Structure And Luminescence Properties Of Ba₉Lu₂Si₆O₂₄ Based Phosphor For Full Spectrum Lighting LED

With the higher requirements of healthy life quality,the full-spectrum white LED has become the focus of the development of lighting field.As for the full-spectrum lighting schemes excited by violet/near-ultraviolet chips,the blue-green emitting phosphor is one of the key material to achieve continuous and uniform spectra.However,the existing blue-green emitting phosphors have narrow emission spectra,resulting in poor spectral continuity of the white LED devices.Therefore,the development of blue-green emitting phosphor with wide emission band which can be excited by violet/near ultraviolet light effectively is an important topic of research significance.In this paper,the multi-site Ba9Lu2Si6O24 is selected as the host.Through the substitution of alkaline earth metal ion Sr2+/Ca2+ in the multi-site host,the occupation of Eu2+/Ce3+activator ions are induced to construct multi-site luminescence centers,which results in the enlargement of emission band and directional regulation of luminescent properties.The effects and their mechanism of local structure regulation on the luminescence characteristics are investigated.The multi-site blue-green emitting phosphors with excellent properties are developed,which can be effectively excited by violet/near ultraviolet light and applied for full-spectrum LED.Based on the investigation of local structure and emission spectrum of Ba9Lu2Si6O24:Eu2+ phosphor,the Eu2+activator ions are proved to occupy Bal,Ba2 and Ba3 sites in multi-site host,that form Eu1,Eu2 and Eu3 characteristic luminescence,respectively.Based on the multi-site regulation strategy,a series of(Bai-xSrx)9Lu2Si6O24:Eu2+phosphors are designed and synthesized.The alkali earth metal ions Sr2+are introduced into the lattice to achieve the tuning of luminescence centers.Through Sr2+substitution,the occupying tendency of activator Eu2+is gradually transferred from Eu1 to Eu2 and Eu3,which results in the enlarged emission band covering the blue-cyan-green regions for the multi-site phosphors(the FWHM of the x=0.2 sample is 139 nm).Due to the superior thermal quenching performance of Eu2 characteristic luminescence,the thermal quenching performance of Sr2+-substituted samples are effectively improved.As for the x=0.3 sample,the emission intensity at 423 K can maintain 73.4%of that at 323 K.The wLED device fabricated by the combination of a 380 nm chip with the(Ba0.8Sr0.2)9Lu2Si6O24:Eu2+sample and(CaSr)AlSiN3:Eu2+red phosphor shows a warm white emission with continuous spectrum and wide coverage(Ra=94.5).By the introduction of Ca2+ to achieve the equivalent/non-equivalent substitution,a series of Ca2+-substituted Ba9Lu2Si6O24:Eu2+phosphors are designed and synthesized.Through the first-principles calculation,XPS,TEM and other tests,the Ca2+ ions are proved to occupy Bal,Ba2,Ba3 and Lu sites in multi-site host,which induces the enlargement of interplanar distance and the expansion of crystal lattice.Ca2+substitution induces the activator Eu2+to occupy Lu crystallographic site with stronger rigidity and higher symmetry,thus achieving rare equivalent/non-equivalent substitution for four crystallographic sites.With the Ca2+ substitution,the Eu(Lu)characteristic luminescence is gradually enhanced,and the Eu(Bal)gradually transfers to Eu(Ba2)and Eu(Ba3)characteristic luminescence,which results in the broadening of the emission band and the enhancement of the luminous efficiency and thermal stability.The external quantum efficiency increased from 45.1%(x=0)to 56.6%(x=0.2),and the thermal quenching performance increased by 42%at 473 K(x=0.25).The cyan-emitting phosphor with broadband emission,high luminous efficiency and excellent thermal stability which can be excited by near ultraviolet light is developed.Based on the investigation of local structure and emission spectrum of Ba9Lu2Si6O24:Ce3+ phosphor,the Ce3+activator ions are proved to occupy Bal,Ba2,Ba3 and Lu sites in multi-site host.The blue emission at 490 nm under the excitation of the 390 nm near ultraviolet light is attributed to Ce[Lu]characteristic luminescence.Based on the multi-site regulation strategy,a series of Ca2+-substituted Ba9Lu2Si6O24:Ce3+phosphors are designed and synthesized.Through Ca2+ substitution,Ce3+at Lu position is oxidized to Ce4+based on the charge balance effect,the occupying tendency of activator Ce3+is transferred from Bal to Ba2 and Ba3 crystallographic sites gradually.The re-absorption phenomenon of Ce[Lu]and Ce[Ba]is effectively inhibited,and the emission band of Ce[Ba]characteristic luminescence is broadened.The external quantum efficiency of the Ca2+-substituted samples is increased from 0.38(x=0)to 0.53(x=0.2).But the thermal stability of Ca2+-substituted samples is declined because the activator Ce3+is less located at Lu crystallographic site with stronger rigidity and higher symmetry.The Ca2+-substituted phosphors achieve the broadening of emission at the short-wavelength region,which can effectively enhance spectral integrity for full-spectrum lighting.