Synthesis And Luminescence Properties Of Phosphors For Alternating Current Driven White Light-emitting Diode

LED is regared as the fourth generation light source for its fantasticfeatures of small size, long life and energy saving. At present, widespreadused LED light sources are DC-driven semiconductor devices commonly workunder low voltage and high current conditions. Compared with traditionalLEDs that are driven by the direct current （DC）2.5-3.2V, the main advantageof AC-LEDs was leaving an AC/DC converter out of the device system. Thiscould economize the cost of AC/DC converter and about15%-30%unnecessary electric power consumption during the AC/DC conversion.Moreover, it would greatly simplify the application system and increase theservice life of LEDs （AC/DC converter is more likely to get breakdown beforeLEDs.）. However, a dark duration （also called dead time） between twoneighboring VFwill be generated periodically with the AC voltage changescyclically, which is highly related to the flickering luminescence. Furthermorethe phosphors used in AC LED are still the traditional fluorescent material, sothat the AC LED with high color temperature, low color rendering. In thispaper, we have synthesized several kinds of aluminate and silicate phosphors,and systematacially studied the synthesis process, structure, optical propertiesand applications to LEDs, and obtained the following results:（1） The phosphor SrAl₂O₄:Eu²⁺co-doped with Ce³⁺, Y³⁺or Dy³⁺wassynthesized via solid-state reaction with H3BO3as flux. The crystal structureand morphology were characterized with XRD and SEM, respectively. Thephotoluminescence spectra were collected with a fluorescence spectrometer.The results showed that appropriate amount of Ce³⁺, Y³⁺or Dy³⁺doped wasbeneficial to suppress the impurity phase of Sr₄Al₁₄O₂₅which easily coexistedwith the SrAl₂O₄phase synthesized by adding H3BO3as flux. However, toomuch Y³⁺or Dy³⁺doped resulted in the formation of the yttrium aluminumgarnet including the Y₃Al₅O₁₂and the Dy₃Al₅O₁₂.Comparatively, the dopedDy³⁺was more helpful in prolonging the persistent luminescence, while Y³⁺was more efficient in enhancing luminescence intensity, Ce³⁺plays asignificant role in susspress the impurity phase of Sr₄Al₁₄O₂₅. To demonstratethe feasibility of the phosphor applied in AC-LED, a nearly white AC-LED was fabricated by coating the phosphor on a blue HV-LED chip. The persistentluminescence was radiated from the AC-LED device after turning power off.Moreover, the contribution of the phosphor on compensating the AC-LEDdark duration through persistent luminescence was identified by using a FastBurst Camera.（2） Sr₄Al₁₄O₂₅: Mn red phosphors were synthesized by high temperaturesolid state reaction in air atmosphere. The influence of the Mn contents andsintering temperature on the phosphor was studied. The strongestluminescence occurs to the content of Mn is0.001, fixed the sinteringtemperature at1300oC. And we combined the combinatorial chemistry methodand orthogonal design to investigate the influence to the photoluminescenceproperties with doping different alkaline earth metals elements and fluxes. TheXPS, EPR and XANES result showed that the red emission comes from Mn4+,and occupied the Al³⁺positionof the AlO6octahedral. In addition, it could befound the stoichiometric ratio is3:5displayed the highest luminous intensity.The XRD patterns showed that its phase is made up of SrAl₂O₄and Sr₄Al₁₄O₂₅.The mechanism for enhancing the luminescence was explored via Ramanspectroscopy and the dielectric constant. Furthermore, we fabricated thewarming LED device using the obtained phosphors assisted with YAG, theCIE coordinates is （0.3305,0.3559）, color rendering index （Ra） is93.23. Theexcellent properties showed that this phosphors could effectively reduce thecolor temperature and improve the color rendering, and would have a goodcommercial prospect.（3） In finally, we investigated the influence of the Eu²⁺concentration,sintering temperature, flux and alkaline earth metal elements to theluminescence properties of the Sr₃MgSi₂O₈:Eu²⁺phosphors, and obtained theoptimal formula and the best sintering process. By the three mixing processesof the hand grinding, ball mill and ball mill assisted with water, we found thatthe process of ball mill assisted with water could effectively improve theluminous efficiency of the samples.