Theoretical Study On Mechanical Properties Of Novel Halide Perovskites

The halide perovskite material ABX₃(A⁺=Cs,CH₃NH₃,CH（NH₂）₂;B²⁺=Pb,Sn,Ge:X^-=Cl,Br,I)has become one of the research hotspots in many fields such as materials,physics,and chemistry due to its controllable physical structure and excellent optoelectronic properties.And its application in light-emitting diodes（LEDs）,lasers,photodetectors and solar cells is also very extensive.In the past nearly ten years,a lot of research work has revolved around how to optimize the photovoltaic performance of halide perovskites,how to improve the stability of their devices,and how to eliminate lead toxicity in material preparation.However,there are relatively few studies on the mechanical properties of halide perovskites.Stress/strain in halide perovskite thin films inevitably exists during the fabrication and operation of solar cells.The mismatched thermal expansion between the perovskite film and the substrate can cause significant stress during the thermal annealing of the material’s fabrication process.The presence of stress can significantly lead to intrinsic instability and degradation of perovskite films.Based on stress,the performance of solar cell devices can be greatly affected by tuning the band gap,phase transition and ion migration of halide perovskites.In this thesis,first-principles calculations are used to investigate the effects of chemical composition,phase transition,structural dimension,octahedral layer thickness and octahedral connectivity on the mechanical properties of all-inorganic-cesium-based halide perovskite materials.The elastic constant Cij and elastic modulus（bulk modulus B,shear modulus G,Young’s modulus E）are used in the paper to determine the elastic properties of halide perovskites,and the geometric factors（i.e.,ionic radius,bond length,etc.）,tolerance factor,relative atomic mass)and electronic factor（i.e.,electronegativity EN,nuclear charge number）to explain the elastic tendency of halide perovskites.The main conclusions of this study are as follows:（1）The effects of different elements on the mechanical properties of the all-inorganiccesium-based halide perovskite material CsBX₃ were studied.It is found that the elastic constants(C₁₁,C₁₂,C₄₄)of the material decrease gradually as the ionic radius of the X-site anion increases[r(C₁₁)<r（Br^-）<r（I^-）],which is due to the decrease in the chemical bond strength of B-X.With the fixed X-site anion unchanged,C₁₁ and C₁₂ were observed to decrease with increasing EN（from Sn to Pb to Ge）,suggesting that the electronic structure of the element plays a key role in the elastic modulus of halide perovskites.（2）The effects of different crystal phases of CsPbI₃ with excellent optoelectronic properties on their mechanical properties were studied.For cubic（α）and tetragonal（β）CsPbI₃,the principal elastic constants(C₁₁,C₂₂,C₃₃)of the crystals show opposite trends to their lattice constants（a,b,c）.For orthorhombic（γ）CsPbI₃,the order of the three lattice constants is b>a>c,while the trend of the elastic constants is C₃₃<C₁₁<C₂₂.This may be since as the symmetry of the phase structure decreases,the structural distortion becomes more dominant in the competition between the B-X bond strength and the structural distortion.The elastic moduli B,G and E all decrease with the increase of the Pb-I bond length,and decrease with the decrease of the symmetry of the crystal structure.In addition,it was found that the structural dimension of halide perovskites can also tune their mechanical anisotropy.（3）The effects of the structural dimensions of CsPbI₃（3D,space group Pm-3m）,Cs₂PbI₄（2D,space group I₄/mcm62）and Cs₄PbBr₆（OD,space group R-3c63）on the mechanical properties of materials were studied.The results show that the relationship between the mechanical change trend of halide perovskite and the dimension is 3D(（4.914 g/cm³）>0D（4.564 g/cm³）>2D(（4.450 g/cm³）,that is,the mechanical change trend of halide perovskite related to its mass density.（4）The effect of layer number n of layered perovskite[n=1（Cs₂PbI₄）,n=2（Cs₃Pb₂I₇）and n=3(Cs₄Pb₃I₁₀)]octahedral[PbI₆]on its mechanical properties was investigated and it was found that the number of layers n can improve their elastic properties.Specifically,when n increases from 1 to 2 to 3,the in-plane and out-of-plane elastic constants not only increase,but also the in-plane elastic constants is about 12 GPa higher than the out-of-plane elastic constants,which is due to in-plane bonding by ionic or/and covalent bonding,while out-ofplane bonding is by weak van der Waals interactions.And the anisotropy of Young’s modulus（E）does not change with the layer thickness n.（5）The effect of the ratio of face sharing of the octahedral[PbI₆]on the mechanical properties of halide perovskites was investigated.The results show that increasing the facesharing ratio of[PbI₆]increases the principal elastic constants and elastic moduli of the corresponding compounds,which may be related to the increased stiffness of their lattices.（6）The smaller the radius of the X-site halogen atom of the double perovskite Cs₂AgBX₆(B³⁺=Bi,Sb,In;X^-=Cl,Br,I)or the smaller the nuclear charge number of the B-site,the more difficult the double perovskite to be compressed;And compared with single perovskites,double perovskites have better flexibility.