Luminescence Mechanism Of Quantum Dots Phosphors、LED Fabrication And Bandwidth Characteristics In VLC

With the development of quantum dots, quantum dot phosphor white LED will become the next generation of lighting source, because of their advantages such as green production, high efficiency, energy saving and high lighting stability, etc. Because the light-emitting phosphors material of Cd Se quantum dot covers most of the visible light region, it is easy to fabricate white LED combining the Ga N chip. However, it contains heavy metal Cd elements and does not conform to the requirement of non-toxic green environmental protection. Therefore, based on the phosphors of ZnCuInS core material, I fabricated a solid white LED. In order to study the characteristics of liquid LED,I fabricated a liquid ZnCuInS/ZnS core/shell quantum dot white LED. Besides, liquid and solid AgInS₂ mixed Yag phosphors white LED have been fabricated and the characteristics of both LED have been compared. In this paper, I studied mechanism of quantum dot phosphor and fabricated four different types of LED devices. Through actual circuit, the quantum dot LED devices have wide application prospect in visible light communication. This paper mainly studies the following five parts:1. Studied the electronic structure and fluorescence radiation mechanism of colloidal quantum dots. Using the theory of effective mass approximation, the colloid model of exciton in quantum dots is constructed, and Hamiltonian is given. With the structure of spherical colloidal quantum dots in the infinite and finite potential well model, I derived out exciton energy level structure, and further deduced the electrons and holes coulomb energy, the electronic structure of colloidal quantum dots. Using Cd Se, Pb Se, CuInS₂ and CuInSe₂ colloidal quantum dots as an example, I calculated the bandgap and electronic structure. The colloidal quantum dots fluorescence radiation characteristics and the radiation energy transfer mechanism have been summarized, respectively.2. Based on CdSe/CdS/ZnS core/shell/shell quantum dot phosphors, I fabricated white LEDs. Through thermal injection method, I synthesized CdSe/CdS/ZnS core/shell/shell quantum dots, with emission peak of 615 nm red light and 545 nm green light, respectively. By vacuum environment and ultrasonic processing technology, a composite white LED was formed by coating the Ga N chip with the red and green phosphors mixed with type OE6630 AB silicone rubber. The characteristics of the LED with different components were investigated by appropriately increasing the red and green fluorescent powder. As a result, white LED changed from cold white（color coordinates（0.283, 0.289）, color temperature 9630K） to warm color yellow（color coordinates（0.367, 0.367）, color temperature 4320K）. I also studied the effect of work voltage on the color coordinates and color temperature. When the work voltage increased from 2.8V to 3.2V, its color coordinates changed from（0.341, 0.384） to（0.285, 0.285）, and the color temperature changed from 5210 K to 9620 K.The stability of the LED device under a working voltage of 2.8V was investigated. The study proved that it is feasible for CdSe/CdS/ZnS core/shell/shell quantum dots working as administered white LED light conversion phosphors.3. Based on ZnCuInS/ZnSe/Zn S core/shell/shell quantum dot phosphors, I fabricated a white LED. Through high temperature injections method, non-toxic ZnCuInS/ZnSe/ZnS quantum dots were synthesized. The size of ZnCuInS/ZnSe/ZnS quantum dots were 2.3nm, 2.7nm and 3.2nm, with photoluminescence peak of 545 nm, 583 nm and 611 nm, respectively. Characterized by the absorption and photoluminescence spectral, the ZnCuInS/ZnSe/Zn S quantum dots had a stokes shift of about 400 me V. The large Stokes shift was mainly induced by the defect level-related radiation recombination mechanism, which has been verified by the temperature-dependent photoluminescence characteristics. The white LED was fabricated by mixing the quantum dots with 583 nm emission peak and EP-3400 rubber to form orange phosphors and spin-coating on the surface of the blue 452 nm Ga N light emitting diode（HXGD-DZ-3w）. I studied the spectral characteristics and heat effect of white LED, while the working voltage changed from 2.5 to 3.2V。4. Fabricated liquid ZnCuInS/ZnS quantum dot white LED of compensation structure. Based on the method reported by Klimov, the ZnCuInS/ZnS quantum dots with the photoluminescence peak at 583 nm were fabricated and mixed with Ga N blue ship to form the white LED. When applied by 2.8V, the LED had color coordinates of（0.3796,0.3284）, color rendering index of 77 and color temperature of 3567 K. Yellow commercial YAG phosphors and the fabricated red glow AgInS₂ quantum dots with the luminescence peak at 640 nm were mixed in BIDE6321 uv glue, respectively. Then the mixture was coated on 460 nm Ga N blue LED chip respectively and formed a solid mixed structure of LED. Also I fabricated a liquid AgInS₂ based white LED. In comparison with the soild white LED, under the same conditions, the luminous intensity and luminous efficiency of liquid white LED increased about 40% and10%, respectively. And the stability increased about 3 times.5. The Zn Cu In S quantum dot white LED is treated as a promising signal transmitter in wireless visible light communication. In this paper, the transmission and receiving circuit were employed to measure the modulation bandwidth of fabricated white devices. From the results, I found out that the bandwidth of Zn Cu In S quantum dots LED（4.1MHz） is wider about one time than the commercial YAG-based white LED. These results suggest that Zn Cu In S is competitive as apromising phosphor in the field of visible light communication.