Synthesis And Properties Of Ti-based Double Perovskite Halides

Recently,all-inorganic lead halide chalcogenide materials have attracted much attention due to their simple synthesis process and high carrier mobility.However,the decomposition of lead halide perovskite produces both lead halide and lead,both of which are water-soluble and can permeate into groundwater,indirectly causing tremendous biological and environmental damage.The all-inorganic lead-free perovskite material has become a new generation of optoelectronic material that can replace lead-based perovskite material due to its advantages of eco-friendly,stability and tunable optoelectronic properties.Presently only a few lead-free perovskites have been synthesized and the exploration of such materials still needs to be carried out extensively.There is still a lot of research space in the field of titanium-based perovskites.In this thesis,the exploration of titanium-based perovskites materials,their stability,photophysical properties,and the most suitable materials for the electron transport layer,is carried out,and the main work is as follows:（1）In this thesis,lead-free perovskite Cs₂TiBr₆ nanocrystals were synthesized by an improved thermal injection method,and nanocrystal films with uniform size and high surface flatness were obtained by Sn Br₄ doping.（2）We have tested the photophysical properties of Cs₂TiBr₆.Through the variable temperature absorption test,we found that the effect of temperature on the band gap of Cs₂TiBr₆ is quite slight,varying by 30 me V.Considering the great potential of Cs₂TiBr₆nanocrystals for optoelectronic applications.We investigated the carrier complexation in Cs₂TiBr₆ nanocrystals when the films were at room temperature and 77 K by femtosecond time-resolved transient absorption spectroscopy technique simultaneously,and we found that the PIA signal at 77 K compared to 295 K has gradually shifted to the lower energy region,and the signal shift is assigned to the variance of the energy structure.Subsequently,we studied the nanocrystal in carrier complexation kinetics and found that the diffusion rate of carriers in the film is very fast,where the single-molecule complexation rate decreases with decreasing temperature and the bimolecular complexation rate increases with dropping temperature.In addition,the nonlinear optical properties of Cs₂TiBr₆ nanocrystals were investigated.Z-scan tests were performed at excitation intensities of 340 n J,530 n J,and 650 n J,respectively,and the excitation intensity interference was excluded,and the TPA cross-section was estimated to be 1070 GM.This provides a comprehensive understanding of the photophysical properties of lead free double perovskite nanomaterials,indicating that such perovskite nanomaterials have great potential in the field of conventional electronic switches.（3）After learning the photophysical properties of nanocrystals,we found that the absorption coefficient and two-photon absorption cross section of nanocrystals are strong and suitable for applications such as solar cells and two-photon photodetectors.Therefore,in part three,we studied in detail the changes that occur when the powder material comes into contact with oxides by simulating the interfacial decomposition of Cs₂TiBr₆ after contact with oxides,and conducted environmental stability and thermal stability tests on Cs₂TiBr₆ powder samples without any oxide doping and samples doped with TiO₂,Sn O₂,and Zn O oxides,respectively.The test results showed that Zn O not only contributed to the stability improvement of Cs₂TiBr₆ powder,but also accelerated the decomposition rate of the powder;while the samples doped with TiO₂and Sn O₂ both reduced the decomposition rate of the material and had a certain degree of improvement for the powder stability.Based on the X-ray diffraction peak intensity ratio analysis we obtained that the reduction of the decomposition rate of the material by TiO₂ is more obvious under the same conditions.The thermogravimetric test results show that the introduction of the two oxides,doped TiO₂ and Sn O₂,does not change the properties of the material,and it is concluded that the most suitable material for the electron transport layer of Cs₂TiBr₆ is TiO₂.