One-step Spin-coating Preparation And The Property Characterization Of The Perovskite CH₃NH₃Sr_xPb_1-xI₃ Films

The perovskite CH₃NH₃Sr_xPb_1-xI₃ films were fabricated on the common glass substrates by using the one-step spin-coating method.The mechanism of the formation of the film was analyzed as well.In addition,the effect of the precursor molar concentration ratio of CH₃NH₃I to PbI₂(R_mol),rotating speed,pre-annealing temperature(T_PA),spin-coating solutions and the amount of Sr were studied on the microstructure and optical properties of the sample films.The main results were shown as below:?1?In the process of the preparation of sol solution,CH₃NH₃ I existed in the form of CH₃NH₃⁺ and I^-,and PbI₂ formed eight surface inorganic metal halide template layer,of which CH₃NH₃⁺filled into the void by hydrogen bonding.Coupled with the DMF molecule in organic solvent,the three formed the PbI₂-DMF-CH₃NH₃⁺ layered structure.Therefore,in addition to CH₃NH₃PbI₃,there existed PbI₂-DMF-CH₃NH₃⁺sandwich structure in the sol solution,because the molecular coordination between PbI₂ and DMF was stronger than that between PbI₂ and CH₃NH₃I,the DMF molecules hindered the reaction of PbI₂ and CH₃NH₃I to form CH₃NH₃PbI₃.Thus,in order to glue the sol and accelerate the volatilization of organic species in the sol solution,the annealing treatment was crucial to the preparation of the films,and at the same time,the PbI₂-DMF-CH₃NH₃⁺ layered structure was collapsed,which promoted the formation of CH₃NH₃PbI₃.?2?With the decrease of R_mol,the absorption edge of the films first blueshifted and then redshifted.The former was caused by the residual CH₃NH₃I,while the latter caused by the film stress arising from the residual PbI₂.Thus,the R_mol of 1:1 was the best molar ratio to prepare the CH₃NH₃PbI₃ films.?3?When T_PA was too low,the reaction of the precursors was not complete,however,when T_PA was too high,CH₃NH₃PbI₃ was decomposed into PbI₂.The average grain size of the samples was increased in the <110> direction at T_PA<140?,and had no obvious change between 200? and 300?.The absorption edge of the film first blueshifted and then redshifted in the range of RT¹40?.The former was attributed to the thermal stress caused by the increase of T_PA,and the latter attributedto the temperature effect in the films which reduced the bond energy of CH₃NH₃PbI₃.Thus,T_PA of 100? was the best pre-annealing temperature for the preparation of CH₃NH₃PbI₃ films by the one-step spin-coating method was concluded.The critical thermal decomposition of the film was approximately 140?.?4?Mixed organic solvents weakened the crystallinity of CH₃NH₃PbI₃ thin films,which was due to the high viscosity and low volatilization rate of DMSO,thus slowing down the reaction rate of PbI₂ and CH₃NH₃I in the process of pre-annealing at 100?.the appropriate increase of the ratio of DMSO could not only effectively adjust the absorption edge of CH₃NH₃PbI₃,but also improve the luminescent intensity of perovskite CH₃NH₃PbI₃.In this experiment,the best perovskite CH₃NH₃PbI₃ films could be prepared at the volume fraction of DMF of 60% in the mixed solvent.?5?The crystal form of the perovskite film did not changed before and after the substitution of a small amount of Sr.In addition,a small amount of Sr made the surface of CH₃NH₃Sr_xPb_1-xI₃ films compact.With the increase of the amount of Sr,the crystallinity of CH₃NH₃Sr_xPb_1-xI₃ became worse,and the absorption edge of the films varied from 1.30 eV to 1.55 eV.The change of the absorption edge of the films was attributed to the stress caused by the incorporation of a small amount of Sr in the perovskite material.The film had excellent luminescent properties at SrI₂:PbI₂=1:9and 2:8.