminescence efficiency of phosphor should be high to guarantee the fluorescence is strong. It depends on the ability of host and activator, and their properties and distributions in the material.
The structure of fluorescence phosphor generally includes host, activator and other components like solvent. The solubility, stability and luminescence efficiency of fluorescence phosphor mainly depend on the structure, composition and crystal structure of it. The performance of the host and activator, as well as their arrangement, will influence the luminescence efficiency and service lifetime of phosphor. Moreover, the dopant species, doping concentration and doping method will have a significant effect on the optical and luminescent properties of the phosphorescence material.
The optical properties of phosphors include color rendering, brightness, light-emitting spectral and spectral shape. The spectral properties of phosphor depends on the energy gap between the host and the activator. The value of energy gap affects the color rendering and brightness of the fluorescence light. The spectral shape is related to the amounts and kinds of activator, as well as the way and degree of doping.
The luminescent decay characteristic of phosphorescence phosphor refers to the length of time in which light appears after excitation and the amount of light emitted over that time period. It will significantly decide the service lifetime of the light source. The luminescent decay characteristic of phosphorescence phosphor is determined by many factors, such as the host and activator structure, the concentration and type of activator and host, and the solubility of counter ion.
In general, the key properties of fluorescence phosphor include stability, luminescence efficiency, color rendering and spectral shape, brightness, luminescent decay characteristic. They all influence the performance of the phosphor. And the fluorescence phosphor with these properties will make great contribution to our daily life.
