Theoretical Study On The Structure And The Stability For BF₄ Substituted CH₃NH₃PbI₃ And CH₃NH₃PbI₃?001?Surface

Since the last century,clean renewable solar energy has received a great deal of attention.Therefore,development of new energy should be established to address energy shortage and environmental woes.As one of the cleanest energy sources,solar energy has many advantages,such as abundant reserves,easy collection,no need for mining and transportation,etc.Therefore,it is of great significance to study how to utilize solar energy effectively.The solar cells with organic-inorganic hybrid perovskite absorbent CH3NH3PbI3 as sensitizer have the advantages of good absorbency and high charge transfer rate.The maximum cell efficiency of this kind of cell has reached 22.1%.However,the stability of CH3NH3PbI3 is poor,and it is easy to degrade in wet environment,which greatly limits the large-scale production of halogenated perovskite solar cells.In order to improve the stability of CH3NH3Pbl3,scientific reseachers have made a lot of efforts.Many studies have shown that the stability of CH3NH3PbI3 can be improved by introducing Br or Cl atoms into the structure.But,the optoelectronic properties of CH3NH3PbI3 are affected when I atoms were replaced by Cl or Br atoms.Therefore,it is necessary to find a new method to improve the stability of perovskite materials.Moreover,the reaction mechanism between water molecules and perovskite materials needs further study and discussion.The method of frist-principle density functional theory has been carried out for investigating the crystal structure,electronic structure and carrier effective mass of CH3NH3PbI(3-x)(BF4)x.The properties of CH3NH3PbI3(001)surface and the effect of BF4 substitution on 001 surface are also studied.Firstly,the structure of CH3NH3Pbl3 crystal was optimized,and the calculated cell parameters were similar with the experimental data.Then the crystal structure of CH3NH3PbI(3-x)(BF4)X is optimized at the same calculation level,and the band structure and carrier effective mass of CH3NH3PbI(3-x)(BF4)x are also discussed.After that,the CH3NH3Pbl3(001)surface which is favorable to the hole transfer was studied.The surface was built by six layers of CH3NH3PbI3 unit cell along c-axis.A vacuum with the thickness of 15A was set along c-axis between the slabs,and the CH3NH3I surface was used as the termination surface.Finally,the CH3NH3I(1-x)(BF4)x-terminated surfaces(x=1,0.5)were simulated,using BF4 to replace all or part of I atoms in CH3NH3I-terminated surface.The adsorption of H2O molecule on CH3NH3I-terminated and CH3NH3I(1-x)(BF4)x-terminated surfaces was simulated,and the influence of BF4 substitution on the stability of CH3NH3PbI3 crystal was discussed.In this paper,the photoelectric performance of CH3NH3Pbl3 was studied from the following three aspects.In the first part,the structure of CH3NH3PbI3 crystal was optimized by DFT-PBE,and the calculated cell parameters were similar with the experimental data.It shows that the PBE method for the CH3NH3PbI3 system is reasonable.Density of state and band structure show that CH3NH3Pbl3 is a semiconductor material.The CH3NH3PbI3 material has a relatively suitable band gap and a large light absorption interval.The carrier effective mass of CH3NH3PbI3 is small,and the effective masses of electron and hole are close.All of that indicate CH3NH3PbI3 has bipolar conductivity and it's a potential semiconductor material.In the second part,the crystal structure,band structure and optoelectronic properties of CH3NH3PbI(3-x)(BF4)x(x=0.083,0.167,0.250,0.333)crystal were studied.The structure of CH3NH3PbI(3-x)(BF4)x crystal is similar to tetragonal phase,but the crystal symmetry has decreased,and the octahedrons are more tilted.But organic ions and octahedrons keep their original stacking mode.The band gap of CH3NH3PbI(3_x)(BF4)x are all greater than 1.52eV,and the band gap increase with the increase of x.It shows that the binding ability to electrons of BF4 is stronger than I.So becomes more and more difficult for electrons to transition from the valence band to the conduction band in CH3NH3PbI(3-x)(BF4)x crystal.The influence of BF4 substitution on the carrier effective mass in G-Z direction is the greatest.With the increase of the x value,the carrier effective mass of G-Z direction is to infinity(the electrons are completely bound),but the effective mass of electron in the direction of G-X and G-Y decreases.In the third part,different adsorption sites when H2O molecule on CH3NH3PbI3(001)surface were investigated.In general,the H2O molecule can interact with both the H atom of organic ion and I atom.Then,the structure of the CH3NH3I surface was destroyed.The interaction between H2O molecule and CH3NH3I(-x)(BF4)x surfaces is stronger than that between H2O molecule and CH3NH3I surface.When the H2O molecule interacts with CH3NH3I(1-x)(BF4)x surface,it is easier to bind to BF4.The results show that the H2O molecule interacts with BF4 group more easily.Introducing BF4 into perovskite structure can protect the inner structure and enhance the stability of the system in wet environment.