Novel Bi³⁺,Eu²⁺,Eu³⁺-Doped Oxide Phosphor Materials:Structure Design,Photoluminescence Tuning And Application In WLED

Phosphor-converted white light-emitting diodes(LEDs)have been widely used in lighting display field due to their advantages of energy conservation,high efficiency,environmental protection,long durability,safety and reliability.A traditional method to obtain white LED is combining blue LED chips with yellow phosphors(YAG:Ce3+),but the white light made from this way exhibits a high correlated color temperature and low color rendering index owing to the lack of red component.An alternative approach to achieve white light of combining single phase white phosphor or tricolor(red,green,blue)phosphors with a near-ultraviolet(n-UV)LED chip have attracted much attention.Accordingly,it is urgent to optimize the photoluminescence(PL)performance of current phosphor materials and explore novel highly efficient phosphor materials.In order to meet the requirements of different PL properties of phosphor materials,it is essential to develop phosphors with tunable PL properties.In this thesis,some design strategies such as cationic substitution,activator concentration regulation,designing energy transfer and valence mixing were utilized to achieve new phosphor materials with tunable PL properties.At the same time,highly efficient red phosphor materials with excellent thermal stability were developed by constructing high symmetry lattice environment.Their crystal structure,photoluminescence properties and practical application were systematically studied.It mainly includes the following three parts:(1)Bi3+ activated BaSrGa4O8,SrLaZnO3.5 and LiCa3MgV3O12 oxide phosphor materials were designed and synthesized.PL properties were regulated and optimized by some strategies.Their crystal structures were determined by XRD and Rietveld refinement.Combined with the diffuse reflectance spectra,PL spectra,temperature-dependent PL spectra and PL lifetime curves of phosphor materials,the influences of crystal structure and local lattice environment on PL properties were analyzed and studied in detail,and the corresponding mechanisms were discussed and verified.A new series of Ba1+xSr1-xGa4O8:Bi3+(x=0-0.7)solid solution phosphors were constructed by cationic equivalent substitution strategy.They exhibited extremely broadband emission,covering almost the entire visible region and extending to the deep-red region.The emission band showed a red-shift with the gradual substitution of Ba for Sr due to crystal field regulation.Combined with the energy transfer between cation sites and a superposition of multiple emission centers,the corresponding full width at half maximum(FWHM)increased from 192 to 283 nm.As a result,the emission could be tuned from cyan to orange across the warm white light region in Ba1+xSr1-xGa4O8:Bi3+(x=0-0.7)systems.Simultaneously,the thermal quenching would decrease with the formation of a solid solution due to the increasing rigidity of the lattice structure.An optical thermometer with good sensitivity and color discrimination was designed based on the different thermal responses of different luminescent centers to increasing temperature,and its application potential was evaluated.Based on an efficient hetero-valent substitution strategy,a series of Sr1-xLa1+xZnO3.5+x/2:Bi3+(0 ?x ?0.4)solid solution phases were successfully constructed.The regulation of local lattice environments and oxygen vacancy of Bi3+promoted the emission tuning from yellow(558 nm)to orange(585 nm)and the quantum efficiency was improved through La3+substitution for Sr2+.Combined with another site-selective excitation strategy,a novel orange-red luminescence can be observed peaking at?609 nm under n-UV light(?390 nm)excitation without visible absorption.The white LED devices fabricated using these yellow-orange-red phosphor materials exhibit excellent electroluminescence performance.A series of single-composition Bi3+-doped LiCa3MgV3O12 garnet-structure phosphor materials were designed.Through adjusting Bi3+ concentration,the emission color under n-UV excitation could be tuned from bluish green(480 nm)to yellow(562 nm)on the basis of local lattice distortion and VO43-?Bi3+energy transfer.Furthermore,full-color PL tuning from bluish green to orangish red across the warm white light region was successfully achieved via the design of VO43-?Bi3+?Eu3+multiple energy transfers.Their thermal stability could be enhanced by the introduction of stable Eu3+luminescence center.(2)Eu2+-doped apatite-type structure Ca9LiGd2/3(PO4)7 phosphor materials was designed and synthesized.By using cationic equivalent substitution strategy,a series of Ca9-xSrxLiGd2/3(PO4)7:Eu2+(x=0-9)solid solution phosphors were constructed.The crystal field regulation around the Eu2+ caused different luminescence behaviors and induced photoluminescence tuning with Sr2+ replacing Ca2+.Under the combined effect of crystal field splitting,energy transfer between Eu2+in cationic sites,nephelauxetic effect,the emission band showed a redshift with emission color tuning from cyan to yellow.In pursuit of precise color luminescence tuning,the Eu2+?Mn2+efficient energy transfer and mixed-valent Eu2+/Eu3+in Ca9-xSrxLiGd2/3(PO4)7:Eu2+(x=0,4.5,9)solid solution were designed by controlling proper synthesis conditions,a wide range emission color tuning from cyan to red including warm white was achieved.Combined with the evolution of crystal micro-environment structure and photoluminescence properties,the corresponding regulation mechanism was analyzed and discussed.(3)Red phosphor materials play a key role in improving the lighting and backlit display quality of white LED.However,the development of a red phosphor with simultaneous high efficiency,excellent thermal stability and high colour purity is still a challenge.Unique non-concentration quenching in Cs3Gd1-xGe3O9:xEu3+(x=0.1-1.0)was successfully developed to achieve a highly efficient red Cs3EuGe3O9 phosphor material.The red-orange ratio of Eu2+ and lattice symmetry were analyzed based on lattice distortion and covalence degree.Cs3EuGe3O9 shows strong red emission at 611 nm when excited by blue light.Due to the negative thermal expansion(NTE)properties,this red phosphor exhibits better thermal stability at higher temperatures(175-250?,I250?/I25??90%)than commercial red K2SiF6:Mn4+ and Y2O3:Eu3+phosphors.The prepared white LED device exhibits a warm white light with high color rendering(CRI=89.7)and low correlation color temperature(CCT=4508K).Based on the crystal structure,the corresponding mechanism of non-concentration quenching and excellent thermal stability induced by NTE property was revealed,which provides a new perspective for developing new phosphor materials.