Study On Physical Properties And High-pressure Regulation Of Cs-based Halide Perovskite Materials

Due to their excellent photoelectric properties,perovskite materials have been widely studied in the direction of photovoltaic devices such as solar cells and light-emitting diodes,and it is a hot topic in scientific and commercial applications at this stage.Among them,halide perovskite has become the focus of exploration due to its high absorption coefficient,tunable band gap and simple preparation process.However,this kind of material has some disadvantages in application.Its stability will be greatly decreased if it is exposed to light or hot and humid environments for a long time,and the process of its practical application is limited.Therefore,it is of great scientific significance to explore the structural stability of halide perovskite materials and the mechanism of its influence on optical properties.It can provide a theoretical basis for solving the stability problem of perovskite materials under photothermal conditions.In this paper,the research work in this area was carried out by additive engineering and high-pressure regulation.By introducing additives into perovskite precursor,the defects can be passivated effectively and the non-radiative recombination of grain boundaries can be inhibited,and this method can obtain a flat,non-porous,and high surface coverage perovskite film.In addition,High-pressure regulation can change the crystal structure of the substance without introducing impurities,so that the perovskite materials undergo phase transition and amorphization,and then obtain perovskite materials with new structure and properties.In this paper,three representative CS-based halide perovskite materials were selected as the research objects.Firstly,the improvement of photoelectric properties of typical halide perovskite material Cs Pb Br₃was explored by using the surfactant passivation method.Secondly,considering the toxicity of Pb,the environmentally friendly and stable halide double perovskite material Cs₂Ag In Cl₆ was studied.Finally,in view of the current situation of insufficient research on low-dimensional perovskite materials,the preparation and physical properties of Pb-free zero-dimensional halide perovskite Cs₃In Br₆ were studied.The main contents of this paper include the following three aspects:（1）Study on physical properties of halide perovskite after surfactant passivationAs the most typical Cs-Pb halide perovskite material,Cs Pb Br₃ has been widely studied However,the defects formed during the growth of halide perovskite films have been one of the key problems to be solved urgently.In this paper,the nonionic surfactant polyethylene glycol p-isooctyl phenyl ether was introduced into the precursor of halide perovskite Cs Pb Br₃ as an additive,which was used to passivate the defects at the grain boundaries to improve the quality of the perovskite film,and then improve the photoelectric performance of the perovskite devices.The results showed that,compared with the non-passivated perovskite film,the perovskite films with additives were closely arranged,the grain boundaries were reduced,and the non-radiative defects at the grain boundaries were highly inhibited.The photoelectric properties and stability of the Pe LED prepared by this method had been improved.Through the optimization of doping concentration,the maximum luminous brightness of the device was 63500cd/m²,the maximum current efficiency was 17.4 cd/A,the maximum external quantum efficiency was 1.03%,and the lifetime of the device had reached 2 h.The results provide an effective way to improve the photoelectric performance of the perovskite devices.（2）Study on Phase structure and optical properties of halide double perovskite under high pressureHalide double perovskite Cs₂In Ag X₆（X=Cl,Br,I）has a direct band gap and a wide emission band width,and is regarded as an ideal lead-free substitute to lead halide perovskite.So,understanding its structure and optical properties is particularly important for its application.In this paper,the structure and optical properties of Cs₂Ag In Cl₆ double perovskite nanocrystals had been systematically investigated by using a variety of in-situ high-pressure testing techniques.The in-situ UV-VIS absorption test method with the highest pressure of 34.3 GPa was used to analyze the change of the material’s band gap under high pressure.During the process from 0 GPa to 5.81 GPa,the direct band gap in the absorption spectrum of nanocrystals gradually redshifted,and when the pressure reached 5.81 GPa,the absorption edge began to show a blue shift and continued to 12.6 GPa.The result of in-situ high-pressure X-ray diffraction（XRD）showed that in the pressure range from 0 GPa to 16 GPa,with the increase of pressure,all Bragg diffraction peaks moved to the direction of large angle significantly,but there was no obvious relative change of diffraction peaks,indicating that the crystal structure did not undergo phase transition during the pressurization process.Under pressure,halide double perovskite nanocrystals exhibited emission blue shift accompanied by red shift of absorption edge,which was attributed to the reduction of the lattice relaxation energy of Ag-Cl and In-Cl bonds during contraction and the increase of metal halide orbital overlap.The findings provide an efficient way to tune the optical properties of halide double perovskite materials.（3）Study on phase structure and optical properties of zero-dimensional halide perovskites under high pressureResearchers have a rich understanding of two-dimensional and three-dimensional halide perovskite,but lack of a systematic understanding of zero-dimensional halide perovskite.In addition,the development of perovskites with blue-light emission lags far behind green-light emission and red-light emission at this stage,which hinders the application of three-primordial light in lighting and panchromatic display.In this paper,zero-dimensional halide perovskite Cs₃In Br₆ nanocrystals with blue light emission were synthesized by thermal injection,and the samples showed intrinsic broadband blue light emission.The hollow structure of Cs₃In Br₆ nanocrystals was regulated by controlling and optimizing the synthesis process.Under UV light excitation,the hollow Cs₃In Br₆ nanocrystals exhibited broadband emission and a large Stokes shift about75nm.In the experiment,the structural stability and optical properties of Cs₃In Br₆nanocrystals were regulated by high-pressure technology,and the samples were characterized by in-situ high-pressure testing,so as to understand the relationship between the structure and physical properties of erovskite materials from a new perspective.The luminous intensity of nanocrystals decreased sharply in the process of pressurization.The luminescence intensity of nanocrystalline decreases sharply during the process of pressure,and the luminous peak changes gently when the pressure increases to 1.84 GPa.In-situ high-pressure XRD results showed that in the pressure range of 0 GPa to 22 GPa,the volume of all octahedral structures would shrink as the pressure increased,resulting in continuous contraction of the unit cell。This contraction would cause the reduction of exciton-phonon coupling strength,which would lead to the reduction of emission intensity,and finally lead to the quenching of the luminescence under pressure.The results further enrich the understanding of the structure and optical properties of zero-dimensional halide perovskite nanocrystals with blue light emission.