The Stability Of Halogen Perovskite Materials And Interfacial Electron Transport Characteristics

Tin（Sn）-based perovskite materials are the best choice to replace the heavy metal element lead（Pb）in inorganic perovskite materials during the last several years.Due to the sensitivity of Sn²⁺ions in the perovskite materials to oxygen,the disadvantage of their short life restricts their commercial applications.In the series studies to improve their stability,doping is believed to be one of the effective approaches to reduce their surface defects,stable the lattice and improve the charge transportation.B-γ-CsSnI₃,a tin-based perovskite material,is easy to be oxidized in the air,and the B-γphase transformed into a relatively stable Cs₂Sn I₆ one with tetravalent tin and high symmetry double layer cubic structure.In this paper,the doping materials and synthesis approaches have been studied to enhance the stability and improve the optoelectronic properties of the tin-based perovskite CsSnI₃ and double-layer perovskite Cs₂Sn I₆ materials and films.Additionally,the electron transport behavior at the interface between the electron transport layer and the perovskite layer were further explored by preparing the multi-layer p-n junction.The purpose of this study is to provide experimental and theoretical support to improve a perovskite cell’s stability and efficiency.The main results are as follows:（1）The phase transition mechanism of the CsSnI₃ phase into the stable double layer perovskite Cs2Sn I6 one in air is investigated.The procedure of the phase transition was slowed down by F^-doping into the X position of the CsSnI₃ lattice.The delaying mechanism of phase transition is that Sn²⁺preferentially binds with F^-and forms a strong covalent bond,which slows down the oxidation procedure of Sn²⁺to Sn⁴⁺.The B-γ-CsSnI（F）₃ phase eventually transformed into Cs₂Sn I（F）₆ one,accompanied by Sn O₂ phase,but the phase transition procedure was delayed by F^-doping.（2）The modification mechanism of the X-site doped double layer perovskite Cs₂Sn I_6-2x（F）_2x films prepared by the modified two-step vapor deposition is discussed.The good crystallinity,flat and complete surface morphology is gained in the doped films DFT calculations show that doping F^-decreases the surface energy of（004）,resulting in a shift from the preferred orientation in the crystal from（222）to the C-axis-preferred（004）for carrier transport,thus enhancing the films conductivity.The bond energy of F^-ions combined with Sn⁴⁺increases and the bond angle decreases,which increases the perovskite band gap,enhances the long-wave transmittance,and effectively reduces the harm caused by the thermal effects on the cells.（3）The modification mechanism of the B-site doped double layer perovskite Cs₂Sn_1-x（Pb）_xI₆ powders synthesized via a modified hydrothermal method is deliberated.Results indicate that the 2at%Pb²⁺doped into Cs₂Sn I₆ facilitates smooth surface,small grain size,thermal stability,large light absorption range,and the photocurrent density as high as,～70μA·cm^-2.A small amount of Pb²⁺reduces the formation energy and surface energy of the compounds,which improves the stability and optoelectronic properties of the double layer perovskite Cs₂Sn I₆.（4）The modification mechanism of the B-site doped double layer perovskite Cs₂Sn_1-x（Mn）_xI₆ thin films prepared by a modified two-step vapor deposition is indagated.With the increase amount of Mn²⁺doping,the surface morphology of the film is complete,the grain size and the transmittance in the infrared region slightly increases,the resistivity decreases by an order of magnitude,and the carrier concentration increases by an order of magnitude.Mn²⁺ion doping ameliorates the nucleation-crystallization process of bilayer perovskite films,and enhances the stability and optoelectronic properties of bilayer perovskite Cs₂Sn I₆.（5）The interfacial characteristics and optoelectronic properties of Cs₂Sn I₆/ETL composite films are investigated.The simple p-n junction composed of three electron transport layers and double-layer perovskite Cs₂Sn I₆ was constructed.The results show that the square resistance and resistivity of the composite films are an order of magnitude lower than that of the perovskite layer.DFT calculation shows that the energy level of Cs₂Sn I₆ is best matched with that of Sn O₂,and the smallest carrier transition barrier,low carrier recombination rate and smallest loss.