Preparation And Optical Emission Properties Of Up/Down Conversion Materials

Trivalent rare earth(RE) ions can realize the conversion of photon energy,which makes the RE ions doped inorganic micro/nano upconversion(UC)and down conversion(DC)luminescence materials have important applications prospects,including solids-state lasers,laser anti-counterfeiting brand,bio-fluorescent probes,three-dimensional display,temperature detectors,photodetectors and solar cells.The luminescence properties of materials are affected by their composition,size,morphology and structure,so it is of great significance to explore the relationship among these features and then design and assemble UC/DC materials with unique properties.The key of UC/DC materials research is to improve the luminescence efficiency and to explore the influence of preparation methods and material combinations on the UC/DC performance.In contrary to the ability of UC process which can convert multiple low-energy photons into a high-energy photon,the quantum cutting(QC)DC luminescent materials convert a high-energy photon into two or more near-infrared photons,improving the spectral response and reducing the carriers heat loss of silicon-based solar cells.In this thesis,Na Y(WO₄)₂ UC materials and Na YF₄ UC/DC materials with different morphologies,sizes and dopants were prepared.The phase structure and UC/DC fluorescence properties were studied.The preparation of RE ions doped high performance conversion film is a key step to promote the conversion of materials from research to application.Therefore,it is of great significance to prepare a conversion film with excellent comprehensive performance by a simple and easy method.In this thesis,the conversion film with the above advantages was prepared by studying the advantages and disadvantages of the one-step method and the two-step method.Finally,the applications of the UC/DC material were explored.In view of the superior chemical stability of oxygenates,we selected the Na Y(WO₄)₂ as the UC matrix material firstly.When the micro-sized Na Y(WO₄)₂ was prepared as UC matrix material,the UC luminescence intensity increased with the increase of the doping amount of Er³⁺.The luminescence intensity of the 10%Er³⁺sample was 13 times higher than that of the 5%Er³⁺sample.With the further increase of the Er³⁺doping concentration,the appearance of the concentration quenching caused the significant decrease of luminescence intensity,indicating that the UC process has energy transfer participation.The UC luminescence of different Er³⁺doping concentration samples in the temperature range of 300 K-30 K were tested.The results showed that the 5%Er³⁺sample's intensity of emission bands located at 530-578 nm region(green emission 1)and 638-690nm(red emission)region at 30 K reach the highest values,and the enhancement ratios were approximately 50 and 3.6 times,respectively.At the same time,the UC luminescence of samples doped with high Er³⁺concentration were less sensitive to temperature.It was found that the introducation of Li⁺ions could change the crystal field environment around Er³⁺and the radiation property of Er³⁺ions due to the greatly diffenrent ionic radius of Li⁺and Er³⁺ions,which promoted the UC efficiency of Er³⁺ions.The UC intensity of 5%Er³⁺-5%Li⁺sample was about 9 times higher than that of sample without Li⁺.The incorporation of Li⁺ions could also increase the luminescent center concentration at the ⁴F_9/2 level of Er³⁺ions,resulting in a higher enhancement of red emission than the green ones.The enhancement of UC intensity of Er³⁺-Li⁺co-doped sample was less than that of the single-doped Er³⁺sample at low temperature due to the increased probability of radiative transition of Er³⁺ions by the addition of Li⁺ions.Er³⁺doped Na Y(WO₄)₂ nano-sized powders with particle sizes between 100-500 nm were also successfully synthesized,and there was no significant difference in the intensities of UC emissions compared with the micro-sized powder.However,some defects on the surface of the nano-powder and the phonon energy of the functional group reduced with the temperature decreasing,which enhanced UC emission of nano-sized powders at low temperatures.Thus,the nano-sized Er³⁺doping Na Y(WO₄)₂ powder exhibited superior temperature sensitivity over micro-sized one.Fluoride has very low vibrational energy,and it's non-radiative relaxation transition could be suppressed and a high quantum efficiency of the desired luminescence could be obtained.Therefore,Na YF₄ was selected as the host matrix material for UC luminescence and DC luminescence materials.However,Na YF₄ has different phase structures(the the cubic phase(?)and the hexagonal phase(?)).It was proved that the pure phase?-Na YF₄ having high luminescence efficiency could be directly prepared by the solvent of alcohol and oleic acid(alcohol-OA)mixture.By studying?-Na YF₄:Er³⁺,Yb³⁺UC materials doped with different concentration of Mn²⁺ions,it was found that the addition of Mn²⁺ions significantly affected the transition of the Na YF₄ phase structure from hexagonal to cubic phase.When the doping concentration of Mn²⁺ions was below 20%,the energy transfer process between Er³⁺and Mn²⁺increased with concentration of Mn²⁺ions,and the intensity of red emission continued to enhance.The futher increase of Mn²⁺ions would cause lattice distortion due to the difference in radius between Mn²⁺and Y³⁺,thereby reducing the UC luminescence intensity.In the low-temperature UC luminescence test,the 20%Mn²⁺sample at 30 K had a significantly higher UC luminescence intensity than the 0%Mn²⁺sample.The?-Na YF₄ UC phosphors with 5%Tb³⁺ions and different Yb³⁺ions doping concentrations were prepared by solvothermal method.With the increase of Yb³⁺ion concentration,the grain sizes became larger,and the characteristic luminescence peak intensity of Tb³⁺ions decreased while that of Yb³⁺ions increased,indicating the existence of energy transfer process from Tb³⁺to Yb³⁺in samples.The transformation of the conversion material powder into a film is of great significance for the subsequent application.Therefore,two methods for preparing the conversion film were studied in this thesis:hydrothermal method(one-step method)and spin coating method(two-step method).The results showed that the Na YF₄:Er³⁺,Yb³⁺films with strong UC luminescence could be prepared through the hydrothermal method easily and quickly.The film region was closely arranged by clusters of Na YF₄ rod-like grains having a diameter of about 300 nm.However,in some regions,grains of different shapes and sizes were generated due to the difference in grain growth environment,and the arrangement was not dense enough.Moreover,the arrangement of the crystal clusters in the film region was not a single layer structure,which directly led to a great decrease to below 80%in transmittance value of films.Thus,the following applications of this film were limited due to its poor transparency,not uniform layer and complicated synthesis parameters.Subsequently,the spin-coating method(two-step method)was used to prepare conversion films.The?-Na YF₄UC/DC powders were dispersed in an anisole solution with dissolved polymethyl methacrylate(PMMA),and?-Na YF₄powders had the great dispersibility in anisole since the surface had OA functional group.At the same time,the passivation and adhesion of PMMA enhanced the green emission intensity of the UC powder and the binding force between the powder and the substrate.The resualts showed an excellent conversion film with high transmittance,no decrease in fluorescence intensity,strong bonding force,and dual function of up/down conversion was successfully prepared on a glass substrate by a simple spin coating method.We explored the application of UC/DC materials in three different fields.Firstly,the research on different UC powders in the field of temperature sensor was carried out.It was found that the phonon energy of matrix material is positively correlated with the?E value of thermal coupling energy level.The UC material with higher phonon energy was more sensitive to room and high measured temperature.However,at low temperature regions below 240 K,the UC materials withi lower phonon energy had the higher sensitivity.In this thesis,the UC film was also applied to CZTS/Cd S heterojunction infrared photodetector.Under 980 nm laser excitation,the short circuit current of the device increased by 26.4%.The device current switch ratio increased from 70.51 to239.33,and the response current also increased from 2.75 m A to 3.59 m A.These results demonstrated that the UC film could improve the performance of the near-infrared photodetectors.Finally,the Si O₂ film with dispersed Na YF₄:Tb³⁺,Yb³⁺DC powder was prepared and then was applied on the crystalline silicon solar cell.Due to the common influence of quantum tailoring of DC materials and the anti-reflection of Si O₂,the short circuit current density of the solar cell increased from 31.97 m A/cm² to 33.39 m A/cm²,and the photoelectric conversion efficiency also increased from 11.57%to 12.35%,which the relative increase rate was 6.74%.