Up-Conversion And Down-Shifting Luminescence Of Rare Earth Doped Lead-Free Double Perovskite And Its Application

Luminescent materials have developed rapidly in the fields of biomedicine,semiconductor,solar energy,illuminating display,sensing and anti-counterfeiting due to their rich variety,colorful,excellent photoelectric performance and various morphology.In these fields,environmentally friendly plant illuminating light-emitting diodes（LEDs）,as the supplementary light source for indoor plant growth,are very important for plant health growth.In addition,high security fluorescent anti-counterfeiting materials,can well safeguard the interests of the people and national information security.Therefore,the development of novel environmental plant lighting and fluorescent anti-counterfeiting materials are particularly important,in line with the national green development strategy.Environmental lead-free double perovskite materials(A₂M⁺M³⁺X₆:A⁺,M⁺,M³⁺are metal ions,X are halogen ions)have great application potential in above two aspects due to their advantages such as non-toxicity,high stability,large lattice tolerance,large absorption cross section and rich molecular structure.However,there are some disadvantages,such as large band gap,low spectrum tunability and single emission mode.Rare earth ions have outstanding advantages,such as rich 4f-4f transition energy levels,narrow emission peaks,high color purity,good stability,long fluorescence lifetime and wide emission wavelength range（including ultraviolet,visible and near infrared emission）,which are ideal dopant.However,the small absorption cross section of rare earth ions has been the bottleneck problem of low luminous efficiency.In view of the advantages and disadvantages of two types of materials.We propose doping rare earth ions into double perovskites,which not only solves the problems of difficult spectrum regulation and single luminescence mode of double perovskites,but also uses the large absorption cross section of double perovskites to sensitize rare earth ions,which solves the problem of small absorption cross section of rare earth ions.Therefore,in this paper,rare earth ions doped lead-free double perovskites are taken as the research object,aiming at obtaining efficient multi-mode and spectrum adjustable fluorescent materials,and explore their potential in LEDs and anti-counterfeiting applications,so as to provide experimental and theoretical basis for their commercialization process.Firstly,rare earth Gd³⁺ions were doped into Cs₂Ag InCl₆double perovskites to obtain highly efficient wide-band red and near-infrared emission,which were applied to plant lighting LEDs.Secondly,rare earth Er³⁺ions were doped into Cs₂NaYb Cl₆double perovskites to realize efficient dual-mode multicolor emission,which were applied to high level of anti-counterfeiting and information encryption.The specific work is as follows:（1）The development of light-emitting diodes（LEDs）that can promote the entire process of plant growth remains a challenging task.Herein,Sb and Gd co-doped Cs₂Ag InCl₆double perovskites microcrystals（DPMCs）with efficient red,far-red and NIR light have been synthesized by solution-processable recrystallization method.Sb³⁺ions doping not only regulates the emission peak of undoped Cs₂Ag InCl₆DPMCs from 600 to 670 nm,but also increases the photoluminescence quantum yields（PLQYs）from 0.5%to 11.7%.Furthermore,the Gd³⁺ions doping further improves the emission intensity,resulting in a PLQYs of 26.3%.This value is 52.6 times higher than that of the undoped sample.The DFT calculation results exhibit the Cs₂Ag InCl₆host has a direct band gap and Sb ions doping slightly reduces the band gap of the host.Finally,LEDs have been prepared by coupling Cs₂Na InCl₆:Sb³⁺（blue）,Cs₂Ag InCl₆:Sb,Gd（red,far-red,NIR）and 365 nm ultraviolet chips.The LEDs simultaneously contain blue,red,far-red and near-infrared light emitting components,which can meet the entire process of plant growth.And there is no energy loss caused by reabsorption between phosphors with different luminescent colors,which solves very challenging problems in the field.This work provides a new idea for the preparation of effective plant lighting LEDs.（2）The development of rare earth-doped luminescent materials with single component,multicolor,and multi-mode still remains a challenge.Here,we fabricated rare earth based double perovskite single crystals（Cs₂NaYb Cl₆DPSCs）with low phonon energy of 134 cm^-1,which offers great potential to serve as highly efficient luminescent host materials.Subsequently,Er³⁺,Tm³⁺and Ho³⁺were successfully incorporated to Cs₂NaYb Cl₆host to form Cs₂Na Ln_xYb_1-xCl₆（Ln=Er,Tm and Ho）DPSCs,realizing efficient dual-mode polychromatic emission.Under 980 nm laser excitation,Cs₂Na Ln_xYb_1-xCl₆DPSCs show high up-conversion（UC）efficiencies of 12.9%,7.2%,and 3.1%for Cs₂Na Er_0.3Yb_0.7Cl₆,Cs₂Na Tm_0.02Yb_0.98Cl₆,and Cs₂Na Ho_0.3Yb_0.7Cl₆,respectively.Meanwhile,Cs₂Na Er_xYb_1-xCl₆DPSCs exhibit the adjustable size（0.5 to 10 mm）and tunable UC or down-shifting emission colors from yellow,green to red upon 980,1540 nm,or 380 nm excitation.Moreover,the DPSCs demonstrate weak concentration and thermal quenching effect,and excellent resistance against oxygen,heat and humidity.Finally,we designed and realized the anti-counterfeiting and information encryption applications based on multicolor emitting DPSCs.