Design,Synthesis,and Luminescence Properties Of Ce³⁺ Doped Nitride Phosphors For Laser Lighting

As a newly emerged high-power solid-state lighting technology,laser lighting attracts great attention in high-luminance lighting and display applications.Usually,white laser light is created by coupling blue laser diode(LD)with luminescent materials(phosphors),that is phosphor-converted white LD(PC-WLD).However,together with the realization of high-luminance white light,the highly intense laser beam also brings color conversion materials tough challenges,i.e.,the failure of conventional organic encapsulation,and the luminance saturation,therefore driving the explosive development of all-inorganic color converters(denoted as laser phosphors later)for laser lighting.Currently,the research of laser phosphors focuses on the Y3Al5O12:Ce3+(YAG:Ce3+)represented garnet phosphors,but their deficiency in red spectral range severely limits the light quality improvement of PC-WLDs.Hence,developing phosphors of various emission colors suitable for laser excitation is of great concern in the community of laser lighting,unfortunately,the achievement is very limited until now.Facing the serious scarcity of laser phosphors of various emission colors,the design,synthesis,and luminescence properties study of a series of novel laser phosphors have been carried out in this dissertation by doping Ce3+activators in nitride hosts.The luminescence performance of these phosphors under high-power laser excitation has been evaluated in a form of phosphor-in-glass(PiG)film.These works are a part of the great efforts to improve the light quality of laser lighting sources.Meanwhile,some new ideas for phosphors design have been proposed.In Chapter 1,the technologies of solid-state lighting and laser lighting are firstly introduced,followed by a basic introduction to phosphors.Then,the luminance saturation behavior and corresponding mechanisms of phosphors under laser excitation are expounded,and the screening and design rules for laser phosphors are proposed.Thirdly,the research progress on laser phosphors(i.e.,luminescent single crystals,bulk PiGs,PiG films,and phosphor ceramics)is highlighted,and finally the opportunities and challenges for the development of laser phosphors are analyzed.In Chapter 2,the raw materials,experimental instruments,and test equipment involved in the research are listed,then some key characterizations and tests,as well as their working principles are briefly introduced.In Chapter 3,an efficient and thermally robust La3Si6N11:Ce3+-in-glass(LSN:Ce3+PiG)film for laser lighting is prepared by cofiring yellow-emitting LSN:Ce3+phosphors and glass frits of low-melting-point on a thermally conductive sapphire substrate.This work creates a high-brightness white laser light using a nitride phosphor for the first time.Due to the excellent chemical stability of LSN:Ce3+,the interfaces between LSN:Ce3+particles and glass matrix are very clear without any detectable interfacial reactions,thus enabling the LSN:Ce3+-PiG film to fully inherit the original thermal stability and high quantum efficiency.The optical performance of the film is optimized by changing the phosphor-to-glass(PtG)ratios and the film thickness,consequently the highest light conversion efficiency is achieved at PtG=2:3 and the thickness of 70 μm,respectively.When coupled to a blue LD,the LSN:Ce3+-PiG film can withstand the maximum laser power density of 12.91 W mm-2 and create a white LD source with a luminous flux of 1076 lm(luminance of 773 Mcd m-2)and a luminous efficacy of 166.05 lm W-1.In Chapter 4,a red-emitting laser phosphor La3Si6N11:Al3+,Ce3+(LSN:Al3+,Ce3+)is designed and synthesized via the interstitial site engineering.The unusual red emission of Ce3+activator(600-665 nm)is realized by an aliovalent substitution of Al3+for Si4+in LSN:Ce3+lattice.The luminescence center and corresponding luminescence mechanism are clarified through the combination of the first-principles calculation and the spectral analysis,which suggests that the red emission originates from the interstitial luminescence center(Ceint3+)formed by the Ce3+occupying at the[Si8N8]void in the c-1/2 layer of LSN lattice.The volume of the void is small,hence causing the long wavelength emission.The short photoluminescence lifetime of LSN:Al3+,Ce3+is 43 ns,which can avoid the non-thermal optical saturation,therefore making it a promising red-emitting phosphor for laser excitation.At last,a dual-layered PiG film composed of yellow-emitting LSN:Ce3+and red-emitting LSN:Al3+,Ce3+is prepared,then it is combined with a blue LD to construct a high color rendition and warm white LD with a color rendering index(Ra)of 78 and a correlated color temperature(CCT)of 3321 K.In Chapter 5,ternary solid solution phosphors Ca1-x-yLixAl1-x-ySi1+x+yN3-yOy:Ce3+(CALSON:Ce3+)are designed and synthesized for high-power laser lighting through a multi-component solid solution strategy.These ternary solid solution phosphors are obtained by simultaneously introducing isostructural LiSi2N3 and Si2N2O into the CaAlSiN3:Ce3+(CASN:Ce3+)lattice.The formartion of solid solutions can effectively broaden the bandgap of CASN:Ce3+(5.21→5.50 eV),thus largely enhancing its thermal stability(the emission intensity retention at 200℃ increases from 75%to 91%)and making it suitable for high-power laser lighting.Under blue light excitation,CALSON:Ce3+shows intense and broadband yellow-orange emission peaking at 580 nm with a full width at half maximum of 135 nm and an external quantum efficiency of 41.5%.Benefiting from the enhanced thermal stability and the broadened bandgap,the luminance saturation threshold of CALSON:Ce3+-PiG film is 17.8 W mm-2,much higher than that of CASN:Ce3+-PiG film(12.9 W mm-2).The white LD constructed with CALSON:Ce3+-PiG film can emit a luminous emittance of 781 lm mm-2 with a CCT of 4349 K and Ra of 70,showing better light quality than those converted by YAG:Ce3+.In Chapter 6,the conclusions for the whole dissertation are presented,and some suggestions on the optimization and application of these prepared laser phosphors are given.Finally,some prospectives for future research on laser phosphors and PC-WLDs are proposed.