Research On Mn⁴⁺-doped Oxide Red Phosphor For Plant Lighting

With the rapid development of artificial illumination technology,phosphor-converted light-emitting-diodes（pc-LEDs）are becoming prevalent in plant lighting for their virtues,such as energy savings,low radiant heat output,high efficiency,tunable luminescence,and so on.Especially for the tunable luminescence,they can provide customized spectra for specific plant growth by combining the suitable phosphors with commercial LED chips.Previous research has revealed that the red light required for plant growth mainly includes deep-red light peaking at 660 nm and far-red light centered at 730 nm.Mn⁴⁺doped oxides usually exhibit wide excitation bands in the range of 200-600 nm and narrow emission bands ranging from 650 to 780 nm,which make them possess broad application prospects in the field of plant lighting.Therefore,Mn⁴⁺ion has been selected as the activator in this thesis.By exploring appropriate oxide matrix materials and studying the influence of component substitution on the luminescence properties,three kinds of high-efficient deep/far red phosphors for plant lighting have finally been prepared by the traditional high-temperature solid-state method.The specific contents are as follows:1)The inequivalent double-site substituted Li₄Al Sb O₆:Mn⁴⁺(LAS:Mn⁴⁺)phosphor was successfully synthesized by a solid-state reaction method.The crystal structure and luminescent properties of LAS:Mn⁴⁺together with the inequivalent double-site substitution of Mn⁴⁺for Al³⁺and Sb⁵⁺ions in LAS were studied.The results show that the LAS compound belongs to the monoclinic C2/c space group.Upon 467 nm excitation,the phosphor emits bright red light centered at 673 nm.The optimal content of Mn⁴⁺in LAS is 0.5 mol%,its corresponding internal quantum yield and color purity are 39.5%and 99.1%,respectively.The photoluminescence（PL）and time-resolved emission spectra prove that Mn⁴⁺ions would occupy both Al³⁺and Sb⁵⁺octahedral sites in the LAS crystal.The device fabricated by combining LAS:0.5%Mn⁴⁺phosphor with a 460 nm LED can emit light about 460 and 673 nm,which provides the blue and deep-red light regions for plant growth at the same time.The LED device packaged with LAS:0.5%Mn⁴⁺and commercial YAG:Mn⁴⁺phosphor exhibits warm white light with low correlated color temperature（CCT=3534 K）.2)The Ca_3-6y（NaLu）_3yLiSbO₆:Mn⁴⁺(CNULS:Mn⁴⁺)far-red phosphor with excellent luminescent properties was synthesized by a solid-state method.The crystal structure and luminescent properties of Ca₃LiSbO₆:Mn⁴⁺(CLS:Mn⁴⁺)phosphor were studied,and the effects of the partial co-substitution strategy of[Na⁺-Lu³⁺]for[Ca²⁺-Ca²⁺]on its crystal structure,luminescence properties,and thermal stability were investigated.The results show that the PL spectrum of CLS:Mn⁴⁺exhibits a far-red emission band peaking at 694 nm.The optimal doping concentration of Mn⁴⁺in CLS is determined to be 0.6 mol%and its corresponding internal and external QEs are 39.5%and 23.1%,respectively.Through the chemical unit co-substitution of[Na⁺-Lu³⁺]for[Ca²⁺-Ca²⁺]in CLS:Mn⁴⁺,its luminescence properties can be improved tremendously and the internal and external QEs for the substitution amount of y=3%are as high as 80.1%and 47.4%,respectively.The large luminescence enhancement is attributed to the increase of structural rigidity induced by lattice contraction.The CNULS:Mn⁴⁺phosphor exhibits excellent thermal stability with the emission intensity at 423 K remaining 87%of the value at 298 K.The LED device fabricated with the CNULS:Mn⁴⁺（y=3%）phosphor can produce bright and stable far-red emission,which matches well with the absorption spectrum of phytochrome P_FR.3)A novel deep-red-emitting Sr KYTeO₆:Mn⁴⁺(SKYT:Mn⁴⁺)phosphor was achieved through the complete co-substitution strategy of[K⁺-Y³⁺]for[Sr²⁺-Ca²⁺]in non-luminous Sr₂CaTeO₆:Mn⁴⁺(SCT:Mn⁴⁺)material.The crystal structure and luminescent properties of SKYT:Mn⁴⁺phosphor were studied,and its luminescent mechanism was discussed.The results show that the SCT compound belongs to the monoclinic P2₁/n（14）space group.PL spectrum of CLS:Mn⁴⁺exhibits a far-red emission band peaking at 694 nm.The optimal content of Mn⁴⁺ions in Sr KYTeO₆ is found to be 0.4 mol%,and the concentration quenching mechanism belongs to electric dipole-dipole interaction.Upon 338 nm excitation,the color purity and IQE of Sr KYTeO₆:0.4%Mn⁴⁺reach as high as 98.5%and 44.3%,respectively.The phosphor also exhibits good thermal stability with aΔE_a of 0.417 e V and the PL intensity at 423K maintaining 56%of the initial value at 298 K.The possible reason for the favorable performances is attributed to the change of local crystal environment induced by distorting the internal[TeO₆]octahedra.Therefore,a co-substitution strategy for further exploring novel Mn⁴⁺-doped perovskite phosphor was proposed.