Research On The Optical Properties And LED Device Applications Of Rare-earth-based Lead-free Perovskite Materials

The all-inorganic lead halide perovskite nanocrystals（CsPbX₃）have broad application prospects and exploration value in the fields of lighting and full-color displays due to their high photoluminescence quantum efficiency（PLQY）,excellent charge carrier transport capability,and tunable emission wavelength.Currently,the external quantum efficiency（EQE）of red,green,blue,and near-infrared light-emitting diode（LED）devices based on lead halide perovskite materials has exceeded 20%.However,the toxicity of lead ions and the stability issues of the materials greatly limit their practical applications and industrialization.Therefore,the development of lead-free perovskite nanocrystals with excellent optoelectronic properties and stability is of great scientific value and prospect.Currently,various lead-free metal halide perovskite materials have been reported as alternatives to Cs Pb X₃ in the fields of optoelectronics and photovoltaics,such as A₂B^I B^IIIX₆(A=Rb⁺,Cs⁺;B^I=K⁺,Ag⁺,Na⁺;B^III=In³⁺,Bi³⁺,Sb³⁺),A₃B₂X₉(B=Bi³⁺,Sb³⁺),and A₂MX₆(M=Sn⁴⁺,Zr⁴⁺,Hf⁴⁺,Te⁴⁺).However,many of these potential materials still face significant challenges in achieving tunable emission wavelengths（from visible light to near-infrared spectrum）,maintaining high photoluminescence quantum efficiency（PLQY）,and stability.Moreover,there are few reports on electroluminescent devices based on lead-free perovskite materials.In the context of this research,based on the aforementioned issues,this paper starts with the design and preparation of lead-free perovskite nanocrystals.It replaces lead ions with trivalent rare earth ions that have similar radii to Pb²⁺ions,which have advantages such as stable luminescence,narrow linewidth,and rich transition spectra（emitting light from ultraviolet to infrared spectrum）.By improving the thermal injection method,efficient and stable rare earth-based lead-free perovskite nanocrystals with multi-color emission were successfully prepared and applied in electroluminescent devices.The specific research contents are as follows:1.By an improved thermal injection method,lanthanide family lead-free Cs₂NaLnX₆（Ln=La,Ce,Sm,Eu,Tb,Er,Yb;X=Cl,Br,I）perovskite nanocrystals were synthesized.Lanthanide ions serve as both the main lattice structure ions and4f-4f emission centers.The mechanism of the broadband emission of Cs₂Na Ln X₆nanocrystals was investigated through the ultrafast dynamics detection,first-principles calculations and experimental analysis.The broadband emission of Cs₂Na Ln X₆nanocrystals originates from self-trapped excitons（STE）.These novel lead-free perovskite nanocrystals based on lanthanide elements can remain stable in water for over eight months and exhibit remarkable thermal stability and photostability.The PLQY of these materials is much higher than that of most double perovskite materials,especially Cs₂Na Eu Cl₆,which can reach 70%.By adjusting the ratio of Tb and Eu,white light emission was achieved in a single host material Cs₂Na Tb Eu Cl₆（Tb:Eu=25:1）.Combined with UV LED chips,white light LED devices with CIE coordinates of（0.334,0.326）and a high color rendering index（CRI）of 91.2 were realized.2.In order to achieve the application of electroluminescent devices,we introduced Li⁺ions to replace Na⁺ions to prepare lanthanide-based Cs₂LiLnCl₆（Ln=La,Sm,Eu,Tb,Er,Yb）perovskite nanocrystals.The rare-earth-based lead-free perovskite nanocrystals exhibited good optical,thermal,and humidity stability.Furthermore,Cs₂Li Ln Cl₆ nanocrystals show high PLQY,especially the PLQY of Cs₂Li Eu Cl₆nanocrystals can reach 82%.By adjusting the ratio of Tb and Eu,white light emission was achieved in a single host material Cs₂Li Tb Eu Cl₆（Tb:Eu=20:1）.This Cs₂Li Tb Eu Cl₆（Tb:Eu=20:1）was successfully applied to white light electroluminescent devices,with a maximum EQE of 0.79%and a maximum brightness of 198 cd m^-2.The T₅₀ of the device reached 84 minutes.3.Currently,it is challenging to achieve efficient,tunable,multi-color,and high color purity narrow-band（within a 20 nm range）electroluminescent devices using rare-earth ions in lead-free perovskite materials.In light of this,we have designed a multifunctional coumarin organic small molecule（EHC）,to modify Cs₃Ln Cl₆perovskite nanocrystals and films.EHC acts as a sensitizer to enhance energy transfer efficiency and reduces non-radiative recombination by passivating defects.Due to the modification with EHC,Cs₃Ln Cl₆ nanocrystals exhibit high PLQY and achieve tunable emission from visible to near-infrared.Notably,Cs₃Eu Cl₆nanocrystals can achieve an impressive PLQY of 92.4%with pure red emission and an exceptional energy transfer efficiency of 93.2%.Furthermore,our work significantly improves the humidity stability of Cs₃Ln Cl₆ nanocrystals.For example,in an environment with humidity exceeding 80%,the fluorescence intensity of EHC modified Cs₃Eu Cl₆nanocrystals can still maintain over 90%of its initial value after three months.Finally,efficient and stable red,green,and blue LED devices were fabricated based on Cs₃Ln Cl₆ rare-earth-based lead-free perovskite materials.The external quantum efficiency（EQE）of the red LED reached 5.17%,with a full width at half maximum（FWHM）of only 18 nm at 617 nm.The maximum brightness was 2373 cd m^-2,and the operating half-life reached 440 h at the brightness of 100 cd m^-2.This represents the highest efficiency achieved so far for lead-free narrow-band emission devices.