Studies On Cyan And Near Infrared Phosphors For Full-spectrum Pc-LEDs

The design of full-spectrum pc-LEDs based on natural spectrum have many advantages such as full solid state,small size,long life,high efficiency,environmental protection and energy saving,etc.As the hotspots in the field of international light source research,the full-spectrum pc-LEDs can not only satisfy the current people-oriented lighting concept,but also be applied to analyze and detect the neighborhood due to its high spectral continuity.Blue-chips are mostly used as excitation light source of commercial pc-LEDs since its lower cost.Therefore,it is of great significance to develop and optimize efficient phosphors suitable for blue light excitation.The full-spectrum pc-LEDs obtained good effects in its visible part as some commercial phosphors,such as the yellow phosphor YAG:Ce³⁺,yellow-green phosphor LuAG:Ce³⁺,red phosphor Sr₂Si₅N₈:Eu²⁺,CaAlSiN₃:Eu²⁺,are widely used in LEDs.However,there is little achievement in the cyan light at 490 nm and the near-infrared（NIR）short-wave at 700-1100 nm,which limits the further development and application of pc-LEDs.In order to obtain vis-NIR pc-LEDs,the cyan phosphors BaSi₂O₂N₂:Eu²⁺（BSON）,which can be used to improve the color index of white LEDs,and the NIR phosphors Ca₂LuZr₂Al₃O₁₂:Cr³⁺(CLZA:Cr³⁺),which can be used in NIR spectroscopy,were studied by co-doping rare earth ions.（1）In BaSi₂O₂N₂:Eu²⁺,the luminescence performance is reduced by the presence of Eu³⁺ions.By doping the trivalent rare earth Ho³⁺ions into BaSi₂O₂N₂:Eu²⁺,the original ionization equilibrium of the lattice breaken.Hence,the trivalent ions were enriched and the production of Eu³⁺were inhibited in the material.As a result,the luminescence performance of cyan phosphors was improved.As the concentration of Ho³⁺reaches 3%,the intensity of Eu²⁺emission at 495 nm reaches the maximum value.This study provides a solution for reducing Eu³⁺in nitride materials.In addition,the phosphor converted white LEDs with color rendering index up to 98.7 and its light efficiency was still up to 94.82 lm/W.In order to simulate the sunlight with high-quality LEDs light,the concept of similarity（SCF）is used to design the LEDs spectral evolution specification after spectral power design（SPD）.Combining the concept of SCF with the current concept of color rendering index（CRI）will hopefully lead to more specification in the filed of full-spectrum pc-LEDs.（2）Super broadband near-infrared（NIR）phosphor converted light-emitting diodes（pc-LEDs）are future light sources in NIR spectroscopy applications such as food testing.At present,a few blue LED excitable super broadband NIR phosphors（bandwidth>300 nm）were developed with low luminous efficiency.Here,we report an efficient super broadband NIR phosphor achieved by doping Nd³⁺in the NIR CLZA:Cr³⁺garnet phosphor we developed previously.The broadband near-infrared emission originated the emission of Nd³⁺and transition metal Cr³⁺.Compared with the f-f transition of rare earth ions,the d-d transition emission efficiency of Cr is relatively low.Therefore,the spectrum broadening was achieved and the internal quantum efficiency（IQE）reached 74.6%by co-doping Nd³⁺luminescent center.The codoped phosphor converted LED produced 38.1 mW NIR output at 100 mA input current.（3）On the basis of the above,Yb³⁺was co-doped in Ca₂LuZr₂Al₃O₁₂:Cr³⁺due to its simpler energy level structure and higher emission efficiency.Here,we report an efficient super broadband NIR phosphor Ca₂LuZr₂Al₃O₁₂:Cr³⁺,Yb³⁺(CLZA:Cr³⁺,Yb³⁺)garnet phosphor.Benefited from the superposition of Cr³⁺emission and highly efficient Yb³⁺emission excited by energy transfer from Cr³⁺,the codoped CLZA:Cr³⁺,Yb³⁺phosphor shows a bandwidth of 320 nm and an internal quantum efficiency（IQE）of 77.2%both higher than that（150 nm and 69.1%）of singly doped CLZA:Cr³⁺phosphor.The codoped phosphor converted LED produced 41.8 mW NIR output at100 mA input current.The pc-LED as a light source was also well applied to the NIR transmission spectra measurement of water.Our results indicate the great potential of CLZA:Cr³⁺,Yb³⁺phosphor in super broadband NIR pc-LED applications.（4）The uniformity of emission spectrum distribution of Cr/Nd and Cr/Yb co-doped samples remains to be improved.Therefore,we prepared ultra broadband NIR garnet phosphors Ca₂LuZr₂Al₃O₁₂:Cr³⁺,Nd³⁺,Yb³⁺(CLZA:Cr³⁺,Nd³⁺,Yb³⁺)due to fortunately the emission spectra of Nd/Yb ions can be complementary.The tri-doped phosphors exhibit a wider NIR emission with a FWHM of 350 nm.To obtain the more uniform spectrum,the doping concentration of Nd³⁺and Yb³⁺was adjusted.In this case,due to the increase of cross-relaxation,the quantum efficiency reduced.However,the luminescence thermal stability of the co-doped samples is still significantly improved.The codoped phosphor converted LED produced 25.3 mW NIR output at 100 mA input current.Finally,the full-spectrum pc-LEDs with vis-NIR emission was realized,packaging by tri-doped NIR phosphors,cyan phosphor BSON,yellow-green phosphor LuAG and red phosphor CaAlSiN₃.