In recent years, white light LEDs have gradually replaced the incandescent bulbs and fluorescent lamps in terms of luminous efficiency, power consumption, longevity and environmental protection, which has led to the elimination of incandescent bulbs and fluorescent lamps. The light bulb schedule has accelerated this trend. 
The mechanism for generating white LEDs can be divided into three types as shown in Figure 1. (a) The conversion of the blue light-emitting chip plus the Nd-YAG phosphor to the white LED is proposed by Nichia Chemical Co., Ltd. [1, 2] . (b) Conversion of the RGB tri-color phosphor to the white LED with the Ziguanglei chip is still in the experimental stage. [3-5] (c) Blending white LEDs using RGB three types of epichips [6, 7]. At present, most of the products on the market are mainly converted into white LEDs by the Blu-ray chip and Nd-YAG phosphors. Therefore, how to improve the luminous efficiency of the Blu-ray chip is crucial for the development of white LEDs.
Figure 1 Mechanism of white LED generation (a) Blue LED +YAG Phosphor (b) UV LED + RGB Phosphor (c) RGB LED
The luminous efficiency of a semiconductor LED depends on the characteristics of the material itself. When the LED is injected with an extra carrier, the composite of the additional carrier is divided into a radiation composite (the additional carrier of the band can be combined to emit light) and the non-radiative composite (phonon composite release). The combination of heat and Oujie), in addition to the defect level between the bands, also captures additional carriers and reduces the chance of additional carrier recombination. Therefore, in recent years, many research teams have studied the illuminating mechanism by analyzing the fluorescence measurement technology in order to study how to improve the luminous efficiency of LEDs.
Fluorescent illumination mechanism
Fluorescence is a phenomenon in which electromagnetic radiation is emitted. For any material, when the incident photon energy is equal to or exceeds the energy band, the valence band electrons are excited across the energy band to reach the conduction band. When the excited state electrons return to the valence band from the conduction band, radiation is generated. The process of radiation generation is mainly divided into three stages as shown in Figure 2. (a) For excitation, additional carrier generation and excitation (b) is energy release and recombination, and the energy of the extra carrier in the excited state is released and combined (c) is a fluorescent photon signal generated by fluorescence generation.
Figure 2 Fluorescence generation process
The way in which the fluorescence is generated is roughly divided into two types, namely, irradiating the sample with photons higher than or equal to the energy of the energy gap to generate additional carriers, or increasing the carrier concentration by electron injection to increase the probability of fluorescent photon generation. In order to increase the intensity of the fluorescent signal. These two types of methods are called photoluminescence (PL) and electro-excitation fluorescence. The principle of LED illumination is electric excitation. However, the measurement of electro-active fluorescence must be embedded in the electrode. Photoexcitation fluorescence must be used in the process prior to embedding the electrode.
Since the laser can be used to provide sufficient power to excite the signal [8], the incident light begins to use the laser source. When the excited state electrons return to the ground state, a photon is generated, and many phonons are generated. It is assumed that the light source used is a continuous wave, and the fluorescent light excited thereby can be regarded as a steady state, and the test piece is continuously irradiated with fluorescence by the light source [9], and the fluorescence spectrum of the laser spectrum and the excitation is shown in Fig. 3.
Figure 3: Fluorescence spectrum of laser and excitation
As can be seen from the Jablonski energy diagram [10] proposed by Alexander Jablonski in Fig. 4, the absorption of incident light is related to the wavelength of the incident photon, that is, the absorption of the material is related to the wavelength of the incident light source.
Figure 4 Jablonski energy diagram [10]
When the sample absorbs the incident light, it excites the electron to a higher energy state. After a period of time, the electron will release energy to a lower energy state. Impurities and defects form various energy levels in the energy gap, and their corresponding energy will be generated by radiation, such as photoexcitation, or by non-radiative recombination process [8][11], such as Phonon emission, defect capture, or the Oujie effect [12].
In addition to the above-mentioned conduction band and the valence band and other energy band conversion will emit fluorescence, the defect will also cause the generation of fluorescence, as shown in Figure 5. Among them, EC, EV and ED are conductive strip, valence band and defect band respectively, wherein the defect band is distributed between EC and EV, the position and quantity depend on the material quality, and (a) is the band in Fig. 5 The electron hole is composited, (b) and (c) are all composites of defects, (b) the electrons of the conduction band are captured by the defects between the energy bands, and (c) the electrons and the valence band for the defect capture. Composite, the emitted fluorescent band depends on the distance between the electron and the hole before the composite.
Figure 5 Radiation recombination (a) Electron hole-to-composite between the energy bands (b) Electron and valence band electro-combination if the defective electrons between the bands are trapped by the defect (c)
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