Preparation And Photoluminescence Of Ce³⁺ Doped Garnet Structure Phosphors

With the development of modern society,people’s requirements for lighting devices are becoming more and more stringent.White light-emitting-diode（LED）have gradually displaced the traditional lighting devices because of their remarkable characteristics,including exergy saving,environment friendly,highly efficiency,long work lifetime,and so on.The combination of blue LED chip coated with commercial Y₃Al₅O₁₂:Ce³⁺ yellow phosphors is a common method to achieve the white LED.Unfortunately,the obtained white light suffers low color rendering index（CRI）and high correlated color temperature due to the lack of red component in the emission spectrum in Y₃Al₅O₁₂:Ce³⁺.In order to improve the CRI of white LED,the researchers always add the additional red phosphors during the fabrication process for making up the defects.However,there is a significant missing emission intensity between the blue light of LED chip and yellow emission of Y₃Al₅O₁₂:Ce³⁺phosphor in the 480-520 nm wavelength region,which is called cyan gap.This so-called cyan gap induces a discontinuous white-light emission spectrum,resulting in the low-quality white light.Therefore,it is important to exploit the efficient phosphors for achieving the high-color-quality white LED.In the view of the aforementioned defects of low CRI and high CCT,the paper demonstrates a series of blue-light-excited Ce³⁺ ions doped garnet phosphors,and systematically study the phase purity,crystal structure,morphology,luminescent properties,thermal stability of as-prepared sample.The main research results as follows:（1）A series of Ce³⁺ doped CaGd₂HfScAl₃O₁₂ garnet phosphors via the high-temperature solid-state reaction method.The as-prepared CaGd₂HfScAl₃O₁₂:Ce³⁺ garnet phosphor possesses a cubic structure with Ia（?）d space group,and the unit cell parameters of the representative CaGd₂HfScAl₃O₁₂:2%Ce³⁺ phosphor are a = b = c = 12.450 （?）,α = β = γ = 90° and V = 1929.59（4）（?）3.Impressively,we find that the CaGd₂HfScAl₃O₁₂:Ce³⁺ garnet phosphor shows an intense absorption band in the 300-500 nm wavelength range with a maximum at 452 nm owing to the 4f（?）5d transition of Ce³⁺ ions.Upon 452 nm excitation,the optimal CaGd₂HfScAl₃O₁₂:2%Ce³⁺ sample exhibits a broad asymmetric yellow emission band in the wavelength range of 470-750 nm with peak at 564 nm and full width at half maximum of 151 nm.The Commission Internationale de l’Eclairage（CIE）chromaticity coordinates and internal quantum efficiency of the CaGd₂HfScAl₃O₁₂:2%Ce³⁺ sample are（0.4485,0.5157）and 30.4%,respectively.Finally,a white LED device is fabricated by combing a 450 nm blue LED chip with commercial Y₃Al₅O₁₂:Ce³⁺ yellow-emitting phosphor,which generates white light with low color rendering index（CRI;Ra = 74.7,R9 =-12.7）and high correlated color temperature（CCT = 6554 K）under the 60 m A driving current.In sharp contrast,another white LED device,which is made by coating our as-prepared CaGd₂Hf ScAl₃O₁₂:2%Ce³⁺ yellow-emitting phosphors onto the surface of a 450 nm blue LED chip,produces white-light emission with high CRI value（Ra = 84.5;R9 = 26.3）and relatively low CCT of 5649 K.This work reveals that the newly discovered broadband yellow-emitting CaGd₂Hf ScAl₃O₁₂Ce³⁺ phosphors can serve as a potential color converter in high-color-quality phosphor-converted white LEDs.（2）In this research,we report an efficient broadband green-emitting CaY₂HfScAl₃O₁₂:Ce³⁺ garnet phosphor enabling full-spectrum white LEDs.Upon 450 nm excitation,the optimal phosphor shows a broad green emission band in the wavelength range of 460-750 nm with peak around 532 nm and full width at half maximum of 121 nm.Compared to the Y₃Al₅O₁₂:Ce³⁺ yellow phosphor,the emission of CaY₂HfScAl₃O₁₂:Ce³⁺ green phosphor covers the cyan spectral range,thus minimizing the so-called cyan gap.Importantly,the internal and external quantum efficiencies of the optimal CaY₂HfScAl₃O₁₂:2%Ce³⁺ sample are 78.9 and46.6%,respectively.Finally,a white LED device is fabricated by utilizing CaY₂HfScAl₃O₁₂:Ce³⁺ green phosphor,CaAlSiN₃:Eu²⁺ red phosphor and a 450 nm blue LED chip,which generates bright full-spectrum warm-white light with ultrahigh color rendering index（Ra = 98.0,R9 = 97.5）and low correlated color temperature of 3602 K under 180 mA driving current.